NKG2D receptor signaling enhances cytolytic activity by virus-specific CD8+ T cells: evidence for a protective role in virus-induced encephalitis

Inoculation with the neurotropic JHM strain of mouse hepatitis virus (JHMV) into the central nervous system (CNS) of mice results in an acute encephalitis associated with an immune-mediated demyelinating disease. During acute disease, infiltrating CD8⁺ T cells secrete gamma interferon (IFN-γ) that controls replication in oligodendrocytes, while infected astrocytes and microglia are susceptible to perforin-mediated lysis. The present study was undertaken to reveal the functional contributions of the activating NKG2D receptor in host defense and disease following JHMV infection. NKG2D ligands RAE-1, MULT1, and H60 were expressed within the CNS following JHMV infection. The immunophenotyping of infiltrating cells revealed that NKG2D was expressed on ~90% of infiltrating CD8⁺ T cells during acute and chronic disease. Blocking NKG2D following JHMV infection resulted in increased mortality that correlated with increased viral titers within the CNS. Anti-NKG2D treatment did not alter T-cell infiltration into the CNS or the generation of virus-specific CD8⁺ T cells, and the expression of IFN-γ was not affected. However, cytotoxic T-lymphocyte (CTL) activity was dependent on NKG2D expression, because anti-NKG2D treatment resulted in a dramatic reduction in lytic activity by virus-specific CD8⁺ T cells. Blocking NKG2D during chronic disease did not affect either T-cell or macrophage infiltration or the severity of demyelination, indicating that NKG2D does not contribute to virus-induced demyelination. These findings demonstrate a functional role for NKG2D in host defense during acute viral encephalitis by selectively enhancing CTL activity by infiltrating virus-specific CD8⁺ T cells.     

Intestinal Intraepithelial Lymphocytes Are Primed for Gamma Interferon and MIP-1β Expression and Display Antiviral Cytotoxic Activity despite Severe CD4+ T-Cell Depletion in Primary Simian Immunodeficiency Virus Infection

Intraepithelial lymphocytes (IEL) are a critical effector component of the gut-associated lymphoid tissue (GALT) and play an important role in mucosal immunity as well as in the maintenance of the epithelial cell integrity and barrier function. The objective of this study was to determine whether simian immunodeficiency virus (SIV) infection of rhesus macaques would cause alterations in the immunophenotypic profiles of IEL and their mitogen-specific cytokine (gamma interferon [IFN-γ] and MIP-1β) responses (by flow cytometry) and virus-specific cytotoxic T-cell (CTL) activity (by the chromium release assay). Virally infected IEL were detected through the entire course of SIV infection by in situ hybridization. Severe depletion of CD4⁺ single-positive and CD4⁺CD8⁺ double-positive T cells occurred early in primary SIV infection, which was coincident with an increased prevalence of CD8⁺ T cells. This was in contrast to a gradual depletion of CD4⁺ T cells in peripheral blood. The CD8⁺ IEL were the primary producers of IFN-γ and MIP-1β and were found to retain their potential to produce both IFN-γ and MIP-1β through the entire course of SIV infection. SIV-specific CTL activity was detected in primary IEL at 1, 2, and 4 weeks post-SIV infection. These results demonstrated that IEL may be involved in generating antiviral immune responses early in SIV infection and in suppressing viral infection thereafter. Alterations in homeostasis in epithelia due to severe CD4⁺ T-cell depletion accompanied by changes in the cytokine and chemokine production by IEL may play a role in the enteropathogenesis of SIV infection.     

Gastrointestinal T Lymphocytes Retain High Potential for Cytokine Responses but Have Severe CD4+ T-Cell Depletion at All Stages of Simian Immunodeficiency Virus Infection Compared to Peripheral Lymphocytes

Gastrointestinal complications in human immunodeficiency virus (HIV) infection are indicative of impaired intestinal mucosal immune system. We used simian immunodeficiency virus (SIV)-infected rhesus macaques as an animal model for HIV to determine pathogenic effects of SIV on intestinal T lymphocytes. Intestinal CD4⁺ T-cell depletion and the potential for cytokine responses were examined during SIV infection and compared with results for lymphocytes from lymph nodes and blood. Flow cytometric analysis demonstrated severe depletion of CD4⁺CD8⁻ single-positive T cells and CD4⁺CD8⁺ double-positive T cells in intestinal lamina propria lymphocytes (LPL) and intraepithelial lymphocytes (IEL) during primary SIV infection which persisted through the entire course of SIV infection. In contrast, CD4⁺ T-cell depletion was gradual in peripheral lymph nodes and blood. Flow cytometric analysis of intracellular gamma interferon (IFN-γ) and interleukin-4 (IL-4) production following short-term mitogenic activation revealed that LPL retained same or higher capacity for IFN-γ production in all stages of SIV infection compared to uninfected controls, whereas peripheral blood mononuclear cells displayed a gradual decline. The CD8⁺ T cells were the major producers of IFN-γ. There was no detectable change in the frequency of IL-4-producing cells in both LPL and peripheral blood mononuclear cells. Thus, severe depletion of CD4⁺ LPL and IEL in primary SIV infection accompanied by altered cytokine responses may reflect altered T-cell homeostasis in intestinal mucosa. This could be a mechanism of SIV-associated enteropathy and viral pathogenesis. Dynamic changes in intestinal T lymphocytes were not adequately represented in peripheral lymph nodes or blood.     

