Cationic lipid/DNA complex-adjuvanted influenza A virus vaccination induces robust cross-protective immunity

Influenza A virus is a negative-strand segmented RNA virus in which antigenically distinct viral subtypes are defined by the hemagglutinin (HA) and neuraminidase (NA) major viral surface proteins. An ideal inactivated vaccine for influenza A virus would induce not only highly robust strain-specific humoral and T-cell immune responses but also cross-protective immunity in which an immune response to antigens from a particular viral subtype (e.g., H3N2) would protect against other viral subtypes (e.g., H1N1). Cross-protective immunity would help limit outbreaks from newly emerging antigenically novel strains. Here, we show in mice that the addition of cationic lipid/noncoding DNA complexes (CLDC) as adjuvant to whole inactivated influenza A virus vaccine induces significantly more robust adaptive immune responses both in quantity and quality than aluminum hydroxide (alum), which is currently the most widely used adjuvant in clinical human vaccination. CLDC-adjuvanted vaccine induced higher total influenza virus-specific IgG, particularly for the IgG2a/c subclass. Higher levels of multicytokine-producing influenza virus-specific CD4 and CD8 T cells were induced by CLDC-adjuvanted vaccine than with alum-adjuvanted vaccine. Importantly, CLDC-adjuvanted vaccine provided significant cross-protection from either a sublethal or lethal influenza A viral challenge with a different subtype than that used for vaccination. This superior cross-protection afforded by the CLDC adjuvant required CD8 T-cell recognition of viral peptides presented by classical major histocompatibility complex class I proteins. Together, these results suggest that CLDC has particular promise for vaccine strategies in which T cells play an important role and may offer new opportunities for more effective control of human influenza epidemics and pandemics by inactivated influenza virus vaccine.     

Viral suppression and immune restoration in the gastrointestinal mucosa of human immunodeficiency virus type 1-infected patients initiating therapy during primary or chronic infection

Although the gut-associated lymphoid tissue (GALT) is an important early site for human immunodeficiency virus (HIV) replication and severe CD4+ T-cell depletion, our understanding is limited about the restoration of the gut mucosal immune system during highly active antiretroviral therapy (HAART). We evaluated the kinetics of viral suppression, CD4+ T-cell restoration, gene expression, and HIV-specific CD8+ T-cell responses in longitudinal gastrointestinal biopsy and peripheral blood samples from patients initiating HAART during primary HIV infection (PHI) or chronic HIV infection (CHI) using flow cytometry, real-time PCR, and DNA microarray analysis. Viral suppression was more effective in GALT of PHI patients than CHI patients during HAART. Mucosal CD4+ T-cell restoration was delayed compared to peripheral blood and independent of the time of HAART initiation. Immunophenotypic analysis showed that repopulating mucosal CD4+ T cells were predominantly of a memory phenotype and expressed CD11 alpha, alpha(E)beta 7, CCR5, and CXCR4. Incomplete suppression of viral replication in GALT during HAART correlated with increased HIV-specific CD8+ T-cell responses. DNA microarray analysis revealed that genes involved in inflammation and cell activation were up regulated in patients who did not replenish mucosal CD4+ T cells efficiently, while expression of genes involved in growth and repair was increased in patients with efficient mucosal CD4+ T-cell restoration. Our findings suggest that the discordance in CD4+ T-cell restoration between GALT and peripheral blood during therapy can be attributed to the incomplete viral suppression and increased immune activation and inflammation that may prevent restoration of CD4+ T cells and the gut microenvironment.     

Interleukin 2 induction of antigen-nonspecific suppressor cells

Although Interleukin 2 (IL-2) is essential to the generation of immune responses it may also be important as a regulator of these same responses, as both primary and secondary anti-SRBC responses are greatly diminished when IL-2 is included in culture. IL-2 must be present within the first 24 hr of culture to affect maximum suppression. This inhibition is mediated by suppressor cells which are expanded by pulsing spleen cells with IL-2 for 48-72 hr. Their development is not antigen dependent and their action is antigen nonspecific. Suppressor cell activity can be generated from either naive or primed animals which are equally effective in inhibiting primary or secondary anti-SRBC responses. Suppressor cells can be propagated for long periods of time in T-cell growth factor-containing medium. These long-term cultured cells retain the ability to inhibit various immune responses such as mitogen- and alloantigen-induced proliferation, the generation of cytotoxic T lymphocytes and humoral responses. These cells suppress these responses by absorbing IL-2, as demonstrated by their ability to remove IL-2 upon incubation at 4 degrees C, and the reversal of suppression by the addition of supraoptimal amounts of IL-2.     

