Critical Role of TLR4 in Human Metapneumovirus Mediated Innate Immune Responses and Disease Pathogenesis

Human metapneumovirus (hMPV) is one of the main causes of acute respiratory tract infections in children, elderly and immunocompromised patients. The mammalian Toll-like receptors (TLR) were identified as critical regulators of innate immunity to a variety of microbes, including viruses. We have recently shown that hMPV-induced cytokine, chemokine and type I interferon secretion in dendritic cells occurs via TLR4, however, its role in hMPV-induced disease is unknown. In this study, wild-type(WT) and TLR4-deficient mice (TLR4^−/−) were infected with hMPV and examined for clinical disease parameters, such as body weight loss and airway obstruction, viral clearance, lung inflammation, dendritic cell maturation, T-cell proliferation and antibody production. Our results demonstrate that absence of TLR4 in hMPV-infected mice significantly reduced the inflammatory response as well as disease severity, shown by reduced body weight loss and airway obstruction and hyperresponsiveness (AHR), compared to WT mice. Levels of cytokines and chemokines were also significantly lower in the TLR4^−/− mice. Accordingly, recruitment of inflammatory cells in the BAL, lungs, as well as in lymph nodes, was significantly reduced in the TLR4^−/− mice, however, viral replication and clearance, as well as T-cell proliferation and neutralizing antibody production, were not affected. Our findings indicate that TLR4 is important for the activation of the innate immune response to hMPV, however it does play a role in disease pathogenesis, as lack of TLR4 expression is associated with reduced clinical manifestations of hMPV disease, without affecting viral protection.     

Memory CD4+ T-cell-mediated protection from lethal coronavirus encephalomyelitis

The antiviral role of CD4⁺ T cells in virus-induced pathologies of the central nervous system (CNS) has not been explored extensively. Control of neurotropic mouse hepatitis virus (JHMV) requires the collaboration of CD4⁺ and CD8⁺ T cells, with CD8⁺ T cells providing direct perforin and gamma interferon (IFN-γ)-mediated antiviral activity. To distinguish bystander from direct antiviral contributions of CD4⁺ T cells in virus clearance and pathology, memory CD4⁺ T cells purified from wild type (wt), perforin-deficient (PKO), and IFN-γ-deficient (GKO) immune donors were transferred to immunodeficient SCID mice prior to CNS challenge. All three donor CD4⁺ T-cell populations controlled CNS virus replication at 8 days postinfection, indicating IFN-γ- and perforin-independent antiviral function. Recipients of GKO CD4⁺ T cells succumbed more rapidly to fatal disease than untreated control infected mice. In contrast, wt and PKO donor CD4⁺ T cells cleared infectious virus to undetectable levels and protected from fatal disease. Recipients of all CD4⁺ T-cell populations exhibited demyelination. However, it was more severe in wt CD4⁺ T-cell recipients. These data support a role of CD4⁺ T cells in virus clearance and demyelination. Despite substantial IFN-γ-independent antiviral activity, IFN-γ was crucial in providing protection from death. IFN-γ reduced neutrophil accumulation and directed macrophages to white matter but did not ameliorate myelin loss.     

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.     

FoxP3+ CD25+ CD8+ T-cell induction during primary simian immunodeficiency virus infection in cynomolgus macaques correlates with low CD4+ T-cell activation and high viral load

