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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Cell Mol Immunol. Author manuscript; available in PMC 2009 October 19.
Published in final edited form as:
PMCID: PMC2763551
NIHMSID: NIHMS110166

Perspectives on the quantitative immunobiology of the IL-7 signaling network

Abstract

Interleukin 7 (IL-7) is an essential cytokine for the development and homeostatic maintenance of T and B cells. Binding of IL-7 to its cognate receptor activates a variety of tightly regulated pathways critical to cell survival, glucose uptake, differentiation and proliferation. There has been much interest in understanding how IL-7 signaling is modulated at multiple interconnected network levels. This review will look at the various strategies used by T and B cells to modulate the strength of the signal through the IL-7 receptor, including the use of shared receptor components, signaling crosstalk, shared interaction domains, feedback loops, integrated gene regulatory control, multimerization and ligand competition. We have illustrated the potential of these network control mechanisms to quantitatively affect the properties of IL-7 signaling and its impact on lymphocyte signaling in health and disease. Analysis of IL-7 signaling at a network level will allow a more informative approach to understand the impact of these processes on lymhocyte biology.

Keywords: IL-7, cytokines, signaling networks, systems biology

1. Introduction

Interleukin 7 is a vital cytokine necessary for the development and survival of T and B cells1. IL-7 is produced primarily by T zone fibroblastic reticular cells in lymphoid organs2 and binds to the IL-7 receptor (IL-7R) complex, a heterodimeric receptor made up of the IL-7R alpha chain (IL-7Rα) and the common gamma chain (γc)3. While IL-7 shares pro-survival and proliferative capacities with related cytokines, including other interleukin family members, it has been shown to play non-redundant roles in T and B cell development and homeostasis1. IL-7Rα is also expressed on dendritic cells (DCs) and monocytes indicating a possible role for IL-7 in multiple hematopoietic lineages4. The molecular analysis of the IL-7 signaling network components has been extensively reviewed5,6 but the details of the IL-7 signaling network behavior as a whole and its interaction with other signaling cascades in cells of the immune system is still under investigation. In this review, we present a case for a detailed quantitative analysis of the IL-7 signaling network by illustrating network behavior using hypothetical examples of interacting network components which have the potential to affect the biology of the IL-7 signaling network in possibly important but non-intuitive ways.

In recent years, there has been a rapid growth in the field of quantitative systems biology of cell signaling networks that can reveal emergent phenomena which are not obvious, based on the study of the individual network components7. Further, it has yielded a molecular understanding of previously observed phenomena at the level of the cell such as cellular responses occurring only over a narrow range of ligand stimulation and at the level of the entire organism such as tissue homeostasis. Of particular relevance to the biology of IL-7 are lymphocyte differentiation checkpoints at various stages of development and the regulation of lymphocyte population size which occurs at the level of the organism. It is becoming increasingly clear that the IL-7 signaling network is regulated at multiple hierarchical levels in the signaling pathway, including receptor expression, signaling pathway cross-talk and transcriptional feedback. What is lacking is an understanding of the relative contributions of the various arms of the IL-7 regulatory network in controlling IL-7 signaling efficacy thereby stimulating survival, proliferation and differentiation throughout lymphoid development. This review aims to illustrate how many of the signaling network components integral to the IL-7 network have the potential to produce complex and non-intuitive behavior, though it will require quantitative experimental and analytical approaches to fully appreciate their contribution to signaling control. The IL-7 signaling pathway has substantial crosstalk with a variety of other receptor-mediated signaling networks. These interactions occur through a combination of shared receptor components, mutual downstream signaling targets and integrated transcriptional control, yielding both inhibitory and synergistic effects. Given the diverse cytokine milieu in which cells reside in vivo, mapping the combinatorial effects of various cytokines in addition to IL-7 on lymphocyte function will be critical to understanding IL-7 signaling network control.

