Intestinal adaptive immune responses influence host health, yet only a few intestinal bacteria species that induce cognate adaptive immune responses during homeostasis have been identified. Here, we show that Akkermansia muciniphila, an intestinal bacterium associated with systemic effects on host metabolism and PD-1 checkpoint immunotherapy, induces immunoglobulin G1 (IgG1) antibodies and antigen-specific T cell responses in mice. Unlike previously characterized mucosal responses, T cell responses to A. muciniphila are limited to T follicular helper cells in a gnotobiotic setting, without appreciable induction of other T helper fates or migration to the lamina propria. However, A. muciniphila–specific responses are context dependent and adopt other fates in conventional mice. These findings suggest that, during homeostasis, contextual signals influence T cell responses to the microbiota and modulate host immune function.

Most, if not all, cytokines activate phosphatidylinositol 3-kinase (PI-3K). Although many cytokine receptors have direct binding sites for the p85 subunit of PI-3K, others, such as the interleukin-3 (IL-3) receptor beta common chain (␤c) and the IL-2 receptor beta chain (IL-2R␤), lack such sites, leaving the mechanism by which they activate PI-3K unclear. Here, we show that the protooncoprotein Shc, which promotes Ras activation by recruiting the Grb2-Sos complex in response to stimulation of cytokine stimulation, also signals to the PI-3K/Akt pathway. Analysis of Y3F and "add-back" mutants of ␤c shows that Y577, the Shc binding site, is the major site required for Gab2 phosphorylation in response to cytokine stimulation. When fused directly to a mutant form of IL-2R␤ that lacks other cytoplasmic tyrosines, Shc can promote Gab2 tyrosyl phosphorylation. Mutation of the three tyrosyl phosphorylation sites of Shc, which bind Grb2, blocks the ability of the Shc chimera to evoke Gab2 tyrosyl phosphorylation. Overexpression of mutants of Grb2 with inactive SH2 or SH3 domains also blocks cytokine-stimulated Gab2 phosphorylation. The majority of cytokine-stimulated PI-3K activity associates with Gab2, and inducible expression of a Gab2 mutant unable to bind PI-3K markedly impairs IL-3-induced Akt activation and cell growth. Experiments with the chimeric receptors indicate that Shc also signals to the PI-3K/Akt pathway in response to IL-2. Our results suggest that cytokine receptors lacking direct PI-3K binding sites activate Akt via a Shc/Grb2/Gab2/PI-3K pathway, thereby regulating cell survival and/or proliferation.The proliferation, differentiation, and survival of hematopoietic cells are controlled by multiple cytokines. Cytokines bind to cell surface receptors and activate receptor-associated Janus family tyrosine kinases (Jaks) (25). Activated Jaks are required for the phosphorylation of multiple sites on receptor cytoplasmic domains. These tyrosyl phosphorylation sites recruit signal relay molecules containing src homology-2 (SH2) and/or phosphotyrosine binding (PTB) domains (26). Signal relay molecules convert receptor-proximal events into the activation of downstream signaling pathways, including the Ras/Raf/mitogen-activated protein kinase (MAPK) and the phosphatidylinositol 3-kinase (PI-3K)/Akt cascades. These pathways culminate in the phosphorylation of key transcription factors and other important cellular regulators (e.g., members of the cell survival/death machinery). Cytokine receptors also bind SH2 domain-containing transcription factors termed STATs, which, upon tyrosyl phosphorylation, activate transcription directly (11). Elucidating the molecular details of these signaling pathways is critical to understanding cytokine action.Much is known about how cytokines activate the MAPK and PI-3K pathways. Most cytokine receptors have direct binding sites for the PTB domain of the adapter protein Shc (7, 26). Shc is recruited to activated receptors, where it becomes tyrosyl phosphorylated on as many as three s...

Proliferative signaling by the IL-2R can occur through two distinct pathways, one mediated by Stat5 and one by the adaptor protein Shc. Although Stat5 induces T cell proliferation by serving as a transcription factor, the mechanism of proliferative signaling by Shc is poorly defined. We examined the roles of two major signaling pathways downstream of Shc, the p44/p42 mitogen-activated protein kinase (extracellular signal-related kinase (Erk)) and phosphatidylinositol 3-kinase (PI3K) pathways, in promitogenic gene induction and proliferation in the IL-2-dependent T cell line CTLL-2. Using IL-2R mutants and specific pharmacologic inhibitors, we found that the PI3K, but not Erk, pathway is required for maximal induction of c-myc, cyclin D2, cyclin D3, cyclin E, and bcl-xL by Shc. To test whether the PI3K pathway is sufficient for proliferative signaling, a tamoxifen-regulated form of PI3K (mp110*ER) was expressed in CTLL-2 cells. Activation of the PI3K pathway through mp110*ER failed to up-regulate expression of the c-myc, cyclin D2, cyclin D3, cyclin E, bcl-2, or bcl-xL genes or down-regulate expression of p27Kip1, even when coactivated with the Janus kinases (Jak) or the Raf/Erk pathway. Moreover, mp110*ER induced modest levels of thymidine incorporation without subsequent cell division. Although insufficient for mitogenesis, mp110*ER enhanced Stat5-mediated proliferative signaling through a mechanism independent of Stat5 transcriptional activity. Thus, in addition to serving a necessary, but insufficient role in Shc-mediated promitogenic gene expression, the PI3K pathway contributes to T cell proliferation by potentiating mitogenic signaling by Stat5.