CD94/NKG2 expression does not inhibit cytotoxic function of lymphocytic choriomeningitis virus-specific CD8+ T cells

LAG-3 sustains TOX expression and regulates the CD94/NKG2-Qa-1b axis to govern exhausted CD8 T cell NK receptor expression and cytotoxicity

Exhausted CD8 T (Tex) cells in chronic viral infection and cancer have sustained co-expression of inhibitory receptors (IRs). Tex cells can be reinvigorated by blocking IRs, such as PD-1, but synergistic reinvigoration and enhanced disease control can be achieved by co-targeting multiple IRs including PD-1 and LAG-3. To dissect the molecular changes intrinsic when these IR pathways are disrupted, we investigated the impact of loss of PD-1 and/or LAG-3 on Tex cells during chronic infection. These analyses revealed distinct roles of PD-1 and LAG-3 in regulating Tex cell proliferation and effector functions, respectively. Moreover, these studies identified an essential role for LAG-3 in sustaining TOX and Tex cell durability as well as a LAG-3-dependent circuit that generated a CD94/NKG2+ subset of Tex cells with enhanced cytotoxicity mediated by recognition of the stress ligand Qa-1b, with similar observations in humans. These analyses disentangle the non-redundant mechanisms of PD-1 and LAG-3 and their synergy in regulating Tex cells.

Preclinical evaluation of two phylogenetically distant arenavirus vectors for the development of novel immunotherapeutic combination strategies for cancer treatment

BACKGROUND

Engineered arenavirus vectors have recently been developed to leverage the body's immune system in the fight against chronic viral infections and cancer. Vectors based on Pichinde virus (artPICV) and lymphocytic choriomeningitis virus (artLCMV) encoding a non-oncogenic fusion protein of human papillomavirus (HPV)16 E6 and E7 are currently being tested in patients with HPV16+ cancer, showing a favorable safety and tolerability profile and unprecedented expansion of tumor-specific CD8+ T cells. Although the strong antigen-specific immune response elicited by artLCMV vectors has been demonstrated in several preclinical models, PICV-based vectors are much less characterized.

METHODS

To advance our understanding of the immunobiology of these two vectors, we analyzed and compared their individual properties in preclinical in vivo and in vitro systems. Immunogenicity and antitumor effect of intratumoral or intravenous administration of both vectors, as well as combination with NKG2A blockade, were evaluated in naïve or TC-1 mouse tumor models. Flow cytometry, Nanostring, and histology analysis were performed to characterize the tumor microenvironment (TME) and T-cell infiltrate following treatment.

RESULTS

Despite being phylogenetically distant, both vectors shared many properties, including preferential infection and activation of professional antigen-presenting cells, and induction of potent tumor-specific CD8+ T-cell responses. Systemic as well as localized treatment induced a proinflammatory shift in the TME, promoting the infiltration of inducible T cell costimulator (ICOS)+CD8+ T cells capable of mediating tumor regression and prolonging survival in a TC-1 mouse tumor model. Still, there was evidence of immunosuppression built-up over time, and increased expression of H2-T23 (ligand for NKG2A T cell inhibitory receptor) following treatment was identified as a potential contributing factor. NKG2A blockade improved the antitumor efficacy of artARENA vectors, suggesting a promising new combination approach. This demonstrates how detailed characterization of arenavirus vector-induced immune responses and TME modulation can inform novel combination therapies.

CONCLUSIONS

The artARENA platform represents a strong therapeutic vaccine approach for the treatment of cancer. The induced antitumor immune response builds the backbone for novel combination therapies, which warrant further investigation.

Cryptic MHC-E epitope from influenza elicits a potent cytolytic T cell response

The extent to which unconventional forms of antigen presentation drive T cell immunity is unknown. By convention, CD8 T cells recognize viral peptides, or epitopes, in association with classical major histocompatibility complex (MHC) class I, or MHC-Ia, but immune surveillance can, in some cases, be directed against peptides presented by nonclassical MHC-Ib, in particular the MHC-E proteins (Qa-1 in mice and HLA-E in humans); however, the overall importance of nonclassical responses in antiviral immunity remains unclear. Similarly uncertain is the importance of 'cryptic' viral epitopes, defined as those undetectable by conventional mapping techniques. Here we used an immunopeptidomic approach to search for unconventional epitopes that drive T cell responses in mice infected with influenza virus A/Puerto Rico/8/1934. We identified a nine amino acid epitope, termed M-SL9, that drives a co-immunodominant, cytolytic CD8 T cell response that is unconventional in two major ways: first, it is presented by Qa-1, and second, it has a cryptic origin, mapping to an unannotated alternative reading frame product of the influenza matrix gene segment. Presentation and immunogenicity of M-SL9 are dependent on the second AUG codon of the positive sense matrix RNA segment, suggesting translation initiation by leaky ribosomal scanning. During influenza virus A/Puerto Rico/8/1934 infection, M-SL9-specific T cells exhibit a low level of egress from the lungs and strong differentiation into tissue-resident memory cells. Importantly, we show that M-SL9/Qa-1-specific T cells can be strongly induced by messenger RNA vaccination and that they can mediate antigen-specific cytolysis in vivo. Our results demonstrate that noncanonical translation products can account for an important fraction of the T cell repertoire and add to a growing body of evidence that MHC-E-restricted T cells could have substantial therapeutic value.

