Independent regulation of lymphocytic choriomeningitis virus-specific T cell memory pools: relative stability of CD4 memory under conditions of CD8 memory T cell loss

Mitigation of influenza-mediated inflammation by immunomodulatory matrix-bound nanovesicles

Cytokine storm describes a life-threatening, systemic inflammatory syndrome characterized by elevated levels of proinflammatory cytokines and immune cell hyperactivation associated with multi-organ dysfunction. Matrix-bound nanovesicles (MBV) are a subclass of extracellular vesicle shown to down-regulate proinflammatory immune responses. The objective of this study was to assess the efficacy of MBV in mediating influenza-induced acute respiratory distress syndrome and cytokine storm in a murine model. Intravenous administration of MBV decreased influenza-mediated total lung inflammatory cell density, proinflammatory macrophage frequencies, and proinflammatory cytokines at 7 and 21 days following viral inoculation. MBV decreased long-lasting alveolitis and the proportion of lung undergoing inflammatory tissue repair at day 21. MBV increased the proportion of activated anti-viral CD4+ and CD8+ T cells at day 7 and memory-like CD62L+ CD44+, CD4+, and CD8+ T cells at day 21. These results show immunomodulatory properties of MBV that may benefit the treatment of viral-mediated pulmonary inflammation with applicability to other viral diseases such as SARS-CoV-2.

Immune Responses to Varicella-Zoster Virus Glycoprotein E Formulated with Poly(Lactic-co-Glycolic Acid) Nanoparticles and Nucleic Acid Adjuvants in Mice

The subunit herpes zoster vaccine Shingrix is superior to attenuated vaccine Zostavax in both safety and efficacy, yet its unlyophilizable liposome delivery system and the limited supply of naturally sourced immunological adjuvant QS-21 still need to be improved. Based on poly(lactic-co-glycolic acid) (PLGA) delivery systems that are stable during the lyophilization and rehydration process and using a double-emulsion (w/o/w) solvent evaporation method, we designed a series of nanoparticles with varicella-zoster virus antigen glycoprotein E (VZV-gE) as an antigen and nucleic acids including polyinosinic-polycytidylic acid (Poly I:C) and phosphodiester CpG oligodeoxynucleotide (CpG ODN), encapsulated as immune stimulators. While cationic lipids (DOTAP) have more potential than neutral lipids (DOPC) for activating gE-specific cell-mediated immunity (CMI) in immunized mice, especially when gE is encapsulated in and presented on the surface of nanoparticles, PLGA particles without lipids have the greatest potential to induce not only the highest gE-specific IgG titers but also the strongest gE-specific CMI responses, including the highest proportions of interferon-γ (IFN-γ)- and interleukin-2 (IL-2)-producing CD4+/CD8+ T cells according to a flow cytometry assay and the greatest numbers of IFN-γ- and IL-2-producing splenocytes according to an enzyme-linked immunospot (ELISPOT) assay. These results showed that immune-stimulating nucleic acids together with the PLGA delivery system showed promise as a safe and economical varicella and zoster vaccine candidate.

Heterologous Immunity: Role in Natural and Vaccine-Induced Resistance to Infections

The central paradigm of vaccination is to generate resistance to infection by a specific pathogen when the vacinee is re-exposed to that pathogen. This paradigm is based on two fundamental characteristics of the adaptive immune system, specificity and memory. These characteristics come from the clonal specificity of T and B cells and the long-term survival of previously-encountered memory cells which can rapidly and specifically expand upon re-exposure to the same specific antigen. However, there is an increasing awareness of the concept, as well as experimental documentation of, heterologous immunity and cross-reactivity of adaptive immune lymphocytes in protection from infection. This awareness is supported by a number of human epidemiological studies in vaccine recipients and/or individuals naturally-resistant to certain infections, as well as studies in mouse models of infections, and indeed theoretical considerations regarding the disproportional repertoire of available T and B cell clonotypes compared to antigenic epitopes found on pathogens. Heterologous immunity can broaden the protective outcomes of vaccinations, and natural resistance to infections. Besides exogenous microbes/pathogens and/or vaccines, endogenous microbiota can also impact the outcomes of an infection and/or vaccination through heterologous immunity. Moreover, utilization of viral and/or bacterial vaccine vectors, capable of inducing heterologous immunity may also influence the natural course of many infections/diseases. This review article will briefly discuss these implications and redress the central dogma of specificity in the immune system.

Virological and Immunological Outcomes of Coinfections

Coinfections involving viruses are being recognized to influence the disease pattern that occurs relative to that with single infection. Classically, we usually think of a clinical syndrome as the consequence of infection by a single virus that is isolated from clinical specimens. However, this biased laboratory approach omits detection of additional agents that could be contributing to the clinical outcome, including novel agents not usually considered pathogens. The presence of an additional agent may also interfere with the targeted isolation of a known virus. Viral interference, a phenomenon where one virus competitively suppresses replication of other coinfecting viruses, is the most common outcome of viral coinfections. In addition, coinfections can modulate virus virulence and cell death, thereby altering disease severity and epidemiology. Immunity to primary virus infection can also modulate immune responses to subsequent secondary infections. In this review, various virological mechanisms that determine viral persistence/exclusion during coinfections are discussed, and insights into the isolation/detection of multiple viruses are provided. We also discuss features of heterologous infections that impact the pattern of immune responsiveness that develops.

