Search tips
Search criteria 


Logo of cshperspectCold Spring Harbor Perspectives in BiologyAboutArchiveSubscribeAlerts
Cold Spring Harb Perspect Biol. 2011 March; 3(3): a002352.
PMCID: PMC3039931

Src-family and Syk Kinases in Activating and Inhibitory Pathways in Innate Immune Cells: Signaling Cross Talk


The response of innate immune cells to growth factors, immune complexes, extracellular matrix proteins, cytokines, pathogens, cellular damage, and many other stimuli is regulated by a complex net of intracellular signal transduction pathways. The majority of these pathways are either initiated or modulated by Src-family or Syk tyrosine kinases present in innate cells. The Src-family kinases modulate the broadest range of signaling responses, including regulating immunoreceptors, C-type lectins, integrins, G-protein-coupled receptors, and many others. Src-family kinases also modulate the activity of other kinases, including the Tec-family members as well as FAK and Pyk2. Syk kinase is required for initiation of signaling involving receptors that utilize immunoreceptor tyrosine activation (ITAM) domains. This article reviews the major activating and inhibitory signaling pathways regulated by these cytoplasmic tyrosine kinases, illuminating the many examples of signaling cross talk between pathways.

Innate immune cells, including macrophages, dendritic cells, granulocytes, and mast cells, function as the first line of defense against pathogens. These cells use a dizzying array of cell-surface receptors, which are connected to an equally complicated intracellular signal transduction network, to sense pathogen molecules and then orchestrate the appropriate immune response. Among the intracellular signaling molecules that are most crucial for innate immune cells are the cytoplasmic tyrosine kinases. Two major kinase families that operate in the proximal intracellular signaling pathways in innate cells are the Src-family kinases and the Syk-ZAP70 family. A third family of kinases, the Tek family, also have important roles in innate cells. They are not discussed in detail in this article, but are reviewed elsewhere in articles on the subject.

There are eight members of the Src family; innate immune cells primarily express Hck, Fgr, Lyn, and to a lesser extent, Src (Lowell 2004). The Syk-ZAP70 family has only two members and only Syk is found in innate cells. Most innate cell types express the same spectrum of kinases with some specific cellular differences. For example, mast cells express a broader range of Src-family kinases than macrophages or dendritic cells (Colgan and Hankel 2010). In general, Src-family and Syk kinases tend to operate together in signaling pathways, with the Src-family being “upstream” or activated first in response to pathogen detection. These enzymes then communicate downstream to Tec-family members. The Tec-family kinases expressed in innate cells include Btk, Bmx, and Tec (Koprulu and Ellmeier 2009; Tohyama and Yamamura 2009). Additionally, Src-family kinases activate yet another family of PTKs, the FAK/Pyk2 tyrosine kinases, which play a major role in integrin signaling (Hauck et al. 2000).

Though primarily studied in activating pathways, Src-family and Syk kinases also activate inhibitory signaling pathways (Nimmerjahn and Ravetch 2008). In many situations, inhibitory signaling often overrides the activating signal. Pathways can also be initiated at different times or rates. Finally, to add even more complexity, activating and inhibitory pathways often interact indirectly, for example, through the production of cytokines and growth factors and not through direct intracellular biochemical interactions; Hence the term signaling “cross talk,” which now appears commonly in the literature (O'Neill 2008; Ivashkiv 2009; Page et al. 2009).


Classical Immunoreceptor Pathways

In the prototypical immunoreceptor pathway, engagement of the receptor leads to activation of Src-family kinases, which in turn phosphorylate immunoreceptor tyrosine-based activation motifs (ITAMs) present on either the receptor or associated subunits (Fig. 1). This leads to recruitment of Syk, by binding of the Syk SH2 protein domain to the phospho-ITAM residues, and activation of Syk allowing it to phosphorylate downstream substrates. One of the enzymes activated downstream is phosphoinositide 3-kinase (PI3-kinase), which generates membrane-associated phosphatidylinositol (3,4,5)-triphosphate (PIP3). The FAK/Pyk2 kinases are activated directly via the Src-family kinases, where they contribute to downstream responses involving cell adhesion and migration. Though usually depicted as a linear signaling pathway with Src-kinases at the top and FAK/Pyk2 at the bottom, there are many points of interaction and cross regulation. Together, these pathways impinge on downstream factors, such as MAPK kinases, which have broad effects on gene transcription; the Rac/Rho pathway to modulate cytoskeletal function; the inositol trisphosphate (IP3), and diacylglycerol pathway (DAG), which regulates Ca2+ entry into cells and activation of various isoforms of protein kinase C (PKC). Overall, the outline of the prototypical immunoreceptor pathway as described here is similar in both innate and adaptive immune cells (Smith-Garvin et al. 2009; Kurosaki et al. 2010).

Figure 1.
Cytoplasmic tyrosine kinases in the activating signaling pathways utilizing ITAM-containing adapters. Examples of immunoreceptors, hemi-ITAM C-type lectin receptors, and nonimmunoreceptors that utilize ITAM-signaling adapters and the cytoplasmic tyrosine ...

New Concepts in Immunoreceptor Signaling: CARD9 and Receptor Diversity

Recent and exciting developments in the immunoreceptor paradigm include recent progress in delineating how this pathway is connected to NF-κB and the demonstration that many innate immune receptors utilize the “immunoreceptor pathway” even though they lack ITAMs and therefore are not technically immunoreceptors.

The adapter protein CARD9, which contains a caspase-recruitment domain (CARD), has now been shown to be the link between a variety of ITAM-containing receptors involved in recognition of fungal and probably other pathogen structures and NF-κB (Fig. 2) (Gross et al. 2006; Gross et al. 2009). CARD9 is closely related to the lymphocyte protein CARMA-1, which forms a complex with the adapter proteins Bcl-10 and MALT1, and thus links the T- and B-cell receptors to the NF-κB pathway (Rawlings et al. 2006). CARD9 forms the same complex in innate cells. In lymphoid cells, CARMA-1 is activated by PKC isoforms (PKCθ in T-cells and PKCβ in B-cells), which phosphorylate CARMA-1, resulting in a conformational change that allows it to interact with IKK, leading to IκB turnover and NF-κB activation. In myeloid cells, it remains unclear if CARD9 activation is directly downstream of PKC activation (Hara and Saito 2009). Nevertheless, it is clear that in the absence of CARD9, receptors involved in fungal pathogen recognition (Dectin-1, Dectin-2) are unable to activate NF-κB, and more importantly, the entire repertoire of ITAM-containing receptors in myeloid cells are uncoupled from NF-κB (Robinson et al. 2006; Yamasaki et al. 2008). This results in profound defects in cytokine responses, which in vivo translates to poor responses to pathogens, specifically fungi such as Candida albicans, bacteria such as Listeria monocytogenes, and M. tuberculosis (Ruland 2008; Dorhoi et al. 2010).

Figure 2.
Tyrosine kinase pathways leading to CARD9 and TLR signaling responses. The central role of the CARD9/Malt1/Bcl10 complex in activating primarily the NF-κB pathway and to a lesser extent the MAPK pathway is shown (see Colonna 2007). Engagement ...

Immunoreceptor signaling has also recently been shown to play a role downstream of innate receptors that lack ITAM sequences. This concept evolved from studies of innate cells lacking the ITAM-signaling adapters DAP12 and the FcεRIγ chain (referred to as FcRγ) (Nimmerjahn and Ravetch 2008; Lanier 2009). Typically, DAP12 and FcRγ are coupled to immunoreceptors through charged amino acid interactions within the transmembrane regions of each protein (Fig. 1). Surprisingly, a number of nonimmunoreceptor pathways, such as neutrophil integrin signaling or IL-3 responses in basophils, are lost in innate cells derived from mice lacking DAP12 or FcRγ (Mocsai et al. 2006; Hida et al. 2009). Examples of receptors that co-opt the classical immunoreceptor pathway are shown in Figure 1. The mechanisms by which these nonimmunoreceptors couple to DAP12 and/or FcRγ to initiate signaling remain unclear.


