Role of CD4 T cell help and costimulation in CD8 T cell responses during Listeria monocytogenes infection

Interferon gamma licensing of human dendritic cells in T-helper-independent CD8+ alloimmunity

The high frequency of allogeneic reactive CD8(+) T cells in human and their resistance to immunosuppression might be one of the reasons why successful tolerance-inducing strategies in rodents have failed in primates. Studies on the requirement for T-helper cells in priming CD8(+) T-cell responses have led to disparate findings. Recent studies have reported CD8(+)-mediated allograft rejection independently of T-helper cells; however, the mechanisms that govern the activation of these T cells are far from being elucidated. In this study, we demonstrated that lipopolysaccharide-treated dendritic cells (DCs) were able to induce proliferation and cytotoxic activity of allogeneic CD8(+) T cells independently of CD4(+) T cells, while adding mycophenolic acid (MPA) to LPS abolished this capacity and resulted in anergic CD8(+) T cells that secreted high levels of interleukin-4 (IL-4), IL-5, IL-10, and transforming growth factor-β. Interestingly, we demonstrated that MPA inhibited the LPS-induced synthesis of tumor necrosis factor-α, IL-12, and interferon-γ (IFN-γ) in DCs. Importantly, we found that adding exogenous IFN-γ to MPA restored both the synthesis of cytokines and the ability to activate CD8(+) T cells. However, adding IL-12 or tumor necrosis factor-α had no effect. These results suggest that IFN-γ has an important role in licensing DCs to prime CD4-independent CD8 allogeneic T cells via an autocrine loop.

4-1BB as a therapeutic target for human disease

4-1BB (CD137) is being thought of as an attractive target for immunotherapy of many human immune diseases based on encouraging results with 4-1BB agonistic antibody treatment in mouse models of cancer, autoimmune disease, asthma and additionally as a means to improve vaccination. In this review, we will summarize the results of basic research on 4-1BB and 4-1BB immunotherapy of disease and provide some potential mechanistic insights into the many stimulatory and regulatory functions of 4-1BB.

Once a killer, always a killer: from cytotoxic T cell to memory cell

The control of the differentiation pathways followed by responding CD8(+) T cells to produce protective memory cells has been intensely studied. Recent developments have identified heterogeneity at the effector cytotoxic T-lymphocyte level within which a bona fide memory cell precursor has emerged. The challenge now is to identify the cellular and molecular factors that control this developmental pathway. This review considers aspects of the regulation of the induction of effectors, the transition of effectors to memory cells, and the dynamics of the memory population.

Memory CD8+ T cell differentiation

In response to infection or effective vaccination, naive antigen-specific CD8+ T cells undergo a dramatic highly orchestrated activation process. Initial encounter with an appropriately activated antigen-presenting cell leads to blastogenesis and an exponential increase in antigen-specific CD8+ T cell numbers. Simultaneously, a dynamic differentiation process occurs, resulting in formation of both primary effector and long-lived memory cells. Current findings have emphasized the heterogeneity of effector and memory cell populations with the description of multiple cellular subsets based on phenotype, function, and anatomic location. Yet, only recently have we begun to dissect the underlying factors mediating the temporal control of the development of distinct effector and memory CD8+ T cell sublineages. In this review we will focus on the requirements for mounting an effective CD8+ T cell response and highlight the elements regulating the differentiation of effector and memory subsets.

CD4+ T cell regulation of CD25 expression controls development of short-lived effector CD8+ T cells in primary and secondary responses

Both CD4(+) T cell help and IL-2 have been postulated to "program" activated CD8(+) T cells for memory cell development. However, the linkage between these two signals has not been well elucidated. Here we have studied effector and memory CD8(+) T cell differentiation following infection with three pathogens (Listeria monocytogenes, vesicular stomatitis virus, and vaccinia virus) in the absence of both CD4(+) T cells and IL-2 signaling. We found that expression of CD25 on antigen-specific CD8(+) T cells peaked 3-4 days after initial priming and was dependent on CD4(+) T cell help, likely through a CD28:CD80/86 mediated pathway. CD4(+) T cell or CD25-deficiency led to normal early effector CD8(+) T cell differentiation, but a subsequent lack of accumulation of CD8(+) T cells resulting in overall decreased memory cell generation. Interestingly, in both primary and recall responses KLRG1(high) CD127(low) short-lived effector cells were drastically diminished in the absence of IL-2 signaling, although memory precursors remained intact. In contrast to previous reports, upon secondary antigen encounter CD25-deficient CD8(+) T cells were capable of undergoing robust expansion, but short-lived effector development was again impaired. Thus, these results demonstrated that CD4(+) T cell help and IL-2 signaling were linked via CD25 up-regulation, which controls the expansion and differentiation of antigen-specific effector CD8(+) T cells, rather than "programming" memory cell traits.

C3 promotes expansion of CD8+ and CD4+ T cells in a Listeria monocytogenes infection

It is known that C3 is required for optimal expansion of T cells during acute viral infections. However, it is not yet determined whether T cell responses to intracellular bacterial infections require C3. Therefore, we have investigated the requirement for C3 to elicit potent T cell responses to Listeria monocytogenes (LM). We show that expansion of Ag-specific CD8 and CD4 T cells during a primary response to LM was markedly reduced in the absence of C3 activity. Further studies indicated that, unlike in an influenza virus infection, the regulation of LM-specific T cell responses by C3 might not involve the downstream effector C5a. Moreover, reduced T cell responses to LM was not linked to defective maturation of dendritic cells or developmental anomalies in the peripheral T cell compartment of C3-deficient mice. Experiments involving adoptive transfer of C3-deficient CD8 T cells into the C3-sufficient environment of wild-type mice showed that these T cells do not have intrinsic proliferative defects, and a paracrine source of C3 will suffice for clonal expansion of CD8 T cells in vivo. However, stimulation of purified C3-deficient CD8 T cells by plastic-immobilized anti-CD3 showed that C3 promotes T cell proliferation directly, independent of its effects on APC. On the basis of these findings, we propose that diminished T cell responses to LM in C3-deficient mice might be at least in part due to lack of direct effects of C3 on T cells. These studies have furthered our understanding of C3-mediated regulation of T cell immunity to intracellular pathogens.

Following TRAIL's path in the immune system

The members of the tumour necrosis factor (TNF) superfamily of cytokines play important roles in the regulation of various immune-cell functions. Likewise, induction of cell death by apoptosis is indispensable for the normal functioning of the immune system. There are two major pathways of apoptosis induction. The intrinsic, or mitochondrial, pathway is regulated by the activation and interaction of members of the Bcl-2 family. The extrinsic, or death receptor, pathway is triggered by certain TNF family members when they engage their respective cognate receptors on the surface of the target cell. Hence, cell-to-cell-mediated death signals are induced by activation of these death receptor-ligand systems. Besides TNF itself and the CD95 (Fas/APO-1) ligand (FasL/Apo1L), the TNF-related apoptosis-inducing ligand (TRAIL/Apo2L) belongs to the subfamily of ligands that is responsible for extrinsic induction of cell death. Depending on their status of stimulation, TRAIL can be expressed by various cells of the immune system, amongst them natural killer (NK) cells, T cells, natural killer T cells (NKT cells), dendritic cells and macrophages. TRAIL has been implicated in immunosuppressive, immunoregulatory and immune-effector functions. With respect to pathological challenges, TRAIL and its receptors have been shown to play important roles in the immune response to viral infections and in immune surveillance of tumours and metastases. In this review we summarize the current knowledge on the role of TRAIL and its receptors in the immune system and, based on this, we discuss future directions of research into the diverse functions of this fascinating receptor-ligand system.

