Identity of the Epstein-Barr virus (EBV) receptor with the complement receptor type 2 (CR2) was established in three sets of experiments using the monoclonal antibodies, HB-5 and anti-B2, which recognize a Mr 145,000 Blymphocyte membrane protein that is CR2. First, the rank order for binding of fluoresceinated EBV to four lymphoblastoid cell lines (SB, JY, Raji, and Molt-4) was identical to the rank order for binding of HB-5 and anti-B2 by analytical flow cytometry. Second, pretreatment of cells with HB-5 followed by treatment with goat F(ab')2 fragments to mouse IgG blocked binding of fluoresceinated EBV on SB, a B-lymphoblastoid cell line. Virus attachment was not inhibited by alone, second antibody alone, rabbit anti-C3b receptor, or UPC10 (an irrelevant monoclonal antibody). Third, transfer of CR2 from SB to protein A-bearing Staphylococcus aureus particles, to which HB-5 had been absorbed, conferred on them the specific ability to bind 1251-labeled EBV. We conclude that CR2 is the EBV receptor of human B lymphocytes.pressed on phagocytes and large granular lymphocytes having natural killer and antibody-dependent cytotoxic activities, but it is not expressed on B lymphocytes (18-21). It consists of two polypeptide chains of Mr 155,000-185,000 and Mr 95,000-105,000 (20,22,23). The C3d receptor or CR2 binds the C3d region of C3d,g, iC3b and, with less affinity, C3b. It is found on mature B lymphocytes and on certain Bcell lines (24-29). It has been characterized as a Mr 140,000-145,000 membrane protein (26,27) that is recognized by the monoclonal antibodies (mAbs) termed anti-B2 (28) and , respectively.In the present study, these mAbs have been used to show that the EBVR and CR2 are quantitatively coexpressed on four cell lines, that binding of antibody to CR2 can prevent attachment of EBV, and that CR2 that has been immunoabsorbed onto particles of Staphylococcus aureus Cowan I strain (SACI) specifically binds EBV.

Kaposi sarcoma is considered a neoplasm of lymphatic endothelium infected with Kaposi sarcoma-associated herpesvirus. It is characterized by the expression of lymphatic lineage-specific genes by Kaposi sarcoma tumor cells. Here we show that infection of differentiated blood vascular endothelial cells with Kaposi sarcoma-associated herpesvirus leads to their lymphatic reprogramming; induction of ∼70% of the main lymphatic lineage-specific genes, including PROX1, a master regulator of lymphatic development; and downregulation of blood vascular genes.Kaposi sarcoma is the most frequently occurring malignant tumor in individuals infected with the human immunodeficiency (HIV) virus and also occurs in HIV-negative immunosuppressed individuals. Kaposi sarcoma mainly affects the skin and forms lesions of various types, including early inflammatory and patch stage lesions, and tumors with a predominant population of spindle cells. Infection with Kaposi sarcoma-associated herpesvirus (KSHV, also known as human herpesvirus-8) is essential for the formation of Kaposi sarcoma tumors 1,2 . Both latent viral genes, such as latency-associated nuclear antigen (LANA), and lytic viral genes, such as viral G-protein-coupled receptor, have been implicated in KSHV-mediated tumorigenesis 3 . In Kaposi sarcoma lesions, cells infected with KSHV characteristically appear spindle-shaped and are associated with slit-like spaces that sometimes contain red blood cells. Kaposi sarcoma is considered to be a neoplasm of KSHV-infected lymphatic endothelium, due to the morphological characteristics of the tumor cells and the expression of several lymphatic lineage-specific proteins, including VEGFR-3 and podoplanin [4][5][6][7] .The homeobox gene PROX1 is a master gene that controls lymphatic vessel development and differentiation 8 , and ectopic expression of PROX1 in differentiated blood vascular endothelial cells leads to lymphatic endothelial reprogramming of these cells 9,10 . Because KSHV can infect blood vascular endothelium, such as human umbilical vein endothelial cells, in vitro, we wondered whether KSHV infection might result in lymphatic reprogramming of blood vascular endothelium, potentially involving upregulation of PROX1.We first characterized the lineage-specific gene expression of cultured human lymphatic endothelial cells (LECs) versus blood vascular endothelial cells (BECs) 11 by Affymetrix HU133A gene arrays. We found that LECs, but not BECs, expressed PROX1, XLKD1 (encoding the hyaluronan receptor LYVE-1) and a number of other lymphatic lineage-specific genes ( Table 1). We then infected human dermal microvascular endothelial cells (HDMECs) with KSHV and carried out two independent transcriptional profiling studies 7 d later. Efficient KSHV infection was confirmed by high levels of expression of LANA mRNA in infected HDMECs. We found that 3-7% of infected HDMECs were ORF59-positive and 1.5-3% were K8.1 positive, in agreement with previous results [12][13][14] . This indicates that <10% of the cells were undergoing lytic reac...

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SummaryFollowing occupancy of the T cell receptor by antigen, T cell proliferation and lymphokine production are determined by a second costimulatory signal delivered by a ligand expressed on antigen presenting cells. The human B cell activation antigen B7, which is expressed on antigen presenting cells including activated B cells and 'Y interferon treated monorytes, has been shown to deliver such a costimulatory signal upon attachment to its ligand on T cells, CD28. We have cloned and sequenced the murine homologue of the human B7 gene . The predicted murine protein has 44% amino acid identity with human B7 . The greatest similarity is in the IgV and Ig-C like domains . Murine B7 mRNA was detected in murine hematopoietic cells of B cell but not T cell origin . Cells transfected with murine B7 provided a costimulatory signal to human CD28+ T lymphocytes. These results demonstrate the costimulatory activity of murine B7 and provide evidence that the ligand attachment site is conserved between the two species .Although occupancy of the TCR complex by antigen in 13, association with the MHC is necessary for the initiation of T cell activation, several lines of evidence suggest that a second costimulatory signal is essential for the induction of proliferation and lymphokine secretion (1-4) . In murine and human systems, this costimulatory signal is delivered by APC and requires cell to cell contact (2, 4) . Cells which can deliver this costimulatory signal include activated, but not resting B lymphocytes (5), INF-y activated monorytes, and dendritic cells (2, 6) .Several recent studies in human systems have provided compelling evidence that the B cell activation antigen B7 can provide one such costimulatory signal (7-9) . B7, a member of the Ig supergene family, has been shown to be a ligand for another member of this family, the T cell surface protein CD28, (10-13) . CD28 is constitutively expressed on 95% of human CD4+ T cells, 50% of CD8+ T cells, and on thymocytes which coexpress CD4 and CD8 (14-16) . Following suboptimal activation of T cells with anti-CD3 mAb (16), anti-CD2 mAb, or phorbol ester (17), crosslinking of CD28 by anti-CD28 mAb results in enhanced T cell proliferation and greatly augments synthesis of multiple lymphokines (18). B7 is likely to be an important regulator of T cell proliferation and lymphokine production as evidenced by its pattern of expression and functional activity. B7 is not expressed on resting B cells (19) but appears following crosslinking of surface Ig (10, 19) or class II MHC (9) . Moreover, B7 is not expressed on unstimulated monorytes and is specifically induced by INF-y but not other stimuli which activate monocytes (20). Human B7 (hB7) 1 transfected cells or recombinant B7-Ig fusion protein augment proliferation and induce IL2, but not IIT4, synthesis in T cells which have been treated with phorbol ester or anti-CD3 mAb (7-9) .In murine systems, a homologue for CD28 has recently been cloned (21); however, a conserved functional activity has not yet been demonstrated . In ...