Classical Inhibitory Pathways

Most inhibitory signals are mediated in a fashion very similar to the immunoreceptor pathway, except that inhibitory pathways are initiated through phosphorylation of immunoreceptor tyrosine-based inhibitory motifs (ITIMs) defined by the sequence amino acids I/V/L/SxYxxL/V (Fig. 3) (Munitz 2010). The Src-family kinase Lyn is primarily responsible for ITIM phosphorylation. Phospho-ITIM domains serve as docking sites for several types of phosphatases, such as SHP-1 (Src homology 2 domain containing protein tyrosine phosphatase-1/2) or SHIP-1 (SH-2-containing inositol phosphatase), which then down-modulate signaling responses by dephosphorylating downstream substrates. Like the activating pathways, this inhibitory pathway was first worked out in lymphocytes and NK cells, mainly through the study of the inhibitory Fc receptor, FcγRIIb (Ravetch and Lanier 2000). The number of newly recognized ITIM-containing inhibitory receptors in innate immune cells has grown to include a number of Ig superfamily molecules (PIR-B, Sirp-1α), sialic binding proteins, referred to as Siglecs, and a few C-type lectin receptors (Robinson et al. 2006; Crocker et al. 2007). In many cases, the ligands for these inhibitory receptors are unknown. In a broad sense, the inhibitory receptors function by limiting innate immune cell functions, including proliferation, cytokine responses, and cell adhesion. Since inhibitory pathways are often engaged at the same time as the activating pathways, the resulting cellular response depends on a balance between the activating signal and the inhibitory signal. Mutagenesis studies suggest that inhibitory signaling is usually dominant; for example, innate immune cells from Lyn kinase-deficient mice display hyperproliferative responses to cytokines and a hyperadhesive phenotype that correlates with the lack of phosphatase activation in these cells (reviewed by Scapini et al. 2009).

Figure 3.
Inhibitory pathways in innate cells utilizing ITIM or “inhibitory” ITAM signaling pathways. Shown to the left are classical inhibitory receptors that contain ITIM binding sequences within their cytoplasmic tails. Like activating receptors, ...

Inhibitory ITAMs

There is increasing evidence that ITAM-coupled activating receptors also contribute to inhibitory signaling (Fig. 3). This was first demonstrated in mast cell experiments that compared engagement of IgE-FcεR receptors by low versus high affinity haptens (Torigoe et al. 1998). The low affinity haptens suppressed the signaling responses. A similar observation was made by Pasquier et al., who found that a Fab fragment of an anti-FcαRI mAb inhibited IgG-mediated phagocytosis in a transfected macrophage cell line, suggesting that low affinity or partial activation of one ITAM-containing receptor can inhibit the function of another ITAM-containing immunoreceptor (Pasquier et al. 2005). Subsequently, this same group demonstrated that monovalent engagement of FcαRI inhibits chemotaxis to MCP-1 and TNF, as well as downstream signal transduction, in an SHP-1-dependent fashion, resulting in the attenuation of tissue injury in an Ab-induced glomerulonephritis model (Kanamaru et al. 2008). This has led to the model whereby low affinity or partial (i.e., monomeric) engagement of activating receptors results in only modest ITAM phosphorylation events, leading to the recruitment of phosphatases (SHP-1 in particular) instead of Syk kinase (Pinheiro da Silva et al. 2008), resulting in an overall diminution of cellular response (Fig. 3).

The activation of ITAM signaling pathways also functions to inhibit other signaling responses, such as TLR signaling (Hamerman et al. 2009). In particular, both macrophages and dendritic cells derived from DAP12 and FcRγ-deficient mice display increased responses to various TLR ligands, such as CpG, endotoxin (LPS), or yeast zymosan (Hamerman et al. 2005; Chu et al. 2008). In macrophages, this inhibitory signaling may be mediated through TREM-2, which is a DAP12 coupled receptor thought to be involved in recognition of various bacterial structures and potentially apoptotic cells (Hamerman et al. 2006; Takahashi et al. 2007; Hsieh et al. 2009; N'Diaye et al. 2009). Additional FcRγ and DAP12-associated receptors on both mouse and human plasmacytoid DCs have also been found to inhibit interferon production following TLR stimulation (Blasius et al. 2004; Fuchs et al. 2005; Swiecki and Colonna 2007).

The molecular mechanisms by which ITAM-based pathways cross-inhibit TLR responses remain unclear (reviewed by Ivashkiv 2008). In addition to phosphatase recruitment, activation of PI3-kinase-mediated pathways that both suppress NF-κB activation and alter recruitment of MyD88 and Mal adaptors to TLRs have been proposed (Kagan and Medzhitov 2006). Activation of calcineurin by ITAM signaling, leading to dephosphorylation of TLR signaling pathway molecules, may also be involved (Kang et al. 2007). Recently, Han et al. have provided an elegant model for ITAM-mediated down regulation of TLR responses (Han et al. 2010). Building on the observation that Mac-1-deficient mice (lacking the integrin αm protein that dimerizes with the β2 chain to form Mac-1) demonstrate increased inflammatory responses to dextran sodium sulfate-induced colitis (Abdelbaqi et al. 2006), this group and others (Wang et al. 2010) found that strong ligation of β2 integrins in macrophages and dendritic cells induces resistance to subsequent stimulation by various TLR ligands. Since signaling through β2 integrins proceeds in a DAP12/FcRγ to Syk pathway, Han et al. (2010) investigated potential intracellular Syk substrates. They found that Syk phosphorylates the TLR adapter proteins MyD88 and TRIF on specific tyrosine residues (Y277 and Y375, respectively), and that phosphorylation of these residues results in recruitment of the E3 ubiquitin ligase Cbl-b, which targets both the adapter proteins and Syk kinase itself for ubiquitin-mediated proteolysis, thus attenuating TLR responses (reviewed by Means and Luster 2010). This unique pathway provides a direct intracellular mechanism for signaling cross talk between ITAM and TLR pathways. It is likely that under physiologic situations of high affinity integrin ligation, such as during diapedisis through vascular endothelium, signaling through this pathway limits innate immune cell activation and reduces inadvertent immunopathology.

Indirect Down-modulation of Signaling: Pathway Cross Talk

While most studies have focused on intracellular mechanisms for inhibitory signaling, there is growing recognition that immunoreceptor activation can also be limited by post-transcriptional events. Following engagement of ITAM pathways in macrophages or dendritic cells, production of inhibitory cytokines such IL-10 and TGF-β can lead to subsequent down-modulation of signaling, both for the ITAM pathway itself and for other pathways, such as the TLRs. For example, engagement of the C-type lectin receptor DC-SIGN by ManLAM, a cell-wall component present in Mycobacterium tuberculosis, down-modulates subsequent stimulation of DCs by LPS (Geijtenbeek et al. 2003). The mechanism involves acetylation of the p65 subunit of NF-κB, leading to prolonged and increased production of IL-10, which in turn inhibits LPS and other TLR responses (Gringhuis et al. 2007). More recently, IL-10 production by neutrophils, following activation through C-type lectin receptors or other ITAM-Syk pathways, has been found to have an immunomodulatory effect on subsequent inflammatory responses (Zhang et al. 2009). Hence, in states of chronic mycobacterial infection, engagement of the C-type lectin/Syk pathway leads to enough neutrophil-derived IL-10 to limit responses of macrophages and DCs. Thus, neutrophil depletion can sometimes increase the immunopathology associated with infection. Similarly, high affinity engagement of β2 integrins on human macrophages can lead to sufficient production of IL-10 to down-modulate subsequent TLR stimulation (Wang et al. 2010). In all of these cases, the activating stimulation (either through the C-type lectins or the β2 integrins) needs to be present for hours to days to allow for sufficient production of IL-10 and other inhibitors. For integrin signaling, this indirect pathway is probably additive with the direct biochemical mechanism of inhibitory cross talk via MyD88/TRIF phosphorylation, but would dominate during prolonged periods of integrin activation potentially to limit chronic inflammation.


