CD8+ cells from HIV-2-infected baboons control HIV replication

The CD8+ T Cell Noncytotoxic Antiviral Responses

The CD8+ T cell noncytotoxic antiviral response (CNAR) was discovered during studies of asymptomatic HIV-infected subjects more than 30 years ago. In contrast to CD8+ T cell cytotoxic lymphocyte (CTL) activity, CNAR suppresses HIV replication without target cell killing. This activity has characteristics of innate immunity: it acts on all retroviruses and thus is neither epitope specific nor HLA restricted. The HIV-associated CNAR does not affect other virus families. It is mediated, at least in part, by a CD8+ T cell antiviral factor (CAF) that blocks HIV transcription. A variety of assays used to measure CNAR/CAF and the effects on other retrovirus infections are described. Notably, CD8+ T cell noncytotoxic antiviral responses have now been observed with other virus families but are mediated by different cytokines. Characterizing the protein structure of CAF has been challenging despite many biologic, immunologic, and molecular studies. It represents a low-abundance protein that may be identified by future next-generation sequencing approaches. Since CNAR/CAF is a natural noncytotoxic activity, it could provide promising strategies for HIV/AIDS therapy, cure, and prevention.

Baboon CD8 T cells suppress SIVmac infection in CD4 T cells through contact-dependent production of MIP-1α, MIP-1β, and RANTES

Simian immunodeficiency virus (SIV) infection in rhesus macaques is often characterized by high viremia and CD4 T cell depletion. By contrast, SIV infection in African nonhuman primate natural hosts is typically nonpathogenic despite active viral replication. Baboons are abundant in Africa and have a geographical distribution that overlaps with natural hosts, but they do not harbor SIVs. Previous work has demonstrated baboons are resistant to chronic SIV infection and/or disease in vivo but the underlying mechanisms remain unknown. Using in vitro SIVmac infections, we sought to identify SIV restriction factors in baboons by comparing observations to the pathogenic rhesus macaque model. SIVmac replicated in baboon PBMC but had delayed kinetics compared to rhesus PBMC. However, SIVmac replication in baboon and rhesus isolated CD4 cells were similar to the kinetics seen for rhesus PBMC, demonstrating intracellular restriction factors do not play a strong role in baboon inhibition of SIVmac replication. Here, we show CD8 T cells contribute to the innate SIV-suppressive activity seen in naïve baboon PBMC. As one mechanism of restriction, we identified higher production of MIP-1α, MIP-1β, and RANTES by baboon PBMC. Contact between CD4 and CD8 T cells resulted in maximum production of these chemokines and suppression of viral replication, whereas neutralization of CCR5-binding chemokines in baboon PBMC increased viral loads. Our studies indicate baboon natural restriction of SIVmac replication is largely dependent on CD4-extrinsinc mechanisms mediated, in part, by CD8 T cells.

Noncytolytic CD8+ Cell Mediated Antiviral Response Represents a Strong Element in the Immune Response of Simian Immunodeficiency Virus-Infected Long-Term Non-Progressing Rhesus Macaques

The ability of long term non progressors to maintain very low levels of HIV/SIV and a healthy state, involves various host genetic and immunological factors. CD8+ non-cytolytic antiviral response (CNAR) most likely plays an important role in this regard. In order to gain a deeper insight into this unique phenomenon, the ability of CD8+ T cells to suppress viral replication in vitro was investigated in 16 uninfected, longitudinally in 23 SIV-infected long-term non-progressing (LTNPs), and 10 SIV-infected rhesus macaques with progressing disease. An acute infection assay utilizing CD4⁺ cells from MHC-mismatched monkeys to avoid cytolytic responses was employed. The study has identified CNAR as a long-term stable activity that inversely correlated with plasma viral load. The activity was also detected in CD8⁺ cells of uninfected macaques, which indicates that CNAR is not necessarily a virus specific response but increases after SIV-infection. Physical contact between CD4⁺ and CD8⁺ cells was mainly involved in mediating viral inhibition. Loss of this activity appeared to be due to a loss of CNAR-expressing CD8+ cells as well as a reduction of CNAR-responsive CD4+ cells. In contrast, in vitro viral replication did not differ in CD4+ cells from un-infected macaques, CNAR(+) and CNAR(-) LTNPs. A role for transitional memory cells in supporting CNAR in the macaque model of AIDS was questionable. CNAR appears to represent an important part of the immune response displayed by CD8+ T cells which might be underestimated up to now.

