Effective immunity requires the coordinated activation of innate and adaptive immune responses. Natural killer (NK) cells are central innate immune effectors, but can also affect the generation of acquired immune responses to viruses and malignancies. How NK cells influence the efficacy of adaptive immunity, however, is poorly understood. Here, we show that NK cells negatively regulate the duration and effectiveness of virus-specific CD4+ and CD8+ T cell responses by limiting exposure of T cells to infected antigen-presenting cells. This impacts the quality of T cell responses and the ability to limit viral persistence. Our studies provide unexpected insights into novel interplays between innate and adaptive immune effectors, and define the critical requirements for efficient control of viral persistence.

Natural killer (NK) cells have been reported to control adaptive immune responses that occur in lymphoid organs at the early stages of immune challenge. The physiological purpose of such regulatory activity remains unclear, because it generally does not confer a survival advantage. We found that NK cells specifically eliminated activated CD4(+) T cells in the salivary gland during chronic murine cytomegalovirus (MCMV) infection. This was dependent on TNF-related apoptosis inducing ligand (TRAIL) expression by NK cells. Although NK cell-mediated deletion of CD4(+) T cells prolonged the chronicity of infection, it also constrained viral-induced autoimmunity. In the absence of this activity, chronic infection was associated with a Sjogren's-like syndrome characterized by focal lymphocytic infiltration into the glands, production of autoantibodies, and reduced saliva and tear secretion. Thus, NK cells are an important homeostatic control that balances the efficacy of adaptive immune responses with the risk of developing autoimmunity.

The mechanism of IFN resistance was examined in three long-term cell lines, SK-MEL-28, SK-MEL-3, and MM96, exhibiting significant variation in responsiveness to the antiproliferative and antiviral effects of type I IFNs. The JAK-STAT components involved in IFN signal transduction were analyzed in detail. After exposure to IFN, activation of the IFN type I receptor-linked tyrosine kinases, JAK-1 and TYK-2, was detected at similar levels in both IFN-sensitive and IFN-resistant cell types, indicating that IFN resistance did not result from a deficiency in signaling at the level of receptor-associated kinase activation. However, analysis of ISGF3 transcription factor components, STAT1, STAT2, and p48-ISGF3␥, revealed that their expression and activation correlated with cellular IFN responsiveness. The analysis was extended to also include IFN-sensitive primary melanocytes, three additional IFN-resistant melanoma cell lines, and seven cell cultures recently established from melanoma patient biopsies. It was consistently observed that the most marked difference in ISGF3 was a lack of STAT1 in the resistant versus the sensitive cells. Transfection of the IFN-resistant MM96 cell line to express increased levels of STAT1 protein partially restored IFN responsiveness in an antiviral assay. We conclude that a defect in the level of STAT1 and possibly all three ISGF3 components in IFN-resistant human melanoma cells may be a general phenomenon responsible for reduced cellular responsiveness of melanomas to IFNs.The interferons (IFNs) comprise a family of multifunctional polypeptides, recognized for their antiviral, antiproliferative, and immunoregulatory functions (reviewed in Refs. 1 and 2). Type I IFNs, IFN-␣ and -␤, exert their biological actions by binding to high-affinity cell-surface receptors that stimulate phosphorylation of tyrosine residues on type I receptor components (3, 4) and on the receptor-associated tyrosine kinases, TYK-2 and JAK-1 (5-7). Following tyrosine phosphorylation, the activated tyrosine kinases then induce the formation and activation by tyrosine phosphorylation of the latent cytoplasmic transcription factor IFN-stimulated gene factor 3␣ (ISGF3␣) (8). The ISGF3␣ transcription factor is a complex of STAT molecules, including STAT1 and STAT2. STAT1 exists in two forms of 91 kDa (STAT1␣) and 84 kDa (STAT1␤), and STAT2 is a 113-kDa protein (reviewed in Refs. 9 and 10). During the classical type I IFN response, the tyrosine-phosphorylated STAT1 and STAT2 proteins form heterodimers (11), which complex with p48-ISGF3␥, a DNA-binding protein. (23), and in many other cancer cell types (24). In addition, melanoma cell lines with a wide variation in their responsiveness to the antiproliferative (25) and antiviral (26) activities of IFNs have been described. We have previously demonstrated that the difference in responsiveness among these melanoma cell lines is related neither to the receptor binding affinity nor to the numbers of IFN receptors on the cell surface (27). In addition, the cDNA sequence encodi...