Endogenous naive CD8+ T cell precursor frequency regulates primary and memory responses to infection

The life cycle of a T cell after vaccination - where does immune ageing strike?

Vaccination is the optimal intervention to prevent the increased morbidity and mortality from infection in older individuals and to maintain immune health during ageing. To optimize benefits from vaccination, strategies have to be developed that overcome the defects in an adaptive immune response that occur with immune ageing. Most current approaches are concentrated on activating the innate immune system by adjuvants to improve the induction of a T cell response. This review will focus upon T cell-intrinsic mechanisms that control how a T cell is activated, expands rapidly to differentiate into short-lived effector cells and into memory precursor cells, with short-lived effector T cells then mainly undergoing apoptosis and memory precursor cells surviving as long-lived memory T cells. Insights into each step of this longitudinal course of a T cell response that takes place over a period of several weeks is beginning to allow identifying interventions that can improve this process of T cell memory generation and specifically target defects that occur with ageing.

IL-10 Regulates Memory T Cell Development and the Balance between Th1 and Follicular Th Cell Responses during an Acute Viral Infection

T cells provide protective immunity against infections by differentiating into effector cells that contribute to rapid pathogen control and by forming memory populations that survive over time and confer long-term protection. Thus, understanding the factors that regulate the development of effective T cell responses is beneficial for the design of vaccines and immune-based therapies against infectious diseases. Cytokines play important roles in shaping T cell responses, and IL-10 has been shown to modulate the differentiation of CD4 and CD8 T cells. In this study, we report that IL-10 functions in a cell-extrinsic manner early following acute lymphocytic choriomeningitis virus infection to suppress the magnitude of effector Th1 responses as well as the generation of memory CD4 and CD8 T cells. We further demonstrate that the blockade of IL-10 signaling during the priming phase refines the functional quality of memory CD4 and CD8 T cells. This inhibition strategy resulted in a lower frequency of virus-specific follicular Th (Tfh) cells and increased the Th1 to Tfh ratio. Nevertheless, neither germinal center B cells nor lymphocytic choriomeningitis virus-specific Ab levels were influenced by the blockade. Thus, our studies show that IL-10 influences the balance between Th1 and Tfh cell differentiation and negatively regulates the development of functionally mature memory T cells.

A Numerically Subdominant CD8 T Cell Response to Matrix Protein of Respiratory Syncytial Virus Controls Infection with Limited Immunopathology

CD8 T cells are involved in pathogen clearance and infection-induced pathology in respiratory syncytial virus (RSV) infection. Studying bulk responses masks the contribution of individual CD8 T cell subsets to protective immunity and immunopathology. In particular, the roles of subdominant responses that are potentially beneficial to the host are rarely appreciated when the focus is on magnitude instead of quality of response. Here, by evaluating CD8 T cell responses in CB6F1 hybrid mice, in which multiple epitopes are recognized, we found that a numerically subdominant CD8 T cell response against D^bM₁₈₇ epitope of the virus matrix protein expressed high avidity TCR and enhanced signaling pathways associated with CD8 T cell effector functions. Each D^bM₁₈₇ T effector cell lysed more infected targets on a per cell basis than the numerically dominant K^dM2₈₂ T cells, and controlled virus replication more efficiently with less pulmonary inflammation and illness than the previously well-characterized K^dM2₈₂ T cell response. Our data suggest that the clinical outcome of viral infections is determined by the integrated functional properties of a variety of responding CD8 T cells, and that the highest magnitude response may not necessarily be the best in terms of benefit to the host. Understanding how to induce highly efficient and functional T cells would inform strategies for designing vaccines intended to provide T cell-mediated immunity.

CD8 T cells play a key role in RSV clearance, immunopathology and disease. Therefore, CD8 T cells can help or harm the host depending on their timing, magnitude, and function. The CD8 T cell response represents a heterogeneous population of cells with phenotypically and functionally diverse subsets, and needs to at least be studied at the level of epitope specificity to understand how to diminish the risk of immunopathology. Studying the bulk response masks distinct contributions of individual CD8 T subsets to immunity and immunopathology. Focusing on CD8 T cell response with the highest magnitude overlooks role of subdominant responses. Here, we studied response to different epitopes and revealed that a numerically subdominant CD8 T cell response against D^bM₁₈₇ epitope of the virus matrix protein controlled virus replication efficiently with limited pulmonary inflammation and illness compared to the previously well-characterized and numerically dominant K^dM2₈₂ T cell response. Our data show that selectively boosting of epitope-specific CD8 T cell responses may be more beneficial than indiscriminant boosting of all available epitopes to achieve rapid viral clearance while limiting immunopathology. This work has implications for antigen design of vaccines intended to induce T-cell-mediated immunity.

