Age-dependent changes in peripheral blood lymphocyte subpopulations in cattle: a longitudinal study.

Immunofluorescence and flow cytometric analyses were used to study the age-dependent changes in the peripheral blood lymphocyte (PBL) subpopulations in cattle. Four healthy Holstein heifer calves (A, B, C and D), 1-2 months of age, were used in this study. Sequential peripheral blood samples were collected once a month for up to 2-2.5 years, and once at approximately 4 years of age. For the first 6 months of age, the calves had similar proportions of CD2+, CD4+, CD8+ T lymphocytes, CD20+ B lymphocytes and MHC class II+ lymphocytes. From 2 months of age up to 2-2.5 years of age, all animals had similar proportions of CD5+ cells; but during the same period, animals A and B had significantly lower proportions of WC1+ gamma delta T cells than animals C and D. After 7 months of age, however, the proportions of CD2+, CD4+ and CD8+ T cells in PBL of animals A and B significantly decreased, whereas the proportions of both CD20+ B lymphocytes and MHC class II+ lymphocytes significantly increased. In contrast, the proportions of the various PBL subpopulations in animals C and D remained virtually unchanged after 7 months of age. For the first 6 months of age, all the calves showed similar absolute counts of PBL. Thereafter, the absolute counts of PBL in animals A and B significantly increased, but remained virtually unchanged in animals C and D. Throughout the study, from 1-2 months up to 2-2.5 years of age the absolute counts of CD2+, CD4+, CD8+ and WC1+ gamma delta T cells in PBL of the four animals were not significantly different from each other. Up to 6 months of age, the CD4+/ CD8+ ratio in all calves was 2.38 +/- 0.46, but significantly decreased thereafter to 1.81 +/- 0.34. However, there were no significant differences in the CD4+/CD8+ ratios among individual animals. The increase in the absolute counts of PBL in animals A and B, after 7 months of age, was due to an increase in the absolute counts of CD5+ cells, CD20+ B lymphocytes and MHC class II+ lymphocytes. Thus, changes in the percent, but not the absolute counts of T lymphocytes, were due to high percent and absolute counts of B lymphocytes, expressing the CD5 and MHC class II antigens.