Secondary CD8+ T-cell responses are controlled by systemic inflammation

Repetitive antigen stimulation in the periphery dictates the composition and recall responses of brain-resident memory CD8+ T cells

The human brain harbors virus-specific, tissue-resident memory (TRM) CD8+ T cells. However, the impact of repeated peripheral viral infection on the generation, phenotype, localization, and recall responses of brain TRM remains elusive. Here, utilizing two murine models of peripheral viral infection, we demonstrate that circulating memory CD8+ T cells with previous antigen exposure exhibit a markedly reduced capacity to form brain TRM compared to naive CD8+ T cells. Repetitively stimulated brain TRM also demonstrate differential inhibitory receptor expression, preserved functionality, and divergent localization patterns compared to primary memory counterparts. Despite these differences, repetitively stimulated brain TRM provide similar protection against intracranial infection as primary populations with superior recall-based recruitment of peripheral lymphocytes. As CD8+ T cells may distinctly seed the brain with each repeated infection of the same host, these findings point to heterogeneity in the brain TRM pool that is dictated by prior peripheral antigen stimulation history.

SARS-CoV-2 inflammation durably imprints memory CD4 T cells

Memory CD4 T cells are critical to human immunity, yet it is unclear whether viral inflammation during memory formation has long-term consequences. Here, we compared transcriptional and epigenetic landscapes of Spike (S)-specific memory CD4 T cells in 24 individuals whose first exposure to S was via SARS-CoV-2 infection or mRNA vaccination. Nearly 2 years after memory formation, S-specific CD4 T cells established by infection remained enriched for transcripts related to cytotoxicity and for interferon-stimulated genes, likely because of a chromatin accessibility landscape altered by inflammation. Moreover, S-specific CD4 T cells primed by infection had reduced proliferative capacity in vitro relative to vaccine-primed cells. Furthermore, the transcriptional state of S-specific memory CD4 T cells was minimally altered by booster immunization and/or breakthrough infection. Thus, infection-associated inflammation durably imprints CD4 T cell memory, which affects the function of these cells and may have consequences for long-term immunity.

Inflammation durably imprints memory CD4+ T cells

Adaptive immune responses are induced by vaccination and infection, yet little is known about how CD4+ T cell memory differs when primed in these two contexts. Notably, viral infection is generally associated with higher levels of systemic inflammation than is vaccination. To assess whether the inflammatory milieu at the time of CD4+ T cell priming has long-term effects on memory, we compared Spike-specific memory CD4+ T cells in 22 individuals around the time of the participants' third SARS-CoV-2 mRNA vaccination, with stratification by whether the participants' first exposure to Spike was via virus or mRNA vaccine. Multimodal single-cell profiling of Spike-specific CD4+ T cells revealed 755 differentially expressed genes that distinguished infection- and vaccine-primed memory CD4+ T cells. Spike-specific CD4+ T cells from infection-primed individuals had strong enrichment for cytotoxicity and interferon signaling genes, whereas Spike-specific CD4+ T cells from vaccine-primed individuals were enriched for proliferative pathways by gene set enrichment analysis. Moreover, Spike-specific memory CD4+ T cells established by infection had distinct epigenetic landscapes driven by enrichment of IRF-family transcription factors, relative to T cells established by mRNA vaccination. This transcriptional imprint was minimally altered following subsequent mRNA vaccination or breakthrough infection, reflecting the strong bias induced by the inflammatory environment during initial memory differentiation. Together, these data suggest that the inflammatory context during CD4+ T cell priming is durably imprinted in the memory state at transcriptional and epigenetic levels, which has implications for personalization of vaccination based on prior infection history.

