LCMV-specific, class II-restricted cytotoxic T cells in beta 2-microglobulin-deficient mice

Internalization of iscom-borne antigens and presentation under MHC class I or class II restriction

Exogenous nonreplicating antigens (Ag) incorporated into immunostimulating complexes (iscoms) induce CTL responses under MHC class I restriction. A requirement for inducing CTL responses is that the Ag is delivered to the cytosol of antigen-presenting cells (APC), a route restricted to endogenously produced Ag. To investigate the mechanisms by which iscoms elicit MHC class I-restricted responses, the intracellular distribution of influenza virus envelope proteins incorporated in iscoms (flu-iscoms) or in micelles (flumicelles) was studied in vitro using murine peritoneal cells (PEC). Ultrathin sections of cells pulsed with biotinylated flu-iscoms or flu-micelles were analyzed by electron microscopy after detection of the biotin label by reaction with streptavidin-gold. PEC pulsed with flu-iscoms showed a pattern of scattered gold particles distributed in clear and dense vesicles as well as in the intracellular space but not associated with organelles. In cells pulsed with flu-micelles, Ag was also detected in most cellular compartments but at a considerably lower concentration. The intracellular distribution of particulate Ag in iscom or micelle form was confirmed by lysis and differential centrifugation of Ag-pulsed APC. Furthermore, P815 cells pulsed with flu-iscoms were lysed by specific immune effectors showing that the iscom-Ag was processed and presented by class I-expressing APC. Flu-iscoms were internalized about 50-fold more efficiently than ovalbumin iscoms (ovaiscoms) suggesting that the nature of the protein and/or the presence of cellular receptors are important factors influencing the capacity of APC to take up iscom-borne proteins. PEC accounted for the most active internalization of iscom-borne Ag, although splenic dendritic cells and B cells also took up fluiscoms with remarkable efficiency.

Human cytomegalovirus inhibits major histocompatibility complex class II expression by disruption of the Jak/Stat pathway

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus that is able to persist for decades in its host. HCMV has evolved protean countermeasures for anti-HCMV cellular immunity that facilitate establishment of persistence. Recently it has been shown that HCMV inhibits interferon gamma (IFN-gamma)-stimulated MHC class II expression, but the mechanism for this effect is unknown. IFN-gamma signal transduction (Jak/Stat pathway) and class II transactivator (CIITA) are required components for IFN-gamma-stimulated MHC class II expression. In this study, we demonstrate that both a clinical isolate and a laboratory strain of HCMV inhibit inducible MHC class II expression at the cell surface and at RNA level in human endothelial cells and fibroblasts. Moreover, reverse transcriptase polymerase chain reaction and Northern blot analyses demonstrate that neither CIITA nor interferon regulatory factor 1 are upregulated in infected cells. Electrophoretic mobility shift assays reveal a defect in IFN-gamma signal transduction, which was shown by immunoprecipitation to be associated with a striking decrease in Janus kinase 1 (Jak1) levels. Proteasome inhibitor studies with carboxybenzyl-leucyl-leucyl-leucine vinyl sulfone suggest an HCMV-associated enhancement of Jak1 protein degradation. This is the first report of a mechanism for the HCMV-mediated disruption of inducible MHC class II expression and a direct virus-associated alteration in Janus kinase levels. These findings are yet another example of the diverse mechanisms by which HCMV avoids immunosurveillance and establishes persistence.

CD4- and TCRalphabeta-positive T lymphocytes predominantly infiltrated into well-moderately differentiated colon adenocarcinoma tissues

Intraepithelial T lymphocytes have been reported as being functional in the growth of epithelial cells and also in the discrimination of aberrant cells, whereas their function against colon adenocarcinoma cells is obscure. The phenotype of colon intraepithelial T lymphocytes has been found in patients with inflammatory bowel diseases but not in patients with colon adenocarcinoma. In this study, we investigated cell surface markers of tumor-infiltrating T lymphocytes of adenocarcinoma of the colon and rectum at various grades of differentiation and intraepithelial T lymphocytes of adjacent normal colon by enzyme immunostaining. Among intraepithelial T lymphocytes of the normal colon, CD8-and TCRgammadelta-positive T lymphocytes were predominant as described. In contrast, tumor-infiltrating T lymphocytes of well-moderately differentiated adenocarcinoma of the colon and rectum were predominantly CD4- and TCRalphabeta-positive. The decrease of TCRgammadelta-positive T lymphocytes and the increase of CD4 and TCRalphabeta-positive T lymphocytes in adenocarcinoma tissues of the colon and rectum suggests that an alteration of the local immune system participates in the formation of adenoma and/or adenocarcinoma of the colon and rectum, resulting in infiltration of CD4-positive T lymphocytes that have certain activity against transformed cells.

Mechanisms of viral clearance and persistence

Using examples predominantly drawn from study of the lymphocytic choriomeningitis virus (LCMV) model system, this review describes the mechanisms involved in control of virus infections by the cell-mediated immune response, and some of the different strategies viruses have evolved to evade such immune clearance so that they can persist in their hosts. The important role played by the CD8+ cytotoxic lymphocyte (CTL) response in clearance of many systemic virus infections is discussed; and it is emphasized that although CD8+ CTL are classically thought of as lymphocytes which mediate lysis of virus-infected target cells, the principal mechanism by which CD8+ T cells effect clearance of persistent and many acute virus infections via production of antiviral cytokines such as tumour necrosis factor-alpha and interferon-gamma, not via destruction of virus-producing cells. To avoid immune-mediated clearance, viruses frequently use a combination of several different strategies. These can be grouped into mechanisms for avoiding recognition by the immune response (such as establishing latent infections, replicating in immune-privileged sites, down-regulating the expression of immune recognition signals on the surface of infected cells, or undergoing antigenic variation); and mechanisms for suppressing the immune response. The latter include generalized immune suppression mechanisms, and strategies for more precisely disabling the specific immune response such as inducing tolerance or exhaustion of virus-specific CTL. The value of understanding both immune clearance mechanisms and viral evasion strategies in the rational design of immune-based therapies to combat persistent virus infections is discussed.

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 muMT 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.