A schematic of the IL-7 signaling network and its connectivity with the T cell signaling network is depicted in Figure 1. We have highlighted the occurrence of the following mechanisms of signaling control: (1) shared receptor components (2) signaling pathway crosstalk, (3) conserved binding motifs, (4) feedback regulation, (5) integrated gene regulatory control, (6) multimerization and (7) ligand competition. In subsequent figures (Figures 2--8),8), we have illustrated the potential of each ‘network motif’ to quantitatively affect the properties of the IL-7 signaling network and thereby the strength and duration of IL-7 signaling. However, as the magnitude of many of the parameters and the relative importance of several putative interactions in signal propagation are yet to be experimentally determined, we have used conservative estimates of the system parameters in presenting the illustrations below. We have discussed the implications of these ‘network motifs’ upon the biology of IL-7 in lymphocyte signaling in health and disease. These predictions are best viewed as hypothetical system properties that will hopefully motivate and guide further quantitative experimental and theoretical research of the IL-7 signaling network.

Fig 1
Control Mechanisms in the IL-7 Signaling Network
Fig 2
Influence of Shared Receptor Components on Signaling
Fig 8
Influence of Multicellular Ligand Competition Effects on Signaling

2. Shared receptor components

Several cytokine receptors are multimeric complexes made up of two or more different component proteins, which are often shared between multiple cytokine receptors. This can impact the relative availability of each receptor component, thereby limiting the extent of cytokine signaling. Common examples of shared cytokine receptor components include sharing of the common gamma chain (γc) between the receptors for IL-2, -4, -7, -9, -15 and -21, the sharing of a common ß chain between the receptors for IL-3 and IL-5 and GM-CSF8 and the sharing of the ß chain between IL-2 and IL-15 receptor complexes9. In the case of the IL-7R complex, both receptor subunits are components of other receptors. As discussed above, the γc is shared with five other cytokine receptors, all of which regulate T cell growth and differentiation. As signaling specificity through the γc receptors is largely controlled by expression of the cytokine-specific α chains (as well as the β chain for IL-2 and IL-15), these subunits are kept under tight transcriptional control throughout development. The IL-7Rα chain can also form a hetero-dimeric receptor complex on binding to the thymic stromal lymphopoietin receptor (TSLPR), which is homologous to the γc chain10. IL7Rα and TSLPR are co-expressed on T cells, pre-B cells and dendritic cells (DCs)4. The ligand for the IL-7Rα:TSLPR complex is TSLP, which is a cytokine that is homologous to IL-7, and is produced by cells of epithelial origin in the thymus, lung, gut and skin11. The presence of shared receptor components among cytokines can result in competition for the shared components among their respective ligands.

The association between the γc and IL-7Rα is required for the generation of a functional receptor complex and is mediated either by the binding of IL-7 or the activation status of T cells12. The presence of competing cytokines can sequester IL-7 receptor components and adversely impact the ability to form functional IL-7R signaling complexes. For instance, IL-7 and TSLP can compete for IL-7Rα. Similarly, IL-7 and IL-15 compete for the γc (Figure 2A). One potential repercussion of shared receptor components among cytokines is therefore an upper limit on the total amount of cytokine-specific signaling that a cell can receive in response to multiple cytokines. For instance, IL-15 can compete for the shared γc component when its expression is limiting13. An example of the quantitative effects of this sharing is shown in Figure 2B, which illustrates how the number of IL-7:IL-7R complexes may decrease on exposure to increasing levels of IL-15 in the presence of a constant concentration of IL-7. However, the net signal strength down any given pathway may depend on the extent to which γc is complexed with IL-7Rα or IL-15Rβ. The use of shared receptor components also partly helps explain the overlap in many cytokine receptor signaling pathways. As will be discussed in subsequent sections, this pleiotropy can also lead to competition for intracellular binding partners and downstream signaling effectors that can affect the propagation of signaling from a particular receptor when multiple cytokines are present.

Aberrant receptor competition may be related to pathological outcomes. For instance, atopic dermatitis and asthma involves pathological Th2 differentiation induced by dendritic cells primed with TSLP14. The effects of TSLP on T cells are largely mediated by modulating the function of DCs15. IL-7 can also modulate DC function. For example, IL-7 is also produced by inflamed synoviocytes in rheumatoid arthritis and it induces cell contact-dependent Th1 cytokine production in cocultures of synovial T cells and monocytes16. Interestingly, skin keratinocytes have been shown to produce both IL-7 and TSLP, especially on exposure to certain pathogens17,18. The competing effects of TSLP and IL-7 on dendritic cells may therefore influence their ability to direct Th2 bias of the CD4+ T cells that they activate. Further, we speculate that the possible ability of IL-7 to modulate the effects of TSLP by ligand competition may open new therapeutic avenues for asthma or atopic dermatitis.