The CD8 antiviral factor (CAF) can suppress HIV-1 transcription from the long terminal repeat (LTR) promoter in the absence of elements upstream of the CATATAA box

Background

The CD8 Antiviral Factor (CAF) suppresses viral transcription from the HIV-1 Long Terminal Repeat (LTR) promoter in a non-cytolytic manner. However, the region on the LTR upon which CAF acts is unknown. Our objective was to determine the region on the LTR upon which CAF acts to suppress HIV-1 transcription.

Methods

Serial deletions of the LTR from the 5’ end and inactivating point mutations were made.

Results

Serial deletions of the LTR from the 5’ end indicated the importance of a short ~120 bp segment, containing the 3 SpI sites, CATA box (used by HIV-1 instead of the TATA box) and TAR region, in the suppressive process. Introduction of deletions or inactivating point mutations in the SpI sites or deletion of the TAR region did not abolish CAF-mediated transcriptional suppression. Yet, CAF-mediated transcriptional suppression was still retained in the HIV-1 CATA-TAR segment.

Conclusion

CAF is able to suppress transcription from the LTR lacking all the elements upstream of the CATA box. Our results suggest that the HIV-1 CATA box may be responsible for CAF-mediated suppression of transcription from the HIV-1 LTR.

Suppression of antigen-specific T cell responses by the Kaposi's sarcoma-associated herpesvirus viral OX2 protein and its cellular orthologue, CD200

Regulating appropriate activation of the immune response in the healthy host despite continual immune surveillance dictates that immune responses must be either self-limiting and therefore negatively regulated following their activation or prevented from developing inappropriately. In the case of antigen-specific T cells, their response is attenuated by several mechanisms, including ligation of CTLA-4 and PD-1. Through the study of the viral OX2 (vOX2) immunoregulator encoded by Kaposi's sarcoma-associated herpesvirus (KSHV), we have identified a T cell-attenuating role both for this protein and for CD200, a cellular orthologue of the viral vOX2 protein. In vitro, antigen-presenting cells (APC) expressing either native vOX2 or CD200 suppressed two functions of cognate antigen-specific T cell clones: gamma interferon (IFN-γ) production and mobilization of CD107a, a cytolytic granule component and measure of target cell killing ability. Mechanistically, vOX2 and CD200 expression on APC suppressed the phosphorylation of ERK1/2 mitogen-activated protein kinase in responding T cells. These data provide the first evidence for a role of both KSHV vOX2 and cellular CD200 in the negative regulation of antigen-specific T cell responses. They suggest that KSHV has evolved to harness the host CD200-based mechanism of attenuation of T cell responses to facilitate virus persistence and dissemination within the infected individual. Moreover, our studies define a new paradigm in immune modulation by viruses: the provision of a negative costimulatory signal to T cells by a virus-encoded orthologue of CD200.

Natural suppression of human immunodeficiency virus type 1 replication is mediated by transitional memory CD8+ T cells

HIV replication is suppressed in vitro by a CD8(+) cell noncytotoxic antiviral response (CNAR). This activity directly correlates with an asymptomatic clinical state. The objective of this study was to identify the phenotype of CD8(+) cell subsets having strong CNAR activity. CD8(+) cell subset frequencies and CNAR levels were measured for human immunodeficiency virus (HIV)-uninfected individuals and three groups of HIV type 1 (HIV-1)-infected individuals: asymptomatic individuals with low-level viremia (vHIV), antiretroviral-drug-treated subjects with undetectable virus levels (TxHIV), and therapy-naïve aviremic elite controllers (EC). CD8(+) cells from the vHIV individuals exhibited the highest HIV-suppressing activity and had elevated frequencies of CD45RA(-) CD27(+) and PD-1(+) (CD279(+)) cells. Functional assessments of CD8(+) cells sorted into distinct subsets established that maximal CNAR activity was mediated by CD45RA(-) CCR7(-) CD27(+) and PD-1(+) CD8(+) cells. T cell receptor (TCR) repertoire profiles of CD8(+) cell subsets having strong CNAR activity exhibited increased perturbations in comparison to those of inactive subsets. Together, these studies suggest that CNAR is driven by HIV replication and that this antiviral activity is associated with oligoclonally expanded activated CD8(+) cells expressing PD-1 and having a transitional memory cell phenotype. The findings better describe the identity of CD8(+) cells showing CNAR and should facilitate the evaluation of this important immune response in studies of HIV pathogenesis, resistance to infection, and vaccine development.

