Enumeration of Listeria monocytogenes-reactive L3T4+ T cells activated during infection.

A limiting dilution system was developed which allows minimal estimates of the number of Listeria monocytogenes-reactive T cells from infected mice. Limiting numbers of T cells were restimulated in vitro with accessory cells in the presence or absence of antigen (heat-killed L. monocytogenes organisms) and proliferative responses determined. The responding T cells resided entirely within the L374+, Lyt2- (helper/inducer) T-cell subset. L. monocytogenes-reactive T cells were not demonstrable in uninfected mice nor during the first 3 or 4 days of infection. In contrast, on days 4 or 5, respectively, of infection approximately 1/1000 T cells showed a response to L. monocytogenes. Their frequency declined only slightly over the subsequent weeks and still was as high as 1/4900 4 weeks after infection when no bacteria were present in the host. The frequency of L. monocytogenes-reactive T cells depended on the number and virulence of the infecting organisms, the highest sublethal dose of virulent bacteria inducing the highest frequency. Chemotherapeutic shortening of infection between days 3 and 4 resulted in a six-fold reduction of reactive T cells. Thus, the frequency of L. monocytogenes-reactive T cells depended on the presence of bacteria in the host during the first 3 to 4 days of infection. These findings may have implications for the rational design of vaccines directed against intracellular bacterial pathogens as they raise the question whether attenuated bacterial strains of low persistence can induce sufficiently high T-cell numbers required for protective immunity.