The human embryonic kidney (HEK293) cell line, commonly used for recombinant adenovirus (Ad) propagation, does not express the Ad coreceptor ␣v␤3 or ␣v␤5 integrins, yet these cells are efficiently infected by Ad vectors. Here we demonstrate that Ad binds to HEK293 cells via the fiber receptor CAR and is subsequently internalized via interaction with integrin ␣v␤1. Function-blocking antibodies directed against ␣v or ␤1, but not ␤3, ␤5, or ␣5, integrin subunits block Ad infection and viral endocytosis. Therefore, ␣v␤1 serves as a coreceptor for Ad infection, and the lack of ␤3 and/or ␤5 but the relatively high expression of ␣v␤1 integrins on certain tumor cell types may explain why these cells are readily transduced by Ad vectors.Adenovirus (Ad) host cell entry requires initial attachment to cells mediated by the fiber interaction with its cellular receptor CAR. The subsequent association of penton base with ␣v␤3 or ␣v␤5 integrins promotes Ad entry (1,21,23). Integrins are heterodimeric receptors for extracellular matrix proteins and cell surface counterreceptors. ␣v integrins (␣v␤1, ␣v␤3, ␣v␤5, ␣v␤6, and ␣v␤8) mediate cell adhesion to various matrix proteins, including fibronectin, vitronectin, and fibrinogen, that contain an arginine-glycine-aspartic acid (RGD) sequence (8,19,24). The interaction of the Ad penton base and ␣v integrin triggers the activation of several signaling molecules (14) that promote actin cytoskeletal reorganization and enhance Ad internalization. Although HEK293 cells have been widely used for propagation of recombinant Ad vectors, the integrin repertoire of these cells has not been clearly established. For example, several reports indicate that HEK293 cells do not express ␣v␤3 and ␣v␤5 integrins (10, 18), while another report indicates that they do (6). Furthermore, ␤5 integrin-deficient mouse fibroblast cells support Ad infection, suggesting that other integrins play a role in Ad infection (11). HEK293 cells were also reported to express ␣5␤1 and ␣v␤1 integrins (2), which have been identified as RGD-dependent receptors for both fibronectin and vitronectin (2,15,22). Considering the fact that soluble fibronectin or RGD-containing peptide reduced Ad infection (23) and that an ␣5␤1-activating antibody has been reported to enhance Ad-mediated gene delivery to certain melanoma cells (4), it was of interest to determine whether Ad uses either ␣v␤1 and/or ␣5␤1 integrins as alternative receptors for virus internalization.The role of CAR and ␣v integrins in Ad infection of HEK293 cells. To investigate whether CAR or integrins promote Ad attachment, we preincubated HEK293 cells with an excess of recombinant Ad type 2 (Ad2) fiber protein or with anti-CAR monoclonal antibody (MAb ) (RmcB), with a function-blocking Fab fragment of a penton base MAb (DAV-1) (20), or with function-blocking ␣v␤5 (P1F6), ␣5 (P1D6), or nonfunctionblocking ␣v (LM142) control MAb prior to measuring the specific binding of 125 I-labeled Ad5 particles (Fig. 1A) as previously described (13). Pretreatment of cells with recombi...