T Cell Vaccinology: Beyond the Reflection of Infectious Responses

Inducing sustained, robust CD8(+) T cell responses is necessary for therapeutic intervention in chronic infectious diseases and cancer. Unfortunately, most adjuvant formulations fail to induce substantial cellular immunity in humans. Attenuated acute infectious agents induce strong CD8(+) T cell immunity, and are thought to therefore represent a good road map for guiding the development of subunit vaccines capable of inducing the same. However, recent evidence suggests that this assumption may need reconsideration. Here we provide an overview of subunit vaccine history as it pertains to instigating T cell responses. We argue that in light of evidence demonstrating that T cell responses to vaccination differ from those induced by infectious challenge, research in pursuit of cellular immunity-inducing vaccine adjuvants should no longer follow only the infection paradigm.

Reduced naïve CD8(+) T-cell priming efficacy in elderly adults

Aging is associated with impaired vaccine efficacy and increased susceptibility to infectious and malignant diseases. CD8⁺ T‐cells are key players in the immune response against pathogens and tumors. In aged mice, the dwindling naïve CD8⁺T‐cell compartment is thought to compromise the induction of de novo immune responses, but no experimental evidence is yet available in humans. Here, we used an original in vitro assay based on an accelerated dendritic cell coculture system in unfractioned peripheral blood mononuclear cells to examine CD8⁺ T‐cell priming efficacy in human volunteers. Using this approach, we report that old individuals consistently mount quantitatively and qualitatively impaired de novo CD8⁺ T‐cell responses specific for a model antigen. Reduced CD8⁺T‐cell priming capacity in vitro was further associated with poor primary immune responsiveness in vivo. This immune deficit likely arises as a consequence of intrinsic cellular defects and a reduction in the size of the naïve CD8⁺ T‐cell pool. Collectively, these findings provide new insights into the cellular immune insufficiencies that accompany human aging.

Heightened self-reactivity associated with selective survival, but not expansion, of naïve virus-specific CD8+ T cells in aged mice

In advanced age, decreased CD8(+) cytotoxic T-lymphocyte (CTL) responses to novel pathogens and cancer is paralleled by a decline in the number and function of naïve CTL precursors (CTLp). Although the age-related fall in CD8(+) T-cell numbers is well established, neither the underlying mechanisms nor the extent of variation for different epitope specificities have been defined. Furthermore, naïve CD8(+) T cells expressing high levels of CD44 accumulate with age, but it is unknown whether this accumulation reflects their preferential survival or an age-dependent driver of CD8(+) T-cell proliferation. Here, we track the number and phenotype of four influenza A virus (IAV)-specific CTLp populations in naïve C57BL/6 (B6) mice during aging, and compare T-cell receptor (TCR) clonal diversity for the CD44hi and CD44lo subsets of one such population. We show differential onset of decline for several IAV-specific CD8(+) T-cell populations with advanced age that parallel age-associated changes in the B6 immunodominance hierarchy, suggestive of distinct impacts of aging on different epitope-specific populations. Despite finding no evidence of clonal expansions in an aged, epitope-specific TCR repertoire, nonrandom alterations in TCR usage were observed, along with elevated CD5 and CD8 coreceptor expression. Collectively, these data demonstrate that naïve CD8(+) T cells expressing markers of heightened self-recognition are selectively retained, but not clonally expanded, during aging.

Strategic Priming with Multiple Antigens can Yield Memory Cell Phenotypes Optimized for Infection with Mycobacterium tuberculosis: A Computational Study

Lack of an effective vaccine results in 9 million new cases of tuberculosis (TB) every year and 1.8 million deaths worldwide. Although many infants are vaccinated at birth with BCG (an attenuated M. bovis), this does not prevent infection or development of TB after childhood. Immune responses necessary for prevention of infection or disease are still unknown, making development of effective vaccines against TB challenging. Several new vaccines are ready for human clinical trials, but these trials are difficult and expensive; especially challenging is determining the appropriate cellular response necessary for protection. The magnitude of an immune response is likely key to generating a successful vaccine. Characteristics such as numbers of central memory (CM) and effector memory (EM) T cells responsive to a diverse set of epitopes are also correlated with protection. Promising vaccines against TB contain mycobacterial subunit antigens (Ag) present during both active and latent infection. We hypothesize that protection against different key immunodominant antigens could require a vaccine that produces different levels of EM and CM for each Ag-specific memory population. We created a computational model to explore EM and CM values, and their ratio, within what we term Memory Design Space. Our model captures events involved in T cell priming within lymph nodes and tracks their circulation through blood to peripheral tissues. We used the model to test whether multiple Ag-specific memory cell populations could be generated with distinct locations within Memory Design Space at a specific time point post vaccination. Boosting can further shift memory populations to memory cell ratios unreachable by initial priming events. By strategically varying antigen load, properties of cellular interactions within the LN, and delivery parameters (e.g., number of boosts) of multi-subunit vaccines, we can generate multiple Ag-specific memory populations that cover a wide range of Memory Design Space. Given a set of desired characteristics for Ag-specific memory populations, we can use our model as a tool to predict vaccine formulations that will generate those populations.

XCR1+ dendritic cells promote memory CD8+ T cell recall upon secondary infections with Listeria monocytogenes or certain viruses

Alexandre et al. demonstrate the XCR1⁺ DCs are instrumental in memory CD8⁺ T cell responses to Listeria, VSV or vaccinia virus infection, but not CMV. Depending on the infection, robust memory CTL responses require cytokine- and chemokine-dependent cross-talk between XCR1⁺ DCs and NK cells or other IFN-γ–producing lymphocytes.

