The Tower of Babel of CD8+ T-cell memory: known facts, deserted roads, muddy waters, and possible dead ends

Coronin 1-dependent cell density sensing and regulation of the peripheral T cell population size

The establishment and maintenance of peripheral T cells is important to ensure appropriate immunity. In mammals, T cells are produced in the thymus before seeding the periphery early in life, and thereafter progressive thymus involution impairs new T cell production. Yet, peripheral T cells are maintained lifelong at approximately similar cell numbers. The question thus arises: what are the mechanisms that enable the maintenance of the appropriate number of circulating T cells, ensuring that T cell numbers are neither too low nor too high? Here, we highlight recent research suggesting a key role for coronin 1, a member of the evolutionarily conserved family of coronin proteins, in both allowing T cells to reach as well as maintain their appropriate cell population size. This cell population size controlling pathway was found to be conserved in amoeba, mice and human. We propose that coronin 1 is an integral part of a cell-intrinsic pathway that couples cell density information with prosurvival signalling thereby regulating the appropriate number of peripheral T cells.

Diverse Virus and Host-Dependent Mechanisms Influence the Systemic and Intrahepatic Immune Responses in the Woodchuck Model of Hepatitis B

Woodchuck infected with woodchuck hepatitis virus (WHV) represents the pathogenically nearest model of hepatitis B and associated hepatocellular carcinoma (HCC). This naturally occurring animal model also is highly valuable for development and preclinical evaluation of new anti-HBV agents and immunotherapies against chronic hepatitis (CH) B and HCC. Studies in this system uncovered a number of molecular and immunological processes which contribute or likely contribute to the immunopathogenesis of liver disease and modulation of the systemic and intrahepatic innate and adaptive immune responses during hepadnaviral infection. Among them, inhibition of presentation of the class I major histocompatibility complex on chronically infected hepatocytes and a role of WHV envelope proteins in this process, as well as augmented hepatocyte cytotoxicity mediated by constitutively expressed components of CD95 (Fas) ligand- and perforin-dependent pathways, capable of eliminating cells brought to contact with hepatocyte surface, including activated T lymphocytes, were uncovered. Other findings pointed to a role of autoimmune response against hepatocyte asialoglycoprotein receptor in augmenting severity of liver damage in hepadnaviral CH. It was also documented that WHV in the first few hours activates intrahepatic innate immunity that transiently decreases hepatic virus load. However, this activation is not translated in a timely manner to induction of virus-specific T cell response which appears to be hindered by defective activation of antigen presenting cells and presentation of viral epitopes to T cells. The early WHV infection also induces generalized polyclonal activation of T cells that precedes emergence of virus-specific T lymphocyte reactivity. The combination of these mechanisms hinder recognition of virus allowing its dissemination in the initial, asymptomatic stages of infection before adaptive cellular response became apparent. This review will highlight a range of diverse mechanisms uncovered in the woodchuck model which affect effectiveness of the anti-viral systemic and intrahepatic immune responses, and modify liver disease outcomes. Further exploration of these and other mechanisms, either already discovered or yet unknown, and their interactions should bring more comprehensive understanding of HBV pathogenesis and help to identify novel targets for therapeutic and preventive interventions. The woodchuck model is uniquely positioned to further contribute to these advances.

CD8+ effector memory T cells induce acute rejection of allogeneic heart retransplants in mice possibly through activating expression of inflammatory cytokines

BACKGROUND

To investigate the effects of CD8⁺ memory T (Tm) cells and CD8⁺ effector memory T (Tem) cells on the results of allogeneic heart retransplantations performed in mice.

METHODS

A skin transplantation model was used to generate sensitized splenic CD8⁺ Tem cells for infusion into BALB/c mice. One week after infusion, the BALB/c mice underwent allogeneic heart transplantation in the abdominal cavity. Cyclosporin A was administered via intraperitoneal injection starting one day prior to transplantation to arrest immunological rejection of the transplanted heart. The effects of sensitized CD8⁺ T_em cells on allogeneic heart graft rejection were examined by monitoring survival of the transplanted hearts, the infiltration of effector memory CD8⁺ T cells into myocardium, and expressions of inflammatory cytokines in blood serum.

RESULTS

Adoptive transfer of sensitized CD8⁺ Tem cells prior to transplantation induced an acute rejection response which decreased the survival of transplanted hearts. The rejection response was accompanied by an infiltration of CD8⁺ Tem cells into the transplanted myocardial tissue. Additionally, infusion of sensitized CD8⁺ Tem cells induced markedly increased expressions of IL-2 and IFN-γ, and decreased expression of TGF-β in the transplanted hearts, as well as higher levels of IFN-γ and CXCL-9 in blood serum.

