T cell memory. Long-term persistence of virus-specific cytotoxic T cells

From concept to cure: The evolution of CAR-T cell therapy

Chimeric antigen receptor (CAR)-T cell therapy has revolutionized cancer immunotherapy in the 21st century, providing innovative solutions and life-saving therapies for previously untreatable diseases. This approach has shown remarkable success in treating various hematological malignancies and is now expanding into clinical trials for solid tumors, such as prostate cancer and glioblastoma, as well as infectious and autoimmune diseases. CAR-T cell therapy involves harvesting a patient's T cells, genetically engineering them with viral vectors to express CARs targeting specific antigens and reinfusing the modified cells into the patient. These CAR-T cells function independently of major histocompatibility complex (MHC) antigen presentation, selectively identifying and eliminating target cells. This review highlights the key milestones in CAR-T cell evolution, from its invention to its clinical applications. It outlines the historical timeline leading to the invention of CAR-T cells, discusses the major achievements that have transformed them into a breakthrough therapy, and addresses remaining challenges, including high manufacturing costs, limited accessibility, and toxicity issues such as cytokine release syndrome and immune effector cell-associated neurotoxicity syndrome. Additionally, the review explores future directions and advances in the field, such as developing next-generation CAR-T cells aiming to maximize efficacy, minimize toxicity, and broaden therapeutic applications.

A replicating LCMV-based vaccine for the treatment of solid tumors

Harnessing the immune system to eradicate tumors requires identification and targeting of tumor antigens, including tumor-specific neoantigens and tumor-associated self-antigens. Tumor-associated antigens are subject to existing immune tolerance, which must be overcome by immunotherapies. Despite many novel immunotherapies reaching clinical trials, inducing self-antigen-specific immune responses remains challenging. Here, we systematically investigate viral-vector-based cancer vaccines encoding a tumor-associated self-antigen (TRP2) for the treatment of established melanomas in preclinical mouse models, alone or in combination with adoptive T cell therapy. We reveal that, unlike foreign antigens, tumor-associated antigens require replication of lymphocytic choriomeningitis virus (LCMV)-based vectors to break tolerance and induce effective antigen-specific CD8+ T cell responses. Immunization with a replicating LCMV vector leads to complete tumor rejection when combined with adoptive TRP2-specific T cell transfer. Importantly, immunization with replicating vectors leads to extended antigen persistence in secondary lymphoid organs, resulting in efficient T cell priming, which renders previously "cold" tumors open to immune infiltration and reprograms the tumor microenvironment to "hot." Our findings have important implications for the design of next-generation immunotherapies targeting solid cancers utilizing viral vectors and adoptive cell transfer.

Survival and division fate programs are preserved but retuned during the naïve to memory CD8+ T-cell transition

Memory T cells are generated from naïve precursors undergoing proliferation during the initial immune response. Both naïve and memory T cells are maintained in a resting, quiescent state and respond to activation with a controlled proliferative burst and differentiation into effector cells. This similarity in the maintenance and response dynamics points to the preservation of key cellular fate programs; however, whether memory T cells have acquired intrinsic changes in these programs that may contribute to the enhanced immune protection in a recall response is not fully understood. Here we used a quantitative model-based analysis of proliferation and survival kinetics of in vitro-stimulated murine naïve and memory CD8+ T cells in response to homeostatic and activating signals to establish intrinsic similarities or differences within these cell types. We show that resting memory T cells display heightened sensitivity to homeostatic cytokines, responding to interleukin (IL)-2 in addition to IL-7 and IL-15. The proliferative response to αCD3 was equal in size and kinetics, demonstrating that memory T cells undergo the same controlled division burst and automated return to quiescence as naïve T cells. However, perhaps surprisingly, we observed reduced expansion of αCD3-stimulated memory T cells in response to activating signals αCD28 and IL-2 compared with naïve T cells. Overall, we demonstrate that although sensitivities to cytokine and costimulatory signals have shifted, fate programs regulating the scale of the division burst are conserved in memory T cells.

ABCB10 Loss Reduces CD4+ T Cell Activation and Memory Formation

T cells must shift their metabolism to respond to infections and tumors and to undergo memory formation. The ATP-binding cassette transporter ABCB10 localizes to the mitochondrial inner membrane, where it is thought to export a substrate important in heme biosynthesis and metabolism, but its role in T cell development and activation is unknown. In this article, we use a combination of methods to study the effect of ABCB10 loss in primary and malignantly transformed T cells. Although Abcb10 is dispensable for development of both CD4⁺ and CD8⁺ T cells, it is required for expression of specific cytokines in CD4⁺, but not CD8⁺, T cells activated in vitro. These defects in cytokine expression are magnified on repeated stimulation. In vivo, CD8⁺ cells lacking ABCB10 expand more in response to viral infection than their control counterparts, while CD4⁺ cells show reductions in both number and percentage. CD4⁺ cells lacking ABCB10 show impairment in Ag-specific memory formation and recall responses that become more severe with time. In malignant human CD4⁺ Jurkat T cells, we find that CRISPR-mediated ABCB10 disruption recapitulates the same cytokine expression defects upon activation as observed in primary mouse T cells. Mechanistically, ABCB10 deletion in Jurkat T cells disrupts the ability to switch to aerobic glycolysis upon activation. Cumulatively, these results show that ABCB10 is selectively required for specific cytokine responses and memory formation in CD4⁺ T cells, suggesting that targeting this molecule could be used to mitigate aberrant T cell activation.

Parallels Between the Antiviral State and the Irradiated State

Improved understanding of host antiviral defense and antitumor immunity have elucidated molecular pathways important to both processes. During viral infection, RNA or DNA in the host cell serves as a danger signal that initiates the antiviral response. Recent studies have elucidated similarities in the signaling pathways activated by viruses and the signaling pathways induced by tumor DNA that is released into the cytoplasm of irradiated tumor cells. Both the host defense to viral infection and the sterile inflammation provoked by radiotherapy induce a type I interferon response that is necessary for pathogen control and immune-mediated tumor control, respectively. These findings have led to the hypothesis that radiotherapy employs a form of viral mimicry.

