A novel PD-L1/BTLA checkpoint constrains CD4⁺ T cell–driven antibacterial humoral immunity via immunometabolic reprogramming in teleost fish

The programmed death-ligand 1/B and T lymphocyte attenuator (PD-L1/BTLA) axis is a newly discovered immune checkpoint that represents a primordial alternative to the mammalian PD-1/PD-L1 pathway in early adaptive immunity. However, the functional roles and molecular mechanisms of this checkpoint in adaptive immunity remain largely unclear. In the present study, we characterize this checkpoint in zebrafish and show that it suppresses CD4⁺ T cell-dependent humoral immune responses and is exploited by E. tarda for immune evasion. We demonstrate that E . tarda infection significantly upregulates PD-L1 on macrophages and BTLA on CD4⁺ T cells. PD-L1–BTLA engagement markedly inhibits macrophage-induced CD4⁺ T-cell activation, suppresses Th1/Th2 cytokine production, and consequently limits IgM⁺ B-cell and IgZ⁺ B-cell activation and antibody production. Disruption of this axis via genetic knockout, blocking antibodies, or soluble proteins reverses these effects, leading to enhanced bacterial clearance and improved host survival. Mechanistically, BTLA ligation recruits SHP-1/2 phosphatases, attenuates the PI3K/AKT/mTORC1 pathway, and suppresses glycolytic metabolism in CD4⁺ T cells, thereby restraining their proliferation and effector function. These findings reveal a previously unrecognized immunometabolic checkpoint mechanism by which E . tarda evades humoral immunity and highlight the evolutionary plasticity of checkpoint-mediated immune regulation, suggesting that metabolic control of T-cell responses represents an ancient strategy of pathogen-driven immunosuppression.