In summary, we identified ATF3 as a regulator of gut TFH cells, which dictates the susceptibility of colitis. Materials and Methods Detailed materials and methods can be found in SI Appendix, including information on mouse strains, human samples, DSS-induced colitis, flow cytometric analysis and Plxnc1 sorting, adoptive transfer, in vitro differentiation of T cells, mixed bone marrow chimera, qRT-PCR, ELISA, ChIP, luciferase assay, statistics, and other methods. All experimental procedures with mice were performed in accordance with the Animal Care and Use Committee of Sun Yat-sen University. not fully elucidated. Here, we found that activating transcription factor 3 (ATF3) protects against colitis by regulating follicular helper T (TFH) cells in the gut. The expression of ATF3 in CD4+ T cells was negatively correlated with the severity of ulcerative colitis in clinical DBCO-NHS ester 2 patients. Mice with ATF3 deficiency in CD4+ T cells (mice compared with littermate controls. The defective TFH cells significantly diminished germinal center formation and IgA production in the gut. Importantly, adoptive transfer of TFH or IgA+ B cells DBCO-NHS ester 2 caused significant remission of colitis in mice, indicating the TFHCIgA axis mediated the effect of ATF3 on gut homeostasis. Mechanistically, B cell lymphoma 6 was identified as a direct transcriptional target of ATF3 in CD4+ T cells. In summary, we exhibited ATF3 as a regulator of TFH cells in the gut, which may represent a potential immunotherapeutic target in colitis. Defects in the gut mucosal immune system play an important role in the pathogenesis of inflammatory bowel diseases (IBDs), including Crohns disease and ulcerative colitis (UC). Gut microbiota promote the development of gut-associated lymphoid tissues (GALTs), which are responsible for the production of secretory IgA (sIgA) in the gut (1, 2). sIgA in lumen functions to maintain the indigenous users of microbiota and prevent the colonization of harmful microbes (3, 4). Once this delicate balance is usually disrupted, the hosts usually suffer from pathogenic conditions, especially IBD. sIgA, therefore, plays a protective role in IBD. The production of IgA could be T cell-independent or T cell-dependent, with the latter as the dominant manner (2, 3). The major site of T cell-dependent IgA production occurs in Peyers patches (PPs), which are the organized follicular structures present along intestinal walls. Indeed, follicular helper T (TFH) cells play a critical role in the facilitation of T cell-dependent production of IgA in PPs, through promoting germinal center (GC) formation and differentiation of B cells into IgA-producing plasmablasts. The plasmablasts then relocate to lamina propria and secrete high-affinity IgA into the intestinal lumen (5). The major biological function of TFH cells is usually to facilitate GC formation, affinity maturation, and antibody production in activated B cells (6). The importance of TFH cells has been well recognized in host defense against viral infections (7), deliberate vaccination DBCO-NHS ester 2 (8), and autoimmune diseases (9). In contrast to rigorous studies on systemic TFH cells, the mechanism regulating gut TFH cells remains poorly comprehended (6, 10). Activating transcription factor 3 (ATF3) is usually a member of the ATF/cAMP response element-binding (ATF/CREB) family (11). ATF3 is usually rapidly induced by a multitude of stimuli which directly or indirectly alter the expression of a variety of DBCO-NHS ester 2 genes in immune cells to limit excessive inflammation (12, 13). The participation of ATF3 in host immune responses against pathogens and certain inflammatory diseases, such as sepsis (12, 13), asthma (14), and hepatic steatosis (15), has been reported. However, its role in gut homeostasis remains to be fully comprehended. Expression of ATF3 was significantly induced in patients with Crohns disease (16). Several studies have indicated the protective role of ATF3 in the maintenance of intestinal barrier function and the pathogenesis of IBD, although unique mechanisms may contribute (17, 18). Here, we recognized ATF3 as a regulator of TFH cells in the gut. Expression of ATF3 in CD4+ T cells was negatively correlated with the severity of UC disease in clinical patients. Deficiency of ATF3 in CD4+ T cells significantly aggravated colitis in mice, which could be rescued by transfer of TFH or IgA+ B cells. We further exhibited that the regulation of TFH cells by ATF3 was intrinsic to T cells and dependent on B cell lymphoma 6 (Bcl6). Collectively, these observations shed light on the contribution of ATF3 to gut mucosal homeostasis, which indicates its potential therapeutic value in IBD. Results ATF3 Deficiency in CD4+ T Cells Aggravates Murine Colitis. Expression profiling of unique tissues revealed that ATF3 was highly expressed in GALTs including colon, PPs, and mesenteric lymph nodes, both in mRNA and protein levels (and and = 10 per group, 400 magnification). The colocalization of CD4 and ATF3 was quantitated using ImageJ software (and mice were challenged with 2.5% (weight per volume) DSS.