Naive CD8⁺ T cell priming during tumor development or many primary infections requires cross-presentation by XCR1⁺ dendritic cells (DCs). Memory CD8⁺ T lymphocytes (mCTLs) harbor a lower activation threshold as compared with naive cells. However, whether their recall responses depend on XCR1⁺ DCs is unknown. By using a new mouse model allowing fluorescent tracking and conditional depletion of XCR1⁺ DCs, we demonstrate a differential requirement of these cells for mCTL recall during secondary infections by different pathogens. XCR1⁺ DCs were instrumental to promote this function upon secondary challenges with Listeria monocytogenes, vesicular stomatitis virus, or Vaccinia virus, but dispensable in the case of mouse cytomegalovirus. We deciphered how XCR1⁺ DCs promote mCTL recall upon secondary infections with Listeria. By visualizing for the first time the in vivo choreography of XCR1⁺ DCs, NK cells and mCTLs during secondary immune responses, and by neutralizing in vivo candidate molecules, we demonstrate that, very early after infection, mCTLs are activated, and attracted in a CXCR3-dependent manner, by NK cell–boosted, IL-12–, and CXCL9-producing XCR1⁺ DCs. Hence, depending on the infectious agent, strong recall of mCTLs during secondary challenges can require cytokine- and chemokine-dependent cross-talk with XCR1⁺ DCs and NK cells.

Regulation of effector and memory CD8(+) T cell function by inflammatory cytokines

Cells communicate with each other through the production and secretion of cytokines, which are integral to the host response to infection. Once recognized by specific cytokine receptors expressed on the cell surface, these exogenous signals direct the biological function of a cell in order to adapt to their microenvironment. CD8(+) T cells are critical immune cells that play an important role in the control and elimination of intracellular pathogens. Current findings have demonstrated that cytokines influence all aspects of the CD8(+) T cell response to infection or immunization. The cytokine milieu induced at the time of activation impacts the overall magnitude and function of the effector CD8(+) T cell response and the generation of functional memory CD8(+) T cells. This review will focus on the impact of inflammatory cytokines on different aspects of CD8(+) T cell biology.

CD40 ligand-expressing recombinant vaccinia virus promotes the generation of CD8(+) central memory T cells

Central memory CD8(+) T cells (TCM ) play key roles in the protective immunity against infectious agents, cancer immunotherapy, and adoptive treatments of malignant and viral diseases. CD8(+) TCM cells are characterized by specific phenotypes, homing, and proliferative capacities. However, CD8(+) TCM -cell generation is challenging, and usually requires CD4(+) CD40L(+) T-cell "help" during the priming of naïve CD8(+) T cells. We have generated a replication incompetent CD40 ligand-expressing recombinant vaccinia virus (rVV40L) to promote the differentiation of human naïve CD8(+) T cells into TCM specific for viral and tumor-associated antigens. Soluble CD40 ligand recombinant protein (sCD40L), and vaccinia virus wild-type (VV WT), alone or in combination, were used as controls. Here, we show that, in the absence of CD4(+) T cells, a single "in vitro" stimulation of naïve CD8(+) T cells by rVV40L-infected nonprofessional CD14(+) antigen presenting cells promotes the rapid generation of viral or tumor associated antigen-specific CD8(+) T cells displaying TCM phenotypic and functional properties. These observations demonstrate the high ability of rVV40L to fine tune CD8(+) mediated immune responses, and strongly support the use of similar reagents for clinical immunization and adoptive immunotherapy purposes.

Review on the identification and role of Toxoplasma gondii antigenic epitopes

Toxoplasma gondii is an obligate intracellular protozoan parasite with a broad range of hosts, and it causes severe toxoplasmasis in both humans and animals. It is well known that the progression and severity of a disease depend on the immunological status of the host. Immunological studies on antigens indicate that antigens do not exert their functions through the entire protein molecule, but instead, specific epitopes are responsible for the immune response. Protein antigens not only contain epitope structures used by B, T, cytotoxic T lymphocyte (CTL), and NK cells to mediate immunological responses but can also contain structures that are unfavorable for protective immunity. Therefore, the study of antigenic epitopes from T. gondii has not only enhanced our understanding of the structure and function of antigens, the reactions between antigens and antibodies, and many other aspects of immunology but it also plays a significant role in the development of new diagnostic reagents and vaccines. In this review, we summarized the immune mechanisms induced by antigen epitopes and the latest advances in identifying T. gondii antigen epitopes. Particular attention was paid to the potential clinical usefulness of epitopes in this context. Through a critical analysis of the current state of knowledge, we elucidated the latest data concerning the biological effects of epitopes and the immune results aimed at the development of future epitope-based applications, such as vaccines and diagnostic reagents.