Evaluation of antiviral T cell responses and TSCM cells in volunteers enrolled in a phase I HIV-1 subtype C prophylactic vaccine trial in India

T cells play an important role in controlling viral replication during HIV infection. An effective vaccine should, therefore, lead to the induction of a strong and early viral-specific CD8⁺ T cell response. While polyfunctional T cell responses are thought to be important contributors to the antiviral response, there is evidence to show that polyfunctional HIV- specific CD8⁺ T cells are just a small fraction of the total HIV-specific CD8⁺ T cells and may be absent in many individuals who control HIV replication, suggesting that other HIV-1 specific CD8⁺ effector T cell subsets may be key players in HIV control. Stem cell-like memory T cells (T_SCM) are a subset of T cells with a long half-life and self-renewal capacity. They serve as key reservoirs for HIV and contribute a significant barrier to HIV eradication. The present study evaluated vaccine-induced antiviral responses and T_SCM cells in volunteers vaccinated with a subtype C prophylactic HIV-1 vaccine candidate administered in a prime-boost regimen. We found that ADVAX DNA prime followed by MVA boost induced significantly more peripheral CD8⁺ T_SCM cells and higher levels of CD8⁺ T cell-mediated inhibition of replication of different HIV-1 clades as compared to MVA alone and placebo. These findings are novel and provide encouraging evidence to demonstrate the induction of T_SCM and cytotoxic immune responses by a subtype C HIV-1 prophylactic vaccine administered using a prime-boost strategy.

Dendritic Cell Regulation of Graft-Vs.-Host Disease: Immunostimulation and Tolerance

Graft-vs.-host disease (GVHD) remains a significant cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Significant progresses have been made in defining the dichotomous role of dendritic cells (DCs) in the development of GVHD. Host-derived DCs are important to elicit allogeneic T cell responses, whereas certain donor-types of DCs derived from newly engrafted hematopoietic stem/progenitor cells (HSPCs) can amply this graft-vs.-host reaction. In contrast, some DCs also play non-redundant roles in mediating immune tolerance. They induce apoptotic deletion of host-reactive donor T cells while promoting expansion and function of regulatory T cells (Treg). Unfortunately, this tolerogenic effect of DCs is impaired during GVHD. Severe GVHD in patients subject to allo-HSCT is associated with significantly decreased number of circulating peripheral blood DCs during engraftment. Existing studies reveal that GVHD causes delayed reconstitution of donor DCs from engrafted HSPCs, impairs the antigen presentation function of newly generated DCs and reduces the capacity of DCs to regulate Treg. The present review will discuss the importance of DCs in alloimmunity and the mechanism underlying DC reconstitution after allo-HSCT.

The Vast Universe of T Cell Diversity: Subsets of Memory Cells and Their Differentiation

The T cell receptor confers specificity for antigen recognition to T cells. By the first encounter with the cognate antigen, reactive T cells initiate a program of expansion and differentiation that will define not only the ultimate quantity of specific cells that will be generated, but more importantly their quality and functional heterogeneity. Recent achievements using mouse model infection systems have helped to shed light into the complex network of factors that dictate and sustain memory T cell differentiation, ranging from antigen load, TCR signal strength, metabolic fitness, transcriptional programs, and proliferative potential. The different models of memory T cell differentiation are discussed in this chapter, and key phenotypic and functional attributes of memory T cell subsets are presented, both for mouse and human cells. Therapeutic manipulation of memory T cell generation is expected to provide novel unique ways to optimize current immunotherapies, both in infection and cancer.

Defining B cell immunodominance to viruses

Immunodominance (ID) defines the hierarchical immune response to competing antigens in complex immunogens. Little is known regarding B cell and antibody ID despite its importance in immunity to viruses and other pathogens. We show that B cells and serum antibodies from inbred mice demonstrate a reproducible ID hierarchy to the five major antigenic sites in the influenza A virus hemagglutinin globular domain. The hierarchy changed as the immune response progressed, and it was dependent on antigen formulation and delivery. Passive antibody transfer and sequential infection experiments demonstrated 'original antigenic suppression', a phenomenon in which antibodies suppress memory responses to the priming antigenic site. Our study provides a template for attaining deeper understanding of antibody ID to viruses and other complex immunogens.

Adoptive immunotherapy for hematological malignancies using T cells gene-modified to express tumor antigen-specific receptors

Accumulating clinical evidence suggests that adoptive T-cell immunotherapy could be a promising option for control of cancer; evident examples include the graft-vs-leukemia effect mediated by donor lymphocyte infusion (DLI) and therapeutic infusion of ex vivo-expanded tumor-infiltrating lymphocytes (TIL) for melanoma. Currently, along with advances in synthetic immunology, gene-modified T cells retargeted to defined tumor antigens have been introduced as “cellular drugs”. As the functional properties of the adoptive immune response mediated by T lymphocytes are decisively regulated by their T-cell receptors (TCRs), transfer of genes encoding target antigen-specific receptors should enable polyclonal T cells to be uniformly redirected toward cancer cells. Clinically, anticancer adoptive immunotherapy using genetically engineered T cells has an impressive track record. Notable examples include the dramatic benefit of chimeric antigen receptor (CAR) gene-modified T cells redirected towards CD19 in patients with B-cell malignancy, and the encouraging results obtained with TCR gene-modified T cells redirected towards NY-ESO-1, a cancer-testis antigen, in patients with advanced melanoma and synovial cell sarcoma. This article overviews the current status of this treatment option, and discusses challenging issues that still restrain the full effectiveness of this strategy, especially in the context of hematological malignancy.

Estimating the diversity, completeness, and cross-reactivity of the T cell repertoire

In order to recognize and combat a diverse array of pathogens the immune system has a large repertoire of T cells having unique T cell receptors (TCRs) with only a few clones specific for any given antigen. We discuss how the number of different possible TCRs encoded in the genome (the potential repertoire) and the number of different TCRs present in an individual (the realized repertoire) can be measured. One puzzle is that the potential repertoire greatly exceeds the realized diversity of naïve T cells within any individual. We show that the existing hypotheses fail to explain why the immune system has the potential to generate far more diversity than is used in an individual, and propose an alternative hypothesis of “evolutionary sloppiness.” Another immunological puzzle is why mice and humans have similar repertoires even though humans have over 1000-fold more T cells. We discuss how the idea of the “protecton,” the smallest unit of protection, might explain this discrepancy and estimate the size of “protecton” based on available precursor frequencies data. We then consider T cell cross-reactivity – the ability of a T cell clone to respond to more than one epitope. We extend existing calculations to estimate the extent of expected cross-reactivity between the responses to different pathogens. Our results are consistent with two observations: a low probability of observing cross-reactivity between the immune responses to two randomly chosen pathogens; and the ensemble of memory cells being sufficiently diverse to generate cross-reactive responses to new pathogens.