CONCLUSIONS

The infusion of sensitized CD8⁺ Tem cells induced an acute graft rejection response and decreased the survival of grafted hearts by regulating the expressions of inflammatory cytokines including CXCL-9, IL-2, and INF-γ. Cyclosporin A had no therapeutic effect on the graft rejection response induced by sensitized CD8⁺ Tem cells.

G0S2 modulates homeostatic proliferation of naïve CD8⁺ T cells and inhibits oxidative phosphorylation in mitochondria

Since its discovery, diverse functions have been attributed to the G₀/G₁ switch gene 2 (G0S2), from lipid metabolism to control of cell proliferation. Our group showed for the first time that G0S2 promotes quiescence in hematopoietic stem cells by interacting with and retaining nucleolin around the nucleus. Herein, we report the role of G0S2 in the differentiation and function of CD8⁺ T cells examined in mice with an embryonic deletion of the G0s2 gene. G0S2 expression in naïve CD8⁺ T cells decreased immediately after T-cell receptor activation downstream of the MAPK, calcium/calmodulin, PI3K, and mTOR pathways. Surprisingly, G0S2-null naïve CD8⁺ T cells displayed increased basal and spare respiratory capacity that was not associated with increased mitochondrial biogenesis but with increased phosphorylation of AMPKα. Naïve CD8⁺ T cells showed increased proliferation in response to in vitro activation and in vivo lymphopenia; however, naïve CD8⁺ T cells expressing the OT-1 transgene exhibited normal differentiation of naïve cells to effector and memory CD8⁺ T cells upon infection with Listeria monocytogenes in a wild type or a G0s2-null environment, with increased circulating levels of free fatty acids. Collectively, our results suggest that G0S2 inhibits energy production by oxidative phosphorylation to fine-tune proliferation in homeostatic conditions.

A mathematical model for a T cell fate decision algorithm during immune response

We formulate and analyze an algorithm of cell fate decision that describes the way in which division vs. apoptosis choices are made by individual T cells during an infection. Such model involves a minimal number of known biochemical mechanisms: it basically relies on the interplay between cell division and cell death inhibitors on one hand, and membrane receptors on the other. In spite of its simplicity, the proposed decision algorithm is able to account for some significant facts in immune response. At the individual level, the existence of T cells that continue to replicate in the absence of antigen and the possible occurrence of T cell apoptosis in the presence of antigen are predicted by the model. Moreover, the latter is shown to yield an emergent collective behavior, the observed delay in clonal contraction with respect to the end of antigen stimulation, which is shown to arise just from individual T cell decisions made according to the proposed mechanism.

Uncoupling T-cell expansion from effector differentiation in cell-based immunotherapy

Adoptive cellular immunotherapy (ACT) is a potentially curative therapy for patients with advanced cancer. Eradication of tumor in mouse models and humans correlates with both a high dose of adoptively transferred cells and cells with a minimally differentiated phenotype that maintain replicative capacity and multipotency. We speculate that response to ACT not only requires transfer of cells with immediate cytolytic effector function to kill the bulk of fast-growing tumor but also transfer of tumor-specific cells that maintain an ability for self-renewal and the capacity to produce a continual supply of cytolytic effector progeny until all malignant cells are eliminated. Current in vitro methods to expand cells to sufficient numbers and still maintain a minimally differentiated phenotype are hindered by the biological coupling of clonal expansion and effector differentiation. Therefore, a better understanding of the physiologic mechanism that couples cell expansion and differentiation in CD8(+) T cells may improve the efficacy of ACT.

Persistence of EBV antigen-specific CD8 T cell clonotypes during homeostatic immune reconstitution in cancer patients

Persistent viruses are kept in check by specific lymphocytes. The clonal T cell receptor (TCR) repertoire against Epstein-Barr virus (EBV), once established following primary infection, exhibits a robust stability over time. However, the determinants contributing to this long-term persistence are still poorly characterized. Taking advantage of an in vivo clinical setting where lymphocyte homeostasis was transiently perturbed, we studied EBV antigen-specific CD8 T cells before and after non-myeloablative lympho-depleting chemotherapy of melanoma patients. Despite more advanced T cell differentiation, patients T cells showed clonal composition comparable to healthy individuals, sharing a preference for TRBV20 and TRBV29 gene segment usage and several co-dominant public TCR clonotypes. Moreover, our data revealed the presence of relatively few dominant EBV antigen-specific T cell clonotypes, which mostly persisted following transient lympho-depletion (TLD) and lymphocyte recovery, likely related to absence of EBV reactivation and de novo T cell priming in these patients. Interestingly, persisting clonotypes frequently co-expressed memory/homing-associated genes (CD27, IL7R, EOMES, CD62L/SELL and CCR5) supporting the notion that they are particularly important for long-lasting CD8 T cell responses. Nevertheless, the clonal composition of EBV-specific CD8 T cells was preserved over time with the presence of the same dominant clonotypes after non-myeloablative chemotherapy. The observed clonotype persistence demonstrates high robustness of CD8 T cell homeostasis and reconstitution.