Bystander hyperactivation of preimmune CD8+ T cells in chronic HCV patients

Chronic infection perturbs immune homeostasis. While prior studies have reported dysregulation of effector and memory cells, little is known about the effects on naïve T cell populations. We performed a cross-sectional study of chronic hepatitis C (cHCV) patients using tetramer-associated magnetic enrichment to study antigen-specific inexperienced CD8(+) T cells (i.e., tumor or unrelated virus-specific populations in tumor-free and sero-negative individuals). cHCV showed normal precursor frequencies, but increased proportions of memory-phenotype inexperienced cells, as compared to healthy donors or cured HCV patients. These observations could be explained by low surface expression of CD5, a negative regulator of TCR signaling. Accordingly, we demonstrated TCR hyperactivation and generation of potent CD8(+) T cell responses from the altered T cell repertoire of cHCV patients. In sum, we provide the first evidence that naïve CD8(+) T cells are dysregulated during cHCV infection, and establish a new mechanism of immune perturbation secondary to chronic infection.

Strong TCR-mediated signals suppress integrated stress responses induced by KDELR1 deficiency in naive T cells

KDEL receptor 1 (KDELR1) regulates integrated stress responses (ISR) to promote naive T-cell survival in vivo. In a mouse line having nonfunctional KDELR1, T-Red (naive T-cell reduced) mice, polyclonal naive T cells show excessive ISR and eventually undergo apoptosis. However, breeding T-Red mice with TCR-transgenic mice bearing relatively high TCR affinity rescued the T-Red phenotype, implying a link between ISR-induced apoptosis and TCR-mediated signaling. Here, we showed that strong TCR stimulation reduces ISR in naive T cells. In mice lacking functional KDELR1, surviving naive T cells expressed significantly higher levels of CD5, a surrogate marker of TCR self-reactivity. In addition, higher TCR affinity/avidity was confirmed using a tetramer dissociation assay on the surviving naive T cells, suggesting that among the naive T-cell repertoire, those that receive relatively stronger TCR-mediated signals via self-antigens survive enhanced ISR. Consistent with this observation, weak TCR stimulation with altered peptide ligands decreased the survival and proliferation of naive T cells, whereas stimulation with ligands having higher affinity had no such effect. These results suggest a novel role of TCR-mediated signals in the attenuation of ISR in vivo.

Asymmetric Cell Division in T Lymphocyte Fate Diversification

Immunological protection against microbial pathogens is dependent on robust generation of functionally diverse T lymphocyte subsets. Upon microbial infection, naïve CD4(+) or CD8(+) T lymphocytes can give rise to effector- and memory-fated progeny that together mediate a potent immune response. Recent advances in single-cell immunological and genomic profiling technologies have helped elucidate early and late diversification mechanisms that enable the generation of heterogeneity from single T lymphocytes. We discuss these findings here and argue that one such mechanism, asymmetric cell division, creates an early divergence in T lymphocyte fates by giving rise to daughter cells with a propensity towards the terminally differentiated effector or self-renewing memory lineages, with cell-intrinsic and -extrinsic cues from the microenvironment driving the final maturation steps.

Clonal Deletion Prunes but Does Not Eliminate Self-Specific αβ CD8(+) T Lymphocytes

It has long been thought that clonal deletion efficiently removes almost all self-specific T cells from the peripheral repertoire. We found that self-peptide MHC-specific CD8(+) T cells in the blood of healthy humans were present in frequencies similar to those specific for non-self antigens. For the Y chromosome-encoded SMCY antigen, self-specific T cells exhibited only a 3-fold lower average frequency in males versus females and were anergic with respect to peptide activation, although this inhibition could be overcome by a stronger stimulus. We conclude that clonal deletion prunes but does not eliminate self-specific T cells and suggest that to do so would create holes in the repertoire that pathogens could readily exploit. In support of this hypothesis, we detected T cells specific for all 20 amino acid variants at the p5 position of a hepatitis C virus epitope in a random group of blood donors.

Extent of Systemic Spread Determines CD8+ T Cell Immunodominance for Laboratory Strains, Smallpox Vaccines, and Zoonotic Isolates of Vaccinia Virus

CD8(+) T cells that recognize virus-derived peptides presented on MHC class I are vital antiviral effectors. Such peptides presented by any given virus vary greatly in immunogenicity, allowing them to be ranked in an immunodominance hierarchy. However, the full range of parameters that determine immunodominance and the underlying mechanisms remain unknown. In this study, we show across a range of vaccinia virus strains, including the current clonal smallpox vaccine, that the ability of a strain to spread systemically correlated with reduced immunodominance. Reduction in immunodominance was observed both in the lymphoid system and at the primary site of infection. Mechanistically, reduced immunodominance was associated with more robust priming and especially priming in the spleen. Finally, we show this is not just a property of vaccine and laboratory strains of virus, because an association between virulence and immunodominance was also observed in isolates from an outbreak of zoonotic vaccinia virus that occurred in Brazil.

