Cytokine expression in vivo during murine listeriosis. Infection with live, virulent bacteria is required for monokine and lymphokine messenger RNA accumulation in the spleen.

To examine the regulation of cytokine synthesis during murine listeriosis, we have monitored IFN-gamma, TNF-alpha, and IL-1 beta mRNA levels in the spleens of C57B1/6 mice after the i.v. infusion of virulent and nonvirulent preparations of Listeria monocytogenes (LM). Messenger RNA coding for TNF, IL-1, or IFN did not become detectable until approximately 12 to 15 h after the infusion of virulent LM. Levels of each cytokine mRNA then increased synchronously reaching peak or near peak levels around 24 h after infection. Levels gradually decreased over the next 4 to 5 days. Unlike virulent LM, neither heat-killed LM, nor nonvirulent LM variants lacking listeriolysin O, stimulated monokine or IFN mRNA accumulation even when administered in very large doses. To gain perspective concerning the response to LM, we examined the early pattern of cytokine mRNA accumulation induced by Salmonella typhimurium (ST), an intracellular pathogen expressing LPS. We noted at least three significant differences between the cytokine responses to LM and ST: 1) monokine mRNA levels increased much more rapidly (within 1 h) after ST infection; 2) unlike LM, ST retained the capacity to stimulate cytokine mRNA production when injected as heat-killed bacteria; 3) in contrast to LM, ST could not trigger the early IFN production characteristic of LM infection. Our data suggest that monokine and IFN production early in listeriosis are critically linked with the process of bacterial invasion of host cells. The timing and pattern of cytokine mRNA accumulation in this setting is qualitatively different from that induced by LPS. The pathway described in these studies may also play a role in the host cytokine response to other intracellular pathogens as well.