A molecular signature of lung-resident CD8+ T cells elicited by subunit vaccination

Natural infection as well as vaccination with live or attenuated viruses elicit tissue resident, CD8+ memory T cell (Trm) response. Trm cells so elicited act quickly upon reencounter with the priming agent to protect the host. These Trm cells express a unique molecular signature driven by the master regulators-Runx3 and Hobit. We previously reported that intranasal instillation of a subunit vaccine in a prime boost vaccination regimen installed quick-acting, CD8+ Trm cells in the lungs that protected against lethal vaccinia virus challenge. It remains unexplored whether CD8+ Trm responses so elicited are driven by a similar molecular signature as those elicited by microbes in a real infection or by live, attenuated pathogens in conventional vaccination. We found that distinct molecular signatures distinguished subunit vaccine-elicited lung interstitial CD8+ Trm cells from subunit vaccine-elicited CD8+ effector memory and splenic memory T cells. Nonetheless, the transcriptome signature of subunit vaccine elicited CD8+ Trm resembled those elicited by virus infection or vaccination. Clues to the basis of tissue residence and function of vaccine specific CD8+ Trm cells were found in transcripts that code for chemokines and chemokine receptors, purinergic receptors, and adhesins when compared to CD8+ effector and splenic memory T cells. Our findings inform the utility of protein-based subunit vaccination for installing CD8+ Trm cells in the lungs to protect against respiratory infectious diseases that plague humankind.

Implications of NKG2A in immunity and immune-mediated diseases

In recent studies, NKG2A is revealed to be a key immune checkpoint for both natural killer (NK) cells and CD8⁺ T cells. It form heterodimer receptors with CD94, and targets the peptide-presenting human leukocyte antigen-E (HLA-E) molecules. Upon crosslinking, NKG2A/CD94 delivers inhibitory signals for NK cells and CD8⁺ T cells, while blocking NKG2A can effectively unleash functions of these cytotoxic lymphocytes. The interaction between NKG2A and HLA-E contributes to tumor immune escape, and NKG2A-mediated mechanisms are currently being exploited to develop potential antitumor therapeutic strategies. In addition, growing evidence shows that NKG2A also plays important roles in other immune-related diseases including viral infections, autoimmune diseases, inflammatory diseases, parasite infections and transplant rejection. Therefore, the current work focuses on describing the effect of NKG2A on immune regulation and exploring its potential role in immune-mediated disorders.

Role of NKG2a/c+CD8+ T cells in pathogenic versus non-pathogenic SIV infections

Some viruses have established an equilibrium with their host. African green monkeys (AGM) display persistent high viral replication in the blood and intestine during Simian immunodeficiency virus (SIV) infection but resolve systemic inflammation after acute infection and lack intestinal immune or tissue damage during chronic infection. We show that NKG2_a/c⁺CD8⁺ T cells increase in the blood and intestine of AGM in response to SIVagm infection in contrast to SIVmac infection in macaques, the latter modeling HIV infection. NKG2_a/c⁺CD8⁺ T cells were not expanded in lymph nodes, and CXCR5⁺NKG2_a/c⁺CD8⁺ T cell frequencies further decreased after SIV infection. Genome-wide transcriptome analysis of NKG2_a/c⁺CD8⁺ T cells from AGM revealed the expression of NK cell receptors, and of molecules with cytotoxic effector, gut homing, and immunoregulatory and gut barrier function, including CD73. NKG2_a/c⁺CD8⁺ T cells correlated negatively with IL-23 in the intestine during SIVmac infection. The data suggest a potential regulatory role of NKG2_a/c⁺CD8⁺ T cells in intestinal inflammation during SIV/HIV infections.

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Molecular determination of NKG2a/c⁺CD8⁺ T cells in two species of nonhuman primates

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Tissue distribution of NKG2a/c⁺CD8⁺ T cell is profoundly sculpted by SIV infections

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Intestinal NKG2a/c⁺CD8⁺ T cells correlated negatively with IL-23 in SIV infection

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NKG2a/c⁺CD8⁺ T cells might play a protective gut barrier function in HIV/SIV infection

The Inhibitory Receptor NKG2A Sustains Virus-Specific CD8⁺ T Cells in Response to a Lethal Poxvirus Infection

CD8(+) T cells and NK cells protect from viral infections by killing virally infected cells and secreting interferon-γ. Several inhibitory receptors limit the magnitude and duration of these anti-viral responses. NKG2A, which is encoded by Klrc1, is a lectin-like inhibitory receptor that is expressed as a heterodimer with CD94 on NK cells and activated CD8(+) T cells. Previous studies on the impact of CD94/NKG2A heterodimers on anti-viral responses have yielded contrasting results and the in vivo function of NKG2A remains unclear. Here, we generated Klrc1(-/-) mice and found that NKG2A is selectively required for resistance to ectromelia virus (ECTV). NKG2A functions intrinsically within ECTV-specific CD8(+) T cells to limit excessive activation, prevent apoptosis, and preserve the specific CD8(+) T cell response. Thus, although inhibitory receptors often cause T cell exhaustion and viral spreading during chronic viral infections, NKG2A optimizes CD8(+) T cell responses during an acute poxvirus infection.

Calcineurin-dependent negative regulation of CD94/NKG2A expression on naive CD8+ T cells

Immune responses lead to expression of immunoregulatory molecules on T cells, including natural killer (NK) receptors, such as CD94/NKG2A on CD8+ T cells; these receptors restrain CD8+ responses, thereby preventing T-cell exhaustion in chronic infections and limiting immunopathology. Here, we examined the requirements for inducing CD94/NKG2A on T cells responding to antigen. In vitro, moderate induction of CD94/NKG2A expression occurred after exposure of naive CD8+ (but not CD4+) cells to CD3 ligation or specific peptide. Surprisingly, expression was inhibited by CD28/B7 costimulation. Such inhibition applied only to CD94/NKG2A and not other NK receptors (NKG2D) and was mediated by IL-2. Inhibition by IL-2 occurred via a NFAT cell-independent component of the calcineurin pathway, and CD94/NKG2A induction was markedly enhanced in the presence of calcineurin blockers, such as FK506 or using calcineurin-deficient T cells, both in vitro and in vivo. In addition to CD28-dependent inhibition by IL-2, CD94/NKG2A expression was impaired by several other cytokines (IL-4, IL-23, and transforming growth factor-β) but enhanced by others (IL-6, IL-10, and IL-21). The complex interplay between these various stimuli may account for the variable expression of CD94/NKG2A during responses to different pathogens in vivo.

