Natural killer (NK) cells have the potential to deliver both direct antimicrobial effects and regulate adaptive immune responses, but NK cell yields have been reported to vary greatly during different viral infections. Activating receptors, including the Ly49H molecule recognizing mouse cytomegalovirus (MCMV), can stimulate NK cell expansion. To define Ly49H's role in supporting NK cell proliferation and maintenance under conditions of uncontrolled viral infection, experiments were performed in Ly49h−/−, perforin 1 (Prf1)−/−, and wild-type (wt) B6 mice. NK cell numbers were similar in uninfected mice, but relative to responses in MCMV-infected wt mice, NK cell yields declined in the absence of Ly49h and increased in the absence of Prf1, with high rates of proliferation and Ly49H expression on nearly all cells. The expansion was abolished in mice deficient for both Ly49h and Prf1 (Ly49h−/−Prf1−/−), and negative consequences for survival were revealed. The Ly49H-dependent protection mechanism delivered in the absence of Prf1 was a result of interleukin 10 production, by the sustained NK cells, to regulate the magnitude of CD8 T cell responses. Thus, the studies demonstrate a previously unappreciated critical role for activating receptors in keeping NK cells present during viral infection to regulate adaptive immune responses.

Natural killer (NK) cells and CD8 ؉ T cells play a prominent role in the clearance of mouse cytomegalovirus (MCMV) infection.The role of NK cells in modulating the CD8 ؉ T-cell response to MCMV infection is still the subject of intensive research. For analyzing the impact of NK cells on mounting of a CD8 ؉ T-cell response and the contribution of these cells to virus control during the first days postinfection (p.i.), we used C57BL/6 mice in which NK cells are specifically activated through the Ly49H receptor engaged by the MCMV-encoded ligand m157. Our results indicate that the requirement for CD8 ؉ T cells in early MCMV control inversely correlates with the engagement of Ly49H. While depletion of CD8 ؉ T cells has only a minor effect on the early control of wild-type MCMV, CD8 ؉ T cells are essential in the control of ⌬m157 virus. The frequencies of virus epitope-specific CD8 ؉ T cells and their activation status were higher in mice infected with ⌬m157 virus. In addition, these mice showed elevated levels of alpha interferon (IFN-␣) and several other proinflammatory cytokines as early as 1.5 days p.i. Although the numbers of conventional dendritic cells (cDCs) were reduced later during infection, particularly in ⌬m157-infected mice, they were not significantly affected at the peak of the cytokine response. Altogether, we concluded that increased antigen load, preservation of early cDCs' function, and higher levels of innate cytokines collectively account for an enhanced CD8 ؉ T-cell response in C57BL/6 mice infected with a virus unable to activate NK cells via the Ly49H-m157 interaction. Mouse cytomegalovirus (MCMV) has been extensively used as a model for studying the role of NK cells in virus control. NK cells play a crucial role in the early stage of MCMV infection, prior to the induction of the adaptive immune response (20). However, the contribution of NK cells in the control of early MCMV infection varies among mouse strains (38; reviewed in reference 43). In C57BL/6 mice, the activating NK cell receptor Ly49H mediates resistance to MCMV infection due to the specific binding of m157, a virally encoded protein (6, 46). NK cell activation through Ly49H-m157 interaction is characterized by perforin-mediated cytotoxicity and specific proliferation of Ly49H ϩ NK cells (13,26,49). Unlike C57BL/6 mice, MCMV-susceptible mouse strains are unable to mount an effective NK cell response to this virus (reviewed in reference 42).In addition to their direct function, which results in the containment of viral infection, a large body of accumulated data suggests that NK cells play a role also in the shaping of the specific immune response (reviewed in reference 50). However, this topic is still controversial and the subject of intense studies. Robbins and colleagues (40) have shown that NK cell activation via the Ly49H-m157 pathway accelerates the CD8 ϩ T-cell response in vivo. According to the proposed scenario, the activation of NK cells via this axis limits alpha/beta interferon (IFN-␣/␤) production by plasmacytoid den...

A major gap in our understanding of infant immunity is why natural killer (NK) cellresponses are deficient, making infants more prone to viral infection. Here we demonstrate that transforming growth factor-β (TGF-β) was responsible for NK cell immaturity during infancy. Higher numbers of fully mature NK cells were found in CD11cdnR mice, whose NK cells lack TGF-βR signaling. Importantly, ontogenic maturation of NK cells progressed faster in the absence of TGF-β signaling, resulting in the formation of mature NK cell pool early in life. As a consequence, infant CD11cdnR mice efficiently controlled viral infections. These data thus demonstrate an unprecedented role for TGF-β in ontogeny that can explain why NK cell responses are deficient early in life.

Cmv1 was the first mouse cytomegalovirus (MCMV) resistance locus identified in C57BL/6 mice. It encodes Ly49H, a NK cell-activating receptor that specifically recognizes the m157 viral protein at the surface of MCMV-infected cells. To dissect the effect of the Ly49h gene in host-pathogen interactions, we generated C57BL/6 mice lacking the Ly49h region. We found that 36 h after MCMV infection, the lack of Ly49h resulted in high viral replication in the spleen and dramatically enhanced proinflammatory cytokine production in the serum and spleen. At later points in time, we observed that MCMV induced a drastic loss in CD8+ T cells in B6.Ly49h−/− mice, probably reflecting severe histological changes in the spleen. Overall, our results indicate that Ly49H+ NK cells contain a systemic production of cytokines that may contribute to the MCMV-induced pathology and play a central role in maintaining normal spleen cell microarchitecture. Finally, we tested the ability of B6.Ly49h−/− mice to control replication of Leishmania major and ectromelia virus. Resistance to these pathogens has been previously mapped within the NK gene complex. We found that the lack of Ly49H+ NK cells is not associated with an altered resistance to L. major. In contrast, absence of Ly49H+ NK cells seems to afford additional protection against ectromelia infection in C57BL/6 mice, suggesting that Ly49H may recognize ectromelia-infected cells with detrimental effects. Taken together, these results confirm the pivotal role of the Ly49H receptor during MCMV infection and open the way for further investigations in host-pathogen interactions.

Plasmacytoid dendritic cells (pDCs) are an important source of type I interferon (IFN) during initial immune responses to viral infections. In mice, pDCs are uniquely characterized by high-level expression of Ly49Q, a C-type lectin-like receptor specific for class I major histocompatibility complex (MHC) molecules. Despite having a cytoplasmic immunoreceptor tyrosine-based inhibitory motif, Ly49Q was found to enhance pDC function in vitro, as pDC cytokine production in response to the Toll-like receptor (TLR) 9 agonist CpG-oligonucleotide (ODN) could be blocked using soluble monoclonal antibody (mAb) to Ly49Q or H-2Kb. Conversely, CpG-ODN–dependent IFN-α production by pDCs was greatly augmented upon receptor cross-linking using immobilized anti-Ly49Q mAb or recombinant H-2Kb ligand. Accordingly, Ly49Q-deficient pDCs displayed a severely reduced capacity to produce cytokines in response to TLR7 and TLR9 stimulation both in vitro and in vivo. Finally, TLR9-dependent antiviral responses were compromised in Ly49Q-null mice infected with mouse cytomegalovirus. Thus, class I MHC recognition by Ly49Q on pDCs is necessary for optimal activation of innate immune responses in vivo.