Early Effector CD8 T Cells Display Plasticity in Populating the Short-Lived Effector and Memory-Precursor Pools Following Bacterial or Viral Infection

Naïve antigen-specific CD8 T cells expand in response to infection and can be phenotypically separated into distinct effector populations, which include memory precursor effector cells (MPECs) and short-lived effector cells (SLECs). In the days before the peak of the T cell response, a third population called early effector cells (EECs) predominate the antigen-specific response. However, the contribution of the EEC population to the CD8 T cell differentiation program during an antimicrobial immune response is not well understood. To test if EEC populations were pre-committed to either an MPEC or SLEC fate, we purified EECs from mice infected with Listeria monocytogenes (LM) or vesicular stomatitis virus (VSV), where the relative frequency of each population is known to be different at the peak of the response. Sorted EECs transferred into uninfected hosts revealed that EECs were pre-programmed to differentiate based on early signals received from the distinct infectious environments. Surprisingly, when these same EECs were transferred early into mismatched infected hosts, the transferred EECs could be diverted from their original fate. These results delineate a model of differentiation where EECs are programmed to form MPECs or SLECs, but remain susceptible to additional inflammatory stimuli that can alter their fate.

Self-recognition drives the preferential accumulation of promiscuous CD4(+) T-cells in aged mice

T-cell recognition of self and foreign peptide antigens presented in major histocompatibility complex molecules (pMHC) is essential for life-long immunity. How the ability of the CD4⁺ T-cell compartment to bind self- and foreign-pMHC changes over the lifespan remains a fundamental aspect of T-cell biology that is largely unexplored. We report that, while old mice (18–22 months) contain fewer CD4⁺ T-cells compared with adults (8–12 weeks), those that remain have a higher intrinsic affinity for self-pMHC, as measured by CD5 expression. Old mice also have more cells that bind individual or multiple distinct foreign-pMHCs, and the fold increase in pMHC-binding populations is directly related to their CD5 levels. These data demonstrate that the CD4⁺ T-cell compartment preferentially accumulates promiscuous constituents with age as a consequence of higher affinity T-cell receptor interactions with self-pMHC.

DOI:http://dx.doi.org/10.7554/eLife.05949.001

The immune system's T cells help the body to recognize and destroy harmful pathogens, such as viruses and bacteria. T cells ‘remember’ immunity-inducing fragments, called antigens, from the pathogens they have encountered. This memory then allows the immune system to quickly fend off infections if those pathogens, or even related pathogens, invade again. Vaccines exploit the ability to form immunological memory by exposing the body to harmless forms of the pathogen, or even just particular antigens from it. This allows the T cells to learn how to identify the pathogen without any risk of illness.

Vaccines have been extremely successful and have helped to virtually eliminate some diseases. However, for reasons that are unclear, the immune systems of older adults become less functional, so vaccines often lose their effectiveness. Paradoxically, as people age T cells become more likely to attack the body's cells, causing autoimmune diseases like arthritis. Understanding what happens to aging T cells to cause these immune changes may help scientists design vaccines that remain effective as people age.

Little is known about what happens to a particular type of T cell—the CD4⁺ T cells—as people age, even though this population plays a critical role in providing other immune cells with detailed instructions on when and how to fight a pathogen. Now, Deshpande et al. show that CD4⁺ T cells undergo a remarkable set of changes in aging mice. Mice that are nearing the end of their natural lifespan have fewer CD4⁺ T cells than younger mice. However, those CD4⁺ T cells that remain are more likely than CD4⁺ T cells from younger mice to be able to recognize multiple antigens. This increase in the proportion of multitasking CD4⁺ T cells corresponds with an increased tendency of these cells to bind to the body's own cells. If similar changes occur in older people, this may help explain some age-related autoimmune diseases. Yet, the relationship between the increase in multitasking CD4⁺ T cells and the decrease in immune function with aging remains to be fully explored.

The challenge for scientists now is to determine how these age-related changes in CD4⁺ T cells affect immune responses to vaccines or pathogens in older individuals. One implication of this work is that CD4⁺ T cell responses may be too robust and out of balance with other arms of the immune system. This could even lead to conditions such as autoimmunity. Alternatively, while there may be more CD4⁺ T cells that can multitask by recognizing multiple antigens, their ability to respond appropriately to infections or vaccinations may be diminished. What is clear from the work of Deshpande et al. is that the rules that have been defined for immunity in adults change with aging. The rules that govern immunity in the elderly must be more clearly defined to realize the goal of designing immunotherapies, such as vaccines, that provide protection throughout the lifespan.