B cells as a therapeutic target in autoimmune disease

Depleting B cells with anti-CD20 monoclonal antibodies emerges as a new therapeutic strategy in autoimmune diseases. Preliminary clinical studies suggest therapeutic benefits in patients with classic autoantibody-mediated syndromes, such as autoimmune cytopenias. Treatment responses in rheumatoid arthritis have opened the discussion about whether mechanisms beyond the removal of potentially pathogenic antibodies are effective in B-cell depletion. B cells may modulate T-cell activity through capturing and presenting antigens or may participate in the neogenesis of lymphoid microstructures that amplify and deviate immune responses. Studies exploring which mechanisms are functional in which subset of patients hold the promise of providing new and rational treatment approaches for autoimmune syndromes.     

Oligomannose-coated liposomes efficiently induce human T-cell leukemia virus-1-specific cytotoxic T lymphocytes without adjuvant

Human T-cell leukemia virus-1 (HTLV-1) causes adult T-cell leukemia/lymphoma, which is an aggressive peripheral T-cell neoplasm. Insufficient T-cell response to HTLV-1 is a potential risk factor in adult T-cell leukemia/lymphoma. Efficient induction of antigen-specific cytotoxic T lymphocytes is important for immunological suppression of virus-infected cell proliferation and oncogenesis, but efficient induction of antigen-specific cytotoxic T lymphocytes has evaded strategies utilizing poorly immunogenic free synthetic peptides. Here, we examined the efficient induction of an HTLV-1-specific CD8+ T-cell response by oligomannose-coated liposomes (OMLs) encapsulating the human leukocyte antigen (HLA)-A*0201-restricted HTLV-1 Tax-epitope (OML/Tax). Immunization of HLA-A*0201 transgenic mice with OML/Tax induced an HTLV-1-specific gamma-interferon reaction, whereas immunization with epitope peptide alone induced no reaction. Upon exposure of dendritic cells to OML/Tax, the levels of CD86, major histocompatibility complex class I, HLA-A02 and major histocompatibility complex class II expression were increased. In addition, our results showed that HTLV-1-specific CD8+ T cells can be efficiently induced by OML/Tax from HTLV-1 carriers compared with epitope peptide alone, and these HTLV-1-specific CD8+ T cells were able to lyse cells presenting the peptide. These results suggest that OML/Tax is capable of inducing antigen-specific cellular immune responses without adjuvants and may be useful as an effective vaccine carrier for prophylaxis in tumors and infectious diseases by substituting the epitope peptide.     

Expression of NK-associated receptors on cytotoxic T cells from melanoma patients: a two-edged sword?

The coexistence of tumor progression with a tumor-specific immune response constitutes a major paradox of tumor immunity. During the last decade, the presence of cytotoxic T lymphocytes (CTLs) recognising melanoma-associated antigens has been unequivocally demonstrated in numerous different in vivo and in vitro models. However, most often these melanoma-specific T lymphocytes do not control tumor growth. Several mechanisms that involve changes in melanoma phenotype and/or in T-cell differentiation and function could explain the inability of the immune response to control melanoma. In the last few years it has been demonstrated that cellular cytotoxicity is the result of a balance between activating signals triggered by the TCR and costimulatory molecules and inhibitory signals triggered by inhibitory receptors expressed by the CTL. Because the final outcome of the immune response against melanoma depends on the balance between activating and inhibitory signals, the expression de novo on melanoma cells of ligands for inhibitory NKRs and the down-regulation of costimulatory molecules may favor the escape of tumor cells from immunosurveillance. In this paper we review how altered expression of molecules required for T-cell costimulation could result in impaired lysis of melanoma. The modulation of antimelanoma T-cell responses by a group of receptors originally described on NK cells (NK-associated receptors) but which are now known also to be expressed on a subset of cytolytic effector cells is reviewed. We hypothesize that the expression of ligands for NKRs on melanoma cells may contribute to T-cell-mediated immune responses against melanoma either enhancing or inhibiting activation and differentiation to effector cells. Blocking inhibitory receptors or increasing activating receptors could result in new strategies to improve T-cell-mediated rejection of melanoma.     

Immunomodulation: suppression, enhancement and autoimmunity.