COVID-19 mRNA Vaccines: The Molecular Basis of Some Adverse Events

Each injection of any known vaccine results in a strong expression of pro-inflammatory cytokines. This is the result of the innate immune system activation, without which no adaptive response to the injection of vaccines is possible. Unfortunately, the degree of inflammation produced by COVID-19 mRNA vaccines is variable, probably depending on genetic background and previous immune experiences, which through epigenetic modifications could have made the innate immune system of each individual tolerant or reactive to subsequent immune stimulations.We hypothesize that we can move from a limited pro-inflammatory condition to conditions of increasing expression of pro-inflammatory cytokines that can culminate in multisystem hyperinflammatory syndromes following COVID-19 mRNA vaccines (MIS-V). We have graphically represented this idea in a hypothetical inflammatory pyramid (IP) and we have correlated the time factor to the degree of inflammation produced after the injection of vaccines. Furthermore, we have placed the clinical manifestations within this hypothetical IP, correlating them to the degree of inflammation produced. Surprisingly, excluding the possible presence of an early MIS-V, the time factor and the complexity of clinical manifestations are correlated to the increasing degree of inflammation: symptoms, heart disease and syndromes (MIS-V).

Sepsis-Induced T Cell Immunoparalysis: The Ins and Outs of Impaired T Cell Immunity

Sepsis results in a deluge of pro- and anti-inflammatory cytokines, leading to lymphopenia and chronic immunoparalysis. Sepsis-induced long-lasting immunoparalysis is defined, in part, by impaired CD4 and CD8 αβ T cell responses in the postseptic environment. The dysfunction in T cell immunity affects naive, effector, and memory T cells and is not restricted to classical αβ T cells. Although sepsis-induced severe and transient lymphopenia is a contributory factor to diminished T cell immunity, T cell-intrinsic and -extrinsic factors/mechanisms also contribute to impaired T cell function. In this review, we summarize the current knowledge of how sepsis quantitatively and qualitatively impairs CD4 and CD8 T cell immunity of classical and nonclassical T cell subsets and discuss current therapeutic approaches being developed to boost the recovery of T cell immunity postsepsis induction.

Defining Memory CD8 T Cell

CD8 T cells comprising the memory pool display considerable heterogeneity, with individual cells differing in phenotype and function. This review will focus on our current understanding of heterogeneity within the antigen-specific memory CD8 T cell compartment and classifications of memory CD8 T cell subsets with defined and discrete functionalities. Recent data suggest that phenotype and/or function of numerically stable circulatory memory CD8 T cells are defined by the age of memory CD8 T cell (or time after initial antigen-encounter). In addition, history of antigen stimulations has a profound effect on memory CD8 T cell populations, suggesting that repeated infections (or vaccination) have the capacity to further shape the memory CD8 T cell pool. Finally, genetic background of hosts and history of exposure to diverse microorganisms likely contribute to the observed heterogeneity in the memory CD8 T cell compartment. Extending our tool box and exploring alternative mouse models (i.e., "dirty" and/or outbred mice) to encompass and better model diversity observed in humans will remain an important goal for the near future that will likely shed new light into the mechanisms that govern biology of memory CD8 T cells.

TLR9-Mediated Conditioning of Liver Environment Is Essential for Successful Intrahepatic Immunotherapy and Effective Memory Recall

Immune defense against hepatotropic viruses such as hepatitis B (HBV) and hepatitis C (HCV) poses a major challenge for therapeutic approaches. Intrahepatic cytotoxic CD8 T cells that are crucial for an immune response against these viruses often become exhausted resulting in chronic infection. We elucidated the T cell response upon therapeutic vaccination in inducible transgenic mouse models in which variable percentages of antigen-expressing hepatocytes can be adjusted, providing mosaic antigen distribution and reflecting the varying viral antigen loads observed in patients. Vaccination-induced endogenous CD8 T cells could eliminate low antigen loads in liver but were functionally impaired if confronted with elevated antigen loads. Strikingly, only by conditioning the liver environment with TLR9 ligand prior and early after peripheral vaccination, successful immunization against high intrahepatic antigen density with its elimination was achieved. Moreover, TLR9 immunomodulation was also indispensable for functional memory recall after high frequency antigen challenge. Together, the results indicate that TLR9-mediated conditioning of liver environment during therapeutic vaccination or antigen reoccurrence is crucial for an efficacious intrahepatic T cell response.

