Antigen-experienced T cells limit the priming of naive T cells during infection with Leishmania major.

One mechanism to control immune responses following infection is to rapidly down-regulate Ag presentation, which has been observed in acute viral and bacterial infections. In this study, we describe experiments designed to address whether Ag presentation is decreased after an initial response to Leishmania major. Naive alphabeta-Leishmania-specific (ABLE) TCR transgenic T cells were adoptively transferred into mice at various times after L. major infection to determine the duration of presentation of parasite-derived Ags. ABLE T cells responded vigorously at the initiation of infection, but the ability to prime these cells quickly diminished, independent of IL-10, regulatory T cells, or Ag load. However, Ag-experienced clonal and polyclonal T cell populations could respond, indicating that the diminution in naive ABLE cell responses was not due to lack of Ag presentation. Because naive T cell priming could be restored by removal of the endogenous T cell population, or adoptive transfer of Ag-pulsed dendritic cells, it appears that T cells that have previously encountered Ag during infection compete with naive Ag-specific T cells. These results suggest that during L. major infection Ag-experienced T cells, rather than naive T cells, may be primarily responsible for sustaining the immune response.