Homeostatic proliferation but not the generation of virus specific memory CD8 T cells is impaired in the absence of IL-15 or IL-15Ralpha

NK-like CD8 T cell: one potential evolutionary continuum between adaptive memory and innate immunity

CD8 T cells are crucial adaptive immune cells with cytotoxicity to fight against pathogens or abnormal self-cells via major histocompatibility complex class I-dependent priming pathways. The composition of the memory CD8 T-cell pool is influenced by various factors. Physiological aging, chronic viral infection, and autoimmune diseases promote the accumulation of CD8 T cells with highly differentiated memory phenotypes. Accumulating studies have shown that some of these memory CD8 T cells also exhibit innate-like cytotoxicity and upregulate the expression of receptors associated with natural killer (NK) cells. Further analysis shows that these NK-like CD8 T cells have transcriptional profiles of both NK and CD8 T cells, suggesting the transformation of CD8 T cells into NK cells. However, the specific induction mechanism underlying NK-like transformation and the implications of this process for CD8 T cells are still unclear. This review aimed to deduce the possible differentiation model of NK-like CD8 T cells, summarize the functions of major NK-cell receptors expressed on these cells, and provide a new perspective for exploring the role of these CD8 T cells in health and disease.

B cells promote CD8 T cell primary and memory responses to subunit vaccines

The relationship between B cells and CD4 T cells has been carefully studied, revealing a collaborative effort in which B cells promote the activation, differentiation, and expansion of CD4 T cells while the so-called “helper” cells provide signals to B cells, influencing their class switching and fate. Interactions between B cells and CD8 T cells are not as well studied, although CD8 T cells exhibit an accelerated contraction after certain infections in B-cell-deficient mice. Here, we find that B cells significantly enhance primary CD8 T cell responses after vaccination. Moreover, memory CD8 numbers and function are impaired in B-cell-deficient animals, leading to increased susceptibility to bacterial challenge. We also show that interleukin-27 production by B cells contributes to their impact on primary, but not memory, CD8 responses. Better understanding of the interactions between CD8 T cells and B cells may aid in the design of more effective future vaccine strategies.

Generating cytotoxic CD8 T cell responses with vaccines can greatly improve their efficacy, but inducing adequate numbers of these cells can be challenging. Klarquist et al. reveal that the magnitude, persistence, and function of CD8 T cell vaccine responses depend on B cells.

cDC1 IL-27p28 Production Predicts Vaccine-Elicited CD8+ T Cell Memory and Protective Immunity

Although adjuvants and formulations are often either empirically derived, or at best judged by their ability to elicit broad inflammation, it would be ideal if specific innate correlates of adaptive immunity could be identified to set a universally applicable benchmark for adjuvant evaluation. Using an IL-27 reporter transgenic mouse model, we show in this study that conventional type 1 dendritic cell IL-27 production in the draining lymph node 12 h after s.c. vaccination directly correlates with downstream CD8⁺ T cell memory and protective immunity against infectious challenge. This correlation is robust, reproducible, predictive, entirely unique to vaccine biology, and is the only innate correlate of CD8⁺ T cell immune memory yet to be identified. Our results provide new insights into the basic biology of adjuvant-elicited cellular immunity and have clear implications for the screening and evaluation of novel adjuvants.

Trans-presentation of interleukin-15 by interleukin-15 receptor alpha is dispensable for the pathogenesis of autoimmune type 1 diabetes

Interleukin-15 (IL-15) is a pro-inflammatory cytokine that is required for the survival and activation of memory CD8⁺T cells, natural killer (NK) cells, innate lymphoid cells, macrophages and dendritic cells. IL-15 is implicated in the pathogenesis of various autoimmune diseases such as rheumatoid arthritis, inflammatory bowel disease, psoriasis and autoimmune type 1 diabetes (T1D). IL-15 receptor (IL-15R) consists of a specific α chain, the β chain that is shared with IL-2R and the common γ chain. IL-15 is unique in the manner in which it binds and signals through its receptor subunits. IL-15 that is complexed with IL-15Rα binds to the βγ receptor complex present on the responding cell to mediate its biological effects through a process referred to as trans-presentation. The trans-presented IL-15 is essential to mediate the biological effects on T lymphocytes and NK cells. Here we show that IL-15, but not IL-15Rα, is required for the development of spontaneous and virus-induced T1D, viral clearance and for antigen cross-presentation to CD8⁺ T lymphocytes. Our findings provide insight into the complexities of IL-15 signalling in the initiation and maintenance of CD8⁺ T cell-mediated immune responses.

IL-2 and IL-15 regulate CD8+ memory T-cell differentiation but are dispensable for protective recall responses

The ability to mount effective secondary responses is a cardinal feature of memory CD8(+) T cells. An understanding of the factors that regulate the generation and recall capacities of memory T cells remains to be ascertained. Several cues indicate that two highly related cytokines, IL-2 and IL-15, share redundant functions in this process. To establish their combined roles in memory CD8(+) T-cell development, maintenance, and secondary responses, we compared the outcome of adoptively transferred IL2Rβ(+/-) or IL2Rβ(-/-) CD8(+) T cells after an acute viral infection in mice. Our results demonstrate that both IL-2 and IL-15 signals condition the differentiation of primary and secondary short-lived effector cells by altering the transcriptional network governing lineage choices. These two cytokines also regulate the homeostasis of the memory T-cell pool, with effector memory CD8(+) T cells being the most sensitive to these two interleukins. Noticeably, the inability to respond to both cytokines limits the proliferation and survival of primary and secondary effectors cells, whereas it does not preclude potent cytotoxic functions and viral control either initially or upon rechallenge. Globally, these results indicate that lack of IL-2 and IL-15 signaling modulates the CD8(+) T-cell differentiation program but does not impede adequate effector functions.