Noncytotoxic suppression of human immunodeficiency virus type 1 transcription by exosomes secreted from CD8+ T cells

CD8(+) T cells display a noncytotoxic activity that suppresses transcription of human immunodeficiency virus type 1 (HIV-1) in an antigen-independent and major histocompatibility complex-unrestricted manner. To date, the precise cellular and molecular factors mediating this CD8(+) T-cell effector function remain unsolved. Despite evidence indicating the dependence of the activity on cell-cell contact, the possibility of a membrane-mediated activity that represses transcription from the viral promoter remains unexplored. We therefore investigated whether this inhibition of HIV-1 transcription might be elicited by a membrane-bound determinant. Using a CD8(+) T-cell line displaying potent noncytotoxic HIV-1 suppression activity, we have identified a membrane-localized HIV-1-suppressing activity that is concomitantly secreted as 30- to 100-nm endosome-derived tetraspanin-rich vesicles known as exosomes. Purified exosomes from CD8(+) T-cell culture supernatant noncytotoxically suppressed CCR5-tropic (R5) and CXCR4-tropic (X4) replication of HIV-1 in vitro through a protein moiety. Similar antiviral activity was also found in exosomes isolated from two HIV-1-infected subjects. The antiviral exosomes specifically inhibited HIV-1 transcription in both acute and chronic models of infection. Our results, for the first time, indicate the existence of an antiviral membrane-bound factor consistent with the hallmarks defining noncytotoxic CD8(+) T-cell suppression of HIV-1.

Changes in soluble factor-mediated CD8+ cell-derived antiviral activity in cynomolgus macaques infected with simian immunodeficiency virus SIVmac251: relationship to biological markers of progression

Cross-sectional studies have shown that the capacity of CD8+ cells from human immunodeficiency virus (HIV)-infected patients and simian immunodeficiency virus (SIV) SIVmac-infected macaques to suppress the replication of human and simian immunodeficiency viruses in vitro depends on the clinical stage of disease, but little is known about changes in this antiviral activity over time in individual HIV-infected patients or SIV-infected macaques. We assessed changes in the soluble factor-mediated noncytolytic antiviral activity of CD8+ cells over time in eight cynomolgus macaques infected with SIVmac251 to determine the pathophysiological role of this activity. CD8+ cell-associated antiviral activity increased rapidly in the first week after viral inoculation and remained detectable during the early phase of infection. The net increase in antiviral activity of CD8+ cells was correlated with plasma viral load throughout the 15 months of follow-up. CD8+ cells gradually lost their antiviral activity over time and acquired virus replication-enhancing capacity. Levels of antiviral activity correlated with CD4+ T-cell counts after viral set point. Concentrations of beta-chemokines and interleukin-16 in CD8+ cell supernatants were not correlated with this antiviral activity, and alpha-defensins were not detected. The soluble factor-mediated antiviral activity of CD8+ cells was neither cytolytic nor restricted to major histocompatibility complex. This longitudinal study strongly suggests that the increase in noncytolytic antiviral activity from baseline and the maintenance of this increase over time in cynomolgus macaques depend on both viral replication and CD4+ T cells.

