CD4+ T-cell induction and effector functions: a comparison of immunity against soluble antigens and viral infections

Computational Identification of Master Regulators Influencing Trypanotolerance in Cattle

African Animal Trypanosomiasis (AAT) is transmitted by the tsetse fly which carries pathogenic trypanosomes in its saliva, thus causing debilitating infection to livestock health. As the disease advances, a multistage progression process is observed based on the progressive clinical signs displayed in the host’s body. Investigation of genes expressed with regular monotonic patterns (known as Monotonically Expressed Genes (MEGs)) and of their master regulators can provide important clue for the understanding of the molecular mechanisms underlying the AAT disease. For this purpose, we analysed MEGs for three tissues (liver, spleen and lymph node) of two cattle breeds, namely trypanosusceptible Boran and trypanotolerant N’Dama. Our analysis revealed cattle breed-specific master regulators which are highly related to distinguish the genetic programs in both cattle breeds. Especially the master regulators MYC and DBP found in this study, seem to influence the immune responses strongly, thereby susceptibility and trypanotolerance of Boran and N’Dama respectively. Furthermore, our pathway analysis also bolsters the crucial roles of these master regulators. Taken together, our findings provide novel insights into breed-specific master regulators which orchestrate the regulatory cascades influencing the level of trypanotolerance in cattle breeds and thus could be promising drug targets for future therapeutic interventions.

Prevalence of Anti-Adeno-Associated Virus Immune Responses in International Cohorts of Healthy Donors

Preexisting immunity against adeno-associated virus (AAV) is a major challenge facing AAV gene therapy, resulting in the exclusion of patients from clinical trials. Accordingly, proper assessment of anti-AAV immunity is necessary for understanding clinical data and for product development. Previous studies on anti-AAV prevalence lack method standardization, rendering the assessment of prevalence difficult. Addressing this need, we used clinical assays that were validated according to guidelines for a comprehensive characterization of anti-AAV1, -AAV2, -AAV5, and -AAV8 immunity in large international cohorts of healthy donors and patients with hemophilia B. Here, we report a higher than expected average prevalence for anti-AAV8 (∼40%) and anti-AAV5 (∼30%) neutralizing antibodies (NAbs), which is supported by strongly correlating anti-AAV IgG antibody titers. A similar anti-AAV8 NAb prevalence was observed in hemophilia B patients. In addition, a high co-prevalence of NAbs against other serotypes makes switching to gene therapy using another serotype difficult. As anti-AAV T cell responses are believed to influence transduction, we characterized anti-AAV T cell responses using interleukin-2 (IL-2) and interferon-γ (IFN-γ) ELISpot assays, revealing a similar prevalence of IFN-γ responses (∼20%) against different serotypes that did not correlate with NAbs. These data, along with the long-term stability of NAbs, emphasize the need to develop strategies to circumvent anti-AAV immunity.

Psychological intervention based on psychoneuroimmunology improves clinical evolution, quality of life, and immunity of children with leukemia: A preliminary study

We conducted a non-randomized, open-label clinical trial to assess whether a psychoneuroimmunology-based intervention enhanced immunity in children with acute lymphoblastic leukemia undergoing chemotherapy. In total, 16 children (44% female) received psychoneuroimmunology-based intervention, whereas 12 (50% female) received health psychoeducation (controls). The primary outcome was immunity markers, being clinical conditions the secondary outcome. Psychoneuroimmunology-based intervention increased immune markers (CD8+ T, B, and natural killer cells, serum immunoglobulin A, and immunoglobulin M) and quality of life, whereas it shortens the duration of fever and use of antipyretics, antibiotics, analgesics, and respiratory therapy. Immunity markers correlated with clinical conditions. Thus, psychoneuroimmunology-based intervention could reduce hospital cost and increase patient well-being.

A Critical Role of IL-21-Induced BATF in Sustaining CD8-T-Cell-Mediated Chronic Viral Control

Control of chronic viral infections by CD8 T cells is critically dependent on CD4 help. In particular, helper-derived IL-21 plays a key role in sustaining the CD8 T cell response; however, the molecular pathways by which IL-21 sustains CD8 T cell immunity remain unclear. We demonstrate that IL-21 causes a phenotypic switch of transcription factor expression in CD8 T cells during chronic viral infection characterized by sustained BATF expression. Importantly, BATF expression during chronic infection is both required for optimal CD8 T cell persistence and anti-viral effector function and sufficient to rescue "unhelped" CD8 T cells. Mechanistically, BATF sustains the response by cooperating with IRF4, an antigen-induced transcription factor that is also critically required for CD8 T cell maintenance, to preserve Blimp-1 expression and thereby sustain CD8 T cell effector function. Collectively, these data suggest that CD4 T cells "help" the CD8 response during chronic infection via IL-21-induced BATF expression.

Shaping successful and unsuccessful CD8 T cell responses following infection

CD8 T cells play a vital role in the immunological protection against intracellular pathogens. Ideally, robust effector responses are induced, which eradicate the pathogen, and durable memory CD8 T cells are also established, which help confer protection against subsequent reinfection. The quality and magnitude of these responses is dictated by multiple factors, including their initial interactions with professional antigen-presenting cells, as well as the cytokine milieu and availability of CD4 T cell help. These factors set the transcriptional landscape of the responding T cells, which in turn influences their phenotypic and functional attributes as well as ultimate fate. Under certain conditions, such as during chronic infections, the development of these usually successful responses becomes subverted. Here we discuss advances in our understanding of the cellular and molecular determinants of T cell quality, and the formation of effector, memory, and exhausted CD8 T cells, during acute and chronic infections.

