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CD48 is a member of the immunoglobulin superfamily whose cell surface expression is strikingly up-regulated on the surface of Epstein-Barr virus-infected B cells. To date, no ligand for human CD48 has been characterized. In this study, we show that human recombinant CD48 binds to the glycosaminoglycan heparan sulfate on the surface of human epithelial cells. We have produced a monoclonal antibody (615) against epithelial cell surfaces that blocks this binding and show that it too recognizes heparan sulfate. The specific epitope on heparan sulfate that is recognized by the antibody and is involved in binding is also expressed in vivo on the basolateral surfaces of mucosal epithelium and lamina propria.CD48 is a cell surface molecule that is expressed by B lymphocytes and other hematopoietic cells. Its expression is upregulated in response to activation signals (1). CD48 is a 40 -45-kDa glycoprotein member of the immunoglobulin superfamily, which is composed of a single polypeptide chain that has a high degree of homology to CD58 (LFA-3) and to a lesser extent CD2, the CD58 ligand. Despite intensive efforts, no ligand for human CD48 has been found, although it is now known to replace CD58 as the ligand for CD2 in rodents, which lack CD58 (2-5). It has been reported that CD48 is an alternate ligand for CD2 in humans (6, 7); however, this was not confirmed by surface plasmon resonance studies, which failed to detect binding of human CD48 to CD2 (K Ͻ 0.5 mM). We have recently presented evidence that there is a ligand for human CD48 on epithelial cells (8). In this study, we show that recombinant CD48 binds to the glycosaminoglycan (GAG) 1 heparan sulfate, which is expressed on epithelial cells in vitro and in vivo.CD48 expression is up-regulated when B cells are driven by Epstein-Barr virus (EBV) infection to become activated, proliferating lymphoblasts (9, 10), and an EBV-responsive element has recently been mapped within the upstream region of the CD48 gene (10). In comparison, we have shown that EBVinfected cells in the peripheral blood are all resting cells (11). We hypothesize that EBV specifically up-regulates CD48 expression on lymphoblastoid B cells in order to cause them to be preferentially retained in the mucosal epithelium through the interaction of CD48 with heparan sulfate, thus explaining the absence of infected lymphoblasts from the peripheral blood. MATERIALS AND METHODSCell Lines and Culture-ER and JY are EBV-immortalized B lymphoblastoid cell lines and were derived in this laboratory. Jurkat, Molt4, Hl-60, and HeLa were obtained from the ATCC. tsA201 was a kind gift from Dr. Brian Seed. Wild type CHO K1 cells and the mutant derivatives pgsA-745, pgsB-650, pgsD-677, and pgsE-606 were kindly supplied by Dr. Jeff Esko and have been characterized in detail elsewhere (12). Lymphoid and HeLa S3 cells were grown in RPMI 1640 supplemented with 10% FCS, 2 mM sodium pyruvate, 55 M ␤-mercaptoethanol, and 50 g/ml gentamicin. The remaining epithelial cells were grown in Dulbecco's modified Eagle's/Ham's F-12 me...

Anti-lymphocyte monoclonal antibody HNK-1 (Leu-7) reacts with the cell surfaces of natural killer (NK) lymphocytes and with myelin-associated glycoprotein (MAG). This antibody reacts intensely with normal and neoplastic adrenal medullary cells. A small proportion of normal pancreatic islet cells, anterior pituitary, and gastroenteropancreatic endocrine cells also show Leu-7 immunoreactivity. In adrenal medulla, ultrastructural immunocytochemical studies and immunoblot analyses reveal that Leu-7 reacts with an intracellular protein of MW 75 KD which is localized within the matrices of the chromaffin granules. The MW of this protein differs from those of MAG and chromogranin A. The findings suggest that Leu-7 immunoreactivity might be a new marker for specific subsets of secretory granules.

Amyloid P component (AP/SAP), a glycoprotein, precipitated with purified snail galactans from Helix pomatia and Ariania arbustorum in a dose‐dependent manner. Radiolabelled AP hinds to human peripheral blood mononuclear cells (PBMC), erythrocytes, and cells derived from human non‐T, non‐B acute lymphocytic leukaemia. The AP cell binding is specific in that it is dose‐dependent and can be blocked both by excess cold AP and by Helix promatia galactan. although it cannot be blocked by an equal amount of the monosaccharide galactose. In vitro studies of human PBMC immune responses demonstrated that AP inhibits PBMC proliferation responses to mycobacterial purified protein derivative and to phytohaemagglutinin and the humoral, antibody response to pokeweed mitogen. The AP‐induced suppression of non‐specific antibody production by human PBMC was dependent on the time at which AP was added to the culture. AP was suppressive if added in the first 48 h of the 7‐day culture, and the suppression could not be reversed by washing the cells after the exposure to AP. The mechanism of AP‐induced immunosuppression is still unclear, but human SAP circulates as a pair of pentameric rings, having ten identical subunits that bind to galactose polymers, and our present data suggest that AP affects the immune response through its properties as a lectin.

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