Human immunodeficiency virus type 2 DNA vaccine provides partial protection from acute baboon infection

We determined if the genetic adjuvants, granulocyte-macrophage colony stimulating factor (GM-CSF) and B7-2, could improve the immunogenicity and efficacy of an HIV-2 DNA vaccine. The vaccine consisted of the HIV-2 tat, nef, gag, and env genes synthesized using optimized codons and formulated with cationic liposomes. Baboons (Papio cynocephalus hamadryas) were immunized by the intramuscular, intradermal, and intranasal routes with these expression constructs and challenged with HIV-2(UC2) by the intravaginal route. In the first month after HIV-2 vaginal challenge, the baboons receiving the HIV-2 DNA vaccine with or without the genetic adjuvants had significant reductions in the viral loads in the peripheral blood mononuclear cells (PBMC) (P = 0.028) while the reductions in their plasma viremia were suggestive of a protective effect (P = 0.1). These data demonstrate that partial protection against HIV-2 vaginal challenge, as measured by reduced viral load, can be achieved using only a DNA vaccine formulation.

The search for the CD8+ cell anti-HIV factor (CAF)

Efforts to control HIV infection have led to the development of several antiretroviral drugs that can limit virus replication, however, these therapies do not offer a long-term solution to the infection. We can learn a great deal from HIV-infected individuals who have lived for more than ten years and remain healthy without receiving antiviral drugs. These long-term survivors or long-term non-progressors have an immune system that can control HIV infection. A major component of this immune response is innate immunity, particularly the CD8(+) cell antiviral non-cytotoxic response (CNAR), mediated by a novel CD8(+) cell antiviral factor (CAF). The characteristics of CNAR and CAF will be described and progress made toward identifying CAF will be reviewed. These studies have uncovered several potentially important natural anti-HIV factors and their relationship to the originally described CAF is considered.

Differential gene expression in CD8+ cells exhibiting noncytotoxic anti-HIV activity

Suppressive subtractive hybridization with polymerase chain reaction was used to identify the gene(s) associated with the CD8+ cell noncytotoxic anti-HIV response. The differences in gene expression profiles of CD8+ cells from a pair of discordant HIV-positive identical twins were studied. Forty-nine genes were identified as expressed at higher levels in the CD8+ cells from the infected twin that inhibited viral replication. The differential expression of these genes was then evaluated using Q-PCR to determine if this gene expression pattern is evident in CD8+ cells from other HIV-positive subjects showing this antiviral activity. Three genes, including one unknown, were found to have significantly increased expression in antiviral CD8+ cells.

Expression patterns of phenotypic markers on lymphocytes from human immunodeficiency virus type 2-infected baboons

The development of AIDS in HIV-1-infected humans is associated with profound changes in the expression patterns of lymphocyte phenotypic markers associated with increased immune activation and with decreased recall immune responses. In assessing these immunologic changes in an animal model, we characterized the expression patterns of immune activation markers on lymphocyte subsets during the acute, chronic, and end stages of HIV-2 infection in baboons. Using flow cytometry, we identified 21 human-specific monoclonal antibodies that were cross-reactive with baboon lymphocytes; however, expression of only 2 of these markers was altered significantly after HIV-2 infection. We found an increase in baboon class II antigen (as measured by anti-HLA-DR) in the CD4(+) T cell subset within 8 weeks of infection (p = 0.045). Moreover, after 1 year of infection, CD11b was downregulated on CD8(+) T lymphocytes (p = 0.027). This downregulation of CD11b was consistently observed in all of the groups of baboons that were chronically infected with three different HIV-2 isolates. In addition, we found substantial downregulation of the interleukin 2 receptor (CD25) and upregulation of class II antigen on CD8(+) lymphocytes in a baboon with an AIDS-like disease. These and other phenotypic markers of immune activation may facilitate characterization of the immunopathogenesis of AIDS in nonhuman primate animal models.

Increased virus replication and virulence after serial passage of human immunodeficiency virus type 2 in baboons

Similar to human immunodeficiency virus type 1 (HIV-1) infection of humans, the natural history of HIV-2 infection in baboons (Papio cynocephalus) is a slow and chronic disease that generally takes several years before an AIDS-like condition develops. To shorten the amount of time to the development of disease, we performed five serial passages of HIV-2(UC2) in baboons by using blood and bone marrow samples during the acute phase of infection when viral loads were at high levels. After these serial passages, virus levels in plasma, peripheral blood mononuclear cells (PBMC) and lymphatic tissues in the acutely infected baboons were increased. Within 1 year of the HIV-2 infection, all of the inoculated baboons showed specific signs of AIDS-related disease progression within the lymphatic tissues, such as vascular proliferation and lymphoid depletion. The HIV-2(UC2) recovered after four serial passages showed increased kinetics of viral replication in baboon PBMC and cytopathicity. This study suggests that the HIV-2 isolate recovered after several serial passages in baboons will be useful in future studies of AIDS pathogenesis and vaccine development by using this animal model.