Adeno-associated virus capsid structure drives CD4-dependent CD8+ T cell response to vector encoded proteins

The immunological sequelae of adeno-associated virus (AAV)-mediated gene transfer in vivo is quite complex. In murine models, most AAV capsids are associated with minimal or dysfunctional T cell responses to antigenic transgene products. In this study we compared T cell activation against AAV2/8 and AAV2/rh32.33 vectors expressing nuclear-targeted LacZ (nLacZ), GFP, or firefly luciferase in murine skeletal muscle. We show that, unlike AAV8, AAVrh32.33 yields qualitatively and quantitatively robust T cell responses to both the capsid and transgene product. AAV2/rh32.33.CB.nLacZ, but not AAV2/8, drives a high degree of cellular infiltration and a loss of detectable transgene expression in C57BL/6 mice. However, cellular immunity to AAVrh32.33 is ablated in the absence of CD4, CD40L, or CD28, permitting stable beta-galactosidase expression. Treatment of CD40L(-/-) mice with the CD40 agonist, FGK45, failed to restore the CD8 response to AAV2/rh32.33.nLacZ, suggesting that additional factors are involved. Our results suggest that specific domains within the AAVrh32.33 capsid augment the adaptive response to both capsid and transgene Ags in a CD4-dependent pathway involving CD40L signaling and CD28 costimulation. Structural comparison of the AAV8 and rh32.33 capsids has identified key differences that may drive differential immunity by affecting tropism, Ag presentation or the activation of innate immunity. This murine model of AAV-mediated cytotoxicity allows us to delineate the mechanism of viral immune activation, which is relevant to the translation of AAV technology in higher order species.

Cognate CD4 help is essential for the reactivation and expansion of CD8 memory T cells directed against the hematopoietic cell–specific dominant minor histocompatibility antigen, H60

In contrast to previous notions of the help-independency of memory CD8 T cells during secondary expansion, here we show that CD4 help is indispensable for the re-expansion of once-helped memory CD8 T cells, using a hematopoietic cell–specific dominant minor histocompatibility (H) antigen, H60, as a model antigen. H60-specific memory CD8 T cells generated during a helped primary response vigorously expanded only when rechallenged under helped conditions. The help requirement for an optimal secondary response was confirmed by a reduction in peak size by CD4 depletion, and was reproduced after skin transplantation. Helpless conditions or noncognate separate help during the secondary response resulted in a significant reduction in the peak size and different response kinetics. Providing CD4 help again during a tertiary challenge restored robust memory expansion; however, the repeated deprivation of help further reduced clonal expansion. Adoptively transferred memory CD8 T cells did not proliferate in CD40L−/− hosts. In the CD40−/− hosts, marginal memory expansion was detected after priming with male H60 cells but was completely abolished by priming with peptide-loaded CD40−/− cells, suggesting the essential role of CD40 and CD40L in memory responses. These results provide insight into the control of minor H antigen-specific CD8 T-cell responses, to maximize the graft-versus-leukemia response.

CD4+ T cells are required for the priming of CD8+ T cells following infection with herpes simplex virus type 1

The role of CD4(+) helper T cells in modulating the acquired immune response to herpes simplex virus type 1 (HSV-1) remains ill defined; in particular, it is unclear whether CD4(+) T cells are needed for the generation of the protective HSV-1-specific CD8(+)-T-cell response. This study examined the contribution of CD4(+) T cells in the generation of the primary CD8(+)-T-cell responses following acute infection with HSV-1. The results demonstrate that the CD8(+)-T-cell response generated in the draining lymph nodes of CD4(+)-T-cell-depleted C57BL/6 mice and B6-MHC-II(-/-) mice is quantitatively and qualitatively distinct from the CD8(+) T cells generated in normal C57BL/6 mice. Phenotypic analyses show that virus-specific CD8(+) T cells express comparable levels of the activation marker CD44 in mice lacking CD4(+) T cells and normal mice. In contrast, CD8(+) T cells generated in the absence of CD4(+) T cells express the interleukin 2 receptor alpha-chain (CD25) at lower levels. Importantly, the CD8(+) T cells in the CD4(+)-T-cell-deficient environment are functionally active with respect to the expression of cytolytic activity in vivo but exhibit a diminished capacity to produce gamma interferon and tumor necrosis factor alpha. Furthermore, the primary expansion of HSV-1-specific CD8(+) T cells is diminished in the absence of CD4(+)-T-cell help. These results suggest that CD4(+)-T-cell help is essential for the generation of fully functional CD8(+) T cells during the primary response to HSV-1 infection.

Effector T cells control lung inflammation during acute influenza virus infection by producing IL-10

Activated antigen-specific T cells produce a variety of effector molecules for clearing infection, but also contribute significantly to inflammation and tissue injury. Here we report an anti-inflammatory property of anti-viral CD8⁺ and CD4⁺ effector T cells (Te) in the infected periphery during acute virus infection. We find that, during acute influenza infection, IL-10 is produced in the infected lungs at high levels -- exclusively by infiltrating virus-specific Te, with CD8⁺ Te contributing a larger fraction of the IL-10 produced. These Te in the periphery simultaneously produce IL-10 and proinflammatory cytokines, and express lineage markers characteristic of conventional Th/c1 cells. Importantly, blocking the action of the Te-derived IL-10 results in enhanced pulmonary inflammation and lethal injury. Our results demonstrate that anti-viral Te exert regulatory functions -- that is, fine-tune the extent of lung inflammation and injury associated with influenza infection by the production of an anti-inflammatory cytokine. The potential implications of these findings for infection with highly pathogenic influenza viruses are discussed.

Cancer immunotherapy targeting the high molecular weight melanoma-associated antigen protein results in a broad antitumor response and reduction of pericytes in the tumor vasculature

The high molecular weight melanoma-associated antigen (HMW-MAA), also known as melanoma chondroitin sulfate proteoglycan, has been used as a target for the immunotherapy of melanoma. This antigen is expressed on the cell surface and has a restricted distribution in normal tissues. Besides its expression in a broad range of transformed cells, this antigen is also found in pericytes, which are important for tumor angiogenesis. We generated a recombinant Listeria monocytogenes (Lm-LLO-HMW-MAA-C) that expresses and secretes a fragment of HMW-MAA (residues 2,160-2,258) fused to the first 441 residues of the listeriolysin O (LLO) protein. Immunization with Lm-LLO-HMW-MAA-C was able to impede the tumor growth of early established B16F10-HMW-MAA tumors in mice and both CD4(+) and CD8(+) T cells were required for therapeutic efficacy. Immune responses to a known HLA-A2 epitope present in the HMW-MAA(2160-2258) fragment was detected in the HLA-A2/K(b) transgenic mice immunized with Lm-LLO-HMW-MAA-C. Surprisingly, this vaccine also significantly impaired the in vivo growth of other tumorigenic cell lines, such as melanoma, renal carcinoma, and breast tumors, which were not engineered to express HMW-MAA. One hypothesis is that the vaccine could be targeting pericytes, which are important for tumor angiogenesis. In a breast tumor model, immunization with Lm-LLO-HMW-MAA-C caused CD8(+) T-cell infiltration in the tumor stroma and a significant decrease in the number of pericytes in the tumor blood vessels. In conclusion, a Lm-based vaccine against HMW-MAA can trigger cell-mediated immune responses to this antigen that can target not only tumor cells but also pericytes in the tumor vasculature.