DOI:http://dx.doi.org/10.7554/eLife.05949.002

Polymicrobial Sepsis Increases Susceptibility to Chronic Viral Infection and Exacerbates CD8+ T Cell Exhaustion

Patients who survive sepsis display suppressed immune functions, often manifested as an increased susceptibility to secondary infections. Recently, using a cecal-ligation and puncture (CLP) model of sepsis, we showed that sepsis induces substantial and long-lasting changes in the available naive CD8(+) T cell repertoire affecting the capacity of the host to respond to newly encountered acute infections. However, the extent to which sepsis changes the host susceptibility to chronic infection and affects CD8(+) T cell responses is currently unknown. In this study, we demonstrate that inbred and outbred mice recovering from a septic event are more susceptible to lymphocytic choriomeningitis virus (LCMV) clone-13 infection exhibited by mortality and viral burden. Primary virus-specific CD8(+) T cells in LCMV clone-13-infected septic mice displayed exacerbated CD8(+) T cell exhaustion illustrated by increased inhibitory molecule expression (e.g., programmed cell death 1, lymphocyte-activation gene 3, and 2B4) and diminished Ag-driven cytokine production (e.g., IFN-γ, TNF-α) compared with similarly infected sham-treated mice. Importantly, therapeutic inhibitory molecule dual blockade (anti-PD-L1 and anti-lymphocyte-activation gene 3) increased the number of circulating LCMV-specific CD8(+) T cells, and improved CD8(+) T cell function and pathogen control in chronically infected septic mice. Together, these results illustrate that polymicrobial sepsis compromises the overall health of the host leading to increased vulnerability to chronic infection and exacerbated CD8(+) T cell exhaustion. Collectively, our findings suggest that septic survivors may be more susceptible and at greater risk for developing exhaustible CD8(+) T cells upon encountering a subsequent chronic infection.

Human and Murine Clonal CD8+ T Cell Expansions Arise during Tuberculosis Because of TCR Selection

The immune system can recognize virtually any antigen, yet T cell responses against several pathogens, including Mycobacterium tuberculosis, are restricted to a limited number of immunodominant epitopes. The host factors that affect immunodominance are incompletely understood. Whether immunodominant epitopes elicit protective CD8+ T cell responses or instead act as decoys to subvert immunity and allow pathogens to establish chronic infection is unknown. Here we show that anatomically distinct human granulomas contain clonally expanded CD8+ T cells with overlapping T cell receptor (TCR) repertoires. Similarly, the murine CD8+ T cell response against M. tuberculosis is dominated by TB10.4_4-11-specific T cells with extreme TCRβ bias. Using a retrogenic model of TB10.4_4-11-specific CD8+ T cells, we show that TCR dominance can arise because of competition between clonotypes driven by differences in affinity. Finally, we demonstrate that TB10.4-specific CD8+ T cells mediate protection against tuberculosis, which requires interferon-γ production and TAP1-dependent antigen presentation in vivo. Our study of how immunodominance, biased TCR repertoires, and protection are inter-related, provides a new way to measure the quality of T cell immunity, which if applied to vaccine evaluation, could enhance our understanding of how to elicit protective T cell immunity.

While T cells are required for protection against Mycobacterium tuberculosis infection, attempts to prevent tuberculosis by vaccines designed to elicit memory T cells have only been partially successful. Several vaccine candidates are in clinical trials, but progress has been slow because their ability to prevent disease must be empirically tested. There is little understanding of why certain antigens are targets of protective immunity. We have characterized an immunodominant CD8⁺ T cell response to the M. tuberculosis antigen TB10.4 (EsxH). CD8⁺ T cells specific for the TB10.4_4–11 epitope are primed early during infection and account for 30–50% of lung CD8⁺ T cells during chronic infection. Now we have used deep sequencing to characterize the TCR repertoire of TB10.4_4-11-specific CD8⁺ T cells in the lungs of infected mice. Interestingly, TB10.4_4-11-specific CD8⁺ T cells exhibit extreme clonal expansion of certain TCRβ with common structural features, most likely because of affinity selection. Affinity selection of T cells is more important when antigen presentation is limiting. Although the lung contains numerous bacteria during infection, antigen-presentation by infected APC may be limiting, mimicking a “low antigen” state. Thus, even T cells that have the potential to mediate protection may function inefficiently because of suboptimal T cell activation.

In vivo RNAi screens: concepts and applications

Functional genomics approaches that leverage the RNAi pathway have been applied in vivo to examine the roles of hundreds or thousands of genes; mainly in the context of cancer. Here, we discuss principles guiding the design of RNAi screens, parameters that determine success and recent developments that have improved accuracy and expanded the applicability of these approaches to other in vivo settings, including the immune system. We review recent studies that have applied in vivo RNAi screens in T cells to examine genes that regulate T cell differentiation during viral infection, and that control their accumulation in tumors in a model of adoptive T cell therapy. In this context, we put forward an argument as to why RNAi approaches in vivo are likely to provide particularly salient insight into immunology.

CD8+ TCR Bias and Immunodominance in HIV-1 Infection

Immunodominance describes a phenomenon whereby the immune system consistently targets only a fraction of the available Ag pool derived from a given pathogen. In the case of CD8(+) T cells, these constrained epitope-targeting patterns are linked to HLA class I expression and determine disease progression. Despite the biological importance of these predetermined response hierarchies, little is known about the factors that control immunodominance in vivo. In this study, we conducted an extensive analysis of CD8(+) T cell responses restricted by a single HLA class I molecule to evaluate the mechanisms that contribute to epitope-targeting frequency and antiviral efficacy in HIV-1 infection. A clear immunodominance hierarchy was observed across 20 epitopes restricted by HLA-B*42:01, which is highly prevalent in populations of African origin. Moreover, in line with previous studies, Gag-specific responses and targeting breadth were associated with lower viral load set-points. However, peptide-HLA-B*42:01 binding affinity and stability were not significantly linked with targeting frequencies. Instead, immunodominance correlated with epitope-specific usage of public TCRs, defined as amino acid residue-identical TRB sequences that occur in multiple individuals. Collectively, these results provide important insights into a potential link between shared TCR recruitment, immunodominance, and antiviral efficacy in a major human infection.