Immune responses in baboons vaccinated with HIV-2 genetic expression libraries

Immunization using genetic expression libraries may be an improvement over conventional DNA immunization using a single gene because more epitopes are simultaneously presented to the immune system. In this study, we evaluated the effectiveness of an HIV-2 vaccine made from a genomic expression library in baboons. We found that HIV-2 expression library immunization induced HIV-2-specific memory responses but low levels of CD8+ cell anti-viral responses and neutralizing antibodies. After intravenous virus challenge using a homologous pathogenic variant, HIV-2UC2/9429, viral loads were similar in the HIV-2-immunized and control baboons. We conclude that although immunization using HIV-2 expression libraries induces immune responses, this approach does not provide protection in baboons against intravenous challenge with HIV-2.

Simian immunodeficiency viruses with defective nef genes show increased susceptibility to the noncytotoxic antiviral activity of CD8+ lymphocytes

The noncytotoxic soluble factor produced by CD8+ T cells inhibits replication of HIV and SIV in vitro and is thought to play a crucial role in combatting infection in vivo. We determined the effect of human CD8+ lymphocytes on the in vitro replication potential of both wild-type and nef-defective mutants of the simian immunodeficiency virus SIVmac251. Although replication of wild-type SIVmac251 in unstimulated human PBMC supplemented with IL-2 was unaffected by the presence of CD8+ T cells, the nef mutants were susceptible to the inhibitory effects. The effect of exogenous IL-2 depended upon the culture conditions: (i) in nonstimulated human PBMC depleted of CD8+ T cells, addition of IL-2 had a positive effect on the growth of the nef-defective viruses; (ii) in total human PBMC, IL-2 appeared to reinforce the CD8+ T-cell-dependent inhibition of the same mutant viruses. This strongly suggests that IL-2 stimulates the noncytotoxic anti-HIV/SIV response of CD8+ cells present in PBMC cultures. PHA stimulation of unfractionated human PBMC overrode the suppression of viral replication by CD8+ T cells. Depletion of activated T cells expressing the IL-2 receptor alpha-chain (CD25+ T cells), present in small amounts in these primary T cell cultures, dramatically reduced viral replication, indicating that the depleted cell population harbors the target cells permissive for viral replication. Furthermore, using neutralizing antibodies we could show that inhibition by the beta-chemokines MIP-1alpha, MIP-1beta, and RANTES and the inhibitory effect of CD8+ lymphocytes on nef mutant SIVmac viruses are harbored on different levels.

The importance of the innate immune system in controlling HIV infection and disease

The innate immune system is the first line of defense against invading pathogens and is particularly important in warding off bacterial and viral infections presenting at the mucosal cell surface. From this primitive immune response, the more sophisticated adaptive immune system was derived. Despite nearly two decades of research directed at inducing adaptive immune responses to HIV, no successful immunological therapy or vaccine has been developed. On the basis of recent observations, it is suggested that instead emphasis should now be placed on the alternative arm of the immune system, the innate immune response. Novel approaches should be developed to elicit this rapidly responding immune activity in HIV infection.

Vaccination with DNA containing tat coding sequences and unmethylated CpG motifs protects cynomolgus monkeys upon infection with simian/human immunodeficiency virus (SHIV89.6P)