Virus-specific MHC-class II-restricted TCR-transgenic mice: effects on humoral and cellular immune responses after viral infection

A transgenic mouse expressing MHC class II-restricted TCR with specificity for a lymphocytic choriomeningitis virus (LCMV) glycoprotein-derived T helper cell epitope was developed to study the role of LCMV-specific CD4+ T cells in virus infection in vivo. The majority of CD4+ T cells in TCR transgenic mice expressed the transgenic receptor, and LCMV glycoprotein-specific TCR transgenic CD4+ T cells efficiently mediated help for the production of LCMV glycoprotein-specific isotype-switched antibodies. In contrast, LCMV glycoprotein-specific TCR transgenic mice exhibited a drastically reduced ability to provide help for the generation of antibody responses specific for the virus-internal nucleoprotein, indicating that intramolecular/intrastructural help is limited to antigens that are accessible to B cells on the viral surface. Antiviral cellular immunity was studied with noncytopathic LCMV and recombinant cytopathic vaccinia virus expressing the LCMV glycoprotein. TCR transgenic mice failed to efficiently control LCMV infection, demonstrating that functional LCMV-specific CD4+ T cells--even if activated and present at extremely high frequencies--cannot directly mediate protective immunity against LCMV. Despite the fact that LCMV-primed CD4+ T cells from TCR transgenic mice as well as from control mice showed low MHC class II-restricted cytotoxic activity in vivo, this did not correlate with protection against LCMV replication in vivo. In contrast, CD4+ T cells from TCR-transgenic mice mediated efficient protection against infection with recombinant vaccinia virus. These results further support the need for different immune effector functions for protective immunity against different viral infections.

Vaccination against persistent viral infection exacerbates CD4+ T-cell-mediated immunopathological disease

Lymphocytic choriomeningitis virus (LCMV) infection of normal mice results in a fatal immunopathologic meningitis mediated by CD8+ cytotoxic T lymphocytes (CTL). We have previously shown that female beta2-microglobulin-deficient (beta2m-/-) mice, which are also deficient in CD8+ T cells, are susceptible to LCMV-induced immune-mediated meningitis, characterized by significant weight loss and mortality. This LCMV disease in beta2m-/- mice is mediated by CD4+ T lymphocytes. Our previous studies have also demonstrated that male beta2m-/- mice are less susceptible than female beta2m-/- mice to LCMV-induced, immune-mediated mortality and weight loss. In this report, we show that vaccination of male beta2m-/- mice enhances immunopathology following intracranial infection with LCMV. We observed increased production of gamma interferon (IFN-gamma), an increase in CD4+ CTL precursor frequency, and an increased frequency of IFN-gamma-producing cells from spleen cells of vaccinated male beta2m-/- mice. Vaccinated male beta2m-/- mice also had significantly increased inflammation in the cerebrospinal fluid (CSF), characterized by a large CD4+ T-cell infiltrate. CSF cells from vaccinated mice showed increased production of IFN-gamma on day 7 postchallenge. Neither vaccinated nor control beta2m-/- mice were able to clear virus, and the two groups had similarly high levels of virus early after infection. These results suggest that the magnitude of the early immune response is more important than the level of virus in the brain in determining the outcome of immunopathology in beta2m-/- mice. We show here that vaccination can increase CD4+ T-cell-dependent immunopathology to a persistent viral infection.

Virus-specific, CD8+ major histocompatibility complex class I-restricted cytotoxic T lymphocytes in lymphocytic choriomeningitis virus-infected beta2-microglobulin-deficient mice

Following infection with lymphocytic choriomeningitis virus (LCMV), normal adult mice generate virus-specific, major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTL) which clear the virus after intraperitoneal infection or cause death following intracranial (i.c.) infection. We have investigated the response of beta2-microglobulin-deficient (beta2m-) mice of the H-2d haplotype (KOD mice) to LCMV infection. Unlike H-2b beta2m- mice, which generate CD4+ MHC class II-restricted CTL in response to LCMV, KOD mice generate high levels of CD8+ MHC class I-restricted, virus-specific CTL. These CTL are specific for the LCMV nucleoprotein epitope (residues 118 to 126) in association with the Ld class I molecule, analogous to the CTL response in wild-type mice. KOD mice are also susceptible to lethal LCM disease, with 75 to 80% of the mice dying 7 to 9 days following i.c. infection with virus. Similar to results with normal mice, lethal LCM disease in KOD mice is prevented by in vivo depletion of CD8+ T cells prior to i.c. infection. In contrast to wild-type mice, however, KOD mice cannot control LCMV and become persistently infected. Overall, these results demonstrate that beta2m is not an absolute requirement for presentation of endogenous antigen on Ld or for induction of virus-specific Ld-restricted CTL in vivo.

Effector CD4+ and CD8+ T-cell mechanisms in the control of respiratory virus infections

The rules for T-cell-mediated control of viruses that infect via the respiratory mucosae show both common themes and differences, depending on the nature of the pathogen. Virus-specific CD8+ cytotoxic T lymphocytes (CTLs) are the key effectors of virus clearance in mice infected with both negative strand RNA viruses (influenza and Sendai) and a DNA virus, the murine gamma-herpesvirus-68 (MHV-68). Recently completed experiments establish that these activated CD8+ T cells indeed operate primarily via contact-dependent lysis. Perforin-mediated cytotoxicity seems to be the preferred mode, though a Fas-based mechanism can apparently serve as an alternative mechanism. Immune CD4+ T cells functioning in the absence of the CD8+ subset cannot eliminate MHV-68 from lung epithelial cells, are somewhat less efficient than the CD8+ CTLs at clearing the RNA viruses, and are generally ineffectual in mice that lack B lymphocytes. Though cytokine secretion by CD4+ and CD8+ T cells in the virus-infected lung may promote both T-cell extravasation and macrophage activation, such processes are not alone sufficient to deal consistently with any of these infections. However, CD4+ T help is mandatory for an effective B-cell response, and can operate to promote the clonal expansion of virus-specific CD8+ T cells in the lymph nodes and spleen. Furthermore, a concurrent CD4+ T-cell response seems to be essential for maintaining continued CD8+ T-cell surveillance and effector capacity through the persistent, latent phase of MHV-68 infection in B cells. Thus, the evidence to date supports a very traditional view; CD8+ T cells function mainly as killers and the CD4+ T cells as helpers in these respiratory virus infections.