The history of tyrosine kinase involvement in innate immune responses to pathogen molecules (LPS, bacterial cell wall components, and foreign DNA) is long and confusing. Many of these studies pre-date the identification of the Toll-like receptors (TLRs) as the major receptors involved in recognition of pathogens. These studies were based on the findings that LPS treatment of innate immune cells increases overall protein tyrosine phosphorylation and various tyrosine kinase inhibitors, particularly those that broadly target Src-family kinases, and could block cellular responses to LPS (Orlicek et al. 1999; Smolinska et al. 2008). The hypothesis that TLRs could activate Src-family kinases were refuted by the finding that macrophages lacking Hck, Fgr, and Lyn displayed normal (or even enhanced) responses to LPS treatment (Meng and Lowell 1997). Indeed, recent studies have suggested that Lyn kinase functions as a negative regulator of TLR responses, based on the finding that Lyn-deficient macrophages show increased cytokine responses to TLR4 and TLR2 stimulation (Keck et al. 2010). The mechanism by which Lyn functions as an inhibitor of TLR responses is still unclear (Kagan and Medzhitov 2006).


Src-family Kinases

The Src-family kinases make up the largest family of cytoplasmic tyrosine kinases expressed in innate cells. Of the nine Src-family kinases, most are present in one type of innate cell or another, with Hck, Fgr, and Lyn kinase being most heavily expressed in monocytes, macrophages, granulocytes, and DCs. These kinases have been implicated as primary signaling molecules downstream of a host of immune cell receptors, including immunoreceptors, cytokine receptors, integrins, and various pathogen receptors (TREMs and Dectins, as examples). Inhibitor studies and use of knockout mice have demonstrated a critical role for Src-family kinases in a variety of host defense and inflammatory conditions (Okutani et al. 2006; Abram and Lowell 2008; Ingley 2008). In some cases, these kinases play clear and direct roles in innate immune signaling, in both activating and inhibitory pathways (Figs. 1 and and3),3), but in other cases their role may be more indirect (for example, by modulating cytokine responses that impact TLR pathways). The Src-family kinases are also responsible for phosphorylation and direct activation of other cytoplasmic tyrosine kinases, in particular Tec-family and FAK/Pyk2 and less directly Syk; hence Src-family members are often at the top of most signaling pathways in innate cells.

Src-family Kinases in Immunoreceptor Pathways

The most well-understood function of these kinases is in the classical immunoreceptor activating (ITAM) pathway (Fig. 1) utilized by many immunoreceptors (Fc receptors, NK activating receptors, and pathogen recognition molecules such as TREM family members), where various Src-family members are known to phosphorylate residues on the receptor associated ITAM adapter to initiate downstream signaling responses. As stated above, the major new finding in this area has been the demonstration that many innate immune receptors that are not classical immunoreceptors in fact utilize this same ITAM-mediated mechanism for intracellular signaling, and hence loss of Src-family activity directly affects these pathways as well. The best example is the integrin pathway—loss of Src-family kinase activity (or removal of the ITAM adapters FcRγ and DAP12, the substrates of the kinases) results in a complete deficiency of β1, β2, and β3 integrin function in innate cells (Abram and Lowell 2009). In all of these pathways, it is uncertain how the Src-family kinases are activated following ligand binding by the receptor. Dephosphorylation of the regulatory C-terminal tyrosine in Src-family kinases, which activates these enzymes, occurs through the action of either receptor tyrosine phosphatases such as CD45 or CD148, or potentially other cytoplasmic tyrosine phosphatases (LMW-PTP and Lyp/PEP, as examples) (Hermiston et al. 2009; Saunders and Johnson 2010; Zambuzzi et al. 2010); however, whether these phosphatases are directly recruited to the signaling receptors remains unclear.

Src-family Kinases in Other Innate Pathways

Src-family kinases also play important roles in pathways where ITAM/immunoreceptor molecules are not involved. A number of studies suggest that Src-family kinases work with Jak kinases in supporting cytokine responses, either by phosphorylation of receptor subunits or potentially phosphorylating Stat molecules (Reddy et al. 2000; Hayakawa and Naoe 2006). For the classical growth factor receptors, such as the G-CSF or the GM-CSF receptor, Src-kinases have been found to be physically associated with the receptor through SH3/receptor interactions involving membrane proximal regions (Sampson et al. 2007; Perugini et al. 2010). In other cytokine responses, Src-family kinases are thought to act more downstream of the receptor, often through interactions with TRAF signaling molecules. Such is the case in the RANK/RANKL signaling response, where interaction of Src and TRAF6 lead to enhancement of downstream responses (particularly activation of PI3-kinase activity and downstream Akt function) (Leibbrandt and Penninger 2008). Formation of TRAF6/Src signaling complexes has also been reported downstream of IL-1 and TNFα signaling pathways, as well as in CD40/CD40L interactions, again with the implication that this complex is involved in stimulating downstream PI3-kinase signaling (Mukundan et al. 2005; Wang et al. 2006).

Src-family kinases have also been shown to be involved in IL-6 signaling pathways, through a direct interaction with the IL-6 receptor signaling protein gp130 (Hallek et al. 1997; Hausherr et al. 2007). This signaling function of Src-family kinases may be particularly important in myeloma cells, where IL-6 serves as a growth factor for these cells. A number of studies have shown involvement of Src-family kinases downstream of the M-CSF receptor in macrophages and myeloid progenitors, with their function being to couple receptor activation to downstream PI3-kinase pathways (Lee and States 2000; Bourgin-Hierle et al. 2008). As in the case of the IL-3 receptor, the M-CSF receptor pathway may also co-opt ITAM-containing adapters to activate downstream functions, since DAP12 has been recently found to be required for optimal M-CSF receptor signaling (Zou et al. 2008). Finally, as in the immunoreceptor pathways, not all the signaling functions of Src kinases in cytokine pathways are activating, since the deficiency of Lyn kinase actually leads to enhanced responses to some cytokines, such as G-CSF, potentially through reduced recruitment of phosphatases (Mermel et al. 2006). The mechanism through which Lyn kinase functions as an inhibitor, while other Src-family members function as activators of cytokine signaling, remains unclear.

Src-family Kinases and GPCRs

Several Src-family kinases have been implicated in regulating chemokine or chemoattractant receptor signaling, which are mediated by G-protein-coupled receptors (GPCRs) (Luttrell and Luttrell 2004). Lyn kinase is activated in macrophages downstream of both CXCR4 and CCR5, and is thought to couple these GPCRs to the MAPK and PI3-kinase pathways (Ptasznik et al. 2002; Tomkowicz et al. 2006; Cheung et al. 2009). Inhibitor studies have also placed Src-family kinases downstream of monocyte-chemoattractant protein (MCP-1) signaling via CCR2 (Arefieva et al. 2005) and IL-8 signaling via CXCR1 (Sai et al. 2008). Similarly, neutrophils derived from hck−/−fgr−/− double mutant mice show significant functional defects following formyl peptide (fMLF) stimulation, which involves signaling through GPCR coupled formyl peptide receptors (Fumagalli et al. 2007). But even in the GPCR signaling pathway, the same paradigm of Src-kinases acting both in a positive and inhibitory fashion seems to be established. While Hck/Fgr-deficient neutrophils have reduced responses to foryml peptide stimulation, they manifest hyper-responsive signaling to chemokines that signal through CCR1 and CXCR2 (Zhang et al. 2005). This inhibitory function in the CCR1/CXCR2 pathways involves phosphorylation of the ITIM-containing inhibitory receptor PIR-B, which in turn is involved in recruiting tyrosine phosphatases that modulate downstream responses from the receptors.

