Differential kinetics of antigen-specific CD4+ and CD8+ T cell responses in the regression of retrovirus-induced sarcomas

MHC multimer technology: current status and future prospects

The detection of antigen-specific T cell responses by MHC multimer staining is rapidly becoming one of the core immunological techniques, and the technology to produce MHC multimers has been optimized substantially in recent years. Furthermore, recent work demonstrates the potential of high-throughput detection of T cell responses and suggests that manipulation of T cell responses through the use of multimeric MHC reagents is also feasible.

Phenotype and homing of CD4 tumor-specific T cells is modulated by tumor bulk

Technical difficulties in tracking endogenous CD4 T lymphocytes have limited the characterization of tumor-specific CD4 T cell responses. Using fluorescent MHC class II/peptide multimers, we defined the fate of endogenous Leishmania receptor for activated C kinase (LACK)-specific CD4 T cells in mice bearing LACK-expressing TS/A tumors. LACK-specific CD44(high)CD62L(low) CD4 T cells accumulated in the draining lymph nodes and had characteristics of effector cells, secreting IL-2 and IFN-gamma upon Ag restimulation. Increased frequencies of CD44(high)CD62L(low) LACK-experienced cells were also detected in the spleen, lung, liver, and tumor itself, but not in nondraining lymph nodes, where the cells maintained a naive phenotype. The absence of systemic redistribution of LACK-specific memory T cells correlated with the presence of tumor. Indeed, LACK-specific CD4 T cells with central memory features (IL-2(+)IFN-gamma(-)CD44(high)CD62L(high) cells) accumulated in all peripheral lymph nodes of mice immunized with LACK-pulsed dendritic cells and after tumor resection. Together, our data demonstrate that although tumor-specific CD4 effector T cells producing IFN-gamma are continuously generated in the presence of tumor, central memory CD4 T cells accumulate only after tumor resection. Thus, the continuous stimulation of tumor-specific CD4 T cells in tumor-bearing mice appears to hinder the systemic accumulation of central memory CD4 T lymphocytes.

CD8+ T-cell dysfunction due to cytolytic granule deficiency in persistent Friend retrovirus infection

Virus-specific CD8+ T cells are critical for the control of acute Friend virus (FV) infections, but are rendered impotent by CD4+ regulatory T cells during the chronic phase of infection. The current study examines this CD8+ T-cell dysfunction by analyzing the production and release of cytolytic molecules by CD8+ T cells. CD8+ T cells with an activated phenotype (CD43+) from acutely infected mice produced all three key components of lytic granules: perforin, granzyme A, and granzyme B. Furthermore, they displayed evidence of recent degranulation and in vivo cytotoxicity. In contrast, activated CD8+ T cells from chronically infected mice were deficient in cytolytic molecules and showed little evidence of recent degranulation and poor in vivo cytotoxicity. Evidence from tetramer-positive CD8+ T cells with known virus specificity confirmed the findings from the activated subset of CD8+ T cells. Interestingly, perforin and granzyme A mRNA levels were not significantly reduced during chronic infection, indicating control at a posttranscriptional level. Granzyme B deficiency was associated with a significant decrease in mRNA levels, but posttranscriptional control also appeared to contribute to deficiency. These results demonstrate a broad impairment of cytotoxic CD8+ T-cell effector function during chronic retroviral infection and explain the inability of virus-specific CD8+ T cells to eliminate persistent virus.

Differential expression regulation of the alpha and beta subunits of the PA28 proteasome activator in mature dendritic cells

Activation of dendritic cells (DC) by Th-dependent (CD40) or -independent (LPS, CpG, or immune complexes) agonistic stimuli strongly enhances the expression of the proteasome activator PA28alphabeta complex. Upon activation of DC, increased MHC class I presentation occurred of the melanocyte-associated epitope tyrosinase-related protein 2(180-188) in a PA28alphabeta-dependent manner. In contrast to other cell types, regulation of PA28alphabeta expression in DC after maturation was found to be IFN-gamma independent. In the present study, we show that expression of PA28alpha and beta subunits was differentially regulated. Firstly, PA28alpha expression is high in both immature and mature DC. In contrast, PA28beta expression is low in immature DC and strongly increased in mature DC. Secondly, we show the presence of a functional NF-kappaB site in the PA28beta promoter, which is absent in the PA28alpha promoter, indicating regulation of PA28beta expression by transcription factors of the NF-kappaB family. In addition, glycerol gradient analysis of DC lysates revealed elevated PA28alphabeta complex formation upon maturation. Thus, induction of PA28beta expression allows proper PA28alphabeta complex formation, thereby enhancing proteasome activity in activated DC. Therefore, maturation of DC not only improves costimulation but also MHC class I processing. This mechanism enhances the CD8(+) CTL (cross)-priming capacity of mature DC.