Role of 4-1BB Receptor in the Control Played by CD8+ T Cells on IFN-γ Production by Mycobacterium tuberculosis Antigen-Specific CD4+ T Cells

Background

Antigen-specific IFN-γ producing CD4⁺ T cells are the main mediators of protection against Mycobacterium tuberculosis infection both under natural conditions and following vaccination. However these cells are responsible for lung damage and poor vaccine efficacy when not tightly controlled. Discovering new tools to control nonprotective antigen-specific IFN-γ production without affecting protective IFN-γ is a challenge in tuberculosis research.

Methods and Findings

Immunization with DNA encoding Ag85B, a candidate vaccine antigen of Mycobacterium tuberculosis, elicited in mice a low but protective CD4⁺ T cell-mediated IFN-γ response, while in mice primed with DNA and boosted with Ag85B protein a massive increase in IFN-γ response was associated with loss of protection. Both protective and non-protective Ag85B-immunization generated antigen-specific CD8⁺ T cells which suppressed IFN-γ-secreting CD4⁺ T cells. However, ex vivo ligation of 4-1BB, a member of TNF-receptor super-family, reduced the massive, non-protective IFN-γ responses by CD4⁺ T cells in protein-boosted mice without affecting the low protective IFN-γ-secretion in mice immunized with DNA. This selective inhibition was due to the induction of 4-1BB exclusively on CD8⁺ T cells of DNA-primed and protein-boosted mice following Ag85B protein stimulation. The 4-1BB-mediated IFN-γ inhibition did not require soluble IL-10, TGF-β, XCL-1 and MIP-1β. In vivo Ag85B stimulation induced 4-1BB expression on CD8⁺ T cells and in vivo 4-1BB ligation reduced the activation, IFN-γ production and expansion of Ag85B-specific CD4⁺ T cells of DNA-primed and protein-boosted mice.

Conclusion/Significance

Antigen-specific suppressor CD8⁺ T cells are elicited through immunization with the mycobacterial antigen Ag85B. Ligation of 4-1BB receptor further enhanced their suppressive activity on IFN-γ-secreting CD4⁺ T cells. The selective expression of 4-1BB only on CD8⁺ T cells in mice developing a massive, non-protective IFN-γ response opens novel strategies for intervention in tuberculosis pathology and vaccination through T-cell co-stimulatory-based molecular targeting.     

Role of human immunodeficiency virus (HIV)-specific T-cell immunity in control of dual HIV-1 and HIV-2 infection

Progressive immune dysfunction and AIDS develop in most cases of human immunodeficiency virus type 1 (HIV-1) infection but in only 25 to 30% of persons with HIV-2 infection. However, the natural history and immunologic responses of individuals with dual HIV-1 and HIV-2 infection are largely undefined. Based on our previous findings, we hypothesized that among patients with dual infection the control of HIV-1 is associated with the ability to respond to HIV-2 Gag epitopes and to maintain HIV-specific CD4⁺ T-cell responses. To test this, we compared the HIV-specific ex vivo IFN-γ enzyme-linked immunospot (ELISPOT) assay responses of 19 dually infected individuals to those of persons infected with HIV-1 or HIV-2 only. Further, we assessed the functional profile of HIV Gag-specific CD4⁺ and CD8⁺ T cells from nine HIV dually infected patients by using a multicolor intracellular cytokine staining assay. As determined by ELISPOT assay, the magnitude and frequency of IFN-γ-secreting T-cell responses to gene products of HIV-1 were higher than those to gene products of HIV-2 (2.64 versus 1.53 log₁₀ IFN-γ spot-forming cells/10⁶ cells [90% versus 63%, respectively].) Further, HIV-1 Env-, Gag-, and Nef- and HIV-2 Gag-specific responses were common; HIV-2 Nef-specific responses were rare. HIV-specific CD4⁺ T helper responses were detected in nine of nine dually infected subjects, with the majority of these T cells producing gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α) and, to a lesser extent, interleukin-2. The HIV-1 plasma viral load was inversely correlated with HIV-2 Gag-specific IFN-γ-/TNF-α-secreting CD4⁺ and HIV-2 Gag-specific IFN-γ-secreting CD8⁺ T cells. In conclusion, the T-cell memory responses associated with containment of single HIV-1 and HIV-2 infection play a similar significant role in the immune control of dual HIV-1 and HIV-2 infection.     

