Persistent antigen is essential for sustaining Leishmania major-specific memory CD4⁺ T cells and long-term immunity

Memory CD4⁺ T cells are central to long-term immunity, yet their persistence in the absence of antigen remains controversial. Lifelong immunity following cutaneous leishmaniasis is primarily mediated by CD4⁺ T cells, but whether persistent parasites are required for sustaining this immunity has not been empirically established. Using a nonpersistent Leishmania major strain (dhfr-ts-deficient), we demonstrate that loss of antigen leads to a decline in Leishmania-specific CD4⁺ T cells and susceptibility to reinfection. To elucidate the mechanism, we developed Leishmania phosphoenolpyruvate carboxykinase (PEPCK)-specific CD4⁺ T cell receptor transgenic (PEG) mice, enabling precise tracking of Leishmania-specific memory CD4⁺ T cells. Both in vitro and in vivo-generated memory PEG cells progressively declined in the absence of antigen, independent of major histocompatibility complex II expression. Mice infected with PEPCK antigen-deficient L. major failed to sustain recall responses and secondary immunity. These findings in a L. major model system argue that persistent antigen may be indispensable for maintaining memory CD4⁺ T cells and ensuring durable immunity against chronic infections.