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

Background: Slik kinase has catalytic activity-dependent and -independent functions. Results: Mutation of activation segment phosphorylation sites abolishes both catalytic and non-catalytic activities; non-catalytic function also depends upon localization via the C-terminal domain. Conclusion: Slik is regulated by both localization and phosphorylation. Significance: Conformational activation can control not only catalytic but also non-catalytic activities of kinases.

No abstract

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.

Talin is the major scaffold protein linking integrin receptors with the actin cytoskeleton. In Drosophila, extended Talin generates a stable link between the sarcomeric cytoskeleton and the tendon matrix at muscle attachment sites. Here, we identify phosphorylation sites on Drosophila Talin by mass spectrometry. Talin is phosphorylated in late embryogenesis when muscles differentiate, especially on T152 in the exposed loop of the F1 domain of the Talin head. Localization of a mutated version of Talin (Talin-T150/T152A) is reduced at muscle attachment sites and can only partially rescue muscle attachment compared with wild-type Talin. We also identify Slik as the kinase phosphorylating Talin at T152. Slik localizes to muscle attachment sites, and the absence of Slik reduces the localization of Talin at muscle attachment sites causing phenotypes similar to Talin-T150/ T152A. Thus, our results demonstrate that Talin phosphorylation by Slik plays an important role in fine-tuning Talin recruitment to integrin adhesion sites and maintaining muscle attachment.

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.

Hippo signaling pathways are evolutionarily conserved signal transduction mechanisms mainly involved in organ size control, tissue regeneration, and tumor suppression. However, in mammals, the primary role of Hippo signaling seems to be regulation of immunity. As such, humans with null mutations in STK4 (mammalian homologue of Drosophila Hippo; also known as MST1) suffer from recurrent infections and autoimmune symptoms. Although dysregulated T cell homeostasis and functions have been identified in MST1-deficient human patients and mouse models, detailed cellular and molecular bases of the immune dysfunction remain to be elucidated. Although the canonical Hippo signaling pathway involves transcriptional coactivator Yes-associated protein (YAP) or transcriptional coactivator with PDZ motif (TAZ), the major Hippo downstream signaling pathways in T cells are YAP/TAZ-independent and they widely differ between T cell subsets. Here we will review Hippo signaling mechanisms in T cell immunity and describe their implications for immune defects found in MST1-deficient patients and animals. Further, we propose that mutual inhibition of Mst and Akt kinases and their opposing roles on the stability and function of forkhead box O and β-catenin may explain various immune defects discovered in mutant mice lacking Hippo signaling components. Understanding these diverse Hippo signaling pathways and their interplay with other evolutionarily-conserved signaling components in T cells may uncover molecular targets relevant to vaccination, autoimmune diseases, and cancer immunotherapies.

Angioimmunoblastic T-cell lymphoma (AITL) is an aggressive peripheral T-cell lymphoma driven by a pool of neoplastic cells originating from T follicular helper (Tfh) cells and concomitant expansion of B cells. Conventional chemotherapies for AITL have shown limited efficacy, and as such, there is a need for improved therapeutic options. Because AITL originates from Tfh cells, we hypothesized that AITL tumors continue to rely on essential Tfh components and intimate T-cell–B-cell (T-B) interactions. Using a spontaneous AITL mouse model (Roquinsan/+ mice), we found that acute loss of Bcl6 activity in growing tumors drastically reduced tumor size, demonstrating that AITL-like tumors critically depend on the Tfh lineage–defining transcription factor Bcl6. Because Bcl6 can upregulate expression of signaling lymphocytic activation molecule–associated protein (SAP), which is known to promote T-B conjugation, we next targeted the SAP-encoding Sh2d1a gene. We observed that Sh2d1a deletion from CD4+ T cells in fully developed tumors also led to tumor regression. Further, we provide evidence that tumor progression depends on T-B cross talk facilitated by SAP and high-affinity LFA-1. In our study, AITL-like tumors relied heavily on molecular pathways that support Tfh cell identity and T-B collaboration, revealing potential therapeutic targets for AITL.