Upregulation of CD94/NKG2A receptors and Qa-1b ligand during murine cytomegalovirus infection of salivary glands

NKG2A is a NK cell exhaustion checkpoint for HCV persistence

Exhaustion of cytotoxic effector natural killer (NK) and CD8⁺ T cells have important functions in the establishment of persistent viral infections, but how exhaustion is induced during chronic hepatitis C virus (HCV) infection remains poorly defined. Here we show, using the humanized C/O^Tg mice permissive for persistent HCV infection, that NK and CD8⁺ T cells become sequentially exhausted shortly after their transient hepatic infiltration and activation in acute HCV infection. HCV infection upregulates Qa-1 expression in hepatocytes, which ligates NKG2A to induce NK cell exhaustion. Antibodies targeting NKG2A or Qa-1 prevents NK exhaustion and promotes NK-dependent HCV clearance. Moreover, reactivated NK cells provide sufficient IFN-γ that helps rejuvenate polyclonal HCV CD8⁺ T cell response and clearance of HCV. Our data thus show that NKG2A serves as a critical checkpoint for HCV-induced NK exhaustion, and that NKG2A blockade sequentially boosts interdependent NK and CD8⁺ T cell functions to prevent persistent HCV infection.

Immune cells may become less responsive, or ‘exhausted’, upon chronic viral infection, but the underlying mechanism and crosstalk are still unclear. Here the authors show that, upon chronic hepatitis C virus (HCV) infection, natural killer cell exhaustion is induced by NKG2A signalling to instruct downstream exhaustion of CD8⁺ T cells and HCV persistence.

Expansion and Protection by a Virus-Specific NK Cell Subset Lacking Expression of the Inhibitory NKR-P1B Receptor during Murine Cytomegalovirus Infection

NK cells play a major role in immune defense against human and murine CMV (MCMV) infection. Although the MCMV genome encodes for MHC class I-homologous decoy ligands for inhibitory NK cell receptors to evade detection, some mouse strains have evolved activating receptors, such as Ly49H, to recognize these ligands and initiate an immune response. In this study, we demonstrate that approximately half of the Ly49H-expressing (Ly49H(+)) NK cells in the spleen and liver of C57BL/6 mice also express the inhibitory NKR-P1B receptor. During MCMV infection, the NKR-P1B(-)Ly49H(+) NK cell subset proliferates to constitute the bulk of the NK cell population. This NK cell subset also confers better protection against MCMV infection compared with the NKR-P1B(+)Ly49H(+) subset. The two populations are composed of cells that differ in their surface expression of receptors such as Ly49C/I and NKG2A/C/E, as well as developmental markers, CD27 and CD11b, and the high-affinity IL-2R (CD25) following infection. Although the NKR-P1B(+) NK cells can produce effector molecules such as IFNs and granzymes, their proliferation is inhibited during infection. A similar phenotype in MCMV-infected Clr-b-deficient mice, which lack the ligand for NKR-P1B, suggests the involvement of ligands other than the host Clr-b. Most interestingly, genetic deficiency of the NKR-P1B, but not Clr-b, results in accelerated virus clearance and recovery from MCMV infection. This study is particularly significant because the mouse NKR-P1B:Clr-b receptor:ligand system represents the closest homolog of the human NKR-P1A:LLT1 system and may have a direct relevance to human CMV infection.

