Siglec-9 defines and restrains a natural killer subpopulation highly cytotoxic to HIV-infected cells

PLoS Pathog. 2021 Nov 11;17(11):e1010034. doi: 10.1371/journal.ppat.1010034. eCollection 2021 Nov.


Siglec-9 is an MHC-independent inhibitory receptor expressed on a subset of natural killer (NK) cells. Siglec-9 restrains NK cytotoxicity by binding to sialoglycans (sialic acid-containing glycans) on target cells. Despite the importance of Siglec-9 interactions in tumor immune evasion, their role as an immune evasion mechanism during HIV infection has not been investigated. Using in vivo phenotypic analyses, we found that Siglec-9+ CD56dim NK cells, during HIV infection, exhibit an activated phenotype with higher expression of activating receptors and markers (NKp30, CD38, CD16, DNAM-1, perforin) and lower expression of the inhibitory receptor NKG2A, compared to Siglec-9- CD56dim NK cells. We also found that levels of Siglec-9+ CD56dim NK cells inversely correlate with viral load during viremic infection and CD4+ T cell-associated HIV DNA during suppressed infection. Using in vitro cytotoxicity assays, we confirmed that Siglec-9+ NK cells exhibit higher cytotoxicity towards HIV-infected cells compared to Siglec-9- NK cells. These data are consistent with the notion that Siglec-9+ NK cells are highly cytotoxic against HIV-infected cells. However, blocking Siglec-9 enhanced NK cells' ability to lyse HIV-infected cells, consistent with the known inhibitory function of the Siglec-9 molecule. Together, these data support a model in which the Siglec-9+ CD56dim NK subpopulation is highly cytotoxic against HIV-infected cells even whilst being restrained by the inhibitory effects of Siglec-9. To harness the cytotoxic capacity of the Siglec-9+ NK subpopulation, which is dampened by Siglec-9, we developed a proof-of-concept approach to selectively disrupt Siglec/sialoglycan interactions between NK and HIV-infected cells. We achieved this goal by conjugating Sialidase to several HIV broadly neutralizing antibodies. These conjugates selectively desialylated HIV-infected cells and enhanced NK cells' capacity to kill them. In summary, we identified a novel, glycan-based interaction that may contribute to HIV-infected cells' ability to evade NK immunosurveillance and developed an approach to break this interaction.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Antigens, CD / genetics
  • Antigens, CD / metabolism*
  • CD56 Antigen / immunology*
  • HIV / physiology*
  • HIV Infections / immunology
  • HIV Infections / metabolism
  • HIV Infections / pathology*
  • HIV Infections / virology
  • Humans
  • Killer Cells, Natural / immunology*
  • Killer Cells, Natural / metabolism
  • Sialic Acid Binding Immunoglobulin-like Lectins / genetics
  • Sialic Acid Binding Immunoglobulin-like Lectins / metabolism*
  • Viral Load*
  • Viremia / immunology
  • Viremia / metabolism
  • Viremia / pathology*
  • Viremia / virology


  • Antigens, CD
  • CD56 Antigen
  • SIGLEC9 protein, human
  • Sialic Acid Binding Immunoglobulin-like Lectins