Seasonal influenza vaccine elicits heterosubtypic immunity against H5N1 that can be further boosted by H5N1 vaccination

Recent findings indicate that seasonal influenza vaccination or infection of healthy humans may contribute to heterosubtypic immunity against new influenza A subtypes, such as H5N1. Here, we investigated whether seasonal influenza vaccination in a mouse model could induce any immunity against the H5N1 subtype. It could be demonstrated that, largely due to the H1N1 component strain A/NewCaledonia/20/99, parenteral immunization of mice with a trivalent seasonal influenza vaccine elicited heterosubtype H5-reactive antibodies able to confer partial protection against H5N1 influenza virus infection. Furthermore, the trivalent seasonal influenza vaccine was found to be compatible with a whole virus H5N1 vaccine in a heterologous prime-boost immunization regimen, achieving superior efficacy compared to a single immunization with an equivalent low-dose of the H5N1 vaccine.

Cross-subtype immunity against avian influenza in persons recently vaccinated for influenza

Seasonal influenza vaccination may induce heterosubtypic immunity against avian influenza virus (H5N1).

Avian influenza virus (H5N1) can be transmitted to humans, resulting in a severe or fatal disease. The aim of this study was to evaluate the immune cross-reactivity between human and avian influenza (H5N1) strains in healthy donors vaccinated for seasonal influenza A (H1N1)/(H3N2). A small frequency of CD4 T cells specific for subtype H5N1 was detected in several persons at baseline, and seasonal vaccine administration enhanced the frequency of such reactive CD4 T cells. We also observed that seasonal vaccination is able to raise neutralizing immunity against influenza (H5N1) in a large number of donors. No correlation between influenza-specific CD4 T cells and humoral responses was observed. N1 may possibly be a target for both cellular and humoral cross-type immunity, but additional experiments are needed to clarify this point. These findings highlight the possibility of boosting cross-type cellular and humoral immunity against highly pathogenic avian influenza A virus subtype H5N1 by seasonal influenza vaccination.

Control of virus-specific CD8+ T-cell exhaustion and immune-mediated pathology by E3 ubiquitin ligase Cbl-b during chronic viral infection

A characteristic feature in the immune response to many persistent viral infections is the dysfunction or deletion of antigen-specific T cells (exhaustion). This down-regulation of virus-specific T-cell response represents a critical control mechanism that exists within T-cell activation pathways to prevent lethal disease by inappropriate responses against disseminating virus infections. However, the molecular mechanisms by which the immune system determines whether to mount a full response to such infections remain largely unexplored. Here, we have established that in the murine lymphocytic choriomeningitis virus (LCMV) model, induction of the T-cell receptor signaling inhibitor molecule E3 ligase Cbl-b is critically involved in this decision. In particular, our data revealed that Cbl-b controls the program responsible for T-cell tolerance (exhaustion) induction during a chronic viral infection. Thus, Cbl-b(-/-) mice infected with a low dose of LCMV Docile mount a strong CD8(+) T-cell response that rapidly clears the infection, and the animals remain healthy; in contrast, down-regulation of the epitope-specific CD8(+) T-cell population in persistently infected Cbl-b(-/-) mice, compared to that in chronically infected B6 mice, was significantly delayed, and this was associated with increased morbidity and eventual death in nearly 20% of the animals. Interestingly, infection of Cbl-b(-/-) mice with a moderate virus dose resulted in rapid death with 100% mortality by 7 to 8 days after infection, caused by a dysregulated antiviral T-cell response, whereas the infected B6 mice survived and remained healthy. In conclusion, our results suggest that Cbl-b is critically involved in T-cell exhaustion and prevention of lethal disease.

CD8 T cell memory development: CD4 T cell help is appreciated

An important goal of vaccination strategies is to elicit long term, effective immunity. Therefore it is imperative to define the parameters that regulate the development and preservation of the numbers and functional quality of cells that confer this property to the host. CD8 T cells are a key component of the host adaptive immune response that helps eradicate invading viruses and other cell-associated pathogens. Once the primary infection is controlled, the CD8 T cells transition from being effector cells into memory cells that act as sentinels of the immune system capable of rapidly purging the host of recurrent infections by the same pathogen. The factors that regulate and orchestrate this transition from effector CD8 T cells into functionally robust memory CD8 T cells are poorly understood. In recent years it has been determined that CD4 T cells play a vital role in the survival and functional responsiveness of memory CD8 T cells. However, the mechanism(s) of this interaction are still unclear.

