Nodular Inflammatory Foci Are Sites of T Cell Priming and Control of Murine Cytomegalovirus Infection in the Neonatal Lung

Neonates, including mice and humans, are highly susceptible to cytomegalovirus (CMV) infection. However, many aspects of neonatal CMV infections such as viral cell tropism, spatio-temporal distribution of the pathogen as well as genesis of antiviral immunity are unknown. With the use of reporter mutants of the murine cytomegalovirus (MCMV) we identified the lung as a primary target of mucosal infection in neonatal mice. Comparative analysis of neonatal and adult mice revealed a delayed control of virus replication in the neonatal lung mucosa explaining the pronounced systemic infection and disease in neonates. This phenomenon was supplemented by a delayed expansion of CD8⁺ T cell clones recognizing the viral protein M45 in neonates. We detected viral infection at the single-cell level and observed myeloid cells forming “nodular inflammatory foci” (NIF) in the neonatal lung. Co-localization of infected cells within NIFs was associated with their disruption and clearance of the infection. By 2-photon microscopy, we characterized how neonatal antigen-presenting cells (APC) interacted with T cells and induced mature adaptive immune responses within such NIFs. We thus define NIFs of the neonatal lung as niches for prolonged MCMV replication and T cell priming but also as sites of infection control.     

Control of Murine Cytomegalovirus in the Lungs: Relative but Not Absolute Immunodominance of the Immediate-Early 1 Nonapeptide during the Antiviral Cytolytic T-Lymphocyte Response in Pulmonary Infiltrates

The lungs are a major organ site of cytomegalovirus (CMV) infection, pathogenesis, and latency. Interstitial CMV pneumonia represents a critical manifestation of CMV disease, in particular in recipients of bone marrow transplantation (BMT). We have employed a murine model for studying the immune response to CMV in the lungs in the specific scenario of immune reconstitution after syngeneic BMT. Control of pulmonary infection was associated with a vigorous infiltration of the lungs, which was characterized by a preferential recruitment and massive expansion of the CD8 subset of α/β T cells. The infiltrate provided a microenvironment in which the CD8 T cells differentiated into mature effector cells, that is, into functionally active cytolytic T lymphocytes (CTL). This gave us the opportunity for an ex vivo testing of the antigen specificities of CTL present at a relevant organ site of viral pathogenesis. The contribution of the previously identified immediate-early 1 (IE1) nonapeptide of murine CMV was evaluated by comparison with the CD3-redirected cytolytic activity used as a measure of the overall CTL response in the lungs. The IE1 peptide was detected by pulmonary CTL, but it accounted for a minor part of the response. Interestingly, no additional viral or virus-induced antigenic peptides were detectable among naturally processed peptides derived from infected lungs, even though infected fibroblasts were recognized in a major histocompatibility complex-restricted manner. We conclude that the antiviral pulmonary immune response is a collaborative function that involves many antigenic peptides, among which the IE1 peptide is immunodominant in a relative sense.     

Engineering Antigen-Specific T Cells from Genetically Modified Human Hematopoietic Stem Cells in Immunodeficient Mice

There is a desperate need for effective therapies to fight chronic viral infections. The immune response is normally fastidious at controlling the majority of viral infections and a therapeutic strategy aimed at reestablishing immune control represents a potentially powerful approach towards treating persistent viral infections. We examined the potential of genetically programming human hematopoietic stem cells to generate mature CD8+ cytotoxic T lymphocytes that express a molecularly cloned, “transgenic” human anti-HIV T cell receptor (TCR). Anti-HIV TCR transduction of human hematopoietic stem cells directed the maturation of a large population of polyfunctional, HIV-specific CD8+ cells capable of recognizing and killing viral antigen-presenting cells. Thus, through this proof-of-concept we propose that genetic engineering of human hematopoietic stem cells will allow the tailoring of effector T cell responses to fight HIV infection or other diseases that are characterized by the loss of immune control.     

