CD9-REGULATED ADHESION - ANTI-CD9 MONOCLONAL-ANTIBODY INDUCE PRE-B-CELL ADHESION TO BONE-MARROW FIBROBLASTS THROUGH DE-NOVO RECOGNITION OF FIBRONECTIN

The modulation of adhesive interaction between lymphocyte progenitors and bone marrow stroma may critically determine the maturation and mig ration of B cell progenitors. mAb against CD9 and beta 1 integrins are reported to induce the homotypic adhesion of pre-B cells. We present evidence that the anti-CD9 mAb 50H.19 and ALB6 but not the proaggregat ory anti-VLA-4 mAb 44H6 also enhance the Fc-independent heterotypic ad hesion of the human pre-B cell lines NALM-6 and HOON to bone-marrow st romal fibroblasts (BM-FB) but not to bone marrow stroma. CD9-enhanced binding of NALM-6 cells to BM-FB was inhibited 58% by the anti-VLA-4 m Ab HP2/1, 36% by the anti-VLA-5 mAb BIIG2, and 99% by their combinatio n. The mAb effectively inhibited adhesion when prebound to NALM-6 cell s but not when prebound to BM-FB. The anti-VCAM-1 mAb E1/6 inhibited C DS-enhanced adhesion by only 14% suggesting the involvement of other l igands. Adhesion was inhibited by mAbs against the COOH-terminus and c entral cell binding domains of fibronectin, as well as by the correspo nding CS1 and RGD peptides. Adhesion was not affected by H-7 and sphin gosine, inhibitors of protein kinase C. These results suggest that per turbation of CD9 on pre-B cells promotes recognition of stromal cell f ibronectin by VLA-4 and VLA-5 and implicates CD9 as a novel regulator of inside-out signaling relevant to B lymphopoiesis.