Recent reports highlight new approaches to immunointervention (suppression or enhancement) based on the known roles of lymphocyte populations, cytokines and their receptors, and adhesion molecules in immune responses. Considerable interest over the past year has focused on immunophilins (cyclophilin and the FK506-binding protein) which appear to play key roles in signal transduction within activated T cells and provide a molecular basis for new advances in immunosuppressive drug therapy.     

The Immune System Strikes Back: Cellular Immune Responses against Indoleamine 2,3-dioxygenase

Background

The enzyme indoleamine 2,3-dioxygenase (IDO) exerts an well established immunosuppressive function in cancer. IDO is expressed within the tumor itself as well as in antigen-presenting cells in tumor-draining lymph nodes, where it promotes the establishment of peripheral immune tolerance to tumor antigens. In the present study, we tested the notion whether IDO itself may be subject to immune responses.

Methods and Findings

The presence of naturally occurring IDO-specific CD8 T cells in cancer patients was determined by MHC/peptide stainings as well as ELISPOT. Antigen specific cytotoxic T lymphocytes (CTL) from the peripheral blood of cancer patients were cloned and expanded. The functional capacity of the established CTL clones was examined by chrome release assays. The study unveiled spontaneous cytotoxic T-cell reactivity against IDO in peripheral blood as well as in the tumor microenvironment of different cancer patients. We demonstrate that these IDO reactive T cells are indeed peptide specific, cytotoxic effector cells. Hence, IDO reactive T cells are able to recognize and kill tumor cells including directly isolated AML blasts as well as IDO-expressing dendritic cells, i.e. one of the major immune suppressive cell populations.

Conclusion

IDO may serve as an important and widely applicable target for anti-cancer immunotherapeutic strategies. Furthermore, as emerging evidence suggests that IDO constitutes a significant counter-regulatory mechanism induced by pro-inflammatory signals, IDO-based immunotherapy holds the promise to boost anti-cancer immunotherapy in general.     

Azathioprine suppresses ezrin-radixin-moesin-dependent T cell-APC conjugation through inhibition of Vav guanosine exchange activity on Rac proteins

We have shown recently that the azathioprine metabolite 6-Thio-GTP causes immunosuppression by blockade of GTPase activation in T lymphocytes. In the present study, we describe a new molecular mechanism by which 6-Thio-GTP blocks GTPase activation. Although 6-Thio-GTP could bind to various small GTPases, it specifically blocked activation of Rac1 and Rac2 but not of closely related Rho family members such as Cdc42 and RhoA in primary T cells upon stimulation with alphaCD28 or fibronectin. Binding of 6-Thio-GTP to Rac1 did not suppress Rac effector coupling directly but blocked Vav1 exchange activity upon 6-Thio-GTP hydrolysis, suggesting that 6-Thio-GTP loading leads to accumulation of 6-Thio-GDP-loaded, inactive Rac proteins over time by inhibiting Vav activity. In the absence of apoptosis, blockade of Vav-mediated Rac1 activation led to a blockade of ezrin-radixin-moesin dephosphorylation in primary T cells and suppression of T cell-APC conjugation. Azathioprine-generated 6-Thio-GTP thus prevents the development of an effective immune response via blockade of Vav activity on Rac proteins. These findings provide novel insights into the immunosuppressive effects of azathioprine and suggest that antagonists of the Vav-Rac signaling pathway may be useful for suppression of T cell-dependent pathogenic immune responses.     

The tumor immunosuppressive microenvironment impairs the therapy of anti-HER2/neu antibody

It has been well established that immune surveillance plays critical roles in preventing the occurrence and progression of tumor. More and more evidence in recent years showed the host anti-tumor immune responses also play important roles in the chemotherapy and radiotherapy of cancers. Our previous study found that tumor- targeting therapy of anti-HER2/neu mAb is mediated by CD8⁺ T cell responses. However, we found here that enhancement of CD8⁺ T cell responses by combination therapy with IL-15R/IL-15 fusion protein or anti-CD40, which are strong stimultors for T cell responses, failed to promote the tumor therapeutic effects of anti-HER2/neu mAb. Analysis of tumor microenviornment showed that tumor tissues were heavily infiltrated with the immunosuppressive macrophages and most tumor infiltrating T cells, especially CD8⁺ T cells, expressed high level of inhibitory co-signaling receptor PD-1. These data suggest that tumor microenvironment is dominated by the immunosuppressive strategies, which thwart anti-tumor immune responses. Therefore, the successful tumor therapy should be the removal of inhibitory signals in the tumor microenvironment in combination with other therapeutic strategies.     