Direct ex vivo detection of HLA-DR3-restricted cytomegalovirus- and Mycobacterium tuberculosis-specific CD4+ T cells

In order to detect epitope-specific CD4+ T cells in mycobacterial or viral infections in the context of human class II major histocompatibility complex protein human leukocyte antigen (HLA)-DR3, two HLA-DR3 tetrameric molecules were successfully produced. One contained an immunodominant HLA-DR3-restricted T-cell epitope derived from the 65-kDa heat-shock protein of Mycobacterium tuberculosis, peptide 1-13. For the other tetramer, we used an HLA-DR3-restricted T-cell epitope derived from cytomegalovirus (CMV) pp65 lower matrix protein, peptide 510-522, which induced high levels of interferon (IFN)-gamma-producing CD4+ T cells in three of four HLA-DR3-positive CMV-seropositive individuals up to 0.84% of CD4+ T cells by intracellular cytokine staining. In peripheral blood mononuclear cells from M. tuberculosis-exposed, Mycobacterium bovis bacille Calmette-Guérin (BCG)-vaccinated, or CMV-seropositive individuals, we were able to directly detect with both tetramers epitope-specific T cells up to 0.62% and 0.45% of the CD4+ T-cell population reactive to M. tuberculosis and CMV, respectively. After a 6-day culture with peptide p510-522, the frequency of CMV-specific tetramer-binding T cells was expanded up to 9.90% tetramer+ CFSElow (5,6-carboxyfluorescein diacetate succinimidyl ester) cells within the CD4+ T-cell population, further confirming the specificity of the tetrameric molecules. Thus, HLA-DR3/peptide tetrameric molecules can be used to investigate HLA-DR3-restricted antigen-specific CD4+ T cells in clinical disease or after vaccination.

Clonal dynamics of tumor-infiltrating lymphocytes

The presence of tumor-infiltrating lymphocytes (TIL) provides important evidence of anti-tumor immunity in vivo. However, TIL are usually not sufficient for inhibiting tumor growth. We explored the spatial and temporal aspects of clonal accumulation of TIL using RT-PCR/single-strand conformation polymorphism analysis. In CMS5 fibrosarcomas in BALB/c mice, accumulated T cell clones were specific in that dominant TIL were identical between distant tumors. Moreover, dominant TIL in the first tumor appeared consistently in the second tumor inoculated after formation of the first tumor. These results suggest that TIL show a certain level of specific tumor surveillance. When we characterized CD4(+) and CD8(+) TIL separately, CD8(+) TIL were highly concentrated and persistently localized at the tumor site, while most CD4(+) TIL clones were less concentrated and less persistent. A functional analysis showed that TIL had a certain degree of anti-tumor activity when CD4(+) and CD8(+) TIL were co-transferred. Co-transfer of CD4(+) and CD8(+) TIL exhibited equivalent anti-tumor activity, irrespective of tumor stage. However, the numbers of TIL did not increase after the early phase of tumor progression. These data suggest that TIL are specific to the tumor and potentially retain anti-tumor activity, although their accumulation in mice is impaired.

Immune surveillance and anti-tumor immune responses: an anatomical perspective

The development of adaptive immune responses against infectious agents relies on the initiation of antigen specific immune responses in secondary lymphoid organs and on the migration of effector cells at the site of infection. Similarly, the development of anti-tumor immunity depends on the recognition of tumor-derived antigens by specific lymphocytes in the context of the lymphoid tissues and on the re-localisation of the cells to the site of cell transformation. Here, we will review the preclinical studies, which have defined the spatial and temporal organisation of anti-tumor immunity, and discuss the implications of these findings in active immunotherapy.