Th2 differentiation in distinct lymph nodes influences the site of mucosal Th2 immune-inflammatory responses

Allergic individuals rarely present with concurrent multiple-organ disease but, rather, with manifestations that privilege a specific site such as the lung, skin, or gastrointestinal tract. Whether the site of allergic sensitization influences the localization of Th2 immune-inflammatory responses and, ultimately, the organ-specific expression of disease, remains to be determined. In this study, we investigated whether both the site of initial Ag exposure and concomitant Th2 differentiation in specific lymph nodes (LNs) privileges Th2 memory responses to mucosal and nonmucosal sites, and whether this restriction is associated with a differential expression in tissue-specific homing molecules. In mice exposed to Ag (OVA) via the peritoneum, lung, or skin, we examined several local and distal LNs to determine the site of Ag-specific proliferation and Th2 differentiation. Whereas respiratory and cutaneous Ag exposure led to Ag-specific proliferation and Th2 differentiation exclusively in lung- and skin-draining LNs, respectively, Ag delivery to the peritoneum evoked responses in gut-associated, as well as distal thoracic, LNs. Importantly, only mice that underwent Th2 differentiation in thoracic- or gut-associated LNs mounted Th2 immune-inflammatory responses upon respiratory or gastric Ag challenge, respectively, whereas cutaneous Th2 recall responses were evoked irrespective of the site of initial sensitization. In addition, we observed the differential expression of gut homing molecules (CCR9, alpha(4), beta(7)) in gut-associated LNs and, unexpectedly, a universal induction of skin-related homing molecules (CCR4, CCR10) in all LNs. These data suggest that the site of initial Th2 differentiation and differential homing molecule expression restricts Th2 immune-inflammatory responses to mucosal, but not cutaneous, tissues.

Why can't the immune system control HIV-1? Defining HIV-1-specific CD4+ T cell immunity in order to develop strategies to enhance viral immunity

Globally, at least 60 million people have been infected with the human immunodeficiency virus type 1 (HIV-1), the majority of whom will develop the acquired immunodeficiency syndrome (AIDS) leading to tremendous morbidity and the mortality. Understanding the immunopathogenesis of AIDS and the immune correlates of viral protection are necessary to develop effective vaccines and immunotherapies. A major focus of our laboratory has been to understand the CD4+ T cell immune response directed against HIV- 1, and to determine mechanisms of T cell dysfunction that lead to viral escape. In addition, we are interested in evaluating the TNF-TNFR family members as potential molecular adjuvants that could be incorporated into vaccines which could be used to further boost T cell immunogenicity in healthy or HIV-1-infected individuals, as many of these molecules have been shown to replace the functions of CD4+ T cell help.

Functional specialization of memory Th cells revealed by expression of integrin CD49b

Infection or immunization induces heterogeneous memory T cell subsets, but their origin and protective value against infection are unclear. In this study, we report the functional characterization of two memory Th subsets, defined by expression of integrin CD49b. Stable CD49b expression is induced in up to one-half of all memory Th cells. More importantly, the CD49b- and CD49b+ subsets display distinct helper activities, typified by the production of IL-10 and TNF-alpha, respectively. Although the inflammatory properties of the CD49b+ subset are protective against intracellular bacterial infection, they are associated with immunopathology in acute viral infection. Modulation of the CD49b-defined memory Th subsets may provide infection type-specific interventions, where either enhancement of the inflammatory response or reduction of immunopathology is essential.

Perforin and Fas cytolytic pathways coordinately shape the selection and diversity of CD8+-T-cell escape variants of influenza virus

Antigenic variation is a viral strategy exploited to promote survival in the face of the host immune response and represents a major challenge for efficient vaccine development. Influenza viruses are pathogens with high transmissibility and mutation rates, enabling viral escape from immunity induced by prior infection or vaccination. Intense selection from neutralizing antibody drives antigenic changes in the surface glycoproteins, resulting in emergence of new strains able to reinfect hosts immune to previously circulating viruses. CD8+ cytotoxic T cells (CTLs) also provide protective immunity from influenza virus infection and may contribute to the antigenic evolution of influenza viruses. Utilizing mice transgenic for an influenza virus NP366-374 peptide-specific T-cell receptor, we demonstrated that the respiratory tract is a suitable site for generation of escape variants of influenza virus selected by CTL in vivo. In this report the contributions of the perforin and Fas pathways utilized by influenza virus-specific CTLs in viral clearance and selection of CTL escape variants have been evaluated. While transgenic CTLs deficient in either perforin- or Fas-mediated pathways are efficient in initial pulmonary viral control, variant virus emergence was observed in all the mice studied, although the spectrum of viral CTL escape variants selected varied profoundly. Thus, a less-restricted repertoire of escape variants was observed in mice with an intact perforin cytotoxic pathway compared with a limited variant diversity in perforin pathway-deficient mice, although maximal variant diversity was observed in mice having both Fas and perforin pathways intact. We conclude that selection of viral CTL escape variants reflects coordinate action between the tightly controlled perforin/granzyme pathway and the more promiscuous Fas/FasL pathway.