Enhanced mucosal and systemic immune responses to intestinal reovirus infection in beta2-microglobulin-deficient mice

Enteric infection of mice with respiratory enteric orphan virus (reovirus) type 1, strain Lang elicits both humoral and cellular immune responses. To investigate the role of CD8+, alpha/beta T-cell receptor (TCR)+ T cells in mucosal immunity to an enteric pathogen, we examined immune responses and viral clearance following enteric reovirus infection in C57BL/6, B6129F2, and beta2-microglobulin-deficient (beta2m-/-) mice. Analysis of Peyer's patch and lamina propria culture supernatants revealed a two- to threefold increase in levels of reovirus-specific immunoglobulin A in beta2m-/- mice compared to normal controls. These data corresponded to a similar increase in the frequency of virus-specific immunoglobulin A-producing cells in Peyer's patches and lamina propria and an increase in immunoglobulin G-producing cells in spleens from beta2m-/- mice compared to controls. These increased humoral immune responses were not due to a difference in B-cell populations because cell counts and flow cytometric analyses showed that beta2m-/- and control mice had similar numbers and percentages of B cells in mucosal and systemic tissues. Analysis of cytokine message by reverse transcriptase-PCR 5 and 10 days after infection revealed no difference in message level for transforming growth factor beta, gamma interferon, interleukin-4, interleukin-5, or interleukin-6 for all mouse strains. Virus tissue titers determined by plaque assay at 5 and 10 days after infection demonstrated that beta2m-/- mice cleared reovirus from the small intestines with the same efficiency as control mice. Collectively, these data suggest that CD8+, alpha/beta TCR+ T cells may regulate mucosal and systemic humoral immune responses to oral infection with reovirus.

Allospecific cytotoxic T cells generated from beta 2m-/- mice in primary MLC: analysis of activation requirements, specificity, and phenotype

It has been demonstrated by several investigators that beta 2m-/- knockout mice are deficient in the expression of MHC Class I molecules but can nevertheless generate CD8(+) allospecific cytotoxic T cells following vigorous in vivo priming. We demonstrate here that in vivo priming is not necessary to generate MHC Class I allospecific CTL from beta 2m-/- mice. When splenocytes from naive unprimed beta 2m-/- mice were provided exogenous cytokines in MHC Class I disparate primary MLC, allospecific cytolytic effectors were generated. beta 2m-/- MHC Class I allospecific CTL that were CD3+ and Thy1.2+ were otherwise heterogeneous in phenotype, including CD8+, CD4+, CD8-CD4-, TCR alpha beta+, and TCR gamma delta+ T cells. This phenotypic variability of beta 2m-/- CTL generated in primary MLC reveals the diversity of CTL precursors that develop in vivo in the absence of MHC Class I.

Characterization of Marek's disease herpesvirus-specific cytotoxic T lymphocytes in chickens inoculated with a non-oncogenic vaccine strain of MDV

Previously we have reported that reticuloendotheliosis virus (REV)-transformed cell lines expressing Marek's disease virus (MDV) genes pp38, meq or gB were lysed by syngeneic MDV-specific splenocytes from major histocompatibility complex (MHC):B9B19 and MHC:B1B21 chickens. In contrast, REV-transformed cell lines expressing the MDV gene ICP4 were only lysed by syngeneic MDV-specific splenocytes from MHC:B21B21 chickens. In this study we report that this syngeneic cell-mediated immune response is induced by cytotoxic T lymphocytes (CTL). Splenocytes from MDV vaccine strain, SB-1 inoculated MHC:B19B19 and MHC:B21B21 chickens, depleted for CD4+, CD8+, TCR gamma delta +, TCR alpha beta 1+ and/or TCR alpha beta 2+ cells, were used as effector cells in chromium-release assays. Effector cells depleted of CD8+ or TCR alpha beta 1+, but not TCR gamma delta + or TCR alpha beta 2+, markedly reduced the MDV-specific release. Depletion of CD4+ effector cells did not influence the specific release significantly. This is the first report on identification of virus-specific CD8+ CTL in chickens inoculated with a non-oncogenic vaccine strain of MDV.

Immunology and immunity studied with viruses

Immunity to viruses is used to define important biological parameters of immunology. Specificity, tolerance and T and B cell memory were analysed with murine model infections. The key parameters of antigen kinetics, localization and patterns of T and B cell response induction in maintaining memory and in causing deletion of reactive lymphocytes were compared for self and for viral foreign antigens. Evidence is reviewed that suggests that B cells essentially recognize antigen patterns, whereas T cells react against antigens newly brought into lymphoid tissues; antigens outside lymphoid tissues are ignored, and antigens always present in, or spreading too fast throughout, lymphoid tissues exhaust and delete T cell responses. Finally, effector mechanisms of antiviral immunity are summarized, as they vary with different viruses. On this basis immunological T and B cell memory against viruses is reviewed. Memory studies suggest that increased precursor frequencies of B and T cells appear to remain in the host independent of antigen persistence. However, in order to protect against cytopathic viruses, memory B cells have to produce antibody to maintain protective elevated levels of antibody: B cell differentiation into plasma cells is driven by persisting antigen. Similarly, to protect against infection with a non-cytopathic virus, cytotoxic T cells have to recirculate through peripheral organs. Activation and capacity to emigrate into solid tissues as well as cytolytic effector function are also dependent upon, and driven by, persisting antigen. Because no convincing evidence is yet available of the existence of identifiable B or T cells with specialized memory characteristics, the phenotype of protective immunological memory correlates best with antigen-driven activation of low frequency effector T cells and plasma cells.

Importance of beta2-microglobulin in murine resistance to mucosal and systemic candidiasis

beta2-Microglobulin knockout (beta2m-/-) mice, which lack major histocompatibility complex class I expression and are deficient in CD8alpha/beta T-cell receptor alpha/beta (TcRalpha/beta) T cells, were as resistant to systemic (intravenous) challenge with Candida albicans as immunocompetent controls. Conversely, the beta2m-/- mutant mice were susceptible to systemic candidiasis of endogenous origin despite the induction of C. albicans-specific antibody and cell-mediated immune responses after colonization with a pure culture of C. albicans. Despite some superficial and transient infections of tongues and esophagi (detected by histology) at 1 to 2 weeks after oral colonization and gastric infections (cardia-antrum section) which were observed at 10 to 12 weeks after oral challenge, C. albicans-colonized beta2m-/- mice showed an overall resistance to candidiasis in other mucosal and cutaneous tissues. These data suggest that immune defects that accompany the loss of beta2-microglobulin play an important role in murine resistance to gastric and disseminated candidiasis of endogenous (intestinal tract) origin and that innate immunity and CD4 TcRalpha/beta as well as CD8alpha/alpha TcRalpha/beta (or -gamma/delta) T cells play an important role in resistance to systemic, cutaneous, and nongastric mucosal tissues.