Induction of impaired antitumor immunity by fusion of MHC class II-deficient dendritic cells with tumor cells

To dissect the role of Ag presentation through MHC class I and/or II pathways by dendritic cell (DC)-tumor fusion cells, we have created various types of DC-tumor fusion cells by alternating fusion cell partners. Fusions of MC38/MUC1 carcinoma cells with DC from wild-type (WT-DC), MHC class I knockout (IKO-DC), class II knockout (IIKO-DC), or class I and II knockout (I/IIKO-DC) mice created WTDC-fusion cells (FC), IKO-FC, IIKO-FC, and I/IIKO-FC, respectively. MHC class II- and MUC1-positive fusion cells were constructed by fusion of B16/MUC1 melanoma cells with IKO-DC (IKO/B16-FC). Immunization of MUC1 transgenic mice with 5 x 10(5) WTDC-FC, IKO-FC, IIKO-FC, or I/IIKO-FC provided 100, 91.7, 61.5, and 15.4% protection, respectively, against tumor challenge with MC38/MUC1 cells. In contrast, all mice immunized with irradiated MC38/MUC1 tumor cells or WT-DC developed tumors. One group of mice was immunized with 5 x 10(5) IKO/B16-FC and then challenged with B16/Ia(+)/MUC1 on one flank and MC38/MUC1 on the other flank. Immunization of these mice with IKO/B16-FC resulted in 100 and 78.6% protection against B16/Ia(+)/MUC1 and MC38/MUC1 tumor challenge, respectively. The antitumor immunity induced by immunization with IKO/B16-FC was able to inhibit the growth of MHC class II-negative tumor. In addition, in vivo results correlated with the induction of Ag-specific CTL. Collectively, the data indicate that MHC class II Ag presentation targeting activation of CD4 T cells is indispensable for antitumor immunity.

Independent roles of perforin, granzymes, and Fas in the control of Friend retrovirus infection

Cytotoxic T-cells (CTL) play a central role in the recovery of mammalian hosts from retroviral infections. However, the molecular pathways that mediate the antiretroviral activity of CTL are still elusive. Here we explore the protective role of the two main cytolytic pathways of CTL, that is, granule exocytosis and Fas/Fas ligand (FasL), in acute and persistent Friend retrovirus (FV) infection of mice. For this purpose, we have used mutant mouse strains with targeted gene defects in one or more components of the two cytolytic pathways including perforin, granzyme A, granzyme B, Fas, and FasL. The important function of CTL in resistance of C57BL/6 (B6) mice to FV is emphasized by the finding that depletion of CD8+ T-cells prior to virus infection resulted in severe splenomegaly and high viral loads in blood and spleen tissue. Analysis of primary FV infection in knockout mice revealed that acute infection was readily controlled in the absence of functional Fas. Most notably in the presence of Fas/FasL each of the three effector molecules of the exocytosis pathway (i.e., perforin, granzyme A, and granzyme B) was capable on its own to mediate suppression of virus replication and protection from leukemia. However, triple knockout mice lacking perforin and the two granzymes were fully susceptible to FV-induced leukemia. In contrast to acute infection the Fas/FasL pathway was mandatory for effective control of FV replication during persistent infection. These findings suggest novel pathways of CTL-mediated viral defense and contribute towards a better understanding of the molecular mechanisms of CTL activity in retroviral infections.

Reduction of retrovirus-induced immunosuppression by in vivo modulation of T cells during acute infection

Chronic infection with Friend retrovirus is associated with suppressed antitumor immune responses. In the present study we investigated whether modulation of T-cell responses during acute infection would restore antitumor immunity in persistently infected mice. T-cell modulation was done by treatments with DTA-1 anti- glucocorticoid-induced tumor necrosis factor receptor monoclonal antibodies. The DTA-1 monoclonal antibody is nondepleting and delivers costimulatory signals that both enhance the activation of effector T cells and inhibit suppression by regulatory T cells. DTA-1 therapy produced faster Th1 immune responses, significant reductions in both acute virus loads and pathology and, most importantly, long-term improvement of CD8(+) T-cell-mediated antitumor responses.