Enhancement of antigen acquisition by dendritic cells and MHC class II-restricted epitope presentation to CD4+ T cells using VP22 DNA vaccine vectors that promote intercellular spreading following initial transfection

Induction of immune responses against microbial antigens using DNA is an attractive strategy to mimic the immunity induced by live vaccines. Although DNA vaccines are efficacious in murine models, the requirement for multiple immunizations using high doses in outbred animals and humans has hindered deployment. This requirement is, in part, a result of poor vaccine spreading and suboptimal DC transfection efficiency. Incorporation of a signal that directs intercellular spreading of a DNA-encoded antigen is proposed to mimic live vaccine spreading and increase dendritic cell (DC) presentation. Bovine herpes virus 1 tegument protein, BVP22, is capable of trafficking to surrounding cells. To test the hypothesis that BVP22 enhances spreading and antigen presentation to CD4+ T cells, a DNA construct containing BVP22, fused in-frame to a sequence encoding a T cell epitope of Anaplasma marginale, was generated. A construct with reversed BVP22 sequence served as a negative control. Immunocytometric analysis of transfected primary keratinocytes, human embryonic kidney 293, COS-7, and Chinese hamster ovary cells showed that BVP22 enhanced intercellular spreading by > or = 150-fold. Flow cytometric analysis of antigen-presenting cells (APCs) positively selected from cocultures of transfected cells and APCs showed that 5% of test APCs were antigen-positive, compared with 0.6% of control APCs. Antigen-specific CD4+ T cell proliferation demonstrated that BVP22 enhanced DC antigen presentation by > or = 20-fold. This first report of the ability of BVP22 to increase DNA-encoded antigen acquisition by DCs and macrophages, with subsequent enhancement of major histocompatibility complex class II-restricted CD4+ T cell responses, supports incorporating a spreading motif in a DNA vaccine to target CD4+ T cell-dependent immunity in outbred animals.

Mucosal immunization of CD4+ T cell-deficient mice with an inactivated virus induces IgG and IgA responses in serum and mucosal secretions

Immunoglobulin (Ig) class switching can occur in the absence of alphabeta+ or gammadelta+ T cells when mice are infected with certain live viruses, although CD4 T helper cells are believed to be essential for induction of a high-affinity antibody response and for efficient isotype switching from IgM to IgG and IgA production. However, little information is available about the immune responses after mucosal immunization of CD4+ T cell-deficient mice with inactivated virus. In this study, we show that intranasal immunization with formalin-inactivated influenza A/PR8/34 virus induces IgG and IgA responses in serum and IgA responses in mucosal secretions in CD4+ T cell-deficient mice. All four subclasses of IgG were produced. IgG1/IgG2a ratios were found to be from 1 to 1.75, indicating that both Th1 and Th2 immune responses are induced by the inactivated influenza virus. The sera and mucosal secretions were found to have neutralizing activity against influenza virus in vitro. In addition, the mucosally immunized CD4+ T cell-deficient mice were protected completely from challenge with a lethal dose of live, pathogenic influenza virus. To our knowledge, this is the first demonstration that mucosal immunization with an inactivated virus induces immune responses in serum and mucosal secretions in CD4+ T cell-deficient mice.

Anatomical Heterogeneity of Memory CD4+ T Cells Due to Reversible Adaptation to the Microenvironment

The memory T cell pool is characterized by a substantial degree of heterogeneity in phenotype and function as well as anatomical distribution, but the underlying mechanisms remain unclear. In this study we confirm that the memory CD4(+) T cell pool in wild-type and TCR-transgenic mice consists of heterogeneous subsets, as defined by surface marker expression or cytokine production. Extralymphoid sites contain significant numbers of memory CD4(+) T cells, which are phenotypically and functionally distinct from their lymphoid counterparts. However, we show in this study that the phenotype of lymphoid and extralymphoid memory T cells is not stable. Instead, the unique properties of extralymphoid memory T cells are acquired upon migration into extralymphoid sites and are lost when memory T cells migrate back into lymphoid organs. Thus, at least some of the extralymphoid properties may represent a transient activation state that can be adopted by T cells belonging to a single memory T cell pool. Furthermore, such intermittent activation during or after migration into extralymphoid sites could provide an important signal, promoting the survival and functional competence of memory T cells in the absence of Ag.

Th cell-independent immune responses to chimeric hemagglutinin/simian human immunodeficiency virus-like particles vaccine

CD4(+) Th cells are believed to be essential for the induction of humoral and cellular immune responses. In this study we tested the effect and possible mechanisms of the major antigenic component in influenza, hemagglutinin (HA), in helping HIV Env to induce immune responses in CD4(+) T cell knockout (CD4 KO) mice. Simian HIV virus-like particles (SHIV VLPs) or phenotypically mixed chimeric influenza HA/SHIV VLPs were used as immunogens to immunize CD4 KO mice either i.p. or intranasally (i.n.). We found that chimeric HA/SHIV VLPs significantly induced a greater IgG Ab response in both i.p. and i.n. immunized mice and a greater IgA Ab response in mucosal washes in i.n. immunized mice compared with SHIV VLPs. Importantly, chimeric HA/SHIV VLPs induced approximately 3-fold higher neutralizing Ab titers against HIV 89.6 than SHIV VLPs in the absence of CD4(+) T cell help. There was also approximately 40% more specific lysis of the HIV Env-expressing target cells in chimeric HA/SHIV VLP-immunized than in SHIV VLP-immunized CD4 KO mouse splenocytes. Moreover, we have found that chimeric HA/SHIV VLPs could efficiently bind and activate dendritic cells and stimulate the activated dendritic cells to secret TNF-alpha and IFN-gamma. Therefore, chimeric HA/SHIV VLPs could efficiently prime and activate APCs, which could, in turn, induce immune responses in a CD4(+) T cell-independent manner. This study suggests a novel adjuvant role of influenza HA as well as a new strategy to develop more effective therapeutic vaccines for AIDS patients with low CD4(+) T cell counts.