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 × C3H)F₁ 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.     

Abnormally High Levels of Virus-Infected IFN-γ+CCR4+CD4+CD25+ T Cells in a Retrovirus-Associated Neuroinflammatory Disorder

Background

Human T-lymphotropic virus type 1 (HTLV-1) is a human retrovirus associated with both HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), which is a chronic neuroinflammatory disease, and adult T-cell leukemia (ATL). The pathogenesis of HAM/TSP is known to be as follows: HTLV-1-infected T cells trigger a hyperimmune response leading to neuroinflammation. However, the HTLV-1-infected T cell subset that plays a major role in the accelerated immune response has not yet been identified.

Principal Findings

Here, we demonstrate that CD4⁺CD25⁺CCR4⁺ T cells are the predominant viral reservoir, and their levels are increased in HAM/TSP patients. While CCR4 is known to be selectively expressed on T helper type 2 (Th2), Th17, and regulatory T (Treg) cells in healthy individuals, we demonstrate that IFN-γ production is extraordinarily increased and IL-4, IL-10, IL-17, and Foxp3 expression is decreased in the CD4⁺CD25⁺CCR4⁺ T cells of HAM/TSP patients as compared to those in healthy individuals, and the alteration in function is specific to this cell subtype. Notably, the frequency of IFN-γ-producing CD4⁺CD25⁺CCR4⁺Foxp3⁻ T cells is dramatically increased in HAM/TSP patients, and this was found to be correlated with disease activity and severity.

Conclusions

We have defined a unique T cell subset—IFN-γ⁺CCR4⁺CD4⁺CD25⁺ T cells—that is abnormally increased and functionally altered in this retrovirus-associated inflammatory disorder of the central nervous system.     

Predominant Clonal Accumulation of CD8+ T Cells with Moderate Avidity in the Central Nervous Systems of Theiler's Virus-Infected C57BL/6 Mice

