INTERACTION OF B-LYMPHOCYTE AND T-LYMPHOCYTE SUBSETS WITH HIGH ENDOTHELIAL VENULES IN THE RAT - BINDING IN-VITRO DOES NOT REFLECT HOMING IN-VIVO

Lymphocytes continuously migrate through the body, and their efficient extravasation from the blood via high endothelial venules (HEV) is es sential for initiating an appropriate immune response. Most investigat ions have focused on the lymphocyte/HEV interaction in vitro. However, to what extent such systems reflect the situation in vivo is not know n. It is also unclear whether lymphocyte subsets immigrate into the HE V in proportion to their presence in the blood, and whether import cap acity is limited by the HEV. When rat mesenteric lymph node lymphocyte s were incubated in vitro on cryostat sections, the well-known prefere ntial binding of B lymphocytes to HEV of Peyer's patches (PP) and T ce lls to HEV of axillary lymph nodes (axLN) was observed (axLN vs. PP: B lymphocytes 21.2 +/- 15.0% vs. 40.6 +/- 11.0%, T lymphocytes 84.6 +/- 6.3% vs. 56.5 +/- 12.9%). However, when labeled mesenteric lymph node lymphocytes were injected and their location within the HEV was analy zed 15 min later, no preferential interaction was seen. After injectio n of labeled thoracic duct lymphocytes, the percentage of labeled cell s among B and T lymphocytes in the blood was significantly different ( 4.4 +/- 0.9% vs. 8.9 +/- 3.6%), whereas that in HEV of axLN (19.0 +/- 6.4% vs. 16.6 +/- 6.0%) and PP (30.6 +/- 6.1% vs. 33.9 +/- 4.4%) was c omparable. Although the number of injected lymphocytes was similar in magnitude to the total blood lymphocyte pool, after injection there wa s no increase in lymphocyte numbers in the HEV. Thus, the adhesion ass ay in vitro does not completely reflect immigration into HEV in vivo. In addition, our data suggest that both the availability of lymphocyte subsets in small venules and the immigration rate into HEV are active ly regulated in vivo.