Lymphocytic choriomeningitis virus infection yields overlapping CD4+ and CD8+ T-cell responses

Activation of CD4(+) T cells helps establish and sustain other immune responses. We have previously shown that responses against a broad set of nine CD4(+) T-cell epitopes were present in the setting of lymphocytic choriomeningitis virus (LCMV) Armstrong infection in the context of H-2(d). This is quite disparate to the H-2(b) setting, where only two epitopes have been identified. We were interested in determining whether a broad set of responses was unique to H-2(d) or whether additional CD4(+) T-cell epitopes could be identified in the setting of the H-2(b) background. To pursue this question, we infected C57BL/6 mice with LCMV Armstrong and determined the repertoire of CD4(+) T-cell responses using overlapping 15-mer peptides corresponding to the LCMV Armstrong sequence. We confirmed positive responses by intracellular cytokine staining and major histocompatibility complex (MHC)-peptide binding assays. A broad repertoire of responses was identified, consisting of six epitopes. These epitopes originate from the nucleoprotein (NP) and glycoprotein (GP). Out of the six newly identified CD4(+) epitopes, four of them also stimulate CD8(+) T cells in a statistically significant manner. Furthermore, we assessed these CD4(+) T-cell responses during the memory phase of LCMV Armstrong infection and after infection with a chronic strain of LCMV and determined that a subset of the responses could be detected under these different conditions. This is the first example of a broad repertoire of shared epitopes between CD4(+) and CD8(+) T cells in the context of viral infection. These findings demonstrate that immunodominance is a complex phenomenon in the context of helper responses.

Generating functional CD8+ T cell memory response under transient CD4+ T cell deficiency: implications for vaccination of immunocompromised individuals

Studies based on either MHC class II-knockout or CD4+ T cell-depleted murine models have demonstrated a critical role for CD4+ T cells in the generation of CD8+ T cell memory. However, it is difficult to extend these findings to immunocompromised humans where a complete loss of CD4+ T cells is rarely observed. Here, we have developed a model setting, which allows studies on the generation of CD8+ T cell memory responses in a transient CD4+ T cell-deficient setting similar to that seen in immunocompromised patients. Immunisation with an adenoviral vaccine under transient helpless or help-deficient conditions showed varying degrees of impact on the priming of CD8+ T cell responses. Antigen-specific T cells generated under normal CD4+ T cell help and transient help-deficient conditions showed similar effector phenotype and were capable of proliferation upon secondary antigen encounter. Most importantly, in spite of CD4+ T cell deficiency, the long-term CD8+ T cell memory response remained functionally stable and showed comparable cytotoxic effector function as seen in CD8+ T cells generated with normal CD4+ T cell numbers. These findings provide evidence that in spite of partially impaired activation of a primary CD8+ T cell response, a fully functional and stable memory CTL response can be induced under conditions of severe transient CD4+ T cell deficiency.

The TRAIL of helpless CD8+ T cells in HIV infection

Our understanding of how CD4(+) T cells can regulate CD8(+) T cell responses in HIV infection is still incomplete. Recent evidence obtained in mice suggests that CD4(+) T cell help is required for efficient CD8(+) T cell-mediated immunity in chronic infection: CD8(+) T cells primed in the absence of such help release the TNF-related apoptosis-inducing ligand TRAIL and undergo apoptosis. Using a novel ELISPOT assay, in the present study we show that CD8(+) T cells are also a source of the antigen-specific TRAIL response in HIV-infected patients with CD4(+) T cell counts below 200. In patients with CD4(+) T cell counts above 200 TRAIL was not detectable. Accordingly, antigens to which patients have likely been exposed when CD4(+) T cell levels were high (e.g., influenza, CMV, and EBV) did not induce TRAIL. Within the HIV-positive donor population with low CD4(+) T cell counts a dissociation of the interferon-gamma (IFN-gamma) and TRAIL response to different HIV peptide epitopes was detectable suggesting impaired immunity to antigens that triggered TRAIL in the absence of IFN-gamma. Our findings emphasize that "helpless" CD8(+) T cells, i.e., cells that have been primed in the absence of CD4(+) T cell help, may play a crucial role in HIV infection. A "helpless" phenotype may impair CD8(+) T cell control of HIV and other infections and possibly contribute to the depletion of CD4(+) T cells via apoptosis. Immunizations and infections in this "helpless" state might result in ineffective CD8(+) T cell responses.

Tumor-specific CD4+ T cells render the tumor environment permissive for infiltration by low-avidity CD8+ T cells

CD4+ T cells enhance tumor destruction by CD8+ T cells. One benefit that underlies CD4+ T cell help is enhanced clonal expansion of newly activated CD8+ cells. In addition, tumor-specific CD4+ help is also associated with the accumulation of greater numbers of CD8+ T cells within the tumor. Whether this too is attributable to the effects of help delivered to the CD8+ cells during priming within secondary lymphoid tissues, or alternatively is due to the action of CD4+ cells within the tumor environment has not been examined. In this study, we have evaluated separately the benefits of CD4+ T cell help accrued during priming of tumor-specific CD8+ T cells with a vaccine, as opposed to the benefits delivered by the presence of cognate CD4+ cells within the tumor. The presence of CD4+ T cell help during priming increased clonal expansion of tumor-specific CD8+ T cells in secondary lymphoid tissue; however, CD8+ T cells that have low avidity for tumor Ag were inefficient in tumor invasion. CD4+ T cells that recognized tumor Ag were required to facilitate accumulation of CD8+ T cells within the tumor and enhance tumor lysis during the acute phase of the response. These experiments highlight the ability of tumor-specific CD4+ T cells to render the tumor microenvironment receptive for CD8+ T cell immunotherapy, by facilitating the accumulation of all activated CD8+ T cells, including low-avidity tumor-specific and noncognate cells.

Control of memory CD8+ T cell differentiation by CD80/CD86-CD28 costimulation and restoration by IL-2 during the recall response

Memory CD8+ T cell responses have been considered to be independent of CD80/CD86-CD28 costimulation. However, recall responses are often severely blunted in CD28-/- mice. Whether this impairment represents a requirement for CD28 costimulation for proper memory CD8+ T cell development or a requirement during the recall response is unknown. Furthermore, how CD28 costimulation affects the phenotype and function of memory CD8+ T cells has not been characterized in detail. In this study, we investigate these questions by studying the role of the CD28 costimulatory pathway in memory CD8+ T cell responses to acute and persistent DNA virus infections. Memory CD8+ T cells against vaccinia virus (VV) infection which develop without CD28 costimulation exhibit lower expression of differentiation markers CD27 and CD122 (IL-15Rbeta). These memory CD8+ T cells also fail to produce IL-2. Our data indicate that for an optimal recall response, CD28 costimulation is required both for T cell priming and also during the recall response. Similar requirements were observed for memory CD8+ T cell responses during persistent infection with murine gammaherpesvirus 68 (MHV-68) infection, indicating CD28 may play the same role in both acute and persistent infections. Finally, we show deficits in the recall response are restored by IL-2 signaling during recall, but not during priming. The data presented show that CD28 costimulation not only controls the magnitude of the primary response but also affects development of memory CD8+ T cells and is required during the recall response in addition to initial T cell priming.

Memory CD8+ T cells require CD28 costimulation

CD8(+) T cells are a critical component of the adaptive immune response against infections and tumors. A current paradigm in immunology is that naive CD8(+) T cells require CD28 costimulation, whereas memory CD8(+) T cells do not. We show here, however, that during viral infections of mice, costimulation is required in vivo for the reactivation of memory CD8(+) T cells. In the absence of CD28 costimulation, secondary CD8(+) T cell responses are greatly reduced and this impairs viral clearance. The failure of CD8(+) T cells to expand in the absence of CD28 costimulation is CD4(+) T cell help independent and is accompanied by a failure to down-regulate Bcl-2 and by cell cycle arrest. This requirement for CD28 costimulation was shown in both influenza A and HSV infections. Thus, contrary to current dogma, memory CD8(+) T cells require CD28 costimulation to generate maximal secondary responses against pathogens. Importantly, this CD28 requirement was shown in the context of real infections were multiple other cytokines and costimulators may be up-regulated. Our findings have important implications for pathogens, such as HIV and measles virus, and tumors that evade the immune response by failing to provide CD28 costimulation. These findings also raise questions about the efficacy of CD8(+) T cell-based vaccines against such pathogens and tumors.

