Blood leucocyte subsets of the rat: expression of adhesion molecules and localization within high endothelial venules.

Although several distinct adhesion pathways are now well characterized, it is not clear whether analysis of adhesion molecule expression on leucocytes is sufficient to predict their interaction with endothelium in vivo. Therefore, in the present study this question was addressed by investigating the interaction between blood leucocyte subsets and high endothelial venules (HEV). The expression of different types of adhesion molecule (CD44, alpha 4-integrins, LFA-1, ICAM-1, CD2 and L-selectin) on lymphocytes, NK cells, monocytes and granulocytes of rat blood was determined by flow cytometry. In the same animals the numbers of blood leucocyte subsets present in the HEV of axillary lymph nodes and Peyer's patches were analysed using immunohistology. In the HEV of both axillary lymph nodes and of Peyer's patches lymphocytes (greater than 10,000 per mm2), as well as small numbers of NK cells and monocytes (less than 500 per mm2), were found. In contrast, granulocytes were not detected here. Lymphocytes, NK cells, monocytes and granulocytes each expressed CD44, alpha 4-integrins, LFA-1, ICAM-1, CD2 and L-selectin in a pattern characteristic to cell type, but this did not correlate with the different ability of the leucocyte subsets to interact with the two types of HEV. In conclusion, determining the expression of CD44, alpha 4-integrins, LFA-1, ICAM-1, CD2 and L-selectin on blood leucocytes alone is not sufficient to predict leucocyte/endothelium interaction in vivo.