Mature B cells are required for acute splenic infection, but not for establishment of latency, by murine gammaherpesvirus 68

Murine gammaherpesvirus 68 (gamma HV-68; also referred to as MHV-68) is a gammaherpesvirus which infects murid rodents. Previous studies showed that CD8 T cells are important for controlling gamma HV-68 replication during the first 2 weeks of infection and suggested a role for B cells in latent or persistent gamma HV-68 infection. To further define the importance of B cells and CD8 T cells during acute and chronic gamma HV-68 infection, we examined splenic infection in mice with null mutations in the transmembrane domain of the mu-heavy-chain constant region (MuMT; B-cell and antibody deficient) or in the beta2-microglobulin gene (beta2 -/-; CD8 deficient). Immunocompetent mice infected intraperitoneally with gamma HV-68 demonstrated peak splenic titers 9 to 10 days postinfection, cleared infectious virus 15 to 20 days postinfection, and harbored low levels of latent virus at 6 weeks postinfection. Beta2-/- mice showed peak splenic gamma HV-68 titers similar to those of normal mice but were unable to clear infectious virus completely from the spleen, demonstrating persistent infectious virus 6 weeks postinfection. These data indicate that CD8 T cells are important for clearing infectious gamma HV-68 from the spleen. Infected MuMT mice did not demonstrate detectable infectious gamma HV-68 in the spleen at any time after infection, indicating that mature B lymphocytes are necessary for acute splenic infection by gamma HV-68. Despite the lack of measurable acute infection, MuMT spleen cells harbored latent virus 6 weeks postinfection at a level about 100-fold higher than that in normal mice. These data demonstrate establishment of latency by a herpesvirus in an organ in the absence of acute viral replication in that organ. In addition, they demonstrate that gamma HV-68 can establish latency in a cell type other than mature B lymphocytes.

Vaccination protects beta 2 microglobulin deficient mice from immune mediated mortality but not from persisting viral infection

Intracranial (i.c.) infection of immunocompetent mice with lymphocytic choriomeningitis virus (LCMV) results in immunopathological lethal meningitis mediated by CD8+ cytotoxic T lymphocytes (CTL). Vaccination of immunocompetent mice elicits a CD8+ CTL response that can protect the mice from lethal meningitis. beta 2 microglobulin-deficient (beta 2m-/-) mice are deficient in CD8+ CTL, exhibit CD4+ CTL, and, after i.c. LCMV infection, undergo a less severe meningitis with decreased mortality and additionally develop a wasting disease. Both wasting disease and mortality in beta 2m-/- mice are mediated by CD4+ T cells. We studied the effects of vaccination and challenge dose on weight loss, mortality and viral clearance after i.c. LCMV infection in beta 2m-/- mice. Unvaccinated beta 2m-/- mice had significant weight loss and mortality at doses of 200 and 10(3) p.f.u. LCMV, while a dose of 10(6) p.f.u. LCMV elicited significant mortality but less weight loss. Vaccination with u.v.-inactivated LCMV in complete Freund's adjuvant or with vaccinia virus expressing the LCMV glycoprotein or nucleoprotein genes protected beta 2m-/- mice from mortality but not weight loss after 200 p.f.u. LCMV challenge. Although protected from mortality, beta 2m-/- mice were unable to clear LCMV from their brains or spleens. Therefore, we show that vaccination can protect against lethal immune-meningitis in the face of persistent infection.

Progressive loss of CD8+ T cell-mediated control of a gamma-herpesvirus in the absence of CD4+ T cells

A unique experimental model has been developed for dissecting the integrity of CD8+ T cell-mediated immunity to a persistent gammaherpesvirus under conditions of CD4+ T cell deficiency. Respiratory challenge of major histocompatibility complex class II -/- and +/+ C57BL/6J mice with the murine gammaherpesvirus 68 (MHV-68) leads to productive infection of both lung and adrenal epithelial cells. Virus titers peak within 5-10 d, and are no longer detected after day 15. Persistent, latent infection is established concurrently in splenic and lymph node B cells, with higher numbers of MHV-68+ lymphocytes being found in all lymphoid sites analyzed from the +/+ mice concurrent with the massive, but transient splenomegaly that occurred only in this group. From day 17, however, the numbers of infected B lymphocytes were consistently higher in the -/- group, while the frequency of this population diminished progressively in the +/+ controls. Infectious MHV-68 was again detected in the respiratory tract and the adrenals of the -/- (but not the +/+) mice from day 22 after infection. The titers in these sites rose progressively, with the majority of the -/- mice dying between days 120 and 133. Even so, some CD8+ effectors were still functioning as late as 100 d after infection. Depletion of CD8+ T cells at this stage led to higher virus titers in the -/- lung, and to the development of wasting in some of the -/- mice. Elimination of the CD8+ T cells from the +/+ group (day 80) increased the numbers of MHV-68+ cells in the spleen, but did not reactivate the infection in the respiratory tract. The results are consistent with the interpretation that CD8+ T cell-mediated control of this persistent gammaherpesvirus is progressively lost in the absence of the CD4+ T cell subset. This parallels what may be happening in AIDS patients who develop Kaposi's sarcoma and various Epstein Barr virus associated disease processes.

CD4+ cytolytic effectors are inefficient in the clearance of Listeria monocytogenes

Cytotoxic T lymphocytes (CTL) recognize and lyse target cells through the interaction of the T-cell receptor complex with the class I or class II major histocompatibility complex (MHC). The production of class I-restricted CTL has been shown to be critical to the elimination of specific pathogens including Listeria monocytogenes. However, the function of class II-restricted CTL in the clearance of intracellular pathogens is poorly understood. H-2b beta 2-microglobulin-deficient mice (beta 2M-/-) are not able to produce CD8+ CTL in response to infection with L. monocytogenes. We used this model to evaluate the efficacy of class II-restricted CTL, in the absence of a class I-restricted response, during a primary infection with L. monocytogenes. We demonstrate that, despite their effectiveness in adoptive transfer of protection, Listeria-specific CD4+ class II-restricted cytotoxic lymphocytes are ineffective in decreasing titres of L. monocytogenes in the spleen was found established infection. In beta 2M-/- mice, persistence of L. monocytogenes in the spleen was found preferentially in class II-negative cells. Surprisingly, class I-restricted CTL from C57BL/6 mice were capable of decreasing bacterial titres during an established infection even in the absence of detectable class I on the surface of cells from beta 2M-/- mice. These data strongly suggest that, in the absence of a class I-restricted response, pathogens that elicit a class II-restricted cytotoxic response may escape prompt eradication by the immune system.