Chronic lymphocytic choriomeningitis virus infection actively down-regulates CD4+ T cell responses directed against a broad range of epitopes

Activation of CD4(+) T cells helps establish and sustain CD8(+) T cell responses and is required for the effective clearance of acute infection. CD4-deficient mice are unable to control persistent infection and CD4(+) T cells are usually defective in chronic and persistent infections. We investigated the question of how persistent infection impacted pre-existing lymphocytic choriomeningitis virus (LCMV)-specific CD4(+) T cell responses. We identified class II-restricted epitopes from the entire set of open reading frames from LCMV Armstrong in BALB/c mice (H-2(d)) acutely infected with LCMV Armstrong. Of nine epitopes identified, six were restricted by I-A(d), one by I-E(d) and two were dually restricted by both I-A(d) and I-E(d) molecules. Additional experiments revealed that CD4(+) T cell responses specific for these epitopes were not generated following infection with the immunosuppressive clone 13 strain of LCMV. Most importantly, in peptide-immunized mice, established CD4(+) T cell responses to these LCMV CD4 epitopes as well as nonviral, OVA-specific responses were actively suppressed following infection with LCMV clone 13 and were undetectable within 12 days after infection, suggesting an active inhibition of established helper responses. To address this dysfunction, we performed transfer experiments using both the Smarta and OT-II systems. OT-II cells were not detected after clone 13 infection, indicating physical deletion, while Smarta cells proliferated but were unable to produce IFN-gamma, suggesting impairment of the production of this cytokine. Thus, multiple mechanisms may be involved in the impairment of helper responses in the setting of early persistent infection.

Effector and memory CTL differentiation

Technological advances in recent years have allowed for an ever-expanding ability to analyze and quantify in vivo immune responses. MHC tetramers, intracellular cytokine staining, an increasing repertoire of transgenic and "knockout" mice, and the detailed characterization of a variety of infectious models have all facilitated more precise and definitive analyses of the generation and function of cytotoxic T lymphocytes (CTL). Understanding the mechanisms behind the differentiation of effector and memory CTL is of increasing importance to develop vaccination strategies against a variety of established and emerging infectious diseases. This review focuses on recent advances in our understanding of how effector and memory CTL differentiate and survive in vivo in response to viral or bacterial infection.

Immunologic evidence for lack of heterologous protection following resolution of HCV in patients with non-genotype 1 infection

Chronic hepatitis C virus (HCV) infection is typically characterized by a lack of virus-specific CD4(+) T-cell-proliferative responses, but strong responses have been described in a subset of persons with persistent viremia. One possible explanation for these responses is that they were primed by an earlier resolved infection and do not recognize the current circulating virus. We defined all targeted epitopes using overlapping peptides corresponding to a genotype 1a strain in 44 patients chronically infected with different HCV genotypes (GT). Surprisingly, more HCV-specific CD4(+) T-cell responses were detected in patients with chronic non-GT1 infection compared with patients with chronic GT1 infection (P = .017). Notably, we found serologic evidence of a previous exposure to GT1 in 4 patients with non-GT1 infection, and these persons also demonstrated significantly more responses than non-GT1 patients in whom genotype and HCV serotype were identical (P < .001). Comparison of recognition of GT1-specific peptides to peptides representing autologous virus revealed the absence of cross-recognition of the autologous circulating virus. These data indicate that persistent HCV infection can occur in the presence of an HCV-specific T-cell response primed against a heterologous HCV strain, and suggest that clearance of 1 GT does not necessarily protect against subsequent exposure to a second GT.

CD40-CD40 ligand interaction between dendritic cells and CD8+ T cells is needed to stimulate maximal T cell responses in the absence of CD4+ T cell help

Stimulation of CD40 on APCs through CD40L expressed on helper CD4+ T cells activates and "licenses" the APCs to prime CD8+ T cell responses. Although other stimuli, such as TLR agonists, can also activate APCs, it is unclear to what extent they can replace the signals provided by CD40-CD40L interactions. In this study, we used an adoptive transfer system to re-examine the role of CD40 in the priming of naive CD8+ T cells. We find an approximately 50% reduction in expansion and cytokine production in TCR-transgenic T cells in the absence of CD40 on all APCs, and on dendritic cells in particular. Moreover, CD40-deficient and CD40L-deficient mice fail to develop endogenous CTL responses after immunization. Surprisingly, the role for CD40 and CD40L are observed even in the absence of CD4+ T cells; in this situation, the CD8+ T cell itself provides CD40L. Furthermore, we show that although TLR stimulation improves T cell responses, it cannot fully substitute for CD40. Altogether, these results reveal a direct and unique role for CD40L on CD8+ T cells interacting with CD40 on APCs that affects the magnitude and quality of CD8+ T cell responses.

Gamma interferon responses of CD4 and CD8 T-cell subsets are quantitatively different and independent of each other during pulmonary Mycobacterium bovis BCG infection

Gamma interferon (IFN-gamma) is a key cytokine in host defense against intracellular mycobacterial infection. It has been believed that both CD4 and CD8 T cells are the primary sources of IFN-gamma. However, the relative contributions of CD4 and CD8 T-cell subsets to IFN-gamma production and the relationship between CD4 and CD8 T-cell activation have not been examined. By using a model of pulmonary mycobacterial infection and various immunodetection assays, we found that CD4 T cells mounted a much stronger IFN-gamma response than CD8 T cells at various times after mycobacterial infection, and this pronounced IFN-gamma production by CD4 T cells was attributed to both greater numbers of antigen-specific CD4 T cells and a greater IFN-gamma secretion capacity of these cells. By using major histocompatibility complex class II-deficient or CD4-deficient mice, we found that the lack of CD4 T cells did not negatively affect primary or secondary CD8 T-cell IFN-gamma responses. The CD8 T cells activated in the absence of CD4 T cells were capable of immune protection against secondary mycobacterial challenge. Our results suggest that, whereas both CD4 and CD8 T cells are capable of IFN-gamma production, the former represent a much greater cellular source of IFN-gamma. Moreover, during mycobacterial infection, CD8 T-cell IFN-gamma responses and activation are independent of CD4 T-cell activation.

Limited role for CD4+ T-cell help in the initial priming of Trypanosoma cruzi-specific CD8+ T cells

Immune control of the protozoan parasite Trypanosoma cruzi requires the activation of both CD4+ and CD8+ T cells. We recently identified two T. cruzi trans-sialidase peptides that are targets of approximately 30% of all CD8+ T cells during acute T. cruzi infection in mice. To determine whether CD4+ T cells are required for generation of these dominant CD8+ T-cell responses, major histocompatibility complex class II (MHC II)-deficient mice were infected with the Brazil strain of T. cruzi and examined for the generation of antigen-specific CD8+ T cells. Strong trans-sialidase TSKB18- and TSKB20-specific CD8+ T-cell responses were generated in both the presence and the absence of CD4+ help. However, the magnitudes of the immunodominant TSKB20-specific CD8+ T-cell responses detectable using class I MHC-peptide tetramers were consistently lower in the blood and spleens of MHC II-deficient mice. Spleen cells from infected MHC II-deficient mice produced gamma interferon after in vitro stimulation with T. cruzi peptides at levels similar to those in wild-type mice, and MHC II-deficient mice displayed strong T. cruzi peptide-specific cytotoxic T-lymphocyte activity in vivo. Thus, primary CD8+ T-cell responses in experimental T. cruzi infection are generated in the absence of CD4+ T cells, providing further evidence that T. cruzi directly activates and licenses antigen-presenting cells. Nevertheless, unhelped CD8+ T cells in T. cruzi-infected mice fail to reach the frequencies achieved in the presence of CD4 T-cell help and are unable to prevent acute-phase death of these mice.

