Distribution of lymphocyte subpopulations in thymus, spleen, and peripheral blood of specific pathogen free pigs from 1 to 40 weeks of age.

Using flow-microfluorometry analysis and cluster determinant (CD) markers, we studied how lymphocyte subpopulations in lymphoid organs of specific-pathogen-free pigs developed in pigs from birth to young adulthood. Cell suspensions of the thymus and spleen were prepared and peripheral blood cells were collected at 1, 4, 10, and 40 weeks of age. Tissue sections of the thymus and spleen were stained with monoclonal antibodies directed against CD2 and immunoglobulin to localize the CD2-Ig- lymphocyte subpopulation. In the thymus, only limited changes were observed in the lymphocyte subpopulations with time. Most thymocytes expressed CD4 or CD8 or both. Most CD2-Ig- cells or, 'null cells', (5-13%) were observed in the medulla of the thymus and probably represented a recirculating cell type. In the spleen and blood the percentage of CD2+ and Ig+ cells increased significantly with time, the former increasing from about 30-60% owing to an increase of CD8+ cells. Therefore, the selective increase of the CD8+ population also caused the CD4/CD8 ratio to change. Although CD2+ cells in the spleen and blood are positive for CD4 or CD8, but not for both, quantities of CD4+ CD8+ cells were also observed. Half of the lymphocytes in the spleen and blood were typed as null cells at 1 week of age and decreased in proportion to the increase of the CD8+ and Ig+ cells. Nevertheless, quantities of null cells were still present in the spleen blood at 40 weeks of age. Almost all these were located in the red pulp of the spleen. This study indicates an effect of age and housing conditions on the distribution of the lymphocyte subpopulations, and especially on the CD8+ subset. Quantities of CD4+CD8+ cells as well as CD4-CD8- were observed in blood, but also in spleen of pigs. The function of high numbers of null cells directly after birth are discussed.