Processing of Listeria monocytogenes antigens and the in vivo T-cell response to bacterial infection

Presentation of antigens to T lymphocytes is a critical step in the clearan ce of pathogens from their hosts and in the establishment of protective imm unity. Several animal models have been developed to study this process, but few have been as informative as the murine immune response to Listeria mon ocytogenes infection. Herein we review the presentation of L. monocytogenes proteins by the MHC class I antigen-processing pathway and the in vivo T-c ell response to these bacterial antigens. These studies demonstrate the fol lowing: 1) The size of a peptide-specific T-cell response does not correlat e with the amount of epitope presented by infected cells; 2) T cells specif ic for dominant epitopes do not, in the case of L. monocytogenes infection, inhibit responses to subdominant epitopes; 3) T cells responding to differ ent epitopes presented by MHC class Ia molecules expand, contract and enter the memory pool synchronously; 4) Repeated in vivo expansion of antigen-sp ecific T-cell populations results in a narrowing of their T-cell receptor r epertoire and in an increase in their affinity for antigen; and 5) T cells restricted by H2-M3 MHC class Ib molecules constitute a major parr of the p rimary response to bacterial infection, but appear to play a relatively sma ller role in memory responses. These studies have provided a novel glimpse of the relationship between antigen processing and in vivo T-cell responses to infection, and provide a foundation for more detailed analyses of T-cel l-mediated adaptive immunity.