The integration of T cell migration, differentiation and function

T cells function locally. Accordingly, T cells' recognition of antigen, their subsequent activation and differentiation, and their role in the processes of infection control, tumour eradication, autoimmunity, allergy and alloreactivity are intrinsically coupled with migration. Recent discoveries revise our understanding of the regulation and patterns of T cell trafficking and reveal limitations in current paradigms. Here, we review classic and emerging concepts, highlight the challenge of integrating new observations with existing T cell classification schemes and summarize the heuristic framework provided by viewing T cell differentiation and function first through the prism of migration.

High avidity cytotoxic T lymphocytes can be selected into the memory pool but they are exquisitely sensitive to functional impairment

High avidity cytotoxic T lymphocytes (CTL) are important in viral clearance and anti-tumor immunity, however, mechanisms for their optimal generation and maintenance in vivo remain unclear. Immunizing mice with an antibody-DNA vaccine encoding a single CTL epitope, induces a 100 fold higher avidity response than peptide vaccination with the identical epitope. The high avidity response is retained into memory and can be efficiently reactivated with an antibody-DNA boost. In contrast, reactivation of high avidity CTL with peptide, stimulated responses with a significant drop in avidity, suggesting loss or conversion of the high avidity CTL to lower avidity. Similarly, high avidity T cells maintained ex vivo were exquisitely sensitive to signaling with low doses of peptide (1 ng/ml) giving optimal TCR stimulation and resulting in retained avidity, proliferation and ability to kill specific targets. In contrast, high avidity T cells maintained ex vivo with supraoptimal TCR stimulation (10 µg/ml peptide) resulted in reduced avidity and failure to kill tumor cells. They also failed to proliferate, showed a significant increase in apoptosis and expressed high levels of the exhaustion marker programmed death-1 (PD-1) and low levels of the lymphocyte-activation gene 3 (LAG-3). This suggests high avidity T cells are recruited to the memory pool but can be lost by supraoptimal stimulation in vitro and in vivo. This is characterized by loss of function and an increase in cell death. The remaining CTL, exhibit low functional avidity that is reflected in reduced anti-tumor activity. This could contribute to failure of the immune system to control the growth of tumors and has implications for vaccination strategies and adoptive transfer of T cells.

Synergistic CD40 signaling on APCs and CD8 T cells drives efficient CD8 response and memory differentiation

The role of CD4 help during CD8 response and memory differentiation has been clearly demonstrated in different experimental models. However, the exact mechanisms of CD4 help remain largely unknown and preclude replacement therapy to develop. Interestingly, studies have shown that administration of an agonist aCD40ab can substitute CD4 help in vitro and in vivo, whereas the targets of this antibody remain elusive. In this study, we address the exact role of CD40 expression on APCs and CD8 T cells using aCD40ab treatment in mice. We demonstrate that aCD40 antibodies have synergetic effects on APCs and CD8 T cells. Full efficiency of aCD40 treatment requires CD40 expression on both populations: if one of these cell populations is CD40-deficient, the CD8 T cell response is impaired. Most importantly, direct CD40 signaling on APCs and CD8 T cells affects CD8 T cell differentiation differently. In our model, CD40 expression on APCs plays an important but dispensable role on CD8 T cell expansion and effector functions during the early phase of the immune response. Conversely, CD40 on CD8 T cells is crucial and nonredundant for their progressive differentiation into memory cells. Altogether, these results highlight that CD40-CD40L-dependent and independent effects of CD4 help to drive a complete CD8 T cell differentiation.

A spatial view of the CD8+ T-cell response: the case of HCV

In viral infections, a memory T-cell population comprises multiple subtypes of cells, distributed in diverse anatomic compartments and possibly re-circulating among them. Accordingly, memory T cells display distinct phenotypes and functions, depending on the nature of the infecting virus, the anatomic location of the infection, and the differences between the sites of active infection and T-cell collection. This paper explores the body compartments where virus-specific CD8(+) T cells have been found during chronic hepatitis C virus infection, describes the cells' memory qualities, and discusses how they are spatially regulated, in comparison with other human viral infections. Understanding the role of compartmentalization and diversity of HCV-specific memory T-cell subsets may be the key to developing effective immunotherapies.