The Attenuated Live Yellow Fever Virus 17D Infects the Thymus and Induces Thymic Transcriptional Modifications of Immunomodulatory Genes in C57BL/6 and BALB/C Mice

Thymus is involved in induction of self-tolerance in T lymphocytes, particularly due to Aire activity. In peripheral tissues, Treg cells and immunomodulatory molecules, like the major histocompatibility complex (MHC) class Ib molecules, are essential for maintenance of autotolerance during immune responses. Viral infections can trigger autoimmunity and modify thymic function, and YFV17D immunization has been associated with the onset of autoimmunity, being contraindicated in patients with thymic disorders. Aiming to study the influence of YFV17D immunization on the transcriptional profiles of immunomodulatory genes in thymus, we evaluated the gene expression of AIRE, FOXP3, H2-Q7 (Qa-2/HLA-G), H2-T23 (Qa-1/HLA-E), H2-Q10, and H2-K1 following immunization with 10,000 LD50 of YFV17D in C57BL/6 and BALB/c mice. The YFV17D virus replicated in thymus and induced the expression of H2-Q7 (Qa-2/HLA-G) and H2-T23 (Qa-1/HLA-E) transcripts and repressed the expression of AIRE and FOXP3. Transcriptional expression varied according to tissue and mouse strain analyzed. Expression of H2-T23 (Qa-1/HLA-E) and FOXP3 was induced in thymus and liver of C57BL/6 mice, which exhibited defective control of viral load, suggesting a higher susceptibility to YFV17D infection. Since the immunization with YFV17D modulated thymus gene expression in genetically predisposed individuals, the vaccine may be related to the onset of autoimmunity disorders.

KLRG1+NKG2A+ CD8 T cells mediate protection and participate in memory responses during γ-herpesvirus infection

Functional CD8 T cell effector and memory responses are generated and maintained during murine γ-herpesvirus 68 (γHV68) persistent infection despite continuous presentation of viral lytic Ags. However, the identity of the CD8 T cell subpopulations that mediate effective recall responses and that can participate in the generation of protective memory to a γ-herpesvirus infection remains unknown. During γHV68 persistence, ∼75% of γHV68-specific CD8 T cells coexpress the NK receptors killer cell lectin-like receptor G1 (KLRG1) and NKG2A. In this study, we take advantage of this unique phenotype to analyze the capacity of CD8 T cells expressing or not expressing KLRG1 and NKG2A to mediate effector and memory responses. Our results show that γHV68-specific KLRG1(+)NKG2A(+) CD8 T cells have an effector memory phenotype as well as characteristics of polyfunctional effector cells such us IFN-γ and TNF-α production, killing capacity, and are more efficient at protecting against a γHV68 challenge than their NKG2A(-)KLRG1(-) counterparts. Nevertheless, γHV68-specific NKG2A(+)KLRG1(+) CD8 T cells express IL-7 and IL-15 receptors, can survive long-term without cognate Ag, and subsequently mount a protective response during antigenic recall. These results highlight the plasticity of the immune system to generate protective effector and proliferative memory responses during virus persistence from a pool of KLRG1(+)NKG2A(+) effector memory CD8 T cells.

Development and function of CD94-deficient natural killer cells

The CD94 transmembrane-anchored glycoprotein forms disulfide-bonded heterodimers with the NKG2A subunit to form an inhibitory receptor or with the NKG2C or NKG2E subunits to assemble a receptor complex with activating DAP12 signaling proteins. CD94 receptors expressed on human and mouse NK cells and T cells have been proposed to be important in NK cell tolerance to self, play an important role in NK cell development, and contribute to NK cell-mediated immunity to certain infections including human cytomegalovirus. We generated a gene-targeted CD94-deficient mouse to understand the role of CD94 receptors in NK cell biology. CD94-deficient NK cells develop normally and efficiently kill NK cell-susceptible targets. Lack of these CD94 receptors does not alter control of mouse cytomegalovirus, lymphocytic choriomeningitis virus, vaccinia virus, or Listeria monocytogenes. Thus, the expression of CD94 and its associated NKG2A, NKG2C, and NKG2E subunits is dispensable for NK cell development, education, and many NK cell functions.