Pathophysiology of airflow limitation in chronic obstructive pulmonary disease

The airflow limitation that defines chronic obstructive pulmonary disease (COPD) is the result of a prolonged time constant for lung emptying, caused by increased resistance of the small conducting airways and increased compliance of the lung as a result of emphysematous destruction. These lesions are associated with a chronic innate and adaptive inflammatory immune response of the host to a lifetime exposure to inhaled toxic gases and particles. Processes contributing to obstruction in the small conducting airways include disruption of the epithelial barrier, interference with mucociliary clearance apparatus that results in accumulation of inflammatory mucous exudates in the small airway lumen, infiltration of the airway walls by inflammatory cells, and deposition of connective tissue in the airway wall. This remodelling and repair thickens the airway walls, reduces lumen calibre, and restricts the normal increase in calibre produced by lung inflation. Emphysematous lung destruction is associated with an infiltration of the same type of inflammatory cells found in the airways. The centrilobular pattern of emphysematous destruction is most closely associated with cigarette smoking, and although it is initially focused on respiratory bronchioles, separate lesions coalesce to destroy large volumes of lung tissue. The panacinar pattern of emphysema is characterised by a more even involvement of the acinus and is associated with alpha1 antitrypsin deficiency. The technology needed to diagnose and quantitate the individual small airway and emphysema phenotypes present in people with COPD is being developed, and should prove helpful in the assessment of therapeutic interventions designed to modify the progress of either phenotype.

Maintenance, loss, and resurgence of T cell responses during acute, protracted, and chronic viral infections

The acute phase of many viral infections is associated with the induction of a pronounced CD8 T cell response which plays a principle role in clearing the infection. By contrast, certain infections are not as readily controlled. In this study, we have used the well-defined system of lymphocytic choriomeningitis virus (LCMV) infection of mice to determine quantitative and qualitative changes in virus-specific CD8 T cell responses that rapidly resolve acute infections, more slowly control protracted infections, or fail to clear chronic infections. Acute LCMV infection elicits potent, functional, multi-epitope-specific CD8 T cell responses. Virus-specific CD8 T cells also expand, albeit to a lesser extent, during protracted LCMV infection. Under these conditions, there is a progressive diminution in the capacity to produce IL-2, TNF-alpha, and IFN-gamma. Changes in cytotoxic activities are also detectable but differ depending upon the specificity of the responding cells. As the infection is slowly resolved, a resurgence of cytokine production by virus-specific CD8 T cells is observed. CD4-deficient mice cannot control infection with certain strains of LCMV, but do mount multi-epitope-specific CD8 T cell responses that also lose effector capabilities; however, they are not maintained indefinitely in an unresponsive state as these cells become deleted over time. Overall, our findings suggest that constant high viral loads result in the progressive diminution of T cell effector functions and subsequent physical loss of the responding cells, whereas if the viral load is brought under control a partial restoration of CD8 T cell functions can occur.

Identification of a protective CD4+ T-cell epitope in p15gag of Friend murine leukemia virus and role of the MA protein targeting the plasma membrane in immunogenicity

Recent studies have demonstrated an essential role of Gag-specific CD4+ T-cell responses for viral control in individuals infected with human immunodeficiency virus type 1. However, little is known about epitope specificities and functional roles of the Gag-specific helper T-cell responses in terms of vaccine-induced protection against a pathogenic retroviral challenge. We have previously demonstrated that immunization with Friend murine leukemia virus (F-MuLV) Gag proteins protects mice against the fatal Friend retrovirus (FV) infection. We report here the structure of a protective T helper cell (Th) epitope, (I)VTWEAIAVDPPP, identified in the p15 (MA) region of F-MuLV Gag. In mice immunized with the Th epitope-harboring peptide or a vaccinia virus-expressed native full-length MA protein, FV-induced early splenomegaly regressed rapidly. In these mice, FV-infected cells were eliminated within 4 weeks and the production of virus-neutralizing antibodies was induced rapidly after FV challenge, resulting in strong protection against the virus infection. Interestingly, mice immunized with the whole MA mounted strong CD4+ T-cell responses to the identified Th epitope, whereas mice immunized with mutant MA proteins that were not bound to the plasma membrane failed to mount efficient CD4+ T-cell responses, despite the presence of the Th epitope. These mutant MA proteins also failed to induce strong protection against FV challenge. These data indicate the importance of the properly processible MA molecule for CD4+ T-cell priming and for the resultant induction of an effective immune response against retrovirus infections.

DNA fusion gene vaccines against cancer: from the laboratory to the clinic

Vaccination against target antigens expressed by cancer cells has now become a realistic goal. DNA vaccines provide a direct link between identification of genetic markers in tumors and vaccine formulation. Simplicity of manufacture facilitates construction of vaccines against disease subsets or even for individual patients. To engage an immune system that exists to fight pathogens, we have developed fusion gene vaccines encoding tumor antigens fused to pathogen-derived sequences. This strategy activates high levels of T-cell help, the key to induction and maintenance of effective immunity. We have dissected the immunogenic tetanus toxin to obtain specific sequences able to activate antibody, CD4+, or CD8+ T cells to attack selected fused tumor antigens. Principles established in preclinical models are now being tested in patients. So far, objective immune responses against idiotypic antigen of neoplastic B cells have been observed in patients with B-cell malignancies and in normal transplant donors. These responses provide a platform for testing physical methods to improve DNA delivery and strategies to boost responses. For cancer, demands are high, because vaccines have to activate powerful immunity against weak antigens, often in a setting of immune damage or tolerance. Vaccination strategies against cancer and against microbes are sharing knowledge and technology for mutual benefit.

