Processing of viable Salmonella typhimurium for presentation of a CD4 T cell epitope from the Salmonella invasion protein C (SipC).

We have identified Salmonella invasion protein C (SipC) as a target antigen for CD4 T cell recognition in mice infected with Salmonella typhimurium. SipC is a product of the type III secretion system encoded by S. typhimurium pathogenicity island 1. A SipC-specific T cell response was induced by infection with either the C5 wild type or attenuated SL3261 vaccine strain of S. typhimurium. We localized the response of T cell lines from infected mice to an epitope near the carboxyl terminus of SipC (SipC(381-394)) and studied the way it was processed from viable S. typhimurium. We demonstrated that CD4 T cell recognition of this epitope required actin-dependent uptake of S. typhimurium. Presentation also occurred when transport of newly synthesized MHC class II from the endoplasmic reticulum was disrupted and when the pH of intracellular compartments was raised, suggesting presentation by mature MHC class II recycled from the macrophage surface into neutral intracellular compartments. Salmonellae are known to colonize macrophages by localizing to compartments that do not make contact with the bactericidal environment of late endosomes or lysosomes, and thus might avoid lysosomal antigen processing. However, we demonstrate that a CD4 T cell response to S. typhimurium-secreted proteins may be induced by an alternative pathway capable of antigen presentation in conditions similar to those in the compartments where Salmonella localize.