Memory CD8(+) T cells colocalize with IL-7(+) stromal cells in bone marrow and rest in terms of proliferation and transcription

It is believed that memory CD8⁺ T cells are maintained in secondary lymphoid tissues, peripheral tissues, and BM by homeostatic proliferation. Their survival has been shown to be dependent on IL-7, but it is unclear where they acquire it. Here we show that in murine BM, memory CD8⁺ T cells individually colocalize with IL-7⁺ reticular stromal cells. The T cells are resting in terms of global transcription and do not express markers of activation, for example, 4-1BB (CD137), IL-2, or IFN-γ, despite the expression of CD69 on about 30% of the cells. Ninety-five percent of the memory CD8⁺ T cells in BM are in G₀ phase of cell cycle and do not express Ki-67. Less than 1% is in S/M/G₂ of cell cycle, according to propidium iodide staining. While previous publications have estimated the extent of proliferation of CD8⁺ memory T cells on the basis of BrdU incorporation, we show here that BrdU itself induces proliferation of CD8⁺ memory T cells. Taken together, the present results suggest that CD8⁺ memory T cells are maintained as resting cells in the BM in dedicated niches with their survival conditional on IL-7 receptor signaling.

Differential effects of IL-15 on the generation, maintenance and cytotoxic potential of adaptive cellular responses induced by DNA vaccination

IL-15 is an important cytokine for the regulation of lymphocyte homeostasis. However, the role of IL-15 in the generation, maintenance and cytotoxic potential of antigen specific T cells is not fully understood. Because the route of antigenic delivery and the vaccine modality could influence the IL-15 requirement for mounting and preserving cytotoxic T cell responses, we have investigated the immunogenicity of DNA-based vaccines in IL-15 KO mice. DNA vaccination with SIV Gag induced antigen-specific CD4(+) and CD8(+) T cells in the absence of IL-15. However, the absolute number of antigen-specific CD8(+) T cells was decreased in IL-15 KO mice compared to WT animals, suggesting that IL-15 is important for the generation of maximal number of antigen-specific CD8(+) T cells. Interestingly, antigen-specific memory CD8 cells could be efficiently boosted 8 months after the final vaccination in both WT and KO strains of mice, suggesting that the maintenance of antigen-specific long-term memory T cells induced by DNA vaccination is comparable in the absence and presence of IL-15. Importantly, boosting by DNA 8-months after vaccination revealed severely reduced granzyme B content in CD8(+) T cells of IL-15 KO mice compared to WT mice. This suggests that the cytotoxic potential of the long-term memory CD8(+) T cells is impaired. These results suggest that IL-15 is not essential for the generation and maintenance of adaptive cellular responses upon DNA vaccination, but it is critical for the preservation of maximal numbers and for the activity of cytotoxic CD8(+) T cells.

IL-15 maintains T-cell survival via S-nitrosylation-mediated inhibition of caspase-3

Caspase activity is critical for both T-cell survival and death. However, little is known regarding what determines caspase activity in cycling T cells. Interleukin (IL)-2 and IL-15 confer very different susceptibilities to T-cell death. We therefore considered that IL-2 and IL-15 differentially regulate caspase activity to influence T-cell survival. We observed that IL-2-cultured primary murine effector T cells manifested elevated levels of caspase-3 activity compared with IL-15-cultured T cells. T cell receptor (TCR) restimulation further increased caspase activity and induced considerable cell death in IL-2-cultured T cells, but provoked only a minimal increase of caspase activity and cell death in IL-15-cultured T cells. IL-2 sensitization to cell death was caspase-3 mediated. Interestingly, increased active caspase-3 levels with IL-2 were independent of active initiator caspase-8 and caspase-9 that were similar with IL-2 and IL-15. Rather, caspase-3 activity was inhibited by posttranslational S-nitrosylation in IL-15-cultured T cells, but not in the presence of IL-2. This paralleled increased reactive nitrogen and oxygen species with IL-15 and reduced glycolysis. Taken together, these data suggest that the metabolic state conferred by IL-15 inhibits T-cell apoptosis in part by maintaining low levels of active caspase-3 via S-nitrosylation.

Asymptomatic memory CD8+ T cells: from development and regulation to consideration for human vaccines and immunotherapeutics

Generation and maintenance of high quantity and quality memory CD8(+) T cells determine the level of protection from viral, bacterial, and parasitic re-infections, and hence constitutes a primary goal for T cell epitope-based human vaccines and immunotherapeutics. Phenotypically and functionally characterizing memory CD8(+) T cells that provide protection against herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2) infections, which cause blinding ocular herpes, genital herpes, and oro-facial herpes, is critical for better vaccine design. We have recently categorized 2 new major sub-populations of memory symptomatic and asymptomatic CD8(+) T cells based on their phenotype, protective vs. pathogenic function, and anatomical locations. In this report we are discussing a new direction in developing T cell-based human herpes vaccines and immunotherapeutics based on the emerging new concept of "symptomatic and asymptomatic memory CD8(+) T cells."

Safety and comparative immunogenicity of an HIV-1 DNA vaccine in combination with plasmid interleukin 12 and impact of intramuscular electroporation for delivery

BACKGROUND

DNA vaccines have been very poorly immunogenic in humans but have been an effective priming modality in prime-boost regimens. Methods to increase the immunogenicity of DNA vaccines are needed.

METHODS

HIV Vaccine Trials Network (HVTN) studies 070 and 080 were multicenter, randomized, clinical trials. The human immunodeficiency virus type 1 (HIV-1) PENNVAX®-B DNA vaccine (PV) is a mixture of 3 expression plasmids encoding HIV-1 Clade B Env, Gag, and Pol. The interleukin 12 (IL-12) DNA plasmid expresses human IL-12 proteins p35 and p40. Study subjects were healthy HIV-1-uninfected adults 18-50 years old. Four intramuscular vaccinations were given in HVTN 070, and 3 intramuscular vaccinations were followed by electroporation in HVTN 080. Cellular immune responses were measured by intracellular cytokine staining after stimulation with HIV-1 peptide pools.

RESULTS

Vaccination was safe and well tolerated. Administration of PV plus IL-12 with electroporation had a significant dose-sparing effect and provided immunogenicity superior to that observed in the trial without electroporation, despite fewer vaccinations. A total of 71.4% of individuals vaccinated with PV plus IL-12 plasmid with electroporation developed either a CD4(+) or CD8(+) T-cell response after the second vaccination, and 88.9% developed a CD4(+) or CD8(+) T-cell response after the third vaccination.

