Targeting Siglec-9/Sialoglycan Interactions to Enhance NK Functions During HIV Infection

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PROJECT SUMMARY: The functions of Natural Killer (NK) cells can be influenced by the cell-surface glycosylation of their target cells. A subset of CD56dim NK cells expresses the Sialic acid-binding protein Siglec- 9. This subset has a high cytolytic activity; however, Siglec-9 itself is an inhibitory receptor that restrains the cytolytic ability of this otherwise highly cytotoxic population. Harnessing the cytotoxic capacity of this population has not been evaluated as an approach for eradicating HIV. In our preliminary studies and focusing first on NK cells, we found that levels of Siglec-9+ CD56dim NK cells inversely correlate with CD4+ T cell-associated HIV DNA during antiretroviral therapy (ART)-suppressed HIV infection. Furthermore, Siglec-9+ CD56dim NK cells exhibited higher cytotoxicity towards HIV+ cells compared to Siglec-9- NK cells. These data are consistent with the highly cytotoxic nature of the Siglec-9+ NK cells. However, consistent with the known inhibitory function of the Siglec-9 molecule itself, blocking Siglec-9 enhanced NK cells' ability to kill HIV+ cells in vitro. Focusing next on target cells, we found that HIV latently-infected CD4+ T cells exhibit high levels of the Siglec-9 ligand, α2-3 Sialic acid, compared to HIV productively-infected or uninfected cells. We also developed a novel approach to block Siglec/Sialic acid interactions during HIV infection by conjugating Sialidase (enzyme cleaves Sialic acid) to four HIV broadly neutralizing antibodies (bNAbs). These conjugates (in hand) can be used in conjunction with drugs that reactivate HIV latently-infected cells to achieve a functional HIV cure. We pilot tested one of these conjugates and found it able to selectively desialylate the surface of HIV+ cells and enhance NK capacity to kill these infected cells in vitro. Together, our data support our central hypothesis that Siglec/sialoglycan interactions contribute to the ability of HIV-infected cells to evade NK immune surveillance and that blocking these interactions, via selective desialylation of HIV-infected cells, will enhance the capacity of NK cells to clear HIV-infected cells. In Aim 1: we will test the hypothesis that Siglec-9/Sialic acid interactions contribute to the ability of HIV latently-infected cells to evade NK immune surveillance. In (1a), we will determine the role of Siglec-9 in the ability of NK cells to kill HIV+ cells, and in (1b), we will determine the role of α2-3 Sialic acid in the ability of HIV latently-infected CD4+ T cells to evade killing by NK cells. In Aim 2: we will test the hypothesis that HIV bNAb- Sialidase conjugates reduce the size of the HIV reservoir (2a) in vitro and (2b) ex vivo, and (2c) delay viral rebound in vivo using a modified version of the splenic-injected primary HIV-infected reservoir (SPHIR-IL15) non-fetal humanized mouse model with high NK longevity. We also will confirm the mechanism by which bNAb- Sialidase conjugates enhance NK cell antiviral function by examining the role of Fc-mediated functions and Siglec-binding in NK targeting. Our interdisciplinary approach is taking advantage of recent advances in the emerging field of glyco-immunology to enhance NK cell capacity to kill HIV+ cells in ART-suppressed individuals. Our goal is to provide a novel mechanism and approach that can be harnessed to functionally cure HIV infection.