Modulation of cytokine response of pneumonic foals by virulent Rhodococcusequi

The ability of Rhodococcus equi to induce pneumonia in foals depends on the presence of an 85- to 90-kb plasmid, In this study, we evaluated whether p lasmid-encoded products mediate virulence by modulating the cytokine respon se of foals. Foals infected intrabronchially with a virulence plasmid-conta ining strain of R. equi had similar gamma interferon (IFN-gamma) and interl eukin-12 (IL-12) p35 but significantly higher IL-1 beta, IL-10, IL-12 p40, and tumor necrosis factor alpha (TNF-alpha) mRNA expression in lung tissue compared to foals infected with the plasmid-cured derivative, IFN-gamma mRN A expression levels in CD4(+) T lymphocytes isolated from bronchial lymph n odes (BLN) were similar for the two groups of R. equi-infected foals on day 3 postinfection. However, on day 14, in association with pneumonia and mar ked multiplication of virulent R. equi but with complete clearance of the p lasmid-cured derivative, IFN-gamma mRNA expression in BLN CD4(+) T lymphocy tes was significantly (P < 0.001) higher in foals infected with the plasmid -cured derivative. These results suggests an immunomodulating role for R. e qui virulence plasmid-encoded products in downregulating IFN-gamma mRNA exp ression by CD4(+) T lymphocytes.