Dual Requirement of Cytokine and Activation Receptor Triggering for Cytotoxic Control of Murine Cytomegalovirus by NK Cells

Natural killer (NK) cells play a critical role in controlling murine cytomegalovirus (MCMV) and can mediate both cytokine production and direct cytotoxicity. The NK cell activation receptor, Ly49H, is responsible for genetic resistance to MCMV in C57BL/6 mice. Recognition of the viral m157 protein by Ly49H is sufficient for effective control of MCMV infection. Additionally, during the host response to infection, distinct immune and non-immune cells elaborate a variety of pleiotropic cytokines which have the potential to impact viral pathogenesis, NK cells, and other immune functions, both directly and indirectly. While the effects of various immune deficiencies have been examined for general antiviral phenotypes, their direct effects on Ly49H-dependent MCMV control are poorly understood. To specifically interrogate Ly49H-dependent functions, herein we employed an in vivo viral competition approach to show Ly49H-dependent MCMV control is specifically mediated through cytotoxicity but not IFNγ production. Whereas m157 induced Ly49H-dependent degranulation, efficient cytotoxicity also required either IL-12 or type I interferon (IFN-I) which acted directly on NK cells to produce granzyme B. These studies demonstrate that both of these distinct NK cell-intrinsic mechanisms are integrated for optimal viral control by NK cells.     

Ly49h-deficient C57BL/6 mice: a new mouse cytomegalovirus-susceptible model remains resistant to unrelated pathogens controlled by the NK gene complex

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.     

Cmv1-independent antiviral role of NK cells revealed in murine cytomegalovirus-infected New Zealand White mice

Ly49H(+) NK cells play a critical role in innate antiviral immune responses to murine CMV (MCMV). Ly49H(b6) recognition of MCMV-encoded m157 on infected cells activates natural killing required for host resistance. We show that mAb 3D10 (anti-Ly49H) recognizes comparable subsets of NK cells from New Zealand White (NZW), New Zealand Black (NZB), and C57BL/6 spleens. However, virus levels in the spleens of MCMV-infected NZW and NZB mice differed greatly. We found that MCMV replication in infected NZW spleens was limited through NK cells. Alternately, NZB mice were profoundly susceptible to MCMV infection. Although 3D10 mAb injections given before infection interfere with Cmv1-type resistance in C57BL/6 mice, similar mAb injections did not affect NZW resistance, likely because NZW NK cell receptors did not bind MCMV-encoded m157. Instead, anti-MCMV host defenses in hybrid NZ offspring were associated with multiple chromosome locations including several putative quantitative trait loci that did not overlap with H-2 or NK gene complex loci. This study revealed a novel pathway used by NK cells to defend against MCMV infection. Thus, the importance of Ly49H in MCMV infection may be shaped by other additional background genes.     

Protection from lethal infection by adoptive transfer of CD8 T cells genetically engineered to express virus-specific innate immune receptor

CMV infection is one of the most common complications in immunocompromised individuals, such as organ and bone marrow transplant patients. Both innate and adaptive immune responses are required for defense against CMV infection. In murine CMV (MCMV) infection, strains harboring the MCMV-specific NK cell activation receptor, Ly49H (Klra8), are resistant. In contrast, MCMV infection of mice lacking Ly49H gene causes early mortality due to uncontrolled viral replication. In this study, we report the successful protection of mice from lethal MCMV infection with gene-transferred polyclonal CD8 T cells. CD8 T cells expressing a chimeric receptor comprising Ly49H extracellular and CD3zeta cytoplasmic domains are capable of killing target cells expressing the MCMV protein, m157. CD8 T cells expressing the chimeric receptor protect mice in vivo from lethality in the acute phase of MCMV infection, leading to the establishment of long-term protection. These data provide proof-of-principle evidence that a novel strategy for harnessing CD8 cytolytic function through TCR-independent yet pathogen-specific receptor can result in effective protection of hosts from pathogens.     