Human mesenchymal stem cells inhibit the differentiation and effector functions of monocytes

Although monocytes represent an essential part of the host defence system, their accumulation and prolonged stimulation could be detrimental and may aggravate chronic inflammatory diseases. The present study has explored the less-understood immunomodulatory effects of mesenchymal stem cells on monocyte functions. Isolated purified human monocytes were co-cultured with human umbilical cord-derived mesenchymal stem cells under appropriate culture conditions to assess monocytes’ vital functions. Based on the surface marker analysis, mesenchymal stem cells halted monocyte differentiation into dendritic cells and macrophages and reduced their phagocytosis functions, which rendered an inability to stimulate T-cell proliferation. The present study confers that mesenchymal stem cells exerted potent immunosuppressive activity on monocyte functions such as differentiation, phagocytosis and Ag presentation; hence, they promise a potential therapeutic role in down-regulating the unwanted monocyte-mediated immune responses in the context of chronic inflammatory diseases.     

Neisseria gonorrhoeae Suppresses Dendritic Cell-Induced, Antigen-Dependent CD4 T Cell Proliferation

Neisseria gonorrhoeae is the second most common sexually transmitted bacterial pathogen worldwide. Diseases associated with N. gonorrhoeae cause localized inflammation of the urethra and cervix. Despite this inflammatory response, infected individuals do not develop protective adaptive immune responses to N. gonorrhoeae. N. gonorrhoeae is a highly adapted pathogen that has acquired multiple mechanisms to evade its host's immune system, including the ability to manipulate multiple immune signaling pathways. N. gonorrhoeae has previously been shown to engage immunosuppressive signaling pathways in B and T lymphocytes. We have now found that N. gonorrhoeae also suppresses adaptive immune responses through effects on antigen presenting cells. Using primary, murine bone marrow-derived dendritic cells and lymphocytes, we show that N. gonorrhoeae-exposed dendritic cells fail to elicit antigen-induced CD4+ T lymphocyte proliferation. N. gonorrhoeae exposure leads to upregulation of a number of secreted and dendritic cell surface proteins with immunosuppressive properties, particularly Interleukin 10 (IL-10) and Programmed Death Ligand 1 (PD-L1). We also show that N. gonorrhoeae is able to inhibit dendritic cell- induced proliferation of human T-cells and that human dendritic cells upregulate similar immunosuppressive molecules. Our data suggest that, in addition to being able to directly influence host lymphocytes, N. gonorrhoeae also suppresses development of adaptive immune responses through interactions with host antigen presenting cells. These findings suggest that gonococcal factors involved in host immune suppression may be useful targets in developing vaccines that induce protective adaptive immune responses to this pathogen.     

Concomitant blockade of A2AR and CTLA-4 by siRNA-loaded polyethylene glycol-chitosan-alginate nanoparticles synergistically enhances antitumor T-cell responses

Inhibitory immune checkpoint (ICP) molecules are important immunosuppressive factors in a tumor microenvironment (TME). They can robustly suppress T-cell-mediated antitumor immune responses leading to cancer progression. Among the checkpoint molecules, cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) is one of the critical inhibitors of anticancer T-cell responses. Besides, the expression of adenosine receptor (A2AR) on tumor-infiltrating T cells potently reduces their function. We hypothesized that concomitant silencing of these molecules in T cells might lead to enhanced antitumor responses. To examine this assumption, we purified T cells from the tumor, spleen, and local lymph nodes of CT26 colon cancer-bearing mice and suppressed the expression of A2AR and CTLA-4 using the small interfering RNA (siRNA)-loaded polyethylene glycol-chitosan-alginate (PCA) nanoparticles. The appropriate physicochemical properties of the produced nanoparticles (NPs; size of 72 nm, polydispersive index [PDI] < 0.2, and zeta potential of 11 mV) resulted in their high efficiency in transfection and suppression of target gene expression. Following the silencing of checkpoint molecules, various T-cell functions, including proliferation, apoptosis, cytokine secretion, differentiation, and cytotoxicity were analyzed, ex vivo. The results showed that the generated nanoparticles had optimal physicochemical characteristics and significantly suppressed the expression of target molecules in T cells. Moreover, a concomitant blockade of A2AR and CTLA-4 in T cells could synergistically enhance antitumor responses through the downregulation of PKA, SHP2, and PP2Aα signaling pathways. Therefore, this combination therapy can be considered as a novel promising anticancer therapeutic strategy, which should be further investigated in subsequent studies.     