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.

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.

Differential microenvironment localization of effector and memory CD8 T cells

CD8 T cells are critical for the clearance of intracellular pathogens. Upon infection, naive CD8 T cells differentiate into effector cells that target and eliminate infected cells. Following clearance of the pathogen, most effector cells die, although a small fraction survives to establish a memory population. Subsequent exposure to the same pathogen induces a rapid response of memory T cells and efficient elimination of the pathogen. Although much is known about the CD8 T cell response, the precise microenvironment location of effector and memory CD8 T cells in secondary lymphoid organs is not well characterized. In this study, we present an in situ analysis of the localization of effector and memory CD8 T cells during the murine immune response to lymphocytic choriomenginits virus. We identified the location of these cells using a transgenic mouse model system in which CD8 T cells are irreversibly tagged with yellow fluorescent protein (YFP) after activation. After infection, YFP+ CD8 T cells were initially observed within T cell zones. Later, these cells were found in the red pulp and a disruption of all CD8 T cell zones was observed. After resolution of the immune response, YFP+ memory CD8 T cells were observed primarily in T cells zones. Thus, in the spleens of mice, effector CD8 T cells localize to the red pulp and memory CD8 T cells localize to the T cell zones. Upon rechallenge, memory CD8 T cells rapidly proliferate and the secondary effector CD8 T cells are found in the red pulp.

Features and distribution of CD8 T cells with human leukocyte antigen class I-specific receptor expression in chronic hepatitis C

CD8(+) T cells represent a sizable component of the liver inflammatory infiltrate in chronic hepatitis C and are thought to contribute to immune-mediated tissue injury. Because chronic stimulation may promote the expression by CD8(+) T cells of distinct human leukocyte antigen class I-specific natural killer cell receptors (NKRs) susceptible to both inhibiting effector functions and promoting cell survival, we examined the distribution and characteristics of CD8(+) T cells with such receptors in chronic hepatitis C patients. NKR CD8(+) T cells were detectable in liver and peripheral blood from hepatitis C virus (HCV)-infected patients but were not major subsets. However, the frequency of NKG2A(+) CD8(+) in the liver and in a lesser extent in the peripheral blood was positively correlated to histological activity in HCV-infected patients. No such correlation was found with KIR(+) T cells in liver in HCV-infected patients and with the both NKR CD8(+) T cells in hepatitis B virus (HBV) infected patients. Circulating CD8(+) T cells expressing KIRs exhibited phenotypic features of memory T cells with exacerbated expression of the senescence marker CD57 in patients. NKG2A(+)CD8(+) T cells were committed T cells that appeared less differentiated than KIR(+)CD8(+) T cells. In HCV-infected patients, their content in perforin was low and similar to that observed in NKG2A(-)CD8(+) T cells; this scenario was not observed in healthy subjects and HBV-infected patients. Both NKG2A and KIRs could inhibit the response of HCV-specific CD8(+) T cells ex vivo.

CONCLUSION

These results support the concept that an accumulation in the liver parenchyma of NKR(+)CD8(+) T cells that have functional alterations could be responsible for liver lesions. They provide novel insights into the complexity of liver-infiltrating CD8(+) T cells in chronic hepatitis C and reveal that distinct subsets of antigen-experienced CD8(+) T cells are differentially sensitive to the pervasive influence of HCV.

Allogeneic differences in the dependence on CD4+ T-cell help for virus-specific CD8+ T-cell differentiation

CD4(+) T-cell help enables antiviral CD8(+) T cells to differentiate into fully competent memory cells and sustains CD8(+) T-cell-mediated immunity during persistent virus infection. We recently reported that mice of C57BL/6 and C3H strains differ in their dependence on CD28 and CD40L costimulation for long-term control of infection by polyoma virus, a persistent mouse pathogen. In this study, we asked whether mice of these inbred strains also vary in their requirement for CD4(+) T-cell help for generating and maintaining polyoma virus-specific CD8(+) T cells. CD4(+) T-cell-depleted C57BL/6 mice mounted a robust antiviral CD8(+) T-cell response during acute infection, whereas unhelped CD8(+) T-cell effectors in C3H mice were functionally impaired during acute infection and failed to expand upon antigenic challenge during persistent infection. Using (C57BL/6 x C3H)F(1) mice, we found that the dispensability for CD4(+) T-cell help for the H-2(b)-restricted polyoma virus-specific CD8(+) T-cell response during acute infection extends to the H-2(k)-restricted antiviral CD8(+) T cells. Our findings demonstrate that dependence on CD4(+) T-cell help for antiviral CD8(+) T-cell effector differentiation can vary among allogeneic strains of inbred mice.

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

Following acute infection, murine cytomegalovirus (MCMV) replicates persistently in the salivary glands, despite the vigorous response of activated CD8 T cells that infiltrate this gland. Virus-specific CD8 T lymphocytes isolated from this organ were found to express the inhibitory CD94/NKG2A receptor that, in some virus models, confers an inhibitory response to cytotoxic T lymphocytes (CTLs). In response to MCMV infection, expression of the CD94/NKG2A ligand, Qa-1b, increased dramatically in the submandibular gland (SMG) prior to upregulation of H-2Dd. However, there was no net negative impact on virus-specific T-cell function, as virus titres were similar in CD94- and CD94+ mice. CD94/NKG2A expression, also known to inhibit apoptosis, did not influence the numbers of accumulated T, NK and NK T cells. These data indicate that expression of inhibitory CD94/NKG2A receptors does not account for the failure of MCMV-specific CTLs to clear the SMG of infection.