CD40L co-stimulation from CD8+ to CD4+ effector memory T cells supports CD4+ expansion

Effector memory T cells (T(EM)) have an important role in immunity against infection. However, little is known about the factors regulating T(EM) maintenance and proliferation. In this study, we investigated the role of direct interactions between CD4(+) and CD8(+) T cells (TC) for human T(EM) expansion. Proliferation of separated or mixed CD4(+) and CD8(+)T(EM) populations was analyzed after polyclonal stimulation in vitro. Compared to each isolated subset mixed T(EM) populations showed increased proliferation and expansion of both CD4(+) and CD8(+)T(EM) subpopulations. Combined activation of CD4(+) and CD8(+) memory T cells (Tmem) induced an increased expression of CD40L and CD40 on both populations. Subsequently, CD40/CD40L caused a bi-directional stimulation of CD40(+)CD4(+)T(EM) by CD40L(+)CD8(+)T(EM) and of CD40(+)CD8(+)T(EM) by CD40L(+)CD4(+)T(EM). Blocking of CD40L on activated CD8(+)T(EM) selectively inhibited proliferation of CD4(+)T(EM), while blocking of CD40L on CD4(+)T(EM) abrogated proliferation of CD8(+)T(EM). Taken together, we demonstrate for the first time that the expression of CD40L is exploited on the one hand by CD8(+)T(EM) to increase the proliferation of activated CD4(+)T(EM) and on the other hand by CD4(+)T(EM) to support the expansion of activated CD8(+)T(EM). Thus, efficient T(EM) expansion requires bi-directional interactions between CD4(+) and CD8(+)T(EM) cells.

The burden of obesity on infectious disease

The world is now experiencing an epidemic of obesity. Although the effects of obesity on the development of metabolic and cardiovascular problems are well studied, much less is known about the impact of obesity on immune function and infectious disease. Studies in obese humans and with obese animal models have repeatedly demonstrated impaired immune function, including decreased cytokine production, decreased response to antigen/mitogen stimulation, reduced macrophage and dendritic cell function, and natural killer cell impairment. Recent studies have demonstrated that the impaired immune response in the obese host leads to increased susceptibility to infection with a number of different pathogens such as community-acquired tuberculosis, influenza, Mycobacterium tuberculosis, coxsackievirus, Helicobacter pylori and encephalomyocarditis virus. While no specific mechanism has been defined for the decreased immune response to infectious disease in the obese host, several obesity-associated changes such as excessive inflammation, altered adipokine signaling, metabolic changes and even epigenetic regulation could affect the immune response. This review will discuss what is currently known about the relationship between obesity and infectious disease.

Profile of a serial killer: cellular and molecular approaches to study individual cytotoxic T-cells following therapeutic vaccination

T-cell vaccination may prevent or treat cancer and infectious diseases, but further progress is required to increase clinical efficacy. Step-by-step improvements of T-cell vaccination in phase I/II clinical studies combined with very detailed analysis of T-cell responses at the single cell level are the strategy of choice for the identification of the most promising vaccine candidates for testing in subsequent large-scale phase III clinical trials. Major aims are to fully identify the most efficient T-cells in anticancer therapy, to characterize their TCRs, and to pinpoint the mechanisms of T-cell recruitment and function in well-defined clinical situations. Here we discuss novel strategies for the assessment of human T-cell responses, revealing in part unprecedented insight into T-cell biology and novel structural principles that govern TCR-pMHC recognition. Together, the described approaches advance our knowledge of T-cell mediated-protection from human diseases.

Transcriptional regulation during CD8 T-cell immune responses

Naïve CD8 T cells differentiate in response to antigen stimulation. They acquire the capacity to express multiple effector molecules and mediate effector functions that contribute to infection control. Once antigen loads are reduced they revert progressively to a less activated status and eventually reach a steady-state referred to as "memory" that is very different from that of naive cells. Indeed, these "memory" cells are "ready-to-go" populations that acquired the capacity to respond more efficiently to antigen stimulation. They modify their cell cycle machinery in order to divide faster; they likely improve DNA repair and other cell survival mechanisms in order to survive during division and thus to generate much larger clones of effector cells; finally, they also mediate effector functions much faster. These modifications are the consequence of changes in the expression of multiple genes, i.e., on the utilization of a new transcription program.

Visualizing early splenic memory CD8+ T cells reactivation against intracellular bacteria in the mouse