The role of CD8+ T cells in the initiation of insulin-dependent diabetes mellitus

While it is generally accepted that T cells are critical for the development of diabetes in the non-obese diabetic (NOD) mouse, the precise functions of the CD4+ and CD8+ subsets remain ill-defined. Transfer experiments have provided evidence that CD4+ cells are the disease initiators, provoking massive mononuclear leukocyte infiltration into the pancreatic islets, while CD8+ cells play an effector role, responsible for the final destruction of islet beta cells. It was surprising, then, to find that NOD mice carrying a null mutation at the beta 2-microglobulin (beta 2-mu) locus, and thereby lacking major histocompatibility complex class I molecules and CD8+ T cells, developed neither insulitis nor diabetes. Here, we argue that the absence of insulitis in these animals results from their lack of CD8+ cells because islet infiltration is also absent when NOD mice are treated with an anti-CD8 monoclonal antibody (mAb) at a young age. Interestingly, the anti-CD8 effect is only observed when the mAb is injected during a discrete age window--2 to 5 weeks after birth. Transfer experiments indicate that the lack of CD8+ cells during this period somehow alters the phenotype of CD4+ cells, preventing them from expressing their insulitis potential. This is not because they are generally immuno-incompetent nor because they are generally more prone to differentiating into cells with Th2 characteristics. Given that neither the beta 2-mu mutation nor anti-CD8 treatment affect insulitis in a T cell receptor transgenic (tg) mouse strain with a CD4+ T cell repertoire highly skewed for an anti-islet cell reactivity, the most straight-forward interpretation of these observations is that CD8+ cells are required for effective priming and expansion of autoreactive CD4+ cells.

Human cytomegalovirus early infection, acute rejection, and major histocompatibility class II expression in transplanted lung. Molecular, immunocytochemical, and histopathologic investigations

The present study aimed to investigate the relationship between acute rejection and human cytomegalovirus (HCMV) infection, as well as the coexpression of HLA-DR and immediate-early (IE) viral antigens, in 143 transbronchial biopsies and bronchoalveolar lavage fluids of 32 lung transplant recipients. We investigated the occurrence of morphologically overt viral infection with conventional histopathology, the expression of IE antigens with single labeling immunohistochemistry, the coexpression of IE antigens and HLA-DR molecules with double labeling techniques, and the presence of viral IE genes with polymerase chain reaction. Histopathologic study showed overt viral infections (12.6%) in 18 of the 143 biopsies; 8 were in a context of pneumonia and 10 were localizations without surrounding inflammatory cells; immunohistochemistry showed IE viral antigen expression in 31 (21.67%); PCR detected viral IE genes in 73/143 lavage fluids and biopsies (51%). The double labeling immunohistochemical technique showed that most IE antigen-expressing, noncytopathic cells were either HLA-DR negative in areas without infiltrates, or HLA-DR positive in those areas where inflammatory infiltrates were consistent, in the absence of viral cytopathy, with acute rejection. The results indicate that, in transplanted lung, the frequency of morphologically occult HCMV infection (as detected by immunohistochemically and/or PCR) is much higher than that of morphologically overt viral infection. The occurrence of inflammatory infiltrates (consistent with acute rejection) around morphologically occult infected cells and the possible lack of inflammation around both early- and late-infected cells suggest that in biopsies with occult infection the infiltrates should be attributed to allograft reaction. This conclusion would be in keeping with the coexpression of HLA-DR and HCMV IE in infiltrate-rich biopsies that are consistent with acute rejection, as well as with the absence of HLA-DR expression in IE antigen-positive cells in infiltrate-free-areas.

Circulating immunoglobulin G can play a critical role in clearance of intestinal reovirus infection

Reoviruses are encapsidated double-stranded RNA viruses that cause systemic disease in mice after peroral (p.o.) inoculation and primary replication in the intestine. In this study, we define components of the immune system involved in the clearing of reovirus from the proximal small intestine. The intestines of immunocompetent adult CB17, 129, and C57BL/6 mice were cleared of reovirus serotype 3 clone 9 (T3C9) within 7 days of p.o. inoculation. Antigen-specific lymphocytes were important for the clearance of intestinal infection, since severe combined immunodeficient (SCID) mice failed to clear T3C9 infection. To define specific immune components required for intestinal clearance, reovirus infection of mice with null mutations in the immunoglobulin M (IgM) transmembrane exon (MuMT; B cell and antibody deficient) or beta 2 microglobulin gene (beta 2-/-; CD8 deficient) was evaluated. beta 2-/- mice cleared reovirus infection with normal kinetics, while MuMT mice showed delayed clearance of T3C9 7 to 11 days after p.o. inoculation. Adoptive transfer of splenic lymphocytes from reovirus-immune CB17 mice inhibited growth of T3C9 in CB17 SCID mouse intestine 11 days after p.o. inoculation. The efficiency of viral clearance by adoptively transferred cells was significantly diminished by depletion of B cells prior to adoptive transfer. Results in SCID and MuMT mice demonstrate an important role for B cells or IgG in clearance of reovirus from the intestines. Polyclonal reovirus-immune rabbit serum, protein A-purified immune IgG, and murine monoclonal IgG2a antibody specific for reovirus outer capsid protein sigma 3 administered intraperitoneally all normalized clearance of reovirus from intestinal tissue in MuMT mice. This result demonstrates an IgA-independent role for IgG in the clearance of intestinal virus infection. Polyclonal reovirus-immune serum also significantly decreased reovirus titers in the intestines of SCID mice, demonstrating a T-cell-independent role for antibody in the clearance of intestinal reovirus infection. B cells and circulating IgG play an important role in the clearance of reovirus from intestines, suggesting that IgG may play a more prominent functional role at mucosal sites of primary viral replication than was previously supposed.

Immune CD4+ T cells promote the clearance of influenza virus from major histocompatibility complex class II -/- respiratory epithelium

The experiments described establish that CD4+ T-cell-dependent effector mechanisms can eliminate an H3N2 influenza A virus from lung cells that are unable to express class II major histocompatibility complex (MHC) glycoproteins. Radiation chimeras were made by using CD4+ T cells and bone marrow from CD8-depleted, MHC class II +/+ mice and irradiated (950 rads) MHC class II -/- recipients. The influenza virus-specific CD4+ T-cell responses in these +/+-->-/- mice were not obviously different from those in the +/+-->+/+ controls: the cytokine profiles, the spectra of plasma cells producing the various immunoglobulin isotypes, and the frequencies of virus-specific CD4+ T cells were similar for the two groups. Expression of class II MHC glycoproteins on stimulator cells, B lymphocytes, and monocytes/macrophages is apparently sufficient for CD4+ T cells to terminate influenza virus infection of MHC class II -/- respiratory epithelium. A possible explanation is that the local spread of this lytic virus in the lung is limited by cytokines and/or antibody.

