ROLE OF BETA-2 INTEGRINS IN THE BINDING OF THYMOCYTES TO RAT THYMIC MACROPHAGES

A role of beta 2 integrins and one of their ligands, ICAM-1, in thymic macrophage (TMF)/thymocyte interactions was studied. TMF were isolate d as adherent cells from 4-day old culture of thymic-cell suspensions either from normal or hydrocortisone-treated rats. Adherent cells were 94-98% positive with ED1 (a pan-macrophage marker). The majority of t hem (75-95%) expressed the CD11b and CD18 molecules, and 60-70% expres sed CD54 (ICAM-1). A low proportion of TMF (10-20%) expressed CD11a (L FA-1). The expression of all these antigens was upregulated by IFN-gam ma and TNF-alpha. The effect of these mAbs on TMF/thymocyte binding wa s studied using a simple rosette assay by incubating unstimulated or I FN-gamma or TNF-alpha stimulated TMF, grown on microscopic slides with resting or ConA+IL-2 activated thymocytes. It was found that LFA-1/CD 18 and ICAM-1 play a significant role in the TMF/thymocyte adhesion. I n addition, a LFA-1-dependent/ICAM-1-independent adhesion pathway was observed, suggesting that LFA-1 might use another ligand. The inhibito ry effect of anti-CD18 mAb (WT-3) was higher than the effect of anti-L FA-1 mAb (WT-1) and was a consequence of blocking the CD18 chain both on thymocytes and TMF. No significant difference in the expression and function of adhesion molecules was found between TMF obtained from no rmal or hydrocortisone-treated rats. The involvement of CD11b in these processes was of lesser importance than the role of the CD11a molecul e. By using mAbs to different epitopes of the CD11b molecule, such as OX-42 (anti-CD11b/CD11c), ED7, and ED8 (anti-CD11b), it was found that they were either slightly or moderately inhibitory under certain expe rimental conditions or did not significantly modulate TMF/thymocyte bi nding. OX-42 was slightly stimulatory in some experiments. Cumulativel y, these results show that beta 2 integrins play a significant role in TMF/thymocyte interactions and probably contribute to T-cell developm ent in vivo.