3. Shared downstream signaling components

Many of the main signaling components of the IL-7 signaling pathway, including both positive and negative regulators of cell proliferation and survival, are shared with other cytokines. During such an interconnected signaling network response, multiple input cues work together through a small set of signaling network effectors that propagate down and spread to a number of downstream targets. Competition for shared signaling mediators can result in a hierarchy of responses controlled at the level of abundance and relative binding affinities to upstream regulators of the response.

The cross-specificity in Jak and Stat activation results is one of the major nodes of cytokine signaling crosstalk. The Jak kinase family is comprised of four members: Jak1, Jak2, Jak3 and Tyk2, each of which is found to be associated with multiple cytokine receptors19. For instance, Jak1 is associated with the α subunits of γc cytokines such as IL-7Rα and IL-4Rα. Jak3 is associated with the γc20,21. Cytokine binding mediates the trans-phosphorylation of receptor associated Jak kinases, which in turn phosphorylate tyrosine residues on the receptors themselves. The receptor phosphotyrosines serve as docking sites for SH2 domain proteins including the Stat family of transcription factors which are activated by Jak-mediated phosphorylation. Signaling crosstalk due to shared Jak kinases is a likely source for many of the redundant signaling activities observed among interleukin family pro-survival cytokines. Cytokines, as well as many other growth factors, activate overlapping subsets of the seven Stat family members (Stat1, 2, 3, 4, 5a, 5b, and 6)19. Thus, phosphorylation of multiple Stats by the Jak kinases also results in considerable cross-talk22. Yet another point of crosstalk exists in the induction of suppressor of cytokine signaling (SOCS) family members. The SOCS proteins include eight family members (SOCS 1−7 and CIS), each of which can inhibit signaling induced by multiple cytokines and growth factors by several mechanisms, including by binding the Jak catalytic site, occupying the receptor Stat docking site, and targeting signaling proteins for degradation 23. SOCS1 is the major SOCS family protein involved in IL-7 signaling regulation, and it is also induced by other cytokines, especially IFNγ. Another major point of signaling crosstalk with the IL-7 signaling network is the PI3K-Akt pathway which is involved in a number of signal transduction networks that regulate cell survival24. Each level of cross-specificity in these pathways, at the level of Jaks, Stats, SOCSs and PI3K-Akt targets, makes it harder to assign protein-specific effects and to deconvolve multi-cytokine responses.

Currently little is known about how pathways signaled by IL-7 are quantitatively regulated, especially in the context of concurrent signals derived from other cytokines or the T cell receptor (TCR). While considering signaling due to multiple cytokines, it is likely that limiting amounts of common downstream targets can result in less than additive activation profiles. This phenomenon is illustrated in Figure 3, which examines the combined activation of the Jak1-Stat5 pathway by IL-15 and IL-7 while assuming first order dephosphorylation. When both cytokine receptors are simultaneously ligated, the combined cytokine signals can be expected to result in an increase in the rate of the response rather than an increase in the magnitude of the response. For simplicity, we have treated IL-15 as a soluble ligand like IL-7 in this example. However, it is now established that IL-15 is primarily presented on IL-15Rα in vivo by IL-15 producing cells25. This is reminiscent of studies on the biological activities of surface-tethered growth factors such as EGF, which induce sustained signals as they prevent the internalization of the receptor complex26. The effects of possible differences in receptor internalization rates of cytokines like IL-7, which is soluble, and IL-15, which is presented by IL-15Rα, have not been investigated. These differences could impact the dynamics of downstream signal competition. Well-designed quantitative experiments at the network level will be required to develop strategies for therapeutic manipulation of the network due to complex interconnected effects of multiple pathway activation.

Fig 3
Influence of Signaling Pathway Crosstalk on Signaling

4. Shared interaction domains

The vast inter-connectivity of signaling networks is largely a result of overlapping binding specificities of multiple proteins for the same target motif, which often occurs in several distinct signaling proteins. Thus, many kinases have numerous substrates, and signaling scaffold proteins can recruit several different signaling mediators to the same domain. Yet, cytokines often predominantly activate only a small subset of these signaling mediators. Simultaneous stimulation with multiple cytokines which share overlapping downstream partners can also influence the strength and dynamics of the signal. Multiple cytokines can compete for the same binding site on a limiting number of receptors. Alternatively, the presence of the same signaling domain on multiple receptors can lead to competition for limiting downstream signaling mediators. Response specificity, timing and prioritization for pathway activation is thus dictated by the relative abundance and binding strength of interacting signaling proteins.

