A CD4⁺ T cell-intrinsic complement C5aR2-prostacyclin-IL-1R2 axis orchestrates Th1 cell contraction

T helper 1 (Th1) cell initiation pathways are well characterized; however, those regulating their contraction are less understood. Here, we define a CD4⁺ T cell-autonomous pathway in which complement C5 orchestrated a shift from prostaglandin E2 (PGE2) dominance to enhanced prostacyclin (PGI2) production via activation of C5a receptor 2 (C5aR2). This pivot in lipid mediators induced autocrine signaling through the PGI2 receptor and expression of the interleukin-1 (IL-1) decoy IL-1 receptor type 2 (IL-1R2), which sequestered Th1 cell-driving intrinsic IL-1β, facilitating Th1 cell contraction. Disruption of this C5aR2-PGI2-R axis was a hallmark of pathologically persistent Th1 cell activity in inflammatory conditions, including cryopyrin-associated periodic syndromes (CAPS), Crohn's disease, and rheumatoid arthritis. Rebalancing this axis through selective PGE2 synthase inhibition rectified the hyperactive Th1 cell phenotype in vitro in T cells from individuals with CAPS. Therefore, complement is a key controller of prostanoid metabolism, and the latter is an intrinsic-and potentially druggable-checkpoint for the cessation of Th1 cell effector responses.