Src-family Kinases and Selectin Signaling

Src-family kinases have been implicated in the innate immune cell response to selectin engagement, primarily recognition of E-selectin by leukocyte PSGL-1 and CD44 counter-receptors. Engagement of these receptors leads to activation of tyrosine phosphorylation that is inhibited by PP2 and other relative Src-family-kinase-specific inhibitors (Hidari et al. 1997; Kumar et al. 2001). The Fgr kinase seems dominant in this signaling response (Zarbock et al. 2008), although there is clearly some redundancy with other Src-family members (Yago et al. 2010). However, like the integrin pathway, it appears that selectin recognition also depends on ITAM adapters, since this signaling is reduced in DAP12-deficient neutrophils (Zarbock et al. 2008).

Src-family Kinases and Membrane Bound Receptors

A number of GPI-linked proteins in innate cells are known to signal through Src-family kinases, including TLR4-associated CD14 and the urokinase plasminogen activator receptor (uPA-R) (Stefanova et al. 1993; D'Alessio and Blasi 2009). In innate immune cells (mainly macrophages), engagement of these receptors leads to increased tyrosine phosphorylation and subsequent signaling events that affect adhesion and migration. It is likely that clustering of these receptors leads to aggregation of lipid raft membrane structures, through the GPI linkages in the receptors, which in turn brings the kinases together, since they too are located in the raft structures.

Src-family Kinase Connections to Tec Kinases

Besides their function in TLR pathways, Tec-family kinases also play important roles in downstream immunoreceptor signaling, where their activation is mediated both through PIP3 generation by PI3-kinase and by direct phosphorylation by Src-family kinases. Monocytes from patients with Bruton's agammaglobulinemia show reduced uptake of both Ig and complement opsonized particles (Amoras et al. 2003; Jongstra-Bilen et al. 2008). Studies in macrophage cell lines suggest that Btk and Tec communicate to the actin polymerization pathway, since these kinases localize to phagocytic cups near sites of actin polymerization. Btk and Tec also play a significant role in macrophage/osteoclast RANK/RANKL signaling. Remarkably, like Src or Src/Hck-deficient mice, the double mutant Tec−/−Btk−/− mice have severe osteopetrosis due to impaired osteoclast maturation (Shinohara et al. 2008). Since it is known that RANK signaling is mediated, in part, through DAP12 (Koga et al. 2004), the effect of Tec/Btk deficiency may reflect impaired activation of downstream actin polymerization responses.

Src-family Kinase Connections to FAK/Pyk2 in Integrin Pathways

Based on studies in nonimmune cells, Src-family kinases can directly modulate the activity of FAK/Pyk2 kinases (Fig. 1). FAK is widely expressed in most cells of the body, while Pyk2 is expressed mainly in the nervous system, T-cells, and various innate cells. FAK kinase has been very extensively studied in fibroblasts and various tumor cell types, where it plays an important role in cell adhesion signaling downstream of integrin activation (Tomar and Schlaepfer 2009). More limited studies of Pyk2 suggest a similar function. Both of these kinases are substrates for Src-family kinases following integrin ligation. Phosphorylation of FAK or Pyk2 by Src-family members leads to protein unfolding and activation of their enzymatic activity. In innate cells, FAK and Pyk2 are found in podosomes, which are the main contact sites in leukocytes (Calle et al. 2006). Macrophages lacking FAK exhibit elevated protrusive activity, altered adhesion dynamics, impaired chemotaxis, and elevated basal Rac1 activity, leading to a marked inability to form stable lamellipodia necessary for directional locomotion (Okigaki et al. 2003; Owen et al. 2007a). These defects point to an alteration in integrin outside-in signaling, consistent with the role of this kinase in fibroblasts and tumor cells. As a result of this defect, recruitment of FAK-deficient monocytes to sites of inflammation is impaired. A similar phenotype is observed in macrophages derived from Pyk2-deficient animals (Okigaki et al. 2003). The involvement of FAK and Pyk2 in macrophage integrin signaling may also affect bacterial phagocytosis by these receptors, since siRNA-mediated knockdown of FAK and/or Pyk2 can reduce uptake of various Yersina strains of bacteria (Owen et al. 2007b). FAK deficiency also reduces neutrophil adhesion, migration, and antibacterial uptake, again potentially through defects in integrin-mediated signaling events (Kasorn et al. 2009). Inhibition of Pyk2 function in neutrophils, through use of inhibitory peptides transduced into the cells, reduces cell adhesion and spreading following integrin activation, again suggesting a similar role for this kinase in neutrophil integrin signaling (Han et al. 2003). Pyk2 has also been implicated in the integrin signaling responses leading to cytokine (in particular IL-10) production (Wang et al. 2010). Despite these initial studies, the specific individual functions of FAK versus Pyk2 in innate immune integrin signaling remains to be defined.

Syk Kinase

The repertoire of signaling pathways that Syk has been implicated in (reviewed by Mocsai et al. 2010) is more restricted than Src-family kinases. Syk kinase is activated by engagement of its two SH2 domains by phospho-ITAM domains; hence it functions only in ITAM-like pathways. This includes, of course, the novel “inhibitory ITAM” pathway described above. It is likely that Syk is only involved in the subset of signaling pathways that also depend on Src-family kinases. However, this doesn't mean that blockade of Syk kinases always produces the same phenotype in a given immunoreceptor signaling response that blockade of Src-family members does. For example, the defect in macrophage phagocytosis in Src-family-kinase-deficient cells is different than in Syk-deficient cells. Lack of Src-family kinases produces a moderate to severe defect in FcR-mediated particle uptake due to a reduction in initial actin polymerization at the phagocytic cup (Fitzer-Attas et al. 2000). However, Syk-deficient cells show a complete block in FcR-mediated particle uptake due to a block in fusion of the arms of the phagocytic cup, a step subsequent to actin polymerization events (Crowley et al. 1997). Hence, it is likely that the more proximal Src-family kinases signal to many other pathways besides just ITAM-mediated Syk activation. Finally, it should be noted that Syk has been implicated in some pathways in which the role of Src-family kinases are unknown. This is particularly true for the C-type lectin receptors in innate cells.

Syk and ITAM Pathways

Syk kinase plays a critical role in all innate immune cell signaling pathways involving ITAM or ITAM-like signaling adapters. This includes signaling from classical immunoreceptors (FcRs, TREMs, Dectin-2, and others), nonimmunoreceptors that co-opt the ITAM adapters for signaling (integrins, selectins, IL-3 receptor), and those C-type lectin receptors that have hemi-ITAM sequences embedded in their C-terminal tails (Dectin-1) (Fig. 1). Since many of the C-type lectin and other immunoreceptor-like pathways converge on Syk, loss of Syk activity produces a more profound block than loss of any specific receptor (LeibundGut-Landmann et al. 2007). These same C-type lectin receptors also are involved in innate immune recognition of mycobacterial and viral pathogen molecules, all of which are affected by Syk blockade in either macrophages or dendritic cells (Chen et al. 2008; Werninghaus et al. 2009).

Syk and the Inflammasome

In addition to signaling downstream through CARD9 to NF-κB, Syk-dependent signaling events have been linked to activation of intracellular pattern recognition receptors, which in turn activate the inflammasome complex leading to IL-1β production. For example, activation of the Nod-like receptor protein NLRP3 requires upstream Syk activity during innate immune cell responses to fungal molecules (Gross et al. 2009). Similarly, Syk-deficient dendritic cells fail to respond to monosodium urate crystals (present in the joints of gout patients), a pathway also known to require NLRP3 (Ng et al. 2008; Martinon 2010). It remains unclear how Syk activation is coupled to the NLRP3/inflammasome complex.

In contrast to Src-family kinases, it is likely that Syk does not signal in cytokine or GPCR-linked pathways. Lack of Syk has no impact on neutrophil, macrophage, or mast cell recognition of various cytokine growth factors or GPCR agonists, such as formyl peptides, ATP, or other agents (Mocsai et al. 2003).