CD40 Signaling Drives Potent Cellular Immune Responses in Heterologous Cancer Vaccinations

Antagonistic antibodies targeting coinhibitory receptors have revolutionized the treatment of cancer by inducing durable immune responses and clinical remissions in patients. In contrast, success of agonistic costimulatory antibodies has thus far been limited because of the insufficient induction of adaptive immune responses. Here, we describe a novel vaccination method consisting of a primary dendritic cell (DC) immunization followed by a composite vaccination, including an agonistic CD40 antibody, soluble antigen, and a TLR3 agonist, referred to as CoAT. In mice, DC/CoAT prime-boost vaccinations targeting either MHC class I or II neoantigens or tumor-associated antigens rendered up to 60% of the total T-cell population specific for a single tumor epitope. DC/CoAT induced durable and complete remissions of large subcutaneous tumors without detectable side effects. Thus, booster vaccinations with agonistic costimulatory antibodies represent an ideal means to amplify DC vaccinations and induce robust T-cell immune responses while providing maximum flexibility regarding the choice of antigen. Cancer Res; 77(8); 1918-26. ©2017 AACR.

Modelling and Simulation of the Dynamics of the Antigen-Specific T Cell Response Using Variable Structure Control Theory

Experimental and mathematical studies in immunology have revealed that the dynamics of the programmed T cell response to vigorous infection can be conveniently modelled using a sigmoidal or a discontinuous immune response function. This paper hypothesizes strong synergies between this existing work and the dynamical behaviour of engineering systems with a variable structure control (VSC) law. These findings motivate the interpretation of the immune system as a variable structure control system. It is shown that dynamical properties as well as conditions to analytically assess the transition from health to disease can be developed for the specific T cell response from the theory of variable structure control. In particular, it is shown that the robustness properties of the specific T cell response as observed in experiments can be explained analytically using a VSC perspective. Further, the predictive capacity of the VSC framework to determine the T cell help required to overcome chronic Lymphocytic Choriomeningitis Virus (LCMV) infection is demonstrated. The findings demonstrate that studying the immune system using variable structure control theory provides a new framework for evaluating immunological dynamics and experimental observations. A modelling and simulation tool results with predictive capacity to determine how to modify the immune response to achieve healthy outcomes which may have application in drug development and vaccine design.

Memory CD8(+) T Cells: Innate-Like Sensors and Orchestrators of Protection

Recent findings have revealed roles for systemic and mucosa-resident memory CD8(+) T cells in the orchestration of innate immune responses critical to host defense upon microbial infection. Here we integrate these findings into the current understanding of the molecular and cellular signals controlling memory CD8(+) T cell reactivation and the mechanisms by which these cells mediate effective protection in vivo. The picture that emerges presents memory CD8(+) T cells as early sensors of danger signals, mediating protective immunity both through licensing of cellular effectors of the innate immune system and via the canonical functions associated with memory T cells. We discuss implications for the development of T cell vaccines and therapies and highlight important areas in need of further investigation.

The Timing of Stimulation and IL-2 Signaling Regulate Secondary CD8 T Cell Responses

Memory CD8 T cells provide protection to immune hosts by eliminating pathogen-infected cells during re-infection. While parameters influencing the generation of primary (1°) CD8 T cells are well established, the factors controlling the development of secondary (2°) CD8 T cell responses remain largely unknown. Here, we address the mechanisms involved in the generation and development of 2° memory (M) CD8 T cells. We observed that the time at which 1° M CD8 T cells enter into immune response impacts their fate and differentiation into 2° M CD8 T cells. Late-entry of 1° M CD8 T cells into an immune response (relative to the onset of infection) not only facilitated the expression of transcription factors associated with memory formation in 2° effector CD8 T cells, but also influenced the ability of 2° M CD8 T cells to localize within the lymph nodes, produce IL-2, and undergo Ag-driven proliferation. The timing of stimulation of 1° M CD8 T cells also impacted the duration of expression of the high-affinity IL-2 receptor (CD25) on 2° effector CD8 T cells and their sensitivity to IL-2 signaling. Importantly, by blocking or enhancing IL-2 signaling in developing 2° CD8 T cells, we provide direct evidence for the role of IL-2 in controlling the differentiation of Ag-driven 2° CD8 T cell responses. Thus, our data suggest that the process of 1° M to 2° M CD8 T cell differentiation is not fixed and can be manipulated, a notion with relevance for the design of future prime-boost vaccination approaches.