Natural Regulation of Immunity to Minor Histocompatibility Antigens

MHC-matched hemopoietic stem cell transplantation is commonly used for the treatment of some forms of leukemia. Conditioning regimens before transplant act to reduce the burden of leukemic cells and the graft-vs-leukemia (GvL) effect can eliminate residual disease. The GvL effect results largely from the recognition of minor histocompatibility Ags by donor T cells on recipient tissues. These Ags are generally widely expressed and also provoke graft-vs-host (GvH) disease. Manipulation of immunity to promote GvL while curtailing GvH would greatly improve clinical outcome. To develop strategies that may achieve this, the parameters which control immunity to minor histocompatibility Ags need to be defined. In this study, we have analyzed responses to the mouse HY minor histocompatibility Ag using hemopoietic cell and skin grafts as surrogate GvL and GvH targets, respectively. We show that natural regulation of CD8 T cell responses to HY operates at multiple levels. First, CD4 T cell help is required for primary CD8 responses directed at hemopoietic cells. However, although CD4 T cells of H2(k) mouse strains recognize HY, they provide ineffective help associated with a proportion of recipients developing tolerance. This was further investigated using TCR-transgenic mice which revealed H2(k)-restricted HY-specific CD4 T cells are highly susceptible to regulation by CD25(+) regulatory T cells which expand in tolerant recipients. A second level of regulation, operating in the context of skin grafts, involves direct inhibition of CD8 T cell responses by CD94/NKG2 engagement of the nonclassical MHC class I molecule Qa1.

The CD94/NKG2 family of receptors: from molecules and cells to clinical relevance

Immune responses must be tightly regulated to avoid hyporesponsiveness on one hand or excessive inflammation and the development of autoimmunity (hyperresponsiveness) on the other hand. This balance is attained through the throttling of activating signals by inhibitory signals that ideally leads to an adequate immune response against an invader without excessive and extended inflammatory signals that promote the development of autoimmunity. The CD94/NKG2 family of receptors is composed of members with activating or inhibitory potential. These receptors are expressed predominantly on NK cells and a subset of CD8+ T cells, and they have been shown to play an important role in regulating responses against infected and tumorigenic cells. In this review, we discuss the current knowledge about this family of receptors, including ligand and receptor interaction, signaling, membrane dynamics, regulation of gene expression and their roles in disease regulation, infections, and cancer, and bone marrow transplantation.

CD94/NKG2A expression is associated with proliferative potential of CD8 T cells during persistent polyoma virus infection

Memory CD8 T cells comprise a critical component of durable immunity because of their capacity to rapidly proliferate and exert effector activity upon Ag rechallenge. During persistent viral infection, memory CD8 T cells repetitively encounter viral Ag and must maintain a delicate balance between limiting viral replication and minimizing immunopathology. In mice infected by polyoma virus, a natural mouse pathogen that establishes long-term persistent infection, the majority of persistence-phase antiviral CD8 T cells express the inhibitory NK cell receptor CD94/NKG2A. In this study, we asked whether CD94/NKG2A expression is associated with Ag-specific recall of polyoma virus-specific CD8 T cells. During the persistent phase of infection, polyoma virus-specific CD8 T cells that express CD94/NKG2A were found to preferentially proliferate; this proliferation was dependent on cognate Ag both in vitro and in vivo. In addition, CD94/NKG2A(+) polyoma-specific CD8 T cells have a markedly enhanced capacity to produce IL-2 upon ex vivo Ag stimulation compared with CD94/NKG2A(-) polyoma-specific CD8 T cells. Importantly, CD94/NKG2A(+) anti-polyoma virus CD8 T cells appear to be essential for Ag-specific recall responses in mice persistently infected by polyoma virus. Because of its higher proliferative potential and capacity to produce IL-2, we propose that the CD94/NKG2A(+) subpopulation represents a less differentiated state than the CD94/NKG2A(-) subpopulation. Identification of proliferation-competent subpopulations of memory CD8 T cells should prove valuable in designing therapeutic vaccination strategies for persistent viral infections.

Qa-1b and CD94-NKG2a interaction regulate cytolytic activity of herpes simplex virus-specific memory CD8+ T cells in the latently infected trigeminal ganglia

After ocular infection, HSV-specific CD8+ T cells migrate to and are specifically retained in the ophthalmic branch of the trigeminal ganglia (TG) even at the time when replicating virus is no longer evident. Virus-specific CD8+ T cells maintain an activation phenotype and secrete IFN-gamma in the latent TG. In this report we demonstrated that activated virus-specific memory CD8+ T cells, although potentially cytolytic, also expressed the CD94-NK cell receptor subfamily G2a inhibitory molecule and were unable to exert cytotoxicity when engaged by Qa-1b expressing targets. Interestingly, many neurons in the latent TG expressed Qa-1b, and blocking of Qa-1b/CD94-NKG2a interaction in an ex vivo TG culture resulted in neuronal cell lysis. The expression of the inhibitory CD94-NKG2a molecule could be induced by TGF-beta1, which was shown to present as a bioactive molecule in the latent TG. Additionally, CD4+ forkhead/winged helix transcription factor 3+ T cells were also determined in the latent TG. Our results demonstrate the operation of a regulatory system in vivo that serves to protect irreplaceable neurons from destruction by the immune system.

