Infection of mice with Salmonella enterica serotype Typhimurium induces a strong Th1 cell response that is central for the control of infection. We infected mice of a resistant background with a virulent strain of S. enterica serovar Typhimurium and analyzed the kinetics and magnitude of the T-cell response. After infection, the majority of CD4(+) and CD8(+) splenocytes acquired an activated phenotype, as indicated by expression levels of CD44 and CD62L. In addition, after 3 to 4 weeks of infection, more than 20% of the CD4(+) and more than 30% of the CD8(+) T cells produced gamma interferon (IFN-gamma) in response to short-term polyclonal stimulation. In contrast, we detected only a moderate (two- to threefold) expansion of both T-cell populations, and BrdU incorporation revealed that there was either no or only a limited increase in the in vivo proliferation of CD4(+) and CD8(+) T cells, respectively. Our results indicate that although an unexpectedly large population of both CD4(+) and CD8(+) T cells is activated and acquires the potential to secrete IFN-gamma, this activation is not paralleled by substantial expansion of these T-cell populations.