Vaccination Expands Antigen-Specific CD4+ Memory T Cells and Mobilizes Bystander Central Memory T Cells

CD4+ T helper memory (Thmem) cells influence both natural and vaccine-boosted immunity, but mechanisms for their maintenance remain unclear. Pro-survival signals from the common gamma-chain cytokines, in particular IL-7, appear important. Previously we showed in healthy volunteers that a booster vaccination with tetanus toxoid (TT) expanded peripheral blood TT-specific Thmem cells as expected, but was accompanied by parallel increase of Thmem cells specific for two unrelated and non cross-reactive common recall antigens. Here, in a new cohort of healthy human subjects, we compare blood vaccine-specific and bystander Thmem cells in terms of differentiation stage, function, activation and proliferative status. Both responses peaked 1 week post-vaccination. Vaccine-specific cytokine-producing Thmem cells were predominantly effector memory, whereas bystander cells were mainly of central memory phenotype. Importantly, TT-specific Thmem cells were activated (CD38High HLA-DR+), cycling or recently divided (Ki-67+), and apparently vulnerable to death (IL-7RαLow and Bcl-2 Low). In contrast, bystander Thmem cells were resting (CD38Low HLA-DR- Ki-67-) with high expression of IL-7Rα and Bcl-2. These findings allow a clear distinction between vaccine-specific and bystander Thmem cells, suggesting the latter do not derive from recent proliferation but from cells mobilized from as yet undefined reservoirs. Furthermore, they reveal the interdependent dynamics of specific and bystander T-cell responses which will inform assessments of responses to vaccines.

Inflammatory IL-15 is required for optimal memory T cell responses

Due to their ability to rapidly proliferate and produce effector cytokines, memory CD8+ T cells increase protection following reexposure to a pathogen. However, low inflammatory immunizations do not provide memory CD8+ T cells with a proliferation advantage over naive CD8+ T cells, suggesting that cell-extrinsic factors enhance memory CD8+ T cell proliferation in vivo. Herein, we demonstrate that inflammatory signals are critical for the rapid proliferation of memory CD8+ T cells following infection. Using murine models of viral infection and antigen exposure, we found that type I IFN-driven expression of IL-15 in response to viral infection prepares memory CD8+ T cells for rapid division independently of antigen reexposure by transiently inducing cell-cycle progression via a pathway dependent on mTOR complex-1 (mTORC1). Moreover, exposure to IL-15 allowed more rapid division of memory CD8+ T cells following antigen encounter and enhanced their protective capacity against viral infection. Together, these data reveal that inflammatory IL-15 promotes optimal responses by memory CD8+ T cells.

Influence of time and number of antigen encounters on memory CD8 T cell development

CD8 T cells are an important part of the adaptive immune system providing protection against intracellular bacteria, viruses, and protozoa. After infection and/or vaccination, increased numbers of antigen-specific CD8 T cells remain as a memory population that is capable of responding and providing enhanced protection during reinfection. Experimental studies indicate that while memory CD8 T cells can be maintained for great lengths of time, their properties change with time after infection and/or vaccination. However, the full scope of these changes and what effects they have on memory CD8 T cell function remain unknown. In addition, memory CD8 T cells can encounter antigen multiple times through either reinfection or prime-boost vaccine strategies designed to increase numbers of protective memory CD8 T cells. Importantly, recent studies suggest that memory CD8 T cell development following infection and/or vaccination is influenced by the number of times they have encountered cognate antigen. Since protection offered by memory CD8 T cells in response to infection depends on both the numbers and quality (functional characteristics) at the time of pathogen re-encounter, a thorough understanding of how time and antigen stimulation history impacts memory CD8 T cell properties is critical for the design of vaccines aimed at establishing populations of long-lived, protective memory CD8 T cells.

Pathogen-specific inflammatory milieux tune the antigen sensitivity of CD8(+) T cells by enhancing T cell receptor signaling

CD8(+) T cells confer host protection through T-cell-receptor (TCR)-mediated recognition of foreign antigens presented by infected cells. Thus, generation of CD8(+) T cell populations with high antigen sensitivity is critical for efficient pathogen clearance. Besides selection of high-affinity TCRs, the molecular mechanisms regulating the antigen sensitivity of CD8(+) T cells remain poorly defined. Herein, we have demonstrated that the antigen sensitivity of effector and memory CD8(+) T cells is dynamically regulated and can be tuned by pathogen-induced inflammatory milieux independently of the selection of cells with higher TCR affinity. Mechanistically, we have demonstrated that the signal-transduction capacity of key TCR proximal molecules is enhanced by inflammatory cytokines, which reduced the antigen density required to trigger antimicrobial functions. Dynamic tuning of CD8(+) T cell antigen sensitivity by inflammatory cytokines most likely optimizes immunity to specific pathogens while minimizing the risk of immunopathology at steady state.

