Experimental infection of neonatal foals with Rhodococcus equi triggers adult-like gamma interferon induction

Abstract

Rhodococcus equi is a facultative intracellular pathogen that causes pneumonia in young foals but does not induce disease in immunocompetent adult horses. Clearance of R. equi depends mainly on gamma interferon (IFN-gamma) production by T lymphocytes, whereas the predominance of interleukin 4 (IL-4) is detrimental. Young foals, like neonates of many other species, are generally deficient in the ability to produce IFN-gamma. The objective of this study was to compare the cytokine profiles, as well as cell-mediated and antibody responses, of young foals to those of adult horses following intrabronchial challenge with R. equi. The lymphoproliferative responses of bronchial lymph node (BLN) cells to concanavalin A were significantly higher in foals than in adult horses. In contrast, adult horses had significantly higher lymphoproliferative responses to R. equi antigens than did foals. Infected foals had significantly lower IL-4 mRNA expression but significantly higher IFN-gamma expression and IFN-gamma/IL-4 ratio in R. equi-stimulated BLN lymphocytes than did infected adults. Infection with R. equi in foals resulted in a significant increase in the percentage of T lymphocytes and CD4(+) T lymphocytes in bronchoalveolar lavage fluid in association with a significant decrease in the percentage of these cell populations in BLNs. Infection of foals also resulted in a marked increase in serum immunoglobulin Ga (IgGa) and IgGb levels, resulting in concentrations in serum that were significantly higher than those of adult horses. This study demonstrates that the immune response to R. equi in foals is not biased toward IL-4 and is characterized by the predominant induction of IFN-gamma.

FIG. 1.

Proliferative responses of BLN cells from five foals and five adult horses at 15 days after challenge with virulent R. equi. Five foals and five adult horses were used as uninfected controls. BLN cells were stimulated either with ConA (A) or with soluble R. equi antigens (B). The change in fluorescence was calculated as the mean fluorescence of the stimulated cells minus that of the same cells cultured without mitogen or antigen. The results are displayed as mean ± SD. Different letters between experimental groups (a, b) indicate a statistically significant difference (P < 0.05).

FIG. 2.

Relative IL-4 (A) and IFN-γ (B) mRNA expression, as well as the IFN-γ/IL-4 ratio (C), following stimulation of BLN cells with soluble R. equi antigens. BLN cells were collected from five foals and five adult horses at 15 days after challenge with virulent R. equi. Five foals and five adult horses were used as uninfected controls. The results are displayed as mean ± SD. Different letters between experimental groups (a, b, c) indicate a statistically significant difference (P < 0.05).

FIG. 3.

Lymphocyte subpopulations in BAL fluid (A) and BLNs (B) from five foals and five adult horses at 15 days after challenge with virulent R. equi. Five foals and five adult horses were used as uninfected controls. The subpopulations are displayed as a percentage of the total number of gated cells (mean ± SD). Different letters between experimental groups (a, b, c) indicate a statistically significant difference (P < 0.05). The difference is not significant when the bars have at least one letter in common.

FIG. 4.

Relative serum IgM and IgG subisotype concentrations in five foals and five adult horses before (solid bars) and 15 days after (dotted bars) challenge with virulent R. equi. Five foals and five adult horses were used as uninfected controls. The results are displayed as mean ± SD. Different letters between experimental groups (a, b, c) indicate a statistically significant difference in preinfection Ig concentrations. Different numbers between experimental groups (1, 2) indicate a statistically significant difference in postinfection Ig concentrations. *, significant difference between pre- and postinfection Ig concentrations for a given group (P < 0.05).