PD-1 deficiency impairs Treg differentiation in Trichinella-infected mice via inhibition of fatty acid oxidation

Helminth infections establish long-term host persistence by inducing regulatory immune responses that limit pathology while promoting parasite survival. Programmed death 1 (PD-1), an inhibitory receptor expressed on activated T cells, plays an essential role in maintaining immune homeostasis during chronic infections. Regulatory T cells (Tregs) expand during helminth infection and are critical for controlling excessive inflammation; however, the metabolic mechanisms linking PD-1 signaling to Treg differentiation during helminth infection remain poorly defined. Here, we investigated the role of PD-1 in regulating Treg differentiation during Trichinella spiralis infection, focusing on metabolic reprogramming in CD4⁺ T cells. Using a murine infection model, we compared wild-type and PD-1–deficient (PD-1 ⁻/⁻ ) mice and evaluated Treg differentiation in vivo and in a bone marrow-derived dendritic cell (BMDC)–CD4⁺ T-cell co-culture system stimulated with muscle larval excretory–secretory products (MES). PD-1 deficiency significantly reduced splenic FoxP3⁺ Treg frequencies following infection and impaired MES-induced Treg differentiation and Treg-associated cytokine production in vitro. Metabolic profiling revealed that CD4⁺ T cells from infected PD-1 ⁻/⁻ mice exhibited markedly reduced fatty acid oxidation (FAO), accompanied by decreased expression of Carnitine Palmitoyltransferase 1A (CPT1A), a key rate-limiting enzyme in FAO. PD-1 deficiency was associated with enhanced PI3K–Akt–mTOR signaling and suppressed AMPK activation. Importantly, pharmacological inhibition of PI3K partially restored FAO activity and rescued Treg differentiation in PD-1 ⁻/⁻ CD4⁺ T cells. These findings suggest that PD-1 supports Treg development during T. spiralis infection, at least in part, through a CPT1A-associated FAO pathway, highlighting a PD-1–associated metabolic pathway that may contribute to helminth-induced immune regulation.