Identification of pulmonary T-lymphocyte and serum antibody isotype responses associated with protection against Rhodococcus equi.

Rhodococcus equi infects and causes pneumonia in foals between 2 and 4 months of age but does not induce disease in immunocompetent adults, which are immune and remain clinically normal upon challenge. Understanding the protective response against R. equi in adult horses is important in the development of vaccine strategies, since those mechanisms likely reflect the protective phenotype that an effective vaccine would generate in the foal. Twelve adult horses were challenged with virulent R. equi and shown to be protected against clinical disease. Stimulation of cells obtained from bronchoalveolar lavage fluid with either R. equi or the vaccine candidate protein VapA resulted in significant proliferation and a significant increase in the level of gamma interferon (IFN-gamma) expression by day 7 postchallenge. The levels of interleukin-4 expression were also increased at day 7 postchallenge; however, this increase was not antigen specific. Anamnestic increases in the levels of binding to R. equi and VapA of all immunoglobulin G (IgG) antibody isotypes [IgGa, IgGb, IgG(T)] examined were detected postchallenge. The levels of R. equi- and VapA-specific IgGa and IgGb antibodies, the IgG isotypes that preferentially opsonize and fix complement in horses, were dramatically enhanced postchallenge. The antigen-specific proliferation of bronchoalveolar lavage fluid cells, the levels of IFN-gamma expression by these cells, and the anamnestic increases in the levels of opsonizing IgG isotypes are consistent with stimulation of a memory response in immune adult horses and represent correlates for vaccine development in foals.