The interface between transcriptional and epigenetic control of effector and memory CD8⁺ T-cell differentiation

Immunity to many intracellular pathogens requires the proliferation, differentiation, and function of CD8(+) cytotoxic T lymphocytes (CTLs). While the majority of effector CTLs die upon clearance of the pathogen, a small proportion of them survive to become long-lived memory CTLs. Memory CTLs can provide protective immunity against re-exposure to the same pathogen and are the principle motivation behind T-cell- based vaccine design. While a large body of cellular immunologic research has proven invaluable to define effector and memory CTLs by their different phenotypes and functions, an emerging focus in the field has been to understand how environmental cues regulate CTL differentiation on a genomic level. Genome-wide studies to profile transcriptional and epigenetic changes during infection have revealed that dynamic changes in DNA methylation patterns and histone modifications accompany transcriptional signatures that define and regulate CTL differentiation states. In this review, we emphasize the importance of epigenetic regulation of CD8(+) T-cell differentiation and the likely role that transcription factors play in this process.

Listeria monocytogenes: a model pathogen to study antigen-specific memory CD8 T cell responses

Memory CD8 T cells play a critical role in providing protection to immune hosts by orchestrating rapid elimination of pathogen-infected cells after re-infection. Systemic bacterial infection with Listeria monocytogenes has been a favored approach for researchers to characterize pathogen-specific CD8 T cell responses, and in-depth understanding of L. monocytogenes biology has provided invaluable experimental tools that have been used to increase our understanding of memory CD8 T cell differentiation. Here, we describe how the tools from this murine model system of infection have been utilized to characterize pathogen-specific CD8 T cells in inbred and genetically diverse outbred hosts as they undergo naïve-to-memory CD8 T cell differentiation in vivo. We also discuss how studying L. monocytogenes-evoked CD8 T cell responses have provided insight on the degree of diminished T cell immunity in clinically relevant conditions such as sepsis and obesity. Overall, this review will highlight how infection with the intracellular pathogen L. monocytogenes has enabled analysis of systemic CD8 T cell responses and greatly contributed to what is known about memory CD8 T cell generation and differentiation.

Mechanisms of Memory T Cell Activation and Effective Immunity

Effective immunization induces the development of populations of robust effector lymphocytes specific for the immunizing antigens. Amongst them are cytotoxic/CD8(+)T lymphocytes, which few will further differentiate into long-lived memory cells persisting in the host and exhibiting improved functional characteristics. The current model is that such memory cells can confer rapid host protection upon cognate antigen-mediated activation and direct killing of infected cells. In this chapter, we discuss work from our group and others that highlight the contribution of inflammatory cytokines to memory CD8(+) T cell activation and of cytolysis-independent mechanisms of host protection.

Of microspheres and microbes: A double-hit strategy for cancer immunotherapy

Efficient immunotherapy relies on the rapid generation of elevated amounts of cancer-specific T lymphocytes. We have recently demonstrated that both rapid and potent tumor-targeting immune responses can be induced with a heterologous prime-boost regimen consisting of poly-lactic co-glycolic acid (PLGA) microsphere-based immunization followed by Listeria monocytogenes infection.