CD4 T-cell memory responses to viral infections of humans show pronounced immunodominance independent of duration or viral persistence

Little is known concerning immunodominance within the CD4 T-cell response to viral infections and its persistence into long-term memory. We tested CD4 T-cell reactivity against each viral protein in persons immunized with vaccinia virus (VV), either recently or more than 40 years ago, as a model self-limited viral infection. Similar tests were done with persons with herpes simplex virus 1 (HSV-1) infection as a model chronic infection. We used an indirect method capable of counting the CD4 T cells in blood reactive with each individual viral protein. Each person had a clear CD4 T-cell dominance hierarchy. The top four open reading frames accounted for about 40% of CD4 virus-specific T cells. Early and long-term memory CD4 T-cell responses to vaccinia virus were mathematically indistinguishable for antigen breadth and immunodominance. Despite the chronic intermittent presence of HSV-1 antigen, the CD4 T-cell dominance and diversity patterns for HSV-1 were identical to those observed for vaccinia virus. The immunodominant CD4 T-cell antigens included both long proteins abundantly present in virions and shorter, nonstructural proteins. Limited epitope level and direct ex vivo data were also consistent with pronounced CD4 T-cell immunodominance. We conclude that human memory CD4 T-cell responses show a pattern of pronounced immunodominance for both chronic and self-limited viral infections and that this pattern can persist over several decades in the absence of antigen.

Primary Epstein-Barr virus infection does not erode preexisting CD8⁺ T cell memory in humans

Acute Epstein-Barr virus (EBV) infection results in an unusually robust CD8(+) T cell response in young adults. Based on mouse studies, such a response would be predicted to result in attrition of preexisting memory to heterologous infections like influenza A (Flu) and cytomegalovirus (CMV). Furthermore, many studies have attempted to define the lymphocytosis that occurs during acute EBV infection in humans, but it is unclear whether bystander T cells contribute to it. To address these issues, we performed a longitudinal prospective study of primary EBV infection in humans. During acute EBV infection, both preexisting CMV- and Flu-specific memory CD8(+) T cells showed signs of bystander activation, including up-regulation of granzyme B. However, they generally did not expand, suggesting that the profound CD8(+) lymphocytosis associated with acute EBV infection is composed largely of EBV-specific T cells. Importantly, the numbers of CMV- and Flu-specific T cells were comparable before and after acute EBV infection. The data support the concept that, in humans, a robust CD8(+) T cell response creates a new memory CD8(+) T cell niche without substantially depleting preexisting memory for heterologous infections.

Attrition of memory CD8 T cells

An important role for the immune system is to maintain protective immunological memory to a wide variety of pathogens encountered over one's lifetime, while still leaving the host able to respond to newly encountered pathogens. Vezys et al. make the interesting observation that it is possible to repeatedly immunize mice in ways that allow for development of high numbers of memory CD8 T cells without depleting pre-existing memory cells specific for other pathogens. This study, which offers promise in developing potent vaccination schemes, is seemingly at odds with work published by us in the 1990s showing a loss in CD8 memory cells after a series of infections. In their reply, Vezys et al. mention that we may have misinterpreted our data because we reported the putative loss of memory T cells as per cent rather than total number, but here we represent the data in those studies as total cell number. We show here in Fig. 1 that a series of infections can indeed reduce the total number of memory cells, indicating that vaccination strategies need to consider this issue.

Human CD4+ memory T cells are preferential targets for bystander activation and apoptosis

There is much evidence that T cells may be activated via mechanisms that act independently of direct TCR ligation. Despite this, the question of whether such forms of bystander T cell activation occur during immune responses is hotly debated. To address some outstanding questions, we set up an in vitro system within which to analyze bystander T cell activation in human T cells, in the absence of the possibility for TCR cross-reactivity. In addition, we have investigated the genetic, phenotypic, and functional characteristics of bystander-activated T cells. In this study, we show that bystander T cell activation is, indeed, observed during a specific immune response, and that it occurs preferentially among CD4(+) memory T cells. Furthermore, bystander-activated T cells display a distinct gene expression profile. The mechanism for bystander T cell activation involves soluble factors, and the outcome is an elevated level of apoptosis. This may provide an explanation for the attrition of T cell memory pools of heterologous specificity during immune responses to pathogens such as viruses.

Immune system derived from homeostatic proliferation generates normal CD8 T-cell memory but altered repertoires and diminished heterologous immune responses

The host responds to lymphopenic environments by acute homeostatic proliferation of T lymphocytes, which acquire phenotypes similar to memory cells. Using T-cell knockout (KO) mice adoptively reconstituted with splenocytes from immunologically naive mice, we examined the immune responses of an immune system derived from homeostatically proliferating (HP) T cells. HP cells mounted relatively normal acute CD8 T-cell responses to lymphocytic choriomeningitis virus (LCMV), but with altered T-cell receptor (TCR) repertoires, and they became functional memory cells capable of recall responses. Although homeostatic proliferation does not normally fully restore T-cell numbers, the CD8(+) T-cell pool was completely restored in T-cell KO mice after LCMV infection. CD4 T-cell responses were lower and not fully restored but seemed sufficient to allow for complete differentiation of CD8 memory T cells. The LCMV-immune HP mouse had an immune repertoire heavily biased with LCMV epitope-specific T cells with oligoclonal expansions. LCMV-immune HP mice had reduced cross-reactive and non-cross-reactive CD8 T-cell responses when challenged with a T cell-cross-reactive virus. Thus, whereas an HP immune system is capable of mounting relatively normal acute and memory CD8 T-cell responses, the narrowing of the T-cell repertoire may reduce immune responses to subsequently encountered pathogens.