The early immune response fails to prevent the establishment of chronic human immunodeficiency virus (HIV) infection but may influence viremia during primary infection, thereby possibly affecting long-term disease progression. CD25⁺ FoxP3⁺ regulatory T cells may contribute to HIV/simian immunodeficiency virus (SIV) pathogenesis by suppressing efficient antiviral responses during primary infection, favoring high levels of viral replication and the establishment of chronic infection. In contrast, they may decrease immune activation during chronic infection. CD4⁺ regulatory T cells have been studied in the most detail, but CD8⁺ CD25⁺ FoxP3⁺ T cells also have regulatory properties. We monitored the dynamics of CD25⁺ FoxP3⁺ T cells during primary and chronic SIVmac251 infection in cynomolgus macaques. The number of peripheral CD4⁺ CD25⁺ FoxP3⁺ T cells paralleled that of memory CD4⁺ T cells, with a rapid decline during primary infection followed by a rebound to levels just below baseline and gradual depletion during the course of infection. No change in the proportion of CD25⁺ FoxP3⁺ T cells was observed in peripheral lymph nodes. A small number of CD4⁺ CD25⁺ FoxP3⁺ T cells at set point was associated with a high plasma viral load. In contrast, peripheral CD8⁺ CD25⁺ FoxP3⁺ T cells were induced a few days after peak plasma viral load during primary infection. The number of these cells was positively correlated with viral load and negatively correlated with CD4⁺ T-cell activation, SIV antigen-specific proliferative responses during primary infection, and plasma viral load at set point, with large numbers of CD8⁺ CD25⁺ FoxP3⁺ T cells being indicative of a poor prognosis.     

Role of PKR and Type I IFNs in Viral Control during Primary and Secondary Infection

Type I interferons (IFNs) are known to mediate viral control, and also promote survival and expansion of virus-specific CD8⁺ T cells. However, it is unclear whether signaling cascades involved in eliciting these diverse cellular effects are also distinct. One of the best-characterized anti-viral signaling mechanisms of Type I IFNs is mediated by the IFN-inducible dsRNA activated protein kinase, PKR. Here, we have investigated the role of PKR and Type I IFNs in regulating viral clearance and CD8⁺ T cell response during primary and secondary viral infections. Our studies demonstrate differential requirement for PKR, in viral control versus elicitation of CD8⁺ T cell responses during primary infection of mice with lymphocytic choriomeningitis virus (LCMV). PKR-deficient mice mounted potent CD8⁺ T cell responses, but failed to effectively control LCMV. The compromised LCMV control in the absence of PKR was multifactorial, and linked to less effective CD8⁺ T cell-mediated viral suppression, enhanced viral replication in cells, and lower steady state expression levels of IFN-responsive genes. Moreover, we show that despite normal expansion of memory CD8⁺ T cells and differentiation into effectors during a secondary response, effective clearance of LCMV but not vaccinia virus required PKR activity in infected cells. In the absence of Type I IFN signaling, secondary effector CD8⁺ T cells were ineffective in controlling both LCMV and vaccinia virus replication in vivo. These findings provide insight into cellular pathways of Type I IFN actions, and highlight the under-appreciated importance of innate immune mechanisms of viral control during secondary infections, despite the accelerated responses of memory CD8⁺ T cells. Additionally, the results presented here have furthered our understanding of the immune correlates of anti-viral protective immunity, which have implications in the rational design of vaccines.     

Primary clearance of murine gammaherpesvirus 68 by PKCtheta-/- CD8 T cells is compromised in the absence of help from CD4 T cells

CD4 T cells are dispensable for acute control of murine gammaherpesvirus 68 (MHV-68) but are necessary for effective long-term control of the virus by CD8 T cells. In contrast, protein kinase C θ (PKCθ) is not essential for either acute or long-term viral control. However, we found that while either CD4 or CD8 T cells could mediate the clearance of MHV-68 from the lungs of PKCθ^+/+ mice, PKCθ^−/− mice depleted of either subset failed to clear the virus. These data suggest that there are two alternative pathways for MHV-68 clearance, one dependent on CD4 T cells and the other on PKCθ. Protection mediated by the latter appears to be short-lived. These observations may help to explain the differential requirement for PKCθ in various models of CD8 T-cell activation and differences in the costimulatory requirements for acute and long-term viral control.     

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.     

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.     