Persistent enteric murine norovirus infection is associated with functionally suboptimal virus-specific CD8 T cell responses

Norovirus (NV) gastroenteritis is a major contributor to global morbidity and mortality, yet little is known about immune mechanisms leading to NV control. Previous studies using the murine norovirus (MNV) model have established a key role for T cells in MNV clearance. Despite these advances, important questions remain regarding the magnitude, location, and dynamics of the MNV-specific T cell response. To address these questions, we identified MNV-specific major histocompatibility complex (MHC) class I immunodominant epitopes using an overlapping peptide screen. One of these epitopes (amino acids 519 to 527 of open reading frame 2 [ORF2(519-527)]) was highly conserved among all NV genogroups. Using MHC class I peptide tetramers, we tracked MNV-specific CD8 T cells in lymphoid and mucosal sites during infection with two MNV strains with distinct biological behaviors, the acutely cleared strain CW3 and the persistent strain CR6. Here, we show that enteric MNV infection elicited robust T cell responses primarily in the intestinal mucosa and that MNV-specific CD8 T cells dynamically regulated the expression of surface molecules associated with activation, differentiation, and homing. Furthermore, compared to MNV-CW3 infection, chronic infection with MNV-CR6 resulted in fewer and less-functional CD8 T cells, and this difference was evident as early as day 8 postinfection. Finally, MNV-specific CD8 T cells were capable of reducing the viral load in persistently infected Rag1(-/-) mice, suggesting that these cells are a crucial component of NV immunity. Collectively, these data provide fundamental new insights into the adaptive immune response to two closely related NV strains with distinct biological behaviors and bring us closer to understanding the correlates of protective antiviral immunity in the intestine.

Memory T cell subsets, migration patterns, and tissue residence

Tissues such as the skin and mucosae are frequently exposed to microbial pathogens. Infectious agents must be quickly and efficiently controlled by our immune system, but the low frequency of naive T cells specific for any one pathogen means dependence on primary responses initiated in draining lymph nodes, often allowing time for serious infection to develop. These responses imprint effectors with the capacity to home to infected tissues; this process, combined with inflammatory signals, ensures the effective targeting of primary immunity. Upon vaccination or previous pathogen exposure, increased pathogen-specific T cell numbers together with altered migratory patterns of memory T cells can greatly improve immune efficacy, ensuring infections are prevented or at least remain subclinical. Until recently, memory T cell populations were considered to comprise central memory T cells (TCM), which are restricted to the secondary lymphoid tissues and blood, and effector memory T cells (TEM), which broadly migrate between peripheral tissues, the blood, and the spleen. Here we review evidence for these two memory populations, highlight a relatively new player, the tissue-resident memory T cell (TRM), and emphasize the potential differences between the migratory patterns of CD4(+) and CD8(+) T cells. This new understanding raises important considerations for vaccine design and for the measurement of immune parameters critical to the control of infectious disease, autoimmunity, and cancer.

The LCMV gp33-specific memory T cell repertoire narrows with age

Background

The memory response to LCMV in mice persists for months to years with only a small decrease in the number of epitope specific CD8 T cells. This long persistence is associated with resistance to lethal LCMV disease. In contrast to studies focused on the number and surface phenotype of the memory cells, relatively little attention has been paid to the diversity of TCR usage in these cells. CD8⁺ T cell responses with only a few clones of identical specificity are believed to be relatively ineffective, presumably due to the relative ease of virus escape. Thus, a broad polyclonal response is associated with an effective anti-viral CD8⁺ T cell response.

Results

In this paper we show that the primary CD8⁺ T cell response to the LCMV gp33-41 epitope is extremely diverse. Over time while the response remains robust in terms of the number of gp33-tetramer⁺ T cells, the diversity of the response becomes less so. Strikingly, by 26 months after infection the response is dominated by a small number TCRβ sequences. In addition, it is of note the gp33 specific CD8⁺ T cells sorted by high and low tetramer binding populations 15 and 22 months after infection. High and low tetramer binding cells had equivalent diversity and were dominated by a small number of clones regardless of the time tested. A similar restricted distribution was seen in NP396 specific CD8⁺ T cells 26 months after infection. The identical TCRVβ sequences were found in both the tetramer^hi and tetramer^lo binding populations. Finally, we saw no evidence of public clones in the gp33-specific response. No CDR3 sequences were found in more than one mouse.

Conclusions

These data show that following LCMV infection the CD8⁺ gp33-specific CD8 T cell response becomes highly restricted with enormous narrowing of the diversity. This narrowing of the repertoire could contribute to the progressively ineffective immune response seen in aging.

Memory type 2 helper T cells induce long-lasting antitumor immunity by activating natural killer cells

Functionally polarized helper T cells (Th cells) play crucial roles in the induction of tumor immunity. There is considerable knowledge about the contributions of IFN-producing Th1 cells that supports the role of cytotoxic cluster of differentiation (CD8) T cells and natural killer (NK) cells, but much less is known about how IL-4-producing Th2 cells contribute to tumor immunity. In this study, we investigated the cellular and molecular mechanisms employed by memory Th2 cells in sustaining tumor immunity by using a mouse model system wherein ovalbumin (OVA) is used as a specific tumor antigen. In this model, we found that OVA-specific memory Th2 cells exerted potent and long-lasting antitumor effects against NK-sensitive OVA-expressing tumor cells, wherein antitumor effects were mediated by NK cells. Specifically, NK cell cytotoxic activity and expression of perforin and granzyme B were dramatically enhanced by the activation of memory Th2 cells. Interleukin 4 (IL-4) produced by memory Th2 cells in vivo was critical for the antitumor effects of the NK cells, which IL-4 directly stimulated to induce their perforin- and granzyme-B-dependent cytotoxic activity. Our findings show that memory Th2 cells can induce potent antitumor immunity through IL-4-induced activation of NK cells, suggesting potential applications in cellular therapy for cancer patients.

Immunization delivered by lentiviral vectors for cancer and infectious diseases

The increasing level of understanding of the lentivirus biology has been instrumental in shaping the design strategy of creating therapeutic lentiviral delivery vectors. As a result, lentiviral vectors have become one of the most powerful gene transfer vehicles. They are widely used for therapeutic purposes as well as in studies of basic biology, due to their unique characteristics. Lentiviral vectors have been successfully employed to mediate durable and efficient antigen expression and presentation in dendritic cells both in vitro and in vivo, leading to the activation of cellular immunity and humoral responses. This capability makes the lentiviral vector an ideal choice for immunizations that target a wide range of cancers and infectious diseases. Further advances into optimizing the vector system and understanding the relationship between the immune system and diseases pathogenesis will only augment the potential benefits and utility of lentiviral vaccines for human health.