Induction of antigen-specific CD8⁺ T cells bearing a high-avidity T-cell receptor (TCR) is thought to be an important factor in antiviral and antitumor immune responses. However, the relationship between TCR diversity and functional avidity of epitope-specific CD8⁺ T cells accumulating in the central nervous system (CNS) during viral infection is unknown. Hence, analysis of T-cell diversity at the clonal level is important to understand the fate and function of virus-specific CD8⁺ T cells. In this study, we examined the Vβ diversity and avidity of CD8⁺ T cells specific to the predominant epitope (VP2_121-130) of Theiler''s murine encephalomyelitis virus. We found that Vβ6⁺ CD8⁺ T cells, associated with epitope specificity, predominantly expanded in the CNS during viral infection. Further investigations of antigen-specific Vβ6⁺ CD8⁺ T cells by CDR3 spectratyping and sequencing indicated that distinct T-cell clonotypes are preferentially increased in the CNS compared to the periphery. Among the epitope-specific Vβ6⁺ CD8⁺ T cells, MGX-Jβ1.1 motif-bearing cells, which could be found at a high precursor frequency in naïve mice, were expanded in the CNS and tightly associated with gamma interferon production. These T cells displayed moderate avidity for the cognate epitope rather than the high avidity normally observed in memory/effector T cells. Therefore, our findings provide new insights into the CD8⁺ T-cell repertoire during immune responses to viral infection in the CNS.Theiler''s murine encephalomyelitis virus (TMEV) is a member of the Cardiovirus genus within the Picornaviridae family (43). This virus is a common enteric pathogen among wild mice but rarely causes neurological disease (57). However, when it infects susceptible mice (e.g., the SJL/J [SJL] strain) intracerebrally, it reproducibly induces a chronic immune-mediated demyelinating disease that has been studied as an infectious model of human multiple sclerosis (MS) (10, 30). In contrast, infection of resistant mice like those of the C57BL/6 (B6) strain results in strong antiviral immune responses that clear the virus effectively and prevent disease development (24, 31). Therefore, immune responses in B6 mice have been often compared to those in susceptible SJL mice to understand the nature of protective versus pathogenic immunity in these mice.It has been shown that the major histocompatibility complex (MHC) H-2D locus is a critical genetic factor for resistance to TMEV-induced demyelinating disease (9, 49). For example, expression of the H-2D^b transgene makes susceptible FVB mice resistant by inducing strong H-2D^b-restricted VP2_121-130-specific CD8⁺ T-cell responses (36). This acquired resistance is abolished when VP2_121-130-specific T cells are tolerized by introducing the VP2 transgene (45). These results strongly suggest that CD8⁺ T cells generated in the presence of H-2D^b are critical for viral clearance from the central nervous system (CNS). Since the cardinal difference between the resistant B6 and susceptible SJL strains is the quantity, not the quality, of virus-specific CD8⁺ T cells (23, 32), strong CD8⁺ T-cell responses are probably required to prevent viral persistence and the consequent development of demyelinating disease. More than threefold more virus-specific CD8⁺ T cells were found in the CNSs of resistant B6 mice than in those of susceptible SJL mice at the acute phase of infection. Thus, the level of virus-specific CD8⁺ T cells at an early phase of the immune response may be a critical factor in resistance to the disease.Many recent investigations indicate that oligoclonal CD8⁺ T cells accumulate in the CNSs of MS patients (4, 38, 51). In addition, CD8⁺ T cells may also induce the development of experimental autoimmune encephalomyelitis (EAE) (54). Therefore, clonal expansion of certain CD8⁺ T cells may be associated with the pathogenesis of demyelinating diseases. However, B6 mice, which are resistant to TMEV-induced demyelinating disease, induce strong CD8⁺ T-cell responses to a single predominant epitope (VP2_121-130), i.e., ≥70% of CNS-infiltrating CD8⁺ T cells (41, 42). These CD8⁺ T cells result in effective viral clearance yet remain at a low level in the CNS more than 120 days postinfection (dpi) without detectable pathology (42). This inconsistency led us to investigate the shape and quality of the T-cell receptor (TCR) repertoire accumulating in the CNSs of B6 mice.The CD8⁺ T-cell responses induced after viral infection have previously been investigated with other animal viruses, including influenza virus, lymphocytic choriomeningitis virus (LCMV), mouse hepatitis virus (MHV), and Borna disease virus (11, 14, 35, 47, 58). Among these models, the detailed T-cell Vβ repertoire in the CNS was described only in the MHV model (46). CD8⁺ T-cell responses against TMEV in B6 mice are primarily against a single predominant epitope (22, 36, 41). However, virtually no study of the TCR Vβ repertoires of virus-specific CD8⁺ T cells has been reported. Furthermore, it is not yet known whether a particular TCR Vβ repertoire is associated with the avidity and/or function of CD8⁺ T cells in the CNS. Since protective versus pathogenic CD8⁺ T cells may correlate with their Vβ repertoire and T-cell function, these studies may help to elucidate the underlying mechanisms of protection versus pathogenesis of CD8⁺ T cells in the CNS.In this study, we have addressed several important questions about the CD8⁺ T-cell repertoire in the CNS. First, what is the pattern of Vβ usage in TMEV-infected B6 mice? Second, are there differences in the antigen-specific CD8⁺ T-cell clonotypes between the CNS and periphery? Third, are the T-cell clonotypes maintained in the CNS during the viral infection? Fourth, what is the functional avidity of T cells accumulating in the CNS during this virus infection? Last, what possible factors are associated with repertoire selection and expansion in the CNS? Our results show that Vβ6⁺ CD8⁺ T cells preferentially expand in the CNS during viral infection. Further analyses of the CDR3 region of antigen-specific Vβ6⁺ CD8⁺ T cells by spectratyping and sequencing indicate that distinct T-cell clonotypes are expanded in the CNS compared to those in the periphery. T cells expressing a particular Vβ6-CDR3-Jβ1.1 sequence are preferentially retained in the CNS during the course of viral infection. Interestingly, these T cells are capable of producing gamma interferon (IFN-γ) upon stimulation and display moderate avidity for the cognate epitope. We believe that our findings will provide important information regarding the CD8⁺ T-cell repertoire during viral infection and that these results may help to provide a better understanding of antiviral CD8⁺ T-cell immunity in the CNS.     

Interferon-dependent immunity is essential for resistance to primary dengue virus infection in mice, whereas T- and B-cell-dependent immunity are less critical

Dengue virus (DEN) causes dengue fever and dengue hemorrhagic fever/dengue shock syndrome, which are major public health problems worldwide. The immune factors that control DEN infection or contribute to severe disease are neither well understood nor easy to examine in humans. In this study, we used wild-type and congenic mice lacking various components of the immune system to study the immune mechanisms in the response to DEN infection. Our results demonstrate that alpha/beta interferon (IFN-α/β) and IFN-γ receptors have critical, nonoverlapping functions in resolving primary DEN infection. Furthermore, we show that IFN-α/β receptor-mediated action limits initial DEN replication in extraneural sites and controls subsequent viral spread into the central nervous system (CNS). In contrast, IFN-γ receptor-mediated responses seem to act at later stages of DEN disease by restricting viral replication in the periphery and eliminating virus from the CNS. Mice deficient in B, CD4⁺ T, or CD8⁺ T cells had no increased susceptibility to DEN; however, RAG mice (deficient in both B and T cells) were partially susceptible to DEN infection. In summary, (i) IFN-α/β is critical for early immune responses to DEN infection, (ii) IFN-γ-mediated immune responses are crucial for both early and late clearance of DEN infection in mice, and (iii) the IFN system plays a more important role than T- and B-cell-dependent immunity in resistance to primary DEN infection in mice.     