Ly49C-Dependent Control of MCMV Infection by NK Cells Is Cis-Regulated by MHC Class I Molecules

Natural Killer (NK) cells are crucial in early resistance to murine cytomegalovirus (MCMV) infection. In B6 mice, the activating Ly49H receptor recognizes the viral m157 glycoprotein on infected cells. We previously identified a mutant strain (MCMV^G1F) whose variant m157 also binds the inhibitory Ly49C receptor. Here we show that simultaneous binding of m157 to the two receptors hampers Ly49H-dependent NK cell activation as Ly49C-mediated inhibition destabilizes NK cell conjugation with their targets and prevents the cytoskeleton reorganization that precedes killing. In B6 mice, as most Ly49H⁺ NK cells do not co-express Ly49C, the overall NK cell response remains able to control MCMVm157^G1F infection. However, in B6 Ly49C transgenic mice where all NK cells express the inhibitory receptor, MCMV infection results in altered NK cell activation associated with increased viral replication. Ly49C-mediated inhibition also regulates Ly49H-independent NK cell activation. Most interestingly, MHC class I regulates Ly49C function through cis-interactions that mask the receptor and restricts m157 binding. B6 Ly49C Tg, β2m ko mice, whose Ly49C receptors are unmasked due to MHC class I deficient expression, are highly susceptible to MCMVm157^G1F and are unable to control a low-dose infection. Our study provides novel insights into the mechanisms that regulate NK cell activation during viral infection.     

Requisite H2k role in NK cell-mediated resistance in acute murine cytomegalovirus-infected MA/My mice

Human CMV infections are a major health risk in patients with dysfunctional or compromised immunity, especially in patients with NK cell deficiencies, as these are frequently associated with high morbidity and mortality. In experimental murine CMV (MCMV) infections, Ly49H activation receptors on C57BL/6 (B6) NK cells engage m157 viral ligands on MCMV-infected cells and initiate dominant virus control. In this study, we report that MCMV resistance in MA/My relies on Ly49H-independent NK cell-mediated control of MCMV infection as NK cells in these mice do not bind anti-Ly49H mAb or soluble m157 viral ligands. We genetically compared MA/My resistance with MCMV susceptibility in genealogically and NK gene complex-Ly49 haplotype-related C57L mice. We found that MCMV resistance strongly associated with polymorphic H2k-linked genes, including MHC and non-MHC locations by analysis of backcross and intercross progeny. The H2b haplotype most frequently, but not absolutely, correlated with MCMV susceptibility, thus confirming a role for non-MHC genes in MCMV control. We also demonstrate a definite role for NK cells in H2k-type MCMV resistance because their removal from C57L.M-H2k mice before MCMV infection diminished immunity. NK gene complex-linked polymorphisms, however, did not significantly influence MCMV control. Taken together, effective NK cell-mediated MCMV control in this genetic system required polymorphic H2k genes without need of Ly49H-m157 interactions.     

The NK cell response to mouse cytomegalovirus infection affects the level and kinetics of the early CD8(+) T-cell response

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 Δm157 virus. The frequencies of virus epitope-specific CD8(+) T cells and their activation status were higher in mice infected with Δm157 virus. In addition, these mice showed elevated levels of alpha interferon (IFN-α) 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 Δm157-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.     

Activation receptor-induced tolerance of mature NK cells in vivo requires signaling through the receptor and is reversible

NK cell responses are determined by signals received through activating and inhibitory cell surface receptors. Ly49H is an NK cell-specific activating receptor that accounts for the genetic resistance to murine CMV (MCMV). The Ly49H receptor has been shown to interact with two adaptor proteins (DAP12 and DAP10). In the context of MCMV infection, interaction of m157 (the MCMV-encoded ligand for Ly49H) with Ly49H results in activation of Ly49H-expressing NK cells. Chronic exposure of Ly49H with m157, however, induces tolerance in these same cells. The mechanism of this tolerance remains poorly understood. Using a transgenic mouse model, we demonstrate that induction of tolerance in Ly49H(+) NK cells by chronic exposure to m157, in vivo, requires signaling through the Ly49H adaptor protein DAP12, but not the DAP10 adaptor protein. Furthermore, mature Ly49H-expressing NK cells from wild-type mice can acquire a tolerant phenotype by 24 h posttransfer into a transgenic C57BL/6 mouse that expresses m157. The tolerant phenotype can be reversed, in vivo, if tolerant NK cells are transferred to mice that do not express the m157 protein. Thus, continuous activating receptor engagement can induce a transient tolerance in mature NK cells in vivo. These observations provide new insight into how activating receptor engagement shapes NK cell function and has important implications in how NK cells respond to tumors and during chronic viral infection.     