T cell receptor cross-reactivity between similar foreign and self peptides influences naive cell population size and autoimmunity

T cell receptor (TCR) cross-reactivity between major histocompatibility complex II (MHCII)-binding self and foreign peptides could influence the naive CD4(+) T cell repertoire and autoimmunity. We found that nonamer peptides that bind to the same MHCII molecule only need to share five amino acids to cross-react on the same TCR. This property was biologically relevant because systemic expression of a self peptide reduced the size of a naive cell population specific for a related foreign peptide by deletion of cells with cross-reactive TCRs. Reciprocally, an incompletely deleted naive T cell population specific for a tissue-restricted self peptide could be triggered by related microbial peptides to cause autoimmunity. Thus, TCR cross-reactivity between similar self and foreign peptides can reduce the size of certain foreign peptide-specific T cell populations and might allow T cell populations specific for tissue-restricted self peptides to cause autoimmunity after infection.

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

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

Paired TCRαβ analysis of virus-specific CD8(+) T cells exposes diversity in a previously defined 'narrow' repertoire

T-cell receptor (TCR) usage has an important role in determining the outcome of CD8(+) cytotoxic T-lymphocyte responses to viruses and other pathogens. However, the characterization of TCR usage from which such conclusions are drawn is based on exclusive analysis of either the TCRα chain or, more commonly, the TCRβ chain. Here, we have used a multiplexed reverse transcription-PCR protocol to analyse the CDR3 regions of both TCRα and β chains from single naive or immune epitope-specific cells to provide a comprehensive picture of epitope-specific TCR usage and selection into the immune response. Analysis of TCR repertoires specific for three influenza-derived epitopes (D(b)NP(366), D(b)PA(224) and D(b)PB1-F2(62)) showed preferential usage of particular TCRαβ proteins in the immune repertoire relative to the naive repertoire, in some cases, resulting in a complete shift in TRBV preference or CDR3 length, and restricted repertoire diversity. The NP(366)-specific TCRαβ repertoire, previously defined as clonally restricted based on TCRβ analysis, was similarly diverse as the PA(224)- and PB1-F2(62)-specific repertoires. Intriguingly, preferred TCR characteristics (variable gene usage, CDR3 length and junctional gene usage) appeared to be able to confer specificity either independently or in concert with one another, depending on the epitope specificity. These data have implications for established correlations between the nature of the TCR repertoire and response outcomes after infection, and suggest that analysis of a subset of cells or a single TCR chain does not accurately depict the nature of the antigen-specific TCRαβ repertoire.

Molecular mechanisms of CD8(+) T cell trafficking and localization

Cytotoxic CD8(+) T cells are potent mediators of host protection against disease due to their ability to directly kill cells infected with intracellular pathogens and produce inflammatory cytokines at the site of infection. To fully achieve this objective, naïve CD8(+) T cells must be able to survey the entire body for the presence of foreign or "non-self" antigen that is delivered to draining lymph nodes following infection or tissue injury. Once activated, CD8(+) T cells undergo many rounds of cell division, acquire effector functions, and are no longer restricted to the circulation and lymphoid compartments like their naïve counterparts, but rather are drawn to inflamed tissues to combat infection. As CD8(+) T cells transition from naïve to effector to memory populations, this is accompanied by dynamic changes in the expression of adhesion molecules and chemokine receptors that ultimately dictate their localization in vivo. Thus, an understanding of the molecular mechanisms regulating CD8(+) T cell trafficking and localization is critical for vaccine design, control of infectious diseases, treatment of autoimmune disorders, and cancer immunotherapy.

Mathematical modeling provides kinetic details of the human immune response to vaccination

With major advances in experimental techniques to track antigen-specific immune responses many basic questions on the kinetics of virus-specific immunity in humans remain unanswered. To gain insights into kinetics of T and B cell responses in human volunteers we combined mathematical models and experimental data from recent studies employing vaccines against yellow fever and smallpox. Yellow fever virus-specific CD8 T cell population expanded slowly with the average doubling time of 2 days peaking 2.5 weeks post immunization. Interestingly, we found that the peak of the yellow fever-specific CD8 T cell response was determined by the rate of T cell proliferation and not by the precursor frequency of antigen-specific cells as has been suggested in several studies in mice. We also found that while the frequency of virus-specific T cells increased slowly, the slow increase could still accurately explain clearance of yellow fever virus in the blood. Our additional mathematical model described well the kinetics of virus-specific antibody-secreting cell and antibody response to vaccinia virus in vaccinated individuals suggesting that most of antibodies in 3 months post immunization were derived from the population of circulating antibody-secreting cells. Taken together, our analysis provided novel insights into mechanisms by which live vaccines induce immunity to viral infections and highlighted challenges of applying methods of mathematical modeling to the current, state-of-the-art yet limited immunological data.