Homeostatic turnover of virus-specific memory CD8 T cells occurs stochastically and is independent of CD4 T cell help

Memory CD8 T cells persist by Ag-independent homeostatic proliferation. To examine the dynamics of this cell turnover, we transferred lymphocytic choriomeningitis virus specific memory CD8 T cells into naive mice and analyzed their in vivo division kinetics longitudinally in individual recipients.Using mathematical modeling, we determined that proliferation of this stably maintained memory CD8 T cell population was homogeneous and stochastic with a small fraction of cells completing division at any given time with an intermitotic interval of 50 d. This homeostatic turnover was comparable between memory CD8 T cells of different viral epitope specificities and also the total memory phenotype (CD44(high)) CD8 T cells. It is well established that CD4 T cell help is critical for maintenance of CD8 T cells during chronic infections, but recent studies have suggested that CD4 T cell help is also required for maintenance of memory CD8 T cells following acute infections. Hence, we assessed the role of CD4 T cells in Ag-independent maintenance of memory CD8 T cells. Consistent with previous reports, we found that memory CD8 T cells declined when transferred into MHC class II-deficient mice. However, their numbers were maintained stably when transferred into CD4 T cell-deficient mice. Interestingly, their homeostatic proliferation, ability to make recall responses, and phenotype were independent of CD4 T cell help because none of these qualities were affected when memory CD8 T cells were transferred and maintained in either MHC class II- or CD4-deficient recipients.

Ratio of HCV structural antigens in protein-based vaccine formulations is critical for functional immune response induction

HCV (hepatitis C virus) infection is among the leading causes of chronic liver disease, but currently there is no vaccine available. Data have accumulated about the importance of targeting different HCV antigens in vaccine candidate preparations. Here, a surface response study to select the optimal ratio of recombinant HCV structural antigens in a vaccine preparation, capable of generating in vivo functional cellular immune response in mice, was performed. The immunogenicity of the selected HCV structural protein mixture (Co-E1-E2) in mice and African green monkeys, after five doses of immunization, was also demonstrated. Specific T-cell proliferative response against HCV structural antigens was induced in vaccinated mice. Moreover, on challenge with recombinant HCV VV (vaccinia virus), all mice controlled the viraemia and 80% were protected. On the other hand, monkeys immunized with Co-E1-E2 developed antibodies, specifically directed to region 412-438 of E2 protein, that include an epitope implicated in HCV neutralization, in addition to a specific proliferative response against HCV Core and E2 proteins. These results indicated that the optimal amount and ratio of HCV recombinant proteins should be taken into account to elicit a successful immune response against HCV and therefore have important implications for vaccine design.

Viral vaccines and CTL response

Immune induction by successful vaccine formulations seems to involve stimulation of both humoral and cellular arms of immunity. Nevertheless, CD8+ CTLs are of critical relevance in the context of intracellular infection and tumor for many reasons. The task of exerting antipathogen activity by CD8+ T cells, which principally function to control and eradicate intracellular pathogens, is enabled by constitutive expression of MHC class-I molecules on all tissue types. CTL induction offers hope for vaccines against pathogens that are resistant to neutralizing activity. This review discusses the mechanism of immune induction by some successful vaccines and based on the accrued evidence suggests ideas for improved design of CTL-inducing vaccines.

T cell responses to mycobacterial catalase-peroxidase profile a pathogenic antigen in systemic sarcoidosis

Sarcoidosis is a systemic granulomatous disease associated with local epithelioid granulomas, CD4(+) T cells, and Th1 cytokines. The tissue Ags that drive this granulomatous inflammation are uncertain. In this study, we used IFN-gamma-ELISPOT assays and flow cytometry to assess lung and blood T cell responses to the candidate pathogenic Ag, Mycobacterium tuberculosis catalase-peroxidase (mKatG) in patients with sarcoidosis from two centers. Despite differences in patient phenotypic, genetic, and prognostic characteristics, we report that T cell responses to mKatG were remarkably similar in these cohorts, with higher frequencies of mKatG-reactive, IFN-gamma-expressing T cells in the blood of sarcoidosis patients compared with nontuberculosis sensitized healthy controls, and (in a subset) in greater numbers than T cells reactive to purified protein derivative. In sarcoidosis, mKatG-reactive CD4(+) Th1 cells preferentially accumulated in the lung, indicating a compartmentalized response. Patients with or without Löfgren syndrome had similar frequencies of mKatG specific IFN-gamma-expressing blood T cells. Circulating mKatG-reactive T cells were found in chronic active sarcoidosis but not in patients with inactive disease. Together, these results demonstrate that T cell responses to mKatG in sarcoidosis fit a profile expected for a pathogenic Ag, supporting an immunotherapeutic approach to this disease.

CD28 and Cd27 Costimulation of Cd8+ T Cells: A Story of Survival

Although the requirement of CD28 and CD27 costimulation has been clearly demonstrated during primary CD8+ T cell responses and this costimulation acts by providing proliferation and survival cues to naive CD8+ T cells, a number of questions also arise from these studies. Is the requirement for CD28 and CD27 costimulation restricted to the initiation of the immune response in the lymph nodes, where presumably the initial contact between naive CD8+ T cell and DC occurs? What is the purpose of the dramatic influx of DC to sites of inflammation such as the lung during influenza virus infection and the formation of inflammatory BALT (iBALT)?(104) Are such DC at the site of inflammation and at later stages of the immune response providing cytokines or costimulation to effector CD8+ T cells? If DC are required for optimal secondary responses (100), is CD28 costimulation the missing signal or is it other members of the B7:CD28 family or TNF family? Given that a number of investigators are actively addressing these questions, the answers we expect will be soon to come and open exciting new opportunities for immune enhancement or dampening strategies and vaccine adjuvants.

A quantitative analysis of the variables affecting the repertoire of T cell specificities recognized after vaccinia virus infection

Many components contribute to immunodominance in the response to a complex virus, but their relative importance is unclear. This was addressed using vaccinia virus and HLA-A*0201 as the model system. A comprehensive analysis of 18 viral proteins recognized by CD8(+) T cell responses demonstrated that approximately one-fortieth of all possible 9- to 10-mer peptides were high-affinity HLA-A*0201 binders. Peptide immunization and T cell recognition data generated from 90 peptides indicated that about one-half of the binders were capable of eliciting T cell responses, and that one-seventh of immunogenic peptides are generated by natural processing. Based on these results, we estimate that vaccinia virus encodes approximately 150 dominant and subdominant epitopes restricted in by HLA-A*0201. However, of all these potential epitopes, only 15 are immunodominant and actually recognized in vivo during vaccinia virus infection of HLA-A*0201 transgenic mice. Neither peptide-binding affinity, nor complex stability, nor TCR avidity, nor amount of processed epitope appeared to strictly correlate with immunodominance status. Additional experiments suggested that vaccinia infection impairs the development of responses directed against subdominant epitopes. This suggested that additional factors, including immunoregulatory mechanisms, restrict the repertoire of T cell specificities after vaccinia infection by a factor of at least 10.