CONCLUSIONS

Use of electroporation after PV administration provided superior immunogenicity than delivery without electroporation. This study illustrates the power of combined DNA approaches to generate impressive immune responses in humans.

The survival of memory CD8 T cells that is mediated by IL-15 correlates with sustained protection against malaria

Ag-specific memory T cell responses elicited by infections or vaccinations are inextricably linked to long-lasting protective immunity. Studies of protective immunity among residents of malaria endemic areas indicate that memory responses to Plasmodium Ags are not adequately developed or maintained, as people who survive episodes of childhood malaria are still vulnerable to either persistent or intermittent malaria infections. In contrast, multiple exposures to radiation-attenuated Plasmodium berghei sporozoites (Pb γ-spz) induce long-lasting protective immunity to experimental sporozoite challenge. We previously demonstrated that sterile protection induced by Pb γ-spz is MHC class I-dependent and CD8 T cells are the key effectors. IFN-γ(+) CD8 T cells that arise in Pb γ-spz-immunized B6 mice are found predominantly in the liver and are sensitive to levels of liver-stage Ag depot and they express CD44(hi)CD62L(lo) markers indicative of effector/effector memory phenotype. The developmentally related central memory CD8 T (TCM) cells express elevated levels of CD122 (IL-15Rβ), which suggests that CD8 TCM cells depend on IL-15 for maintenance. Using IL-15-deficient mice, we demonstrate in this study that although protective immunity is inducible in these mice, protection is short-lived, mainly owing to the inability of CD8 TCM cells to survive in the IL-15-deficient milieu. We present a hypothesis consistent with a model whereby intrahepatic CD8 TCM cells, being maintained by IL-15-mediated survival and basal proliferation, are conscripted into the CD8 effector/effector memory T cell pool during subsequent infections.

Use it or lose it: establishment and persistence of T cell memory

Pre-existing T cell memory provides substantial protection against viral, bacterial, and parasitic infections. The generation of protective T cell memory constitutes a primary goal for cell-mediated vaccines, thus understanding the mechanistic basis of memory development and maintenance are of major importance. The widely accepted idea that T cell memory pools are directly descended from the effector populations has been challenged by recent reports that provide evidence for the early establishment of T cell memory and suggest that the putative memory precursor T cells do not undergo full expansion to effector status. Moreover, it appears that once the memory T cells are established early in life, they can persist for the lifetime of an individual. This is in contrast to the reported waning of naïve T cell immunity with age. Thus, in the elderly, immune memory that was induced at an early age may be more robust than recently induced memory, despite the necessity for long persistence. The present review discusses the mechanisms underlying the early establishment of immunological memory and the subsequent persistence of memory T cell pools in animal models and humans.

The bone marrow functions as the central site of proliferation for long-lived NK cells

NK cells play an important role in the early defense against invading pathogens. Although it is well established that infection leads to a substantial, local increase in NK cell numbers, little is known about the mechanisms that trigger their proliferation and migration. In this study, we investigated the dynamics of NK cell responses after intranasal respiratory virus infection. We show that NK cell numbers increased in the airways after influenza virus infection but find no evidence of proliferation either at the site of infection or in the draining lymph nodes. Instead, we find that the bone marrow (BM) is the primary site of proliferation of both immature and mature NK cells during infection. Using an adoptive transfer model, we demonstrate that peripheral, long-lived and phenotypically mature NK cells migrate back to the BM and proliferate there, both homeostatically and in response to infection. Thus, the BM is not only a site of NK cell development but also an important site for proliferation of long-lived mature NK cells.

Functions of IL-15 in anti-viral immunity: multiplicity and variety

An effective immune response to an invading viral pathogen requires the combined actions of both innate and adaptive immune cells. For example, NK cells and cytotoxic CD8 T cells are capable of the direct engagement of infected cells and the mediation of antiviral responses. Both NK and CD8 T cells depend on common gamma chain (γc) cytokine signals for their development and homeostasis. The γc cytokine IL-15 is very well characterized for its role in promoting the development and homeostasis of NK cells and CD8 T cells, but emerging literature suggests that IL-15 mediates the anti-viral responses of these cell populations during an active immune response. Both NK cells and CD8 T cells must become activated, migrate to sites of infection, survive at those sites, and expand in order to maximally exert effector functions, and IL-15 can modulate each of these processes. This review focuses on the functions of IL-15 in the regulation of multiple aspects of NK and CD8 T cell biology, investigates the mechanisms by which IL-15 may exert such diverse functions, and discusses how these different facets of IL-15 biology may be therapeutically exploited to combat viral diseases.

Safety and immunogenicity of an HIV-1 gag DNA vaccine with or without IL-12 and/or IL-15 plasmid cytokine adjuvant in healthy, HIV-1 uninfected adults

Background

DNA vaccines are a promising approach to vaccination since they circumvent the problem of vector-induced immunity. DNA plasmid cytokine adjuvants have been shown to augment immune responses in small animals and in macaques.

Methodology/Principal Findings

We performed two first in human HIV vaccine trials in the US, Brazil and Thailand of an RNA-optimized truncated HIV-1 gag gene (p37) DNA derived from strain HXB2 administered either alone or in combination with dose-escalation of IL-12 or IL-15 plasmid cytokine adjuvants. Vaccinations with both the HIV immunogen and cytokine adjuvant were generally well-tolerated and no significant vaccine-related adverse events were identified. A small number of subjects developed asymptomatic low titer antibodies to IL-12 or IL-15. Cellular immunogenicity following 3 and 4 vaccinations was poor, with response rates to gag of 4.9%/8.7% among vaccinees receiving gag DNA alone, 0%/11.5% among those receiving gag DNA+IL-15, and no responders among those receiving DNA+high dose (1500 ug) IL-12 DNA. However, after three doses, 44.4% (4/9) of vaccinees receiving gag DNA and intermediate dose (500 ug) of IL-12 DNA demonstrated a detectable cellular immune response.