Several conserved interaction domains are found in the intracellular domain of the IL-7Rα chain. The cytoplasmic tail of IL-7Rα has two regions, Box1 and Y449, which are thought to be of particular importance for signal propagation regulating survival, proliferation and thymocyte development. Box1 is an 8 amino acid membrane proximal motif that binds Jak1 and is found in all type I cytokine receptors. Y449 is a one of three tyrosines in IL-7Rα, which is conserved between humans and mice, and recruits SH2 domain-containing Stat family members when it is phosphorylated by receptor-associated Jak1. Although Stat5 is the major Stat recruited to the Y449 site on IL-7 signaling, SH2 domain homology with other Stat family members could lead to competition among the Stats for binding to the Y449 site (Figure 4A). In particular, Stat1, 3 and 5 have been shown to be activated by IL-7 signaling22,27. A mutation at the Y449 site does not completely abrogate Stat1 and Stat3 signaling27, suggesting additional routes for their activation by IL-7. Additional phosphotyrosine binding proteins like the Shc adaptor protein and Insulin Receptor Substrate proteins may also compete with Stat5 for binding to the Y449 site 28. Competition at the Y449 site affecting Stat5 access could alter the timing and magnitude of Stat signaling. A hypothetical example of such competition between Stat5 and Stat3 this is shown in Figure 4B. We have assumed that the Stat5:pY449 interaction is 10-fold stronger than the Stat3:pY449 binding and that the rates of Jak-mediated phosphorylation are equal for both Stats. The resulting delay in the kinetics of Stat3 phosphorylation, given fixed amounts of pY449 binding sites, is illustrated. Conversely, the presence of phosphotyrosines on other proteins that can also bind the SH2 domain of Stat5 could sequester Stat5 and hinder its binding to IL-7Rα. It has also been proposed that a second survival signal originating from the Y449 site arises from the recruitment of PI3K 29. The difference in binding kinetics of PI3K and Stat5 to the Y449 site could regulate the extent and timing down these two pathways, which may influence the downstream integration of survival signals. Quantitative experiments of signal dynamics under varying IL-7Rα, PI3K or Stat5 levels will be needed to determine how competition for binding sites impacts propagation of survival signals in lymphocytes.

Fig 4
Influence of Conserved Binding Domains on Signaling

5. Signaling feedback control

Feedback loops comprise key mechanisms by which signal inhibition and propagation is controlled within cells. Signaling through a receptor may lead to signal inhibition via receptor internalization, induction of inhibitory phosphatases or transcriptional changes in receptor or regulator expression. Likewise, positive signaling feedback can be generated by inducing transcription of the receptor or its positive regulators or by autocrine secretion of a stimulatory ligand. Signaling feedback control is an important regulatory process in IL-7 signaling that allows avoidance of pathway saturation, establishment of signaling thresholds and fine tuning of the signal at an optimal level for cell survival. Understanding the balance of positive and negative feedback loops will be essential for a complete understanding of cytokine responses.

Negative feedback loops are particularly important in regulation of IL-7Rα expression. Receptor ligation leads to endocytic loss of the receptor from the surface, contributing to signal attenuation. In addition to receptor loss from internalization, several γc cytokines, including IL-7, activate both negative and positive feedback loops to modulate receptor mRNA expression. In CD8+ T cells, downregulation is mediated by the transcriptional repressor Gfi1, which is upregulated upon IL-7 signaling, as well as signaling by other interleukin family members30. A second transcriptionally mediated negative feedback loop involves upregulation of SOCS1 expression upon cytokine signaling (Figure 5). SOCS1 can directly inhibit Jaks by acting as a pseudosubstrate through its kinase inhibitory region, as well as by ubiquitin-mediated degradation of the signaling complex itself23. In addition, cytokine-independent regulation of SOCS1 also plays critical roles in regulating signaling by IL-7 and other γc cytokines throughout development. For example, SOCS1 is expressed at high levels in DP cells during thymic development to prevent IL-7 signaling and possible aberrant positive selection31. SOCS1 knockout mice show spontaneous activation of lymphocytes even in a pathogen free environment32. A number of negative regulators of the Stats have also been identified such as the PIAS family of proteins33. However, the relative contribution of these mechanisms of signal feedback inhibition to the overall control of IL-7 signal attenuation is yet to be elucidated.