CD8brightCD56+ T cells are cytotoxic effectors in patients with active Behcet's uveitis

Behçet's uveitis, characterized by chronic recurrent uveitis and obliterating retinal vasculitis, frequently causes bilateral blindness. Intraocular infiltration of TCRalphabeta+CD8brightCD56+ cells was a distinct feature in Behçet's uveitis. However, phenotypic natures and effector functions of the cells have remained elusive. This study was conducted to determine phenotypic and functional characteristics and cytotoxic mechanisms of CD8brightCD56+ T cells in Behçet's uveitis. CD11b+CD27-CD62L- phenotypes of CD8brightCD56+ T cells were increased in patients with active Behçet's uveitis compared with inactive Behcet's patients and normal controls. Interestingly, CD45RAdimCD45RO- phenotypes were expanded, and CD94 expression was markedly up-regulated in contrast to the down-regulation of NKG2D. Furthermore, these subsets were polarized to produce IFN-gamma and contained high amounts of preformed intracellular perforin while exclusively expressing surface FasL upon PI stimulation. Moreover, the cytolytic functions of freshly isolated CD8brightCD56+ T cells were up-regulated against both K562 (NK-sensitive) and Raji (NK-resistant) cells, which were effectively inhibited by perforin inhibitor (concanamycin A). Their cytolytic activity against HUVECs was also increased and was effectively suppressed by Fas ligand inhibitor (brefeldin A) and partly by perforin inhibitor. Furthermore, cytolytic functions of PMA and ionomycin-stimulated CD8brightCD56+ T cells against HUVECs were greatly enhanced, by pretreatment of recombinant human IFN-gamma on HUVECs. Therefore, CD8brightCD56+ T cells in Behçet's uveitis are characterized by cytotoxic effector phenotypes with functional NK receptors and function as strong cytotoxic effectors through both Fas ligand-dependent and perforin-dependent pathways.

Amelioration of acute graft-versus-host disease by NKG2A engagement on donor T cells

Acute graft-versus-host disease (aGVHD) remains a major complication of allogeneic bone marrow transplantation, which is caused by donor T cells specific for host alloantigens. In a murine model, we found that donor T cells expressed a natural killer cell inhibitory receptor, CD94/NKG2A, during the course of aGVHD. Administration of an anti-NKG2A mAb markedly inhibited the expansion of donor T cells and ameliorated the aGVHD pathologies. These results suggested that the CD94/NKG2A inhibitory receptor expressed on host-reactive donor T cells can be a novel target for the amelioration of aGVHD.

NK cell recognition

The integrated processing of signals transduced by activating and inhibitory cell surface receptors regulates NK cell effector functions. Here, I review the structure, function, and ligand specificity of the receptors responsible for NK cell recognition.

The role of TCR stimulation and TGF-beta in controlling the expression of CD94/NKG2A receptors on CD8 T cells

Following antigen recognition, murine CD8 T cells express CD94/NKG2A receptors. Our results show that this up-regulation occurs rapidly in vitro and is accompanied by an approximately 8-fold increase in CD94 and approximately 125-fold increase in NKG2A mRNA. In contrast, only a twofold increase in NKG2C mRNA is noted. The addition of TGF-beta, but not IL-10, IL-12 or IL-15, leads to a further increase in cell membrane expression of these receptors, as well as a approximately 6-fold increase in mRNA for both chains. TGF-beta also increases CD94/NKG2A expression on memory CD8 T cells that are re-exposed to antigen. The effect of TGF-beta on increasing CD94/NKG2A expression on both naive and memory CD8 T cells occurs only when there is a concurrent stimulation through the TCR. In contrast, TGF-beta does not increase expression of CD94/NKG2A on resting or activated NK cells. We also show by using purified CD8 T cells, that TGF-beta acts directly on these cells. These results implicate a role for both antigen and TGF-beta in increasing expression of inhibitory CD94/NKG2A receptors on CD8 T cells.

The role of CD94/NKG2 in innate and adaptive immunity

CD94/NKG2 is a heterodimer expressed on natural killer (NK) and a small subset of T cells. This receptor varies in function as an inhibitor or activator depending on which isoform of NKG2 is expressed. The ligand for CD94/NKG2 is HLA-E in human and its homolog, Qa1 in mouse, which are both nonclassical class I molecules that bind leader peptides from other class I molecules. Although <5% of CD8 T cells express the receptor in a naïve mouse, its expression is upregulated upon specific recognition of antigen. Similar to NK cells, most CD8 T cells that express high levels of CD94 co-express NKG2A, the inhibitory isoform. The engagement of this receptor can lead to a blocking of cytotoxicity. However, these receptors have also been implicated in the cell survival of both NK and CD8 T cells. The level of CD94 expression is inversely correlated with the level of apoptosis in culture. Thus, CD94/NKG2 receptors may regulate effector functions and cell survival of NK cells and CD8 T cells, thereby playing a crucial role in the innate and adaptive immune response to a pathogen.

Activating CD94:NKG2C and inhibitory CD94:NKG2A receptors are expressed by distinct subsets of committed CD8+ TCR alphabeta lymphocytes

A subset of CD8(+) T cells express the natural killer cell receptors CD94:NKG2A or CD94:NKG2C. We found that although many CD8(+) T cells transcribe CD94 and NKG2C, expression of a functional CD94:NKG2C receptor is restricted to highly differentiated effector cells. CD94:NKG2A is expressed by a different subset consisting of CCR7(+) memory cells and CCR7(-) effector cells. Since NKG2A can only be induced on naive CD8(+) T cells while CD94(-) memory cells are refractory, it is likely that commitment to the CD94:NKG2A(+) subset occurs during the first encounter with antigen. CCR7(+)CD94:NKG2A(+) T cells recirculate through lymph nodes where upon activation, they produce large quantities of IFN-gamma. These cells occur as a separate CD94:NKG2A(+) T cell lineage with a distinct TCR repertoire that differs from that of the other CD8(+)CD94(-) T cells activated in situ.