Gamma/Delta T-cell functional responses differ after pathogenic human immunodeficiency virus and nonpathogenic simian immunodeficiency virus infections

The objective of this study was to functionally assess gamma/delta (γδ) T cells following pathogenic human immunodeficiency virus (HIV) infection of humans and nonpathogenic simian immunodeficiency virus (SIV) infection of sooty mangabeys. γδ T cells were obtained from peripheral blood samples from patients and sooty mangabeys that exhibited either a CD4-healthy (>200 CD4⁺ T cells/μl blood) or CD4-low (<200 CD4 cells/μl blood) phenotype. Cytokine flow cytometry was utilized to assess production of Th1 cytokines tumor necrosis factor alpha and gamma interferon following ex vivo stimulation with either phorbol myristate acetate/ionomycin or the Vδ2 γδ T-cell receptor agonist isopentenyl pyrophosphate. Sooty mangabeys were observed to have higher percentages of γδ T cells in their peripheral blood than humans did. Following stimulation, γδ T cells from SIV-positive (SIV⁺) mangabeys maintained or increased their ability to express the Th1 cytokines regardless of CD4⁺ T-cell levels. In contrast, HIV-positive (HIV⁺) patients exhibited a decreased percentage of γδ T cells expressing Th1 cytokines following stimulation. This dysfunction is primarily within the Vδ2⁺ γδ T-cell subset which incurred both a decreased overall level in the blood and a reduced Th1 cytokine production. Patients treated with highly active antiretroviral therapy exhibited a partial restoration in their γδ T-cell Th1 cytokine response that was intermediate between the responses of the uninfected and HIV⁺ patients. The SIV⁺ sooty mangabey natural hosts, which do not proceed to clinical AIDS, provide evidence that γδ T-cell dysfunction occurs in HIV⁺ patients and may contribute to HIV disease progression.     

Protective Role of Gamma Interferon during the Recall Response to Influenza Virus

During secondary immune responses to influenza virus, virus-specific T memory cells are a major source of gamma interferon (IFN-γ). We assessed the contribution of IFN-γ to heterologous protection against the A/WSN/33 (H1N1) virus of wild-type and IFN-γ−/− mice previously immunized with the A/HK/68 (H3N2) virus. The IFN-γ−/− mice displayed significantly reduced survival rates subsequent to a challenge with various doses of the A/WSN/33 virus. This was associated with an impaired ability of the IFN-γ−/− mice to completely clear the pulmonary virus by day 7 after the challenge, although significant reduction of the virus titers was noted. However, the IFN-γ−/− mice developed type A influenza virus cross-reactive cytotoxic T lymphocytes (CTLs) similar to the wild-type mice, as demonstrated by both cytotoxicity and a limiting-dilution assay for the estimation of CTL precursor frequency. The pulmonary recruitment of T cells in IFN-γ−/− mice was not dramatically affected, and the percentage of CD4⁺ and CD8⁺ T cells was similar to that of wild-type mice. The T cells from IFN-γ−/− mice did not display a significant switch toward a Th2 profile. Furthermore, the IFN-γ−/− mice retained the ability to mount significant titers of WSN and HK virus-specific hemagglutination-inhibiting antibodies. Together, these results are consistent with a protective role of IFN-γ during the heterologous response against influenza virus independently of the generation and local recruitment of cross-reactive CTLs.     

Perforin-mediated effector function within the central nervous system requires IFN-gamma-mediated MHC up-regulation

CD8(+) T cells infiltrating the CNS control infection by the neurotropic JHM strain of mouse hepatitis virus. Differential susceptibility of infected cell types to clearance by perforin or IFN-gamma uncovered distinct, nonredundant roles for these antiviral mechanisms. To separately evaluate each effector function specifically in the context of CD8(+) T cells, pathogenesis was analyzed in mice deficient in both perforin and IFN-gamma (PKO/GKO) or selectively reconstituted for each function by transfer of CD8(+) T cells. Untreated PKO/GKO mice were unable to control the infection and died of lethal encephalomyelitis within 16 days, despite substantially higher CD8(+) T cell accumulation in the CNS compared with controls. Uncontrolled infection was associated with limited MHC class I up-regulation and an absence of class II expression on microglia, coinciding with decreased CD4(+) T cells in CNS infiltrates. CD8(+) T cells from perforin-deficient and wild-type donors reduced virus replication in PKO/GKO recipients. By contrast, IFN-gamma-deficient donor CD8(+) T cells did not affect virus replication. The inability of perforin-mediated mechanisms to control virus in the absence of IFN-gamma coincided with reduced class I expression. These data not only confirm direct antiviral activity of IFN-gamma within the CNS but also demonstrate IFN-gamma-dependent MHC surface expression to guarantee local T cell effector function in tissues inherently low in MHC expression. The data further imply that IFN-gamma plays a crucial role in pathogenesis by regulating the balance between virus replication in oligodendrocytes, CD8(+) T cell effector function, and demyelination.     