Memory CD8(+) T cells represent an important effector arm of the immune response in maintaining long-lived protective immunity against viruses and some intracellular bacteria such as Listeria monocytogenes (L.m). Memory CD8(+) T cells are endowed with enhanced antimicrobial effector functions that perfectly tail them to rapidly eradicate invading pathogens. It is largely accepted that these functions are sufficient to explain how memory CD8(+) T cells can mediate rapid protection. However, it is important to point out that such improved functional features would be useless if memory cells were unable to rapidly find the pathogen loaded/infected cells within the infected organ. Growing evidences suggest that the anatomy of secondary lymphoid organs (SLOs) fosters the cellular interactions required to initiate naive adaptive immune responses. However, very little is known on how the SLOs structures regulate memory immune responses. Using Listeria monocytogenes (L.m) as a murine infection model and imaging techniques, we have investigated if and how the architecture of the spleen plays a role in the reactivation of memory CD8(+) T cells and the subsequent control of L.m growth. We observed that in the mouse, memory CD8(+) T cells start to control L.m burden 6 hours after the challenge infection. At this very early time point, L.m-specific and non-specific memory CD8(+) T cells localize in the splenic red pulp and form clusters around L.m infected cells while naïve CD8(+) T cells remain in the white pulp. Within these clusters that only last few hours, memory CD8(+) T produce inflammatory cytokines such as IFN-gamma and CCL3 nearby infected myeloid cells known to be crucial for L.m killing. Altogether, we describe how memory CD8(+) T cells trafficking properties and the splenic micro-anatomy conjugate to create a spatio-temporal window during which memory CD8(+) T cells provide a local response by secreting effector molecules around infected cells.

CCR6: a biomarker for Alzheimer's-like disease in a triple transgenic mouse model

The inflammatory status of the brain in patients as well as animal models of Alzheimer's disease (AD) has been extensively studied. Accumulation of activated microglia producing tumor necrosis factor-α and monocyte chemotactic protein-1 contribute to the pathology of the disease. However, little is known about the changes in the spleen and associated peripheral immunity that might contribute to AD pathology. The goal of this study was to characterize phenotypic and functional changes in spleen, blood and brain cell populations that contribute to development of an AD-like disease in a triple transgenic (3xTg-AD) mouse model. The 3xTg-AD mice had increased percentages of brain Gr-1+ granulocytes, dendritic cells and macrophages, spleen and blood derived CD8+Ly6C+ memory T cells and CCR6+ B cells, as well as increased levels of secreted interleukin-6. Brain tissue from older 12 month old symptomatic 3xTg-AD female mice exhibited highly elevated mRNA expression of CCR6 compared to wild-type mice. Importantly, this pronounced increase in expression of CCR6 was also detected in brain and spleen tissue from pre-symptomatic 5--6 month old 3xTg-AD females and males. Our data demonstrate increased expression of CCR6 in the brain and peripheral immune organs of both pre-symptomatic and symptomatic 3xTg-AD mice, strongly suggesting an ongoing inflammatory process that precedes onset of clinical AD-like disease.

Single-unit dominance after double-unit umbilical cord blood transplantation coincides with a specific CD8+ T-cell response against the nonengrafted unit

We investigated the potential role of an immune reaction in mediating the dominant engraftment of 1 cord blood unit in 14 patients who received a double-unit cord blood transplantation (CBT). In 10 patients, dominant engraftment of a single donor unit emerged by day 28 after CBT. In 9 of these 10 patients, a significant subset of CD8(+) CD45RO(+/-)CCR7(-) T cells, present in peripheral blood mononuclear cells and derived from the engrafting cord blood unit, produced interferon-gamma (IFN-gamma) in response to the nonengrafting unit. No significant population of IFN-gamma-secreting cells was detectable when posttransplantation peripheral blood mononuclear cells were stimulated against cells from the engrafted unit (P < .001) or from a random human leukocyte antigen disparate third party (P = .003). Three patients maintained persistent mixed chimerism after CBT, and no significant IFN-gamma-secreting cells were detected after similar stimulations in these patients (P < .005). Our data provide the first direct evidence in human double-unit CBT recipients that immune rejection mediated by effector CD8(+) T cells developing after CBT from naive precursors is responsible for the failure of 1 unit to engraft. Future investigations based on these findings may result in strategies to predict a dominant unit and enhance graft-versus-leukemia effect.

Primary occult hepadnavirus infection induces virus-specific T-cell and aberrant cytokine responses in the absence of antiviral antibody reactivity in the Woodchuck model of hepatitis B virus infection

Although the virological features of serologically silent hepadnaviral primary occult infection (POI) have been relatively well recognized in the woodchuck model of hepatitis B virus infection, the characteristics of accompanying immune responses remain unknown. In this study, the kinetics of woodchuck hepatitis virus (WHV)-specific and generalized (mitogen-induced) T-cell proliferative responses and cytokine expression profiles in circulating lymphoid cells and the liver, along with WHV-specific antibody responses, were investigated during experimentally induced POI and subsequent challenge with a liver-pathogenic dose (>10(3) virions) or liver-nonpathogenic dose (50 virions) of the same virus. The data revealed that POI, which does not prompt WHV surface antigenemia, antiviral antibody response, and hepatitis or protect from challenge with a liver-pathogenic virus dose, was accompanied by the appearance of a strong WHV-specific T-cell response directed against multiple viral epitopes that intermittently persisted at low levels for up to 10-months during follow-up. Furthermore, immediately after exposure to a liver-nonpathogenic dose of WHV, lymphocytes acquired a heightened capacity to proliferate in response to mitogenic stimuli and displayed augmented expression of alpha interferon, interleukin-12 (IL-12), and IL-2, but not tumor necrosis factor alpha. Overall, the kinetics of WHV-specific and mitogen-induced T-cell proliferative and cytokine responses in POI were closely comparable to those seen in infection induced by liver-pathogenic viral doses. The data demonstrated that virus-specific T-cell proliferative reactivity is a very sensitive indicator of exposure to hepadnavirus, even to small amounts inducing serologically mute infection. They also showed that hepadnaviral POI is not only a molecularly but also an immunologically identifiable and distinctive entity.

