Non-random recirculation of small, CD4+ and CD8+ T lymphocytes in sheep: evidence for lymphocyte subset-specific lymphocyte-endothelial cell recognition

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Abstract
The migratory properties of small, CD4 + and CD8 + T lymphocyte subsets have been examined in sheep under physiological conditions. Lymphocytes obtained free-floating In lymph were directly labeled with fluorochrome In vitro and returned to the blood of the same animal. Over the next 48 h the lymph collection bottles were replaced at various times. The cells in these collections, as well as an aliquot of the cells used for direct fluorescent labeling, were then phenotyped with monoclonal antibodies (mAbs) which define the mutually exclusive CD4 +and CD8 + T lymphocyte subsets in sheep. Binding of mAbs was detected by using secondary reagents labeled with a fluorochrome of a different colour. All cell samples were analyzed by flow cytometry and CD4/CD8 ratios were determined for the reclrculated, fluorochrome-labeled population In each lymph collection. The mean CD4/CD8 ratio was then calculated and compared with the CD4/CD8 ratio of the intravenously infused starting population. In three experiments employing efferent prescapular lymph cells, three experiments employing efferent intestinal lymph cells and two experiments employing afferent Intestinal lymph cells, the mean CD4/CD8 ratio of the recirculated, fluorochrome-labeled population was different from the CD4/CD8 ratio of the starting population, thereby indicating that non-random migration of these subsets had occurred. The finding that the CD4/CD8 ratio of the reclrculated population was higher than the CD4/CD8 ratio of the transfused starting population in every case provides strong experimental support for the hypothesis that small, CD4 + T cells are extracted from the blood by specialized vascular endothelium with greater efficiency than small, CD8 + T cells. The finding that the migratory patterns did not depend on the lymph compartment examined cannot be satisfactorily explained by Invoking previously described tissue-specific, lymphocyte - endothelial cell recognition mechanisms. Therefore, these studies implicate the existence of an additional, lymphocyte subset-specific mechanism of lymphocyte-endothelial cell recognition which, In co-operation with tissue-specific lymphocyte-endothelial cell recognition mechanisms, is involved In regulating the reassortment of immunocompetent cells serving diverse immunological functions.

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