Effector CD4+ T-cell involvement in clearance of infectious herpes simplex virus type 1 from sensory ganglia and spinal cords

In primary infection, CD8⁺ T cells are important for clearance of infectious herpes simplex virus (HSV) from sensory ganglia. In this study, evidence of CD4⁺ T-cell-mediated clearance of infectious HSV type 1 (HSV-1) from neural tissues was also detected. In immunocompetent mice, HSV-specific CD4⁺ T cells were present in sensory ganglia and spinal cords coincident with HSV-1 clearance from these sites and remained detectable at least 8 months postinfection. Neural CD4⁺ T cells isolated at the peak of neural infection secreted gamma interferon, tumor necrosis factor alpha, interleukin-2 (IL-2), or IL-4 after stimulation with HSV antigen. HSV-1 titers in neural tissues were greatly reduced over time in CD8⁺ T-cell-deficient and CD8⁺ T-cell-depleted mice, suggesting that CD4⁺ T cells could mediate clearance of HSV-1 from neural tissue. To examine possible mechanisms by which CD4⁺ T cells resolved neural infection, CD8⁺ T cells were depleted from perforin-deficient or FasL-defective mice. Clearance of infectious virus from neural tissues was not significantly different in perforin-deficient or FasL-defective mice compared to wild-type mice. Further, in spinal cords and brains after vaginal HSV-1 challenge of chimeric mice expressing both perforin and Fas or neither perforin nor Fas, virus titers were significantly lower than in control mice. Thus, perforin and Fas were not required for clearance of infectious virus from neural tissues. These results suggest that HSV-specific CD4⁺ T cells are one component of a long-term immune cell presence in neural tissues following genital HSV-1 infection and play a role in clearance of infectious HSV-1 at neural sites, possibly via a nonlytic mechanism.     

Attenuation of antigen-induced airway hyperresponsiveness and inflammation in CXCR3 knockout mice

Background

CD8+ T cells participate in airway hyperresponsiveness (AHR) and allergic pulmonary inflammation that are characteristics of asthma. CXCL10 by binding to CXCR3 expressed preferentially on activated CD8+ T cells, attracts T cells homing to the lung. We studied the contribution and limitation of CXCR3 to AHR and airway inflammation induced by ovalbumin (OVA) using CXCR3 knockout (KO) mice.

Methods

Mice were sensitized and challenged with OVA. Lung histopathological changes, AHR, cellular composition and levels of inflammatory mediators in bronchoalveolar lavage (BAL) fluid, and lungs at mRNA and protein levels, were compared between CXCR3 KO mice and wild type (WT) mice.

Results

Compared with the WT controls, CXCR3 KO mice showed less OVA-induced infiltration of inflammatory cells around airways and vessels, and less mucus production. CXCR3 KO mice failed to develop significant AHR. They also demonstrated significantly fewer CD8+ T and CD4+ T cells in BAL fluid, lower levels of TNFα and IL-4 in lung tissue measured by real-time RT-PCR and in BAL fluid by ELISA, with significant elevation of IFNγ mRNA and protein expression levels.

Conclusions

We conclude that CXCR3 is crucial for AHR and airway inflammation by promoting recruitment of more CD8+ T cells, as well as CD4+ T cells, and initiating release of proinflammatory mediators following OVA sensitization and challenge. CXCR3 may represent a novel therapeutic target for asthma.     

Reduced protection from simian immunodeficiency virus SIVmac251 infection afforded by memory CD8+ T cells induced by vaccination during CD4+ T-cell deficiency

Adaptive CD4⁺ and CD8⁺ T-cell responses have been associated with control of human immunodeficiency virus/simian immunodeficiency virus (HIV/SIV) replication. Here, we have designed a study with Indian rhesus macaques to more directly assess the role of CD8 SIV-specific responses in control of viral replication. Macaques were immunized with a DNA prime-modified vaccinia virus Ankara (MVA)-SIV boost regimen under normal conditions or under conditions of antibody-induced CD4⁺ T-cell deficiency. Depletion of CD4⁺ cells was performed in the immunized macaques at the peak of SIV-specific CD4⁺ T-cell responses following the DNA prime dose. A group of naïve macaques was also treated with the anti-CD4 depleting antibody as a control, and an additional group of macaques immunized under normal conditions was depleted of CD8⁺ T cells prior to challenge exposure to SIV_mac251. Analysis of the quality and quantity of vaccine-induced CD8⁺ T cells demonstrated that SIV-specific CD8⁺ T cells generated under conditions of CD4⁺ T-cell deficiency expressed low levels of Bcl-2 and interleukin-2 (IL-2), and plasma virus levels increased over time. Depletion of CD8⁺ T cells prior to challenge exposure abrogated vaccine-induced protection as previously shown. These data support the notion that adaptive CD4⁺ T cells are critical for the generation of effective CD8⁺ T-cell responses to SIV that, in turn, contribute to protection from AIDS. Importantly, they also suggest that long-term protection from disease will be afforded only by T-cell vaccines for HIV that provide a balanced induction of CD4⁺ and CD8⁺ T-cell responses and protect against early depletion of CD4⁺ T cells postinfection.     