Conclusions/Significance

This combination of HIV gag DNA with plasmid cytokine adjuvants was well tolerated. There were minimal responses to HIV gag DNA alone, and no apparent augmentation with either IL-12 or IL-15 plasmid cytokine adjuvants. Despite the promise of DNA vaccines, newer formulations or methods of delivery will be required to increase their immunogenicity.

Trial Registration

Clinicaltrials.gov NCT00115960NCT00111605

TLR agonists and/or IL-15 adjuvanted mucosal SIV vaccine reduced gut CD4⁺ memory T cell loss in SIVmac251-challenged rhesus macaques

Adjuvant plays an important role in increasing and directing vaccine-induced immune responses. In a previous study, we found that a mucosal SIV vaccine using a combination of IL-15 and TLR agonists as adjuvant mediated partial protection against SIVmac251 rectal challenge, whereas neither IL-15 nor TLR agonists alone as an adjuvant impacted the plasma viral loads. In this study, dissociation of CD4(+) T cell preservation with viral loads was observed in the animals vaccinated with adjuvants. Significantly higher levels of memory CD4(+) T cell numbers were preserved after SIVmac251 infection in the colons of the animals vaccinated with vaccine containing any of these adjuvants compared to no adjuvant. When we measured the viral-specific CD8(+) tetramer responses in the colon lamina propria, we found significantly higher levels of gag, tat, and pol epitope tetramer(+) T cell responses in these animals compared to ones without adjuvant, even if some of the animals had similarly high viral loads. Furthermore, this CD4(+) T preservation was positively correlated with increased levels of gag and Tat, but not pol tetramer(+) T cell responses, and inversely correlated with beta-chemokine expression. The pre-challenged APOBEC3G expression level, which has previously been shown inversely associated with viral loads, was further found positively correlated with CD4(+) T cell number preservation. Overall, these data highlight one unrecognized role of adjuvant in HIV vaccine development, and show that vaccines can produce a surprising discordance between CD4(+) T cell levels and SIV viral load.

Aberrant CD8+ T-cell responses and memory differentiation upon viral infection of an ataxia-telangiectasia mouse model driven by hyper-activated Akt and mTORC1 signaling

Immune system-related pathology is common in ataxia-telangiectasia (A-T) patients and mice that lack the protein kinase, A-T mutated (ATM). However, it has not been studied how ATM influences immune responses to a viral infection. Using the lymphocytic choriomeningitis virus (LCMV) infection model, we show that ATM(-/-) mice, despite having fewer naïve CD8⁺ T cells, effectively clear the virus. However, aberrant CD8⁺ T-cell responses are observed, including defective expansion and contraction, effector-to-memory differentiation, and a switch in viral-epitope immunodominance. T-cell receptor-activated, but not naïve, ATM(-/-) splenic CD8⁺ T cells have increased ribosomal protein S6 and Akt phosphorylation and do not proliferate well in response to IL-15, a cytokine important for memory T-cell development. Accordingly, pharmacological Akt or mammalian target of rapamycin complex 1 (mTORC1) inhibition during T-cell receptor activation alone rescues the IL-15 proliferation defect. Finally, rapamycin treatment during LCMV infection in vivo increases the number of memory T cells in ATM(-/-) mice. Altogether, these results show that CD8⁺T cells lacking ATM have hyperactive Akt and mTORC1 signaling in response to T-cell receptor activation, which results in aberrant cytokine responses and memory T-cell development. We speculate that similar signaling defects contribute to the immune system pathology of A-T, and that inhibition of Akt and/or mTORC1 may be of therapeutic value.

Increased numbers of preexisting memory CD8 T cells and decreased T-bet expression can restrain terminal differentiation of secondary effector and memory CD8 T cells

Memory CD8 T cells acquire effector memory cell properties after reinfection and may reach terminally differentiated, senescent states ("Hayflick limit") after multiple infections. The signals controlling this process are not well understood, but we found that the degree of secondary effector and memory CD8 T cell differentiation was intimately linked to the amount of T-bet expressed upon reactivation and preexisting memory CD8 T cell number (i.e., primary memory CD8 T cell precursor frequency) present during secondary infection. Compared with naive cells, memory CD8 T cells were predisposed toward terminal effector (TE) cell differentiation because they could immediately respond to IL-12 and induce T-bet, even in the absence of Ag. TE cell formation after secondary (2°) or tertiary infections was dependent on increased T-bet expression because T-bet(+/-) cells were resistant to these phenotypic changes. Larger numbers of preexisting memory CD8 T cells limited the duration of 2° infection and the amount of IL-12 produced, and consequently, this reduced T-bet expression and the proportion of 2° TE CD8 T cells that formed. Together, these data show that over repeated infections, memory CD8 T cell quality and proliferative fitness is not strictly determined by the number of serial encounters with Ag or cell divisions, but is a function of the CD8 T cell differentiation state, which is genetically controlled in a T-bet-dependent manner. This differentiation state can be modulated by preexisting memory CD8 T cell number and the intensity of inflammation during reinfection. These results have important implications for vaccinations involving prime-boost strategies.

Dissecting the requirements for maintenance of the CMV-specific memory T-cell pool

Cytomegalovirus (CMV) infection promotes a broad T-cell response, with the resulting memory cells displaying diverse phenotypes. CMV establishes lifelong persistence/latency, and it is thought that viral antigens expressed during this period may regulate the expansion and/or maintenance of "inflationary" CD8 T-memory populations that display an effector memory phenotype. We show here that mouse CMV (MCMV)-specific inflationary memory T cells do not decrease in number after thymectomy, indicating that recent thymic emigrants are not strictly required for their maintenance. Furthermore, persistent MCMV replication in the salivary gland does not significantly impact the T-cell memory compartment, as surgical removal did not alter its composition. These results shed light upon the mechanisms required for maintenance of the large, MCMV-specific T-cell memory pool.