Tumor rejection induced by CD70-mediated quantitative and qualitative effects on effector CD8+ T cell formation

In vivo priming of antigen-specific CD8+ T cells results in their expansion and differentiation into effector T cells followed by contraction into a memory T cell population that can be maintained for life. Recent evidence suggests that after initial antigenic stimulation, the magnitude and kinetics of the CD8+ T cell response are programmed. However, it is unclear to what extent CD8+ T cell instruction in vivo is modulated by costimulatory signals. Here, we demonstrate that constitutive ligation of the tumor necrosis factor receptor family member CD27 by its ligand CD70 quantitatively augments CD8+ T cell responses to influenza virus infection and EL-4 tumor challenge in vivo by incrementing initial expansion and maintaining higher numbers of antigen-specific T cells in the memory phase. Concomitantly, the quality of antigen-specific T cells improved as evidenced by increased interferon (IFN)-gamma production and a greater cytotoxic potential on a per cell basis. As an apparent consequence, the superior effector T cell formation induced by CD70 protected against a lethal dose of poorly immunogenic EL4 tumor cells in a CD8+ T cell- and IFN-gamma-dependent manner. Thus, CD70 costimulation enhances both the expansion and per cell activity of antigen-specific CD8+ T cells.

Essential role for virus-neutralizing antibodies in sterilizing immunity against Friend retrovirus infection

The current experiments use the Friend retrovirus model to demonstrate that vaccine-primed B cells are essential for sterilizing immunity, and the results indicate that the requisite function of these cells is the production of virus-neutralizing antibodies rather than priming or reactivation of T cells. B cell-deficient mice were poorly protected by vaccination, but adoptive transfer experiments showed that the T cells from B cell-deficient mice were primed as well as those from wild-type mice. Furthermore, passive transfer of virus-neutralizing antibodies completely compensated for B cell deficiency. The presence of virus-neutralizing antibodies at the time of infection was crucial for vaccine efficacy. Interestingly, virus-neutralizing antibodies worked synergistically with vaccine-primed T cells to provide a level of protection many orders of magnitude greater than either antibodies or immune T cells alone. Nonneutralizing antibodies also contributed to protection and acted cooperatively with neutralizing antibodies to reduce infection levels. These results emphasize the importance of inducing both T cell responses and virus-neutralizing antibody responses for effective retroviral vaccine protection.

Analysis of autoreactive CD4 T cells in experimental autoimmune encephalomyelitis after primary and secondary challenge using MHC class II tetramers

Experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, is primarily mediated by CD4 T cells specific for Ags in the CNS. Using MHC class II tetramers, we assessed expansion and phenotypic differentiation of polyclonal self-reactive CD4 T cells during EAE after primary and secondary challenge with the specific Ag. After EAE induction in SJL mice with proteolipid protein 139-151, CNS-specific T cells up-regulated activation markers and expanded in the draining lymph nodes and in the spleen. Less than 20% of total autoreactive T cells entered the CNS simultaneously with Th cells of other specificities. Almost all tetramer-positive cells in the CNS were activated and phenotypically distinct from the large peripheral pool. When EAE was induced in Ag-experienced mice, disease symptoms developed earlier and persisted longer; autoreactive T cells were more rapidly activated and invaded the CNS earlier. In striking contrast to specific CTLs that respond after secondary viral challenge, the absolute numbers of autoreactive CD4 T cells were not increased, indicating that the accelerated autoreactivity in Ag-experienced mice is not related to higher frequencies of autoreactive CD4 T cells.

Properties of CD4+ T cells in human cytomegalovirus infection

The correlates of protective immunity to disease-inducing viruses in man remain to be elucidated. We determined the kinetics and properties of cytomegalovirus (CMV)-specific CD4(+) T cells in healthy individuals and renal transplant recipients during different stages of CMV infection. Our data reveal that circulating CMV-specific CD4(+) T cells displayed an effector-memory phenotype, and produced the T helper 1 cytokines interferon-gamma and tumor necrosis factor-alpha. In addition, they lacked molecules for secondary lymphoid organ homing and expressed the cytotoxic molecule granzyme B, inferring a direct role of these cells at target sites of infection. In asymptomatic individuals the CMV-specific CD4(+) T-cell response preceded CMV-specific CD8(+) T-cell responses, whereas in symptomatic individuals the CMV-specific effector memory CD4(+) T-cell response was delayed and only detectable after antiviral therapy. The appearance of disease symptoms in these patients suggests that functional CD8(+) T cell and antibody responses are insufficient to control viral replication and that formation of effector memory CD4(+) T cells is necessary for recovery of infection.