Principles of adoptive T cell cancer therapy

The transfusion of T cells, also called adoptive T cell therapy, is an effective treatment for viral infections and has induced regression of cancer in early-stage clinical trials. However, recent advances in cellular immunology and tumor biology are guiding new approaches to adoptive T cell therapy. For example, use of engineered T cells is being tested as a strategy to improve the functions of effector and memory T cells, and manipulation of the host to overcome immunotoxic effects in the tumor microenvironment has led to promising results in early-stage clinical trials. Challenges that face the field and must be addressed before adoptive T cell therapy can be translated into routine clinical practice are discussed.

Molecular mimicry in multiple sclerosis

One of the most common demyelinating central nervous system (CNS) diseases in humans is multiple sclerosis (MS). The disease can be very debilitating with vision loss, motor and sensory disturbances, and cognitive impairment. The clinical course may present as a relapsing-remitting disease course, a progressive disease course, or a combination thereof. The etiology of MS is unknown. Though many viruses have been shown to be associated with MS, no one virus has ever been demonstrated to be the cause of MS. In addition, MS is thought to have an autoimmune component. Molecular mimicry is one hypothesis put forth which could reconcile the diverse pathology and etiology of MS. Molecular mimicry occurs when peptides from pathogens share sequence or structural similarities with self-antigens. Infection with various pathogens, each with its individual molecular mimic to a CNS antigen, may explain the inability of investigators to link one specific virus to MS. Molecular mimicry may be mediated through human leukocyte antigen class I- and class II-restricted T cells and antibodies, which may explain the diversity in phenotype. Aspects of molecular mimicry will be discussed in relation to each of these immune system components. Examples of various molecular mimics will be discussed with a particular focus on the CNS and MS. Molecular mimicry alone may not be able to induce disease; priming of the immune system by infection with a pathogen that carries a molecular mimic to self may have to be followed by a later nonspecific immunologic challenge in order for disease to be initiated. Recent research into this priming and triggering of disease will be discussed in relation to an animal model for MS.

Confronting complexity: real-world immunodominance in antiviral CD8+ T cell responses

Antiviral CD8(+) T cells respond to only a minute fraction of the potential peptide determinants encoded by viral genomes. Immunogenic determinants can be ordered into highly reproducible hierarchies based on the magnitude of cognate CD8(+) T cell responses. Until recently, this phenomenon, termed immunodominance, was largely defined and characterized in model systems utilizing a few strains of inbred mice infected with a handful of viruses with limited coding capacity. Here, I review work that has extended immunodominance studies to viruses of greater complexity and to the real world of human antiviral immunity.

Protective 'immunity' by pre-existent neutralizing antibody titers and preactivated T cells but not by so-called 'immunological memory'

The idea of immunological memory originally arose from the observation that survivors of infections were subsequently resistant to disease caused by the same infection. While most immunologists accept a special 'remembering' memory quality, we have argued previously and document here that increased resistance against re-infection, i.e. immunity, reflects low-level antigen-driven T- and B-cell responses, resulting in elevated serum or mucosal titers of protective antibodies or of activated T cells, respectively. Periodic antigen re-exposure is from within, by persisting infection (long-term) or by immune complexes (short-term), or from without, by low-level re-infections. This simple concept is supported by clinical evidence and model experiments but is often ignored, although this concept, but not so-called 'immunological memory', as defined in textbooks (i.e. earlier and better responses of a primed host), is compatible with evolutionary maternal antibody transfer of protection as well as immunity against existing infections. The concept of 'immunity without immunological remembering memory' explains why it is easy to generate vaccines against acute cytopathic infections, particularly those of early childhood, where neutralizing antibodies are the key to protection, because it has been validated by adoptive transfer of maternal antibodies. It also explains why we have not succeeded (yet?) to generate truly protective vaccines against persisting infections, because we cannot imitate 'infection immunity' that is long-lasting, generating protective T- and B-cell stimulation against variable infections without causing disease by either immunopathology or tolerance.

Not all CD4 + memory T cells are long lived

Memory T cells are thought to have several properties that distinguish them from their naïve precursors. They are found in parts of the body that rarely house naïve cells, they respond to antigen with faster proliferation and more rapid progression to effector function, they are less sensitive to the absence of their selecting major histocompatibility complex (MHC), and, above all, they are long lived. Here we show that this last property may not be universal. Some CD4+ T cells that have surface proteins characteristic of memory cells have the same half-life in vivo as naïve cells. The description of these cells as memory cells therefore depends on our definition of the word 'memory'.

Immunodominance and Immunodomination: Critical Factors in Developing Effective CD8+ T‐Cell–Based Cancer Vaccines

The focusing of cellular immunity toward one, or just a few, antigenic determinant, even during immune responses to complex microorganisms or antigens, is known as immunodominance. Although described in many systems, the mechanisms of determinant immunodominance are only just beginning to be appreciated, especially in relation to the interplay between T cells of differing specificities and the interactions between T cells and the antigen-presenting cells (APCs). The outcome of these cellular interactions can lead to a form of immune suppression of one specificity by another-described as "immunodomination". The specific and detailed mechanisms involved in this process are now partly defined. A full understanding of all the factors that control immunodominance and influence immunodomination will help us to develop better viral and cancer vaccines.

