The impact of HIV-1 subtype C Envelope N-glycosylation on DC-SIGN meditated modulation of DC function to facilitate transmission or enhance viral pathogenesis
N-glycosylation plays an important role in Envelope (Env) function and may be involved in the modulation of the immune response to HIV-1 infection. In this study, we hypothesized that Env N-glycosylation may affect viral pathogenesis by influencing Env structure and function. Furthermore, we also postulated that differences in Env glycosylation could affect interactions between Env and DC-SIGN of dendritic cells (DCs), activating alternative signalling pathways which stimulate the release of different immune modulators. We generated pseudovirus of eighteen Env clones (PSVs) with variable number and position of potential N-glycan sites (PNGs) and compared their ability to infect TZM-bl cells, bind to Raji+ DC-SIGN cells, trans-infect TZM-bl cells when captured by either Raji-DC-SIGN cells or monocyte-derived dendritic cells (MDDCs) and modulate MDDC signaling by investigating the release of Interleukin-10 (IL-10) and other immune modulatory cytokines and MAPK activation. Entry efficiency, DC-SIGN binding and trans-infection varied widely across all clones. The level of IL-10 secreted by MDDCs in response to PSV stimulation varied 32-fold. The induction of IL-10 secretion by purified gp140 confirmed that Env was the viral component that stimulated the secretion of IL-10 via interaction with DC-SIGN and potentially other undefined receptors. PSV and purified gp140 stimulated MDDC signaling via ERK and JNK phosphorylation, while p38 was not activated. The addition of recombinant DC-SIGN lowered the levels of secreted IL-10 and ERK /JNK phosphorylation, suggesting that DC-SIGN plays a role in these responses. As Env mannosylation correlated with DC-SIGN binding, five highly conserved Env PNGs (241, 262, 386, 392, and 448) previously identified to carry high mannose type N-glycans and hence thought to be involved in DC-SIGN binding were deleted in two Env clones by site-directed mutagenesis to confirm their importance in Env function. The potential role of these PNGs in Env entry efficiency, DC-SIGN binding, trans-infection, induction of MDDC IL-10 secretion and activation of MAPK phosphorylation was determined. Deletion of these sites significantly affected the entry efficiency, DC-SIGN binding, trans-infection and MDDC IL-10 secretion, with one Env clone proving to be more sensitive to mutation than the other. This suggests that PNGs influence Env function in a clone-specific manner. As deletion of highly conserved PNGs abrogated Env function we used sequence analysis to identify PNGs involved in binding DC-SIGN and inducing MDDC IL-10 secretion. We grouped PSVs based on the presence or absence of specific PNGs in Env sequences and compared entry efficiency, DC-SIGN binding, trans-infection, stimulation of MDDC IL-10 secretion and induction of MAPK phosphorylation. Three Env PNGs were significantly associated with entry efficiency (N356, N392, and N674), and three sites (N289, N356 and 674) were significantly associated with trans-infection while N674 also influenced DCSIGN binding. The majority of MDDC donors secreted higher levels of IL-10 when stimulated with PSVs that carried PNGS at N130 (p = 0.0016) and N332 (p = 0.0039) and lacked N674 (p = 0.033). When Envs were graded on whether they had 0, 1, 2 or 3 of the PNGs (e.g. -130, -332, +674; -130, +332 and +674, etc.) those that carried either one of the PNGs or the entire induction motif (N130+ N332+ N674-) significantly stimulated MDDCs to secrete higher levels of IL-10 than those that completely lacked the motif (p = 0.0335 and p = 0.0304, respectively). As the presence of N674 was linked to reduction in all functions of Env, it is likely that the presence of an N-glycan at this site affected Env structure and could skew the analysis. Excluding N674 indicated that the presence of PNGs at position 130 and 332 was sufficient to induce significantly higher IL-10 release than those that had either none or one of these sites (p = 0.0053). When we determined whether N130 and N332 were enriched in subtype C acute infection Envs, these sequences were not enriched with PNGs at either N130 or N332 compared to chronic infection viruses. However, when IL-10 levels were compared between MDDC donors stimulated with PSV of either acute or chronic infection clones, those from early infection significantly enhanced MDDC secretion of IL-10 (p = 0.0039). This suggests that even though PNGs at 130 and N332 could be involved in inducing MDDC IL-10 secretion, it is not the only requirement for enhanced stimulation. Although Env differentially activated ERK and JNK phosphorylation, ERK phosphorylation did not correlate with IL-10 secretion, suggesting that this MAPK signaling pathway was not solely responsible for triggering the release of MDDC IL-10 and other regulatory cytokines. PSVs also stimulated the release of TNFα, IL-1β, IL-6, IL-8, MIP-1a, and MIP-1b while having no effect on IL-12 levels. This suggests that HIV-1 binding to DCs in the genital tract could change the dynamics of DC immune responses, deregulating their cytokines secretion and destabilising the Th0 cell differentiation to facilitate viral survival and thus productive clinical infection. We therefore conclude that HIV-1 variants differentially stimulate MDDCs to release immunosuppressive IL-10 and that transmitted founders could be better at modulating immune responses in the genital tract compared to chronic infection variants