The longevity of memory CD8 T cell responses after repetitive antigen stimulations

In experimental models in which the Ag-stimulation history of memory CD8 T cell populations was clearly defined (adoptive transfer of a known number of TCR-transgenic memory CD8 T cells), all facets of the ensuing CD8 T cell responses, including proliferative expansion, duration and extent of contraction, diversification of memory CD8 T cell transcriptomes, and life-long survival, were dependent on the number of prior Ag encounters. However, the extent to which sequential adoptive-transfer models reflect the physiological scenario in which memory CD8 T cells are generated by repetitive Ag challenges of individual hosts (no adoptive transfer involved) is not known. Direct comparison of endogenous memory CD8 T cell responses generated in repetitively infected hosts revealed that recurrent homologous boosting was required to preserve the numbers and increase the phenotypic and functional complexity of the developing memory CD8 T cell pool. Although life-long survival of the memory CD8 T cells was not impacted, phenotype (i.e., upregulation of CD62L) and function (i.e., homeostatic turnover, Ag-stimulated IL-2 production) of repeatedly stimulated memory CD8 T cells were dependent on time after last Ag encounter. Therefore, repetitive Ag challenges of individual hosts can substantially influence the numerical and functional attributes of polyclonal memory CD8 T cells, a notion with important implications for the design of future vaccination strategies aimed at increasing the number of protective memory CD8 T cells.

Lung airway-surveilling CXCR3(hi) memory CD8(+) T cells are critical for protection against influenza A virus

Inducing memory CD8(+) T cells specific for conserved antigens from influenza A virus (IAV) is a potential strategy for broadly protective vaccines. Here we show that memory CD8(+) T cells in the airways played an important role in early control of IAV. Expression of the chemokine receptor CXCR3 was critical for memory CD8(+) T cells to populate the airways during the steady state and vaccination approaches were designed to favor the establishment of memory CD8(+) T cells in the airways. Specifically, we found that interleukin-12 (IL-12) signaling shortly after immunization limited CXCR3 expression on memory CD8(+) T cells. Neutralization of IL-12 or adjuvants that did not induce high amounts of IL-12 enhanced CXCR3 expression, sustained airway localization of memory CD8(+) T cells, and resulted in superior protection against IAV.

A self-help program for memory CD8+ T cells: positive feedback via CD40-CD40L signaling as a critical determinant of secondary expansion

The ability of memory CD8+ T cells to rapidly proliferate and acquire cytolytic activity is critical for protective immunity against intracellular pathogens. The signals that control this recall response remain unclear. We show that CD40L production by memory CD8+ T cells themselves is an essential catalyst for secondary expansion when systemic inflammation is limited. Secondary immunization accompanied by high levels of systemic inflammation results in CD8+ T cell secondary expansion independent of CD4+ T cells and CD40-CD40L signaling. Conversely, when the inflammatory response is limited, memory CD8+ T cell secondary expansion requires CD40L-producing cells, and memory CD8+ T cells can provide this signal. These results demonstrate that vaccination regimens differ in their dependence on CD40L-expressing CD8+ T cells for secondary expansion, and propose that CD40L-expression by CD8+ T cells is a fail-safe mechanism that can promote memory CD8+ T cell secondary expansion when inflammation is limited.

Increase in IFNγ(-)IL-2(+) cells in recent human CD4 T cell responses to 2009 pandemic H1N1 influenza

Human CD4 T cell recall responses to influenza virus are strongly biased towards Type 1 cytokines, producing IFNγ, IL-2 and TNFα. We have now examined the effector phenotypes of CD4 T cells in more detail, particularly focusing on differences between recent versus long-term, multiply-boosted responses. Peptides spanning the proteome of temporally distinct influenza viruses were distributed into pools enriched for cross-reactivity to different influenza strains, and used to stimulate antigen-specific CD4 T cells representing recent or long-term memory. In the general population, peptides unique to the long-circulating influenza A/New Caledonia/20/99 (H1N1) induced Th1-like responses biased toward the expression of IFNγ(+)TNFα(+) CD4 T cells. In contrast, peptide pools enriched for non-cross-reactive peptides of the pandemic influenza A/California/04/09 (H1N1) induced more IFNγ(-)IL-2(+)TNFα(+) T cells, similar to the IFNγ(-)IL-2(+) non-polarized, primed precursor T cells (Thpp) that are a predominant response to protein vaccination. These results were confirmed in a second study that compared samples taken before the 2009 pandemic to samples taken one month after PCR-confirmed A/California/04/09 infection. There were striking increases in influenza-specific TNFα(+), IFNγ(+), and IL-2(+) cells in the post-infection samples. Importantly, peptides enriched for non-cross-reactive A/California/04/09 specificities induced a higher proportion of Thpp-like IFNγ(-)IL-2(+)TNFα(+) CD4 T cells than peptide pools cross-reactive with previous influenza strains, which induced more Th1 (IFNγ(+)TNFα(+)) responses. These IFNγ(-)IL-2(+)TNFα(+) CD4 T cells may be an important target population for vaccination regimens, as these cells are induced upon infection, may have high proliferative potential, and may play a role in providing future effector cells during subsequent infections.

