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

<i>Rhodococcus equi</i> is an intracellular bacterium that causes suppurative pneumonia in foals. T-helper (Th) 1 cells play an important role in the protective response against <i>R. equi</i>. 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 <i>R. equi</i>-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 <i>R. equi</i>, 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 <i>R. equi</i> infection.