Clearance of an Influenza A Virus by CD4+ T Cells Is Inefficient in the Absence of B Cells

The primary CD8⁺ T-cell response protected most B-cell-deficient μMT mice against intranasal infection with the HKx31 influenza A virus. Prior exposure did not prevent reinfection upon homologous challenge, and the recall CD8⁺ T-cell response cleared the virus from the lung within 7 days. Depleting the CD8⁺ T cells substantially reduced the capacity of these primed mice to deal with the infection, in spite of evidence for established CD4⁺ T-cell memory. Thus, the control of this relatively mild influenza virus by both primary and secondary CD4⁺ T-cell responses is relatively inefficient in the absence of B cells and CD8⁺ T cells.     

Polyfunctional Type-1, -2, and -17 CD8+ T Cell Responses to Apoptotic Self-Antigens Correlate with the Chronic Evolution of Hepatitis C Virus Infection

Caspase-dependent cleavage of antigens associated with apoptotic cells plays a prominent role in the generation of CD8⁺ T cell responses in various infectious diseases. We found that the emergence of a large population of autoreactive CD8⁺ T effector cells specific for apoptotic T cell-associated self-epitopes exceeds the antiviral responses in patients with acute hepatitis C virus infection. Importantly, they endow mixed polyfunctional type-1, type-2 and type-17 responses and correlate with the chronic progression of infection. This evolution is related to the selection of autoreactive CD8⁺ T cells with higher T cell receptor avidity, whereas those with lower avidity undergo prompt contraction in patients who clear infection. These findings demonstrate a previously undescribed strict link between the emergence of high frequencies of mixed autoreactive CD8⁺ T cells producing a broad array of cytokines (IFN-γ, IL-17, IL-4, IL-2…) and the progression toward chronic disease in a human model of acute infection.     

Expansion of human cytomegalovirus (HCMV) immediate-early 1-specific CD8+ T cells and control of HCMV replication after allogeneic stem cell transplantation

Recovery of human cytomegalovirus (HCMV)-specific T immunity is critical for protection against HCMV disease in the early phase after allogeneic stem cell transplantation (SCT). Using an enzyme-linked immunospot assay with overlapping 15-mer peptides spanning pp65 and immediate-early 1 HCMV proteins, we investigated which HCMV-specific CD8⁺ gamma interferon-positive (IFN-γ⁺) T-cell responses against pp65 and IE-1 were associated with control of HCMV replication in 48 recipients of unmanipulated HLA-matched allografts at 3 months (M3) and 6 months (M6) after SCT and in 23 donors. At M3 after SCT, the magnitude of the pp65-specific IFN-γ-producing CD8⁺ T-cell response was greater in recipients than in donors, regardless of HCMV status. In contrast, expansion of IE-1-specific CD8⁺ T cells at M3 was associated with protection against HCMV, and no patient with this expansion had HCMV replication at M3. At M6, the number of HCMV-specific CD8⁺ T cells against both pp65 and IE-1 had expanded in all recipients, regardless of their previous levels of HCMV replication. The recipients' HCMV-specific CD8⁺ T cells already detectable in related donors were predominantly targeting pp65. In contrast, in 40% of the cases, the HCMV-specific CD8⁺ T cells in recipients involved new CD8⁺ T-cell specificities undetectable in their related donors and preferentially targeting IE-1. Taken together, these results showed that the delay in reconstituting IE-1-specific CD8⁺ T cells is correlated with the lack of protection against HCMV in the first 3 months after SCT. They also show that IE-1 is a major antigenic determinant of the early restoration of protective immunity to HCMV after SCT.     

Distinct APC Subtypes Drive Spatially Segregated CD4+ and CD8+ T-Cell Effector Activity during Skin Infection with HSV-1

Efficient infection control requires potent T-cell responses at sites of pathogen replication. However, the regulation of T-cell effector function in situ remains poorly understood. Here, we show key differences in the regulation of effector activity between CD4⁺ and CD8⁺ T-cells during skin infection with HSV-1. IFN-γ-producing CD4⁺ T cells disseminated widely throughout the skin and draining lymph nodes (LN), clearly exceeding the epithelial distribution of infectious virus. By contrast, IFN-γ-producing CD8⁺ T cells were only found within the infected epidermal layer of the skin and associated hair follicles. Mechanistically, while various subsets of lymphoid- and skin-derived dendritic cells (DC) elicited IFN-γ production by CD4⁺ T cells, CD8⁺ T cells responded exclusively to infected epidermal cells directly presenting viral antigen. Notably, uninfected cross-presenting DCs from both skin and LNs failed to trigger IFN-γ production by CD8⁺ T-cells. Thus, we describe a previously unappreciated complexity in the regulation of CD4⁺ and CD8⁺ T-cell effector activity that is subset-specific, microanatomically distinct and involves largely non-overlapping types of antigen-presenting cells (APC).     

