Interleukin (IL)-4 and IL-13 are related cytokines that regulate many aspects of allergic inflammation. They play important roles in regulating the responses of lymphocytes, myeloid cells, and non-hematopoietic cells. In T-cells, IL-4 induces the differentiation of naïve CD4 T cells into Th2 cells, in B cells, IL-4 drives the immunoglobulin (Ig) class switch to IgG1 and IgE, and in macrophages, IL-4 and IL-13 induce alternative macrophage activation. This review gives a short insight into the functional formation of these cytokine receptors. I will discuss both the binding kinetics of ligand/receptor interactions and the expression of the receptor chains for these cytokines in various cell types; both of which are crucial factors in explaining the efficiency by which these cytokines induce intracellular signaling and gene expression. Work initiated in part by William (Bill) E. Paul on IL-4 some 30 years ago has now grown into a major building block of our current understanding of basic immunology and the immune response. This knowledge on IL-4 has growing clinical importance, as therapeutic approaches targeting the cytokine and its signal transduction are becoming a part of the clinical practice in treating allergic diseases. Just by reading the reference list of this short review, one can appreciate the enormous input Bill has had on shaping our understanding of the pathophysiology of allergic inflammation and in particular the role of IL-4 in this process.

Although interleukin (IL)-4 and IL-13 participate in allergic inflammation and share a receptor subunit (IL-4Rα), differential functions for these cytokines have been reported. Therefore, we compared cells expressing type I and II IL-4 receptors with cells expressing only type II receptors for their responsiveness to these cytokines. IL-4 induced highly efficient, γC-dependent tyrosine phosphorylation of insulin receptor substrate 2 (IRS-2), whereas IL-13 was less effective, even when phosphorylation of signal transducer and activator of transcription 6 (STAT6) was maximal. Only type I receptor-γC+ signaling induced efficient association of IRS-2 with p85 or GRB2. IL-4 signaling through type I receptor complexes induced more robust expression of a subset of genes associated with alternatively activated macrophages than did IL-13, despite equivalent activation of STAT6. Thus, IL-4 activates signaling pathways through the type I receptor complex, qualitatively differently from IL-13, which cooperate to induce optimal gene expression.

The family of cytoplasmic Janus (Jak) tyrosine kinases plays an essential role in cytokine signal transduction, regulating cell survival and gene expression. Ligand-induced receptor dimerization results in phosphorylation of Jak2 on activation loop tyrosine Y1007 and stimulation of its catalytic activity, which, in turn, results in activation of several downstream signaling cascades. Recently, the catalytic activity of Jak2 has been found to be subject to negative regulation through various mechanisms including association with SOCS proteins. Here we show that the ubiquitin-dependent proteolysis pathway is involved in the regulation of the turnover of activated Jak2. In unstimulated cells Jak2 was monoubiquitinated, and interleukin-3 or gamma interferon stimulation induced polyubiquitination of Jak2. The polyubiquitinated Jak2 was rapidly degraded through proteasomes. By using different Jak2 mutants we show that tyrosine-phosphorylated Jak2 is preferentially polyubiquitinated and degraded. Furthermore, phosphorylation of Y1007 on Jak2 was required for proteasomal degradation and for SOCS-1-mediated downregulation of Jak2. The proteasome inhibitor treatment stabilized the Jak2-SOCS-1 protein complex and inhibited the proteolysis of Jak2. In summary, these results indicate that the ubiquitin-proteasome pathway negatively regulates tyrosine-phosphorylated Jak2 in cytokine receptor signaling, which provides an additional mechanism to control activation of Jak2 and maintain cellular homeostasis.

SUMMARY The transcription factor GATA3 is crucial for the differentiation of naïve CD4+ T cells into T helper 2 (Th2) cells. Here we show that deletion of Gata3 allowed the appearance of interferon γ (IFNγ)-producing cells in the absence of interleukin-12 (IL-12) and IFNγ. Such IFNγ production was transcription factor T-bet-independent. Another T-box-containing transcription factor Eomes, but not T-bet, was induced both in GATA3-deficient CD4+ T cells differentiated under Th2 cell conditions, and in Th2 cells with enforced Runx3 expression, contributing to IFNγ production. GATA3 over-expression blocked Runx3-mediated Eomes induction and IFNγ production, and GATA3 protein physically interacted with Runx3 protein. Furthermore, we found that Runx3 directly bound to multiple regulatory elements of the Ifng gene, and that blocking Runx3 function in either Th1 or GATA3-deficient “Th2” cells results in diminished IFNγ production by these cells. Thus, the Runx3-mediated pathway, actively suppressed by GATA3, induces IFNγ production in a STAT4 and T-bet-independent manner.