Immunogenicity of candidate DNA vaccine based on subtype A of human immunodeficiency virus type 1 predominant in Russia

Human immunodeficiency virus (HIV)-1 subtype A strains circulating among the majority of HIVinfected individuals in the former Soviet Union (FSU) countries demonstrate low genetic diversity. The consensus sequence of the FSU region-specific isolate has been used for the candidate DNA vaccine development. We constructed recombinant plasmids with four viral genes: env (gp140), gag, pol (reverse transcriptase), and nef. We immunized BALB/c mice intramuscularly using equimolar mixture of four recombinant plasmids, and observed significant cytotoxic T lymphocyte response and specific CD8(+) cell production against cells presenting HIV-1 peptides. Overall, the Th1 pathway of immune response clearly dominated. Immunological properties of this candidate DNA vaccine against HIV-1 suggest the possibility of its further study in clinical trials.

Immunity and immunological memory following smallpox vaccination

The smallpox vaccine consists of live vaccinia virus and is generally considered the gold standard of vaccines, since it is the only one that has led to the complete eradication of an infectious disease from the human population. Renewed fears that smallpox might be deliberately released in an act of bioterrorism have led to resurgence in the study of immunity and immunological memory to vaccinia virus and other poxviruses. Here we review our current understanding of memory T-cell, memory B-cell, and antibody responses to vaccinia and related poxviruses, both in animal models and human subjects. Of particular interest are recent advances in understanding protective immunity to poxviruses, quantifying immunological memory to the smallpox vaccine in humans, and identifying major vaccinia-specific T-cell and B-cell epitopes. In addition, potential mechanisms for maintenance of immunological memory are discussed.

Memory of mice and men: CD8+ T-cell cross-reactivity and heterologous immunity

Summary:  The main functions of memory T cells are to provide protection upon re‐exposure to a pathogen and to prevent the re‐emergence of low‐grade persistent pathogens. Memory T cells achieve these functions through their high frequency and elevated activation state, which lead to rapid responses upon antigenic challenge. The significance and characteristics of memory CD8⁺ T cells in viral infections have been studied extensively. In many of these studies of T‐cell memory, experimental viral immunologists go to great lengths to assure that their animal colonies are free of endogenous pathogens in order to design reproducible experiments. These experimental results are then thought to provide the basis for our understanding of human immune responses to viruses. Although these findings can be enlightening, humans are not immunologically naïve, and they often have memory T‐cell populations that can cross‐react with and respond to a new infectious agent or cross‐react with allo‐antigens and influence the success of tissue transplantation. These cross‐reactive T cells can become activated and modulate the immune response and outcome of subsequent heterologous infections, a phenomenon we have termed heterologous immunity. These large memory populations are also accommodated into a finite immune system, requiring that the host makes room for each new population of memory cell. It appears that memory cells are part of a continually evolving interactive network, where with each new infection there is an alteration in the frequencies, distributions, and activities of memory cells generated in response to previous infections and allo‐antigens.

Not-so-great expectations: re-assessing the essence of T-cell memory

We are often taught that secondary, or memory, responses by lymphocytes are more vigorous than primary responses. An expectation commonly associated with this notion is that the initial encounter with a pathogen should result in immunity to re-infection. Although this outcome is sometimes the case, it is not universally true. In this review, we propose a unified model of T-cell memory to explain the apparent successes and failures of eliciting vaccine-like protection from prior encounters with pathogens. We speculate that memory T cells arise as an invariant consequence of clonal selection during an immune response. The quality of memory T cells, however, seems to vary in the degree to which they have acquired effector characteristics and, thus, their ability to confer immunity to re-infection. Although not all memory T cells possess the embellished attributes of fully developed effector cells, they all seem to share the rudimentary quality of preserving an antigen specificity that has proven itself useful. We suggest that the ability to maintain the integrity of the T-cell repertoire, more than establishing immunity to re-infection, may represent the fundamental form of memory for the adaptive immune system.

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'.

Naive and memory CD4+ T cell survival controlled by clonal abundance

Immunity to a plethora of microbes depends on a diverse repertoire of naïve lymphocytes and the production of long-lived memory cells. We present evidence here that low clonal abundance in a polyclonal repertoire favors the survival and activation of naïve CD4(+) T cells as well as the survival of their memory cell progeny. The inverse relation between clonal frequency and survival suggests that intraclonal competition could help maintain an optimally diverse repertoire of T cells and an optimal environment for the generation of long-lived memory cells.

Persistent Cytomegalovirus-Specific Memory Responses in the Lung Allograft and Blood following Primary Infection in Lung Transplant Recipients

