Tpl2 is essential for TNF production in models of septic shock (13
); however, the role of Tpl2 in host defense against a gram positive organism such as Listeria monocytogenes
has not been previously assessed. In this study, we report that Tpl2−/−
mice are more susceptible to primary L. monocytogenes
infection, due to a primary failure in innate host defense likely involving defects in both PRR signaling and cytokine production. Although Tpl2 is not required for systemic TNF production and has distinct roles in different innate immune cells, Tpl2 is critical for production of IL-1β. Furthermore, a range of structurally distinct PRRs evidently utilize Tpl2 for the induction of IL-1β. Thus, the present study establishes a new role for Tpl2 in host defense and immunoregulation.
We were very surprised to find that Tpl2−/−
mice infected with L. monocytogenes
produced abundant TNF. Upon further analysis, we found that Tpl2 is required in macrophages but is not essential in DCs for TNF induction. Moreover, production of this cytokine in response to some ligands was entirely independent of Tpl2. What might account for the cell type- and receptor-specific requirements for Tpl2 in TNF production? It is abundantly clear that Tpl2 is essential in macrophages for linking all TLRs to ERK, and our data are consistent with this conclusion (13
); however, other signaling pathways are activated by TLRs, and we found that in the absence of Tpl2, these pathways are differentially activated in macrophages and BMDCs. The absence of Tpl2 resulted in increased NF-κB activation in BMDCs but not in macrophages. Another PRR commonly activated by bacterial products is Nod2 (4
). Our results show that Tpl2 is required for ERK activation in response to the Nod2 ligand, MDP. Tpl2 also plays a role in TNF production in response to MDP in both macrophages and BMDCs. It will be useful to ascertain whether Tpl2 is involved in signaling by other intracellular pathogen sensors and to dissect how Tpl2 is biochemically linked to these diverse PRRs. In contrast, we found that ligands that engage dectin-1 activate ERK and TNF production independent of Tpl2.
The data provided in the present study indicate that Tpl2 is also critical for IL-1β production – even more so than TNF. We show that Tpl2 is required for maximal production of IL-1β in macrophages and BMDCs, independent of the PRR engaged. Tpl2 regulates the transcription of IL-1β. Further work is needed to dissect where Tpl2 fits into the pathway leading to IL-1β production. It has previously been shown that inhibition of ERK interferes with TLR-induced IL-1β production (46
). This might lead one to suspect that this would be the mechanism by which Tpl2 contributes to IL-1β induction; however, zymosan-dependent IL-1β production is also disrupted by the absence of Tpl2, even though ERK activation is normal. This implies that Tpl2 has an additional, ERK-independent role in linking PRRs to the regulation of IL-1β. Regulation of IL-12 production by Tpl2 is also thought to be independent of ERK (14
). Clearly, identification of additional downstream targets of Tpl2 will be an important area for future investigation.
TNF is absolutely essential for defense against L. monocytogenes
), and the fact that some cell types are not able to produce TNF in the absence of Tpl2 may contribute to susceptibility of infection. Importantly, responsiveness to TNF is also altered in Tpl2−/−
antigen presenting cells (22
). While IL-1 is not absolutely required for host defense against L. monocytogenes
, it also clearly plays a role (37
). Likely, the absence of IL-1β production and the loss of TNF in certain cell types in combination with diminished responsiveness to TNF by Tpl2−/−
cells contribute to increased susceptibility to L. monocytogenes
mice. It is also possible that Tpl2 plays an important role for other cytokines as well.
Our findings have important implications in terms of signal transduction, but also have very practical implications. The treatment of rheumatoid arthritis (RA) and inflammatory bowel disease (IBD) has been revolutionized by the use of TNF antagonists (48
). Because of its profound role in models of septic shock, Tpl2 has been considered an important target for the generation of new immunosuppressive drugs (49
). Consistent with this notion, mice bearing a deletion in the 3′ AU-rich element of TNF mRNA (TnfΔARE
), over-produce TNF and develop IBD with a histopathologic phenotype similar to Crohn's disease (54
). When the TnfΔARE
mice are crossed onto a Tpl2−/−
background, onset of intestinal inflammation is significantly delayed (55
); however, the work presented herein provides a cautionary note with respect to the idea that Tpl2 antagonism will necessarily block TNF production, even though TNF signal transduction might be impaired in some cell types (22
). Our results clearly show that different PRRs have distinct dependence upon Tpl2 for TNF production; in fact, with some ligands, TNF production is entirely normal in the absence of Tpl2. Moreover, some cells, like DCs only display partial reductions in TNF in the absence of Tpl2 in response to TLR stimulation. Gut flora express multiple PRRs that are thought to play a role in disease pathogenesis of IBD (56
). Clearly, it will be of interest to examine the role of Tpl2 in models of IBD and to determine if
cytokine production is affected. Conversely, our results argue that inhibition of Tpl2 could be an effective means of antagonizing IL-1β. This could be useful in the spectrum of inflammatory diseases characterized by overproduction of IL-1β, like neonatal onset multisystem inflammatory disease and other cryopyrinopathies, Still's disease, and possibly diabetes (57