The broad involvement of viral sialic acids in HIV-1 infection is evident when MDM were infected with sodium periodate-treated pseudoviruses from multiple strains of HIV-1. Disrupting sialic acid with sodium periodate reduced the R5-pseudoviruses infection by 20�C60%. Since periodate-treated gp120 exhibited a reduced binding to MDM, the observed decrease in infection by periodate-treated virus is likely a result of reduced host attachment mediated by viral sialic acid. The involvement of carbohydrate recognition in HIV-1 infection has been controversial largely due to the use of cell lines in early research discoveries. This is also supported by the known antiviral effects of many carbohydrate binding agents, such as bacterial cyanovirin-N and some plant lectins. Consistent with previous publications, our infection experiment with JRFL INCB28060 c-Met inhibitor pseudovirus was potently inhibited by CVN; VSV infection of MDM was not affected. While CVN binding likely blocks viral access to both I-type and C-type lectin receptors on macrophages, we evaluated the potential contribution of C-type lectin receptors on MDM using mannose, mannan, and EDTA. Compared to the sialylated compounds, compounds specific for C-type lectin receptors exhibited less inhibition to JRFL-pseudovirus infection even though EDTA potently inhibited VSV pseudovirus infection. This suggests a unique role for Siglecs recognition of viral sialylated glycans in HIV-1 attachment and entry. To further investigate the contribution of individual Siglec receptors to viral attachment, we carried out pseudovirus infections in the presence of blocking antibodies against the major macrophage-expressed Siglecs, Siglec-1, -3 and -9. Blocking Siglec-1 or -3 reduced JRFL infection of MDM to 20% and 70%, respectively, compared to the PBS control. Blocking Siglec-9, however, did not affect the infection, even at an antibody concentration of 100 mg/ml. These data demonstrate a preferential involvement of Siglec-1 in HIV-1 infection of MDM. This is also consistent with the high affinity binding of Siglec-1 to gp120 relative to other Siglecs. The striking difference between blocking Siglec-1 and -9 is interesting even though both recombinant proteins bind gp120 well in solution. Their preferential usage by HIV-1 may be related to their differential masking by cell surface cis-sialic acid. This is consistent with our gp120 binding experiment, which shows that the binding of gp120 to Oligomycin A ATPase inhibitor Siglec-9 transfected CHO cells requires neuraminidase treatment, unlike Siglec-1. Siglec-1 has 17 extracellular domains and is likely less masked than the three-domain Siglec-9. To further address the potential contribution of masked Siglec receptors on macrophages, we treated MDM with neuraminidase prior to HIV-1 infection. Indeed, treating MDM with neuraminidase significantly increased luciferase virus infection for multiple strains of pseudotyped HIV-1, suggesting a potential contribution by masked Siglecs to the viral infection under certain conditions.