Modulation of cytokine response of pneumonic foals by virulent Rhodococcus equi

Abstract

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 plasmid-encoded products mediate virulence by modulating the cytokine response of foals. Foals infected intrabronchially with a virulence plasmid-containing strain of R. equi had similar gamma interferon (IFN-gamma) and interleukin-12 (IL-12) p35 but significantly higher IL-1beta, 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 mRNA expression levels in CD4+ T lymphocytes isolated from bronchial lymph nodes (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 marked multiplication of virulent R. equi but with complete clearance of the plasmid-cured derivative, IFN-gamma mRNA expression in BLN CD4+ T lymphocytes was significantly (P < 0.001) higher in foals infected with the plasmid-cured derivative. These results suggests an immunomodulating role for R. equi virulence plasmid-encoded products in downregulating IFN-gamma mRNA expression by CD4+ T lymphocytes.

FIG. 1

LS mean IL-1β, IL-10, IL-12 p35, IL-12 p40, IFN-γ, and TNF-α mRNA concentrations in the lungs of control foals (solid bars; n = 6) and foals infected intrabronchially with either virulent plasmid-containing R. equi 103⁺ (dotted bars; n = 8) or its plasmid-cured derivative 103⁻ (striped bars; n = 8). Half of the foals in each group (103⁺, 103⁻, and controls) were euthanized at 3 days postinfection, and the remainder were euthanized at 14 days postinfection. mRNA expression was quantitated by RT-competitive PCR. Lowercase letters differing between groups indicate a statistically significant difference in mRNA expression (P ≤ 0.05); capital letters differing between groups indicate a trend toward a statistically significant difference in mRNA expression (P ≤ 0.10). When at least one letter is common between two groups, the difference is not statistically significant.

FIG. 2

LS mean IL-2, IL-4, IL-6, IL-10, IL-12 p35, and IFN-γ mRNA concentrations in BLN CD4⁺ T lymphocytes of control foals (solid bars; n = 6) and foals infected intrabronchially with either virulent plasmid-containing R. equi 103⁺ (dotted bars; n = 8) or its plasmid-cured derivative 103⁻ (striped bars; n = 8). For details, see the legend to Fig. 1.