BZIP Transcription Factors BATF and c-Maf are Essential for Type-2 Inflammation
Helminth exposure, allergy and asthma each induce cellular responses in lymphoid and peripheral tissues that give rise to type-2 inflammation. Essential molecular mediators of this response are type-2 cytokines interleukin(IL)-4 and IL-13 derived from various subsets of immune cells. In lymphoid tissues, CD4+ Tfh cells make IL-4 to elicit IgE and high-affinity IgG1 production. In peripheral sites of infection, group 2 innate lymphoid (ILC2) cells make IL-13 and Th2 cells make both IL-13 and IL-4. Together, these cells mediate smooth muscle contraction, mucus production and recruitment of other innate effector cells, all of which are hallmarks of type-2 inflammation. As central mediators of type-2 inflammation, understanding the cell-specific expression and molecular regulation of type-2 cytokines in CD4+ T cells and ILC2 cells may lead to new therapies that ameliorate allergic disease and helminth infections.
The AP-1 factor basic leucine zipper transcription factor ATF-like (BATF) has been identified as a pioneer factor in in vitro-generated Th17 cells. BATF facilitates chromatin remodeling at the IL-17 locus as well as loci of key Th17-associated lineage specifying factors. It has also been deemed essential to the generation of functional humoral immunity through the development of follicular helper T (Tfh) cells and germinal center B cells. However, the role of BATF in the development and function of other CD4+ T helper subsets and innate immune cells in vivo has remained unclear. I show here that mice deficient in BATF do not develop type-2 inflammation after exposure to the parasitic helminth Nippostongylus brasiliensis. Since type-2 cytokine expression by Th2 and ILC2 cells is essential for expedient helminth expulsion, I hypothesized that BATF likely has a role in the development and/or induction of cytokine expression in CD4+ Th2 cells and ILC2 cells. Consistent with this hypothesis, I found that BATF utilizes a novel mechanism to control Th2 cytokine expression in Th2 cells. Specifically, BATF promotes permissive epigenetic modifications to alter the chromatin landscape early during Th2 cell differentiation. In addition, my data show that BATF deficiency inhibits the activation of ILC2 cells, preventing ILC2-mediated helminth clearance.
In addition to uncovering BATF-mediated regulations of type-2 inflammation, my work has revealed new insight into the role of a second bZIP transcription factor, cMaf, during type-2 immunity. As mentioned above, helminth exposure elicits IL-4 production by both CD4+ Tfh and Th2 cells. Although type-2 cytokine transcription has been well characterized in Th2 cells, Tfh cell-mediated IL-4 production has yet to be fully defined. Importantly, I show that IL-4 production by Tfh cells is sustained upon deletion of classical IL-4 regulatory factors signal transducer and activator of transcription 6 (STAT6) and STAT5 and is not dependent on high GATA-3 expression. In sum, Tfh-driven IL-4 production is induced independent of classical pathways in Th2 cells.
Presently, the non-canonical transcription factors involved in IL-4 production by Tfh cells remain unclear. C-Maf works with BCL6, the master regulator of Tfh cells, to elicit Tfh formation. However, the precise role of c-Maf in Tfh cell fate and function remains unclear. So far, it has been shown that in Th2 cells, c-Maf binds to the IL-4 promoter and in Tfh cells, c-Maf binds to the CNS2 enhancer of the IL-4 locus to regulate IL-4 expression. Therefore, I hypothesized that c-Maf is important in non-canonical, GATA-3-independent IL-4 production by Tfh cells.
Here, I show that Tfh cells lacking canonical Th2 pathways for IL-4 expression express high levels of c-Maf and IL-4 transcript. Deletion of c-Maf in CD4+ T cells resulted in normal induction of BCL6 expression. Thus the initial stages of Tfh cell generation were induced. However, cMaf-deficient CD4+ T cells did not express important molecules associated with Tfh cell migration. Immunohistochemistry also confirmed that c-Maf deficiency inhibited CD4+ T cell migration from the paracortex into the B cell follicle.
These defects did not inhibit cMaf-deficient CD4+ T cells from making IL-4 transcript; however, IL-4 protein production was significantly impaired. Together, these results demonstrate that c-Maf is essential for Tfh cell-mediated immunity by promoting CD4+ T cell migration to the B cell follicles and the production of IL-4 protein in the germinal centers.
Collectively, the objective of my thesis research is to define the roles of the bZIP transcription factors BATF and c-Maf in type-2 inflammation. My data demonstrate that BATF is essential for the differentiation and function of Tfh, Th2, and ILC2 cells during helminth infection. Additionally, I have shown that c-Maf is required for Tfh function and CD4+ T cell migration to the B cell follicle. Thus, BATF and c-Maf are central to the development of humoral and peripheral type-2 inflammatory responses against helminth infection. Given the wide spectrum of disorders associated with type-2 inflammation, the identification of factors relevant to the development and function of Th2-, ILC2- and Tfh-driven allergic pathologies is broadly relevant. A comprehensive characterization of core factors like BATF and c-Maf provide new avenues in which to explore novel therapies to modulate type-2 inflammatory responses.
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