Primary CMV infection in lung transplant recipients (LTRs) is associated with increased mortality. We studied 22 donor CMV-positive, recipient-negative (D(+)R(-)) LTRs for the development of posttransplant CMV-specific immunity. We found that 13 of 22 D(+)R(-) LTRs (59.1%) seroconverted (CMV IgG Ab(+)). Using pooled peptides of the immunodominant CMV Ags pp65 and IE1, we detected CMV-specific CD8(+)IFN-gamma(+) T cells in the PBMC of 90% of seroconverted individuals following primary infection by intracellular cytokine staining. In contrast, few seroconverters had detectable CMV-specific CD4(+)IFN-gamma(+) T cells during viral latency. However, the majority of IgG(+) LTRs demonstrated CMV-specific CD4(+) and CD8(+) T cell proliferative responses from PBMC, with CD4(+)IFN-gamma(+) T cells detectable upon re-expansion. Examination of lung allograft mononuclear cells obtained by bronchoalveolar lavage revealed both CMV-specific CD4(+) and CD8(+)IFN-gamma(+) T cells, including patients from whom CD4(+)IFN-gamma(+) T cells were simultaneously undetectable in the PBMC, suggesting differential effector memory populations between these compartments. Moreover, both responses in the PBMC and lung allograft were found to persist, despite substantial immunosuppression, long after primary infection. Clinical correlation in this cohort demonstrated that the acquisition of CMV immunity was associated with freedom from CMV disease (p < or = 0.009) and preservation of allograft function (p < or = 0.02) compared with those who failed to develop CMV immunity. Together, our data reveal immunologic heterogeneity in D(+)R(-) LTRs, with the development and persistence of primary CMV responses that may provide clinical benefit.

Homeostasis of T cell numbers: from thymus production to peripheral compartmentalization and the indexation of regulatory T cells

A system under homeostatic control tends to maintain its structure and functions by establishing dynamic equilibriums controlled by multiple regulatory mechanisms. We have shown that this is the case for immune system. Several different mechanisms seem to participate in the homeostatic control of T cell numbers and population distribution. In other words, besides a quantitative dimension, there is also a qualitative dimension in T cell homeostasis. This is achieved through competition by driving the specialization of sub-populations of lymphocytes to occupy specific niches in the peripheral pool and by developing independent homeostatic mechanisms for each particular cell sub-set. Thus, the sizes of the naïve and memory T cell compartments are governed by independent homeostatic mechanisms, which preserve the capacity to deal with any novel infection (conferred by the presence of naïve T cells) whilst ensuring the efficacy of memory responses when dealing with recurring antigens. Peripheral T cell homeostasis also depends on the integrity of sub-population structure and the presence of regulatory CD4+ CD25+ T cells. The indexation of regulatory CD4+ CD25+ T cell numbers to the numbers of peripheral activated CD4+ T cells is another mechanism of homeostasis that has major advantages in the control of immune responses. It ensures continuous regulation of T cell numbers throughout immune responses, allowing for increases in cell numbers as long as the proportion of CD4+ CD25+ regulatory T cells is kept.

Immunology and mathematics: crossing the divide

'It's high time molecular biology became quantitative, it cries out to a physicist ... for modeling. Modeling isn't a crutch, it's the opposite; it's a way of suggesting experiments to do, to fill gaps in your understanding.' John Maddox, Editor of Nature 1966-73, and 1980-95.

The role of models in understanding CD8+ T-cell memory

Immunological memory - the ability to 'remember' previously encountered pathogens and respond faster on re-exposure - is a central feature of the immune response of vertebrates. We outline how mathematical models have contributed to our understanding of CD8(+) T-cell memory. Together with experimental data, models have helped to quantitatively describe and to further our understanding of both the generation of memory after infection with a pathogen and the maintenance of this memory throughout the life of an individual.

Duration of Infection and Antigen Display Have Minimal Influence on the Kinetics of the CD4+T Cell Response toListeria monocytogenesInfection

The T cell response to infection consists of clonal expansion of effector cells, followed by contraction to memory levels. It was previously thought that the duration of infection determines the magnitude and kinetics of the T cell response. However, recent analysis revealed that transition between the expansion and contraction phases of the Ag-specific CD8+ T cell response is not affected by experimental manipulation in the duration of infection or Ag display. We studied whether the duration of infection and Ag display influenced the kinetics of the Ag-specific CD4+ T cell response to Listeria monocytogenes (LM) infection. We found that truncating infection and Ag display with antibiotic treatment as early as 24 h postinfection had minimal impact on the expansion or contraction of CD4+ T cells; however, the magnitudes of the Ag-specific CD4+ and CD8+ T cell responses were differentially affected by the timing of antibiotic treatment. Treatment of LM-infected mice with antibiotics at 24 h postinfection did not prevent generation of detectable CD4+ and CD8+ memory T cells at 28 days after infection, vigorous secondary expansion of these memory T cells, or protection against a subsequent LM challenge. These results demonstrate that events within the first few days of infection stimulate CD4+ and CD8+ T cell responses that are capable of carrying out the full program of expansion and contraction to functional memory, independently of prolonged infection or Ag display.

CD4 T-cell memory

CD4 T-cell memory is in some ways more enigmatic than CD8 T-cell memory. This is mostly due to the fact that CD4 T cells tend to expand far less in response to antigenic stimuli, thereby thwarting attempts at their detection during the course of an immune response. Nevertheless, there is a wide range of experimental models that have provided information regarding the survival and maintenance of CD4 memory cells, their functional capacities, their differentiation states and program of development following activation. The emerging picture is one of great versatility and functional heterogeneity as befits their central position within the immune system.