Differential tissue-specific regulation of antiviral CD8+ T-cell immune responses during chronic viral infection

The hallmarks of the immune response to viral infections are the expansion of antigen-specific CD8(+) cytotoxic T lymphocytes (CTLs) after they encounter antigen-presenting cells in the lymphoid tissues and their subsequent redistribution to nonlymphoid tissues to deal with the pathogen. Control mechanisms exist within CTL activation pathways to prevent inappropriate CTL responses against disseminating infections with a broad distribution of pathogen in host tissues. This is demonstrated during overwhelming infection with the noncytolytic murine lymphocytic choriomeningitis virus, in which clonal exhaustion (anergy and/or deletion) of CTLs prevents immune-mediated pathology but allows persistence of the virus. The mechanism by which the immune system determines whether or not to mount a full response to such infections is unknown. Here we present data showing that the initial encounter of specific CTLs with infected cells in lymphoid tissues is critical for this decision. Whether the course of the viral infection is acute or persistent for life primarily depends on the degree and kinetics of CTL exhaustion in infected lymphoid tissues. Virus-driven CTL expansion in lymphoid tissues resulted in the migration of large quantities of CTLs to nonlymphoid tissues, where they persisted at stable levels. Surprisingly, although virus-specific CTLs were rapidly clonally exhausted in lymphoid tissues under conditions of chronic infection, a substantial number of them migrated to nonlymphoid tissues, where they retained an effector phenotype for a long time. However, these cells were unable to control the infection and progressively lost their antiviral capacities (cytotoxicity and cytokine secretion) in a hierarchical manner before their eventual physical elimination. These results illustrate the differential tissue-specific regulation of antiviral T-cell responses during chronic infections and may help us to understand the dynamic relationship between antigen and T-cell populations in many persistent infections in humans.

Ablation of CD8 and CD4 T cell responses by high viral loads

To evaluate the impact of sustained viral loads on anti-viral T cell responses we compared responses that cleared acute lymphocytic choriomeningitis virus infection with those that were elicited but could not resolve chronic infection. During acute infection, as replicating virus was cleared, CD8 T cell responses were down-regulated, and a pool of resting memory cells developed. In chronically infected hosts, the failure to control the infection was associated with pronounced and prolonged activation of virus-specific CD8 T cells. Nevertheless, there was a progressive diminution of their effector activities as their capacity to produce first IL-2, then TNF-alpha, and finally IFN-gamma was lost. Chronic lymphocytic choriomeningitis virus infection was also associated with differential contraction of certain CD8 T cell responses, resulting in altered immunodominance. However, this altered immunodominance was not due to selective expansion of T cells expressing particular TCR Vbeta segments during chronic infection. High viral loads were not only associated with the ablation of CD8 T cell responses, but also with impaired production of IL-2 by virus-specific CD4 T cells. Taken together, our data show that sustained exposure to high viral loads results in the progressive functional inactivation of virus-specific T cell responses, which may further promote virus persistence.

Characterization of human immunodeficiency virus type 1 (HIV-1) Gag- and Gag peptide-specific CD4(+) T-cell clones from an HIV-1-seronegative donor following in vitro immunization

Substantial evidence argues that human immunodeficiency virus type 1 (HIV-1)-specific CD4(+) T cells play an important role in the control of HIV-1 replication in infected individuals. Moreover, it is increasingly clear that an HIV vaccine should elicit potent cytotoxic lymphocyte and antibody responses that will likely require an efficient CD4(+) T-cell response. Therefore, understanding and characterizing HIV-specific CD4(+) T-cell responses is an important aim. Here we describe the generation of HIV-1 Gag- and Gag peptide-specific CD4(+) T-cell clones from an HIV-1-seronegative donor by in vitro immunization with HIV-1 Gag peptides. The Gag peptides were able to induce a strong CD4(+) T-cell immune response in peripheral blood mononuclear cells from the HIV-1-seronegative donor. Six Gag peptide-specific CD4(+) T-cell clones were isolated and their epitopes were mapped. The region of p24 between amino acids 201 and 300 of Gag was defined as the immunodominant region of Gag. A new T helper epitope in the p6 protein of Gag was identified. Two clones were shown to recognize Gag peptides and processed Gag protein, while the other four clones reacted only to Gag peptides under the experimental conditions used. Functional analysis of the clones indicated that both Th1 and Th2 types of CD4(+) T cells were obtained. One clone showed direct antigen-specific cytotoxic activity. These clones represent a valuable tool for understanding the cellular immune response to HIV-1, and the study provides new insights into the HIV-1-specific CD4(+) T-cell response and the induction of an anti-Gag and -Gag peptide cellular primary immune response in vitro.