The roles of cytoplasmic tyrosine kinases in innate immune responses are complex, ranging from direct signaling involvement in very defined pathways (such as Syk in C-type lectin receptor signaling) to more diffuse interactions (Src-family kinases somehow regulating GPCR responses) to indirect secondary effects (Src-family and Syk function in immunoreceptor pathways leading to IL-10 production that feeds back on TLR-mediated responses). It is clear that Src-family members have the broadest effects on overall signaling, while Syk has more defined roles. This diffuse, versus very defined, functional role is mirrored in the fact that deficiencies in single Src-family kinases tend to produce limited signaling defects in innate cells, while loss of Syk has very defined broad functional effects. Because of redundancy, studying the role of Src-kinases in any pathway often requires blocking multiple family members, after which one often finds that many signaling responses are affected. This overall broad function for Src-family members is analogous to how a rheostat controls lighting: It dials up and dials down responses in a graded fashion. In contrast, Syk acts more like a signaling switch: It is critically required in an absolute way in a limited number of pathways.

When viewed in this fashion, it becomes obvious that if we are to design therapeutics that target these kinases for use in inflammatory or autoimmune disease, we are better off focusing on the switch kinases (Syk) rather than the rheostat kinases (Src-family), since targeting the former will produce defined effects. Indeed, a number of companies are close to producing/releasing highly active Syk kinase inhibitors that have strong potential for treatment of immune-mediated disease (Cohen and Fleischmann 2010; Colonna et al. 2010). In contrast, most of the Src-family inhibitors produced and used clinically so far are rather broadly acting, and hit enzymes besides just Src-family members. This may be useful for when treating cancer, which is where most of the anti-Src kinase drugs have been used (Kim et al. 2009), but is likely problematic for chronic treatment of immune-mediated disease. Moreover, distinguishing between individual Src-family members may be chemically impossible to achieve. Nevertheless, given our ever-expanding understanding of kinase-mediated signaling pathways in innate cells, it is quite likely that in the very near future we will see highly active drugs that block these pathways, resulting potentially in clinical benefit.


CAL is supported by NIH grants AI065495 and AI068150. We thank Clare Abram, Patrizia Scapini, and Chrystelle Lamagna for critical reading of the manuscript.