Recent evidence suggests that a CD8-mediated cytotoxic T cell response against the Tat protein of human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) controls primary infection after pathogenic virus challenge, and correlates with the status of long-term nonprogressor in humans. Due to the presence of unmethylated CpG sequences, DNA vaccination can boost the innate immunity driving more potent T cell-mediated immune responses. Therefore, cynomolgus monkeys were vaccinated with a tat-expressing vector containing defined unmethylated CpG sequences (pCV-tat). Here it is shown that the intramuscular inoculation of the pCV-tat contained primary infection with the highly pathogenic SHIV89.6P virus preventing the CD4(+) T cell decline in all the vaccinated monkeys. Undetectable virus replication and negative virus isolation correlated in all cases with the presence of anti-Tat CTLs. However, a CD8-mediated non cytolytic antiviral activity was also present in all protected animals. Of note, this activity was absent in the controls but was present in the monkey inoculated with the CpG-rich vector alone that was partially protected against viral challenge (i.e. no virus replication but positive virus isolation). These results suggest that a CTL response against Tat protects against primary infection by blocking virus replication at its early stage, in the absence of sterilizing immunity. Nevertheless, the boost of the innate immunity by CpG sequences can contribute to this protection both by driving more potent CTL responses and by inducing other CD8-mediated antiviral activities. Thus, the CpG-rich tat DNA vaccine may represent a promising candidate for preventive and therapeutic vaccination against AIDS.

Role of CD8+ cell-produced anti-viral factors in protective immunity in HIV-2-exposed but seronegative macaques resistant to intrarectal SIVsm challenge

The cell-mediated immune response is likely to be important in controlling HIV/SIV infection. There is evidence that beta-chemokines and other, as yet unknown, anti-viral factors play a role in host defence against HIV infection. We reported previously that HIV-2 exposed but seronegative cynomolgus macaques developed SIV-specific cytotoxic T lymphocytes and were resistant to mucosal SIV challenge. The aim of this study was to examine CD8+ cell-dependent production of beta-chemokines and other anti-viral factors in these macaques. The animals, selected from among 17 monkeys enrolled in two separate experiments, were either treated with an anti-viral drug or immunized passively with HIV-2 antibody-positive serum. Three of these monkeys were protected against repeated HIV-2 challenge and were also able to control SIV infection 3 years later. Control samples were obtained from four macaques that became SIV infected and from 39 naïve animals. The three resistant monkeys showed significantly higher production of RANTES and MIP-1alpha than the 39 naïve animals. In addition, SIV infection was suppressed by CD8+ cell culture supernatants of these monkeys. However, antibodies to chemokines only partially neutralized CD8+ cell-mediated SIV suppression indicating that the anti-viral activity observed in these monkeys was the result of combined action of several inhibitory factors.

CD8(+)-Cell antiviral factor activity is not restricted to human immunodeficiency virus (HIV)-specific T cells and can block HIV replication after initiation of reverse transcription

CD8(+) lymphocytes from human immunodeficiency virus (HIV)-infected patients can suppress in vitro HIV replication in CD4(+) T cells by a noncytolytic mechanism involving secreted CD8(+)-cell antiviral factor(s) (CAF). Using an HIV Nef-specific cytotoxic-T-lymphocyte (CTL) line and autologous CD4(+) T cells infected with a nef-deleted HIV-1 virus, we demonstrated that, after a priming antigenic stimulation, this suppression does not require the presence of the specific antigen during the effector phase. Furthermore, using an Epstein-Barr virus (EBV)-specific CTL line from an HIV-seronegative donor, we demonstrated that the ability to inhibit HIV replication in a noncytolytic manner is not restricted to HIV-specific effector cells; indeed, EBV-specific CTL were as efficient as HIV-specific effectors in suppressing R5 or X4 HIV-1 strain replication in vitro. This HIV-suppressive activity mediated by a soluble factor(s) present in the culture supernatant was detectable for up to 14 days following stimulation of EBV-specific CD8(+) cells with the cognate epitope peptide. Following acute infection of CEM cells with an X4 strain of HIV-1, EBV-specific CTL line supernatant containing HIV-suppressive activity did not block virus entry but was shown to interfere with virus replication after the first template switching of reverse transcription. Our results suggest that the noncytolytic control of HIV replication by EBV-specific CD8(+) T lymphocytes corresponded to a CAF-like activity and thus demonstrate that CAF production may not be restricted to CTL induced during HIV disease. Moreover, CAF acts after reverse transcription at least for X4 isolate replication inhibition.

