Patients with autosomal dominant hyper-IgE syndrome (AD-HIES) share a clinical tendency to develop increased T helper 2 (Th2) cell responses and hallmarks of Th2-mediated immune responses, such as elevated serum IgE levels, eosinophilia, and allergic manifestations in the skin, respiratory tract and at the systemic level. The first genetic etiology identified was the loss-of-function mutations in STAT3 ^{1, 2}. Since then, three other genetic etiologies have since been reported; all three share similar manifestations of elevated Th2 cytokines, eosinophilia, and high IgE than STAT3 deficiency: (1) ZNF341 deficiency (ZNF341 being a transcription factor required for the expression and activity of STAT3 ^{3, 4}, (2) deficiency of the common receptor chain gp130 (encoded by IL6ST; ^{5, 6}), and (3) IL-6R deficiency ⁷. IL-6R and gp130 form the IL-6 receptor complex, which transmits the IL-6 signal through STAT3 activation. Therefore, the unifying mechanism underlying the clinical features of allergy in AD-HIES patients is the disruption of IL-6–dependent STAT3 activation. However, although these studies in AD-HIES patients have hinted at a role for IL-6 in controlling Th2 bias, the specific contribution of IL-6 and the underlying mechanism remains undefined.

In our study ⁸, we aimed to understand the role of IL-6 in suppressing Th2 cell differentiation and Th2 cell-mediated inflammation and whether our findings might be relevant to developing interventions for preventing unwanted Th2 cell-mediated responses. For this, we used a clinically relevant house dust mite (HDM)-induced allergic airway inflammation mouse model, which allows for generating or preventing Th2 cell responses and associated airway inflammation simply by varying the amount of bacterial lipopolysaccharide (LPS) contained in the inhalant HDM allergen ^{9, 10}. We found that IL-6 is produced in response to LPS present in HDM allergens. Using wild-type (WT) and IL-6-deficient (IL-6^−/−) mice and a conditional knockout model with selective IL-6 receptor deletion in T cells, we found that when IL-6 is produced, Th2 cell responses to HDM allergens are prevented. Particularly, IL-6 signaling in responding T cells is intrinsically required to suppress the Th2 cell differentiation program ⁸.

To understand the underlying mechanism by which IL-6 signaling suppresses the commitment of naïve T cells to the Th2 cell pathway, we performed RNA-seq to compare the transcriptomes of T cells primed in WT and IL-6-deficient environments. Ingenuity Pathway Analysis (IPA) predicted an enhanced IL-2-dependent gene regulation in the absence of IL-6 signaling. Since IL-2 plays a critical role in the polarization of Th2 cells ^{11, 12}, we predicted that IL-6-dependent inhibition of IL-2 signaling in T cells is a mechanism to suppress the Th2 cell differentiation program in response to allergen exposure. In vivo and in vitro analysis of antigen-specific T cells that do or do not receive IL-6 and IL-2 signaling confirmed that IL-6 suppresses IL-2 signaling in T cells and, as such, represses signals necessary for Th2 cell differentiation ⁸.

We further wanted to define better the mechanism by which IL-6 controls IL-2 responsiveness, so we examined gene expression differences between T cells primed in WT and IL-6-deficient environments. One of the genes most significantly downregulated in T cells that did not receive IL-6 signaling was Socs3. SOCS proteins are generally negative-feedback inhibitors of signaling induced by cytokines that act via the JAK/STAT pathway, such as IL-2-dependent STAT5 activation. Using a conditional knockout model with selective SOCS3 deletion in T cells, we found that SOCS3 is strongly induced by IL-6 in antigen receptor-activated CD4⁺ T cells and potently inhibits IL-2-induced STAT5 activation and Th2 cell differentiation in response to HDM allergens ⁸. Finally, we aimed to find a drug mimicking the SOCS3-mediated inhibition of IL-2 signaling that could therefore be used to prevent Th2 cell differentiation. The binding of IL‐2 to the IL‐2 receptor activates JAK1 and JAK3 kinases, a process that is required to subsequently activate STAT5. SOCS3 inhibits the kinase activity of JAK1. Thus, we tested whether the treatment of mice with the JAK1 selective inhibitor upadacitinib could block IL-2 signaling in recently activated T cells and hence their polarization to Th2 cells and found a positive result with this drug ⁸.

Overall, we have discovered a novel and previously unrecognized role for IL-6 in preventing the development of unwanted Th2 cell responses and associated diseases. Our work supports a model in which IL-6 is produced in response to the detection of pathogen-associated molecular patterns (PAMPs) contained in allergens and its signaling in activated antigen-specific T cells upregulates the expression of SOCS3, thereby limiting IL-2 signaling and thus restricting the acquisition of a Th2 cell differentiation program. This work further outlines possible preventive interventions and points to JAK1 inhibition as an attractive strategy to deter Th2 cell immunity.

REFERENCES

1. Holland, S.M. et al. STAT3 mutations in the hyper-IgE syndrome. N Engl J Med 357, 1608-1619 (2007).

2. Minegishi, Y. et al. Dominant-negative mutations in the DNA-binding domain of STAT3 cause hyper-IgE syndrome. Nature 448, 1058-1062 (2007).

3. Beziat, V. et al. A recessive form of hyper-IgE syndrome by disruption of ZNF341-dependent STAT3 transcription and activity. Sci Immunol 3 (2018).

4. Frey-Jakobs, S. et al. ZNF341 controls STAT3 expression and thereby immunocompetence. Sci Immunol 3 (2018).

5. Schwerd, T. et al. A biallelic mutation in IL6ST encoding the GP130 co-receptor causes immunodeficiency and craniosynostosis. J Exp Med 214, 2547-2562 (2017).

6. Beziat, V. et al. Dominant-negative mutations in human IL6ST underlie hyper-IgE syndrome. J Exp Med217 (2020).

7. Spencer, S. et al. Loss of the interleukin-6 receptor causes immunodeficiency, atopy, and abnormal inflammatory responses. J Exp Med 216, 1986-1998 (2019).

8. Bachus, H. et al. IL-6 prevents Th2 cell polarization by promoting SOCS3-dependent suppression of IL-2 signaling. Cell Mol Immunol (2023).

9. Bachus, H. et al. Impaired Tumor-Necrosis-Factor-alpha-driven Dendritic Cell Activation Limits Lipopolysaccharide-Induced Protection from Allergic Inflammation in Infants. Immunity 50, 225-240 e224 (2019).

10. Kaur, K. et al. GM-CSF production by non-classical monocytes controls antagonistic LPS-driven functions in allergic inflammation. Cell Rep 37, 110178 (2021).

11. Cote-Sierra, J. et al. Interleukin 2 plays a central role in Th2 differentiation. Proc Natl Acad Sci U S A101, 3880-3885 (2004).

12. Liao, W. et al. Priming for T helper type 2 differentiation by interleukin 2-mediated induction of interleukin 4 receptor alpha-chain expression. Nat Immunol 9, 1288-1296 (2008).