Hippo Pathway Kinase Mst1 Is Required for Long-Lived Humoral Immunity.

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 naive T cells but, paradoxically, mildly elevated serum Ab titers. Nonetheless, the role of Mst1 in humoral immunity remains poorly understood. In this study, we found that early T cell-dependent IgG1 responses in young adult Mst1-deficient mice were largely intact with signs of impaired affinity maturation. However, the established Ag-specific IgG1 titers in Mst1-deficient mice decayed more readily because of a loss of Ag-specific but not the overall bone marrow plasma cells. Despite the impaired affinity and longevity of Ag-specific Abs, 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 Ab replacement therapy can be beneficial to the patients.

ICOS Signaling Controls Induction and Maintenance of Collagen-Induced Arthritis.

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 Tyr181-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 Tyr181-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.

Inducible costimulator (ICOS) potentiates TCR-induced calcium flux by augmenting PLCγ1 activation and actin remodeling.

The inducible costimulator (ICOS) is a T cell costimulatory receptor that plays crucial roles in T cell differentiation and function. So far, ICOS has been shown to activate three signaling components: phosphoinositide 3-kinase (PI3K), intracellular calcium mobilization, and TANK binding kinase 1 (TBK1). By generating a knock-in strain of mice in which the ICOS gene is modified such that the ICOS-mediated PI3K pathway is selectively abrogated while the capacity of ICOS to mobilize intracellular calcium remains intact, we have shown that ICOS-mediated PI3K activation is required for some but not all T cell responses. This suggests that the ICOS-calcium signaling axis may explain some of the PI3K-independent ICOS functions. Further, a recent in vivo imaging study indicated that ICOS-dependent intracellular calcium flux facilitates cognate T cell-B cell interactions within the germinal center. However, how ICOS promotes TCR-mediated calcium flux has not been clear. Here we identified a membrane proximal motif in the cytoplasmic tail of ICOS that is essential for ICOS-assisted calcium signaling and demonstrate that ICOS can induce calcium flux independently of other signaling motifs. We also provide evidence that ICOS potentiates phospholipase Cγ1 (PLCγ1) activation to enhance calcium release from the intracellular pool. In parallel, acute ligation of ICOS without TCR co-engagement leads to activation of small GTPases, RhoA and Cdc42, consistent with the capacity of ICOS to induce actin remodeling. Importantly, interruption of actin dynamics during acute TCR or TCR-ICOS co-ligation severely impairs calcium flux in T cells even in the presence of activated PLCγ1. Thus, ICOS potentiates TCR-induced calcium flux by enhancing PLCγ1 activation and actin remodeling in a coordinated manner.