Anti-feline immunodeficiency virus (FIV) soluble factor(s) produced from antigen-stimulated feline CD8(+) T lymphocytes suppresses FIV replication

Feline immunodeficiency virus (FIV) causes AIDS-like symptoms in infected cats. Concanavalin A (ConA)-stimulated peripheral blood mononuclear cells (PBMC) from chronically FIV strain PPR-infected cats readily expressed FIV. In contrast, when PBMC from these animals were stimulated with irradiated, autologous antigen-presenting cells (APC), at least a 10-fold drop in viral production was observed. In addition to FIV-specific cytotoxic T lymphocytes, anti-FIV activity was demonstrated in the cell-free supernatants of effector T lymphocytes stimulated with APC. The FIV-suppressive activity was induced from APC-stimulated PBMC of either FIV-infected or uninfected cats but not from ConA-stimulated PBMC. Suppression of FIV strain PPR replication was observed for both autologous and heterologous feline PBMC, was dose dependent, and demonstrated cross-reactivity and cell specificity. It was also demonstrated that the anti-FIV activity originated from CD8(+) T lymphocytes and was mediated by a noncytolytic mechanism.

Suppression of feline immunodeficiency virus replication in vitro by a soluble factor secreted by CD8+ T lymphocytes

Mitogen-activated lymphoblasts isolated from the blood and lymph nodes, but not the spleen, of domestic cats acutely infected with the Petaluma or Glasgow8 isolates of feline immunodeficiency virus (FIV), suppressed the replication of FIV in the MYA-1 T-cell line in a dose-dependent manner. This effect was not limited to the homologous isolate of FIV. The suppressor activity declined with progression to chronic infection, with lower levels of activity detectable only in the lymph nodes. Immunization of domestic cats with whole inactivated FIV vaccine elicited profound suppressor activity in both the blood and lymph nodes. The suppressor activity was associated with the CD8+ T-cell subpopulation, the effect did not appear to be major histocompatibility complex-restricted, and was mediated by a soluble factor(s). This activity may be associated with the control of virus replication during both the asymptomatic stages of FIV infection, and in the protective immunity observed in cats immunized with whole inactivated virus vaccines.

HLA compatibility requirements for CD8(+)-T-cell-mediated suppression of human immunodeficiency virus replication

CD8(+) T cells from human immunodeficiency virus (HIV)-infected individuals can suppress HIV replication in cultured CD4(+) cells by a noncytotoxic mechanism. Efficient suppression of HIV replication (>90% reduction) does not require HLA class I or class II histocompatibility between the effector CD8(+) T cells and the infected target CD4(+) T cells. However, maximal control of HIV production occurs when the CD8(+) effector cells and CD4(+) target cells are syngeneic. In some cases, more than 20-fold fewer syngeneic CD8(+) T cells were required to achieve the same degree of HIV inhibition as HLA-mismatched CD8(+) T cells. The increased antiviral activity seen in the syngeneic setting did not map exclusively to either the HLA class I or class II locus. These findings suggest that genetic compatibility (potentially, but not necessarily, at the HLA class I and class II loci) regulates CD8(+) T-cell noncytotoxic antiviral activity against infected CD4(+) T cells.

Binding of HIV-2 envelope glycoprotein to CD8 molecules and related chemokine production

We recently found that human immunodeficiency virus (HIV)-2 envelope glycoprotein, but not that of HIV-1, could bind to CD4 and CD8 molecules on T cells, and that the binding site of HIV-2 envelope glycoprotein was located on the alpha-chain (but not the beta-chain) of CD8. This study showed that the binding of HIV-2 envelope glycoprotein could induce phosphorylation of protein tyrosine kinase p56lck in CD8+ T cells. We also found that production of beta-chemokines in response to HIV-2 envelope glycoprotein was significantly higher than that in response to HIV-1 envelope glycoprotein, and that CD8+ T cells were the main source of beta-chemokines production among the T-cell population. These findings indicate the possibility that the binding of envelope glycoprotein to CD8 molecules are related to signal transduction into CD8+ T cells and the resultant beta-chemokine production in HIV-2 infection. Our results may help to explain the differences in disease manifestations between HIV-1 and HIV-2, including the lower virulence of HIV-2 and the longer survival of HIV-2-infected individuals.