CD8+ T Cells Cause Disability and Axon Loss in a Mouse Model of Multiple Sclerosis

Background

The objective of this study was to test the hypothesis that CD8⁺ T cells directly mediate motor disability and axon injury in the demyelinated central nervous system. We have previously observed that genetic deletion of the CD8⁺ T cell effector molecule perforin leads to preservation of motor function and preservation of spinal axons in chronically demyelinated mice.

Methodology/Principal Findings

To determine if CD8⁺ T cells are necessary and sufficient to directly injure demyelinated axons, we adoptively transferred purified perforin-competent CD8⁺ spinal cord-infiltrating T cells into profoundly demyelinated but functionally preserved perforin-deficient host mice. Transfer of CD8⁺ spinal cord-infiltrating T cells rapidly and irreversibly impaired motor function, disrupted spinal cord motor conduction, and reduced the number of medium- and large-caliber spinal axons. Likewise, immunodepletion of CD8⁺ T cells from chronically demyelinated wildtype mice preserved motor function and limited axon loss without altering other disease parameters.

Conclusions/Significance

In multiple sclerosis patients, CD8⁺ T cells outnumber CD4⁺ T cells in active lesions and the number of CD8⁺ T cells correlates with the extent of ongoing axon injury and functional disability. Our findings suggest that CD8⁺ T cells may directly injure demyelinated axons and are therefore a viable therapeutic target to protect axons and motor function in patients with multiple sclerosis.     

Critical Role for CD4+ T Cells in Controlling Retrovirus Replication and Spread in Persistently Infected Mice

Reactivations of persistent viral infections pose a significant medical problem in immunocompromised cancer, transplant, and AIDS patients, yet little is known about how persistent viral infections are immunologically controlled. Here we describe a mouse model for investigating the role of the immune response in controlling a persistent retroviral infection. We demonstrate that, following recovery from acute Friend virus infection, a small number of B cells evade immunological destruction and harbor persistent virus. In vivo depletions of T-cell subsets in persistently infected mice revealed a critical role for CD4⁺ T cells in controlling virus replication, spread to the erythroid lineage, and induction of erythroleukemia. The CD4⁺ T-cell effect was independent of CD8⁺ T cells and in some cases was also independent of virus-neutralizing antibody responses. Thus, the CD4⁺ T cells may have had a direct antiviral effect. These results may have relevance for human immunodeficiency virus (HIV) infections where loss of CD4⁺ T cells is associated with an increase in HIV replication, reactivation of persistent viruses, and a high incidence of virus-associated cancers.     

MyD88 Intrinsically Regulates CD4 T-Cell Responses

Myeloid differentiation factor 88 (MyD88) is an essential adaptor protein in the Toll-like receptor-mediated innate signaling pathway, as well as in interleukin-1 receptor (IL-1R) and IL-18R signaling. The importance of MyD88 in the regulation of innate immunity to microbial pathogens has been well demonstrated. However, its role in regulating acquired immunity to viral pathogens and neuropathogenesis is not entirely clear. In the present study, we examine the role of MyD88 in the CD4⁺ T-cell response following lymphocytic choriomeningitis virus (LCMV) infection. We demonstrate that wild-type (WT) mice developed a CD4⁺ T-cell-mediated wasting disease after intracranial infection with LCMV. In contrast, MyD88 knockout (KO) mice did not develop wasting disease in response to the same infection. This effect was not the result of MyD88 regulation of IL-1 or IL-18 responses since IL-1R1 KO and IL-18R KO mice were not protected from weight loss. In the absence of MyD88, naïve CD4⁺ T cells failed to differentiate to LCMV-specific CD4 T cells. We demonstrated that MyD88 KO antigen-presenting cells are capable of activating WT CD4⁺ T cells. Importantly, when MyD88 KO CD4⁺ T cells were reconstituted with an MyD88-expressing lentivirus, the rescued CD4⁺ T cells were able to respond to LCMV infection and support IgG2a antibody production. Overall, these studies reveal a previously unknown role of MyD88-dependent signaling in CD4⁺ T cells in the regulation of the virus-specific CD4⁺ T-cell response and in viral infection-induced immunopathology in the central nervous system.     

