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Human natural killer (NK) cells require an HLA-DR+ accessory cell (AC) population to lyse herpes simplex virus-type 1 (HSV-1)-infected fibroblasts (HSV-Fs) but not K562 target cells. It has been postulated that ACs may function by producing interferon-alpha (IFN-alpha), which stimulates NK cells. Using a sequential enrichment protocol, ACs were found to coenrich with the interferon-producing cells (IPCs). Treatment of the ACs with a protein synthesis inhibitor, emetine, inhibited both their IFN production and AC function, results that support a central role for IFN in AC activity. In contrast, when the arginine analogue canavanine was added to NK assays, no IFN-alpha was produced and NK(HSV-Fs) activity was only partially inhibited. Consistent with IFN-independent AC function, treatment with either polyclonal sheep or bovine anti-IFN-alpha neutralized all the IFN-alpha produced during the NK assays but caused either no or partial reduction of NK(HSV-Fs) activity, respectively. However, when limiting numbers of ACs were used, the bovine antiserum neutralized both IFN-alpha and NK(HSV-Fs) activity. We further found that HLA-DR+ cells are required for cell clustering, suggesting a role for cell contact. Finally, fixation of activated ACs prevented the accessory function. Together, these results demonstrate that ACs can provide help to NK cells in both IFN-alpha-dependent and -independent manners.

Herpes simplex virus type 1 (HSV-1)-infected human fibroblast (HSV-FS) targets are susceptible to lysis by natural killer (NK) cells, whereas uninfected FS are resistant to lysis. Studies were undertaken to determine the mechanism of this preferential susceptibility. HSV-FS were not intrinsically less stable than FS, as determined by a 5"Cr release assay under hypotonic shock in the presence of rat granule cytolysin and by sensitivity to anti-human leukocyte antigen class I antibody plus complement. Single-cell assays in agarose demonstrated that although similar numbers of large granular lymphocytes bound to the HSV-FS and FS targets, the conjugates with HSV-FS were lysed at a much higher frequency than those with FS. These results suggested that both targets are bound by the NK cells but only the HSV-FS were able to trigger lysis. The requirement for active virus expression was demonstrated by failure of emetine-treated HSV-FS targets or targets infected with UV-inactivated HSV to be lysed by NK effectors. To evaluate the role of viral glycoproteins in conferring susceptibility to lysis, Fab were prepared from HSV-l-seropositive sera; these Fab were unable to block lysis of the HSV-FS. Furthermore, incubation in phosphonoacetic acid failed to reduce NK(HSV-FS) activity despite * Corresponding author.

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