Yet another role for natural killer cells: cytotoxicity in immune regulation and viral persistence.

A t early times during viral infections, natural killer (NK) cells can contribute to direct antiviral defense (1, 2). The effect is most dramatic when they are stimulated to release the contents of their cytolytic granules for lysis of virus-infected target cells. Type 1 interferons (IFNs), produced during innate responses to infections, induce elevated NK cell cytotoxicity. Killing, however, is ultimately dependent on signaling through NK activating receptors by ligands expressed on target cell surfaces. A complex repertoire of germ-line genes encodes NK receptors (2). Some of these are conserved, whereas others are polygenic and polymorphic. At later times, adaptive immunity, such as that delivered by cytotoxic CD8 T cells, mediates protection. The kinetics and magnitudes of viral replication compared with developing T-cell responses can shift the balance between the virus and an infected host through three states: viral clearance with long-lived immunity, immune-mediated pathology with life-threatening conditions, and functional exhaustion of T cells with resulting chronic infection. The report by Lang et al. in PNAS (3) provides evidence of a role for NK cell cytotoxicity in limiting CD8 T-cell responses, to result in persistent viral infection as well as elevated infection-induced disease. This immunoregulatory function suggests that NK cells direct the balance between the virus and the host, but the results must be considered in the context of the targets of NK cell-mediated lysis, the race in establishing states of infection, and the complexities associated with the experimental systems used to identify the effects. A potential role for NK cell-mediated cytotoxicity in immunoregulation was first considered because although NK cells are induced to have elevated killing whenever type 1 IFNs are induced, they are important in early direct defense against some but not all viruses (1, 4). The best evidence for their direct antiviral function comes from studies of murine cytomegalovirus (MCMV) infections of mice. Here, their maximal protective effect depends on cytotoxicity and on an NK cell-activating receptor present in MCMV-resistant but not -sensitive strains of mice, Ly49H, with a virus-induced ligand, m157, expressed on infected target cells (2). In the case of infections of mice with the lymphocytic choriomeningitis virus (LCMV), however, type 1 IFNs are induced and stimulate NK cell cytotoxic functions, but an NK cell contribution to early resistance is difficult to detect (4). The absence of an NKactivating receptor and/or a virus-induced ligand for an activating receptor may account for an inability to access NK cell killing for direct LCMV defense. Why, however, would a pathway inducing NK cell-mediated cytotoxicity be preserved without benefit to antiviral defense? Studies aimed at addressing this issue have shown that the presence of NK cells can limit T-cell responses to either MCMV or LCMV infection (5–7) and adaptive memory responses to antigen delivered by a nonreplicating adenovirus vector (8). Conversely, NK cell lysis of target cells has been reported to support T-cell responses by providing antigens for presentation (9), and direct NK cell antiviral defense has been shown to promote early CD8 T-cell responses to MCMV by limiting the magnitudes of other innate responses (10). Thus, data are accumulating for immunoregulatory functions mediated through NK cell cytotoxicity, but the results suggest effects both inhibiting and enhancing adaptive immunity, and the pathways to their delivery remain poorly defined. Lang et al. (3) now investigate LCMV infection in C57BL/6 mice rendered NK cell deficient throughout the infection. Challenge viruses were different LCMV strains, including relatively mild as well as more aggressive variants that can establish chronic infections. The most remarkable aspect of the work is that NK cell deficiencies resulted in lower viral burdens. When the more aggressive conditions of infection were examined at day 10 of infection during transition to persistence, the differences in the NK cell deplete compared with the replete groups were dramatic, with up to 6 log decreases and even clearance from multiple organs. The elevated resistance to infection was accompanied by increases in the proportions of functioning, antigen-specific CD8 T cells induced at these times. The results complement those recently reported by Waggoner et al. (11). Thus, the presence of NK cells limits CD8 T-cell responses to LCMV, prevents elimination of virus, and sets up to promote viral persistence. What is the mechanism used by NK cells to mediate the effects? Here, the studies from both groups conclude that NK cell cytotoxicity is responsible, but they disagree on the immune cell target. Lang et al. argue that the CD8 T cells are themselves killed off by NK cells, whereas Waggoner et al. build a case that CD4 T cells helping the CD8 T-cell responses are killed (Fig. 1). Both groups evaluated the in vivo role for NK cell cytotoxicity by examining recovery of cells adoptively transferred into perforin (prf)-deficient and control infected mice. Differences in viral replication resulting from deficiencies in CD8 T as well as NK cell-mediated cytotoxicity may have confounded both studies, but theLang et al. and Waggoner et al. reports also examined different cells and conditions for sensitivity to in vivo NK cell-mediated killing; either congenically marked LCMV-specific T-cell receptor transgenic CD8 T cells were transferred before and evaluated at day 8 after infection, or fluorescently labeled Fig. 1. NK cell-mediated cytotoxicity in the regulation of immune responses to viral infections. New work is revealing a role forNK cell-mediatedkilling in immune regulation during LCMV infection. The effects have been reported to be delivered to either CD8 (3) or CD4 (11) T cells. The observed NK cell-mediated immunoregulatory effects are accompanied by reduced resistance and/or persistence of the viral infection. Studies of MCMV infection, with an important contribution of NK cell-mediated killing to antiviral defense, have shown increased viral persistence in thepresenceofNKcell defense (6). Together, these observations are establishing a paradigm with the NK cell as the “top dog” in regulating host–virus relationships and challenging the understanding of the race between the virus and the host.