Background:  Dendritic cells (DC) and natural killer (NK) cell frequencies and function are impaired in HIV infection. This study analyzes the role of plasmacytoid (PDC) and myeloid DC (MDC) in regulating NK cytotoxic activity to virally infected and tumor targets in HIV-1-infected individuals.

Methods:  PBMC from ART-suppressed (< 50 RNA copies/mL) and viremic (> 100,000 copies/mL) HIV-1-infected individuals and uninfected controls were used as sources of DC and NK cell subsets. DC subsets and NK were enriched using magnetic beads or erythrocyte rosetting. HLA-DR⁺ cells were depleted from PBMC by complement-mediated lysis. NK cytotoxicity was tested in a ⁵¹Cr-release assay using HSV-infected fibroblasts or K562 target cells. Direct cell contact requirements between NK and DC subsets were assessed using trans-well cultures:  NK cell activation was evaluated measuring CD69 surface expression by flow cytometry after co-culture with DC primed with or without CpG-2216, IFN-a, or Influenza-PR8.

Results:  HLA-DR⁺ cells were required for NK cell-mediated lysis of herpes-infected targets (35.45% vs 6.75% for HLA-DR^–); no requirement for HLA-DR⁺ cells was observed with K562 target cells (48.75% vs 41.8% for HLA-DR^–. The addition of HLA-DR⁺ cells from suppressed HIV⁺ donors (< 50 copies/mL), but not from viremic donors (> 100,000 copies/mL) to HLA-DR^-from HIV^– controls restored NK cytotoxicity against infected targets (38.6% compared to 48.5% for total PBMC). Determination of the cell subset regulating NK responses was addressed by testing the effects of highly enriched PDC and MDC cells derived from suppressed individuals as compared to controls. Our results indicate that PDC had a greater permissive role than MDC for NK-mediated killing of infected targets (PDC = 29.5%, MDC = 13.9%, total PBMC = 35.9 % for HIV⁺ donors; PDC = 33.31%, MDC = 9.47%; total PBMC = 44.64% for controls). The study of CD69 expression on NK cells in permeable membrane (trans-well) based culture experiments supported a major role for soluble factors (IFN-a) as the mediator of PDC accessory function in NK cell cytotoxicity.

Conclusions:  Our data indicate that in HIV infection, NK responses against infected targets depend on the presence of both mature NK and functional PDC. Our results support a critical role for HLA-DR⁺ cells/PDC in regulating NK cell cytotoxic responses against virally infected cells, but not tumor targets, where no evidence of HLA-DR⁺ cell-mediated help was observed.

Keywords: Plasmacytoid dendritic cells; NK cells; Cytotoxic activity