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Platelets can engulf human immunodeficiency virus type 1 (HIV-1), and a significant amount of HIV-1 in the blood of infected individuals is associated with these cells. However, it is unclear how platelets capture HIV-1 and whether platelet-associated virus remains infectious. DC-SIGN and other lectins contribute to capture of HIV-1 by dendritic cells (DCs) and facilitate HIV-1 spread in DC/T-cell cocultures. Here, we show that platelets express both the C-type lectin-like receptor 2 (CLEC-2) and low levels of DC-SIGN. CLEC-2 bound to HIV-1, irrespective of the presence of the viral envelope protein, and facilitated HIV-1 capture by platelets. However, a substantial fraction of the HIV-1 binding activity of platelets was dependent on DC-SIGN. A combination of DC-SIGN and CLEC-2 inhibitors strongly reduced HIV-1 association with platelets, indicating that these lectins are required for efficient HIV-1 binding to platelets. Captured HIV-1 was maintained in an infectious state over several days, suggesting that HIV-1 can escape degradation by platelets and might use these cells to promote its spread. Our results identify CLEC-2 as a novel HIV-1 attachment factor and provide evidence that platelets capture and transfer infectious HIV-1 via DC-SIGN and CLEC-2, thereby possibly facilitating HIV-1 dissemination in infected patients.

The severe acute respiratory syndrome-associated coronavirus (SARS-CoV) causes severe pneumonia with a fatal outcome in approximately 10% of patients. SARS-CoV is not closely related to other coronaviruses but shares a similar genome organization. Entry of coronaviruses into target cells is mediated by the viral S protein. We functionally analyzed SARS-CoV S using pseudotyped lentiviral particles (pseudotypes). The SARS-CoV S protein was found to be expressed at the cell surface upon transient transfection. Coexpression of SARS-CoV S with human immunodeficiency virus-based reporter constructs yielded viruses that were infectious for a range of cell lines. Most notably, viral pseudotypes harboring SARS-CoV S infected hepatoma cell lines but not T- and B-cell lines. Infection of the hepatoma cell line Huh-7 was also observed with replication-competent SARS-CoV, indicating that hepatocytes might be targeted by SARS-CoV in vivo. Inhibition of vacuolar acidification impaired infection by SARS-CoV S-bearing pseudotypes, indicating that S-mediated entry requires low pH. Finally, infection by SARS-CoV S pseudotypes but not by vesicular stomatitis virus G pseudotypes was efficiently inhibited by a rabbit serum raised against SARS-CoV particles and by sera from SARS patients, demonstrating that SARS-CoV S is a target for neutralizing antibodies and that such antibodies are generated in SARS-CoV-infected patients. Our results show that viral pseudotyping can be employed for the analysis of SARS-CoV S function. Moreover, we provide evidence that SARS-CoV infection might not be limited to lung tissue and can be inhibited by the humoral immune response in infected patients.

The C-type lectins DC-SIGN and DC-SIGNR (collectively referred to as DC-SIGN/R) bind to the ebolavirus glycoprotein (EBOV-GP) and augment viral infectivity. DC-SIGN/R strongly enhance infection driven by the GP of EBOV subspecies. Zaire (ZEBOV) but have a much less pronounced effect on infection mediated by the GP of EBOV subspecies. Sudan (SEBOV). For this study, we analyzed the determinants of the differential DC-SIGN/R interactions with ZEBOV- and SEBOV-GP. The efficiency of DC-SIGN engagement by ZEBOV-GP was dependent on the rate of GP incorporation into lentiviral particles, while appreciable virion incorporation of SEBOV-GP did not allow robust DC-SIGN/R usage. Forced incorporation of high-mannose carbohydrates into SEBOV-GP augmented the engagement of DC-SIGN/R to the levels observed with ZEBOV-GP, indicating that appropriate glycosylation of SEBOV-GP is sufficient for efficient DC-SIGN/R usage. However, neither signals for N-linked glycosylation unique to SEBOV- or ZEBOV-GP nor the highly variable and heavily glycosylated mucin-like domain modulated the interaction with DC-SIGN/R. In contrast, analysis of chimeric GPs identified the signal peptide as a determinant of DC-SIGN/R engagement. Thus, ZEBOV- but not SEBOV-GP was shown to harbor high-mannose carbohydrates, and GP modification with these glycans was controlled by the signal peptide. These results suggest that the signal peptide governs EBOV-GP interactions with DC-SIGN/R by modulating the incorporation of high-mannose carbohydrates into EBOV-GP. In summary, we identified the level of GP incorporation into virions and signal peptide-controlled glycosylation of GP as determinants of attachment factor engagement.