Both Conventional and Interferon Killer Dendritic Cells Have Antigen-Presenting Capacity during Influenza Virus Infection

Natural killer cells are innate effector cells known for their potential to produce interferon-γ and kill tumour and virus-infected cells. Recently, B220⁺CD11c^intNK1.1⁺ NK cells were found to also have antigen-presenting capacity like dendritic cells (DC), hence their name interferon-producing killer DC (IKDC). Shortly after discovery, it has already been questioned if IKDC really represent a separate subset of NK cells or merely represent a state of activation. Despite similarities with DCs, in vivo evidence that they behave as bona fide APCs is lacking. Here, using a model of influenza infection, we found recruitment of both conventional B220⁻ NK cells and IKDCs to the lung. To study antigen-presenting capacity of NK cell subsets and compare it to cDCs, all cell subsets were sorted from lungs of infected mice and co-cultured ex vivo with antigen specific T cells. Both IKDCs and conventional NK cells as well as cDCs presented virus-encoded antigen to CD8 T cells, whereas only cDCs presented to CD4 T cells. The absence of CD4 responses was predominantly due to a deficiency in MHCII processing, as preprocessed peptide antigen was presented equally well by cDCs and IKDCs. In vivo, the depletion of NK1.1-positive NK cells and IKDCs reduced the expansion of viral nucleoprotein-specific CD8 T cells in the lung and spleen, but did finally not affect viral clearance from the lung. In conclusion, we found evidence for APC function of lung NK cells during influenza infection, but this is a feature not exclusive to the IKDC subset.     

Dendritic cells are less susceptible to human immunodeficiency virus type 2 (HIV-2) infection than to HIV-1 infection

Human immunodeficiency virus type 1 (HIV-1) infection of dendritic cells (DCs) has been documented in vivo and may be an important contributor to HIV-1 transmission and pathogenesis. HIV-1-specific CD4⁺ T cells respond to HIV antigens presented by HIV-1-infected DCs and in this process become infected, thereby providing a mechanism through which HIV-1-specific CD4⁺ T cells could become preferentially infected in vivo. HIV-2 disease is attenuated with respect to HIV-1 disease, and host immune responses are thought to be contributory. Here we investigated the susceptibility of primary myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) to infection by HIV-2. We found that neither CCR5-tropic primary HIV-2 isolates nor a lab-adapted CXCR4-tropic HIV-2 strain could efficiently infect mDCs or pDCs, though these viruses could infect primary CD4⁺ T cells in vitro. HIV-2-exposed mDCs were also incapable of transferring virus to autologous CD4⁺ T cells. Despite this, we found that HIV-2-specific CD4⁺ T cells contained more viral DNA than memory CD4⁺ T cells of other specificities in vivo. These data suggest that either infection of DCs is not an important contributor to infection of HIV-2-specific CD4⁺ T cells in vivo or that infection of DCs by HIV-2 occurs at a level that is undetectable in vitro. The frequent carriage of HIV-2 DNA within HIV-2-specific CD4⁺ T cells, however, does not appear to be incompatible with preserved numbers and functionality of HIV-2-specific CD4⁺ T cells in vivo, suggesting that additional mechanisms contribute to maintenance of HIV-2-specific CD4⁺ T-cell help in vivo.     

