Lymphoid tissue- and inflammation-specific endothelial cell differentiation defined by monoclonal antibodies.

Endothelial cells play an essential role in immune responses by regulating the entry of leukocytes into lymphoid tissues and sites of inflammation. As an initial approach to analyzing endothelial cell specialization in relation to such immune function, we have produced monoclonal antibodies (MAB) against mouse lymph node endothelium. Three antibodies were selected: MECA-20, recognizing the endothelium of all blood vessels in lymphoid as well as non-lymphoid organs; MECA-217, which stains the endothelium lining large elastic arteries, but among small vessels is specific for post-capillary venules within lymphoid organs and tissues exposed to exogenous antigen, such as skin and uterus; and MECA-325, an antibody that demonstrates specificity for the specialized high endothelial venules (HEV) that control lymphocyte homing into lymph nodes and Peyer's patches. MECA-325 failed to stain vessels in any non-lymphoid organs tested. Immunoperoxidase studies of HEV in lymph node frozen sections, and of isolated high endothelial cells in suspensions, demonstrated that the antigens recognized by all three antibodies are expressed at the cell surface; those defined by MECA-20 and MECA-325 are also present in the cytoplasm. To study the regulation of the antigens defined by these MAB in relation to extra-lymphoid immune reactions, we assessed their expression in induced s.c. granulomas as a model for chronic inflammation. Small vessels in the granulomas were already stained by MECA-217 in the first days of development. In contrast MECA-325 detected postcapillary venules (which frequently displayed the morphologic characteristics of HEV) only from approximately 1 wk, in parallel with the development of a persistent mononuclear cell infiltrate including numerous lymphocytes. The selective appearance of the MECA-325 antigen on vascular endothelium supporting lymphocyte traffic in both lymphoid and extra-lymphoid sites suggests that this antigen may play an important role in the process of lymphocyte extravasation. The demonstration of lymphoid organ- and inflammation-specific microvascular antigens offers direct evidence for a complex specialization of endothelium in relation to immune stimuli, and supports the concept that microvascular differentiation may play an important role in local immune responses.