IFN-gamma clearly plays an important role in the murine host response against Listeria monocytogenes, but the time course of its production and its precise role in immunity remain controversial. To address these issues, we sequentially monitored IFN production and bacterial accumulation in vivo in C57B1/6 mice during primary listeriosis. IFN-gamma mRNA levels (measured by Northern blot analysis of freshly isolated splenic RNA) and serum IFN (measured by ELISA) were both maximal on day 1 of infection, decreasing steadily after day 2 to barely detectable levels by days 4 to 6. Significantly, there was no direct relationship between IFN levels and listericidal activity in vivo. Between days 1 and 3, the period of maximal IFN production, host bacterial load (assessed by quantitating live L. monocytogenes/spleen) increased approximately 10- to 50-fold. On the other hand, during the immune phase of infection (between days 5 and 7), a period when both IFN mRNA and protein were barely detectable, the host bacterial load decreased 1,000- to 10,000-fold. The paucity of IFN production in vivo during the immune phase was unexpected in light of previous reports demonstrating abundant in vitro lymphokine release by splenocytes isolated during the same time period. By direct comparisons of IFN production in vivo and in vitro, however, we could show that the late (days 6-7) peak of IFN release observed in Ag-stimulated cultures was an in vitro artifact. By contrast the pattern of spontaneous IFN release (obtained when freshly isolated cells were incubated in the absence of Ag) conformed more closely to that observed in vivo. Because listerial Ag stimulated vigorous lymphokine release in vitro, we sought to determine whether an analogous effect could be observed in vivo. In fact, even the infusion of very large doses of live bacteria (5-20,000,000/mouse) did not stimulate endogenous IFN-gamma production in mice infected for 6 to 7 days. These studies suggest three major conclusions: 1) IFN production in vivo occurs primarily during the early phase of listeriosis; 2) the dramatic decrease in bacterial numbers observed late in infection cannot be directly attributed to increased IFN production by LM-immune T cells; 3) although Ag-driven cultures of freshly isolated cells can provide useful information about the potential lymphokine-producing capabilities of Ag-specific T cells, these results have limited relevance in understanding patterns of T cell lymphokine production in vivo.

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