Fig 5
Influence of Feedback Control on Signaling

IL-7 signaling also elicits positive feedback loops which contribute to signal amplification and sharp response thresholds. In developing B cells, IL-7 signaling causes upregulation of the transcription factors EBF and E2A, which in turn upregulate IL-7Rα, leading to a self-sustaining positive feedback loop34. Feedback loops in IL-7 signaling play a critical role in B cell development by maintaining B cell lineage commitment among differentiating common lymphoid progenitors. In addition, sustained IL-7 signaling is necessary for survival of pro-B cell35. EBF and E2A coordinately regulate the initiation of the B cell gene expression program as well as rearrangement of the immunoglobulin heavy chain loci33. The signal feedback loops between these proteins ensure normal development of B cell precursors through various checkpoints in B cell development.

It was recently shown that in macrophages, Tat protein produced by the human immunodeficiency virus (HIV) can cause upregulation of IL-7Rα and increase IL-7 signaling. Increased signaling in turn promotes early infection events including viral entry, and ultimately efficient viral production36. Interestingly, the effect of HIV Tat protein on CD8+ T cells was the opposite of that seen in macrophages, where it instead decreased IL-7Rα expression, inhibiting cell survival signaling37. A detailed analysis of the complex, cell-specific feedback mechanisms will help better understanding how the IL-7 signaling network is exploited by pathogens.

6. Integrated gene regulatory control

Interaction between signaling pathways at the gene regulatory level gives rise to a coordinated response and synergy in outcomes. Survival, activation and proliferation programs that are driven by antigen-receptor signaling and various γc cytokines are characteristic of lymphocytes. Microarray profiling has allowed for the high-throughput querying of gene expression programs regulated by cytokine and TCR signaling and their relationships with a variety of biological processes. High levels of overlap have been observed amongst the several hundred genes regulated by IL-2, IL-7 and IL-15. However, approximately 73% of these genes are also regulated by TCR signaling, and less than 20% of genes are unique to cytokine stimulation38,39. SOCS1 and Gfi1 are genes that are known to be transcriptionally induced during T cell activation as well as by pro-survival cytokine signaling. SOCS1 and Gfi1 inhibit the IL-7 signaling pathway at the post-translational and transcriptional levels respectively. IL7Rα itself is transcriptionally downregulated by antigen-receptor signaling. In fact, it has been suggested that there is a greater overlap among TCR and interleukin-induced genes than amongst the genes induced by interleukin family members themselves38. This strongly suggests that gaining a complete understanding of IL-7 induced signaling will require consideration of how related cytokines and TCR signaling influence the transcriptional network response.

As a specific example of the impact of interacting gene regulatory control, we have illustrated the phenomenon of coreceptor tuning which allows CD8+ T cells to maintain their antigen-receptor signaling at levels just below the threshold of autoimmunity (Figure 6A)40. The levels of CD8 are a critical determinant of the responsiveness of a T cell to self-peptide MHC (spMHC) as CD8 promotes the kinetics of binding of TCR to a spMHC complex41. Interestingly, CD8α is transcriptionally induced by IL-7 signaling while at the same time, IL-7 signals are inhibited by spMHC-induced TCR signal transduction. This leads to a mutual feedback loop at the level of gene regulation which results in co-regulation of T cell survival and antigen responsiveness40. This allows CD8+ T cells to adapt to the self-specificity of their unique TCRs so that they receive sufficient survival signals without losing self-tolerance. Figure 6B illustrates how the combined TCR and IL-7 survival signal would be expected to change as a function of the strength of interaction between TCR and spMHC in the presence and absence of the coreceptor tuning effect. In the absence of coreceptor tuning, cells with higher self reactivity would receive greater survival stimuli, making them potentially autoreactive. Coreceptor tuning may also allow T cells with a weak responsiveness to spMHC to better receive other pro-survival signals from IL-7.