Requirement for CD70 in CD4+ Th cell-dependent and innate receptor-mediated CD8+ T cell priming

Dendritic cell (DC) conditioning by CD4+ T cells, or via engagement of innate receptors, is thought to be essential for CD8+ T cell priming. However, the molecular features that distinguish a conditioned DC from an unconditioned DC are poorly defined. In this study, we investigate the role of CD70, a member of the TNF superfamily that is expressed on activated DC, in CD4+ Th-dependent and -independent CD8+ T cell responses. We demonstrate that CD70 is required for CD4+ T cell-dependent priming of CD8+ T cells as well as priming mediated by the viral signature, dsRNA. Accordingly, mice that were subjected to CD70 blockade during the primary response fail to generate a memory CD8+ T cell response. Furthermore, we find that CD70 is dispensable for CD4+ T cell expansion and help for B cells, thus suggesting a direct role for CD70 in CD8+ T cell priming. Our results show that the innate and adaptive (CD4+ T cells) arms of the immune system use a common signaling pathway in driving CD8+ T cell responses and suggest that expression of CD70 on DC represents the hallmark of conditioned DC.

H2-M3-Restricted T Cells Participate in the Priming of Antigen-Specific CD4+ T Cells

H2-M3-restricted CD8+ T cells provide early protection against bacterial infections. In this study, we demonstrate that activated H2-M3-restricted T cells provide early signals for efficient CD4+ T cell priming. C57BL/6 mice immunized with dendritic cells coated with the MHC class II-restricted listeriolysin O peptide LLO(190-201) (LLO) generated CD4+ T cells capable of responding to Listeria monocytogenes (LM) infection. Inclusion of a H2-M3-restricted formylated peptide fMIGWII (fMIG), but not MHC class Ia-restricted peptides, during immunization with LLO significantly increased IFN-gamma-producing CD4+ T cell numbers, which was associated with increased protection against LM infection. Studies with a CD4+ T cell-depleting mAb indicate that the reduction in bacterial load in fMIG plus LLO immunized mice is likely due to augmented numbers of LLO-specific CD4+ T cells, generated with the help of H2-M3-restricted CD8+ T cells. We also found that augmentation of LLO-specific CD4+ T lymphocytes with H2-M3-restricted T cells requires presentation of LLO and fMIG by the same dendritic cells. Interestingly, the augmented CD4+ T cell response generated with fMIG also increased primary LM-specific responses by MHC class Ia-restricted CD8 T cells. Coimmunization with LLO and fMIG also increases the number of memory Ag-specific CD4+ T cells. We also demonstrate that CD8 T cells restricted to another MHC class Ib molecule, Qa-1, whose human equivalent is HLA-E, are also able to enhance Ag-specific CD4+ T cell responses. These results reveal a novel function for H2-M3- and Qa-1-restricted T cells; provision of help to CD4+ Th cells during the primary response.

Functional Dichotomy between OX40 and 4-1BB in Modulating Effector CD8 T Cell Responses

Members of the TNFR family are thought to deliver costimulatory signals to T cells and modulate their function and survival. In this study, we compare the role of two closely related TNFR family molecules, OX40 and 4-1BB, in generating effector CD8 T cells to Ag delivered by adenovirus. OX40 and 4-1BB were both induced on responding naive CD8 T cells, but 4-1BB exhibited faster and more sustained kinetics than OX40. OX40-deficient CD8 T cells initially expanded normally; however, their accumulation and survival at late times in the primary response was significantly impaired. In contrast, 4-1BB-deficient CD8 T cells displayed hyperresponsiveness, expanding more than wild-type cells. The 4-1BB-deficient CD8 T cells also showed enhanced maturation attributes, whereas OX40-deficient CD8 T cells had multiple defects in the expression of effector cell surface markers, the synthesis of cytokines, and in cytotoxic activity. These results suggest that, in contrast to current ideas, OX40 and 4-1BB can have a clear functional dichotomy in modulating effector CD8 T cell responses. OX40 can positively regulate effector function and late accumulation/survival, whereas 4-1BB can initially operate in a negative manner to limit primary CD8 responses.

CD4(+) T cells stimulate memory CD8(+) T cell expansion via acquired pMHC I complexes and costimulatory molecules, and IL-2 secretion

The rapid and efficient expansion of CD8(+) memory T cells after the second encounter with a pathogen constitutes a hallmark trait of adaptive immunity. Yet, the contribution of CD4(+) T cells to the expansion of memory CD8(+) T cells remains the subject of controversy. Here, we show that, antigen-specific CD4(+) T cells, once activated by dendritic cells (DC) in vitro, have the capacity to stimulate expansion of memory CD8(+) T cells in vivo. The memory CD8(+) T cell expansion triggered by active CD4(+) T cells are mediated through DC-derived MHC I/peptide complexes and CD80 molecules displayed on the active CD4(+) T cells, with the involvement of IL-2 secreted by the active CD4(+) T cells. These results highlight a previously undescribed role of active CD4(+) T cells in triggering expansion of memory CD8(+) T cells.

Differential requirements of T cell subsets for CD40 costimulation in immunity to Blastomyces dermatitidis

Cell-mediated immunity and production of type 1 cytokines are the main defenses against pathogenic fungi. Ligation of CD40 by CD40L on T cells is critical for the induction of these immune responses in vivo. We explored the role of CD40/CD40L interactions in vaccine immunity to Blastomyces dermatitidis by immunizing CD40(-/-) and CD40L(-/-) mice and analyzing their resistance to reinfection in a murine pulmonary model. In the absence of CD40 or CD40L, CD4(+) cells failed to get primed or produce type 1 cytokine and impaired the generation of CD8(+) T1 cells. The CD8(+) T cell defect was not due to regulatory T cells or impaired APC maturation or Ag presentation to T cells. If CD4(+) cells were first eliminated, vaccination of CD40(-/-) and CD40L(-/-) mice restored priming of CD8(+) cells, type 1 cytokine production, and resistance. Hence, CD4(+) and CD8(+) cells differ sharply in their requirement for CD40/CD40L interaction during the generation of antifungal immunity. Despite the plasticity of T cell subsets in vaccine immunity, in absence of CD40/CD40L interaction, CD4(+) cells may impede the priming of CD8(+) cells at the cost of host survival against a lethal infectious disease.

Monitoring the antitumor response of naive and memory CD8 T cells in RAG1-/- mice by positron-emission tomography

Therapeutic antitumor immunity depends on a highly migratory CTL population capable of activation and trafficking between lymphoid and tumor-bearing microanatomic sites. We recently adapted positron-emission tomography gene expression imaging for noninvasive, longitudinal localization and quantitation of antitumor T lymphocyte migration in vivo. In this study, we apply this system to enumerate the temporal accumulation of naive vs memory T cells. Naive or memory OT-1 CD8(+) T cells, retrovirally marked with the sr39TK gene, were adoptively transferred into RAG1(-/-) animals bearing EL-4 or EG.7 (an OVA-expressing subline), and repetitively imaged by microPET over several weeks. Memory cells demonstrated early accumulation and apparent proliferation, with large T cell numbers at the Ag-positive tumor as early as day 1 after T cell transfer. Naive T cells did not accumulate in the E.G7 tumor until day 8, and reached only 25% of the peak levels achieved by memory T cells. Both naive and memory cells eradicated the Ag-expressing tumor at a comparable density of intratumoral T cells (2-4 x 10(6)/g). However, due to the slower rate of T cell expansion and continued tumor growth, naive cells required approximately 10-fold higher Ag-specific precursor frequency to reach a tumoricidal cell density. As recently reported, memory but not naive T cells accumulated in local lymph nodes and lungs, where they persisted as a resident population after tumor eradication. Positron-emission tomography-based immunologic imaging is a noninvasive modality providing unique and meaningful information on the dynamics of the antitumor CTL response.

