hold the patent (Patent # WO2022/075474) for anti-human-TIGIT agonistic antibody. restoring the imbalance of T cells that are activated in autoimmune diseases and suggest potential clinical applications for anti-hu-TIGIT agonistic mAbs as therapeutic agents. Subject terms: Systemic lupus erythematosus, Rheumatoid arthritis A monoclonal human T cell immunoreceptor with Ig and ITIM domains (TIGIT) agonistic antibody in a TIGIT knock-in mouse model suppresses follicular and peripheral helper T cell activation and enhances suppression effects of naive regulatory T cells. Introduction Treatment of systemic autoimmune diseases has improved with the advent of molecular targeted Cyclosporin H therapies1C3, but some patients still cannot control disease activity. The autoantibodies are characteristic of some autoimmune diseases, which are produced by the cooperation of self-reactive T cells and B cells. Follicular helper T (Tfh) and peripheral helper T (Tph) cells were reported to assist B cells4,5, and they are associated with autoimmune disease pathogenesis6C9. Because activation and inactivation of T cells are strictly regulated by signalling mediated by T cell receptors and costimulatory/inhibitory molecules10, we hypothesised that coinhibitory molecules might be a therapeutic target. T cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory motif domains (TIGIT) is a unique coinhibitory receptor expressed on effector T, memory T, regulatory T (Treg), and natural killer (NK) cells. TIGIT binds to two ligands, CD112 (PVRL2, nectin-2) and CD155 (poliovirus receptor, PVR), which are expressed on antigen-presenting cells, fibroblasts, endothelial cells, and some cancer cells11C14. TIGIT signalling inhibits non-Treg and NK cell activation12,15. TIGIT-deficient or TIGIT-inhibited mice are known to exhibit exacerbated symptoms of experimental autoimmune encephalomyelitis (EAE) and collagen-induced arthritis (CIA)16,17, and conversely, anti-mouse-TIGIT agonistic monoclonal antibodies (mAbs) have been reported to improve EAE symptoms by inhibiting T cell activation18. TIGIT signalling is also known to have the potential to enhance the suppressive function of Treg cells19,20. Agonistic mAbs to human-TIGIT (hu-TIGIT) were reported to induce to Treg cell effector molecule fibrinogen-like protein 219, CD350 but there are no reports directly demonstrating the in vivo effect of anti-hu-TIGIT agonistic mAb or analysing its functions. In this study, we developed anti-hu-TIGIT agonistic mAbs and hu-knock-in (KI) mice and explored how our mAb acts on a mouse model and human cells in detail. Our findings indicate that anti-hu-TIGIT agonistic mAb can manipulate T cell imbalance, and it has a potential clinical application as therapeutic agents for autoimmune diseases. Results Correlation between CD4+ T cell subsets and disease activity in patients with systemic autoimmune diseases To confirm in detail whether TIGIT-expressing cells are involved in the pathogenesis of autoimmune diseases, we first checked TIGIT expression in T cells in patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and Sj?grens syndrome (SjS), where TIGIT expression is known to be changed in peripheral T cells21C23. First, we compared the proportions of various T cell subsets and the expression of TIGIT and programmed cell death-1 (PD-1), known as a coinhibitory molecule24, on each cell subset in those diseases by performing immunophenotyping of peripheral blood (PB) Cyclosporin H by flow cytometry. Specifically, these features were compared among 10 patients with untreated active RA, 10 patients with active SLE, 20 patients with untreated SjS and 15 healthy controls (HCs) (Supplementary Table?1). Generally, in some patient groups versus the HCs, the proportions of Tfh and Tph cells were significantly higher Cyclosporin H in the CD4+ T cell population, while the proportion of CD45RA+ effector memory T (Temra) cells was significantly higher in the CD8+ T cell population (Supplementary Fig.?1). Within the CD4+ T cell compartment, many TIGIT-expressing cells were.