Inability of Plasmacytoid Dendritic Cells To Directly Lyse HIV-Infected Autologous CD4+ T Cells despite Induction of Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand

The function of plasmacytoid dendritic cells (PDC) in chronic human immunodeficiency virus type 1 (HIV-1) infection remains controversial with regard to its potential for sustained alpha interferon (IFN-α) production and induction of PDC-dependent tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-mediated cytotoxicity of HIV-infected cells. We address these areas by a study of chronically HIV-1-infected subjects followed through antiretroviral therapy (ART) interruption and by testing PDC cytolytic function against autologous HIV-infected CD4⁺ T cells. Rebound in viremia induced by therapy interruption showed a positive association between TRAIL and viral load or T-cell activation, but comparable levels of plasma IFN-α/β were found in viremic ART-treated and control subjects. While PDC from HIV-infected subjects expressed less interferon regulator factor 7 (IRF-7) and produced significantly less IFN-α upon Toll-like receptor 7/9 (TLR7/9) engagement than controls, membrane TRAIL expression in PDC from HIV⁺ subjects was increased. Moreover, no significant increase in death receptor 5 (DR5) expression was seen in CD4⁺ T cells from viremic HIV⁺ subjects compared to controls or following in vitro infection/exposure to infectious and noninfectious virus or exogenous IFN-α, respectively. Although activated PDC killed the DR5-expressing HIV-infected Sup-T1 cell line, PDC did not lyse primary autologous HIV⁺ CD4⁺ T cells yet could provide accessory help for NK cells in killing HIV-infected autologous CD4⁺ T cells. Taken together, our data show a lack of sustained high levels of soluble IFN-α in chronic HIV-1 infection in vivo and document a lack of direct PDC cytolytic activity against autologous infected or uninfected CD4⁺ T cells.Human immunodeficiency virus (HIV) infection is associated with chronic immune activation, progressive immune suppression, and deletion of memory adaptive responses, resulting in increased susceptibility to opportunistic infections (23, 51, 52). Loss of CD4⁺ T cells is the hallmark of HIV infection, with multiple mechanisms proposed as contributing to this loss (activation-induced cell death, direct cytopathic effect, immune cells, and death receptor-mediated apoptosis induction) (reviewed in references 33 and 34). One of the most puzzling observations in AIDS pathogenesis has been the progressive depletion of bystander T cells, especially in lymphoid tissues (25, 33, 34, 55). While antiretroviral therapy (ART) initiated in the early stages of HIV infection, when CD4⁺ T-cell counts are high (>500 cells/μl), may prevent the destruction of lymph node (LN) tissue and the massive depletion of CD4⁺ T lymphocytes by decreasing the rate of virally induced apoptosis (20), a persistent, albeit decreased, level of apoptosis of peripheral blood CD4⁺ and CD8⁺ T cells is seen in ART-treated HIV⁺ subjects despite long-term viral suppression (36).A member of the tumor necrosis factor (TNF) family, TNF-related apoptosis-inducing ligand (TRAIL), has been shown to be involved in HIV-1-associated T-cell apoptosis (33, 34). TRAIL (soluble or membrane bound) induces apoptosis upon binding to death receptor 4 (DR4; also named TRAIL-R1) or DR5 (also named TRAIL-R2, TRICK2, or Killer/DR5).On the basis of the in vitro observation that alpha interferon (IFN-α) and interferon regulator factor 7 (IRF-7) are increased in plasmacytoid dendritic cells (PDC) exposed to HIV-1 (40), the hypothesis that PDC activation by HIV-1 is responsible for an increased level of IFN-α throughout chronic disease has been proposed. It has also been proposed that the activation of the PDC compartment by HIV-1 participates in the initial immune activation following acute infection and contributes to CD4⁺ T-cell depletion by inducing, through IFN-α, the production of TRAIL, which mediates apoptosis of DR5-expressing CD4⁺ T cells following HIV-1 infection (37, 38, 40). However, several lines of evidence question the direct involvement of PDC in the loss of T cells during HIV infection, as PDC numbers are depleted during chronic HIV infection and PDC remaining in circulation are functionally impaired (10). Recent data show that circulating PDC in HIV-infected subjects, although unable to secrete IFN-α after Toll-like receptor (TLR)-mediated activation, constitutively express an increased level of IFN-α mRNA, indicating that during HIV infection PDC are activated yet impaired (71). Rodriguez et al. demonstrated the prevention of spontaneous apoptosis of CD4⁺ and CD8⁺ T cells by IFN-α (63), a major product of PDC following HIV-1 stimulation (3, 28). In addition, Audige et al. (2) showed that blockade of IFN-α and IFN-α receptor during in vitro HIV infection of CD4⁺ T cells isolated from human tonsils did not prevent apoptosis or TRAIL production, suggesting a lack of a central link between IFN-α production and apoptosis of tonsillar CD4⁺ T cells in HIV-1 infection. These data are also consistent with the observation that, in the human peripheral blood lymphocyte-transplanted SCID mouse (hu-PBL-SCID) model, IFN-α efficiently increases the survival of CD4⁺ T cells (49). Thus, controversy remains on the role of IFN-α as an indirect or direct inducer of apoptosis of CD4⁺ T cells through PDC/TRAIL induction, whereas the possibility that IFN-α acts as an antiviral agent by controlling HIV-1 replication and thus reducing virally mediated T-cell loss appears to be supported by several studies (reviewed in references 26, 47, and 58). In this regard, endogenous IFN-α produced by PDC has been shown to play an important role in controlling HIV infection in the human thymus (35), upregulating host antiviral factors such as APOBEC (1, 32, 44, 70) and stimulating NK cell-mediated cytotoxic activity against autologous HIV-infected targets (72).In this report, we investigated the in vivo correlates of viremia in chronically infected subjects by studying the relationship between therapy interruption-associated viremia and plasma IFN-α and TRAIL levels. We also tested in vitro the functional outcome of HIV-1-activated PDC in terms of their ability to mediate lysis of primary autologous CD4 T cells (infected or not with HIV-1), compared to indirect PDC-mediated lysis effects on the NK-dependent antiviral cytotoxic response.     