Cross-Presentation of a Spread-Defective MCMV Is Sufficient to Prime the Majority of Virus-Specific CD8+ T Cells

CD8+ T cells can be primed by peptides derived from endogenous proteins (direct presentation), or exogenously acquired protein (cross-presentation). However, the relative ability of these two pathways to prime CD8+ T cells during a viral infection remains controversial. Cytomegaloviruses (CMVs) can infect professional antigen presenting cells (APCs), including dendritic cells, thus providing peptides for direct presentation. However, the viral immune evasion genes profoundly impair recognition of infected cells by CD8+ T cells. Nevertheless, CMV infection elicits a very strong CD8+ T cell response, prompting its recent use as a vaccine vector. We have shown previously that deleting the immune evasion genes from murine cytomegalovirus (MCMV) that target class I MHC presentation, has no impact on the size or breadth of the CD8+ T cell response elicited by infection, suggesting that the majority of MCMV-specific CD8+ T cells in vivo are not directly primed by infected professional APCs. Here we use a novel spread-defective mutant of MCMV, lacking the essential glycoprotein gL, to show that cross-presentation alone can account for the majority of MCMV-specific CD8+ T cell responses to the virus. Our data support the conclusion that cross-presentation is the primary mode of antigen presentation by which CD8+ T cells are primed during MCMV infection.     

Memory CD8+ T Cells: Orchestrators and Key Players of Innate Immunity?

Over the past decades, the dichotomy between innate and adaptive immune responses has largely dominated our understanding of immunology. Upon primary encounter with microbial pathogens, differentiation of adaptive immune cells into functional effectors usually takes several days or even longer, making them contribute to host protection only late during primary infection. However, once generated, antigen-experienced T lymphocytes can persist in the organism and constitute a pool of memory cells that mediate fast and effective protection to a recall infection with the same microbial pathogen. Herein, we challenge this classical paradigm by highlighting the “innate nature” of memory CD8⁺ T cells. First, within the thymus or in the periphery, naïve CD8⁺ T cells may acquire phenotypic and functional characteristics of memory CD8⁺ T cells independently of challenge with foreign antigens. Second, both the “unconventional” and the “conventional” memory cells can rapidly express protective effector functions in response to sets of inflammatory cytokines and chemokines signals, independent of cognate antigen triggering. Third, memory CD8⁺ T cells can act by orchestrating the recruitment, activation, and licensing of innate cells, leading to broad antimicrobial states. Thus, collectively, memory CD8⁺ T cells may represent important actors of innate immune defenses.     

A Role for Perforin in Downregulating T-Cell Responses during Chronic Viral Infection

Cytotoxic T cells secrete perforin to kill virus-infected cells. In this study we show that perforin also plays a role in immune regulation. Perforin-deficient (perf −/−) mice chronically infected with lymphocytic choriomeningitis virus (LCMV) contained greater numbers of antiviral T cells compared to persistently infected +/+ mice. The enhanced expansion was seen in both CD4 and CD8 T cells, but the most striking difference was in the numbers of LCMV-specific CD8 T cells present in infected perf −/− mice. Persistent LCMV infection of +/+ mice results in both deletion and anergy of antigen-specific CD8 T cells, and our results show that this peripheral “exhaustion” of activated CD8 T cells occurred less efficiently in perf −/− mice. This excessive accumulation of activated CD8 T cells resulted in immune-mediated damage in persistently infected perf −/− mice; ~50% of these mice died within 2 to 4 weeks, and mortality was fully reversed by in vivo depletion of CD8 T cells. This finding highlights an interesting dichotomy between the role of perforin in viral clearance and immunopathology; perforin-deficient CD8 T cells were unable to clear the LCMV infection but were capable of causing immune-mediated damage. Finally, this study shows that perforin also plays a role in regulating T-cell-mediated autoimmunity. Mice that were deficient in both perforin and Fas exhibited a striking acceleration of the spontaneous lymphoproliferative disease seen in Fas-deficient (lpr) mice. Taken together, these results show that the perforin-mediated pathway is involved in downregulating T-cell responses during chronic viral infection and autoimmunity and that perforin and Fas act independently as negative regulators of activated T cells.     