Cutting edge: IL-15-independent maintenance of mucosally generated memory CD8 T cells

Effective vaccines against intracellular pathogens rely on the generation and maintenance of memory CD8 T cells (T(mem)). Hitherto, evidence has indicated that CD8 T(mem) use the common γ-chain cytokine IL-15 for their steady-state maintenance in the absence of Ag. This evidence, however, has been amassed predominantly from models of acute, systemic infections. Given that the route of infection can have significant impact on the quantity and quality of the resultant T(mem), reliance on limited models of infection may restrict our understanding of long-term CD8 T(mem) survival. In this article, we show IL-15-independent generation, maintenance, and function of CD8 T(mem) after respiratory infection with influenza virus. Importantly, we demonstrate that alternating between mucosal and systemic deliveries of the identical virus prompts this change in IL-15 dependence, necessitating a re-evaluation of the current model of CD8 T(mem) maintenance.

Cytomegalovirus infection induces the accumulation of short-lived, multifunctional CD4+CD45RA+CD27+ T cells: the potential involvement of interleukin-7 in this process

The relative roles that ageing and lifelong cytomegalovirus (CMV) infection have in shaping naive and memory CD4+ T-cell repertoires in healthy older people is unclear. Using multiple linear regression analysis we found that age itself is a stronger predictor than CMV seropositivity for the decrease in CD45RA+ CD27+ CD4+ T cells over time. In contrast, the increase in CD45RA⁻ CD27⁻ and CD45RA+ CD27⁻ CD4+ T cells is almost exclusively the result of CMV seropositivity, with age alone having no significant effect. Furthermore, the majority of the CD45RA⁻ CD27⁻ and CD45RA+ CD27⁻ CD4+ T cells in CMV-seropositive donors are specific for this virus. CD45RA+ CD27⁻ CD4+ T cells have significantly reduced CD28, interleukin-7 receptor α (IL-7Rα) and Bcl-2 expression, Akt (ser473) phosphorylation and reduced ability to survive after T-cell receptor activation compared with the other T-cell subsets in the same donors. Despite this, the CD45RA+ CD27⁻ subset is as multifunctional as the CD45RA⁻ D27+ and CD45RA⁻ CD27⁻ CD4+ T-cell subsets, indicating that they are not an exhausted population. In addition, CD45RA+ CD27⁻ CD4+ T cells have cytotoxic potential as they express high levels of granzyme B and perforin. CD4+ memory T cells re-expressing CD45RA can be generated from the CD45RA⁻ CD27+ population by the addition of IL-7 and during this process these cells down-regulated expression of IL-7R and Bcl-2 and so resemble their counterparts in vivo. Finally we showed that the proportion of CD45RA+ CD27⁻ CD4+ T cells of multiple specificities was significantly higher in the bone marrow than the blood of the same individuals, suggesting that this may be a site where these cells are generated.

Novel roles for IL-15 in T cell survival

Interleukin-15 (IL-15) is generally considered to be a regulator of T cell homeostasis because it works with other common gamma-chain cytokines like IL-2 and IL-7 to control the maintenance of naive and memory T cell populations. However, recent reports highlight new roles for IL-15 during the primary immune responses that involve promoting the survival of antigen-specific CD8(+) T cells. These findings illuminate a previously unanticipated role for IL-15 in the generation and resolution of the effector CD8(+) T cell response to pathogens.

Distinct roles for IL-2 and IL-15 in the differentiation and survival of CD8+ effector and memory T cells

IL-2 provides a memory differentiation signal to CD8+ T cells during the primary response that impacts the ability of the subsequent memory pool to mount a successful recall response. In this study, we find that although primary effector CTL development is modestly decreased in the absence of IL-2, the persistence of short-term and long-term effector memory CD8+ T cells on pathogen clearance is greatly diminished. Furthermore, secondary challenge of CD8+ memory T cells lacking the high-avidity IL-2R results in a failure to repopulate the effector pool. The role of IL-2 in promoting effector differentiation is not shared with the highly related cytokine, IL-15. Although IL-15 supports the survival of effector CD8+ T cells after pathogen clearance, its absence does not impair either primary or secondary effector CTL differentiation, nor does it impact the differentiation of long-term effector memory CD8+ T cells. These findings indicate a unique role for IL-2, but not IL-15, in promoting the differentiation not only of primary effector CD8+ T cells, but also of CD8+ memory T cells capable of secondary effector differentiation.

IL-15 has innate anti-tumor activity independent of NK and CD8 T cells

The innate immune system is crucial for host defense and immunosurveillance against pathogens and tumor cells. IL-15 is a pleiotropic cytokine with important effects on cells of the innate and adaptive immune systems. The NK cell- and CD8(+) T cell-mediated functions of IL-15 against tumor cells have been well documented. However, it has not been established whether IL-15 has innate anti-tumor functions independent of these cells. Here, we explored the innate anti-tumor potential of IL-15 using a B16F10 melanoma tumor model. IL-15tg mice exhibited significantly more resistance to tumor growth and metastasis compared to B6 mice, and to IL-15(-/-) mice, which exhibited increased susceptibility to B16F10 challenge. In vivo depletion of NK cells and CD8(+) T cells abrogated the innate resistance to B16F10 cells in B6 but not in IL-15tg mice. In addition, lung macrophages from IL-15tg mice produced significantly higher levels of NO and IL-12 compared with macrophages from B6 or IL-15(-/-) mice. To examine whether IL-15 has innate anti-tumor activity independent of NK cells and CD8(+) T cells, we developed Ad-Op-hIL-15; this resulted in significantly higher levels of biologically active hIL-15. Delivery of Ad-Op-hIL-15 into RAG-2(-/-)/gamma(c)(-/-) mice significantly suppressed tumor burden in the lungs compared with the control adenovirus vector. Our results show that IL-15 can have innate anti-tumor activity independent of NK cells and CD8(+) T cells and the common gamma(c)R.