Antiviral therapy during primary simian immunodeficiency virus infection fails to prevent acute loss of CD4+ T cells in gut mucosa but enhances their rapid restoration through central memory T cells

Gut-associated lymphoid tissue (GALT) is an early target of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) and a site for severe CD4+ T-cell depletion. Although antiretroviral therapy (ART) is effective in suppressing HIV replication and restoring CD4+ T cells in peripheral blood, restoration in GALT is delayed. The role of restored CD4+ T-cell help in GALT during ART and its impact on antiviral CD8+ T-cell responses have not been investigated. Using the SIV model, we investigated gut CD4+ T-cell restoration in infected macaques, initiating ART during either the primary stage (1 week postinfection), prior to acute CD4+ cell loss (PSI), or during the chronic stage at 10 weeks postinfection (CSI). ART led to viral suppression in GALT and peripheral blood mononuclear cells of PSI and CSI animals at comparable levels. CSI animals had incomplete CD4+ T-cell restoration in GALT. In PSI animals, ART did not prevent acute CD4+ T-cell loss by 2 weeks postinfection in GALT but supported rapid and complete CD4+ T-cell restoration thereafter. This correlated with an accumulation of central memory CD4+ T cells and better suppression of inflammation. Restoration of CD4+ T cells in GALT correlated with qualitative changes in SIV gag-specific CD8+ T-cell responses, with a dominance of interleukin-2-producing responses in PSI animals, while both CSI macaques and untreated SIV-infected controls were dominated by gamma interferon responses. Thus, central memory CD4+ T-cell levels and qualitative antiviral CD8+ T-cell responses, independent of viral suppression, were the immune correlates of gut mucosal immune restoration during ART.     

Histamine releasing factor and elongation factor 1 alpha secreted via malaria parasites extracellular vesicles promote immune evasion by inhibiting specific T cell responses

Protozoan pathogens secrete nanosized particles called extracellular vesicles (EVs) to facilitate their survival and chronic infection. Here, we show the inhibition by Plasmodium berghei NK65 blood stage‐derived EVs of the proliferative response of CD4⁺ T cells in response to antigen presentation. Importantly, these results were confirmed in vivo by the capacity of EVs to diminish the ovalbumin‐specific delayed type hypersensitivity response. We identified two proteins associated with EVs, the histamine releasing factor (HRF) and the elongation factor 1α (EF‐1α) that were found to have immunosuppressive activities. Interestingly, in contrast to WT parasites, EVs from genetically HRF‐ and EF‐1α‐deficient parasites failed to inhibit T cell responses in vitro and in vivo. At the level of T cells, we demonstrated that EVs from WT parasites dephosphorylate key molecules (PLCγ1, Akt, and ERK) of the T cell receptor signalling cascade. Remarkably, immunisation with EF‐1α alone or in combination with HRF conferred a long‐lasting antiparasite protection and immune memory. In conclusion, we identified a new mechanism by which P. berghei‐derived EVs exert their immunosuppressive functions by altering T cell responses. The identification of two highly conserved immune suppressive factors offers new conceptual strategies to overcome EV‐mediated immune suppression in malaria‐infected individuals.     

Suppression by cyclosporin A of murine T-cell-mediated immunity against viruses in vivo and in vitro

The immunosuppressive effect of Cyclosporin A on T-cell-mediated antiviral immune responses was examined. When administered intraperitoneally CS-A abrogated anti-vaccinia virus, anti-lymphocytic choriomeningitis virus (LCMV), and anti-vesicular stomatitis virus (VSV) T-cell responses in a dose-dependent fashion. Usually 50-60 mg/kg were efficient in suppressing primary T-cell responses completely. In contrast, 10-20 mg/kg often enhanced T-cell responses significantly when compared with controls. Suppression was observed if CS-A treatment was started before virus injection and up to 12 hr after infection; CS-A given 24 hr after the virus still suppressed T-cell activity partially. A 50 mg/kg dose of CS-A suppressed secondary anti-vaccinia virus or anti-VSV T-cell responses in vivo by a factor of about 10. This dose suppressed the primary T-cell-dependent footpad swelling induced by local LCMV infection and prevented T-cell-mediated immunopathological death due to LCM when LCMV was injected intracerebrally. In addition, clearance of LCMV was delayed drastically by CS-A treatment. When added to cultures of in vivo-primed antiviral T cells that were restimulated in vitro, CS-A inhibited both proliferation as well as generation of virus-specific cytotoxic T cells in a dose-dependent way. The results show that in CS-A-treated mice primary and secondary antiviral T-cell responses are strongly inhibited; acute viral infections with cytopathic viruses may therefore be more dramatic. In contrast immunopathological T-cell-mediated disease caused by noncytopathic viruses such as LCMV may be prevented or attenuated.     

