P10.08
Background: Although HIV-1 primarily infects T-cells through the interaction of viral envelope with CD4 and a co-receptor CCR5/CXCR4, CD4 expresses in low level in macrophages. Recently, sialic acid-binding Ig-like lectin 1 (Siglec-1, CD169), which is highly expressed on the cell surface of macrophages, has been identified as a major receptor for HIV-1 infection. It is known that Siglec-1 binds to glycans containing sialic acid. To further understand the mechanism; we determined the binding affinity/kinetics of monomeric gp120 and trimeric gp145 to assess if there were differences in their ability to bind to Siglec-1. We further narrowed down the env-binding motif to the V2 loop of gp120. Finally, we conducted an inhibition assay in the presence/absence of lactose (LA), 2, 6′-Sialyllactose (6′-SL) and sialic acid (SA) to identify the nature of the interactions.
Methods: Siglec-1 was immobilized on a CM5 chip on a Biacore T200. JRFL and SF162 (clade B) gp120, gp145, and V2 loop proteins were captured. In additional experiments, these env proteins were immobilized, while Siglec-1 was injected over that surface. For the inhibition assays, LA, SL and SA were mixed with the env proteins or with Siglec-1 and then injected onto the immobilized Siglec-1 or env proteins, respectively.
Results: The gp120 and gp145 binds to Siglec-1 with high affinity, 4.13 nM (JRFL gp120), 0.28-2.9 nM (JRFL gp145) and 0.44-8.26 nM (SF162 gp145). Importantly, we found that the interaction of env protein to Siglec-1 took place through the V2 loop (4.34 nM JRFL V2; 1.96 nM SF162 V2). The presence of SA completely blocked the interaction of the env and the V2 loop protein with Siglec-1, suggesting thereby that the interaction occurred via a SA motif.
Conclusions: The binding of gp120, gp145, and V2 loop to Siglec-1 is a high affinity interaction. Binding occurs through the V2 loop and requires sialic acid. This suggests that the vaccine design should aim to induce antibodies against V2-sialic acid complexes to prevent HIV-1 infections.