Natural Killer Cell Sensing of Infected Cells Compensates for MyD88 Deficiency but Not IFN-I Activity in Resistance to Mouse Cytomegalovirus

In mice, plasmacytoid dendritic cells (pDC) and natural killer (NK) cells both contribute to resistance to systemic infections with herpes viruses including mouse Cytomegalovirus (MCMV). pDCs are the major source of type I IFN (IFN-I) during MCMV infection. This response requires pDC-intrinsic MyD88-dependent signaling by Toll-Like Receptors 7 and 9. Provided that they express appropriate recognition receptors such as Ly49H, NK cells can directly sense and kill MCMV-infected cells. The loss of any one of these responses increases susceptibility to infection. However, the relative importance of these antiviral immune responses and how they are related remain unclear. In humans, while IFN-I responses are essential, MyD88 is dispensable for antiviral immunity. Hence, a higher redundancy has been proposed in the mechanisms promoting protective immune responses against systemic infections by herpes viruses during natural infections in humans. It has been assumed, but not proven, that mice fail to mount protective MyD88-independent IFN-I responses. In humans, the mechanism that compensates MyD88 deficiency has not been elucidated. To address these issues, we compared resistance to MCMV infection and immune responses between mouse strains deficient for MyD88, the IFN-I receptor and/or Ly49H. We show that selective depletion of pDC or genetic deficiencies for MyD88 or TLR9 drastically decreased production of IFN-I, but not the protective antiviral responses. Moreover, MyD88, but not IFN-I receptor, deficiency could largely be compensated by Ly49H-mediated antiviral NK cell responses. Thus, contrary to the current dogma but consistent with the situation in humans, we conclude that, in mice, in our experimental settings, MyD88 is redundant for IFN-I responses and overall defense against a systemic herpes virus infection. Moreover, we identified direct NK cell sensing of infected cells as one mechanism able to compensate for MyD88 deficiency in mice. Similar mechanisms likely contribute to protect MyD88- or IRAK4-deficient patients from viral infections.     

Deficient major histocompatibility complex-linked innate murine cytomegalovirus immunity in MA/My.L-H2b mice and viral downregulation of H-2k class I proteins

NK cells are key effectors of innate immunity and host survival during cytomegalovirus (CMV) infection. Innate murine CMV (MCMV) resistance in MA/My mice requires Ly49H/m157-independent H-2k-linked NK cell control. Here we show that replacement of MA/My H-2k with C57L H-2b susceptibility genes led to a remarkable loss of innate virus immunity, though NK gamma interferon was induced in H-2b and H-2k strains shortly after infection. Thus, H-2b genes expressed in C57L or MA/My.L-H2b are sufficient in alerting NK cells to intrusion but fail to support NK restraint of viral infection. In addition, novel H-2 recombinant strains were produced and utilized in a further refinement of a critical genetic interval controlling innate H-2k-linked MCMV resistance. Importantly, this analysis excluded the gene interval from Kk class I through class II. The responsible gene(s) therefore resides in an interval spanning Dk class Ia and more-distal major histocompatibility complex (MHC) nonclassical class Ib genes. Recently, the NK activation receptor Ly49P and MHC class I Dk proteins were genetically implicated in MCMV resistance, in part because Ly49P-expressing reporter T cells could specifically bind Dk molecules on MCMV-infected mouse embryonic fibroblasts (MEFs). However, as we found that H-2k innate resistance differs in the C57L or MA/My backgrounds and because MCMV very efficiently downregulates H-2k class I proteins in L929 cells and primary MEFs shortly after infection, a Ly49P/Dk model should not fully explain H-2k-linked MCMV resistance.     