CD4-negative cytotoxic T cells with a T cell receptor alpha/beta intermediate expression in CD8-deficient mice

Targeted disruption of the CD8 gene results in a profound block in cytotoxic T cell (CTL) development. Since CTL are major histocompatibility complex (MHC) class I restricted, we addressed the question of whether CD8-/- mice can reject MHC class I-disparate allografts. Studies have previously shown that skin allografts are rejected exclusively by T cells. We therefore used the skin allograft model to answer our question and grafted CD8-/- mice with skins from allogeneic mice deficient in MHC class II or in MHC class I (MHC-I or MHC-II-disparate, respectively). CD8-/- mice rejected MHC-I-disparate skin rapidly even if they were depleted of CD4+ cells in vivo (and were thus deficient in CD4+ and CD8+ T cells). By contrast, CD8+/+ controls depleted of CD4+ and CD8+ T cells in vivo accepted the MHC-I-disparate skin. Following MHC-I, but not MHC-II stimulation, allograft-specific cytotoxic activity was detected in CD8-/- mice even after CD4 depletion. A population expanded in both the lymph nodes and the thymus of grafted CD8-/- animals which displayed a CD4-8-3intermediateTCR alpha/betaintermediate phenotype. Indeed its T cell receptor (TCR) density was lower than that of CD4+ cells in CD8-/- mice or of CD8+ cells in CD8+/+ mice. Our data suggest that this CD4-8- T cell population is responsible for the CTL function we have observed. Therefore, MHC class I-restricted CTL can be generated in CD8-/- mice following priming with MHC class I antigens in vivo. The data also suggest that CD8 is needed to up-regulate TCR density during thymic maturation. Thus, although CD8 plays a major role in the generation of CTL, it is not absolutely required.

Gene knockout mice establish a primary protective role for major histocompatibility complex class II-restricted responses in Chlamydia trachomatis genital tract infection

Mice with disrupted beta 2-microglobulin (beta 2m-/-), I-A (class II-/-), or CD4 (CD4-/-) genes were examined for their capacity to resolve Chlamydia trachomatis genital tract infection. C57BL/6 and beta 2m-/- mice resolved infection similarly and were culture negative by 4 to 5 weeks following infection. Conversely, major histocompatibility complex (MHC) class II-/- mice failed to resolve infection, and CD4-/- mice showed a significant delay (2 weeks). Secondary challenge of C57BL/6, beta 2m-/-, and CD4-/- mice established that acquired protective immunity, which was characterized by an infection of shortened duration and reduced shedding of infectious organisms, developed. Serological analysis of C57BL/6 and beta 2m-/- mice by enzyme-linked immunosorbent assays revealed no striking differences in the immunoglobulin subclass specificity of the anti-Chlamydia response, although some differences were observed in the magnitude of the immunoglobulin G2a (IgG2a) and IgG2b responses. Class II-/- mice produced lower-titered serum anti-Chlamydia antibodies of all isotypes. The serum antibody responses of CD4-/- mice were similar to those of C57BL/6 mice, except that the anti-Chlamydia IgA response was delayed by approximately 3 weeks. Analysis of vaginal washes for Chlamydia-reactive antibodies revealed the presence of IgG2a, IgG2b, and IgA in C57BL/6 and beta 2m-/- mice and primarily of IgA in CD4-/- mice. Vaginal washes from class II-/- mice were consistently antibody negative. Interestingly, the Chlamydia-specific IgA response in the vaginal washes of CD4-/- mice was delayed, but its appearance coincided with decreased shedding of infectious organisms and resolution of infection. Our results demonstrate that MHC class II-restricted T-cell responses are necessary for the development of protective immunity to Chlamydia genital tract infection and that local (vaginal) anti-Chlamydia IgA antibody coincides with the resolution of infection. A substantive role for MHC class I-restricted T-cell responses in protective immunity to Chlamydia genital tract infection was not confirmed.

The role of CD8+ T lymphocytes in coxsackievirus B3-induced myocarditis

Coxsackievirus infections have previously been shown to cause acute or chronic myocarditis in humans, and several mouse models have been established to study the pathology of this disease. Myocardial injury may result from direct viral effects and/or may be immune mediated. To determine the relative roles of these processes in pathogenesis, we have compared coxsackievirus B3 (CVB3) infections of normal and immuno-compromised transgenic knockout (ko) mice. CVB3 was able to infect all strains used (C57BL/6, CD4ko, and beta-microglobulin ko [beta 2Mko]), and following intraperitoneal injection, two disease processes could be distinguished. First, the virus caused early (3 to 7 days postinfection) death in a viral dose-dependent manner. Immunocompetent C57BL/6 mice were highly susceptible (50% lethal dose = 70 PFU), while immunodeficient transgenic ko mice were less susceptible, showing 10- and 180-fold increases in the 50% lethal dose (for CD4ko and beta 2Mko mice, respectively). Second, a histologic examination of surviving CD4ko mice at 7 days postinfection revealed severe myocarditis; the inflammatory infiltrate comprised 40 to 50% macrophages, 30 to 40% NK cells, and 10 to 20% CD8+ T lymphocytes. The infiltration resolved over the following 2 to 3 weeks, with resultant myocardial fibrosis. In vivo depletion of CD8+ T lymphocytes from these CD4ko mice led to a marked reduction in myocarditis and an increase in myocardial virus titers. beta 2Mko mice, which lack antiviral CD8+ T cells, are much less susceptible to early death and to the development of myocarditis. We conclude that our data support a strong immunopathologic component in CVB3-induced disease and implicate both CD4+ and CD8+ T cells. Compared with immunocompetent animals, (i) mice lacking CD4+ T cells (CD4ko) were more resistant to virus challenge, and (ii) mice lacking CD8+ T cells (beta 2Mko and in vivo-depleted CD4ko) showed enhanced survival and a reduced incidence of the later myocarditis. Nevertheless, the picture is complex, since (iii) removal of the CD4+ component, while protecting against early death, greatly magnified the severity of myocarditis, and (iv) removal of the CD8+ cells from CD4ko mice, although protecting against early death and later myocarditis, led to markedly increased virus titers in the heart. These data underscore the complex balance between the costs and benefits of effective antiviral immune responses.