HTLV-1-mediated immunopathological CNS disease

HAM/TSP is a chronic inflammatory disease of the spinal cord. It is rather rare in HTLV-1-infected individuals. Immunogenetic factors of the HLA complex have been identified that support or prevent the development of the disease. In HAM/TSP patients a characteristic constellation of high proviral loads and increased cellular and humoral immune responses have been established. Immune dysfunction in HAM/TSP patients might be partly explained by HTLV-1 tax p40 transactivation of cellular genes in infected CD4+ T lymphocytes. The oligoclonal expansion of infected T lymphocytes, the variation of tax p40 within HTLV-1 carriers, and the regulation of proviral gene expression are possible determinants for disease development and need to be clarified in future studies.

Impairment of immunological memory in the absence of MHC despite survival of memory T cells

The mechanisms by which immunological memory is maintained after infection or vaccination are still a matter of debate. Long-term survival of memory T cells does not require major histocompatibility complex (MHC) contact. We show here that compared with memory CD4+ T cells that maintain contact with MHC class II, memory CD4+ T cells deprived of MHC class II contact show distinct functional defects upon antigen re-encounter. Thus, in contrast to their survival, maintenance of the typical quality of memory T cells crucially depends on MHC-derived signals.

CD85/LIR-1/ILT2 and CD152 (cytotoxic T lymphocyte antigen 4) inhibitory molecules down-regulate the cytolytic activity of human CD4+ T-cell clones specific for Mycobacterium tuberculosis

Antigen-specific cytolytic CD4+ T lymphocytes control Mycobacterium tuberculosis infection by secreting cytokines and by killing macrophages that have phagocytosed the pathogen. However, lysis of the latter cells promotes microbial dissemination, and other macrophages engulf the released bacteria. Subsequently, CD4+ T-cell-mediated killing of macrophages goes on, and this persistent process may hamper control of infection, unless regulatory mechanisms maintain a subtle balance between lysis of macrophages by cytolytic CD4+ cells and activation of cytolytic CD4+ cells by infected macrophages. We asked whether inhibitory molecules expressed by CD4+ cytolytic T lymphocytes could play a role in such a balance. To this end, human CD4+ T-cell clones specific for M. tuberculosis were produced that displayed an autologous major histocompatibility complex class II-restricted lytic ability against purified protein derivative (PPD)-pulsed antigen-presenting cells. All T-cell clones expressed CD152 (cytotoxic T-lymphocyte antigen 4 [CTLA-4]) and CD85/leukocyte immunoglobulin-like receptor 1 (LIR-1)/immunoglobulin-like transcript 2 (ILT2) inhibitory receptors, but not CD94 and the killer inhibitory receptor (or killer immunoglobulin-like receptor [KIR]) p58.2. CD3-mediated activation of the clones was inhibited in a redirected killing assay in which CD152 and CD85/LIR-1/ILT2 were cross-linked. Specific antigen-mediated proliferation of the clones was also sharply reduced when CD152 and CD85/LIR-1/ILT2 were cross-linked by specific monoclonal antibody (MAb) followed by goat anti-mouse antiserum. In contrast, blockade of the receptors by specific MAb only increased their proliferation. Production of interleukin 2 (IL-2) and gamma interferon (IFN-gamma) by the T-cell clones was also strongly reduced when CD152 and CD85/LIR-1/ILT2 were cross-linked. The lytic activity of the T-cell clones against PPD-pulsed autologous monocytes or Epstein-Barr virus-activated B cells was increased by blockade and decreased by cross-linking of the receptors. These results indicate that CD152 and CD85/LIR-1/ILT2 play a role in the regulation of the antigen-specific activity of CD4+ cytolytic T lymphocytes against PPD-presenting cells.

Different levels of T-cell receptor triggering induce distinct functions in hepatitis B and hepatitis C virus-specific human CD4(+) T-cell clones

CD4(+) T cells play a major role in the host defense against viruses and intracellular microbes. During the natural course of such an infection, specific CD4(+) T cells are exposed to a wide range of antigen concentrations depending on the body compartment and the stage of disease. While epitope variants trigger only subsets of T-cell effector functions, the response of virus-specific CD4(+) T cells to various concentrations of the wild-type antigen has not been systematically studied. We stimulated hepatitis B virus core- and hepatitis C virus NS3-specific CD4(+) T-cell clones which had been isolated from patients with acute hepatitis during viral clearance with a wide range of specific antigen concentrations and determined the phenotypic changes and the induction of T-cell effector functions in relation to T-cell receptor internalization. A low antigen concentration induced the expression of T-cell activation markers and adhesion molecules in CD4(+) T-cell clones in the absence of cytokine secretion and proliferation. The expression of CD25, HLA-DR, CD69, and intercellular cell adhesion molecule 1 increased as soon as T-cell receptor internalization became detectable. A 30- to 100-fold-higher antigen concentration, corresponding to the internalization of 20 to 30% of T-cell receptor molecules, however, was required for the induction of proliferation as well as for gamma interferon and interleukin-4 secretion. These data indicate that virus-specific CD4(+) T cells can respond to specific antigen in a graded manner depending on the antigen concentration, which may have implications for a coordinate regulation of specific CD4(+) T-cell responses.