Fig 6
Influence of Integrated Gene Regulatory Control on Signaling

Studying the circuitry of gene regulatory programs may help bring important new insights into the contribution of IL-7 signaling to autoimmunity and cancer. Exposure of CD8+ T cells to low levels of IL-7 in vitro for a few hours can pre-dispose them to an enhanced syngeneic mixed lymphocyte reaction with syngeneic dendritic cells40. This phenomenon suggests that autoimmune phenotypes may involve deregulation of coreceptor tuning machinery. It has also been suggested that elevated responses to pro-survival cytokines may contribute to the ability to avoid death by lymphoblastic leukemia cells42. The altered responses of these leukemic cells likely reflect critical changes in gene expression. Predicting potential therapeutic targets in the IL-7 signaling pathway will therefore require a comprehensive understanding of how normal control is achieved, and the transcriptional processes contributing to its pathological deregulation.

7. Multimeric signaling complexes and combinatorial complexity

The Stat proteins (Stat1, Stat3, Stat 4, Stat 5a, Stat 5b and Stat 6) are transcription factors involved in γc cytokine signaling that are expressed constitutively at high levels in the cytosol of resting cells which facilitates their ability to induce a rapid response on phosphorylation by Jak kinases19. Tyrosine phosphorylation of Stats leads to their dimerization through their SH2 domain interactions. Both homodimers (all Stats except Stat2) and heterodimers (Stat1 and Stat2, Stat1 and Stat3, and Stat5a and Stat5b) are formed19,28. Furthermore, tetramerization of Stats has been shown to occur in the case of Stat3, Stat4 and Stat5. Such higher order complex formation is mediated by the N-terminal domains of Stat proteins. The Stat dimers and corresponding tetramers could have differences in their respective DNA binding specificities43.

The presence of several Stat protein multimerization states (monomer, dimer and tetramer) simultaneously present in the cell, each with different cellular function, complicates the ability to directly relate Stat phosphorylation state to activity and phenotypic outputs. Multimerization kinetics can also cause higher order oligomerization states to be turned on over narrower ranges of input signal (Figure 7). IL-7 has been shown to promote survival versus proliferation at different concentrations (0.1 and 1ng/mL respectively) in recent thymic emigrants, but the effect is not correlated with total Stat544. One potential explanation could be that Stat dimers and tetramers could preferentially induce cell survival or cell cycle entry respectively. As shown in Figure 7B, higher levels of Stat5 dimers are predicted to be generated by a weak IL-7 signal than tetramers.

Fig 7
Influence of Component Multimerization on Signaling

Due to cooperative effects, Stat tetramers may selectively increase the activity of certain promoters which have lower affinity binding sites for Stats. Indeed, mutational analysis of Stat5a-binding DNA oligonucleotides has demonstrated that it is possible to introduce specific mutations that virtually abolish Stat5a dimer binding without affecting binding as tetramers. Increased promoter occupancy may change the threshold for transcriptional activity while widening the gene transcription spectrum43. Some responses to cytokine signaling may be explained by the presence of conditions that favor the generation of dimers over tetramers or vice versa. In fact, the presence of high levels of Stat5 tetramers has been consistently seen in some leukemias44. Furthermore, combinatorial complexity in the dimerization of Stats can lead to additional diversity in the nature of signals transduced by IL-7. Stat5a and Stat5b are two closely related Stats with subtle differences in their DNA binding specificity. These differences are likely to be reflected in the DNA-binding specificities of Stat5a/b homodimers and heterodimers and also in their higher order complexes. Combinatorial complexity can also arise during the heterodimerization of other Stats (Stat1/Stat3 or Stat1/Stat2).

8. Multicellular ligand competition

Lymphocyte homeostasis refers to the maintenance of the numbers and diversity of lymphocytes through an organism's lifetime. The homeostasis of naïve T cells is maintained by the competition for limiting amounts of pro-survival cytokines such as IL-7 and regular contact with spMHC45. Despite stiff competition for pro-survival ligands among T cells, the homeostasis of T cell numbers as well as diversity is maintained46. This can be better explained by studying how features of the IL-7 signaling network affect the behavior of the entire cell population.