Strain-Specific Properties and T Cells Regulate the Susceptibility to Papilloma Induction by Mus musculus Papillomavirus 1

The immunocytes that regulate papillomavirus infection and lesion development in humans and animals remain largely undefined. We found that immunocompetent mice with varying H-2 haplotypes displayed asymptomatic skin infection that produced L1 when challenged with 6×10¹⁰ MusPV1 virions, the recently identified domestic mouse papillomavirus (also designated “MmuPV1”), but were uniformly resistant to MusPV1-induced papillomatosis. Broad immunosuppression with cyclosporin A resulted in variable induction of papillomas after experimental infection with a similar dose, from robust in Cr:ORL SENCAR to none in C57BL/6 mice, with lesional outgrowth correlating with early viral gene expression and partly with reported strain-specific susceptibility to chemical carcinogens, but not with H-2 haplotype. Challenge with 1×10¹² virions in the absence of immunosuppression induced small transient papillomas in Cr:ORL SENCAR but not in C57BL/6 mice. Antibody-induced depletion of CD3⁺ T cells permitted efficient virus replication and papilloma formation in both strains, providing experimental proof for the crucial role of T cells in controlling papillomavirus infection and associated disease. In Cr:ORL SENCAR mice, immunodepletion of either CD4⁺ or CD8⁺ T cells was sufficient for efficient infection and papillomatosis, although deletion of one subset did not inhibit the recruitment of the other subset to the infected epithelium. Thus, the functional cooperation of CD4⁺ and CD8⁺ T cells is required to protect this strain. In contrast, C57BL/6 mice required depletion of both CD4⁺ and CD8⁺ T cells for infection and papillomatosis, and separate CD4 knock-out and CD8 knock-out C57BL/6 were also resistant. Thus, in C57BL/6 mice, either CD4⁺ or CD8⁺ T cell-independent mechanisms exist that can protect this particular strain from MusPV1-associated disease. These findings may help to explain the diversity of pathological outcomes in immunocompetent humans after infection with a specific human papillomavirus genotype.     

Renewal of peripheral CD8+ memory T cells during secondary viral infection of antibody-sufficient mice

Kinetic studies and short pulses of injected 5-bromo-2-deoxyuridine have been used to analyze the development and renewal of peripheral CD8(+) memory T cells in the lungs during primary and secondary respiratory virus infections. We show that developing peripheral CD8(+) memory T cells proliferate during acute viral infection with kinetics that are indistinguishable from those of lymphoid CD8(+) memory T cells. Secondary exposure to the same virus induces a new round of T cell proliferation and extensive renewal of the peripheral and lymphoid CD8(+) memory T cell pools in both B cell-deficient mice and mice with immune Abs. In mice with virus-specific Abs, CD8(+) T cell proliferation takes place with minimal inflammation or effector cell recruitment to the lungs. The delayed arrival of CD8(+) memory T cells to the lungs of these animals suggests that developing memory cells do not require the same inflammatory signals as effector cells to reach the lung airways. These studies provide important new insight into mechanisms that control the maintenance and renewal of peripheral memory T cell populations during natural infections.     

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.     

CCR2+ Inflammatory Dendritic Cells and Translocation of Antigen by Type III Secretion Are Required for the Exceptionally Large CD8+ T Cell Response to the Protective YopE69-77 Epitope during Yersinia Infection

During Yersinia pseudotuberculosis infection of C57BL/6 mice, an exceptionally large CD8⁺ T cell response to a protective epitope in the type III secretion system effector YopE is produced. At the peak of the response, up to 50% of splenic CD8⁺ T cells recognize the epitope YopE_69-77. The features of the interaction between pathogen and host that result in this large CD8⁺ T cell response are unknown. Here, we used Y. pseudotuberculosis strains defective for production, secretion and/or translocation of YopE to infect wild-type or mutant mice deficient in specific dendritic cells (DCs). Bacterial colonization of organs and translocation of YopE into spleen cells was measured, and flow cytometry and tetramer staining were used to characterize the cellular immune response. We show that the splenic YopE_69-77-specific CD8⁺ T cells generated during the large response are polyclonal and are produced by a “translocation-dependent” pathway that requires injection of YopE into host cell cytosol. Additionally, a smaller YopE_69-77-specific CD8⁺ T cell response (~10% of the large expansion) can be generated in a “translocation-independent” pathway in which CD8α⁺ DCs cross present secreted YopE. CCR2-expressing inflammatory DCs were required for the large YopE_69-77-specific CD8⁺ T cell expansion because this response was significantly reduced in Ccr2^-/- mice, YopE was translocated into inflammatory DCs in vivo, inflammatory DCs purified from infected spleens activated YopE_69-77-specific CD8⁺ T cells ex vivo and promoted the expansion of YopE_69-77-specific CD8⁺ T cells in infected Ccr2^-/- mice after adoptive transfer. A requirement for inflammatory DCs in producing a protective CD8⁺ T cell response to a bacterial antigen has not previously been demonstrated. Therefore, the production of YopE_69-77-specific CD8⁺ T cells by inflammatory DCs that are injected with YopE during Y. pseudotuberculosis infection represents a novel mechanism for generating a massive and protective adaptive immune response.     