Gamma Interferon (IFN-γ) Receptor Restricts Systemic Dengue Virus Replication and Prevents Paralysis in IFN-α/β Receptor-Deficient Mice

We previously reported that mice lacking alpha/beta and gamma interferon receptors (IFN-α/βR and -γR) uniformly exhibit paralysis following infection with the dengue virus (DENV) clinical isolate PL046, while only a subset of mice lacking the IFN-γR alone and virtually no mice lacking the IFN-α/βR alone develop paralysis. Here, using a mouse-passaged variant of PL046, strain S221, we show that in the absence of the IFN-α/βR, signaling through the IFN-γR confers approximately 140-fold greater resistance against systemic vascular leakage-associated dengue disease and virtually complete protection from dengue-induced paralysis. Viral replication in the spleen was assessed by immunohistochemistry and flow cytometry, which revealed a reduction in the number of infected cells due to IFN-γR signaling by 2 days after infection, coincident with elevated levels of IFN-γ in the spleen and serum. By 4 days after infection, IFN-γR signaling was found to restrict DENV replication systemically. Clearance of DENV, on the other hand, occurred in the absence of IFN-γR, except in the central nervous system (CNS) (brain and spinal cord), where clearance relied on IFN-γ from CD8⁺ T cells. These results demonstrate the roles of IFN-γR signaling in protection from initial systemic and subsequent CNS disease following DENV infection and demonstrate the importance of CD8⁺ T cells in preventing DENV-induced CNS disease.     

Collaboration of Antibody and Inflammation in Clearance of Rabies Virus from the Central Nervous System

To investigate the involvement of various cellular and humoral aspects of immunity in the clearance of rabies virus from the central nervous system, (CNS), we studied the development of clinical signs and virus clearance from the CNS in knockout mice lacking either B and T cells, CD8⁺ cytotoxic T cells, B cells, alpha/beta interferon (IFN-α/β) receptors, IFN-γ receptors, or complement components C3 and C4. Following intranasal infection with the attenuated rabies virus CVS-F3, normal adult mice of different genetic backgrounds developed a transient disease characterized by loss of body weight and appetite depression which peaked at 13 days postinfection (p.i.). While these animals had completely recovered by day 21 p.i., mice lacking either B and T cells or B cells alone developed a progressive disease and succumbed to infection. Mice lacking either CD8⁺ T cells, IFN receptors, or complement components C3 and C4 showed no significant differences in the development of clinical signs by comparison with intact counterparts having the same genetic background. However, while infectious virus and viral RNA could be detected in normal control mice only until day 8 p.i., in all of the gene knockout mice studied except those lacking C3 and C4, virus infection persisted through day 21 p.i. Analysis of rabies virus-specific antibody production together with histological assessment of brain inflammation in infected animals revealed that clearance of CVS-F3 by 21 days p.i. correlated with both a strong inflammatory response in the CNS early in the infection (day 8 p.i.), and the rapid (day 10 p.i.) production of significant levels of virus-neutralizing antibody (VNA). These studies confirm that rabies VNA is an absolute requirement for clearance of an established rabies virus infection. However, for the latter to occur in a timely fashion, collaboration between VNA and inflammatory mechanisms is necessary.