Editors: Lawrence E. Samelson and Andrey S. Shaw

Additional Perspectives on Immunoreceptor Signaling available at


  • Abdelbaqi M, Chidlow JH, Matthews KM, Pavlick KP, Barlow SC, Linscott AJ, Grisham MB, Fowler MR, Kevil CG 2006. Regulation of dextran sodium sulfate induced colitis by leukocyte beta 2 integrins. Lab Invest 86: 380–390 [PubMed]
  • Abram CL, Lowell CA 2008. The diverse functions of Src family kinases in macrophages. Front Biosci 13: 4426–4450 [PubMed]
  • Abram CL, Lowell CA 2009. The ins and outs of leukocyte integrin signaling. Annu Rev Immunol 27: 339–362 [PMC free article] [PubMed]
  • Amoras AL, Kanegane H, Miyawaki T, Vilela MM 2003. Defective Fc-, CR1- and CR3-mediated monocyte phagocytosis and chemotaxis in common variable immunodeficiency and X-linked agammaglobulinemia patients. J Investig Allergol Clin Immunol 13: 181–188 [PubMed]
  • Arefieva TI, Kukhtina NB, Antonova OA, Krasnikova TL 2005. MCP-1-stimulated chemotaxis of monocytic and endothelial cells is dependent on activation of different signaling cascades. Cytokine 31: 439–446 [PubMed]
  • Blasius A, Vermi W, Krug A, Facchetti F, Cella M, Colonna M 2004. A cell-surface molecule selectively expressed on murine natural interferon-producing cells that blocks secretion of interferon-alpha. Blood 103: 4201–4206 [PubMed]
  • Bourgin-Hierle C, Gobert-Gosse S, Therier J, Grasset MF, Mouchiroud G 2008. Src-family kinases play an essential role in differentiation signaling downstream of macrophage colony-stimulating factor receptors mediating persistent phosphorylation of phospholipase C-gamma2 and MAP kinases ERK1 and ERK2. Leukemia 22: 161–169 [PubMed]
  • Calle Y, Burns S, Thrasher AJ, Jones GE 2006. The leukocyte podosome. Eur J Cell Biol 85: 151–157 [PubMed]
  • Chen ST, Lin YL, Huang MT, Wu MF, Cheng SC, Lei HY, Lee CK, Chiou TW, Wong CH, Hsieh SL 2008. CLEC5A is critical for dengue-virus-induced lethal disease. Nature 453: 672–676 [PubMed]
  • Cheung R, Malik M, Ravyn V, Tomkowicz B, Ptasznik A, Collman RG 2009. An arrestin-dependent multi-kinase signaling complex mediates MIP-1beta/CCL4 signaling and chemotaxis of primary human macrophages. J Leukoc Biol 86: 833–845 [PubMed]
  • Chu CL, Yu YL, Shen KY, Lowell CA, Lanier LL, Hamerman JA 2008. Increased TLR responses in dendritic cells lacking the ITAM-containing adapters DAP12 and FcRγ. Eur J Immunol 38: 166–173 [PMC free article] [PubMed]
  • Cohen S, Fleischmann R 2010. Kinase inhibitors: a new approach to rheumatoid arthritis treatment. Curr Opin Rheumatol 22: 330–335 [PubMed]
  • Colgan JD, Hankel IL 2010. Signaling pathways critical for allergic airway inflammation. Curr Opin Allergy Clin Immunol 10: 42–47 [PMC free article] [PubMed]
  • Colonna L, Catalano G, Chew C, D'Agati V, Thomas JW, Wong FS, Schmitz J, Masuda ES, Reizis B, Tarakhovsky A, et al. 2010. Therapeutic Targeting of Syk in Autoimmune Diabetes. J Immunol 185: 1532–1543 [PMC free article] [PubMed]
  • Colonna M 2007. All roads lead to CARD9. Nat Immunol 8: 554–555 [PubMed]
  • Crocker PR, Paulson JC, Varki A 2007. Siglecs and their roles in the immune system. Nat Rev Immunol 7: 255–266 [PubMed]
  • Crowley MT, Costello PS, Fitzer-Attas CJ, Turner M, Meng F, Lowell C, Tybulewicz VL, DeFranco AL 1997. A critical role for Syk in signal transduction and phagocytosis mediated by Fcgamma receptors on macrophages. J Exp Med 186: 1027–1039 [PMC free article] [PubMed]
  • D'Alessio S, Blasi F 2009. The urokinase receptor as an entertainer of signal transduction. Front Biosci 14: 4575–4587 [PubMed]
  • Dorhoi A, Desel C, Yeremeev V, Pradl L, Brinkmann V, Mollenkopf HJ, Hanke K, Gross O, Ruland J, Kaufmann SH 2010. The adaptor molecule CARD9 is essential for tuberculosis control. J Exp Med 207: 777–792 [PMC free article] [PubMed]
  • Fitzer-Attas CJ, Lowry M, Crowley MT, Finn AJ, Meng F, DeFranco AL, Lowell CA 2000. Fcγ receptor-mediated phagocytosis in macrophages lacking the Src family tyrosine kinases Hck, Fgr, and Lyn. J Exp Med 191: 669–682 [PMC free article] [PubMed]
  • Fuchs A, Cella M, Kondo T, Colonna M 2005. Paradoxic inhibition of human natural interferon-producing cells by the activating receptor NKp44. Blood 106: 2076–2082 [PubMed]
  • Fumagalli L, Zhang H, Baruzzi A, Lowell CA, Berton G 2007. The Src family kinases Hck and Fgr regulate neutrophil responses to N-formyl-methionyl-leucyl-phenylalanine. J Immunol 178: 3874–3885 [PubMed]
  • Geijtenbeek TB, Van Vliet SJ, Koppel EA, Sanchez-Hernandez M, Vandenbroucke-Grauls CM, Appelmelk B, Van Kooyk Y 2003. Mycobacteria target DC-SIGN to suppress dendritic cell function. J Exp Med 197: 7–17 [PMC free article] [PubMed]
  • Gilfillan AM, Rivera J 2009. The tyrosine kinase network regulating mast cell activation. Immunol Rev 228: 149–169 [PMC free article] [PubMed]
  • Graham LM, Brown GD 2009. The Dectin-2 family of C-type lectins in immunity and homeostasis. Cytokine 48: 148–155 [PMC free article] [PubMed]
  • Gringhuis SI, den Dunnen J, Litjens M, van Het Hof B, van Kooyk Y, Geijtenbeek TB 2007. C-type lectin DC-SIGN modulates Toll-like receptor signaling via Raf-1 kinase-dependent acetylation of transcription factor NF-kappaB. Immunity 26: 605–616 [PubMed]
  • Gross O, Gewies A, Finger K, Schafer M, Sparwasser T, Peschel C, Forster I, Ruland J 2006. Card9 controls a non-TLR signalling pathway for innate anti-fungal immunity. Nature 442: 651–656 [PubMed]
  • Gross O, Poeck H, Bscheider M, Dostert C, Hannesschlager N, Endres S, Hartmann G, Tardivel A, Schweighoffer E, Tybulewicz V, et al. 2009. Syk kinase signalling couples to the Nlrp3 inflammasome for anti-fungal host defence. Nature 459: 433–436 [PubMed]
  • Hallek M, Neumann C, Schaffer M, Danhauser-Riedl S, von Bubnoff N, de Vos G, Druker BJ, Yasukawa K, Griffin JD, Emmerich B 1997. Signal transduction of interleukin-6 involves tyrosine phosphorylation of multiple cytosolic proteins and activation of Src-family kinases Fyn, Hck, and Lyn in multiple myeloma cell lines. Exp Hematol 25: 1367–1377 [PubMed]
  • Hamerman JA, Jarjoura JR, Humphrey MB, Nakamura MC, Seaman WE, Lanier LL 2006. Cutting edge: inhibition of TLR and FcR responses in macrophages by triggering receptor expressed on myeloid cells (TREM)-2 and DAP12. J Immunol 177: 2051–2055 [PubMed]
  • Hamerman JA, Ni M, Killebrew JR, Chu CL, Lowell CA 2009. The expanding roles of ITAM adapters FcRgamma and DAP12 in myeloid cells. Immunol Rev 232: 42–58 [PMC free article] [PubMed]
  • Hamerman JA, Tchao NK, Lowell CA, Lanier LL 2005. Enhanced Toll-like receptor responses in the absence of signaling adaptor DAP12. Nat Immunol 6: 579–586 [PMC free article] [PubMed]
  • Han C, Jin J, Xu S, Liu H, Li N, Cao X 2010. Integrin CD11b negatively regulates TLR-triggered inflammatory responses by activating Syk and promoting degradation of MyD88 and TRIF via Cbl-b. Nat Immunol 11: 734–742 [PubMed]
  • Han H, Fuortes M, Nathan C 2003. Critical role of the carboxyl terminus of proline-rich tyrosine kinase (Pyk2) in the activation of human neutrophils by tumor necrosis factor: separation of signals for the respiratory burst and degranulation. J Exp Med 197: 63–75 [PMC free article] [PubMed]
  • Hara H, Saito T 2009. CARD9 versus CARMA1 in innate and adaptive immunity. Trends Immunol 30: 234–242 [PubMed]
  • Hauck CR, Klingbeil CK, Schlaepfer DD 2000. Focal adhesion kinase functions as a receptor-proximal signaling component required for directed cell migration. Immunol Res 21: 293–303 [PubMed]
  • Hausherr A, Tavares R, Schaffer M, Obermeier A, Miksch C, Mitina O, Ellwart J, Hallek M, Krause G 2007. Inhibition of IL-6-dependent growth of myeloma cells by an acidic peptide repressing the gp130-mediated activation of Src family kinases. Oncogene 26: 4987–4998 [PubMed]
  • Hayakawa F, Naoe T 2006. SFK-STAT pathway: an alternative and important way to malignancies. Ann N Y Acad Sci 1086: 213–222 [PubMed]