All is fair in virus-host interactions: NK cells and cytomegalovirus

The infection of mice with mouse cytomegalovirus (MCMV) as a model of human cytomegalovirus (HCMV) infection has been particularly informative in elucidating the role of innate and adaptive immune response mechanisms during infection. Millions of years of co-evolution between cytomegaloviruses (CMV) and their hosts has resulted in numerous attempts to overwhelm each other. CMVs devote many genes to modulating the host natural killer (NK) cell response and NK cells employ many strategies to cope with CMV infection. While focusing on these attack-counterattack measures, this review will discuss several novel mechanisms of immune evasion by MCMV, the role of Ly49 receptors in mediating resistance to MCMV, and the impact of the initial NK cell response on the shaping of adaptive immunity.     

Gain of virulence caused by loss of a gene in murine cytomegalovirus

Mouse strains are either resistant or susceptible to murine cytomegalovirus (MCMV). Resistance is determined by the Cmv1(r) (Ly49h) gene, which encodes the Ly49H NK cell activation receptor. The protein encoded by the m157 gene of MCMV has been defined as a ligand for Ly49H. To find out whether the m157 protein is the only Ly49H ligand encoded by MCMV, we constructed the m157 deletion mutant and a revertant virus. Viruses were tested for susceptibility to NK cell control in Ly49H+ and Ly49H- mouse strains. Deletion of the m157 gene abolished the viral activation of Ly49H+ NK cells, resulting in higher virus virulence in vivo. Thus, in the absence of m157, Ly49H+ mice react like susceptible strains. 129/SvJ mice lack the Ly49H activation NK cell receptor but express the inhibitory Ly49I NK cell receptor that binds to the m157 protein. The Deltam157 inhibitory phenotype was weak because MCMV encodes a number of proteins that mediate NK inhibition, whose contribution could be shown by another mutant.     

Cmv4, a new locus linked to the NK cell gene complex, controls innate resistance to cytomegalovirus in wild-derived mice

CMV can cause life-threatening disease in immunodeficient hosts. Experimental infection in mice has revealed that the genetically determined natural resistance to murine CMV (MCMV) may be mediated either by direct recognition between the NK receptor Ly49H and the pathogen-encoded glycoprotein m157 or by epistatic interaction between Ly49P and the host MHC H-2D(k). Using stocks of wild-derived inbred mice as a source of genetic diversity, we found that PWK/Pas (PWK) mice were naturally resistant to MCMV. Depletion of NK cells subverted the resistance. Analysis of backcrosses to susceptible BALB/c mice revealed that the phenotype was controlled by a major dominant locus effect linked to the NK gene complex. Haplotype analysis of 41 polymorphic markers in the Ly49h region suggested that PWK mice may share a common ancestral origin with C57BL/6 mice; in the latter, MCMV resistance is dependent on Ly49H-m157 interactions. Nevertheless, PWK mice retained viral resistance against m157-defective mutant MCMV. These results demonstrate the presence of yet another NK cell-dependent viral resistance mechanism, named Cmv4, which most likely encodes for a new NK activating receptor. Identification of Cmv4 will expand our understanding of the specificity of the innate recognition of infection by NK cells.     

Comparing the kinetics of NK cells, CD4, and CD8 T cells in murine cytomegalovirus infection

