Lymphocyte adhesion to high endothelium is mediated by two beta 1 integrin receptors for fibronectin, alpha 4 beta 1 and alpha 5 beta 1.

Using a rat model we have previously proposed a role for fibronectin as an adhesive ligand on high endothelial cells (HEC) for recirculating lymphocytes. Lymphocyte adhesion to high endothelial cells was blocked by CS1 peptide (from the type III connecting segment of fibronectin) and RGD-containing peptides using two different in vitro assays of lymphocyte-HEC recognition, the frozen section assay and cultured HEC. In order to study the receptors utilised by lymphocytes to bind to HEC we have developed a xenogeneic model in which the adhesion of human lymphocytes to HEC cultured from rat lymph nodes is measured. The basic properties of lymphocyte-HEC interaction were retained using human lymphocytes. CS1 peptide and RGD-containing peptides gave similar profiles of inhibition of lymphocyte adhesion as found previously using rat cells. FACS analysis showed that the majority of peripheral blood lymphocytes expressed two beta 1 integrin receptors, alpha 4 beta 1 and alpha 5 beta 1, which are known to recognise distinct adhesion domains in fibronectin. A subpopulation of lymphocytes also expressed alpha 3 beta 1, which, like alpha 5 beta 1, has been reported to be an RGD-dependent adhesion receptor for the central cell binding domain of fibronectin. Anti-alpha 4 and anti-alpha 5 subunit monoclonal antibodies maximally inhibited adhesion to HEC by 60% and 65%, respectively. Monoclonal antibodies to the common beta 1 subunit gave slightly higher inhibition at 70%. These results suggest that lymphocytes employ one or both of two different beta 1 integrin fibronectin receptors to bind to HEC. The simultaneous or alternate engagement of two fibronectin receptors on the lymphocyte surface by immobilised fibronectin in the endothelial layer may contribute to the stabilisation of adhesive contacts or to the subsequent transendothelial migration of lymphocytes. In contrast to lymphocytes, peripheral blood neutrophils did not express any members of the beta 1 integrin family. The selective expression of beta 1 integrins by lymphocytes and not neutrophils contrasted with the widespread distribution of the other homing-associated adhesion molecules, LECAM-1, CD44 and LFA-1, on these two cell types. It is thus possible that the selective expression of beta 1 integrins regulates the constitutive migration of lymphocytes but not neutrophils into organised lymphoid tissues.