  • Hermiston ML, Zikherman J, Zhu JW 2009. CD45, CD148, and Lyp/Pep: critical phosphatases regulating Src family kinase signaling networks in immune cells. Immunol Rev 228: 288–311 [PMC free article] [PubMed]
  • Hida S, Yamasaki S, Sakamoto Y, Takamoto M, Obata K, Takai T, Karasuyama H, Sugane K, Saito T, Taki S 2009. Fc receptor γ-chain, a constitutive component of the IL-3 receptor, is required for IL-3-induced IL-4 production in basophils. Nat Immunol 10: 214–222 [PubMed]
  • Hidari KI, Weyrich AS, Zimmerman GA, McEver RP 1997. Engagement of P-selectin glycoprotein ligand-1 enhances tyrosine phosphorylation and activates mitogen-activated protein kinases in human neutrophils. J Biol Chem 272: 28750–28756 [PubMed]
  • Hsieh CL, Koike M, Spusta SC, Niemi EC, Yenari M, Nakamura MC, Seaman WE 2009. A role for TREM2 ligands in the phagocytosis of apoptotic neuronal cells by microglia. J Neurochem 109: 1144–1156 [PMC free article] [PubMed]
  • Ingley E 2008. Src family kinases: regulation of their activities, levels and identification of new pathways. Biochim Biophys Acta 1784: 56–65 [PubMed]
  • Ivashkiv LB 2008. A signal-switch hypothesis for cross-regulation of cytokine and TLR signalling pathways. Nat Rev Immunol 8: 816–822 [PMC free article] [PubMed]
  • Ivashkiv LB 2009. Cross-regulation of signaling by ITAM-associated receptors. Nat Immunol 10: 340–347 [PMC free article] [PubMed]
  • Jongstra-Bilen J, Puig Cano A, Hasija M, Xiao H, Smith CI, Cybulsky MI 2008. Dual functions of Bruton's tyrosine kinase and Tec kinase during Fcgamma receptor-induced signaling and phagocytosis. J Immunol 181: 288–298 [PubMed]
  • Kagan JC, Medzhitov R 2006. Phosphoinositide-mediated adaptor recruitment controls Toll-like receptor signaling. Cell 125: 943–955 [PubMed]
  • Kanamaru Y, Pfirsch S, Aloulou M, Vrtovsnik F, Essig M, Loirat C, Deschenes G, Guerin-Marchand C, Blank U, Monteiro RC 2008. Inhibitory ITAM signaling by FcαRI-FcRγ chain controls multiple activating responses and prevents renal inflammation. J Immunol 180: 2669–2678 [PubMed]
  • Kanazawa N 2007. Dendritic cell immunoreceptors: C-type lectin receptors for pattern-recognition and signaling on antigen-presenting cells. J Dermatol Sci 45: 77–86 [PubMed]
  • Kang YJ, Kusler B, Otsuka M, Hughes M, Suzuki N, Suzuki S, Yeh WC, Akira S, Han J, Jones PP 2007. Calcineurin negatively regulates TLR-mediated activation pathways. J Immunol 179: 4598–4607 [PubMed]
  • Kasorn A, Alcaide P, Jia Y, Subramanian KK, Sarraj B, Li Y, Loison F, Hattori H, Silberstein LE, Luscinskas WF, et al. 2009. Focal adhesion kinase regulates pathogen-killing capability and life span of neutrophils via mediating both adhesion-dependent and -independent cellular signals. J Immunol 183: 1032–1043 [PubMed]
  • Keck S, Freudenberg M, Huber M 2010. Activation of murine macrophages via TLR2 and TLR4 is negatively regulated by a Lyn/PI3K module and promoted by SHIP1. J Immunol 184: 5809–5818 [PubMed]
  • Kerrigan AM, Brown GD 2010. Syk-coupled C-type lectin receptors that mediate cellular activation via single tyrosine based activation motifs. Immunol Rev 234: 335–352 [PubMed]
  • Kim LC, Song L, Haura EB 2009. Src kinases as therapeutic targets for cancer. Nat Rev Clin Oncol 6: 587–595 [PubMed]
  • Koga T, Inui M, Inoue K, Kim S, Suematsu A, Kobayashi E, Iwata T, Ohnishi H, Matozaki T, Kodama T, et al. 2004. Costimulatory signals mediated by the ITAM motif cooperate with RANKL for bone homeostasis. Nature 428: 758–763 [PubMed]
  • Koprulu AD, Ellmeier W 2009. The role of Tec family kinases in mononuclear phagocytes. Crit Rev Immunol 29: 317–333 [PubMed]
  • Kumar P, Hosaka S, Koch AE 2001. Soluble E-selectin induces monocyte chemotaxis through Src family tyrosine kinases. J Biol Chem 276: 21039–21045 [PubMed]
  • Kurosaki T, Shinohara H, Baba Y 2010. B cell signaling and fate decision. Annu Rev Immunol 28: 21–55 [PubMed]
  • Lanier LL 2009. DAP10- and DAP12-associated receptors in innate immunity. Immunol Rev 227: 150–160 [PMC free article] [PubMed]
  • Lee AW, States DJ 2000. Both src-dependent and -independent mechanisms mediate phosphatidylinositol 3-kinase regulation of colony-stimulating factor 1-activated mitogen-activated protein kinases in myeloid progenitors. Mol Cell Biol 20: 6779–6798 [PMC free article] [PubMed]
  • Leibbrandt A, Penninger JM 2008. RANK/RANKL: regulators of immune responses and bone physiology. Ann N Y Acad Sci 1143: 123–150 [PubMed]
  • LeibundGut-Landmann S, Gross O, Robinson MJ, Osorio F, Slack EC, Tsoni SV, Schweighoffer E, Tybulewicz V, Brown GD, Ruland J, et al. 2007. Syk- and CARD9-dependent coupling of innate immunity to the induction of T helper cells that produce interleukin 17. Nat Immunol 8: 630–638 [PubMed]
  • Lowell CA 2004. Src-family kinases: rheostats of immune cell signaling. Mol Immunol 41: 631–643 [PubMed]
  • Luttrell DK, Luttrell LM 2004. Not so strange bedfellows: G-protein-coupled receptors and Src family kinases. Oncogene 23: 7969–7978 [PubMed]
  • Martinon F 2010. Mechanisms of uric acid crystal-mediated autoinflammation. Immunol Rev 233: 218–232 [PubMed]
  • Mashima R, Hishida Y, Tezuka T, Yamanashi Y 2009. The roles of Dok family adapters in immunoreceptor signaling. Immunol Rev 232: 273–285 [PubMed]
  • Means TK, Luster AD 2010. Integrins limit the Toll. Nat Immunol 11: 691–693 [PubMed]
  • Meng F, Lowell CA 1997. Lipopolysaccharide (LPS)-induced macrophage activation and signal transduction in the absence of Src-family kinases Hck, Fgr, and Lyn. J Exp Med 185: 1661–1670 [PMC free article] [PubMed]
  • Mermel CH, McLemore ML, Liu F, Pereira S, Woloszynek J, Lowell CA, Link DC 2006. Src family kinases are important negative regulators of G-CSF-dependent granulopoiesis. Blood 108: 2562–2568 [PubMed]
  • Mocsai A, Abram CL, Jakus Z, Hu Y, Lanier LL, Lowell CA 2006. Integrin signaling in neutrophils and macrophages uses adaptors containing immunoreceptor tyrosine-based activation motifs. Nat Immunol 7: 1326–1333 [PubMed]
  • Mocsai A, Ruland J, Tybulewicz VL 2010. The SYK tyrosine kinase: a crucial player in diverse biological functions. Nat Rev Immunol 10: 387–402 [PubMed]
  • Mocsai A, Zhang H, Jakus Z, Kitaura J, Kawakami T, Lowell CA 2003. G-protein-coupled receptor signaling in Syk-deficient neutrophils and mast cells. Blood 101: 4155–4163 [PubMed]
  • Mukundan L, Bishop GA, Head KZ, Zhang L, Wahl LM, Suttles J 2005. TNF receptor-associated factor 6 is an essential mediator of CD40-activated proinflammatory pathways in monocytes and macrophages. J Immunol 174: 1081–1090 [PubMed]
  • Munitz A 2010. Inhibitory receptors on myeloid cells: new targets for therapy? Pharmacol Ther 125: 128–137 [PubMed]
  • N'Diaye EN, Branda CS, Branda SS, Nevarez L, Colonna M, Lowell C, Hamerman JA, Seaman WE 2009. TREM-2 (triggering receptor expressed on myeloid cells 2) is a phagocytic receptor for bacteria. J Cell Biol 184: 215–223 [PMC free article] [PubMed]
  • Ng G, Sharma K, Ward SM, Desrosiers MD, Stephens LA, Schoel WM, Li T, Lowell CA, Ling CC, Amrein MW, et al. 2008. Receptor-independent, direct membrane binding leads to cell-surface lipid sorting and Syk kinase activation in dendritic cells. Immunity 29: 807–818 [PMC free article] [PubMed]
  • Nimmerjahn F, Ravetch JV 2008. Fcγ receptors as regulators of immune responses. Nat Rev Immunol 8: 34–47 [PubMed]
  • O'Neill LA 2008. When signaling pathways collide: positive and negative regulation of toll-like receptor signal transduction. Immunity 29: 12–20 [PubMed]
  • Okigaki M, Davis C, Falasca M, Harroch S, Felsenfeld DP, Sheetz MP, Schlessinger J 2003. Pyk2 regulates multiple signaling events crucial for macrophage morphology and migration. Proc Natl Acad Sci U S A 100: 10740–10745 [PubMed]
