The trajectories of relative concentrations of antirhodococcal immunoglobulin A differ from those of immunoglobulin G subisotypes in bronchoalveolar lavage fluid but not serum of foals

Objective: To characterize age-related trajectories of the relative concentrations of anti-virulence-associated protein A (VapA) immunoglobulin (Ig)-G1, IgG3/5, IgG4/7, and IgA in serum and bronchoalveolar lavage fluid (BALF) as indicators of systemic and intrapulmonary immune responses in foals naturally exposed to Rhodococcus equi. Methods: Blood and BALF samples were collected at ages 4, 14, 28, and 56 days from healthy Quarter Horse foals (n = 19) naturally exposed to R equi. Samples were tested via ELISA for relative concentration of IgG1, IgG3/5, IgG4/7, and IgA recognizing the VapA of R equi. Data were analyzed using linear mixed-effects modeling to compare VapA ELISA results by age for IgG subisotypes and IgA. Results: Serum relative concentrations of VapA-specific IgG1, IgG3/5, IgG4/7, and IgA of 19 foals did not differ significantly during the first 28 days of age but increased nearly 10-fold by age 56 days. Similarly, BALF relative concentrations of VapA-specific IgG1, IgG3/5, and IgG4/7 in these foals did not differ significantly during the first 28 days of age but increased by about 10-fold by age 56 days. In contrast, BALF relative concentrations of VapA-specific IgA increased progressively with age. Conclusions: Natural exposure results in progressive increase in BALF concentration of IgA but not IgGs, indicating intrapulmonary IgA response to virulent R equi. Clinical Relevance: Foals do not mount strong antibody responses to R equi when they are most vulnerable to infection. Further investigation is warranted to determine if increasing intrapulmonary antirhodococcal IgA is a correlate of protective immunity or merely a marker of exposure.