The regulation of lymphocyte activation by inhibitory receptors

The ability of activating immune recognition receptors on lymphocytes to regulate cellular activation and function can be profoundly altered by co-stimulation with inhibitory receptors. Inhibitory receptors, such as the MHC-recognizing inhibitory receptors expressed on NK cells and subpopulations of activated T cells, can fully block the generation of any cytotoxic function by targeting proximal signals. Inhibitory Fc receptors on B cells, macrophages and mast cells can influence their threshold for activation, but the induction of inhibitory Fc receptors also appears to play a major role in the attenuation of ongoing responses. The three identified groups of inhibitory B7-recognizing receptors (CTLA-4, PD-1 and BTLA) are only expressed on activated hematopoietic cells, thus exclusively regulating ongoing immune responses in lymphoid organs and the periphery. In each case, the integrated positive and negative regulatory events determine the nature of the functional response.

Pathogen-host standoff: immunity to polyomavirus infection and neoplasia

Polyomaviruses establish persistent infection in a variety of hosts, including humans, where they pose an oncogenic threat under conditions of depressed immune function. Control of persistent infection by these DNA tumor viruses requires continuous immunosurveillance by functionally competent antiviral CD8+ T cells. Repetitive antigen encounter by these T cells, however, often leads to their deletion or inactivation. Elucidation of the in vivo mechanisms that sustain antigen-specific CD8+ T-cell effector activity in the face of persistent antigen is essential for devising immunotherapeutic strategies against viral oncogenesis.

KLRL1, a novel killer cell lectinlike receptor, inhibits natural killer cell cytotoxicity

Natural killer (NK) cell inhibitory receptors play important roles in the regulation of target susceptibility to natural killing. Here, we report the molecular cloning and functional characterization of a novel NK cell receptor, KLRL1, from human and mouse dendritic cells. KLRL1 is a type II transmembrane protein with an immunoreceptor tyrosine-based inhibitory motif and a C-type lectinlike domain. The KLRL1 gene is located in the central region of the NK gene complex in both humans and mice, on human chromosome 12p13 and mouse chromosome 6F3, adjacent to the other KLR genes. KLRL1 is preferentially expressed in lymphoid tissues and immune cells, including NK cells, T cells, dendritic cells, and monocytes or macrophages. Western blot and fluorescence confocal microscopy analyses indicated that KLRL1 is a membrane-associated glycoprotein, which forms a heterodimer with an as yet unidentified partner. Human and mouse KLRL1 are both predicted to contain putative immunoreceptor tyrosine-based inhibitory motifs (ITIMs), and immunoprecipitation experiments demonstrated that KLRL1 associates with the tyrosine phosphatases SHP-1 (SH2-domain-containing protein tyrosine phosphatase 1) and SHP-2. Consistent with its potential inhibitory function, pretreatment of target cells with human KLRL1-Fc fusion protein enhances NK-mediated cytotoxicity. Taken together, our results demonstrate that KLRL1 belongs to the KLR family and is a novel inhibitory NK cell receptor.

Murine cytomegalovirus interference with antigen presentation has little effect on the size or the effector memory phenotype of the CD8 T cell response

As with most herpesviruses, CMVs encode viral genes that inhibit Ag presentation to CD8 T cells (VIPRs). VIPR function has been assumed to be essential for CMV to establish its characteristic lifetime infection of its host. We compared infection of C57BL/6 mice with wild-type murine CMV (MCMV) and a virus lacking each of MCMV's three known VIPRs: m4, m6, and m152. During acute infection, there was very little difference between the two viruses with respect to the kinetics of viral replication and clearance, or in the size and kinetics of the virus-specific CD8 T cell response. During chronic infection, a large, effector memory, virus-specific CD8 T cell population (CD8(low)CD62L(-)CD11c(+)NKG2A(+)) was maintained in both infections; the size and phenotype of the CD8 T cell response to both viruses was remarkably similar. The characteristic effector memory phenotype of the CD8 T cells suggested that both wild-type and Deltam4+m6+m152 virus continued to present Ag to CD8 T cells during the chronic phase of infection. During the chronic phase of infection, MCMV cannot be isolated from immunocompetent mice. However, upon immunosuppression, both Deltam4+m6+m152 and wild-type virus could be reactivated from mice infected for 6 wk. Thus, restoring the ability of CD8 T cells to detect MCMV had little apparent effect on the course of MCMV infection and on the CD8 T cell response to it. These results challenge the notion that VIPR function is necessary for CMV persistence in the host.

CD1d1-Dependent Control of the Magnitude of an Acute Antiviral Immune Response

CD1d1-restricted NK T (NKT) cells rapidly secrete both Th1 and Th2 cytokines upon activation and are therefore thought to play a regulatory role during an immune response. In this study we examined the role of CD1d1 molecules and NKT cells in regulating virus-induced cytokine production. CD1d1-deficient (CD1KO) mice, which lack NKT cells, were infected with lymphocytic choriomeningitis virus, and spontaneous cytokine release from splenocytes was measured. We found that CD1KO mice produce significantly higher amounts of IL-2, IL-4, and IFN-gamma compared with wild-type controls postinfection. Depletion studies of individual lymphocyte subpopulations suggested that CD4+ T cells are required; however, isolation of specific lymphocyte populations indicated that CD4+ T cells alone are not sufficient for the increase in cytokine production in CD1KO mice. Splenocytes from lymphocytic choriomeningitis virus-infected CD1KO mice continued to produce enhanced cytokine levels long after viral clearance and cleared viral RNA faster than wild-type mice. There was no difference in the number of splenocytes between uninfected wild-type and CD1KO mice, whereas the latter knockout mice had an increased number of splenocytes after infection. Collectively, these data provide clear evidence that the expression of CD1d1 molecules controls the magnitude of the cell-mediated immune response to an acute viral infection.