Ligand-independent exhaustion of killer immunoglobulin-like receptor-positive CD8+ T cells in human immunodeficiency virus type 1 infection

Virus-specific CD8⁺ T cells play a central role in the control of viral infections, including human immunodeficiency virus type 1 (HIV-1) infection. However, despite the presence of strong and broad HIV-specific CD8⁺ T-cell responses in chronic HIV-1 infection, these cells progressively lose critical effector functions and fail to clear the infection. Mounting evidence suggests that the upregulation of several inhibitory regulatory receptors on the surface of CD8⁺ T cells during HIV-1 infection may contribute directly to the impairment of T-cell function. Here, we investigated the role of killer immunoglobulin receptors (KIR), which are expressed on NK cells and on CD8⁺ T cells, in regulating CD8⁺ T-cell function in HIV-1 infection. KIR expression was progressively upregulated on CD8⁺ T cells during HIV-1 infection and correlated with the level of viral replication. Expression of KIR was associated with a profound inhibition of cytokine secretion, degranulation, proliferation, and activation by CD8⁺ T cells following stimulation with T-cell receptor (TCR)-dependent stimuli. In contrast, KIR⁺ CD8⁺ T cells responded potently to TCR-independent stimulation, demonstrating that these cells are functionally competent. KIR-associated suppression of CD8⁺ T-cell function was independent of ligand engagement, suggesting that these regulatory receptors may constitutively repress TCR activation. This ligand-independent repression of TCR activation of KIR⁺ CD8⁺ T cells may represent a significant barrier to therapeutic interventions aimed at improving the quality of the HIV-specific CD8⁺ T-cell response in infected individuals.     

Evidence of CD8+ T-cell-mediated selective pressure on human immunodeficiency virus type 1 nef in HLA-B*57+ elite suppressors

Elite suppressors (ES) are human immunodeficiency virus type 1 (HIV-1)-infected patients who maintain viral loads of <50 copies/ml without treatment. The observation that the HLA-B*57 allele is overrepresented in these patients implies that HIV-1-specific CD8⁺ T cells play a key role in suppressing viral replication. We have previously shown that while CD8⁺ T-cell escape mutations are rarely seen in proviral Gag sequences in resting CD4⁺ T cells from peripheral blood, they are present in every clone amplified from the low levels of free virus in the plasma of HLA-B*57⁺ ES. In this study, we compared the pattern of mutations in Nef sequences amplified from peripheral blood CD4⁺ T cells and from plasma virus. We show that Nef mutations are present in plasma virus but are rare in the cellular sequences and provide evidence that these plasma Nef variants represent novel escape mutants. The results provide further evidence of CD8⁺ T-cell-mediated selective pressure on plasma virus in ES and suggest that there must be ongoing HIV-1 replication in spite of the very low viral loads seen for these patients.     

Requirement for CD4+ T Cells in the Friend Murine Retrovirus Neutralizing Antibody Response: Evidence for Functional T Cells in Genetic Low-Recovery Mice

Recovery from infection with the Friend murine leukemia retrovirus complex (FV) requires T-helper cells and cytotoxic T cells as well as neutralizing antibodies. Several host genes, including genes of the major histocompatibility complex (H-2) and an H-2-unlinked gene, Rfv-3, influence these FV-specific immune responses. (B10.A × A/Wy)F₁ mice, which have the H-2^a/a Rfv-3^r/s genotype, fail to mount a detectable FV-specific T-cell proliferative response but nevertheless produce FV-specific neutralizing immunoglobulin M (IgM) antibodies and can eliminate FV viremia. Thus, this IgM response, primarily influenced by the Rfv-3 gene, may be T-cell independent. To test this idea, mice were depleted of either CD4⁺ or CD8⁺ T-cell populations in vivo and were monitored for the effect on the neutralizing antibody response following FV infection. Surprisingly, mice in which CD4⁺ cells were depleted showed undetectable FV-neutralizing antibody responses and high viremia levels compared to nondepleted or CD8-depleted animals. In addition to knocking out the FV antibody response, CD4⁺ T-cell depletion reduced survival time significantly, further indicating the importance of CD4⁺ T cells. These studies revealed the first evidence for a functional T-cell response following FV infection in these low-recovery mice and showed that CD4⁺ T-helper cells are required for the Rfv-3-controlled FV antibody response.