CD4+ T cell-independent maintenance and expansion of memory CD8+ T cells derived from in vitro dendritic cell activation

CD4+ T cells are essential for the maintenance of CD8+ memory T (Tm) cells following acute infection, but the importance of CD4+ T cells for the maintenance and expansion of CD8+ Tm cells to non-infectious antigens remains mostly unknown. Here, we showed that ovalbumin (OVA)-specific CD8+ Tm cell precursors derived from in vitro stimulation of TCR transgenic OT I CD8+ T cells with OVA protein-pulsed bone marrow-derived dendritic cells (DCOVA) can give rise to functional CD8+ Tm cells after adoptively transferred into mice. These CD8+ Tm cells can be maintained and remain fully functional in CD4+ T cell-absent environments in vivo. Furthermore, CD4+ T cells are not essential for the expansion of these CD8+ Tm cells. Finally, these in vitro DCOVA-activated CD8+ Tm cells maintained in CD4-deficient mice are also able to confer fully protective immunity against a later challenge of OVA-expressing tumor cells. Collectively, these findings demonstrate that in contrast to acute infections, maintenance and expansion of CD8+ Tm cells after priming with OVA protein-pulsed dendritic cells are independent of CD4+ T cells.

Transcutaneous immunization in mice: Induction of T-helper and cytotoxic T lymphocyte responses and protection against human papillomavirus-induced tumors

Previous reports have shown that transcutaneous immunization (TCI) with proteins or peptides in combination with adjuvants efficiently induces specific cellular and humoral immune responses. However, depending on the kind of skin pretreatment, induction of cellular immune responses was restricted to generation of either specific cytotoxic T lymphocytes (CTLs) or T-helper (Th) cells. In this study, we induced antigen-specific CTL responses together with the appropriate Th responses by TCI of C57BL/6 mice. We applied ovalbumin protein or an ovalbumin-derived fusion peptide containing a CTL and Th epitope together with a combination of cholera toxin (CT) and CpG oligodeoxynucleotide (CpG) onto cold wax-depilated and hydrated bare skin. TCI with the ovalbumin fusion peptide induced more robust CTL and Th responses than that with ovalbumin protein. The fusion peptide in combination with the nontoxic CT derivative CTA1-D2D1 and CpG induced an antigen-specific CTL response, albeit less efficiently than in combination with complete CT. Further, we compared the potency of HPV-16 E7 oncoprotein-derived peptides containing single (CTL) or multiple (CTL + Th + B cell) epitopes to induce effective CTL responses. Strong E7-specific CTL responses were detected only after TCI with the E7 multiepitope peptide. This peptide was also shown to protect mice against tumor growth after challenge with HPV-16 E7-positive tumor cells. TCI with E7 protein and CT/CpG led to formation of an E7-specific humoral immune response.

TNF Receptor Type 2 (p75) Functions as a Costimulator for Antigen-Driven T Cell Responses In Vivo

Naive T cells require costimulation for robust Ag-driven differentiation and survival. Members of the TNFR family have been shown to provide costimulatory signals conferring survival at distinct phases of the T cell response. In this study, we show that CD4 and CD8 T cells depend on TNFR type 2 (p75) for survival during clonal expansion, allowing larger accumulation of effector cells and conferring protection from apoptosis for a robust memory pool in vivo. We demonstrate using the MHC class I-restricted 2C TCR and MHC class II-restricted AND TCR transgenic systems that TNFR2 regulates the threshold for clonal expansion of CD4 and CD8 T cell subsets in response to cognate Ag. Using a novel recombinant Listeria monocytogenes (rLM) expressing a secreted form of the 2C agonist peptide (SIY) to investigate the role of TNFR2 for T cell immunity in vivo, we found that TNFR2 controls the survival and accumulation of effector cells during the primary response. TNFR2-/- CD8 T cells exhibit loss of protection from apoptosis that is correlated with diminished survivin and Bcl-2 expression. Null mutant mice were more susceptible to rLM-SIY challenge at high doses of primary infection, correlating with impaired LM-specific T cell response in the absence of TNFR2-mediated costimulation. Moreover, the resulting memory pools specific for SIY and listeriolysin O epitopes derived from rLM-SIY were diminished in TNFR2-/- mice. Thus, examination of Ag-driven T cell responses revealed a hitherto unknown costimulatory function for TNFR2 in regulating T cell survival during the differentiation program elicited by intracellular pathogen in vivo.

SIV DNA vaccine co-administered with IL-12 expression plasmid enhances CD8 SIV cellular immune responses in cynomolgus macaques

Current evidence suggests that a strong induced CD8 human immunodeficiency virus type 1 (HIV-1)-specific cell mediated immune response may be an important aspect of an HIV vaccine. The response rates and the magnitude of the CTL responses induced by current DNA vaccines in humans need to be improved and cellular immune responses to DNA vaccines can be enhanced in mice by co-delivering DNA plasmids expressing immune modulators. Two reported to work well in the mouse systems are interleukin (IL)-12 and CD40L. We sought to compare these molecular adjuvants in a primate model system. The cDNA for macaque IL-12 and CD40L were cloned into DNA vectors. Groups of cynomolgus macaques were immunized with 2 mg of plasmid expressing SIVgag alone or in combination with either IL-12 or CD40L. CD40L did not appear to enhance the cellular immune response to SIVgag antigen. However, more robust results were observed in animals co-injected with the IL-12 molecular adjuvant. The IL-12 expanded antigen-specific IFN-gamma positive effector cells as well as granzyme B production. The vaccine immune responses contained both a CD8 component as well a CD4 component. The adjuvanted DNA vaccines illustrate that IL-12 enhances a CD8 vaccine immune response, however, different cellular profiles.

CD28 is required for optimal induction, but not maintenance, of vaccine-induced immunity to Blastomyces dermatitidis

Cellular immunity mediated by T lymphocytes, in particular CD4+ and CD8+ type 1 cells, is the main defense against pathogenic fungi. Here, CD28-deficient (CD28-/-) mice were used to study the role of costimulation for the generation and maintenance of T-cell-mediated, type 1 cytokine-dependent mechanisms of vaccine immunity to Blastomyces dermatitidis infection. Disruption of CD28 costimulation reduced the number of type 1 CD4 and CD8 cells generated and impaired resistance to infection. Type 1 T-cell subsets generated in vaccinated CD28-/- mice were durable and protected mice for at least 3 months after vaccination. Our findings suggest that CD28 is required for the induction of optimal, protective T-cell responses to B. dermatitidis infection but may be dispensable for the maintenance of T-cell memory.

Mycobacterial immune reconstitution inflammatory syndrome in HIV-1 infection after antiretroviral therapy is associated with deregulated specific T-cell responses: beneficial effect of IL-2 and GM-CSF immunotherapy

BACKGROUND

With the advent of antiretroviral therapy (ART) cases of immune reconstitution inflammatory syndrome (IRIS) have increasingly been reported. IRIS usually occurs in individuals with a rapidly rising CD4 T-cell count or percentage upon initiation of ART, who develop a deregulated immune response to infection with or without reactivation of opportunistic organisms. Here, we evaluated rises in absolute CD4 T-cells, and specific CD4 T-cell responses in 4 HIV-1+ individuals presenting with mycobacterial associated IRIS who received in conjunction with ART, IL-2 plus GM-CSF immunotherapy.

METHODS

We assessed CD4 T-cell counts, HIV-1 RNA loads, phenotype for naïve and activation markers, and in vitro proliferative responses. Results were compared with those observed in 11 matched, successfully treated asymptomatic clinical progressors (CP) with no evidence of opportunistic infections, and uninfected controls.

RESULTS

Median CD4 T-cell counts in IRIS patients rose from 22 cells/microl before initiation of ART, to 70 cells/microl after 8 months of therapy (median 6.5 fold increase). This coincided with IRIS diagnosis, lower levels of naïve CD4 T-cells, increased expression of immune activation markers, and weak CD4 T-cell responses. In contrast, CP had a median CD4 T-cell counts of 76 cells/microl at baseline, which rose to 249 cells/microl 6 months post ART, when strong T-cell responses were seen in > 80% of patients. Higher levels of expression of immune activation markers were seen in IRIS patients compared to CP and UC (IRIS > CP > UC). Immunotherapy with IL-2 and GM-CSF paralleled clinical recovery.