Human CD94 gene expression: dual promoters differing in responsiveness to IL-2 or IL-15

CD94 is a C-type lectin required for the dimerization of the CD94/NKG2 family of receptors, which are expressed on NK cells and T cell subsets. Little is known about CD94 gene expression and the elements that regulate CD94 transcription. In this study, we report that CD94 gene expression is regulated by distal and proximal promoters that transcribe unique initial exons specific to each promoter. This results in two species of transcripts; the previously described CD94 mRNA and a novel CD94C mRNA. All NK cells and CD94(+), CD8(+) alphabeta T cells transcribe CD94 mRNA. Stimulation of NK and CD8(+) alphabeta T cells with IL-2 or IL-15 induced the transcription of CD94C mRNA. The distal and proximal promoters both contain elements with IFN-gamma-activated and Ets binding sites, known as GAS/EBS. Additionally, an unknown element, termed site A, was identified in the proximal promoter. EMSA analyses showed that constitutive factors could bind to oligonucleotide probes containing each element. After treatment of primary NK cells with IL-2 or IL-15, separate inducible complexes could be detected with oligonucleotide probes containing either the proximal or distal GAS/EBS elements. These elements are highly conserved between mice and humans, which suggests that both species regulate CD94 gene expression via mechanisms that predate their evolutionary divergence.

Transcriptional control of murine CD94 gene: differential usage of dual promoters by lymphoid cell types

The CD94 gene product is involved in controlling NK cell activation, and is one of a family of immune receptors that is found in the NK gene complex in both humans and mice, adjacent to members of the NKG2 family. CD94 forms a heterodimeric complex with several members of the NKG2 family on the surface of NK, T, and NKT cells. These complexes recognize the nonclassical MHC class I molecules HLA-E and Qa-1(b) in humans and mice, respectively. The mechanism for cell type-specific expression of CD94 and other genes from the NK gene complex has not yet been elucidated. In the current study, we show that the murine CD94 gene has two promoters, one of which is upstream of a previously unidentified exon. We illustrate by quantitative real-time PCR that lymphoid cell types use these two promoters differentially and that the promoter usage seen in adult cells is already established during fetal development. We determined that the differential promoter usage by NK cells appears to be susceptible to perturbation, as both the murine NK cell line LNK, as well as cultured C57BL/6 NK cells showed altered promoter usage relative to fresh NK cells. Furthermore, the promoter activity observed in transfection assays did not correlate with expression of the endogenous CD94 gene, suggesting the involvement of chromatin structure/methylation in transcriptional regulation. Our detection of DNase I hypersensitive sites at the CD94 locus that are present only in a cell line expressing endogenous CD94 supports this hypothesis.

Qa-1, a nonclassical class I histocompatibility molecule with roles in innate and adaptive immunity

Qa-1, a nonclassical class I histocompatibility molecule expressed in mice, predominantly assembles with a single nonameric peptide, Qdm, derived from the signal sequence of certain class Ia molecules. The Qa-1/Qdm complex is the primary ligand for CD94/NKG2A inhibitory receptors expressed on a major fraction of natural killer (NK) cells. Cells become susceptible to killing by NK cells under conditions where surface expression of the Qa-1/Qdm inhibitory ligand is reduced. The CD94/NKG2 "missing-self" recognition system serves as mechanism for removing cells that have abnormalities in the intracellular machinery required for assembly and expression of class I-peptides complexes, as a consequence of viral infection, for example. Despite its highly focused peptide-binding specificity, Qa-1 also has a capacity to act as an antigen-presentation molecule for CD8+ T cells. It appears that a small subpopulation of these T cells undergoes positive selection by interaction with Qa-1 in the thymus, and they maintain their specificity for Qa-1 after maturation. The role of these unusual T cells in adaptive immune responses remains to be defined.

Caveats in the design of MHC class I tetramer/antigen-specific T lymphocytes dissociation assays

Relative avidities of antigen-specific T cells for major histocompatibility complex peptide complexes (MHCp) have recently been measured by MHC tetramer dissociation assays, but there is no consensus on the methodologies used for these experiments. While we do not question the conclusions reached in previous studies, in this paper we discuss the caveats that are present in the design of all MHCp tetramer dissociation protocols, and we propose a set of criteria that should be met in the evaluation of appropriate methodologies. We find that it is necessary to use specific reagents to compete with rebinding of the labeled tetramer, but that when either intact anti-MHC antibodies or cold MHC tetramers are used, the dissociation rates are dependent upon the concentrations of the competitor. In contrast, we demonstrate that apparent dissociation rates are independent of the competitor concentration when blocking anti-MHC Fab fragments are used, suggesting that these are the most appropriate reagents to use for tetramer dissociation experiments.

A transgenic mouse model genetically tags all activated CD8 T cells

Identifying and characterizing Ag-specific CD8+ T cells are central to the study of immunological memory. Although powerful strategies such as MHC tetramers and peptide-induced cytokine production assays exist for identifying Ag-specific CD8+ T cells, alternate strategies that are not dependent upon a priori knowledge of the immunodominant and subdominant antigenic epitopes, as well as the MHC background of the animal are of obvious utility. In this study, we present a transgenic mouse model that uses Cre-loxP recombination to permanently mark all activated CD8+ T cells with beta-galactosidase. We used the lymphocytic choriomeningitis virus infection model to track the dynamics of the antiviral CD8+ T cell responses. We show that in this transgenic mouse model system, all of the antiviral effector and memory CD8+ T cells are contained within the beta-gal-marked CD8+ T cell population.