Upregulation of class I major histocompatibility complex antigens by interferon gamma is necessary for T-cell-mediated elimination of recombinant adenovirus-infected hepatocytes in vivo

Recombinant adenoviruses are attractive vehicles for liver-directed gene therapy because of the high efficiency with which they transfer genes to hepatocytes in vivo. First generation recombinant adenoviruses deleted of E1 sequences also express recombinant and early and late viral genes, which lead to development of destructive cellular immune responses. Previous studies indicated that class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocytes (CTLs) play a major role in eliminating virus-infected cells. The present studies utilize mouse models to evaluate the role of T-helper cells in the primary response to adenovirus-mediated gene transfer to the liver. In vivo ablation of CD4+ cells or interferon gamma (IFN-gamma) was sufficient to prevent the elimination of adenovirus-transduced hepatocytes, despite the induction of a measurable CTL response. Mobilization of an effective TH1 response as measured by in vitro proliferation assays was associated with substantial upregulation of MHC class I expression, an effect that was prevented in IFN-gamma-deficient animals. These results suggest that elimination of virus-infected hepatocytes in a primary exposure to recombinant adenovirus requires both induction of antigen-specific CTLs as well as sensitization of the target cell by TH1-mediated activation of MHC class I expression.

Increased gamma-delta T-lymphocyte response to Mycobacterium bovis BCG in major histocompatibility complex class I-deficient mice

Mice with a homologous deletion of the beta 2-microglobulin gene (beta 2m-) are deficient in class I major histocompatibility complex molecules (MHC) and consequently are deficient in CD8+ T cells. These beta 2m- mutant mice control the intraperitoneal growth of an avirulent vaccine strain of mycobacteria, Mycobacterium bovis BCG, after intraperitoneal infection similarly to normal mice. We show that beta 2m- mice have an increased gamma-delta (gamma delta) T-cell response after infection with live avirulent mycobacteria. beta 2m- mice have an earlier and more sustained rise in the proportion of intraperitoneal gamma delta T cells, averaging 17% of T cells, compared with 6% in normal mice, at 28 days after infection. Compared with the population in normal mice, gamma delta T cells in the spleens of beta 2m- mice averaged a higher proportion of the total T-cell population of the spleen on days 5, 8, and 14 after intraperitoneal infection. These data document the kinetics of gamma delta T cells reactive to mycobacterial antigens in vivo without class I MHC restriction and support a role for class I MHC and CD8+ T cells in the in vivo regulation of gamma delta T cells.

Beta 2-microglobulin-deficient mice demonstrate class II MHC restricted anti-viral CD4+ but not CD8+ CTL against influenza-sensitized autologous splenocytes

Mice transgenic for beta 2-microglobulin gene deletion (beta 2M-/-) can clear respiratory pathogens but at a slower rate than control mice. How these mice eliminate virus is not known, but the process is believed to involve CD4+ T cells. Recent studies from other laboratories have suggested a role for CD8+ cytotoxic T lymphocytes (CTLs) in both recognition of beta 2M deficient cells by allogeneic mice and rejection of MHC-incompatible tumor cells by beta 2M-/- mice. After influenza inoculation, we found no evidence for anti-influenza CD8 CTL activity from the lungs or spleens of beta 2M-/- mice. Anti-influenza CD4+, class-II restricted CTL activity was demonstrated from both the lungs and spleens. We next used mitogen-stimulated splenocytes from beta 2M-/- mice for targets in an in vitro CTL assay. This method for determining MHC class II CTL activity showed that the lungs and spleens of influenza-infected beta 2M-/- mice contained precursors to CD4+, but not CD8+, effector CTLs. The data indicate that CD8+ CTLs have no role in anti-viral activity in beta 2M-/- mice. Development of anti-tumor CTLs and anti-viral CTLs may arise via different mechanisms.

Mouse hepatitis virus A59-induced demyelination can occur in the absence of CD8+ T cells

Mouse hepatitis virus causes a chronic demyelinating disease in C57BL/6 mice. While early studies suggested demyelination is due to direct cytolytic effects of virus on oligodendrocytes, there is increasing evidence for the involvement of the immune system in the mechanism of demyelination. In this study we have asked whether demyelination can occur in the absence of functional MHC class I expression and CD8+ T cells. We infected transgenic mice lacking expression of beta 2 microglobulin (beta 2 M -/- mice) with MHV-A59. In beta 2M-/- mice, virus was much more lethal than in either of the parental strains used to produce the mice; furthermore, while clearance from the CNS did occur in beta 2M-/- mice, it was slower than in C57BL/6 mice. This is consistent with the importance of CD8+ cells in viral clearance. Because of the increased sensitivity of the beta 2M-/- mice to infection, only low levels of virus could be used to evaluate chronic disease. Even at these low levels, demyelination did occur in some animals. To compare infection in beta 2M-/- and C57BL/6 mice we used a higher dose of an attenuated variant of MHV-A59, C12. The attenuated variant induced less demyelination in C57BL/6 mice compared to wild type A59, but the levels observed were not significantly different from those seen in beta 2M-/- mice. Thus, MHV-induced demyelination can occur in some animals in the absence of MHC class I and CD8+ cells.

Role of CD8 T cells in primary Chlamydia infection

The role of CD4 and CD8 T cells in primary Chlamydia trachomatis pneumonia was investigated by using in vivo depletion techniques to eliminate T-cell populations. Reduction of either CD4 T cells or CD8 T cells caused a significant increase in organism burden in the lungs, as measured by both quantitative culture and detection of chlamydial antigen on day 14 postinfection. Chlamydia-specific antibody levels in plasma or antigen-induced gamma interferon (IFN-gamma) production by spleen cells was dramatically reduced by depletion of CD4 cells. The reduction in IFN-gamma achieved by depletion of CD8 cells did not reach statistical significance. In the survival studies, depletion of CD4 cells led to a significant increase in mortality. Although there was a trend toward higher mortality, depletion of CD8 cells did not significantly increase mortality. The role of CD8 T cells in host defense was clarified in studies using beta 2-microglobulin-deficient (major histocompatibility class I antigen-deficient, C1D) mice which are defective in CD8 T-cell function. In this model, a significant increase in organism burden was seen during infection in C1D mice compared with that C57BL/6 controls and a significant increase in mortality was observed as well. However, surviving C1D mice were able to clear the infection by day 34. C1D mice had increased numbers of CD4 T cells in both the spleen and the lungs during infection compared with those of C57BL/6 controls. IFN-gamma in C57BL/6 mice was produced by both CD4 and CD8 cells. Thus, there is a protective role for both CD4 and CD8 cells in host defense against Chlamydia infection, but the former appear to be dominant.