Viral vaccines and CTL response

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

Reversible blockade of thymic output: an inherent part of TLR ligand-mediated immune response

TLRs constitute a first set of sensors that detect viral nucleic acids including dsRNA which triggers TLR3. We report the early, direct, and detrimental effect of polyinosine-polycytidilic acid treatment on T cell development. Inhibition of thymopoiesis was targeted to several thymocyte subpopulations. First, both a blockade of the double negative (DN)1-DN2 transition and a severe down-regulation of DN3-DN4 thymocyte proliferation were observed. In addition, an important decrease in the absolute numbers of double-positive thymocytes, concomitant with an increase in frequencies of apoptotic cells in this population were shown. This inhibition of thymopoiesis resulted in a reduced thymic output, as evidenced by a drop of the absolute numbers of naive T cells and TCR excision circles levels. The decrease in thymic cellularity and defects in thymic development were severely reduced, but not completely abolished in IFN-alpha/betaR(-/-) mice, showing a direct contribution of type I IFNs, known to be massively up-regulated in viral infections, to the inhibition of T cell development. Strikingly, the TCR repertoire in treated mice was biased toward shorter CDR3 lengths as a result of a decreased expression of TdT and Rag2. However, thymic integrity remained intact since thymopoiesis was restored both quantitatively and qualitatively 14 days after the cessation of polyinosine-polycytidilic acid treatment. These results demonstrate a novel immunomodulatory role for virally encoded TLR ligands and RNA sensors; they further illustrate the diversity of mechanisms that viruses use to interfere with the development of a pathogen-specific immune responses.

Therapeutic vaccination with simian immunodeficiency virus (SIV)-DNA + IL-12 or IL-15 induces distinct CD8 memory subsets in SIV-infected macaques

DNA vaccination is an invaluable approach for immune therapy in that it lacks vector interference and thus permits repeated vaccination boosts. However, by themselves, DNA-based vaccines are typically poor inducers of Ag-specific immunity in humans and non-human primates. Cytokines, such as IL-12 and IL-15, have been shown to be potent adjuvants for the induction and maintenance of cellular immune responses, in particular during HIV infection. In this study, we examined the ability of therapeutic vaccination with SIV-DNA+IL-12 or IL-15 as molecular adjuvants to improve DNA vaccine potency and to enhance memory immune responses in SIV-infected macaques. Our results demonstrate that incorporating IL-12 into the vaccine induces SIV-specific CD8 effector memory T cell (T(EM)) functional responses and enhances the capacity of IFN-gamma-producing CD8 T(EM) cells to produce TNF. Lower levels of PD-1 were expressed on T cells acquiring dual function upon vaccination as compared with mono-functional CD8 T(EM) cells. Finally, a boost with SIV-DNA+IL-15 triggered most T cell memory subsets in macaques primed with either DNA-SIV or placebo but only CD8 T(EM) in macaques primed with SIV-DNA+IL-12. These results indicate that plasmid IL-12 and IL-15 cytokines represent a significant addition to enhance the ability of therapeutic DNA vaccines to induce better immunity.

Human effector and memory CD8+ T cell responses to smallpox and yellow fever vaccines

To explore the human T cell response to acute viral infection, we performed a longitudinal analysis of CD8(+) T cells responding to the live yellow fever virus and smallpox vaccines--two highly successful human vaccines. Our results show that both vaccines generated a brisk primary effector CD8(+) T cell response of substantial magnitude that could be readily quantitated with a simple set of four phenotypic markers. Secondly, the vaccine-induced T cell response was highly specific with minimal bystander effects. Thirdly, virus-specific CD8(+) T cells passed through an obligate effector phase, contracted more than 90% and gradually differentiated into long-lived memory cells. Finally, these memory cells were highly functional and underwent a memory differentiation program distinct from that described for human CD8(+) T cells specific for persistent viruses. These results provide a benchmark for CD8(+) T cell responses induced by two of the most effective vaccines ever developed.

IL-15 as a mediator of CD4+ help for CD8+ T cell longevity and avoidance of TRAIL-mediated apoptosis

CD4+ helper T cells contribute to the induction and maintenance of antigen-specific CD8+ T cells. Their absence results in short-lived antigen-specific CD8+ T cells and defective secondary CD8+ T cell responses because of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis. Here, we show that IL-15 codelivered with vaccines can overcome CD4+ T cell deficiency for promoting longevity of antigen-specific CD8+ T cells and avoidance of TRAIL-mediated apoptosis. In both priming and secondary responses, IL-15 down-regulates proapoptotic Bax, an intermediate in TRAIL-mediated apoptosis, and increases anti-apoptotic Bcl-X(L) in CD8+ T cells. Thus, IL-15 is sufficient to mimic CD4+ T cell help. Antigen-specific CD4+ T cells induce dendritic cells (DCs) to produce IL-15. IL-15 is also necessary for optimal help, because helper cells do not deliver effective help through IL-15-/- DCs. Therefore, IL-15 codelivered with vaccines can overcome CD4+ helper T cell deficiency for induction of functionally efficient CD8+ T cells and maintenance of CD8+ cytotoxic T lymphocytes (CTLs), and IL-15 is probably one of the natural mediators of help. These findings suggest new vaccine strategies against infections and cancers, especially in individuals with CD4-deficiency.

Non-redundant role for IL-7R signaling for the survival of CD8+ memory T cells

IL-7 and IL-15 are important cytokines for CD8 memory T cells. However, the extent that IL-7 is essential for CD8 T cell memory remains unclear because blocking IL-7 in vivo results in near complete inhibition of T cell development with the few mature T cells exhibiting functional abnormalities. To bypass this complication, CD8 memory development was examined utilizing a mouse model where transgenic IL-7Ralpha was selectively expressed in the thymus of IL-7Ralpha(-/-) mice. T cell development was corrected but the resulting peripheral T cells were essentially IL-7 non-responsive. Activation of IL-7R-defective OT-I CD8(+) T cells with OVA(257-264) and IL-2 readily yielded CTL. Upon further culture with IL-15, these CTL expressed phenotypic and functional properties of central memory-like cells. Thus, IL-7R-defective CD8(+) T cells do not exhibit intrinsic defects in effector or memory development. When IL-7R-defective OT-I CTL were adoptively transferred into normal or IL-15(-/-) recipient mice in a non-inflammatory setting, they converted into memory-like cells, but did not persist, which was even more striking in IL-15(-/-) recipients. This poor persistence was rescued after expression of transgenic Bcl-2 in IL-7R-defective OT-I T cells. Collectively, these data indicate that IL-7 is non-redundantly required for the survival of CD8 memory T cells.