NK cells recognize virus-infected cells with germline-encoded activating and inhibitory receptors that do not undergo genetic recombination or mutation. Accordingly, NK cells are often considered part of the innate immune response. The innate response comprises rapid early defenders that do not form immune memory. However, there is increasing evidence that experienced NK cells provide increased protection to secondary infection, a hallmark of the adaptive response. In this study, we compare the dynamics of the innate and adaptive immune responses by examining the kinetic profiles of the NK and T cell response to murine CMV infection. We find that, unexpectedly, the kinetics of NK cell proliferation is neither earlier nor faster than the CD4 or CD8 T cell response. Furthermore, early NK cell contraction after the peak of the response is slower than that of T cells. Finally, unlike T cells, experienced NK cells do not experience biphasic decay after the response peak, a trait associated with memory formation. Rather, NK cell contraction is continuous, constant, and returns to below endogenous preinfection levels. This indicates that the reason why Ag-experienced NK cells remain detectable for a prolonged period after adoptive transfer and infection is in part due to the high precursor frequency, slow decay rate, and low background levels of Ly49H(+) NK cells in recipient DAP12-deficient mice. Thus, the quantitative contribution of Ag-experienced NK cells in an endogenous secondary response, with higher background levels of Ly49H(+) NK cells, may be not be as robust as the secondary response observed in T cells.     

<Emphasis Type="Italic">Ly49h</Emphasis> is necessary for genetic resistance to murine cytomegalovirus

Natural killer (NK) cells play critical roles in antiviral immunity. While the importance of effector mechanisms such as interferons has been demonstrated through knockout mice, specific mechanisms of how viruses are recognized and controlled by NK cells are less well defined. Previous genetic studies have mapped the resistance genes for murine cytomegalovirus (MCMV), herpes simplex virus-1 (HSV-1), and ectromelia virus to the NK gene complex on murine chromosome 6, a region containing the polymorphic Ly49 and Nkrp1 families. Genetic resistance to MCMV in C57BL/6 has been attributed to Ly49H, an activation receptor, through susceptibility of the recombinant inbred strain BXD-8 that lacks Ly49h (also known as Klra8) but derived about half of its genome from its DBA/2 progenitor. However, it remained possible that epigenetic effects could account for the MCMV phenotype in BXD-8 mice. Herein, we report the generation of a novel congenic murine strain, B6.BXD8-Klra8 ( Cmv1-del )/Wum, on the C57BL/6 genetic background to evaluate the effect of deletion of a single NK activation receptor, Ly49H. Deletion of Ly49H rendered mice much more susceptible to MCMV infection. This increase in susceptibility did not appear to be a result of a difference in NK cell expansion or interferon-gamma (IFN-gamma) production between the C57BL/6 and the B6.BXD8 strains. On the other hand, the deletion of Ly49h did not otherwise affect NK cell maturation or Ly49D expression and had no effect on susceptibility to HSV-1 or ectromelia virus. In conclusion, Ly49h is necessary for genetic resistance to MCMV, but not HSV-1 or ectromelia virus.     

Functional consequences of natural sequence variation of murine cytomegalovirus m157 for Ly49 receptor specificity and NK cell activation

The Ly49H activating receptor on C57BL/6 (B6) NK cells plays a key role in early resistance to murine cytomegalovirus (MCMV) infection through specific recognition of the MCMV-encoded MHC class I-like molecule m157 expressed on infected cells. The m157 molecule is also recognized by the Ly49I inhibitory receptor from the 129/J mouse strain. The m157 gene is highly sequence variable among MCMV isolates, with many m157 variants unable to bind Ly49H(B6). In this study, we have sought to define if m157 variability leads to a wider spectrum of interactions with other Ly49 molecules and if this modifies host susceptibility to MCMV. We have identified novel m157-Ly49 receptor interactions, involving Ly49C inhibitory receptors from B6, BALB/c, and NZB mice, as well as the Ly49H(NZB) activation receptor. Using an MCMV recombinant virus in which m157(K181) was replaced with m157(G1F), which interacts with both Ly49H(B6) and Ly49C(B6), we show that the m157(G1F)-Ly49C interactions cause no apparent attenuating effect on viral clearance in B6 mice. Hence, when m157 can bind both inhibitory and activation NK cell receptors, the outcome is still activation. Thus, these data indicate that whereas m157 variants predominately interact with inhibitory Ly49 receptors, these interactions do not profoundly interfere with early NK cell responses.     

