Part of the neuraminidase effect is to unmask Siglec receptors on the cell surface thus increasing their binding to viral sialic acids. Siglec-1 transfected CHO cells bound to gp120 without neuraminidase treatment, suggesting that most of Siglec-1 exists in an unmasked state, likely due to its large size with 17 predicted immunoglobulin domains. In contrast, Siglec-9, although a high affinity receptor for sialic acid, displayed significant adhesion only after neuraminidase treatment, suggesting most of the receptors are masked on cell surface. Consistent with the binding results, HIV-1 R5-pseudovirus KU-0059436 molecular weight infections of MDM were preferentially inhibited by sialo- but not their asialo-compounds and by sodium periodate treatment of the viruses. However, the effects of the sialyllated compounds in X4 infections were less conclusive as X4-viruses infect macrophages at a very low level due to the preferential usage of CXCR4. The results of Siglec blocking antibodies on R5-pseudovirus infection further defined the contribution of individual Siglec receptors in HIV-1 infection of macrophages. The preferential involvement of Siglec-1 and, to a lesser extent, Siglec-3 was observed in JRFL pseudovirus infections blocked by Siglec antibodies, as well as sialyllactose-inhibited HIV-1BaL infections. These results are also consistent with gp120 binding experiments using Siglec-transfected CHO cells. Thus, the contribution of Siglec-1 in HIV-1 infection reflects its high binding affinity to sialic acid and favorable availability for viral attachment. While the exact distributions of viral sialic acids on HIV-1 isolates remain unknown, their predicted N-glycan sites vary significantly both in positions and numbers. In contrast to conserved CD4 binding site on gp120, half of the glycosylation sites in gp120 variable loop regions are not conserved among different strains. The variations in viral envelope sialyloglycan distribution may very well result in variations in Siglec-dependent viral attachment, consistent with the observed variations in sialic acid-dependent infections among JRFL, DH125, AD8 and HW1 strains of HIV-1. In addition, host factors could influence HIV-1 attachment. Interestingly, the expression of Siglec-1 is up-regulated by inflammation and infection, including HIV-1 infection. This raises the possibility that increased Siglec-1 expression on macrophages at inflammatory sites may further enhance HIV replication and thus contribute to viral dissemination. This is also consistent with a recent finding of an association between immune HhAntag691 activation and progressive SIV infection in pigtail macaques. Thus, viral envelope glycan heterogeneities together with host Siglec expressions could lead to a wide range of sialic acid-dependent host susceptibilities to HIV infection. In conclusion, we suggest that HIV-1 viruses utilize their sialoglycans on the envelope protein to facilitate viral attachment to macrophages through binding to Siglec receptors, in particular Siglec-1, and thus enhancing their binding to CD4 for productive entry.