Distinct time effects of vaccination on long-term proliferative and IFN-gamma-producing T cell memory to smallpox in humans

Residual immunity to the smallpox virus raises key questions about the persistence of long-term immune memory in the absence of antigen, since vaccination ended in 1980. IFN-γ–producing effector–memory and proliferative memory T cells were compared in 79 vaccinees 13–25 yr after their last immunization and in unvaccinated individuals. Only 20% of the vaccinees displayed both immediate IFN-γ–producing effector–memory responses and proliferative memory responses at 6 d; 52.5% showed only proliferative responses; and 27.5% had no detectable vaccinia-specific responses at all. Both responses were mediated by CD4 and CD8 T cells. The vaccinia-specific IFN-γ–producing cells were composed mainly of CD4Pos CD45RANeg CD11aHi CD27Pos and CCR7Neg T cells. Their frequency was low but could be expanded in vitro within 7 d. Time since first immunization affected their persistence: they vanished 45 yr after priming, but proliferative responses remained detectable. The number of recalls did not affect the persistence of residual effector–memory T cells. Programmed revaccination boosted both IFN-γ and proliferative responses within 2 mo of recall, even in vaccinees with previously undetectable residual effector–memory cells. Such long-term maintenance of vaccinia-specific immune memory in the absence of smallpox virus modifies our understanding of the mechanism of persistence of long-term memory to poxviruses and challenges vaccination strategies.

Immunological memory to viral infections

The purpose of immunological memory is to protect the host from reinfection, to control persistent infections, and, through maternal antibody, to protect the host's immunologically immature offspring from primary infections. Immunological memory is an exclusive property of the acquired immune system, where in the presence of CD4 T cell help, T cells and B cells clonally expand and differentiate to provide effector systems that protect the host from pathogens. Here we describe how T and B cell memory is generated in response to virus infections and how these cells respond when the host is infected again by similar or different viruses.

Long-term follow-up after successful interferon therapy of acute hepatitis C

Early treatment of acute hepatitis C infection with interferon alfa-2b (IFN-alpha-2b) prevents chronicity in almost all patients. So far, no data are available on the long-term outcome after interferon (IFN) therapy of acute hepatitis C. The aim of this study was to assess the clinical, virological, and immunological long-term outcome of 31 successfully treated patients with acute hepatitis C infection who were followed for a median of 135 weeks (52-224 weeks) after end of therapy. None of the individuals had clinical evidence of liver disease. Alanine aminotransferase (ALT) levels were normal in all but 1 patient. Serum hepatitis C virus (HCV) RNA was negative throughout follow-up, even when investigated with the highly sensitive transcription-mediated amplification (TMA) assay (cutoff 5-10 IU/mL). In addition, no HCV RNA was detected in peripheral blood mononuclear cells (PBMC) of 15 cases tested. The patients' overall quality-of-life scores as determined by the SF-36 questionnaire did not differ from the German reference control cohort. Ex vivo interferon gamma (IFN-gamma) ELISPOT analysis detected HCV-specific CD4(+) T-helper cell reactivity in only 35% of cases, whereas HCV-specific CD8(+) T-cell responses were found in 4 of 5 HLA-A2-positive individuals. Anti-HCV antibody levels decreased significantly during and after therapy in all individuals. In conclusion, early treatment of symptomatic acute hepatitis C with IFN-alpha-2b leads to a long-term virological, biochemical, and clinical response. Waning of anti-HCV humoral immunity and presence of HCV-specific CD8(+) (but not CD4(+)) T cells highlights the complexity of T-cell and B-cell memory to HCV, which might be significantly altered by IFN treatment.

Immunological memory in humans

Specificity and memory are the defining characteristics of adaptive immune responses. Vaccines are predicated on the existence of immune memory, and the robustness of immune memory is a primary determinant of vaccine efficacy. How is immune memory maintained? Much progress has been made in this area over the past several years, and new human studies have added key insights into the longevity of B and T cell immune memory in the absence of antigen.

Interleukin 7 regulates the survival and generation of memory CD4 cells

Cytokines, particularly those of the common gamma chain receptor family, provide extrinsic signals that regulate naive CD4 cell survival. Whether these cytokines are required for the maintenance of memory CD4 cells has not been rigorously assessed. In this paper, we examined the contribution of interleukin (IL) 7, a constitutively produced common gamma chain receptor cytokine, to the survival of resting T cell receptor transgenic memory CD4 cells that were generated in vivo. IL-7 mediated the survival and up-regulation of Bcl-2 by resting memory CD4 cells in vitro in the absence of proliferation. Memory CD4 cells persisted for extended periods upon adoptive transfer into intact or lymphopenic recipients, but not in IL-7- mice or in recipients that were rendered deficient in IL-7 by antibody blocking. Both central (CD62L+) and effector (CD62L-) memory phenotype CD4 cells required IL-7 for survival and, in vivo, memory cells were comparable to naive CD4 cells in this regard. Although the generation of primary effector cells from naive CD4 cells and their dissemination to nonlymphoid tissues were not affected by IL-7 deficiency, memory cells failed to subsequently develop in either the lymphoid or nonlymphoid compartments. The results demonstrate that IL-7 can have previously unrecognized roles in the maintenance of memory in the CD4 cell population and in the survival of CD4 cells with a capacity to become memory cells.

Similarities and differences in CD4+ and CD8+ effector and memory T cell generation

Naive CD4+ and CD8+ T cells undergo unique developmental programs after activation, resulting in the generation of effector and long-lived memory T cells. Recent evidence indicates that both cell-intrinsic and cell-extrinsic factors regulate memory T cell differentiation. This review compares and contrasts how naive CD4+ and CD8+ T cells make the transition to effector and/or memory cells and discusses the implications of these findings for vaccine development.