T cells have evolved strategies to maximize the efficiency of utilization of limiting amounts of IL-7 to maintain homeostasis. IL-7Rα downregulation, which is induced by IL-7 signaling, has been proposed to serve as a mechanism to maximize population size among cells competing for limiting IL-7 by allowing the maximum possible number of cells to receive pro-survival signals30. In this ‘altruistic’ model of IL-7 signaling, cells signaled by IL-7 transiently downregulate surface expression of IL-7Rα and thus become unresponsive to the continued presence of IL-7. The remaining IL-7 in the extracellular milieu can then be used to signal as yet unsignaled cells. Cells are hypothesized to indefinitely cycle between signaled and unsignaled states with low and high receptor expression respectively3. It is not known if such cycling occurs in vivo and if it does occur, what the time scales of the cycling in receptor levels are. A further point of contention that remains unexplored is whether ‘altruistic’ behavior must be realized by two populations of ‘signaled’ and ‘unsignaled’ cells in vivo, driven for example by their respective arrival at the IL-7 production sites in the lymph node, or whether the entire population is maintained at constant receptor levels in equilibrium with extracellular IL-7. Furthermore, it is possible that the comparatively faster processes of IL-7Rα internalization are sufficient to regulate signal strength and depletion.

This ‘altruistic’ behavior has been extended to explain the need for IL-7Rα downregulation upon T cell activation (Figure 8)30. Activated T cells undergo rapid expansion and their numbers can occasionally reach almost half of the total number of T cells during some acute infections47. On activation, T cells gain responsiveness to other cytokines such as IL-2 and downregulate IL-7Rα and IL-7 consumption, presumably in order to preserve the naïve repertoire that is critically dependent on IL-7. Figure 8B shows the possible decrease in total naïve T cell population numbers that might be expected in the absence of IL-7Rα downregulation in the activated pool as compared to what would be seen for an ‘altruistic’ activated pool. A similar interaction is believed to exist between double negative (DN) and double positive (DP) thymocytes. The DN population of lymphocyte precursors in the thymus is critically dependent on IL-7 and gives rise to DP cells that constitute a large fraction of the cells in the thymus. As forced expression of IL-7Rα on DP thymocytes results in reduced numbers in the thymus, it has been suggested that the physiological downregulation of IL-7Rα on DP cells results in sparing of the limiting IL-7 resource for the DN population48.

IL-7 has been tested as a therapy for lymphopenic disorders49. A population level model of the IL-7 signaling network can be used to estimate optimal IL-7 therapeutic dosing regimens that minimize simultaneous activation of negative feedback loops. This will require careful determination of the effects of both receptor downregulation at increasing doses in combination with consideration of the effects of multi-cellular competition.

9. Conclusions

We have discussed several examples of pathological perturbations in the IL-7 signaling network. However, the involvement of IL-7 signaling in many diseases is not completely understood. For instance, a likely causal SNP (rs6897932) in the IL7Rα gene, was recently identified in a population of multiple sclerosis (MS) patients50. This SNP results in aberrant expression of a soluble isoform of the IL-7Rα by putatively disrupting an exonic splicing silencer. Simultaneously, the level of the normal membrane-bound IL-7Rα is reduced. This may have an impact on the pathogenesis of MS by altering the dynamics of the IL-7 or TSLP signaling network in multiple cell types. Notably, TSLP-activated DCs are involved in CD4+ T cell homeostasis51 and T regulatory cell development in the thymus52. In another recent development, a low level of IL-7Rα was shown to be an excellent marker for human T regulatory cells and Foxp3 expression53. This further suggests that there may be hitherto unrecognized roles for the IL-7 signaling network in human diseases.

The IL-7 signaling network provides a good example of the utility of a systems perspective in examining how multiple levels and mechanisms of regulation interact to produce a dynamic cellular response. A view of the pathway that is focused on single components provides an incomplete picture of the regulatory network because IL-7Rα simultaneously signals through multiple shared pathways and IL7Rα signaling is modulated through multiple concurrent mechanisms. We can reliably model the complex biology of IL-7 signaling only by integrating what is known about all the factors involved in signal propagation, repression and modulation. A comprehensive quantitative evaluation of the IL-7 signal network will extend our ability to understand the complex relationships involved in lymphocyte development and homeostasis. In addition, a detailed network analysis of IL-7 signaling may shed light on other cytokine signaling networks as well because it shares many components with other cytokine signaling networks. New technologies that allow quantitative network level analysis of signaling provide us with powerful tools that will improve our understanding of the immunobiology of IL-7 signaling.

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