CONCLUSION

These data suggest that mycobacterial IRIS is associated with inadequate immune reconstitution rather than vigorous specific T-cell responses, and concomitant administration of IL-2 and GM-CSF immunotherapy with effective ART may correct/augment T-cell immunity in such setting resulting in clinical benefit.

Simultaneous Th1-type cytokine expression is a signature of peritoneal CD4+ lymphocytes responding to infection with Listeria monocytogenes

The robust murine response to infection with Listeria monocytogenes makes an excellent model to study the functional development of immune cells. We investigated the cellular immune response to i.p. infection using intracellular cytokine staining to identify Ag-specific lymphocytes. CD4(+) peritoneal exudate cells obtained 10 days postinfection predominantly coexpressed TNF-alpha, IFN-gamma, and IL-2 after polyclonal or Ag stimulation. A population of cells simultaneously making TNF-alpha and IFN-gamma was also detected but at a lower frequency. By following the kinetics of the response to Listeria, we found that CD4(+) lymphocytes coexpressing TNF-alpha and IFN-gamma dominated on day 6 postinfection and then declined. From days 10-27, TNF-alpha(+)IFN-gamma(+)IL-2(+) (triple-positive) was the most prevalent cytokine phenotype, and the frequency steadily declined. These characteristic cytokine expression patterns were observed in both primary and secondary responses to Listeria infection and developed even when infection was terminated with antibiotic treatment. A cytokine-assisted immunization procedure resulted in both double- and triple-positive cells, but the clear predominance of triple-positive cells required Listeria infection. Triple-positive cells were preferentially noted in the peritoneal cavity tissue site; spleen cells displayed a predominant population of double-positive T cells (TNF-alpha(+)IFN-gamma(+)). We speculate that the appearance of triple-positive cells represents a functionally significant subset important in host defense at nonlymphoid tissue sites.

NK cells help to induce CD8(+)-T-cell immunity against Toxoplasma gondii in the absence of CD4(+) T cells

CD8(+) T-cell immunity plays an important role in protection against intracellular infections. Earlier studies have shown that CD4(+) T-cell help was needed for launching in vivo CD8(+) T-cell activity against these pathogens and tumors. However, recently CD4(+) T-cell-independent CD8 responses during several microbial infections including those with Toxoplasma gondii have been described, although the mechanism is not understood. We now demonstrate that, in the absence of CD4(+) T cells, T. gondii-infected mice exhibit an extended NK cell response, which is mediated by continued interleukin-12 (IL-12) secretion. This prolonged NK cell response is critical for priming parasite-specific CD8(+) T-cell immunity. Depletion of NK cells inhibited the generation of CD8(+) T-cell immunity in CD4(-/-) mice. Similarly neutralization of IL-12 reduces NK cell numbers in infected animals and leads to the down-regulation of CD8(+) T-cell immunity against T. gondii. Adoptive transfer of NK cells into the IL-12-depleted animals restored their CD8(+) T-cell immune response, and animals exhibited reduced mortality. NK cell gamma interferon was essential for cytotoxic T-lymphocyte priming. Our studies for the first time demonstrate that, in the absence of CD4(+) T cells, NK cells can play an important role in induction of primary CD8(+) T-cell immunity against an intracellular infection. These observations have therapeutic implications for immunocompromised individuals, including those with human immunodeficiency virus infection.

The atheroprotective effect of 17beta-estradiol depends on complex interactions in adaptive immunity

Estradiol prevents fatty streak formation in chow-fed atherosclerosis-prone apolipoprotein E (ApoE)-deficient mice. We previously reported that fatty streak development of immunodeficient ApoE(-/-)/recombination activating gene 2 (RAG-2(-/-)) double-deficient mice was insensitive to estradiol. In the present work, we demonstrate that the reconstitution of ApoE(-/-)/RAG-2(-/-) with bone marrow from immunocompetent ApoE(-/-)/RAG-2(+/+) mice restores the protective effect of estradiol on fatty streak constitution. We extended this demonstration to the model of low-density lipoprotein receptor-deficient mice, establishing the obligatory role of mature lymphocytes in this process. We then investigated whether the protective effect of estradiol was mediated by a specific lymphocyte subpopulation by studying the hormonal effect on fatty streak constitution in recently developed models of ApoE(-/-) mice deficient in selective T-lymphocyte subsets (either TCRalphabeta+, CD4+, CD8+, or TCRgammadelta+ lymphocytes) or B lymphocytes. In all these specifically immunodeficient mice, estradiol administration to ovariectomized mice conferred protection as in immunocompetent ApoE(-/-) mice, clearly demonstrating that no single lymphocyte subpopulation was specifically required for this effect. These results point to additional lymphocyte-dependent mechanisms such as modulating the interactions among lymphocytes and between lymphocytes and endothelial and/or antigen-presenting cells.

Altered Cytokine Responses of Dengue-Specific CD4+T Cells to Heterologous Serotypes

The interplay of different inflammatory cytokines induced during a dengue (DEN) virus infection plays a role in either protection or increased disease severity. We measured the frequencies and characterized the cytokine responses of DEN virus-specific memory CD4+ T cells in PBMC of six volunteers who received experimental live attenuated monovalent DEN vaccines. IFN-gamma and TNF-alpha responses to inactivated DEN Ags were detected in up to 0.54 and 1.17% of total circulating CD4+ T cells, respectively. Ags from the homologous serotype elicited the highest IFN-gamma response. The ratio of TNF-alpha- to IFN-gamma-producing CD4+ T cells was higher after stimulation with Ags from heterologous DEN serotypes. Peptide-specific CD4+ T cell frequencies of up to 0.089% were detected by direct staining using HLA class II tetramers. IFN-gamma and TNF-alpha responses to individual HLA class II-restricted peptide epitopes were detected in up to 0.05 and 0.27% of CD4+ T cells, respectively. Peptide sequences from the homologous serotype elicited a variety of cytokine response patterns. TNF-alpha- to IFN-gamma-positive CD4+ T cell ratios varied between peptides, but the ratio of the sum of responses was highest against heterologous serotypes. These results demonstrate epitope sequence-specific differences in T cell effector function. These patterns of effector responses may play a role in the immunopathogenesis of DEN hemorrhagic fever.

Synergism between active listeriolysin O and dimethyldioctadecylammonium bromide to activate CD8+ T cells

Purified recombinant listeriolysin O (LLO) was assessed for its ability to induce T cell responses in mice. Intraperitoneal immunisation with LLO, as a fusion with glutathione-S-transferase (GST), induced the production of LLO-specific CD8(+) T cells, but not LLO-specific CD4(+) T cells. The generation of this response could be blocked by pre-treatment with cholesterol, indicating a requirement for LLO pore formation. An increase in the LLO-specific response of both CD8(+) and CD4(+) T cells could be detected following the addition of dimethyldioctadecylammonium bromide (DDA), although the generation of this response was not dependent upon LLO pore formation, suggesting that DDA might change the presentation pathway of LLO leading to activation of the CD8(+) T cells. However, this response was dependent upon the presence of structurally intact LLO, suggesting a requirement for the innate recognition of LLO in the activation of the CD4(+) and CD8(+) T cells. Therefore, DDA and LLO can act synergistically to induce the production of a CD8(+) T cell response.

TNF/TNFR family members in costimulation of T cell responses

Several members of the tumor necrosis factor receptor (TNFR) family function after initial T cell activation to sustain T cell responses. This review focuses on CD27, 4-1BB (CD137), OX40 (CD134), HVEM, CD30, and GITR, all of which can have costimulatory effects on T cells. The effects of these costimulatory TNFR family members can often be functionally, temporally, or spatially segregated from those of CD28 and from each other. The sequential and transient regulation of T cell activation/survival signals by different costimulators may function to allow longevity of the response while maintaining tight control of T cell survival. Depending on the disease condition, stimulation via costimulatory TNF family members can exacerbate or ameliorate disease. Despite these complexities, stimulation or blockade of TNFR family costimulators shows promise for several therapeutic applications, including cancer, infectious disease, transplantation, and autoimmunity.