Melanoma-specific memory T cells are functionally active in Ret transgenic mice without macroscopic tumors

We previously reported that bone marrows of breast cancer patients contained tumor antigen-specific CD8(+) T cells with central or effector memory phenotype. Using a recently developed ret transgenic mouse melanoma model, we now show that bone marrows and tumors of transgenic mice contain high frequencies of CD8(+) T cells specific for the melanoma antigen tyrosinase-related protein 2 and showing mostly effector memory phenotype. Moreover, increased numbers of bone marrow tyrosinase-related protein-2-specific effector memory CD8(+) T cells are also detected in transgenic animals older than 20 weeks with disseminated melanoma cells in the bone marrow and lymph nodes but showing no visible skin tumors and no further melanoma progression. After a short-term coincubation with dendritic cells generated from the bone marrow and pulsed with melanoma lysates, bone marrow memory T cells from mice without macroscopic melanomas produced IFN-gamma in vitro and exerted antitumor activity in vivo after adoptive transfer into melanoma-bearing mice. Our data indicate that functionally active bone marrow-derived melanoma-specific memory T cells are detectable at the phase of microscopic tumor load, suggesting that thereby they could control disseminated melanoma cells.

Phenotype and functional characterization of long-term gp100-specific memory CD8+ T cells in disease-free melanoma patients before and after boosting immunization

PURPOSE

Effective cancer vaccines must both drive a strong CTL response and sustain long-term memory T cells capable of rapid recall responses to tumor antigens. We sought to characterize the phenotype and function of gp100 peptide-specific memory CD8+ T cells in melanoma patients after primary gp100(209-2M) immunization and assess the anamnestic response to boosting immunization.

EXPERIMENTAL DESIGN

Eight-color flow cytometry analysis of gp100-specific CD8+ T cells was done on peripheral blood mononuclear cells collected shortly after the primary vaccine regimen, 12 to 24 months after primary vaccination, and after boosting immunization. The anamnestic response was assessed by comparing the frequency of circulating gp100-specific T cells before and after boosting. Gp100 peptide-induced in vitro functional avidity and proliferation responses and melanoma-stimulated T-cell CD107 mobilization were compared for cells from all three time points for multiple patients.

RESULTS

The frequency of circulating gp100-specific memory CD8+ T cells was comparable with cytomegalovirus-specific and FLU-specific T cells in the same patients, and the cells exhibited anamnestic proliferation after boosting. Their phenotypes were not unique, and individual patients exhibited one of two distinct phenotype signatures that were homologous to either cytomegalovirus-specific or FLU-specific memory T cells. Gp100-specific memory T cells showed some properties of competent memory T cells, such as heightened in vitro peptide-stimulated proliferation and increase in central memory (TCM) differentiation when compared with T-cell responses measured after the primary vaccine regimen. However, they did not acquire enhanced functional avidity usually associated with competent memory T-cell maturation.

CONCLUSIONS

Although vaccination with class I-restricted melanoma peptides alone can break tolerance to self-tumor antigens, it did not induce fully competent memory CD8+ T cells--even in disease-free patients. Data presented suggest other vaccine strategies will be required to induce functionally robust long-term memory T cells.

Donor-reactive CD8 memory T cells infiltrate cardiac allografts within 24-h posttransplant in naive recipients

Normal immune responses stimulated by pathogenic and environmental antigens generate memory T cells that react with donor antigens and no currently used immunosuppressive drug completely inhibits memory T-cell function. While donor-reactive memory T cells clearly compromise graft outcomes, mechanisms utilized by memory T cells to promote rejection are largely unknown. In this study, we investigated how early endogenous memory cells infiltrate and express effector function in cardiac allografts. Endogenous CD8 memory T cells in nonsensitized recipients distinguish syngeneic versus allogeneic cardiac allografts within 24 h of reperfusion. CD8-dependent production of IFN-gamma and CXCL9/Mig was observed 24 to 72 h posttransplant in allografts but not isografts. CXCL9 was produced by donor cells in response to IFN-gamma made by recipient CD8 T cells reactive to donor class I major histocompatibility complex (MHC) molecules. Activated CD8 T cells were detected in allografts at least 3 days before donor-specific effector T cells producing IFN-gamma were detected in the recipient spleen. Early inflammation mediated by donor-reactive CD8 memory T cells greatly enhanced primed effector T-cell infiltration into allografts. These results suggest that strategies for optimal inhibition of alloimmunity should include neutralization of infiltrating CD8 memory T cells within a very narrow window after transplantation.