CD8 T cell responses to infectious pathogens

CD8 T cells respond to viral infections but also participate in defense against bacterial and protozoal infections. In the last few years, as new methods to accurately quantify and characterize pathogen-specific CD8 T cells have become available, our understanding of in vivo T cell responses has increased dramatically. Pathogen-specific T cells, once thought to be quite rare following infection, are now known to be present at very high frequencies, particularly in peripheral, nonlymphoid tissues. With the ability to visualize in vivo CD8 T cell responses has come the recognition that T cell expansion is programmed and, to a great extent, independent of antigen concentrations. Comparison of CD8 T cell responses to different pathogens also highlights the intricate relationship between microbially induced innate inflammatory responses and the kinetics, magnitude, and character of long-term T cell responses. This review describes recent progress in some of the major murine models of CD8 T cell-mediated immunity to viral, bacterial, and protozoal infection.

Kinetics of CD4+ and CD8+ memory T-cell responses during hepatitis C virus rechallenge of previously recovered chimpanzees

The immunological correlates of hepatitis C virus (HCV)-specific immunity are not well understood. Antibodies to HCV structural proteins do not appear to play a key role in clearance of the virus and do not persist after recovery. Here, we studied the kinetics of the cellular immune responses of three HCV-recovered chimpanzees during rechallenge with increasing doses of homologous HCV. Although HCV envelope antibodies remained undetectable throughout the rechallenge, all animals mounted rapid HCV-specific T-cell responses. The pattern of the cellular immune response in blood and liver correlated with the virological outcome. The animal that most rapidly cleared circulating HCV as determined by nested reverse transcription-PCR (RT-PCR) displayed the most vigorous and sustained response of gamma interferon (IFN-gamma)-producing and proliferating CD4(+) T cells in the blood. Vigorous CD4(+) T-cell proliferation during viremia was followed by an increased frequency and a phenotypic and functional change of the tetramer(+) CD8(+) T-cell population. The second animal cleared HCV initially with strong peripheral and intrahepatic CD4(+) T-cell responses but experienced low-level HCV recrudescence 12 weeks later, when HCV-specific T cells became undetectable. The third animal maintained minute amounts of circulating HCV, detectable only by nested RT-PCR, in the face of a weak IFN-gamma(+) T-cell response. Collectively, the results suggest protective rather than sterilizing immunity after recovery from hepatitis C. The rate of HCV clearance following reexposure depends on the cellular immune response, the quality and quantity of which may vary among chimpanzees that recovered from HCV infection.

Cytokine control of memory T-cell development and survival

Evidence has accumulated that cytokines have a fundamental role in the differentiation of memory T cells. Here, we follow the CD8+ T cell from initial activation to memory-cell generation, indicating the checkpoints at which cytokines determine the fate of the T cell. Members of the common cytokine-receptor gamma-chain (gammac)-cytokine family--in particular, interleukin-7 (IL-7) and IL-15--act at each stage of the immune response to promote proliferation and survival. In this manner, a stable and protective, long-lived memory CD8+ T-cell pool can be propagated and maintained.

Regulation of the generation and maintenance of T-cell memory: a direct, default pathway from effectors to memory cells

Memory T cells are derived directly from effector cells without need for additional antigen, TcR triggering or induced cytokines. A large fraction of effectors can become memory cells without division, supporting a default pathway with little further differentiation. This suggests that the same signals during infection/vaccination determine the extent and nature of both effector and memory cell development.

Cross-reactive antigen is required to prevent erosion of established T cell memory and tumor immunity: a heterologous bacterial model of attrition

Induction and maintenance of T cell memory is critical for the control of intracellular pathogens and tumors. Memory T cells seem to require few "maintenance signals," though often such studies are done in the absence of competing immune challenges. Conversely, although attrition of CD8(+) T cell memory has been characterized in heterologous viral models, this is not the case for bacterial infections. In this study, we demonstrate attrition of T cell responses to the intracellular pathogen Listeria monocytogenes (LM) following an immune challenge with a second intracellular bacterium, Mycobacterium bovis (bacillus Calmette-Guérin, BCG). Mice immunized with either LM or recombinant LM (expressing OVA; LM-OVA), develop a potent T cell memory response. This is reflected by peptide-specific CTL, IFN-gamma production, and frequency of IFN-gamma-secreting T cells to native or recombinant LM Ags. However, when the LM-infected mice are subsequently challenged with BCG, there is a marked reduction in the LM-specific T cell responses. These reductions are directly attributable to the effects on CD4(+) and CD8(+) T cells and the data are consistent with a loss of LM-specific T cells, not anergy. Attrition of the Ag (OVA)-specific T cell response is prevented when LM-OVA-immunized mice are challenged with a subsequent heterologous pathogen (BCG) expressing OVA, demonstrating memory T cell dependence on Ag. Although the reduction of the LM-specific T cell response did not impair protection against a subsequent LM rechallenge, for the first time, we show that T cell attrition can result in the reduction of Ag-specific antitumor (B16-OVA) immunity previously established with LM-OVA immunization.

