LYMPHOCYTE MIGRATION ACROSS HIGH ENDOTHELIUM IS ASSOCIATED WITH INCREASES IN ALPHA-4-BETA-1 INTEGRIN (VLA-4) AFFINITY

The constitutive recirculation of lymphocytes between the widely distr ibuted organs of the immune system is essential for host defence. We h ave developed an in vitro model of lymphocyte migration from the blood into lymph nodes which employs primary cultures of high endothelial c ells (HEC). HEC-adherent lymphocytes adopt one of two distinct morphol ogies which correlates with their position in the endothelial layer; t ype I cells are bound to the surface of HEC and type II cells are unde rneath the endothelial layer. In a previous study we reported that the numbers of type I and type II cells are independently regulated, howe ver the relationship between these two lymphocyte populations was not determined. In this study we have carried out detailed kinetic, phenot ypic and functional analyses of type I and type II lymphocytes and det ermined their relationship. Using allotype marked lymphocytes from the PVG.RT7a and PVG.RT7b rat strains in a pulse-chase analysis, type I a nd type II lymphocytes were found to represent the same population of lymphocytes at different stages of interaction with the endothelial la yer, rather than representing two independent lymphocyte populations. Migration was an irreversible event and the efficiency of migration (i .e. transition from type I to type II) was related to the concentratio n of lymphocytes plated on to the HEC layer. Following transmigration lymphocytes showed an increased ability to migrate across HEC layers a nd to bind to immobilised CSI peptide. The increased binding to CSI pe ptide was transient and fell to control levels over a 3 hour time peri od. The expression of alpha4 integrin subunit on lymphocytes was uncha nged following migration which suggests that the affinity of the CSI r eceptor, alpha4beta1, is upregulated by interaction with HEC. Together these results suggest that transendothelial migration is regulated by increases in the affinity of alpha4beta1 integrin on lymphocytes foll owing contact with HEC.