ICAM-3 INTERACTS WITH LFA-1 AND REGULATES THE LFA-1 ICAM-1 CELL-ADHESION PATHWAY/

The interaction of lymphocyte function-associated antigen-1 (LFA-1) wi th its ligands mediates multiple cell adhesion processes of capital im portance during immune responses. We have obtained three anti-ICAM-3 m Abs which recognize two different epitopes (A and B) on the intercellu lar adhesion molecule-3 (ICAM-3) as demonstrated by sequential immunop recipitation and cross-competitive mAb-binding experiments. Immunoaffi nity purified ICAM-3-coated surfaces were able to support T lymphoblas t attachment upon cell stimulation with both phorbol esters and cross- linked CD3, as well as by mAb engagement of the LFA-1 molecule with th e activating anti-LFA1 NKI-L16 mAb. T cell adhesion to purified ICAM-3 was completely inhibited by cell pretreatment with mAbs to the LFA-1a lpha (CD11a) or the LFA-beta (CD18) integrin chains. Anti-ICAM-3 mAbs specific for epitope A, but not those specific for epitope B, were abl e to trigger T lymphoblast homotypic aggregation. ICAM-3-mediated cell aggregation was dependent on the LFA-1/ICAM-1 pathway as demonstrated by blocking experiments with mAbs specific for the LFA-1 and ICAM-1 m olecules. Furthermore, immunofluorescence studies on ICAM-3-induced ce ll aggregates revealed that both LFA-1 and ICAM-1 were mainly located at intercellular boundaries. ICAM-3 was located at cellular uropods, w hich in small aggregates appeared to be implicated in cell-cell contac ts, whereas in large aggregates it appeared to be excluded from cell-c ell contact areas. Experiments of T cell adhesion to a chimeric ICAM-1 -Fc molecule revealed that the proaggregatory anti-ICAM-3 HP2/19 mAb w as able to increase T lymphoblast attachment to ICAM-1, suggesting tha t T cell aggregation induced by this mAb could be mediated by increasi ng the avidity of LFA-1 for ICAM-1. Moreover, the HP2/19 mAb was costi mulatory with anti-CD3 mAb for T lymphocyte proliferation, indicating that enhancement of T cell activation could be involved in ICAM-3-medi ated adhesive phenomena. Altogether, our results indicate that ICAM-3 has a regulatory role on the LFA-1/ICAM-1 pathway of intercellular adh esion.