Age-Related Expansion of Tim-3 Expressing T Cells in Vertically HIV-1 Infected Children

As perinatally HIV-1-infected children grow into adolescents and young adults, they are increasingly burdened with the long-term consequences of chronic HIV-1 infection, with long-term morbidity due to inadequate immunity. In progressive HIV-1 infection in horizontally infected adults, inflammation, T cell activation, and perturbed T cell differentiation lead to an “immune exhaustion”, with decline in T cell effector functions. T effector cells develop an increased expression of CD57 and loss of CD28, with an increase in co-inhibitory receptors such as PD-1 and Tim-3. Very little is known about HIV-1 induced T cell dysfunction in vertical infection. In two perinatally antiretroviral drug treated HIV-1-infected groups with median ages of 11.2 yr and 18.5 yr, matched for viral load, we found no difference in the proportion of senescent CD28⁻CD57⁺CD8⁺ T cells between the groups. However, the frequency of Tim-3⁺CD8⁺ and Tim-3⁺CD4⁺ exhausted T cells, but not PD-1⁺ T cells, was significantly increased in the adolescents with longer duration of infection compared to the children with shorter duration of HIV-1 infection. PD-1⁺CD8⁺ T cells were directly associated with T cell immune activation in children. The frequency of Tim-3⁺CD8⁺ T cells positively correlated with HIV-1 plasma viral load in the adolescents but not in the children. These data suggest that Tim-3 upregulation was driven by both HIV-1 viral replication and increased age, whereas PD-1 expression is associated with immune activation. These findings also suggest that the Tim-3 immune exhaustion phenotype rather than PD-1 or senescent cells plays an important role in age-related T cell dysfunction in perinatal HIV-1 infection. Targeting Tim-3 may serve as a novel therapeutic approach to improve immune control of virus replication and mitigate age related T cell exhaustion.     

Murine model of interstitial cytomegalovirus pneumonia in syngeneic bone marrow transplantation: persistence of protective pulmonary CD8-T-cell infiltrates after clearance of acute infection

Interstitial pneumonia (IP) is a severe organ manifestation of cytomegalovirus (CMV) disease in the immunocompromised host, in particular in recipients of bone marrow transplantation (BMT). Diagnostic criteria for the definition of CMV-IP include clinical evidence of pneumonia together with CMV detected in bronchoalveolar lavage or lung biopsy. We have used the model of syngeneic BMT and simultaneous infection of BALB/c mice with murine CMV for studying the pathogenesis of CMV-IP by controlled longitudinal analysis. A disseminated cytopathic infection of the lungs with fatal outcome was observed only when reconstituting CD8 T cells were depleted. Neither CD8 nor CD4 T cells mediated an immunopathogenesis of acute CMV-IP. By contrast, after efficient hematolymphopoietic reconstitution, viral replication in the lungs was moderate and focal. The histopathological picture was dominated by preferential infiltration of CD8 T cells confining viral replication to inflammatory foci. Notably, after clearance of acute infection, CD62L(lo) and CD62L(hi) subsets of CD44(+) memory CD8 T cells were found to persist in lung tissue. One can thus operationally distinguish an early CMV-positive IP (phase 1) and a late CMV-negative IP (phase 2). According to the definition, phase 2 histopathology would not be diagnosed as a CMV-IP and could instead be misinterpreted as a CMV-induced immunopathology. We document here that phase 1 as well as phase 2 pulmonary CD8 T cells are capable of exerting effector functions and are effectual in protecting against productive infection. We propose that antiviral "stand-by" memory-effector T cells persist in the lungs to prevent virus recurrence from latency.     

