Persistent antigen is essential for sustaining <i>Leishmania major</i> –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 <i>Leishmania major</i> strain (<i>dhfr-ts</i>-deficient), we demonstrate that loss of antigen leads to a decline in <i>Leishmania</i>-specific CD4⁺ T cells and susceptibility to reinfection. To elucidate the mechanism, we developed <i>Leishmania</i> phosphoenolpyruvate carboxykinase (PEPCK)-specific CD4⁺ T cell receptor transgenic (PEG) mice, enabling precise tracking of <i>Leishmania</i>-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 <i>L. major</i> failed to sustain recall responses and secondary immunity. These findings in a <i>L. major</i> model system argue that persistent antigen may be indispensable for maintaining memory CD4⁺ T cells and ensuring durable immunity against chronic infections.