Human patients with homozygous null mutations in the ICOS gene suffer from recurrent infections due to humoral immune defects. Studies on human patients and mouse models have shown that inducible T-cell co-stimulator (ICOS)-deficient individuals cannot form T follicular helper (Tfh) cells, a group of CD4 T cells that migrate into B cell follicles and facilitate germinal center (GC) reactions. ICOS-induced phosphoinositide 3-kinase signaling pathways have been shown to play critical roles in Tfh programming, migration of Tfh cells into the GC, and delivery of T cell help during Tfh-GC B cell conjugation. These processes are also assisted by ICOS-mediated intracellular calcium mobilization and TANK-binding kinase 1 signaling. However, ICOS signaling also has stimulatory roles in T regulatory cells and innate lymphoid cells (ILCs), providing another layer of complexity. In this review, we discuss celltype-specific signaling mechanisms utilized by ICOS in Tfh cells, T regulatory cells, and ILCs. Whenever relevant, we compare the roles and signaling pathways of ICOS and CD28. Understanding ICOS signal transduction mechanisms used by distinct immune subsets at different stages of immune responses or disease progression may help improve vaccination protocols, treat autoimmune diseases, and enhance cancer immunotherapy. K E Y W O R D S calcium, ICOS, phosphoinositide 3-kinase, signal transduction, T follicular helper cell

ICOS is a key costimulatory receptor facilitating differentiation and function of follicular helper T cells and inflammatory T cells. Rheumatoid arthritis patients were shown to have elevated levels of ICOS T cells in the synovial fluid, suggesting a potential role of ICOS-mediated T cell costimulation in autoimmune joint inflammation. In this study, using ICOS knockout and knockin mouse models, we found that ICOS signaling is required for the induction and maintenance of collagen-induced arthritis (CIA), a murine model of rheumatoid arthritis. For the initiation of CIA, the Tyr-based SH2-binding motif of ICOS that is known to activate PI3K was critical for Ab production and expansion of inflammatory T cells. Furthermore, we found that Tyr-dependent ICOS signaling is important for maintenance of CIA in an Ab-independent manner. Importantly, we found that a small molecule inhibitor of glycolysis, 3-bromopyruvate, ameliorates established CIA, suggesting an overlap between ICOS signaling, PI3K signaling, and glucose metabolism. Thus, we identified ICOS as a key costimulatory pathway that controls induction and maintenance of CIA and provide evidence that T cell glycolytic pathways can be potential therapeutic targets for rheumatoid arthritis.

MYSM1 is a chromatin-binding histone deubiquitinase. mutations in humans result in lymphopenia whereas loss of in mice causes severe hematopoietic abnormalities, including an early arrest in B cell development. However, it remains unknown whether MYSM1 is required at later checkpoints in B cell development or for B cell-mediated immune responses. We analyzed conditional mouse models Tg.mb1-cre,Tg.CD19-cre, and Tg.CD21-cre with inactivation of at prepro-B, pre-B, and follicular B cell stages of development. We show that loss of at the prepro-B cell stage inTg.mb1-cre mice results in impaired B cell differentiation, with an ∼2-fold reduction in B cell numbers in the lymphoid organs. Tg.mb1-cre B cells also showed increased expression of activation markers and impaired survival and proliferation. In contrast, was largely dispensable from the pre-B cell stage onward, with Tg.CD19-cre andTg.CD21-cre mice showing no alterations in B cell numbers and largely normal responses to stimulation. MYSM1, therefore, has an essential role in B cell lineage specification but is dispensable at later stages of development. Importantly, MYSM1 activity at the prepro-B cell stage of development is important for the normal programming of B cell responses to stimulation once they complete their maturation process.

The protein kinase Mst1 is a key component of the evolutionarily conserved Hippo pathway that regulates cell survival, proliferation, differentiation, and migration. In humans, Mst1-deficiency causes primary immunodeficiency. Patients with MST1-null mutations show progressive loss of naïve T cells but, paradoxically, mildly elevated serum antibody titers. Nonetheless, the role of Mst1 in humoral immunity remains poorly understood. Here we found that early T-dependent IgG1 responses in young adult Mst1-deficient mice were largely intact with signs of impaired affinity maturation. However, the established antigen-specific IgG1 titers in Mst1-deficient mice decayed more readily due to a loss of antigen-specific, but not the overall, bone marrow plasma cells. Despite the impaired affinity and longevity of antigen-specific antibodies, Mst1-deficient mice produced plasma cells displaying apparently normal maturation markers with intact migratory and secretory capacities. Intriguingly, in immunized Mst1-deficient mice, T follicular helper cells were hyperactive expressing higher levels of IL-21, IL-4, and surface CD40L. Accordingly, germinal center B cells progressed more rapidly into the plasma cell lineage, presumably forgoing rigorous affinity maturation processes. Importantly, Mst1-deficient mice had elevated levels of CD138+Blimp1+ splenic plasma cell populations yet the size of the bone marrow plasma cell population remained normal. Thus, overproduced low affinity plasma cells from dysregulated germinal centers seem to underlie humoral immune defects in Mst1-deficiency. Our findings imply that vaccination of Mst1-deficient human patients, even at the early stage of life, may fail to establish long-lived high affinity humoral immunity and that prophylactic antibody replacement therapy can be beneficial to the patients.