Emergence of CD8+ T cells expressing NK cell receptors in influenza A virus-infected mice

Both innate and adaptive immune responses play an important role in the recovery of the host from viral infections. In the present report, a subset of cells coexpressing CD8 and NKR-P1C (NK1.1) was found in the lungs of mice infected with influenza A virus. These cells were detected at low numbers in the lungs of uninfected mice, but represented up to 10% of the total CD8(+) T cell population at day 10 postinfection. Almost all of the CD8(+)NK1.1(+) cells were CD8alphabeta(+)CD3(+)TCRalphabeta(+) and a proportion of these cells also expressed the NK cell-associated Ly49 receptors. Interestingly, up to 30% of these cells were virus-specific T cells as determined by MHC class I tetramer staining and by intracellular staining of IFN-gamma after viral peptide stimulation. Moreover, these cells were distinct from conventional NKT cells as they were also found at increased numbers in influenza-infected CD1(-/-) mice. These results demonstrate that a significant proportion of CD8(+) T cells acquire NK1.1 and other NK cell-associated molecules, and suggests that these receptors may possibly regulate CD8(+) T cell effector functions during viral infection.     

Cytomegalovirus-Specific T Cells Persist at Very High Levels during Long-Term Antiretroviral Treatment of HIV Disease

Background

In healthy, HIV seronegative, CMV seropositive adults, a large proportion of T cells are CMV-specific. High-level CMV-specific T cell responses are associated with accelerated immunologic aging (“immunosenesence”) in the elderly population. The impact of untreated and treated HIV infection on the frequency of these cells remains undefined.

Methodology/Principal Findings

We measured the proportion of CD4+ and CD8+ T cells responding to CMV pp65 and IE proteins was measured using flow cytometry in 685 unique HIV seronegative and seropositive individuals. The proportion of CMV-specific CD8+ T cells was consistently higher in the HIV-seropositive subjects compared to the HIV-seronegative subjects. This HIV effect was observed even in patients who lacked measurable immunodeficiency. Among the HIV-seropositive subjects, CMV-specific CD8+ T cell responses were proportionately lower during recent infection, higher during chronic untreated infection and higher still during long-term antiretroviral treated infection. The CD8+ T cell response to just two CMV proteins (pp65 and IE) was approximately 6% during long-term therapy, which was over twice that seen in HIV-seronegative persons. CMV-specific CD4+ T cell responses followed the same trends, but the magnitude of the effect was smaller.

Conclusions/Significance

Long-term successfully treated HIV infected patients have remarkably high levels of CMV-specific effector cells. These levels are similar to that observed in the elderly, but occur at much younger ages. Future studies should focus on defining the potential role of the CMV-specific inflammatory response in non-AIDS morbidity and mortality, including immunosenescence.     

Plasmacytoid Dendritic Cells Mediate the Regulation of Inflammatory Type T Cell Response for Optimal Immunity against Respiratory Chlamydia Pneumoniae Infection

Chlamydia pneumoniae (Cpn) infection is a leading cause for a variety of respiratory diseases and has been implicated in the pathogenesis of chronic inflammatory diseases. The regulatory mechanisms in host defense against Cpn infection are less understood. In this study, we investigated the role of plasmacytoid dendritic cells (pDCs) in immune regulation in Cpn respiratory tract infection. We found that in vivo depletion of pDCs increased the severity of infection and lung pathology. Mice depleted of pDC had greater body weight loss, higher lung bacterial burden and excessive tissue inflammation compared to the control mice. Analysis of specific T cell cytokine production pattern in the lung following Cpn infection revealed that pDC depleted mice produced significantly higher amounts of inflammatory cytokines, especially TNF-α, but lower IL-10 compared to the controls. In particular, pDC depleted mice showed pathogenic T cell responses characterized by inflammatory type-1 (CD8 and CD4) and inflammatory Th2 cell responses. Moreover, pDC depletion dramatically reduced CD4 regulatory T cells (Tregs) in the lungs and draining lymph nodes. Furthermore, pDC-T cell co-culture experiments showed that pDCs isolated from Cpn infected mice were potent in inducing IL-10 producing CD4 Tregs. Together, these findings provide in vivo evidence for a critical role of pDCs in homeostatic regulation of immunity during Cpn infection. Our findings highlight the importance of a ‘balanced’ immune response for host protective immunity and preventing detrimental immunopathology during microbial infections.     