Characterizing the anti-tumor function of adoptively transferred NK cells in vivo

Natural killer (NK) cells represent a promising cell type to utilize for effective adoptive immunotherapy. However, little is known about the important cytolytic molecules and signaling pathways used by NK cells in the adoptive transfer setting. To address this issue, we developed a novel mouse model to investigate the trafficking and mechanism of action of these cells. We demonstrate that methylcholanthrene-induced RKIK sarcoma cells were susceptible to NK cell-mediated lysis in vitro and in vivo following adoptive transfer of NK cells in C57BL/6 RAG-2(-/-)gammac(-/-) mice. Cytotoxic molecules perforin, granzymes B and M as well as the death ligand TRAIL and pro-inflammatory cytokine IFN-gamma were found to be important in the anti-tumor effect mediated by adoptively transferred NK cells. Importantly, we demonstrate that adoptively transferred NK cells could traffic to the tumor site and persisted in vivo which correlated with the anti-tumor effect observed. Overall, the results of this study have important implications for enhancing NK cell-based immunotherapies.

CD4 T-cell help programs a change in CD8 T-cell function enabling effective long-term control of murine gammaherpesvirus 68: role of PD-1-PD-L1 interactions

We previously showed that agonistic antibodies to CD40 could substitute for CD4 T-cell help and prevent reactivation of murine gammaherpesvirus 68 (MHV-68) in the lungs of major histocompatibility complex (MHC) class II(-/-) (CII(-/-)) mice, which are CD4 T cell deficient. Although CD8 T cells were required for this effect, no change in their activity was detected in vitro. A key question was whether anti-CD40 treatment (or CD4 T-cell help) changed the function of CD8 T cells or another cell type in vivo. To address this question, in the present study, we showed that adoptive transfer of CD8 T cells from virus-infected wild-type mice or anti-CD40-treated CII(-/-) mice caused a significant reduction in lung viral titers, in contrast to those from control CII(-/-) mice. Anti-CD40 treatment also greatly prolonged survival of infected CII(-/-) mice. This confirms that costimulatory signals cause a change in CD8 T cells enabling them to maintain effective long-term control of MHV-68. We investigated the nature of this change and found that expression of the inhibitory receptor PD-1 was significantly increased on CD8 T cells in the lungs of MHV-68-infected CII(-/-), CD40(-/-), or CD80/86(-/-) mice, compared with that in wild-type or CD28/CTLA4(-/-) mice, correlating with the level of viral reactivation. Furthermore, blocking PD-1-PD-L1 interactions significantly reduced viral reactivation in CD4 T-cell-deficient mice. In contrast, the absence of another inhibitory receptor, NKG2A, had no effect. These data suggest that CD4 T-cell help programs a change in CD8 T-cell function mediated by altered PD-1 expression, which enables effective long-term control of MHV-68.

Impaired immune response in severe human lower tract respiratory infection by respiratory syncytial virus

BACKGROUND

Respiratory syncytial virus (RSV) is a major cause of acute lower respiratory infection in infants. The immune response plays a leading role in the severity of the disease. We hypothesized that severe RSV disease is associated with an impaired immune response characterized by low circulating T lymphocytes and plasma cytokine concentrations.

METHODS

We evaluate the in vivo immune responses of previously healthy infants with their first proven RSV-acute lower respiratory infection that required hospitalization. According to the clinical severity, defined by using a strict scoring system, the in vivo immune response was compared through the analysis of plasma cytokine values and the phenotyping of peripheral blood lymphocyte and natural killer (NK) cells.

RESULTS

Absolute blood cell counts of CD4+, CD8+, and CD19+ lymphocytes and NK cells were lower in subjects with RSV than in control infants. Lowest cell counts were observed in more severe RSV-infected infants. Significant low values were obtained in CD8+ lymphocytes (P = 0.03) and nonactive NK cells, that express CD94 antigen (P = 0.046). In contrast, activated NK cells that do not express CD94 molecules were significantly higher in RSV infected infants than in healthy controls (% of cells: P = 0.004). The interferon-gamma and tumor necrosis factor-alpha values in RSV infected patients were lower than in controls subjects. Interleukin-17 cytokine was not detected in healthy infants and the largest concentration was found in moderately ill patients as compared with severe cases (P = 0.033). RSV infection showed significantly higher interleukin-8 chemokine than in control infants (P = 0.024).