DNA-Launched Alphavirus Replicons Encoding a Fusion of Mycobacterial Antigens Acr and Ag85B Are Immunogenic and Protective in a Murine Model of TB Infection

There is an urgent need for effective prophylactic measures against Mycobacterium tuberculosis (Mtb) infection, particularly given the highly variable efficacy of Bacille Calmette-Guerin (BCG), the only licensed vaccine against tuberculosis (TB). Most studies indicate that cell-mediated immune responses involving both CD4+ and CD8+ T cells are necessary for effective immunity against Mtb. Genetic vaccination induces humoral and cellular immune responses, including CD4+ and CD8+ T-cell responses, against a variety of bacterial, viral, parasitic and tumor antigens, and this strategy may therefore hold promise for the development of more effective TB vaccines. Novel formulations and delivery strategies to improve the immunogenicity of DNA-based vaccines have recently been evaluated, and have shown varying degrees of success. In the present study, we evaluated DNA-launched Venezuelan equine encephalitis replicons (Vrep) encoding a novel fusion of the mycobacterial antigens α-crystallin (Acr) and antigen 85B (Ag85B), termed Vrep-Acr/Ag85B, for their immunogenicity and protective efficacy in a murine model of pulmonary TB. Vrep-Acr/Ag85B generated antigen-specific CD4+ and CD8+ T cell responses that persisted for at least 10 wk post-immunization. Interestingly, parenterally administered Vrep-Acr/Ag85B also induced T cell responses in the lung tissues, the primary site of infection, and inhibited bacterial growth in both the lungs and spleens following aerosol challenge with Mtb. DNA-launched Vrep may, therefore, represent an effective approach to the development of gene-based vaccines against TB, particularly as components of heterologous prime-boost strategies or as BCG boosters.     

Immunodominance in virus-induced CD8(+) T-cell responses is dramatically modified by DNA immunization and is regulated by gamma interferon

The phenomenon whereby the host immune system responds to only a few of the many possible epitopes in a foreign protein is termed immunodominance. Immunodominance occurs not only during microbial infection but also following vaccination, and clarification of the underlying mechanism may permit the rational design of vaccines which can circumvent immunodominance, thereby inducing responses to all epitopes, dominant and subdominant. Here, we show that immunodominance affects DNA vaccines and that the effects can be avoided by the simple expedient of epitope separation. DNA vaccines encoding isolated dominant and subdominant epitopes induce equivalent responses, confirming a previous demonstration that coexpression of dominant and subdominant epitopes on the same antigen-presenting cell (APC) is central to immunodominance. We conclude that multiepitope DNA vaccines should comprise a cocktail of plasmids, each with its own epitope, to allow maximal epitope dispersal among APCs. In addition, we demonstrate that subdominant responses are actively suppressed by dominant CD8(+) T-cell responses and that gamma interferon (IFN-gamma) is required for this suppression. Furthermore, priming of CD8(+) T cells to a single dominant epitope results in strong suppression of responses to other normally dominant epitopes in immunocompetent mice, in effect rendering these epitopes subdominant; however, responses to these epitopes are increased 6- to 20-fold in mice lacking IFN-gamma. We suggest that, in agreement with our previous observations, IFN-gamma secretion by CD8(+) T cells is highly localized, and we propose that its immunosuppressive effect is focused on the APC with which the dominant CD8(+) T cell is in contact.     

The importance of the back-signal from T cells into antigen-presenting cells in determining susceptibility to parasites.

It has long been known that certain MHC class II genes can dominantly suppress immune responses and so increase susceptibility to parasite infections, but the mechanism has been unclear. Recent work has revealed one way in which this form of suppression may operate, through gating by MHC class II molecules of the back-signal from activated T cells into macrophages. The two known suppressive genes of the mouse are expressed in macrophages more extensively than are other class II genes. This is associated with suppression of IL-4 production resulting, we infer, from overproduction in the macrophages of IL-12, the counter-cytokine to IL-4. The lack of IL-4 may itself be immunosuppressive, even for Th2 responses, and excess IL-12 can overinduce the antiproliferative cytokine IFN-gamma. Although this mechanism requires further substantiation, we believe that it offers a reasonable answer to an old conundrum.