Acute primary infection with cytomegalovirus (CMV) in kidney transplant recipients results in the appearance of a phenotypically aberrant CD8+ T cell population

Human cytomegalovirus (CMV) is a beta-herpesvirus that causes a chronic subclinical infection in healthy man. The immune system is unable to eliminate the virus completely, allowing virus to persist in a latent state. In the immunocompromised host, this equilibrium is disturbed, resulting in a clinical infection. In immunocompromised rats, clinical CMV infection is associated with an increase in NK cells and CD8+ T cells, including a phenotypically aberrant CD8+ T cell population. Using flow cytometry, we examined the effect of acute CMV infection on the composition of leukocyte subsets in immunocompromised patients. Therefore, we used peripheral blood of CMV seronegative patients receiving a kidney from a seronegative (control group) or a seropositive donor. Of the patients receiving a seropositive kidney, only the patients undergoing acute CMV infection were included (experimental group). Special attention was paid to the phenotype of the cytotoxic T cells. The development of acute CMV infection resulted in an increased NK cell number and an activation of both CD4+ and CD8+ T cells, as determined by HLA-DR expression. An aberrant CD8+ T cell subset with decreased expression of CD8 and TCR alphabeta appeared in the infected patients. Furthermore, the size of this subpopulation of CD8+ T cells is positively correlated with the viral load.     

Random T-cell receptor recruitment in human immunodeficiency virus type 1 (HIV-1)-specific CD8+ T cells from genetically identical twins infected with the same HIV-1 strain

Human immunodeficiency virus type 1 (HIV-1) cytotoxic T-lymphocyte escape mutations represent both a major reason for loss of HIV immune control and a considerable challenge for HIV-1 vaccine design. Previous data suggest that initial HIV-1-specific CD8⁺ T-cell responses are determined largely by viral and host genetics, but the mechanisms influencing the subsequent viral evolution are unclear. Here, we show a random recruitment of T-cell receptor (TCR) alpha and beta clonotypes of the initial HIV-1-specific CD8⁺ T cells during primary infection in two genetically identical twins infected simultaneously with the same virus, suggesting that stochastic TCR recruitment of HIV-1-specific CD8⁺ T cells contributes to the diverse and unpredictable HIV-1 sequence evolution.     

Kinetics of ocular and systemic antigen-specific T-cell responses elicited during murine cytomegalovirus retinitis

Cytomegalovirus (CMV) reactivation in the retina of immunocompromized patients is a cause of significant morbidity as it can lead to blindness. The adaptive immune response is critical in controlling murine CMV (MCMV) infection in MCMV-susceptible mouse strains. CD8(+) T cells limit systemic viral replication in the acute phase of infection and are essential to contain latent virus. In this study, we provide the first evaluation of the kinetics of anti-viral T-cell responses after subretinal infection with MCMV. The acute response was characterized by a rapid expansion phase, with infiltration of CD8(+) T cells into the infected retina, followed by a contraction phase. MCMV-specific T cells displayed biphasic kinetics with a first peak at day 12 and contraction by day 18 followed by sustained recruitment of these cells into the retina at later time points post-infection. MCMV-specific CD8(+) T cells were also observed in the draining cervical lymph nodes and the spleen. Presentation of viral epitopes and activation of CD8(+) T cells was widespread and could be detected in the spleen and the draining lymph nodes, but not in the retina or iris. Moreover, after intraocular infection, antigen-specific cytotoxic activity was detectable and exhibited kinetics equivalent to those observed after intraperitoneal infection with the same viral dose. These data provide novel insights of how and where immune responses are initiated when viral antigen is present in the subretinal space.     

Specific recognition of the viral protein UL18 by CD85j/LIR-1/ILT2 on CD8+ T cells mediates the non-MHC-restricted lysis of human cytomegalovirus-infected cells

Immune evasion mechanisms of human CMV are known; however, the immune control of infection remains poorly elucidated. We show that interaction between the viral protein UL18 on infected cells and the invariant receptor CD85j/LIR-1/ILT2 expressed on CTL is relevant for the control of infection. Resting and activated CD8(+) T cells lysed UL18 expressing cells, whereas cells infected with CMV defective for UL18 were not killed. Lysis was not dependent on CD8(+) T cell Ag specificity, MHC-unrestricted and specifically blocked by anti-CD85j and anti-UL18 mAb. Moreover, soluble recombinant UL18Fc immunoprecipitated CD85j from T cells. Activation is mediated by CD85j and its pathway is unrelated to CD3/TCR engagement. UL18 is detected in immunocompromised patients with productive infection and the mechanism used in vivo by human CMV to ensure survival of the immunocompetent host may be mediated by activation signals delivered by infected cells to T lymphocytes via UL18/CD85j interactions.