Mechanisms of T cell epitope immunodominance analyzed in murine listeriosis.

We demonstrate conclusively that the bacterial exotoxin listeriolysin O (LLO) is a target Ag for eliciting CD4+ T cell responses following infection with Listeria monocytogenes. The minimal I-Ek-restricted immunodominant CD4+ T cell epitope was identified as peptide 215-226 (p215-226). Most LLO-specific T cell hybridomas recognized p203-226, p208-226, p215-226, and p215-234, although each exhibited a characteristic pattern of preferential reactivity. One hybridoma (IIIC5) reacted to p203-226 but not to p208-226 or any other LLO peptide tested. With APCs from B10 congenic mice and cells transfected with either I-Ak or I-Ek, IIIC5 recognized p203-216 with I-Ak, while a different hybridoma (IB5) recognized p215-226 with I-Ek. Competitive binding studies demonstrated that of 15 LLO peptides tested, only p203-226, p215-226, and p215-234 had high affinity for I-Ek, while p203-226 could also bind to isolated I-Ak. Of nine LLO peptides tested, only p215-234 bound multiple class II MHC alleles. These findings suggest that the immunodominance of p203-226 may be due in part to the presence of multiple T cell epitopes with I-Ek- and I-Ak-binding capability. Many of the rules of immunodominance observed with model Ags are also operative in our murine model of bacterial infectious disease. Furthermore, a novel mechanism of immunodominance based on newly defined structural features of MHC molecules is implicated. This information is crucial for rational vaccine development.