Comparative analysis of the magnitude, quality, phenotype, and protective capacity of simian immunodeficiency virus gag-specific CD8+ T cells following human-, simian-, and chimpanzee-derived recombinant adenoviral vector immunization

Recombinant adenoviral vectors (rAds) are the most potent recombinant vaccines for eliciting CD8(+) T cell-mediated immunity in humans; however, prior exposure from natural adenoviral infection can decrease such responses. In this study we show low seroreactivity in humans against simian- (sAd11, sAd16) or chimpanzee-derived (chAd3, chAd63) compared with human-derived (rAd5, rAd28, rAd35) vectors across multiple geographic regions. We then compared the magnitude, quality, phenotype, and protective capacity of CD8(+) T cell responses in mice vaccinated with rAds encoding SIV Gag. Using a dose range (1 × 10(7)-10(9) particle units), we defined a hierarchy among rAd vectors based on the magnitude and protective capacity of CD8(+) T cell responses, from most to least, as: rAd5 and chAd3, rAd28 and sAd11, chAd63, sAd16, and rAd35. Selection of rAd vector or dose could modulate the proportion and/or frequency of IFN-γ(+)TNF-α(+)IL-2(+) and KLRG1(+)CD127(-)CD8(+) T cells, but strikingly ∼30-80% of memory CD8(+) T cells coexpressed CD127 and KLRG1. To further optimize CD8(+) T cell responses, we assessed rAds as part of prime-boost regimens. Mice primed with rAds and boosted with NYVAC generated Gag-specific responses that approached ∼60% of total CD8(+) T cells at peak. Alternatively, priming with DNA or rAd28 and boosting with rAd5 or chAd3 induced robust and equivalent CD8(+) T cell responses compared with prime or boost alone. Collectively, these data provide the immunologic basis for using specific rAd vectors alone or as part of prime-boost regimens to induce CD8(+) T cells for rapid effector function or robust long-term memory, respectively.

FoxP3⁺ regulatory CD4 T cells control the generation of functional CD8 memory

During the primary immune response, CD8 memory emerges from an environment of strong immune activation. The FoxP3⁺ regulatory CD4 T-cell subset (Treg) is known as a key suppressive component of the immune system. Here we report that Tregs are required for the generation of functional CD8 memory. In the absence of Tregs during priming, the resulting memory cells proliferate poorly and fail to differentiate into functional cytotoxic secondary effectors following antigen reactivation. We find that the Tregs act early, during the expansion phase of primary CD8 effectors, by fine tuning interleukin-2 exposure of CD8 memory precursors. This crucial new role of Tregs has implications for optimal vaccine development.

The role of CD4⁺ T cells in the generation of memory CD8⁺ T cells is not fully understood. In this study, the exposure of CD8 memory precursors to interleukin-2 during early antigen priming is shown to be controlled by FoxP3⁺ regulatory CD4⁺ T cells, resulting in the production of functional memory cells.

Transcriptional control of effector and memory CD8+ T cell differentiation

During an infection, T cells can differentiate into multiple types of effector and memory T cells, which help to mediate pathogen clearance and provide long-term protective immunity. These cells can vary in their phenotype, function and location, and in their long-term fate in terms of their ability to populate the memory T cell pool. Over the past decade, the signalling pathways and transcriptional programmes that regulate the formation of heterogeneous populations of effector and memory CD8(+) T cells have started to be characterized, and this Review discusses the major advances in these areas.