Differential regulation of killer cell lectin-like receptor G1 expression on T cells

The killer cell lectin-like receptor G1 (KLRG1) is the mouse homologue of the rat mast cell function-associated Ag and contains a tyrosine-based inhibitory motif in its cytoplasmic domain. It has been demonstrated that KLRG1 is induced on activated NK cells and that KLRG1 can inhibit NK cell effector functions. In this study, we show that in naive C57BL/6 mice KLRG1 is expressed on a subset of CD44(high)CD62L(low) T cells. KLRG1 expression can be detected on a small number of V(alpha)14i NK T cells but not on CD8alphaalpha(+) intraepithelial T cells that are either TCRgammadelta(+) or TCRalphabeta(+). We also show that KLRG1 expression is dramatically induced on approximately 50% of the CD8(+) T cells during both a viral and a parasitic infection. Interestingly, during Toxoplasma gondii infection, KLRG1 is up-regulated on CD4(+) T cells. Although KLRG1 expression can be induced on both NK cells and T cells, the molecular mechanism leading to the induction of KLRG1 differs in these two subsets of cells. Indeed, the up-regulation of KLRG1 on NK cells can be driven in vivo by cytokines, whereas KLRG1 cannot be induced on CD8(+) T cells by cytokines. In addition, although induction of KLRG1 on T cells appears to require TCR engagement in vivo, TCR engagement is not sufficient for KLRG1 induction in vitro. Taken together, these data suggest that the expression and induction of KLRG1 on T cells are tightly regulated. This could have important biological consequences on T cell activation and homeostasis.

A peptide from heat shock protein 60 is the dominant peptide bound to Qa-1 in the absence of the MHC class Ia leader sequence peptide Qdm

The MHC class Ib molecule Qa-1 binds specifically and predominantly to a single 9-aa peptide (AMAPRTLLL) derived from the leader sequence of many MHC class Ia proteins. This peptide is referred to as Qdm. In this study, we report the isolation and sequencing of a heat shock protein 60-derived peptide (GMKFDRGYI) from Qa-1. This peptide is the dominant peptide bound to Qa-1 in the absence of Qdm. A Qa-1-restricted CTL clone recognizes this heat shock protein 60 peptide, further verifying that it binds to Qa-1 and a peptide from the homologous Salmonella typhimurium protein GroEL (GMQFDRGYL). These observations have implications for how Qa-1 can influence NK cell and T cell effector function via the TCR and CD94/NKG2 family members, and how this effect can change under conditions that cause the peptides bound to Qa-1 to change.

The gp49B1 inhibitory receptor regulates the IFN-gamma responses of T cells and NK cells

The magnitude and diversity of Ag-specific T cell effector activity have been proposed to be controlled by an integration of positive signals transduced by the TCR and negative signals originating from inhibitory cell surface molecules. Although the lectin family of NK cell-associated inhibitory receptors has been reported to regulate the function of murine CTLs, gp49B1, the Ig superfamily member is not known to be expressed on T cells. Moreover, the consequences of the lack of an endogenously expressed NK cell-associated inhibitory receptor on T cell functions are not known. We report that gp49B1 is expressed by nearly all activated CD8 and CD4 T cells in addition to NK cells during an immune response to viral, bacterial, or tumor challenge. Kinetics of gp49B1 expression parallel functional capability and subside in the memory phase. Following vaccinia viral infection, IFN-gamma production by both subsets of T cells and NK cells is enhanced in gp49B1-deficient mice compared with gp49B1(+/+) mice. The stimulation threshold for IFN-gamma production is also lower in gp49B1-deficient T cells. In contrast, no significant differences were observed in the cytotoxic responses. We conclude that gp49B1 is a unique inhibitory receptor that is induced in multiple lineages of innate and adaptive immune cells during an infection and controls their IFN-gamma, but not cytotoxic responses.

Immune functions encoded by the natural killer gene complex

There has been marked progress in our understanding of the role of natural killer (NK) cells in immune responses, mainly due to the identification of NK-cell receptors and their ligands. The genes encoding many NK-cell receptors are located in the NK-gene complex (NKC). Here, we review the properties of NKC-encoded receptors, and provide a genomic and conceptual framework for an insight into NK-cell function and biology.

Preferential survival of CD8 T and NK cells expressing high levels of CD94

The Qa-1(b)/Qdm tetramer binds to CD94/NKG2 receptors expressed at high levels on approximately 50% of murine NK cells. Although very few CD8 T cells from naive mice express CD94/NKG2 receptors, approximately 50% of CD8 T cells taken from mice undergoing a secondary response against Listeria monocytogenes (LM) are CD94(high) and bind the tetramer. Although CD94(int) NK cells do not bind the tetramer, CD94(int) CD8 T cells do, and this binding is dependent on the CD8 coreceptor. We found that the extent of apoptosis in CD8 T and NK cells was inversely related to the expression of CD94, with lower levels of apoptosis seen in CD94(high) cells after 1-3 days of culture. The difference in CD8 T cell survival was evident as early as 6 h after culture and persisted until nearly all the CD94(neg/int) cells were apoptotic by 48 h. In contrast, expression of inhibitory Ly-49A,G2,C/I molecules was associated with higher levels of apoptosis. Cross-linking CD94/NKG2 receptors on CD8 T cells from a mouse undergoing an LM infection further reduced the percentage of apoptotic cells on the CD94-expressing populations, while cross-linking Ly-49I had no effect on CD8 T cells expressing Ly-49I. Cross-linking CD3 on CD8 T cells from a mouse undergoing a secondary LM infection increases the extent of apoptosis, but this is prevented by cross-linking CD94/NKG2 receptors at the same time. Similar results were observed with NK cells in that the CD94(high) population displayed less apoptosis than CD94(int) cells after 1-3 days in culture. Therefore, the expression of CD94/NKG2 is correlated with a lower level of apoptosis and may play an important role in the maintenance of CD8 T and NK cells.

Regulation of antiviral CD8+ T cells by inhibitory natural killer cell receptors

Recent evidence indicates that CD8(+) T cells express natural killer cell receptors that constrain the range and magnitude of their activities. For virus-specific CD8(+) T cells, upregulation of these receptors serves to control infection, while concurrently minimizing bystander pathology. Dysregulated expression of these receptors, however, may foster the establishment of persistent virus infection.