Aberrant lymphocyte activation precedes delayed virus-specific T-cell response after both primary infection and secondary exposure to hepadnavirus in the woodchuck model of hepatitis B virus infection

The contribution of virus-specific T lymphocytes to the outcome of acute hepadnaviral hepatitis is well recognized, but a reason behind the consistent postponement of this response remains unknown. Also, the characteristics of T-cell reactivity following reexposure to hepadnavirus are not thoroughly recognized. To investigate these issues, healthy woodchucks (Marmota monax) were infected with liver-pathogenic doses of woodchuck hepatitis virus (WHV) and investigated unchallenged or after challenge with the same virus. As expected, the WHV-specific T-cell response appeared late, 6 to 7 weeks postinfection, remained high during acute disease, and then declined but remained detectable long after the resolution of hepatitis. Interestingly, almost immediately after infection, lymphocytes acquired a heightened capacity to proliferate in response to mitogenic (nonspecific) stimuli. This reactivity subsided before the WHV-specific T-cell response appeared, and its decline coincided with the cells' augmented susceptibility to activation-induced death. The analysis of cytokine expression profiles confirmed early in vivo activation of immune cells and revealed their impairment of transcription of tumor necrosis factor alpha and gamma interferon. Strikingly, reexposure of the immune animals to WHV swiftly induced hyperresponsiveness to nonspecific stimuli, followed again by the delayed virus-specific response. Our data show that both primary and secondary exposures to hepadnavirus induce aberrant activation of lymphocytes preceding the virus-specific T-cell response. They suggest that this activation and the augmented death of the cells activated, accompanied by a defective expression of cytokines pivotal for effective T-cell priming, postpone the adaptive T-cell response. These impairments likely hamper the initial recognition and clearance of hepadnavirus, permitting its dissemination in the early phase of infection.

Expansion of human cytomegalovirus-specific T lymphocytes from unfractionated peripheral blood mononuclear cells with artificial antigen-presenting cells

BACKGROUND

The aim of this study was to find a simple and feasible method for ex vivo expansion of human cytomegalovirus (CMV)-specific cytotoxic T cells from unfractionated peripheral blood mononuclear cells (PBMNCs).

STUDY DESIGN AND METHODS

Unfractionated PBMNCs from three HLA-A*0201-CMV-seropositive donors were stimulated with CMVpp65(495-503) peptide-loaded HLA-A*0201-immunoglobulin fusion protein (HLA-A2-Ig) based artificial antigen-presenting cells (aAPCs) on Day 1. Once a week the CMV-specific T cells were harvested and restimulated with fresh aAPCs. T-cell cultures were maintained for 28 days and then analyzed.

RESULTS

With aAPCs and starting with 1x10(7) freshly isolated PBMNCs that were less than 0.1 percent CMV-specific, more than 1x10(7) T cells with a CMV-specific frequency greater than 93 percent in all donors tested were generated. Expanded CD8+ cytotoxic T lymphocytes were functionally active and showed antigen-specific secretion of interferon-gamma and cytotoxic activity. No alloreactivity against unpulsed HLA-A*0201-positive cells was detected.

CONCLUSION

Herein is reported the successful in vitro expansion of CMV-specific cytotoxic CD8+ T cells from unfractionated PBMNCs of healthy CMV-seropositive blood donors by the use of HLA-A2-Ig-based aAPCs. This study demonstrates that more than 1x10(7) CMV-specific T cells can be generated from approximately 1x10(7) unfractionated PBMNCs within 1 month under highly reproducible conditions.

Assessing ageing of individual T lymphocytes: mission impossible?

Effector T lymphocytes are the progeny of a limited number of antigen-specific precursor cells and it has been estimated that clonotypic human T cells may expand million fold on their way reaching high cell numbers that are sufficient for immune protection. Moreover, memory T cell responses are characterized by repetitive expansion of antigen-specific T cell clonotypes, and limitations in the proliferative capacity could lead to immune senescence. Because telomeres progressively shorten as a function of cell division, telomere length is a powerful indicator of the replicative in vivo history of human T lymphocytes. In this review, we summarize observations made over the last decade on telomere length dynamics of well-defined T cell populations derived from healthy donors and patients with infectious disease or cancer. We focus on T cell differentiation, T cell ageing, and natural and vaccine induced immune responses. We also discuss the scientific evidence for in vivo replicative senescence of antigen-specific T cells, and evaluate the available methods for measuring telomere lengths and telomerase activity, and their potential and limitations to increase our understanding of T cell physiology.