Persistent infection with ebola virus under conditions of partial immunity

Ebola hemorrhagic fever in humans is associated with high mortality; however, some infected hosts clear the virus and recover. The mechanisms by which this occurs and the correlates of protective immunity are not well defined. Using a mouse model, we determined the role of the immune system in clearance of and protection against Ebola virus. All CD8 T-cell-deficient mice succumbed to subcutaneous infection and had high viral antigen titers in tissues, whereas mice deficient in B cells or CD4 T cells cleared infection and survived, suggesting that CD8 T cells, independent of CD4 T cells and antibodies, are critical to protection against subcutaneous Ebola virus infection. B-cell-deficient mice that survived the primary subcutaneous infection (vaccinated mice) transiently depleted or not depleted of CD4 T cells also survived lethal intraperitoneal rechallenge for >/==" BORDER="0">25 days. However, all vaccinated B-cell-deficient mice depleted of CD8 T cells had high viral antigen titers in tissues following intraperitoneal rechallenge and died within 6 days, suggesting that memory CD8 T cells by themselves can protect mice from early death. Surprisingly, vaccinated B-cell-deficient mice, after initially clearing the infection, were found to have viral antigens in tissues later (day 120 to 150 post-intraperitoneal infection). Furthermore, following intraperitoneal rechallenge, vaccinated B-cell-deficient mice that were transiently depleted of CD4 T cells had high levels of viral antigen in tissues earlier (days 50 to 70) than vaccinated undepleted mice. This demonstrates that under certain immunodeficiency conditions, Ebola virus can persist and that loss of primed CD4 T cells accelerates the course of persistent infections. These data show that CD8 T cells play an important role in protection against acute disease, while both CD4 T cells and antibodies are required for long-term protection, and they provide evidence of persistent infection by Ebola virus suggesting that under certain conditions of immunodeficiency a host can harbor virus for prolonged periods, potentially acting as a reservoir.

Persistent alterations in the T-cell repertoires of HIV-1-infected and at-risk uninfected men

OBJECTIVE

We examined the association between immunogenic exposure and T-cell receptor (TCR) diversity to more clearly assess the impact of HIV-1 infection on the T-cell repertoire.

METHODS

: To estimate the extent of T-cell clonality attributable to HIV-1 infection, we evaluated T-cell repertoires in low-risk and at-risk seronegative men and HIV-1 seropositive men by assessment of T-cell receptor beta-chain (TCR beta) complimentary determining region 3 (CDR3) lengths.

RESULTS

The frequency of T-cell clonality in both HIV-1 infected and at-risk uninfected men was elevated in comparison to low-risk uninfected men. Among low-risk and at-risk seronegative, and HIV-1 seropositive men, clonal expansions were present in 3, 8, and 10% of CD4+ CDR3 lengths, and 18, 22, and 28% of CD8+ CDR3 lengths respectively. In addition, the longitudinal conservation of clonal expansions was observed in at-risk seronegative men. Based on comparisons to at-risk seronegative men, we estimate that at-risk seropositive men with chronic HIV-1 infection exhibit a 27% increase in the number of expanded CD8+ CDR3 lengths.

CONCLUSION

These findings provide an approximation of the magnitude of the T-cell response in individuals undergoing chronic HIV-1 infection and demonstrate a significant association between the history of immunogenic challenge and the magnitude of clonality within the T-cell repertoire. In addition, these findings underscore the necessity of selecting controls with similar antigenic exposure histories when investigating T-cell dynamics in HIV-infected individuals.

Effector HIV-specific cytotoxic T-lymphocyte activity in long-term nonprogressors: associations with viral replication and progression

Ex vivo effector cytotoxic T-lymphocyte (CTL) activity was assessed in 27 members of the Australian Long-Term Nonprogressor cohort and correlated with genetic, virological, and immunological markers. The 27 individuals were antiretroviral naive with CD4(+) T-cell counts of >500 cells/ microl for more than 8 years after human immunodeficiency virus type 1 (HIV-1) infection. Effector CTL activity was determined using a standard ex vivo chromium release assay. Individuals with CTL activity (HIV-1 env(IIIB) or pol or gag) were then compared to those without CTL activity in relation to plasma HIV-1 RNA, ICD p24 antigen, beta(2)-microglobulin, CD4 and CD8 T-cell counts, CCR5 and CCR2b genotypes, and progression to CD4 <500 cells/microl or commencement of antiretroviral treatment. Of the 27 individuals examined, 19 had no detectable effector CTL activity. The eight individuals with detectable CTL activity had significantly higher plasma levels of HIV-1 RNA (P = 0.014), immune complex dissociated p24 antigen (P = 0.006), and beta(2)-microglobulin (P = 0.009). There was increased risk of progression within 4 years of study entry in individuals with detectable effector CTL activity, higher plasma levels of HIV-1 RNA, higher beta(2)-microglobulin levels, and higher immune complex dissociated p24 antigen levels at enrollment (P = 0.017, P = 0.004, P = 0.027, P = 0.008 respectively). Multivariate analysis demonstrated viral load remained the strongest predictor of disease progression within this group (P = 0.017). There were no significant associations between CTL response and chemokine receptor genotype. These findings demonstrate the importance of HIV replication in generating an effector CTL response and show that effector CTL activity may be an early predictor of progression in people with long-term asymptomatic HIV infection.

On differences between immunity and immunological memory

The evolutionary benefits of immunological memory are important: whereas antibodies can be transmitted to offspring by their mother and thereby benefit the species, T cell memory may function to help the individual combat persistent infection in peripheral tissues. Although experimental immunological memory is largely maintained antigen-independently, protective immunity is antigen-dependent.

Generation of functional antigen-specific T cells in defined genetic backgrounds by retrovirus-mediated expression of TCR cDNAs in hematopoietic precursor cells

We have developed an alternative to transgenesis for producing antigen-specific T cells in vivo. In this system, clonal naive T cells with defined antigen specificity are generated by retrovirus-mediated expression of T cell antigen receptor cDNAs in RAG1-deficient murine hematopoietic precursor cells. These T cells can be stimulated to proliferate and produce cytokines by exposure to antigen in vitro, and they become activated and expand in vivo after immunization. IL-2-deficient T cells generated by this technique show decreased proliferation and cytokine production, both of which can be rescued by exogenous addition of this growth factor. Thus, retrovirus-mediated expression of T cell antigen receptor cDNAs in hematopoietic precursor cells permits the rapid and efficient analysis of the life history of antigen-specific T cells in different genetic backgrounds and may allow for the long-term production of antigen-specific T cells with different functional properties for prophylactic and therapeutic purposes.

The quest for an AIDS vaccine: is the CD8+ T-cell approach feasible?

The rationale for developing anti-HIV vaccines that stimulate cytotoxic T-lymphocyte responses is given. We argue that such vaccines will work, provided that attention is paid to the development of memory T-cell responses that are strong and preferably activated. Furthermore, the vaccine should match the prevailing virus clade as closely as possible. Vaccines will have to stimulate a wide range of responses, but it is not clear how this can be achieved.