Analysis of the relationship between viral infection and autoimmune disease

The clinical association between viral infection and onset or exacerbation of autoimmune disorders remains poorly understood. Here, we examine the relative roles of molecular mimicry and nonspecific inflammatory stimuli in progression from infection to autoimmune disease. Murine herpes virus 1 (HSV-1 KOS) infection triggers T cell-dependent autoimmune reactions to corneal tissue. We generated an HSV-1 KOS point mutant containing a single amino acid exchange within the putative mimicry epitope as well as mice expressing a TCR transgene specific for the self-peptide mimic to allow dissection of two pathogenic mechanisms in disease induction. These experiments indicate that viral mimicry is essential for disease induction after low-level viral infection of animals containing limited numbers of autoreactive T cells, while innate immune mechanisms become sufficient to provoke disease in animals containing relatively high numbers of autoreactive T cells.

Neutralizing antiviral antibody responses

Neutralizing antibodies are evolutionarily important effectors of immunity against viruses. Their evaluation has revealed a number of basic insights into specificity, rules of reactivity (tolerance), and memory—namely, (1) Specificity of neutralizing antibodies is defined by their capacity to distinguish between virus serotypes; (2) B cell reactivity is determined by antigen structure, concentration, and time of availability in secondary lymphoid organs; and (3) B cell memory is provided by elevated protective antibody titers in serum that are depending on antigen stimulation. These perhaps slightly overstated rules are simple, correlate with in vivo evidence as well as clinical observations, and appear to largely demystify many speculations about antibodies and B cell physiology. The chapter also considers successful vaccines and compares them with those infectious diseases where efficient protective vaccines are lacking, it is striking to note that all successful vaccines induce high levels of neutralizing antibodies (nAbs) that are both necessary and sufficient to protect the host from disease. Successful vaccination against infectious diseases such as tuberculosis, leprosy, or HIV would require induction of additional long-lasting T cell responses to control infection.

The role of CD4+ T cell help and CD40 ligand in the in vitro expansion of HIV-1-specific memory cytotoxic CD8+ T cell responses

CD4(+) T cells have been shown to play a critical role in the maintenance of an effective anti-viral CD8(+) CTL response in murine models. Recent studies have demonstrated that CD4(+) T cells provide help to CTLs through ligation of the CD40 receptor on dendritic cells. The role of CD4(+) T cell help in the expansion of virus-specific CD8(+) memory T cell responses was examined in normal volunteers recently vaccinated to influenza and in HIV-1 infected individuals. In recently vaccinated normal volunteers, CD4(+) T cell help was required for optimal in vitro expansion of influenza-specific CTL responses. Also, CD40 ligand trimer (CD40LT) enhanced CTL responses and was able to completely substitute for CD4(+) T cell help in PBMCs from normal volunteers. In HIV-1 infection, CD4(+) T cell help was required for optimal expansion of HIV-1-specific memory CTL in vitro in 9 of 10 patients. CD40LT could enhance CTL in the absence of CD4(+) T cell help in the majority of patients; however, the degree of enhancement of CTL responses was variable such that, in some patients, CD40LT could not completely substitute for CD4(+) T cell help. In those HIV-1-infected patients who demonstrated poor responses to CD40LT, a dysfunction in circulating CD8(+) memory T cells was demonstrated, which was reversed by the addition of cytokines including IL-2. Finally, it was demonstrated that IL-15 produced by CD40LT-stimulated dendritic cells may be an additional mechanism by which CD40LT induces the expansion of memory CTL in CD4(+) T cell-depleted conditions, where IL-2 is lacking.

Targeted inactivation of the tetraspanin CD37 impairs T-cell-dependent B-cell response under suboptimal costimulatory conditions

CD37 is a membrane protein of the tetraspanin superfamily, which includes CD9, CD53, CD63, CD81, and CD82. Many of these molecules are expressed on leukocytes and have been implicated in signal transduction, cell-cell interactions, and cellular activation and development. We generated and analyzed mice deficient for CD37. Despite the high expression of CD37 on cells of the immune system, no changes in development and cellular composition of lymphoid organs were observed in mice lacking CD37. Analyses of humoral immune responses revealed a reduced level of immunoglobulin G1 (IgG1) in the sera of nonimmunized mice and an alteration of responses to T-cell-dependent antigens. Antibody responses to model antigen administered in the absence of adjuvant and to viral infections were generally poor in CD37-deficient mice. These poor antibody responses could be overcome by the immunization of antigen together with adjuvant. These results suggest a role for CD37 in T-cell-B-cell interactions which manifests itself under suboptimal costimulatory conditions.

Induction of CD4(+) T-cell-independent immunoglobulin responses by inactivated influenza virus

Through cognate interaction between antigen-specific B-cell and CD4(+) alphabeta T cells, the CD4(+) alphabeta T cells secrete cytokines that initiate immunoglobulin (Ig) class switching from IgM to IgG. In this study, we show that formalin-inactivated influenza PR8 virus induces virus-specific IgM and IgG responses in the absence of CD4(+) T cells and that all four subclasses of IgG are produced. The immunized CD4-deficient mice were also found to be completely protected against lethal infection with live, pathogenic influenza virus. The ability of CD4(+) T-cell-deficient mice to generate these IgG responses was not found to be impaired when these mice were depleted of CD8(+) T cells with an anti-CD8 monoclonal antibody. In contrast, alphabeta T-cell-deficient mice (TCRbeta(-/-)) were not found to produce significant amounts of IgG upon immunization with formalin-inactivated PR8 virus. These results suggest that CD4(-) CD8(-) double-negative alphabeta T cells are playing a role in regulating Ig class switching in the absence of CD4(+) T cells.