A small molecule inhibitor of CD28 costimulation restrains pathogenic T-cell responses in inflammatory bowel disease

CD28 costimulation contributes to pathogenic T cell responses in inflammatory bowel disease (IBD), but current B7-directed blockade also limits CTLA-4 signaling. Using a sensitive NanoBiT split-luciferase screening platform, we identified and optimized CA-23, a small molecule antagonist that directly binds human and mouse CD28 without measurable binding to CD80, CD86, or CTLA-4. CA-23 inhibited CD28-B7 engagement and CD28-dependent T cell activation without agonist activity in human whole blood and peripheral blood mononuclear cells. CA-23 achieved exposure in the colon and mesenteric lymph nodes and reduced disease severity, histologic injury, and pathogenic Th1 and Th17 responses in a T cell transfer model of colitis. In PBMCs from donors with ulcerative colitis or Crohns disease, CA-23 suppressed inflammatory cytokine production and T cell activation to a degree matching or exceeding Abatacept. In human intestinal epithelial-PBMC co-cultures, CA-23 preserved Treg suppressive activity and epithelial barrier integrity, whereas Abatacept reduced Treg function. CA-23 did not alter CD80 or CD86 expression on autologous antigen-presenting cells and showed no substantial off-target activity in the tested selectivity panel. These findings support direct CD28 antagonism as a mechanistically differentiated alternative to B7-directed co-stimulation blockade for suppressing pathogenic T cell responses in preclinical models of IBD. One Sentence SummaryA CD28-selective small molecule blocks pathogenic T cell activation and preserves Treg function unlike Abatacept in IBD models.