  • Okutani D, Lodyga M, Han B, Liu M 2006. Src protein tyrosine kinase family and acute inflammatory responses. Am J Physiol Lung Cell Mol Physiol 291: 129–141 [PubMed]
  • Orlicek SL, Hanke JH, English BK 1999. The src family-selective tyrosine kinase inhibitor PP1 blocks LPS and IFN-gamma-mediated TNF and iNOS production in murine macrophages. Shock 12: 350–354 [PubMed]
  • Owen KA, Pixley FJ, Thomas KS, Vicente-Manzanares M, Ray BJ, Horwitz AF, Parsons JT, Beggs HE, Stanley ER, Bouton AH 2007a. Regulation of lamellipodial persistence, adhesion turnover, and motility in macrophages by focal adhesion kinase. J Cell Biol 179: 1275–1287 [PMC free article] [PubMed]
  • Owen KA, Thomas KS, Bouton AH 2007b. The differential expression of Yersinia pseudotuberculosis adhesins determines the requirement for FAK and/or Pyk2 during bacterial phagocytosis by macrophages. Cell Microbiol 9: 596–609 [PubMed]
  • Page TH, Smolinska M, Gillespie J, Urbaniak AM, Foxwell BM 2009. Tyrosine kinases and inflammatory signalling. Curr Mol Med 9: 69–85 [PubMed]
  • Pasquier B, Launay P, Kanamaru Y, Moura IC, Pfirsch S, Ruffie C, Henin D, Benhamou M, Pretolani M, Blank U, et al. 2005. Identification of FcαRI as an inhibitory receptor that controls inflammation: dual role of FcRγ ITAM. Immunity 22: 31–42 [PubMed]
  • Perugini M, Brown AL, Salerno DG, Booker GW, Stojkoski C, Hercus TR, Lopez AF, Hibbs ML, Gonda TJ, D'Andrea RJ 2010. Alternative modes of GM-CSF receptor activation revealed using activated mutants of the common beta-subunit. Blood 115: 3346–3353 [PubMed]
  • Pinheiro da Silva F, Aloulou M, Benhamou M, Monteiro RC 2008. Inhibitory ITAMs: a matter of life and death. Trends Immunol 29: 366–373 [PubMed]
  • Ptasznik A, Urbanowska E, Chinta S, Costa MA, Katz BA, Stanislaus MA, Demir G, Linnekin D, Pan ZK, Gewirtz AM 2002. Crosstalk between BCR/ABL oncoprotein and CXCR4 signaling through a Src family kinase in human leukemia cells. J Exp Med 196: 667–678 [PMC free article] [PubMed]
  • Ravetch JV, Lanier LL 2000. Immune inhibitory receptors. Science 290: 84–89 [PubMed]
  • Rawlings DJ, Sommer K, Moreno-Garcia ME 2006. The CARMA1 signalosome links the signalling machinery of adaptive and innate immunity in lymphocytes. Nat Rev Immunol 6: 799–812 [PubMed]
  • Reddy EP, Korapati A, Chaturvedi P, Rane S 2000. IL-3 signaling and the role of Src kinases, JAKs and STATs: a covert liaison unveiled. Oncogene 19: 2532–2547 [PubMed]
  • Robinson MJ, Sancho D, Slack EC, LeibundGut-Landmann S, Reis e Sousa C 2006. Myeloid C-type lectins in innate immunity. Nat Immunol 7: 1258–1265 [PubMed]
  • Ruland J 2008. CARD9 signaling in the innate immune response. Ann N Y Acad Sci 1143: 35–44 [PubMed]
  • Sai J, Raman D, Liu Y, Wikswo J, Richmond A 2008. Parallel phosphatidylinositol 3-kinase (PI3K)-dependent and Src-dependent pathways lead to CXCL8-mediated Rac2 activation and chemotaxis. J Biol Chem 283: 26538–26547 [PMC free article] [PubMed]
  • Sampson M, Zhu QS, Corey SJ 2007. Src kinases in G-CSF receptor signaling. Front Biosci 12: 1463–1474 [PubMed]
  • Saunders AE, Johnson P 2010. Modulation of immune cell signalling by the leukocyte common tyrosine phosphatase, CD45. Cell Signal 22: 339–348 [PubMed]
  • Scapini P, Pereira S, Zhang H, Lowell CA 2009. Multiple roles of Lyn kinase in myeloid cell signaling and function. Immunol Rev 228: 23–40 [PMC free article] [PubMed]
  • Shinohara M, Koga T, Okamoto K, Sakaguchi S, Arai K, Yasuda H, Takai T, Kodama T, Morio T, Geha RS, et al. 2008. Tyrosine kinases Btk and Tec regulate osteoclast differentiation by linking RANK and ITAM signals. Cell 132: 794–806 [PubMed]
  • Sly LM, Ho V, Antignano F, Ruschmann J, Hamilton M, Lam V, Rauh MJ, Krystal G 2007. The role of SHIP in macrophages. Front Biosci 12: 2836–2848 [PubMed]
  • Smith-Garvin JE, Koretzky GA, Jordan MS 2009. T cell activation. Annu Rev Immunol 27: 591–619 [PMC free article] [PubMed]
  • Smolinska MJ, Horwood NJ, Page TH, Smallie T, Foxwell BM 2008. Chemical inhibition of Src family kinases affects major LPS-activated pathways in primary human macrophages. Mol Immunol 45: 990–1000 [PubMed]
  • Stefanova I, Corcoran ML, Horak EM, Wahl LM, Bolen JB, Horak ID 1993. Lipopolysaccharide induces activation of CD14-associated protein tyrosine kinase p53/56lyn. J Biol Chem 268: 20725–20728 [PubMed]
  • Swiecki MK, Colonna M 2007. Running to stand still: BCR-like signaling suppresses type I IFN responses in pDC. Eur J Immunol 37: 3327–3329 [PubMed]
  • Takahashi K, Prinz M, Stagi M, Chechneva O, Neumann H 2007. TREM2-transduced myeloid precursors mediate nervous tissue debris clearance and facilitate recovery in an animal model of multiple sclerosis. PLoS Med 4: e124. [PMC free article] [PubMed]
  • Tohyama Y, Yamamura H 2009. Protein tyrosine kinase, syk: a key player in phagocytic cells. J Biochem 145: 267–273 [PubMed]
  • Tomar A, Schlaepfer DD 2009. Focal adhesion kinase: switching between GAPs and GEFs in the regulation of cell motility. Curr Opin Cell Biol 21: 676–683 [PMC free article] [PubMed]
  • Tomkowicz B, Lee C, Ravyn V, Cheung R, Ptasznik A, Collman RG 2006. The Src kinase Lyn is required for CCR5 signaling in response to MIP-1beta and R5 HIV-1 gp120 in human macrophages. Blood 108: 1145–1150 [PubMed]
  • Torigoe C, Inman JK, Metzger H 1998. An unusual mechanism for ligand antagonism. Science 281: 568–572 [PubMed]
  • Wang KZ, Wara-Aswapati N, Boch JA, Yoshida Y, Hu CD, Galson DL, Auron PE 2006. TRAF6 activation of PI 3-kinase-dependent cytoskeletal changes is cooperative with Ras and is mediated by an interaction with cytoplasmic Src. J Cell Sci 119(Pt 8): 1579–1591 [PubMed]
  • Wang L, Gordon RA, Huynh L, Su X, Park Min KH, Han J, Arthur JS, Kalliolias GD, Ivashkiv LB 2010. Indirect inhibition of Toll-like receptor and type I interferon responses by ITAM-coupled receptors and integrins. Immunity 32: 518–530 [PMC free article] [PubMed]
  • Werninghaus K, Babiak A, Gross O, Holscher C, Dietrich H, Agger EM, Mages J, Mocsai A, Schoenen H, Finger K, et al. 2009. Adjuvanticity of a synthetic cord factor analogue for subunit Mycobacterium tuberculosis vaccination requires FcRγ-Syk-Card9-dependent innate immune activation. J Exp Med 206: 89–97 [PMC free article] [PubMed]
  • Yago T, Shao B, Miner JJ, Yao L, Klopocki AG, Maeda K, Coggeshall KM, McEver RP 2010. E-selectin engages PSGL-1 and CD44 through a common signaling pathway to induce integrin αLβ2-mediated slow leukocyte rolling. Blood 116: 485–494 [PubMed]
  • Yamasaki S, Ishikawa E, Sakuma M, Hara H, Ogata K, Saito T 2008. Mincle is an ITAM-coupled activating receptor that senses damaged cells. Nat Immunol 9: 1179–1188 [PubMed]
  • Zambuzzi WF, Milani R, Teti A 2010. Expanding the role of Src and protein-tyrosine phosphatases balance in modulating osteoblast metabolism: lessons from mice. Biochimie 92: 327–332 [PubMed]
  • Zarbock A, Abram CL, Hundt M, Altman A, Lowell CA, Ley K 2008. PSGL-1 engagement by E-selectin signals through Src kinase Fgr and ITAM adapters DAP12 and FcRγ to induce slow leukocyte rolling. J Exp Med 205: 2339–2347 [PMC free article] [PubMed]
  • Zhang H, Meng F, Chu CL, Takai T, Lowell CA 2005. The Src family kinases Hck and Fgr negatively regulate neutrophil and dendritic cell chemokine signaling via PIR-B. Immunity 22: 235–246 [PubMed]
  • Zhang X, Majlessi L, Deriaud E, Leclerc C, Lo-Man R 2009. Coactivation of Syk kinase and MyD88 adaptor protein pathways by bacteria promotes regulatory properties of neutrophils. Immunity 31: 761–771 [PubMed]
  • Zou W, Reeve JL, Liu Y, Teitelbaum SL, Ross FP 2008. DAP12 couples c-Fms activation to the osteoclast cytoskeleton by recruitment of Syk. Mol Cell 31: 422–431 [PMC free article] [PubMed]

Articles from Cold Spring Harbor Perspectives in Biology are provided here courtesy of Cold Spring Harbor Laboratory Press