Murine CD8+ T cells that specifically delete autologous CD4+ T cells expressing V beta 8 TCR: a role of the Qa-1 molecule

Interactions mediated by TCRs expressed on different T cell subsets may play a role in immunoregulation. To investigate this idea, we studied the regulation of superantigen-induced TCR V beta-restricted responses. We asked whether the in vivo regulation of CD4+ V beta 8+ T cells following SEB injection is controlled by CD8+ T cells. We found that in mice deficient in CD8+ T cells, the down-regulation of CD4+ V beta 8+ T cells below baseline is not observed. Moreover, following SEB administration, CD8+ T cells emerge that preferentially kill subpopulations of activated CD4+ V beta 8+ but not CD4+ V beta 8- T cells in vitro. This TCR V beta-specific cytotoxicity is dependent on beta 2-microglobulin and is inhibited by antisera specific for Qa-1 but not by antibody to MHC class Ia. These data suggest the idea that the specificity of immune regulation may involve CD8+ T cell recognition of TCR V beta determinants and Qa-1 molecules expressed on CD4+ T cells.

The role of interferon beta in human cytomegalovirus-mediated inhibition of HLA DR induction on endothelial cells

Human cytomegalovirus (HCMV), a member of the virus family Herpesviridae that is associated with extensive worldwide morbidity and mortality in immunocompromised hosts, inhibits interferon-gamma (IFN gamma)-mediated induction of human leukocyte antigen (HLA) class II antigens on endothelial cells. In this study, the ability of HCMV-infected endothelial cells to synthesize interferon-beta (IFN beta), and the role of IFN beta in HCMV-mediated inhibition of HLA class II induction, was investigated. As determined by an encephalomyocarditis virus protection assay, HCMV-infected endothelial cell culture supernatants contained 240 IU/ml of IFN type I activity, of which 99.9% was IFN beta, as compared to the absence of IFN beta in mock-infected culture supernatants. UV-irradiated supernatants from HCMV-infected cultures inhibited induction of HLA class II in noninfected cultures by 24%. This inhibition could be abolished with 500 NU/ml of anti-IFN beta antibody. Addition of anti-IFN beta antibody directly to HCMV-infected cultures mitigated but did not abolish HLA class II antigen inhibition. Dual immunohistochemistry for HCMV and HLA DR demonstrated that infected cells, in contrast to noninfected cells, were rarely induced to express HLA class II even in the presence of anti-IFN beta antibody. These findings suggest that HCMV inhibits induction of HLA class II antigens by IFN beta dependent and independent mechanisms.

Clearance of Sendai virus by CD8+ T cells requires direct targeting to virus-infected epithelium

Minimal numbers of CD8+ T cells are found in bronchoalveolar lavage (BAL) populations recovered from Sendai virus-infected mice that are homozygous (-/-) for a beta 2-microglobulin (beta 2-m) gene disruption. The prevalence of the CD8+ set was substantially increased in the pneumonic lungs of 8-12-week radiation chimeras made using substantially class I major histocompatibility complex (MHC) glycoprotein-negative beta 2-m (-/-) recipients and normal beta 2-m (+/+) bone marrow. Even so, the CD8+ (but not the CD4+) lymphocyte counts were still much lower than in the (+/+)-->(+/+) controls. The (+/+)-->(+/+) and (+/+)-->(-/-) chimeras cleared Sendai virus and potent virus-immune CD8+ cytotoxic T lymphocytes (CTL) specific for H-2Kb+viral nucleoprotein peptide were found in the BAL from both groups. However, following in vivo depletion of the CD4+ population, only the (+/+)-->(+/+) mice were able to deal with the infection. Similarly, adoptively transferred, H-2Kb-restricted CD8+ T cells from previously-primed (+/+) mice also failed to clear virus from the lungs of (+/+)-->(-/-) chimeras infected within 2 weeks of reconstitution with bone marrow, though they were effective in the (+/+)-->(+/+) controls. Sendai virus-immune CD8+ T cells are thus unable to eliminate virus-infected beta 2-m (-/-) lung epithelial cells that might be thought to be expressing very small amounts of either isolated class I heavy chain, or class I MHC glycoprotein that has bound beta 2-m derived from beta 2-m (+/+) T cells or macrophages present in the pneumonic lung. Furthermore, the CD8+ CTL that are being exposed to beta 2-m (+/+) stimulators in the BAL population cannot operate in some bystander mode to clear virus from respiratory epithelium.

CD4+ T cells are required to sustain CD8+ cytotoxic T-cell responses during chronic viral infection

In this study, we have examined the relative contributions of CD4+ and CD8+ T cells in controlling an acute or chronic lymphocytic choriomeningitis virus (LCMV) infection. To study acute infection, we used the LCMV Armstrong strain, which is cleared by adult mice in 8 to 10 days, and to analyze chronic infection, we used a panel of lymphocyte-tropic and macrophage-tropic variants of LCMV that persist in adult mice for several months. We show that CD4+ T cells are not necessary for resolving an acute LCMV infection. CD4+ T-cell-depleted mice were capable of generating an LCMV-specific CD8+ cytotoxic T-lymphocyte (CTL) response and eliminated virus with kinetics similar to those for control mice. The CD8+ CTL response was critical for resolving this infection, since beta 2-microglobulin knockout (CD8-deficient) mice were unable to control the LCMV Armstrong infection and became persistently infected. In striking contrast to the acute infection, even a transient depletion of CD4+ T cells profoundly affected the outcome of infection with the macrophage- and lymphocyte-tropic LCMV variants. Adult mice given a single injection of anti-CD4 monoclonal antibody (GK1.5) at the time of virus challenge became lifelong carriers with high levels of virus in most tissues. Unmanipulated adult mice infected with the different LCMV variants contained virus for prolonged periods (> 3 months) but eventually eliminated infection from most tissues, and all of these mice had LCMV-specific CD8+ CTL responses. Although the level of CTL activity was quite low, it was consistently present in all of the chronically infected mice that eventually resolved the infection. These results clearly show that even in the presence of an overwhelming viral infection of the immune system, CD8+ CTL can remain active for long periods and eventually resolve and/or keep the virus infection in check. In contrast, LCMV-specific CTL responses were completely lost in chronically infected CD4-depleted mice. Taken together, these results show that CD4+ T cells are dispensable for short-term acute infection in which CD8+ CTL activity does not need to be sustained for more than 2 weeks. However, under conditions of chronic infection, in which CD8+ CTLs take several months or longer to clear the infection, CD4+ T-cell function is critical. Thus, CD4+ T cells play an important role in sustaining virus-specific CD8+ CTL during chronic LCMV infection. These findings have implications for chronic viral infections in general and may provide a possible explanation for the loss of human immunodeficiency virus-specific CD8+ CTL activity that is seen during the late stages of AIDS, when CD4+ T cells become limiting.