Memory CD8+ T cell differentiation in viral infection: A cell for all seasons

Chronic viral infections such as hepatitis B virus (HBV), hepatitis C virus (HCV) and human immunodeficiency virus (HIV) are major global health problems affecting more than 500 million people worldwide. Virus-specific CD8+ T cells play an important role in the course and outcome of these viral infections and it is hypothesized that altered or impaired differentiation of virus-specific CD8+ T cells contributes to the development of persistence and/or disease progression. A deeper understanding of the mechanisms responsible for functional differentiation of CD8+ T cells is essential for the generation of successful therapies aiming to strengthen the adaptive component of the immune system.

IL-2 regulates expansion of CD4+ T cell populations by affecting cell death: insights from modeling CFSE data

It is generally accepted that IL-2 influences the dynamics of populations of T cells in vitro and in vivo. However, which parameters for cell division and/or death are affected by IL-2 is not well understood. To get better insights into the potential ways of how IL-2 may influence the population dynamics of T cells, we analyze data on the dynamics of CFSE-labeled polyclonal CD4(+) T lymphocytes in vitro after anti-CD3 stimulation at different concentrations of exogenous IL-2. Inferring cell division and death rates from CFSE-delabeling experiments is not straightforward and requires the use of mathematical models. We find that to adequately describe the dynamics of T cells at low concentrations of exogenous IL-2, the death rate of divided cells has to increase with the number of divisions cells have undergone. IL-2 hardly affects the average interdivision time. At low IL-2 concentrations 1) fewer cells are recruited into the response and successfully complete their first division; 2) the stochasticity of cell division is increased; and 3) the rate, at which the death rate increases with the division number, increases. Summarizing, our mathematical reinterpretation suggests that the main effect of IL-2 on the in vitro dynamics of naive CD4(+) T cells occurs by affecting the rate of cell death and not by changing the rate of cell division.

Four functionally distinct populations of human effector-memory CD8+ T lymphocytes

In humans, the pathways of memory and effector T cell differentiation remain poorly defined. We have dissected the functional properties of ex vivo effector-memory (EM) CD45RA-CCR7- T lymphocytes present within the circulating CD8+ T cell pool of healthy individuals. Our studies show that EM T cells are heterogeneous and are subdivided based on differential CD27 and CD28 expression into four subsets. EM(1) (CD27+CD28+) and EM(4) (CD27-CD28+) T cells express low levels of effector mediators such as granzyme B and perforin and high levels of CD127/IL-7Ralpha. EM(1) cells also have a relatively short replicative history and display strong ex vivo telomerase activity. Therefore, these cells are closely related to central-memory (CD45RA-CCR7+) cells. In contrast, EM(2) (CD27+CD28-) and EM(3) (CD27-CD28-) cells express mediators characteristic of effector cells, whereby EM(3) cells display stronger ex vivo cytolytic activity and have experienced larger numbers of cell divisions, thus resembling differentiated effector (CD45RA+CCR7-) cells. These data indicate that progressive up-regulation of cytolytic activity and stepwise loss of CCR7, CD28, and CD27 both characterize CD8+ T cell differentiation. Finally, memory CD8+ T cells not only include central-memory cells but also EM(1) cells, which differ in CCR7 expression and may therefore confer memory functions in lymphoid and peripheral tissues, respectively.

CD8 single-cell gene coexpression reveals three different effector types present at distinct phases of the immune response

To study in vivo CD8 T cell differentiation, we quantified the coexpression of multiple genes in single cells throughout immune responses. After in vitro activation, CD8 T cells rapidly express effector molecules and cease their expression when the antigen is removed. Gene behavior after in vivo activation, in contrast, was quite heterogeneous. Different mRNAs were induced at very different time points of the response, were transcribed during different time periods, and could decline or persist independently of the antigen load. Consequently, distinct gene coexpression patterns/different cell types were generated at the various phases of the immune responses. During primary stimulation, inflammatory molecules were induced and down-regulated shortly after activation, generating early cells that only mediated inflammation. Cytotoxic T cells were generated at the peak of the primary response, when individual cells simultaneously expressed multiple killer molecules, whereas memory cells lost killer capacity because they no longer coexpressed killer genes. Surprisingly, during secondary responses gene transcription became permanent. Secondary cells recovered after antigen elimination were more efficient killers than cytotoxic T cells present at the peak of the primary response. Thus, primary responses produced two transient effector types. However, after boosting, CD8 T cells differentiate into long-lived killer cells that persist in vivo in the absence of antigen.