Inflammatory monocytes activate memory CD8(+) T and innate NK lymphocytes independent of cognate antigen during microbial pathogen invasion

Memory CD8(+) T cells induced upon immunization exhibit improved functional features that contribute to protection of immunized hosts. Although both cognate antigen recognition and inflammation are important for memory CD8(+) T cell reactivation, the relative contribution of these factors and the cell types providing these signals in vivo are poorly defined. Here, we show that Ly6C(+)CCR2(+) inflammatory monocytes, a subset of monocytes, largely orchestrate memory CD8(+) T and NK lymphocytes activation by differentiating into interleukin-18 (IL-18)- and IL-15-producing cells in an inflammasome and type I interferon-IRF3-dependent manner. Memory CD8(+) T cells became potent effector cells by sensing inflammation from monocytes independently of their cognate antigen. Like NK cells, they underwent rapid mobilization, upregulated intense and sustained effector functions during bacterial, viral, and parasitic infections, and contributed to innate responses and protection in vivo. Thus, inflammatory monocyte-derived IL-18 and IL-15 are critical to initiate memory CD8(+) T and NK lymphocytes differentiation into antimicrobial effector cells.

Population dynamics of naive and memory CD8 T cell responses after antigen stimulations in vivo

The extent to which the progeny of one primary memory CD8 T cell differs from the progeny of one naive CD8 T cell of the same specificity remains an unresolved question. To explore cell-autonomous functional differences between naive and memory CD8 T cells that are not influenced by differences in the priming environment, an experimental model has been developed in which physiological numbers of both populations of cells were cotransferred into naive hosts before Ag stimulation. Interestingly, naive CD8 T cells undergo greater expansion in numbers than do primary memory CD8 T cells after various infections or immunizations. The intrinsic ability of one naive CD8 T cell to give rise to more effector CD8 T cells than one memory CD8 T cell is independent of the number and quality of primary memory CD8 T cells present in vivo. The sustained proliferation of newly activated naive CD8 T cells contributed to their greater magnitude of expansion. Additionally, longitudinal analyses of primary and secondary CD8 T cell responses revealed that on a per-cell basis naive CD8 T cells generate higher numbers of long-lived memory cells than do primary memory CD8 T cells. This enhanced "memory generation potential" of responding naive CD8 T cells occurred despite the delayed contraction of secondary CD8 T cell responses. Taken together, the data in this study revealed previously unappreciated differences between naive and memory CD8 T cells and will help further define the functional potential for both cell types.

Pathogen-induced inflammatory environment controls effector and memory CD8+ T cell differentiation

In response to infection, CD8(+) T cells integrate multiple signals and undergo an exponential increase in cell numbers. Simultaneously, a dynamic differentiation process occurs, resulting in the formation of short-lived effector cells (SLECs; CD127(low)KLRG1(high)) and memory precursor effector cells (CD127(high)KLRG1(low)) from an early effector cell that is CD127(low)KLRG1(low) in phenotype. CD8(+) T cell differentiation during vesicular stomatitis virus infection differed significantly than during Listeria monocytogenes infection with a substantial reduction in early effector cell differentiation into SLECs. SLEC generation was dependent on Ebi3 expression. Furthermore, SLEC differentiation during vesicular stomatitis virus infection was enhanced by administration of CpG-DNA, through an IL-12-dependent mechanism. Moreover, CpG-DNA treatment enhanced effector CD8(+) T cell functionality and memory subset distribution, but in an IL-12-independent manner. Population dynamics were dramatically different during secondary CD8(+) T cell responses, with a much greater accumulation of SLECs and the appearance of a significant number of CD127(high)KLRG1(high) memory cells, both of which were intrinsic to the memory CD8(+) T cell. These subsets persisted for several months but were less effective in recall than memory precursor effector cells. Thus, our data shed light on how varying the context of T cell priming alters downstream effector and memory CD8(+) T cell differentiation.