Regulation of memory antibody levels: the role of persisting antigen versus plasma cell life span

Protective Ab levels can be maintained for years upon infection or vaccination. In this study, we studied the duration of Ab responses as a function of the life span of plasma cells and tested the role of persisting Ag in maintaining B cell memory. Our analysis of B cell responses induced in mice immunized with virus-like particles demonstrates the following: 1) Ab titers are long-lived, but decline continuously with a t(1/2) of approximately 80 days, which corresponds to the life span of plasma cells; 2) the germinal center (GC) reaction, which lasts for up to 100 days, is dependent on Ag associated with follicular dendritic cells; and 3) early GCs produce massive numbers of plasma and memory B cell precursors, whereas the late Ag-dependent GCs are dispensable for the maintenance of Ab levels and B cell memory.

IL-15 protects intestinal epithelial cells

IL-15, a T-cell growth factor, has been shown to be increased in inflammatory bowel disease (IBD). It has been suggested that neutralization of IL-15 could protect from T cell-dependent autoimmune inflammation. On the other hand, an anti-apoptotic effect of IL-15 has been demonstrated in kidney epithelial cells during nephritis. We therefore tested the role of IL-15 in two different experimental models of colitis in vivo, and in models of intestinal epithelial cell (IEC) apoptosis in vitro. IL-15 blockade in chronic dextran sulphate sodium-induced colitis resulted in aggravation of the disease with a significantly 2.1-fold increased epithelial damage score compared to controls. TUNEL staining clearly revealed increased apoptosis. IL-6, TNF and IFN-gamma secretion by mesenteric lymph node cells were increased. In the T cell-dependent SCID transfer model of colitis IL-15 neutralization reduced the inflammatory infiltration and proinflammatory cytokine production. Despite that, the intestinal epithelial damage was not reduced. In vitro, IL-15 pre-incubation prevented up to 75% of CH11 antibody-induced apoptosis in SW-480 cells and reduced caspase-3 activity. According to this, endogenously produced IL-15 in chronic colitis does not only act as a proinflammatory cytokine but has at the same time the potential to reduce mucosal damage by preventing IEC apoptosis.

Monoculture-derived T lymphocytes specific for multiple viruses expand and produce clinically relevant effects in immunocompromised individuals

Immunocompromised individuals are at high risk for life-threatening diseases, especially those caused by cytomegalovirus (CMV), Epstein-Barr virus (EBV) and adenovirus. Conventional therapeutics are primarily active only against CMV, and resistance is frequent. Adoptive transfer of polyclonal cytotoxic T lymphocytes (CTLs) specific for CMV or EBV seems promising, but it is unclear whether this strategy can be extended to adenovirus, which comprises many serotypes. In addition, the preparation of a specific CTL line for each virus in every eligible individual would be impractical. Here we describe genetic modification of antigen-presenting cell lines to facilitate the production of CD4(+) and CD8(+) T lymphocytes specific for CMV, EBV and several serotypes of adenovirus from a single cell culture. When administered to immunocompromised individuals, the single T lymphocyte line expands into multiple discrete virus-specific populations that supply clinically measurable antiviral activity. Monoculture-derived multispecific CTL infusion could provide a safe and efficient means to restore virus-specific immunity in the immunocompromised host.

Establishment and recall of CD8 + T‐cell memory in a model of localized transient infection

The influenza A virus model of localized, transient respiratory infection provides a well-defined experimental system for dissecting the induction and maintenance of CD8+ T-cell memory. This review focuses on quantitative and qualitative aspects of the prominent D(b)NP366- and D(b)PA224-specific CD8+ T-cell responses in virus-infected B6 mice. The different virus-specific effector and memory sets are compared by phenotypic [CD62L, interleukin-7 receptor-alpha (IL-7Ralpha), and IL-15Rbeta expression] and functional [interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and IL-2 production] analyses. Most clonotypes [defined by T-cell receptor (TCR) CDR3beta sequence] generated during the acute phase of infection survive into memory, with those expressing the more consensus 'canonical' TCRs being the major contributors to the recall response. The extent of clonal expansion and the size of memory CD8+ T-cell populations has been characterized for mice challenged with either wildtype or mutant viruses, where broadly equivalent D(b)NP366 and D(b)PA224 expression was achieved by disabling the peptides in their native configuration, then expressing them in the viral neuraminidase protein. Combining the clonotypic and antigen dose analyses led to a somewhat mechanistic conclusion that the magnitude of any virus-specific CD8+ T-cell response will be a direct function of antigen dose and the size of the naïve or memory CD8+ T-cell precursor pool.

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'.

Lack of IL-15 results in the suboptimal priming of CD4+ T cell response against an intracellular parasite

IFN-gamma-producing CD4+ T cells, although important for protection against acute Toxoplasma gondii infection, can cause gut pathology, which may prove to be detrimental for host survival. Here we show that mice lacking IL-15 gene develop a down-regulated IFN-gamma-producing CD4+ T cell response against the parasite, which leads to a reduction in gut necrosis and increased level of survival against infection. Moreover, transfer of immune CD4+ T cells from WT to IL-15-/- mice reversed inhibition of gut pathology and caused mortality equivalent to levels of parental WT mice. Down-regulated CD4+ T cell response in the absence of IL-15, manifested as reduced antigen-specific proliferation, was due to defective priming of the T cell subset by dendritic cells (DCs) of these animals. When stimulated with antigen-pulsed DCs from WT mice, CD4+ T cells from IL-15-/- mice were primed optimally, and robust proliferation of these cells was observed. A defect in the DCs of knockout mice was further confirmed by their reduced ability to produce IL-12 upon stimulation with Toxoplasma lysate antigen. Addition of exogenous IL-15 to DC cultures from knockout mice led to increased IL-12 production by these cells and restored their ability to prime an optimal parasite-specific CD4+ T cell response. To our knowledge, this is the first demonstration of the role of IL-15 in the development of CD4+ T cell immunity against an intracellular pathogen. Furthermore, based on these observations, targeting of IL-15 should have a beneficial effect on individuals suffering from CD4+ T cell-mediated autoimmune diseases.