HIV-1 Vpr does not inhibit CTL-mediated apoptosis of HIV-1 infected cells

HIV-1 infected persons develop a robust CTL response to HIV antigens, yet HIV-1 is able to evade this host response and successfully replicate. The mechanism(s) of evasion is not completely defined but has been suggested to include resistance of infected cells to CTL-mediated apoptosis. The HIV-1 Vpr protein induces G2 arrest by indirectly inhibiting activation of cyclin B/p34cdc2 kinase. Granzyme B, the principle mediator of CTL-induced apoptosis, prematurely activates this same kinase complex. Therefore, we assessed the susceptibility of HIV-1 infected cells to CTL-mediated apoptosis to determine whether the expression of Vpr protected the infected cells from CTL-induced apoptosis. Antigen-specific CD8(+) CTL were able to induce apoptosis in HIV-1 infected cells and cells labeled with peptide corresponding to the CTL epitope with equivalent efficiency. This demonstrates that neither HIV-1 Vpr nor any other HIV protein directly inhibits CTL effector functions. Furthermore, we confirm that HIV-1 Nef is able to provide partial protection from CTL recognition of infected cells. Thus, the inability of CTL to control HIV-1 infection is likely not due to direct inhibition of CTL-mediated apoptosis.

Genetic immunization of wild-type and hepatitis C virus transgenic mice reveals a hierarchy of cellular immune response and tolerance induction against hepatitis C virus structural proteins

To study the effect of genetic immunization on transgenic expression of hepatitis C virus (HCV) proteins, we evaluated the immunological response of HCV transgenic mice to HCV expression plasmids. FVB/n transgenic mice expressing HCV structural proteins (core, E1, and E2) and wild-type (WT) FVB/n mice were immunized intramuscularly with plasmids expressing core (pHCVcore) or core/E1/E2 (pHCVSt). After immunization, HCV-specific humoral and cellular immune response was studied. Both WT and transgenic mice immunized with either HCV construct produced antibodies and exhibited T-cell proliferative responses against core or envelope. In WT mice immunized with pHCVSt, cytotoxic T-lymphocyte (CTL) activities were detected against E2 but not against core or E1, whereas strong CTL activities against core could be detected in WT mice immunized with pHCVcore. In pHCVSt-immunized, transgenic mice, CTL activities against the core or envelope were completely absent, but core-specific CTL activities could be detected in pHCVcore-immunized transgenic mice. A similar pattern of immune responses was also observed in other mouse strains, including a transgenic line expressing human HLA-A2.1 molecules (AAD mice). Despite the presence of a peripheral cellular immunity against HCV, no liver pathology or lymphocytic infiltrate was observed in these transgenic mice. Our study suggests a hierarchy of CTL response against the HCV structural proteins (E2 > core > E1) in vivo when the proteins are expressed as a polyprotein. The HCV transgenic mice can be induced by DNA immunization to generate anti-HCV antibodies and anticore CTLs. However, they are tolerant at the CTL level against the E2 protein despite DNA immunization.

Protective long-term antibody memory by antigen-driven and T help-dependent differentiation of long-lived memory B cells to short-lived plasma cells independent of secondary lymphoid organs

Memory is a hallmark of immunity. Memory carried by antibodies is largely responsible for protection against reinfection with most known acutely lethal infectious agents and is the basis for most clinically successful vaccines. However, the nature of long-term B cell and antibody memory is still unclear. B cell memory was studied here after infection of mice with the rabies-like cytopathic vesicular stomatitis virus, the noncytopathic lymphocytic choriomeningitis virus (Armstrong and WE), and after immunization with various inert viral antigens inducing naive B cells to differentiate either to plasma cells or memory B cells in germinal centers of secondary lymphoid organs. The results show that in contrast to very low background levels against internal viral antigens, no significant neutralizing antibody memory was observed in the absence of antigen and suggest that memory B cells (i) are long-lived in the absence of antigen, nondividing, and relatively resistant to irradiation, and (ii) must be stimulated by antigen to differentiate to short-lived antibody-secreting plasma cells, a process that is also efficient in the bone marrow and always depends on radiosensitive, specific T help. Therefore, for vaccines to induce long-term protective antibody titers, they need to repeatedly provide, or continuously maintain, antigen in minimal quantities over a prolonged time period in secondary lymphoid organs or the bone marrow for sufficient numbers of long-lived memory B cells to mature to short-lived plasma cells.

T cells compete for access to antigen-bearing antigen-presenting cells

These studies tested whether antigenic competition between T cells occurs. We generated CD8(+) T cell responses in H-2(b) mice against the dominant ovalbumin epitope SIINFEKL (ova8) and subdominant epitope KRVVFDKL, using either vaccinia virus expressing ovalbumin (VV-ova) or peptide-pulsed dendritic cells. CD8(+) T cell responses were visualized by major histocompatibility complex class I-peptide tetrameric molecules. Transfer of transgenic T cells with high affinity for ova8 (OT1 T cells) completely inhibited the response of host antigen-specific T cells to either antigen, demonstrating that T cells can directly compete with each other for response to antigen. OT1 cells also inhibited CD8(+) T cell responses to an unrelated peptide, SIYRYGGL, providing it was presented on the same dendritic cells as ova8. These inhibitions were not due to a more rapid clearance of virus or antigen-presenting cells (APCs) by the OT1 cells. Rather, the inhibition was caused by competition for antigen and antigen-bearing cells, since it could be overcome by the injection of large numbers of antigen-pulsed dendritic cells. These results imply that common properties of T cell responses, such as epitope dominance and secondary response affinity maturation, are the result of competitive interactions between antigen-bearing APC and T cell subsets.

Expression of intracellular IFN-gamma in HSV-1-specific CD8+ T cells identifies distinct responding subpopulations during the primary response to infection

Cutaneous infection in the footpads of C57BL/6 mice with HSV-1 results in an accumulation of activated (CD44high CD25+) CD8+ T cells within the draining popliteal lymph node (PLN). These studies were undertaken to evaluate the frequency and phenotype of the CD8+ T cell population within the PLN, recognizing the single immunodominant HSV-1 epitope derived from the viral envelope glycoprotein, glycoprotein B (gB), using an intracellular IFN-gamma-staining assay. It revealed that approximately 6% of the CD8+ T cells were specific for the gB epitope. Phenotypic analysis of the IFN-gamma-producing gB-specific CD8+ T cells generated in the PLN during the course of the acute infection expressed the CD44high CD25+ phenotype on days 3-5 postinfection. Surprisingly, IFN-gamma-producing CD8+ T cells expressed the CD44high CD25- phenotype on days 5-8 postinfection, in contrast to expectations for a CD8+ effector T cell. IFN-gamma-producing CD25- CD8+ T cells were detected in the PLN on day 21 postinfection, long after infectious virus had been cleared. Throughout the response, the spleen was found to be the major reservoir of gB-specific CD8+ T cells, even during the peak of the response. In contrast to the gB-specific CD8+ T cell population within the PLN, the entire gB-specific CD8+ T cell population within the spleen was CD25-. Collectively, these results suggest the generation of subpopulations of virus-specific CD8+ T cells, distinguished by the expression of CD25, during the acute phase of the primary response to a localized viral infection.