Functional impairment of CD8(+) T cells by regulatory T cells during persistent retroviral infection

The establishment of viral persistence generally requires evasion of the host CD8(+) T cell response. Here we describe a form of evasion wherein the CD8(+) T cells are fully capable of recognizing their cognate antigen but their effector functions are suppressed by regulatory T cells. Virus-specific CD8(+) T cells adoptively transferred into mice persistently infected with Friend virus proliferated and appeared activated, but failed to produce IFNgamma or reduce virus loads. Cotransfer experiments revealed that a subpopulation of CD4(+) T cells from persistently infected mice suppressed IFNgamma production by the CD8(+) T cells. Treatment of persistently infected mice with anti-GITR antibody to ameliorate suppression by regulatory T cells significantly improved IFNgamma production by transferred CD8(+) T cells and allowed a significant reduction in viral loads. The results indicate that CD4(+) regulatory T cells contribute to viral persistence and demonstrate an immunotherapy for treating chronic retroviral infections.

Lymphocyte activation and hepatic cellular infiltration in immunocompetent mice infected by dengue virus

Activation and expansion of dengue virus-specific T cells and abnormal liver functions in dengue patients have been documented. However, it remains to be determined whether T cells are involved in the pathogenic mechanism of dengue virus infection. In this study, immunocompetent C57BL/6 mice were employed to study dengue virus-induced T cell activation. Mice were inoculated with 10(8) PFU dengue virus serotype 2 strain 16681 by the intravenous route. Dengue viral core RNA was detected by RT-PCR in mouse serum, liver, spleen, and brain at different time points after infection. Splenic T cells were activated as evidenced by their expression of CD69 and O-glycosylated CD43 at as early as day 3 after infection. Splenic T cell expression of O-glycosylated CD43 and IFN-gamma production coordinately peaked at day 5. Coincided with the peak of splenic T cell activation was hepatic lymphocyte infiltration and elevation of liver enzymes. Flow cytometric analysis revealed the infiltrating CD8(+) T cell to CD4(+) T cell ratio was 5/3. After a second inoculation of dengue virus, hepatic T cell infiltration and liver enzyme levels increased sharply. The infiltrating hepatic CD8(+) T cell to CD4(+) T cell ratio increased to 5.8/1. A strong correlation was found between T cell activation and hepatic cellular infiltration in immunocompetent mice infected with dengue virus. The kinetics of liver enzyme elevation also correlated with that of T cell activation. These data suggest a relationship between T cell infiltration and elevation of liver enzymes.

Interference with T cell receptor-HLA-DR interactions by Epstein-Barr virus gp42 results in reduced T helper cell recognition

Epstein-Barr virus (EBV) persists lifelong in infected hosts despite the presence of antiviral immunity. Many viral antigens are expressed during lytic infection. Thus, for EBV to spread, it must have evolved effective ways to evade immune recognition. Here, we report that HLA class II-restricted antigen presentation to T helper cells is hampered in the presence of the lytic-phase protein gp42. This interference with T cell activation involves association of gp42 with class II peptide complexes. Using HLA-DR tetramers, we identify a block in T cell receptor (TCR)-class II interactions imposed by gp42 as the underlying mechanism. EBV gp42 sterically clashes with TCR Valpha-domains as visualized by superimposing the crystal structures for gp42-HLA-DR1 and TCR-MHC class II complexes. Blocking TCR recognition provides a previously undescribed strategy for viral immune evasion.

Effective postexposure treatment of retrovirus-induced disease with immunostimulatory DNA containing CpG motifs

Therapeutic strategies for the treatment of acute retroviral infections have relied mainly on antiviral drugs. In this study we used the Friend virus model system to demonstrate that enhancement of the immune system can also have dramatic therapeutic effects. Since resistance to Friend virus-induced leukemia in mice is associated with T helper cell type 1 (Th1) immune responses, we enhanced these responses in susceptible mice by treatment with synthetic oligodeoxynucleotides containing unmethylated CpG motifs (CpG-ODN). Treatments begun at 4 days postinfection increased recovery from 6% in the control group to 74% in the CpG-treated group. CpG-mediated recovery was associated with a significant reduction of viral loads in the blood and spleens of treated mice compared to those of control animals. The treatment promoted Th1-type cytokine production by splenocytes of Friend virus-infected mice and augmented Friend virus-specific cytotoxic T-cell responses, but no influence on the virus-specific neutralizing antibody response was observed. Friend virus-specific CD8(+) T cells were critical for effective treatment with CpG-ODN, since in vivo depletion of these cells from treated mice prevented their recovery. Our results demonstrate that CpG-ODN therapy can significantly enhance virus-specific cellular immune responses and prevent retrovirus-induced disease. These findings may have implications for antiviral therapy in general.