Immunophenotypic analysis of foal bronchoalveolar lavage lymphocytes

The purpose of this study was to define the normal immunophenotype of equine lymphocytes present within the pulmonary air spaces, and to determine if this changes as foals age from one to ten weeks. Six pairs of mares and foals underwent sequential bronchoalveolar lavage (BAL) between 1 and 10 weeks of age. Data were grouped according to foal age (1, 1-3, 3-6, or 6-10 weeks of age) and were compared to adult control values obtained from the mares. BAL cells were harvested and stained with antibodies to the equine homologues of CD5, CD4, CD8, CD44, MHC I, MHC II and to equine IgG. Data, including percent positive staining and mean fluorescence intensity, were acquired on a flow cytometer gated for viable lymphocytes. All foals had significantly fewer CD5+ lymphocytes than mares, with the largest differences in the youngest animals. The percentage of CD4+ lymphocytes increased as the foals aged, approaching adult levels by 3 weeks of age, while the percentage of CD8+ lymphocytes increased more slowly and approached adult levels by 10 weeks of age. The CD4:CD8 ratio changed from 1.26 at one week of age to 0.78 by 10 weeks of age, compared to an adult value of 0.66. Lymphocytes from foals less than 6 weeks of age expressed MHC II and CD44 at lower levels than adults. The lymphocytic populations within the airways of foals are significantly different from adult animals. This may account for the susceptibility of foals to certain respiratory infections during the first few months of life.