The role of antigen-independent persistence of memory cytotoxic T lymphocytes

We use mathematical models to analyze the role of a memory cytotoxic T lymphocyte (CTL) response in viral infections. The model predicts that antigen-independent persistence of an elevated number of precursor CTL (CTLp) does not protect the host from clinical symptoms upon re-infection. Instead, we find that antigen-independent long-term persistence of CTLp is required to clear virus infections. This mechanism also applies to infection in hosts that have never experienced the pathogen before. Requirement of antigen for the long-term maintenance of CTLp results in failure to clear the infection, even in hosts characterized by a high CTL responsiveness. We compare the CTL model to a B cell model. In keeping with experimentally established findings, B cells are efficient in protecting against re-infection, but are unlikely to clear viral infections unless the virus is cytopathic. We conclude that the role of 'memory CTLp' is different from the role of memory B cells in viral infections: antigen-independent long-term persistence of CTLp is a pre-requisite to ensure clearance of infection.

A new theory of cytotoxic T-lymphocyte memory: implications for HIV treatment

We use simple mathematical models to examine the dynamics of primary and secondary cytotoxic T-lymphocyte (CTL) responses to viral infections. In particular, we are interested in conditions required to resolve the infection and to protect the host upon secondary challenge. While protection against reinfection is only effective in a restricted set of circumstances, we find that resolution of the primary infection requires persistence of CTL precursors (GTLp), as well as a fast rate of activation of the CTLp. Since these are commonly the defining characteristics of CTL memory, we propose that CTL memory may have evolved in order to clear the virus during primary challenge. We show experimental data from lymphocytic choriomeningitis virus infection in mice, supporting our theory on CTL memory. We adapt our models to HIV and find that immune impairment during the primary phase of the infection may result in the failure to establish CTL memory which in turn leads to viral persistence. Based on our models we suggest conceptual treatment regimes which ensure establishment of CTL memory. This would allow the immune response to control HIV in the long term in the absence of continued therapy.

Status of Cytomegalovirus Prevention and Treatment in 2000

Cytomegalovirus (CMV) infection continues to be a problem in selected populations following hematopoietic stem cell transplantation (SCT). Although there have been no new antiviral agents for management of this infection in recent years, the methods for using the existing agents have improved with newer assays for detection of virus. In addition, our understanding of immunity to CMV has undergone considerable expansion. This paper will address these new aspects relating to CMV infection in the setting of SCT.

In Section I Dr. Zaia reviews the pathogenesis of CMV and the current epidemiology of CMV disease following marrow or blood allo-SCT with emphasis on late-onset disease. The current lab tests available for preemptive management are summarized including the role for conventional shell vial cultures, and a comparison of the CMV antigenemia assay with the new nucleic acid-based assays, including the hybrid capture assay, the NASBA assay, and “real-time” PCR assays. Use of antiviral agents with these tests in the preemptive management of CMV infection is discussed.

Ultimately, what is necessary is restoration of adequate CMV immunity, and that requires understanding the basics of the CMV-specific immune response. In Section II, Dr. Sissons traces the evolution of the CTL response from primary infection into memory and reviews recent advances in the understanding of cytotoxic T cell based immunity to CMV, based on the use of T cell clonotypic analysis and markers of T cell memory and activation, with conventional CTL functional assays.

In Section III Dr. Riddell presents approaches to correction of the problem of CMV pathogenesis, namely direct restoration of the CMV-specific cellular immune deficiency. Attempts at passive therapies will be reviewed with the focus on current problems and approaches to these problems.

In Section IV, Dr. Diamond presents work on the identification of multiple HLA-allele specific cytotoxic T cell epitopes specific for CMV-pp65 and - pp150. Specific epitopes are recognized by CMV-seropositive individuals including healthy donors, SCT recipients, and AIDS patients, indicating their potential usefulness as vaccines. One of these epitopes is recognized by most individuals who express the HLA A*0201 Class I allele. Pre-clinical evaluation in HLA2.1 transgenic mice of vaccine structures utilizing this epitope, and alternative delivery systems are described. Possible methods for vaccination of donor and/or recipient of a SCT as well as their limitations, utilizing synthetic or viral vaccines, are discusseed.

Status of Cytomegalovirus Prevention and Treatment in 2000

Cytomegalovirus (CMV) infection continues to be a problem in selected populations following hematopoietic stem cell transplantation (SCT). Although there have been no new antiviral agents for management of this infection in recent years, the methods for using the existing agents have improved with newer assays for detection of virus. In addition, our understanding of immunity to CMV has undergone considerable expansion. This paper will address these new aspects relating to CMV infection in the setting of SCT.

In Section I Dr. Zaia reviews the pathogenesis of CMV and the current epidemiology of CMV disease following marrow or blood allo-SCT with emphasis on late-onset disease. The current lab tests available for preemptive management are summarized including the role for conventional shell vial cultures, and a comparison of the CMV antigenemia assay with the new nucleic acid-based assays, including the hybrid capture assay, the NASBA assay, and “real-time” PCR assays. Use of antiviral agents with these tests in the preemptive management of CMV infection is discussed.

Ultimately, what is necessary is restoration of adequate CMV immunity, and that requires understanding the basics of the CMV-specific immune response. In Section II, Dr. Sissons traces the evolution of the CTL response from primary infection into memory and reviews recent advances in the understanding of cytotoxic T cell based immunity to CMV, based on the use of T cell clonotypic analysis and markers of T cell memory and activation, with conventional CTL functional assays.

In Section III Dr. Riddell presents approaches to correction of the problem of CMV pathogenesis, namely direct restoration of the CMV-specific cellular immune deficiency. Attempts at passive therapies will be reviewed with the focus on current problems and approaches to these problems.

In Section IV, Dr. Diamond presents work on the identification of multiple HLA-allele specific cytotoxic T cell epitopes specific for CMV-pp65 and - pp150. Specific epitopes are recognized by CMV-seropositive individuals including healthy donors, SCT recipients, and AIDS patients, indicating their potential usefulness as vaccines. One of these epitopes is recognized by most individuals who express the HLA A*0201 Class I allele. Pre-clinical evaluation in HLA2.1 transgenic mice of vaccine structures utilizing this epitope, and alternative delivery systems are described. Possible methods for vaccination of donor and/or recipient of a SCT as well as their limitations, utilizing synthetic or viral vaccines, are discusseed.