Dendritic cells maximize the memory CD8 T cell response to infection

Costimulatory signals from dendritic cells (DCs) are required for naive T cells to respond to antigenic stimulation. To what extent DCs reactivate memory T cells during recall responses is not known. Here, an in vivo depletion system has been used to analyze the role of DCs in reactivating CD8 memory T cells during recall responses to three different microbial infections. We show a profound decrease in the numbers of responding memory CD8 T cells in both lymphoid and nonlymphoid tissues during the recall responses to infection with vesicular stomatitis virus, Listeria monocytogenes (Lm), or influenza virus. These data show that interaction with DCs is a major mechanism driving T cell reactivation in vivo, even during a tissue-specific infection of the respiratory tract.

Coimmunization with an Optimized IL-15 Plasmid Results in Enhanced Function and Longevity of CD8 T Cells That Are Partially Independent of CD4 T Cell Help

DNA vaccines are a promising technology for the induction of Ag-specific immune responses, and much recent attention has gone into improving their immune potency. In this study we test the feasibility of delivering a plasmid encoding IL-15 as a DNA vaccine adjuvant for the induction of improved Ag-specific CD8(+) T cellular immune responses. Because native IL-15 is poorly expressed, we used PCR-based strategies to develop an optimized construct that expresses 80-fold higher than the native IL-15 construct. Using a DNA vaccination model, we determined that immunization with optimized IL-15 in combination with HIV-1gag DNA constructs resulted in a significant enhancement of Ag-specific CD8(+) T cell proliferation and IFN-gamma secretion, and strong induction of long-lived CD8(+) T cell responses. In an influenza DNA vaccine model, coimmunization with plasmid expressing influenza A PR8/34 hemagglutinin with the optimized IL-15 plasmid generated improved long term CD8(+) T cellular immunity and protected the mice against a lethal mucosal challenge with influenza virus. Because we observed that IL-15 appeared to mostly adjuvant CD8(+) T cell function, we show that in the partial, but not total, absence of CD4(+) T cell help, plasmid-delivered IL-15 could restore CD8 secondary immune responses to an antigenic DNA plasmid, supporting the idea that the effects of IL-15 on CD8(+) T cell expansion require the presence of low levels of CD4 T cells. These data suggest a role for enhanced plasmid IL-15 as a candidate adjuvant for vaccine or immunotherapeutic studies.

Distinctin vivo dendritic cell activation by live versus killedListeria monocytogenes

Immunization of mice with live or heat-killed Listeria monocytogenes (HKLM) efficiently primes pathogen-specific CD8(+) T cells. T lymphocytes primed by HKLM, however, undergo attenuated proliferation and do not fully differentiate. Thus, only infection with live bacteria induces long-term, CD8(+) T cell-mediated protective immunity. In this study we demonstrate that live and heat-killed bacteria, while both associating with Mac-3(+)CD11b(hi) cells, localize to distinct splenic areas following intravenous inoculation. While HKLM localize to the marginal zone and the splenic red pulp, live L. monocytogenes are carried to the T cell zone of splenic white pulp. Despite these differences, in vivo depletion of CD11c-expressing cells prevents priming of naive T cells by either HKLM or live L. monocytogenes. Analysis of CD11c(hi) dendritic cells (DC) reveals that infection with live L. monocytogenes induces higher levels of CD40, CD80 and CD86 expression than immunization with HKLM. Our results suggest that CD8(+) T cell priming following HKLM immunization or live infection is mediated by DC and that the disparate outcomes of priming can be attributed to suboptimal conditioning of DC in the absence of live, cytosol-invasive bacteria.

CD1d-independent regulation of NKT cell migration and cytokine production upon Listeria monocytogenes infection

Natural killer T (NKT) cells are a unique T-cell population that is positively selected by CD1d-expressing cells. In this study, we examined the kinetics of conventional CD4+TCRbeta+ and CD4-TCRbeta+ cells along with various NKT cell populations from WT and CD1d KO mice after oral Listeria monocytogenes (Lm) infection at different time points in tissue compartments. We found that CD4+TCRbeta+ cells expressing NK1.1+ (NKT) were constitutively expressed in the lung of both strains of mice, but disappeared after infection. In contrast, CD4-TCRbeta+ NK1.1+ cells migrated to the spleen. Here, we demonstrated that endogenous IL-12 was predominantly expressed in the spleen of CD1d KO mice 2 days after infection, whereas IL-4 was predominantly expressed in the liver of WT mice. Higher levels of IFN-gamma were expressed in MLN of CD1d KO but not in WT mice on day 5. Thus, tissue-specific ligands orchestrate the localization and activation of NKT cells to control immune response to Listeria, which may explain the difference in disease susceptibility.

Induction of CD70 on dendritic cells through CD40 or TLR stimulation contributes to the development of CD8+ T cell responses in the absence of CD4+ T cells

The expansion of CD8(+) T cells in response to Ag can be characterized as either dependent or independent of CD4(+) T cells. The factors that influence this dichotomy are poorly understood but may be dependent upon the degree of inflammation associated with the Ag. Using dendritic cells derived from MHC class II-deficient mice to avoid interaction with CD4(+) T cells in vivo, we have compared the immunogenicity of peptide-pulsed dendritic cells stimulated with molecules associated with infection to those stimulated via CD40. In the absence of CD4(+) T cell help, the expansion of primary CD8(+) T cells after immunization with TNF-alpha- or poly(I:C)-stimulated dendritic cells was minimal. In comparison, LPS- or CpG-stimulated dendritic cells elicited substantial primary CD8(+) T cell responses, though not to the same magnitude generated by immunization with CD40L-stimulated dendritic cells. Remarkably, mice immunized with any stimulated dendritic cell population generated fully functional recall CD8(+) T cells without the aid of CD4(+) T cell help. The observed hierarchy of immunogenicity was closely correlated with the expression of CD70 (CD27L) on the stimulated dendritic cells, and Ab-mediated blockade of CD70 substantially prevented the CD4(+) T cell-independent expansion of primary CD8(+) T cells. These results indicate that the expression of CD70 on dendritic cells is an important determinant for helper-dependence of primary CD8(+) T cell expansion and provide an explanation for the ability of a variety of pathogens to stimulate primary CD8(+) T cell responses in the absence of CD4(+) T cells.

The CD4 molecule on CD8+ T lymphocytes directly enhances the immune response to viral and cellular antigens

CD8+ T lymphocytes play a major role in cellular-mediated immune responses to foreign antigen. We have previously demonstrated that costimulation of purified human CD8+ T cells induces de novo expression of the CD4 molecule and that ligation of CD4 on this cell type modulates CD8+ T cell activity in vitro. Herein, we investigate how the CD4 molecule expressed on murine CD8+ T cells contributes to CD8+ cell responses in vivo by employing adoptive transfer of CD8 cells from CD4 knockout mice into severe combined immunodeficient (SCID) recipients. Transfer of these cells into syngeneic SCID mice resulted in a decreased immune response to infection by lymphocytic choriomeningitis virus. These decreased responses occurred even in the presence of CD4+ T cells, indicating that this was truly a CD8-cell defect. Similarly, transfer of CD8+ T cells incapable of expressing CD4 into allogeneic SCID mice resulted in a decreased response to alloantigens compared with that of normal CD8+ T cells. Therefore, CD4 expression on CD8 T lymphocytes modulates cytotoxic T lymphocyte function and is critical in vivo for optimal cell-mediated immunity to viral and alloantigens.