No one is naive: the significance of heterologous T-cell immunity

Memory T cells that are specific for one virus can become activated during infection with an unrelated heterologous virus, and might have roles in protective immunity and immunopathology. The course of each infection is influenced by the T-cell memory pool that has been laid down by a host's history of previous infections, and with each successive infection, T-cell memory to previously encountered agents is modified. Here, we discuss evidence from studies in mice and humans that shows the importance of this phenomenon in determining the outcome of infection.

Sustained high frequencies of specific CD4 T cells restricted to a single persistent virus

Replication of cytomegalovirus (CMV) is largely controlled by the cellular arm of the immune response. In this study the CMV-specific CD4 T-cell response was characterized in a cohort of apparently healthy individuals. In 11% of all individuals, extremely high frequencies, between 10 and 40%, were found. High-level frequencies of CMV-specific CD4 T cells persisted over several months and were not the result of an acute infection. Specific T cells were oligoclonal and were phenotypically and functionally characterized as mature effector cells, with both cytokine-secreting and proliferative potential. These high-level frequencies do not seem to compromise the immune response towards heterologous infections, and no signs of immunopathology were observed. Whereas a large temporary expansion of virus-specific T cells is well known to occur during acute infection, we now show that extremely high frequencies of virus-specific T cells may continuously exist in chronic CMV infection without overtly compromising the remaining protective immunity.

Depletion of human NK and CD8 cells prior to in vitro H1N1 flu vaccine stimulation increases the number of gamma interferon-secreting cells compared to the initial undepleted population in an ELISPOT assay

In order to study the respective roles of CD4, CD8, and CD56 (NK) cells in gamma interferon (IFN-gamma) production after in vitro stimulation with flu vaccine in a healthy adult human population, we depleted these cellular subtypes before stimulation with antigen (inactivated split vaccine, A/Texas H1N1, or A/Sydney H3N2). We observed that while CD4 cells were required for IFN-gamma secretion in both conditions in vitro, CD56 (NK) cells and, to a lesser extent, CD8 cells had a negative effect on such synthesis upon H1N1 stimulation, as judged by an increased number of spots compared to the initial undepleted population. This regulation of IFN-gamma secretion was associated with an increase in ICAM-1 expression, in particular on T and B cells. This study points out the importance of evaluating in vitro immune responses on a whole-cell population in addition to isolated subtypes if one needs to address potential cellular interactions occurring in vivo in some situations (H1N1 stimulation in the present case). Such cross-regulations occur even in vitro during the antigenic stimulation step.

Identification of broadly recognized, T helper 1 lymphocyte epitopes in an equine lentivirus

Equine infectious anaemia virus (EIAV) is a horse lentivirus causing lifelong, persistent infection. During acute infection, CD8(+) cytotoxic T lymphocytes (CTL) are probably involved in terminating plasma viraemia. However, only a few EIAV CTL epitopes, restricted to fewer horse major histocompatibility complex (MHC) class I alleles, are known. As interferon-gamma (IFN-gamma)-secreting CD4(+), T helper 1 (Th1) lymphocytes promote CTL activity and help maintain memory CTL, identifying broadly recognized EIAV Th1 epitopes would contribute significantly to vaccine strategies seeking to promote strong CTL responses among horses with varying class I haplotypes. To this end, peripheral blood mononuclear cells (PBMC) from 10 MHC disparate, EIAV-infected horses were tested in T-lymphocyte proliferation assays for recognition of peptides from the Gag p26 capsid region and a portion of Pol. Both regions are highly conserved among EIAV isolates, and this Pol region is 51-63% homologous to other lentiviral Pol proteins. Seven of 10 horses recognized peptide Gag 221-245, and peptides Gag 242-261 and Pol 323-344 were recognized by five and four horses, respectively. Furthermore, the Gag peptides were recognized by two additional horses after resolving their initial plasma viraemia, indicating that these two peptides can be immunodominant early in infection. Gag peptide-responsive PBMC produced only IFN-gamma, indicating a Th1 response, while Pol 323-344-responsive PBMC produced IFN-gamma both with and without interleukin-4. PBMC from uninfected horses failed to either proliferate or secrete cytokines in response to peptide stimulation. Finally, CD4(+) T lymphocytes were required for proliferation responses, as shown by assays using CD4- versus CD8-depleted PBMC.

Divergence in the degree of clonal expansions in inflammatory T cell subpopulations mirrors HLA-associated risk alleles in genetically and clinically distinct subtypes of childhood arthritis

Clinically distinct forms of childhood arthritis are associated with different risk alleles of polymorphic loci within the MHC, which code for the antigen-presenting class I or class II molecules. We have compared the TCR diversity of synovial T cells from children with enthesitis-related (HLA-B27(+)) arthritis and oligoarticular arthritis (with class II MHC risk allele associations) in parallel with peripheral blood T cells from each child, using a high-resolution heteroduplex TCR analysis. We demonstrate that multiple clonal T cell expansions are present and persistent within the joint in both groups, but that there is disease-specific divergence in the dominant T cell subset containing these expansions. Thus, the largest clonotypes within the inflamed joints of children with class II-associated arthritis are within the CD4(+) synovial T cell population, while the dominant clones from children with enthesitis-related arthritis (associated with a class I allele) are within the CD8(+) synovial T cell population. These data provide powerful data to support the concept that recognition of MHC-peptide complexes by T cells plays a role in the pathogenesis of juvenile arthritis.