IL-7Ralpha expression on CD4+ T lymphocytes decreases with HIV disease progression and inversely correlates with immune activation

Many factors can influence the rate of HIV disease progression, including those that maintain T cell homeostasis. One key homeostatic regulator is the IL-7 receptor (IL-7R). Previous studies have shown IL-7R expression levels decrease in HIV infection, but effects on memory subtypes, CD4(+) T cells, and cell function have not been explored. The present study examined the expression of the IL-7Ralpha chain on naïve and memory T lymphocyte subsets of both HIV-positive and HIV-negative individuals from Nairobi, Kenya to assess the role of IL-7Ralpha in HIV disease. Expression of IL-7Ralpha was significantly reduced in all CD4(+) and CD8(+) T cell subsets in HIV-positive individuals. This reduction was further enhanced in those with advanced HIV progression. Expression of IL-7Ralpha was inversely correlated to immune activation, and apoptosis, and was positively correlated with CD4 count in both bivariate and multivariate analysis. Expression of IL-7Ralpha did not correlate with HIV viral loads, indicating the elevated immune activation seen in HIV-infected individuals may be impacting expression of IL-7Ralpha, independent of viral loads. Signaling via the IL-7R is essential for T cell homeostasis and maintenance of T cell memory. Reduction of this receptor may contribute to the homeostatic disruption seen in HIV.

Shared alterations in NK cell frequency, phenotype, and function in chronic human immunodeficiency virus and hepatitis C virus infections

Human immunodeficiency virus (HIV) and hepatitis C virus (HCV) cause clinically important persistent infections. The effects of virus persistence on innate immunity, including NK cell responses, and the underlying mechanisms are not fully understood. We examined the frequency, phenotype, and function of peripheral blood CD3- CD56+ NK subsets in HIV+ and HCV+ patients and identified significantly reduced numbers of total NK cells and a striking shift in NK subsets, with a marked decrease in the CD56(dim) cell fraction compared to CD56(bright) cells, in both infections. This shift influenced the phenotype and functional capacity (gamma interferon production, killing) of the total NK pool. In addition, abnormalities in the functional capacity of the CD56(dim) NK subset were observed in HIV+ patients. The shared NK alterations were found to be associated with a significant reduction in serum levels of the innate cytokine interleukin 15 (IL-15). In vitro stimulation with IL-15 rescued NK cells of HIV+ and HCV+ patients from apoptosis and enhanced proliferation and functional activity. We hypothesize that the reduced levels of IL-15 present in the serum during HIV and HCV infections might impact NK cell homeostasis, contributing to the common alterations of the NK pool observed in these unrelated infections.

Interleukin-15-deficient mice develop protective immunity to Toxoplasma gondii

Previous studies have suggested an important role for interleukin-15 (IL-15) in resistance to and memory for Toxoplasma gondii infection. The studies presented here reveal that IL-15 is not required for infection-induced expansion of NK or CD8+ T cells. Furthermore, IL-15-/- mice develop long-term protective immunity to this pathogen.

IL-15 is superior to IL-2 in the generation of long-lived antigen specific memory CD4 and CD8 T cells in rhesus macaques

Using tetanus toxoid (TT) and influenza (Flu) immunization of rhesus macaques as a model, the effect of IL-2 and IL-15 on the generation and maintenance of antigen specific memory T cells was evaluated following primary and secondary immunization. Daily cytokine administration expanded primarily effector but not memory cells, while spacing cytokine administration to q3-7 days markedly enhanced TT and Flu specific memory responses. Following primary immunization, TT specific CD4 and influenza matrix protein (Flu-MP) specific CD8 effector responses were enhanced by IL-2 administration but CD8 specific memory responses were no different from cytokine non-treated monkeys. In contrast, expansion of Flu specific CD8 cells with IL-15 was only modest but resulted in significantly elevated levels of memory cells at 6 months. IL-15 also significantly enhanced early and late TT specific CD4 responses. The highest levels of primary effector and memory T cells were observed following alternate administration of both IL-2 and IL-15. Following booster immunization, however, only IL-15 appeared able to enhance CD8 T cell responses while IL-2 or IL2/IL-15 administration were less effective.

The Size and Phenotype of Virus-Specific T Cell Populations Is Determined by Repetitive Antigenic Stimulation and Environmental Cytokines

Based on the expression of the TNFR SFP CD27, two Ag-primed CD8(+) T cell subsets can be discerned in the circulation of healthy individuals: CD27(+) T cells that produce a variety of cytokines but do not display immediate cytolytic activity; and cytotoxic CD27(-) T cells, which secrete only IFN-gamma and TNF-alpha. The mechanism that controls the generation of these different phenotypes is unknown. We show that CMV reactivation not only increases the number of virus-specific T cells but also induces their transition from a CD27(+) to a CD27(-) phenotype. In support of a relation between pool size and phenotype in a cohort of latently infected individuals, the number of Ag-specific CD27(-) CD8(+) T cells was found to be linearly related to the total number of CMV-specific CD8(+) T cells. In vitro studies revealed that the acquisition of the CD27(-) phenotype on CMV-specific T cells depended on the interaction of CD27 with its cellular ligand, CD70. Expression of CD70 was proportional to the amount of antigenic stimulation and blocked by the CD4(+) T cell-derived cytokine IL-21. Thus, induction of CD70, which may vary in distinct viral infections, appears to be a key factor in determining the size and phenotype of the CMV-specific T cell population in latently infected individuals.

Cutting Edge: Effector Memory CD8+ T Cells in the Lung Airways Retain the Potential to Mediate Recall Responses

Previous studies have shown that long-lived memory CD8(+) T cells persist in the lung airways following the resolution of a murine Sendai virus infection. These cells are CD11a(low), noncytolytic, and do not proliferate in the lung airways raising the possibility that they are "end stage" or terminally differentiated memory cells. In this current report, we investigated the functional characteristics of these cells by analyzing their capacity to respond to secondary viral infection outside of the lung environment. We show that, after transfer into the bloodstream, CD11a(low) memory T cells from the lung airways can return to the secondary lymphoid tissue and respond to a secondary viral challenge. Furthermore, these cells re-express CD11a, which may contribute to their migratory and proliferative capacity. These data demonstrate that lung airway memory CD8(+) T cells are not terminally differentiated cells and retain the capacity to mediate recall responses to infection.