Our studies suggest that the vascular endothelium is activated in patients with sickle cell anemia, regardless of the patients' clinical status. Adhesion proteins on activated endothelial cells may have a role in the vascular pathology of sickle cell disease.

have identified a new fibronectin receptor that is identical to the integrin receptor a4B1. mAbs P3E3, P4C2, and P4G9 recognized epitopes on the tx4 subunit and completely inhibited the adhesion of peripheral blood and cultured T lymphocytes to a 38-kD tryptic fragment of plasma fibronectin containing the carboxy-terminal Heparin II domain and part of the type III connecting segment (IIICS). The ligand in IrlCS for ot4B1 was the CS-1 region previously defined as an adhesion site for melanoma cells. The functionally defined mAbs to or4 partially inhibited T lymphocyte adhesion to intact plasma fibronectin and had no effect on their attachment to an 80-kD tryptic fragment containing the RGD (arg-glyasp) adhesion sequence, mAbs (P1D6 and P1F8) to the previously described fibronectin receptor, ot5B1, completely inhibited T lymphocyte adhesion to the 80-kD fragment but had no effect on their attachment to the 38-kD fragment or to CS-1. Both ot4/~l and ot5/31 localized to focal adhesions when fibroblasts that express these receptors were grown on fibronectin-coated surfaces. These findings demonstrated a specific interaction of both receptors with fibronectin at focal contacts.In conclusion, these findings show clearly that cultured T lymphocytes use two independent receptors during attachment to fibronectin and that (a) ~x5~l is the receptor for the RGD containing cell adhesion domain, and (b) ot4~l is the receptor for a carboxyterminal cell adhesion region containing the Heparin II and IIICS domains. Furthermore, these data also show that T lymphocytes express a clear preference for a region of molecular heterogeneity in IIICS (CS-1) generated by alternative splicing of fibronectin pre-mRNA and that o~4~1 is the receptor for this adhesion site.

Abstract. Using monoclonal antibody technology and affinity chromatography we have identified four distinct classes of cell surface receptors for native collagen on a cultured human fibrosarcoma cell line, HT-1080. Two classes of monoclonal antibodies prepared against HT-1080 cells inhibited adhesion to extracellular matrix components. Class I antibodies inhibited cell adhesion to collagen, fibronectin, and iaminin. These antibodies immunoprecipitated two noncovalently linked proteins (subunits) with molecular masses of 147 and 125 kD, termed a and 13, respectively. Class II antibodies inhibited cell adhesion to native collagen only and not fibronectin or laminin. Class II antibodies immunoprecipitated a single cell surface protein containing two noncovalently linked subunits with molecular masses of 145 and 125 kD, termed a and 13, respectively. The two classes of antibodies did not cross-react with the same cell surface protein and recognized epitopes present on the ct subunits. Pulsechase labeling studies with [35S]methionine indicated that neither class I nor II antigen was a metabolic precursor of the other. Comparison of the a and 13 subunits of the class I and II antigens by peptide mapping indicated that the 13 subunits were identical while the ct subunits were distinct. In affinity chromatography experiments HT-1080 cells were extracted with Triton X-100 or octylglucoside detergents and chromatographed on insoluble fibronectin or native type I or VI collagens. A single membrane protein with the biochemical characteristics of the class I antigen was isolated on fibronectin-Sepharose and could be immunoprecipitated with the class I monoclonal antibody.The class I antigen also specifically bound to type I and VI collagens, consistent with the observation that the class I antibodies inhibit cell adhesion to types VI and I collagen and fibronectin. The class II antigen, however, did not bind to collagen (or fibronectin) even though class II monoclonal antibodies completely inhibited adhesion of HT-1080 cells to types I and III-VI collagen. The class I13 and 1113 subunits were structurally related to the 13 subunit of the fibronectin receptor described by others. However, none of these receptors shared the same ct subunits. Additional membrane glycoprotein(s) with molecular mass ranges of 80-90 and 35-45 kD, termed the class III and IV receptors, respectively, bound to types I and VI collagen but not to fibronectin. Monoclonal antibodies prepared against the class III receptor had no consistent effect on cell attachment or spreading, suggesting that it is not directly involved in adhesion to collagen-coated substrates.These results suggest that the class I and II receptors are two new members of a family of cell surface receptors for the extracellular matrix involved in mediating cell adhesion and shall be referred to as ECMRI and II. Each member of this family possesses a common 13 subunit and a unique a subunit. The class II receptor appears to be a primary mediator of specific cell adhesion to collagen. Th...