To look for a possible correlation between the virulence of Rhodococcus equi and its cytokine-inducing capacity, we evaluated intracellular survival and measured cytokine induction by mouse macrophages infected with a virulent strain containing an 85-kb plasmid and expressing VapA (103+), its avirulent plasmid-cured derivative (103−), and heat-killed 103+ (HK). After incubation with similar numbers of bacteria, macrophages infected with 103− contained significantly more organisms than those infected with 103+ or HK. The number of bacteria in the macrophages infected with 103− and HK decreased progressively, whereas the 103+ numbers remained constant over 48 h. Interleukin 1β (IL-1β), IL-6, IL-10, IL-12 p40, and tumor necrosis factor alpha (TNF-α) mRNA induction peaked at 4 h and returned to baseline between 12 and 48 h postinfection. IL-1β, IL-6, IL-10, and TNF-α concentrations assessed by enzyme-linked immunosorbent assay generally agreed well with mRNA expression; IL-12 could, however, not be detected. For all the cytokines detected, mean concentrations in the supernatants were consistently higher in the 103−-infected monolayers than in those infected with 103+, although, with the exception of IL-1β, the differences were not statistically significant. R. equi HK was a poor inducer of cytokine production. In conclusion, virulent and avirulent R. equi strains induced similar levels of cytokine synthesis. The slightly greater induction of most cytokines observed following infection with 103− is likely secondary to greater uptake by macrophages rather than to a direct role of VapA or another plasmid-encoded product in downregulating cytokine induction.
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