CONCLUSION

We propose that severe RSV infection in very young infants is associated with poor blood proinflammatory cytokine production, low counts of CD8+ T cells and with a greater activity of a group of NK cells, that are independent of the major histocompatibility complex class Ib recognition system.

Activation of intracellular signaling pathways by the murine cytomegalovirus G protein-coupled receptor M33 occurs via PLC-{beta}/PKC-dependent and -independent mechanisms

The presence of numerous G protein-coupled receptor (GPCR) homologs within the herpesvirus genomes suggests an essential role for these genes in viral replication in the infected host. Such is the case for murine cytomegalovirus (MCMV), where deletion of the M33 GPCR or replacement of M33 with a signaling defective mutant has been shown to severely attenuate replication in vivo. In the present study we utilized a genetically altered version of M33 (termed R131A) in combination with pharmacological inhibitors to further characterize the mechanisms by which M33 activates downstream signaling pathways. This R131A mutant of M33 fails to support salivary gland replication in vivo and, as such, is an important tool that can be used to examine the signaling activities of M33. We show that M33 stimulates the transcription factor CREB via heterotrimeric G(q/11) proteins and not through promiscuous coupling of M33 to the G(s) pathway. Using inhibitors of signaling molecules downstream of G(q/11), we demonstrate that M33 stimulates CREB transcriptional activity in a phospholipase C-beta and protein kinase C (PKC)-dependent manner. Finally, utilizing wild-type and R131A versions of M33, we show that M33-mediated activation of other signaling nodes, including the mitogen-activated protein kinase family member p38alpha and transcription factor NF-kappaB, occurs in the absence of G(q/11) and PKC signaling. The results from the present study indicate that M33 utilizes multiple mechanisms to modulate intracellular signaling cascades and suggest that signaling through PLC-beta and PKC plays a central role in MCMV pathogenesis in vivo.

The salivary glands as a privileged site of cytomegalovirus immune evasion and persistence

The salivary glands (SG) provide a haven for persistent cytomegalovirus replication, and in this regard are a privileged site of virus immune evasion. The murine cytomegalovirus (MCMV) model has provided insight into the immunological environment of the SG and the unqiue virus-host relationship of this organ. In response to MCMV infection, a robust T cell-mediated immune response is elicited, comprised predominantly of CD8+ T cells that phenotypically and functionally appear activated. However, they fail to clear virus by an unknown evasion mechanism that is independent of inhibitory NKG2A- or Programmed Death 1-mediated signaling. Virus is eventually eliminated from the SG by effector CD4+ T cells expressing antiviral cytokines. However, this mechanism is severely dampened by high levels of the immunosuppressive cytokine IL-10, selectively expressed by SG CD4+ T cells.

Cutting edge: engagement of NKG2A on CD8+ effector T cells limits immunopathology in influenza pneumonia

Influenza pneumonia results in considerable lung injury, a significant component of which is mediated by CD8+ T cell Ag recognition in the distal airways and alveoli. TNF-alpha produced by Ag-specific CD8+ T cells appears primarily responsible for this immunopathology, and we have examined the negative regulation of CD8+ TNF production by CD94/NKG2A engagement with its receptor, Qa-1b. TNF production by antiviral CD8+ T cells was significantly enhanced by NKG2A blockade in vitro, and mice deficient in the NKG2A ligand, Qa-1b, manifested significantly greater pulmonary pathology upon CD8+ T cell-mediated clearance in influenza pneumonia. Furthermore, blockade of NKG2A ligation resulted in the enhancement of lung injury induced by CD8+ effector cell recognition of alveolar Ag in vivo in the absence of infectious virus. These data demonstrate that CD94/NKG2A transduces a biologically important signal in vivo to activated CD8+ T cells that limits immunopathology in severe influenza infection.