The Crk-associated substrate, p130 CAS , has been implicated in the regulation of the actin cytoskeleton following ligation of cell integrins with the extracellular matrix. Integrin-mediated cell adhesion involves p130 CAS association with focal adhesion kinase (p125 FAK ). Internalization/cell entry of type 2 and type 5 adenoviruses (Ad) is also mediated by ␣ v integrins. However, expression of dominant negative forms of p125 FAK does not alter virus entry, and Ad entry occurs normally in p125 FAK -deficient fibroblasts. We now provide evidence that Ad internalization, a process which is mediated by ␣ v integrins, also requires p130 CAS and phosphatidylinositol-3-OH kinase (PI 3-kinase). Ad induces p130 CAS phosphorylation and inhibition of p130 CAS phosphorylation by tyrphostin and genistein, or expression of the substrate domain deleted p130 CAS blocks Ad internalization. p130 CAS was also found to associate with the p85 subunit of PI 3-kinase through its proline-rich domain during virus internalization and expression of p130 CAS containing a deleted proline-rich domain (PRD) inhibited adenovirus cell entry. We showed further that the RPLPSPP motif in the proline-rich region of p130 CAS interacts with the SH3 domain of p85/PI 3-kinase. These studies reveal the molecular basis by which p130 CAS coordinates the signaling pathways involved in integrin-mediated Ad endocytosis.The identification of cellular receptors that mediate entry of different human pathogens (Salmonella, Listeria, human immunodeficiency virus, herpes simplex virus type-1, adenovirus) has provided an opportunity to investigate the precise biochemical events involved in their pathogenesis (1, 2). Adenovirus has proved to be a useful tool for uncovering the signaling events involved in integrin-mediated viral endocytosis. Adenoviruses are associated with mild/acute infections of the eye, upper respiratory and gastrointestinal tracts (3). Adenovirus types 2/5 entry into host cells occurs after initial binding via the adenovirus fiber protein to a 46-kDa cell receptor (4, 5). Subsequent interaction of the Ad2/5 penton base with ␣ v integrins mediates virus internalization (6) via the clathrin-coated pit pathway (7,8). Adenovirus internalization also requires PI 3-kinase 1 activation (9) and the Rho family of small GTPases (10). These signaling molecules promote reorganization of the actin cytoskeleton, a process required for efficient virus entry.Integrins are a family of heterodimeric membrane glycoproteins that mediate the formation of focal adhesion complexes via interaction with extracellular matrix proteins. This process also involves signaling events including the rapid phosphorylation of focal adhesion kinase (p125 FAK ) and colocalization with actin-associated proteins such as vinculin, ␣-actinin, paxillin, tensin, and others, which help bridge integrins and actin (11). A number of signaling molecules, including c-Src, COOH-terminal Src kinase, p130 CAS , phospholipase C-␥, PI 3-kinase, PKC, and the Ras/Rho family of small GTP-binding pro...

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SummaryA series of human CD8 transgenic (hCD8 Tg) mice with differential expression in the thymus and periphery were produced to investigate CD8 coreceptor regulation of repertoire selection and T cell responses. Expression of hCD8 markedly enhanced responses to both HLA class I molecules and hybrid A2/K b molecules providing functional evidence for a second interaction site, outside of the ct3 domain, which is essential for optimal coreceptor function. Peripheral T cell expression ofhCD8 was sufficient to augment responsiveness to HLA class I, as hCD8 Tg mice which lacked thymic expression responded as well as mice expressing hCD8 in the thymus and periphery. Both murine CD8 + and CD4 + T cells expressing hCD8 transgenes exhibited markedly enhanced responses to foreign HLA class I, revealing the ability of T cell receptor repertoires selected on either murine class I or class II to recognize human class I major histocompatibility complex (MHC). In contrast to recognition of foreign class I, thymic expression of hCD8 transgenes was absolutely required to enhance recognition of antigenic peptide restricted by self-HLA class I. Thus, our studies revealed disparate requirements for CD8 coreceptor expression in the thymus for selection of a T cell repertoire responsive to foreign MHC and to antigenic peptides bound to self-MHC, providing a novel demonstration of positive selection that is dependent on human CD8.T cells have evolved to recognize foreign antigens displayed on the cell surface in association with class I or class II MHC proteins. The T cell receptor ligand being recognized is a binary MHC-peptide complex. However, TCR binding to MHC peptide is generally not sufficient for activation of T cells. Functional recognition usually requires concurrent binding of class I or class II MHC-peptide complexes by TCR and CD8 or CD4 coreceptors, respectively (1-4). Whereas the molecular basis for TCR binding to the class I MHC-peptide complexes involves specific interactions with peptide bound by the 0el, ct2 domains of MHC molecules (5, 6), CD8 coreceptors interact with a nonpolymorphic region within the % domain of class I MHC proteins (7,8 class I MHC proteins generally express CD8 coreceptors, whereas T cells that express CD4 coreceptors generally recognize antigenic peptide presented by class II MHC proteins (reviewed in [91).The interaction of TCR and coreceptors with MHCpeptide ligands is essential for the initiation ofthymic selection, alloreactivity, and recognition of antigenic peptide presented by self-MHC-restricting elements. Furthermore, development of immune cells capable of mediating these functional responses requires expression of both TCR (10, 11) and coreceptors (12, 13). However, it has been difficult to assess the relative contribution of the TCI~ and coreceptors in directing the specificity of these functional responses. We generated human CD8 transgenic (hCD8 Tg) 1 mice to determine the specific affects of CD8 coreceptor binding to MHC class I on positive selection and peripheral T cell respon...

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