IgG Subtype Response against Virulence-Associated Protein A in Foals Naturally Infected with Rhodococcus equi

Abstract

Rhodococcus equi is an intracellular bacterium that causes suppurative pneumonia in foals. T-helper (Th) 1 cells play an important role in the protective response against R. equi. In mice and humans, the directionality of IgG switching reflects the polarization of Th-cell responses, but this has not been fully elucidated in horses. In this 4-year study, we classified R. equi-infected foals into surviving and non-surviving group and investigated differences in IgG subclass response to virulence-associated protein A, the main virulence factor of R. equi, between the groups. IgGa, IgGb, and IgG(T) titers were significantly higher in the non-surviving group compared with the surviving group. The titers of IgGa and IgG(T), IgGb and IgG(T), and IgGa and IgGb, respectively, were positively correlated, and the IgG(T)/IgGb ratio in the non-surviving group was significantly higher than that in the surviving group. The IgG(T) titer tended to increase more than the IgGa and IgGb titers in the non-surviving group compared with the surviving group. Our findings suggest that the IgG(T) bias in IgG subclass responses reflects the immune status, which exacerbates R. equi infection.

Keywords: IgG subtype; Rhodococcus equi; polarization of Th-cell response; prognosis.

Figure 1

Comparison of the Ig, IgGa, IgGb, and IgG(T) titers between the non-surviving and surviving groups. Values are expressed as ELISA units. Asterisks indicate statistically significant differences (p < 0.05). NS, not significant.

Figure 2

Comparison of ratios of IgG(T)/IgGa, IgG(T)/IgGb, and IgGb/IgGa in the non-surviving and surviving groups. Asterisks indicate statistically significant differences (p < 0.05). NS, not significant.

Figure 3

Correlation between serum levels of IgG subtypes in the non-surviving and surviving groups. The regression line (blue dots) and multiple correlation coefficient (R) are indicated. Results of tests of no correlation are expressed as p-values. Red dots indicate the regression line of the non-surviving group for each IgG subtype.