Natural Killer Cell Dependent Within-Host Competition Arises during Multiple MCMV Infection: Consequences for Viral Transmission and Evolution

It is becoming increasingly clear that many diseases are the result of infection from multiple genetically distinct strains of a pathogen. Such multi-strain infections have the capacity to alter both disease and pathogen dynamics. Infection with multiple strains of human cytomegalovirus (HCMV) is common and has been linked to enhanced disease. Suggestions that disease enhancement in multi-strain infected patients is due to complementation have been supported by trans-complementation studies in mice during co-infection of wild type and gene knockout strains of murine CMV (MCMV). Complementation between naturally circulating strains of CMV has, however, not been assessed. In addition, many models of multi-strain infection predict that co-infecting strains will compete with each other and that this competition may contribute to selective transmission of more virulent pathogen strains. To assess the outcome of multi-strain infection, C57BL/6 mice were infected with up to four naturally circulating strains of MCMV. In this study, profound within-host competition was observed between co-infecting strains of MCMV. This competition was MCMV strain specific and resulted in the complete exclusion of certain strains of MCMV from the salivary glands of multi-strain infected mice. Competition was dependent on Ly49H⁺ natural killer (NK) cells as well as the expression of the ligand for Ly49H, the MCMV encoded product, m157. Strains of MCMV which expressed an m157 gene product capable of ligating Ly49H were outcompeted by strains of MCMV expressing variant m157 genes. Importantly, within-host competition prevented the shedding of the less virulent strains of MCMV, those recognized by Ly49H, into the saliva of multi-strain infected mice. These data demonstrate that NK cells have the strain specific recognition capacity required to meditate within-host competition between strains of MCMV. Furthermore, this within-host competition has the capacity to shape the dynamics of viral shedding and potentially select for the transmission of more virulent virus strains.     

Coordinate expression of cytokines and chemokines by NK cells during murine cytomegalovirus infection

Cytokines and chemokines activate and direct effector cells during infection. We previously identified a functional group of five cytokines and chemokines, namely, IFN-gamma, activation-induced T cell-derived and chemokine-related cytokine/lymphotactin, macrophage-inflammatory protein 1alpha, macrophage-inflammatory protein 1beta, and RANTES, coexpressed in individual activated NK cells, CD8(+) T cells, and CD4(+) Th1 cells in vitro and during in vivo infections. However, the stimuli during infection were not known. In murine CMV (MCMV) infection, the DAP12/KARAP-associated Ly49H NK cell activation receptor is crucial for resistance through recognition of MCMV-encoded m157 but NK cells also undergo in vivo nonspecific responses to uncharacterized stimuli. In this study, we show that Ly49H ligation by m157 resulted in a coordinated release of all five cytokines/chemokines from Ly49H(+) NK cells. Whereas other cytokines also triggered the release of these cytokines/chemokines, stimulation was not confined to the Ly49H(+) population. At the single-cell level, the production of the five mediators showed strong positive correlation with each other. Interestingly, NK cells were a major source of these five cytokines/chemokines in vitro and in vivo, whereas infected macrophages produced only limited amounts of macrophage-inflammatory protein 1alpha, macrophage-inflammatory protein1beta, and RANTES. These findings suggest that both virus-specific and nonspecific NK cells play crucial roles in activating and directing other inflammatory cells during MCMV infection.     

DAP12 signaling directly augments proproliferative cytokine stimulation of NK cells during viral infections

NK cells vigorously proliferate during viral infections. During the course of murine CMV infection, this response becomes dominated by the preferential proliferation of NK cells that express the activation receptor Ly49H. The factors driving such selective NK cell proliferation have not been characterized. In this study, we demonstrate that preferential NK cell proliferation is dependent on DAP12-mediated signaling following the binding of Ly49H to its virally encoded ligand, m157. Ly49H signaling through DAP12 appears to directly augment NK cell sensitivity to low concentrations of proproliferative cytokines such as IL-15. The impact of Ly49H-mediated signaling on NK cell proliferation is masked in the presence of high concentrations of proproliferative cytokines that nonselectively drive all NK cells to proliferate.