T cells recognizing a 11mer influenza peptide complexed to H-2D(b) show promiscuity for peptide length

T-cell repertoire is selected according to self peptide-MHC (major histocompatibility complex) complexes in the thymus. Although most peripheral T cells recognize specific pathogen-derived peptides complexed to self-MHC exclusively, some possess cross-reactivity to other self or foreign peptides presented by self-MHC molecules; a phenomenon often termed T-cell receptor (TCR) promiscuity or degeneracy. TCR promiscuity has been attributed to various autoimmune conditions. On the other hand, it is considered a mechanism for a relatively limited TCR repertoire to deal with a potentially much larger antigenic peptide repertoire. Such property has also been utilized to bypass self-tolerance for cancer vaccine development. Although many studies explored such degeneracy for peptide of the same length, few studies reported such properties for peptides of different length. In this study, we finely characterized the CD8(+) T-cell response specific for a 11mer peptide derived from influenza A viral polymerase basic protein 2. The short-term T-cell line, despite possessing highly biased TCR, was able to react with multiple peptides of different length sharing the same core sequence. Out data clearly showed the importance of detailed and quantitative assessments for such T-cell specificity. Our data also emphasize the importance of biochemical demonstration of the naturally presented minimal peptide.

Fixed expression of single influenza virus-specific TCR chains demonstrates the capacity for TCR α- and β-chain diversity in the face of peptide-MHC class I specificity

The characteristics of the TCR repertoire expressed by epitope-specific CD8(+) T cells can be an important determinant of the quality of immune protection against virus infection. Most studies of epitope-specific TCR repertoires focus solely on an analysis of TCR β-chains, rather than the combined TCRαβ heterodimers that confer specificity. Hence, the importance of complementary α- and β-chain pairing in determining TCR specificity and T cell function is not well understood. Our earlier study of influenza-specific TCR repertoires in a C57BL/6J mouse model described a structural basis for preferred TCRαβ pairing that determined exquisite specificity for the D(b)PA224 epitope from influenza A virus. We have now extended this analysis using retrogenic mice engineered to express single TCR α- or β-chains specific for the D(b)NP366 or D(b)PA224 epitopes derived from influenza A virus. We found that particular TCRαβ combinations were selected for recognition of these epitopes following infection, indicating that pairing of certain α- and β-chain sequences is key for determining TCR specificity. Furthermore, we demonstrated that some TCRαβ heterodimers were preferentially expanded from the naive repertoire in response to virus infection, suggesting that appropriate αβ pairing confers optimal T cell responsiveness to Ag.

Plasmodium suppresses expansion of T cell responses to heterologous infections

Plasmodium remains a major pathogen causing malaria and impairing defense against other infections. Defining how Plasmodium increases susceptibility to heterologous pathogens may lead to interventions that mitigate the severity of coinfections. Previous studies proposed that reduced T cell responses during coinfections are due to diminished recruitment of naive T cells through infection-induced decreases in chemokine CCL21. We found that, although Listeria infections reduced expression of CCL21 in murine spleens, lymphocytic choriomeningitis virus (LCMV)-specific T cell responses were not impaired during Listeria + LCMV coinfection, arguing against a major role for this chemokine in coinfection-induced T cell suppression. In our experiments, Plasmodium yoelii infection led to a reduced CD8(+) T cell response to a subsequent Listeria infection. We propose an alternative mechanism whereby P. yoelii suppresses Listeria-specific T cell responses. We found that Listeria-specific T cells expanded more slowly and resulted in lower numbers in response to coinfection with P. yoelii. Mathematical modeling and experimentation revealed greater apoptosis of Listeria-specific effector T cells as the main mechanism, because P. yoelii infections did not suppress the recruitment or proliferation rates of Listeria-specific T cells. Our results suggest that P. yoelii infections suppress immunity to Listeria by causing increased apoptosis in Listeria-specific T cells, resulting in a slower expansion rate of T cell responses.

In vivo RNA interference screens identify regulators of antiviral CD4(+) and CD8(+) T cell differentiation

Classical genetic approaches to examine the requirements of genes for T cell differentiation during infection are time consuming. Here we developed a pooled approach to screen 30-100+ genes individually in separate antigen-specific T cells during infection using short hairpin RNAs in a microRNA context (shRNAmir). Independent screens using T cell receptor (TCR)-transgenic CD4(+) and CD8(+) T cells responding to lymphocytic choriomeningitis virus (LCMV) identified multiple genes that regulated development of follicular helper (Tfh) and T helper 1 (Th1) cells, and short-lived effector and memory precursor cytotoxic T lymphocytes (CTLs). Both screens revealed roles for the positive transcription elongation factor (P-TEFb) component Cyclin T1 (Ccnt1). Inhibiting expression of Cyclin T1, or its catalytic partner Cdk9, impaired development of Th1 cells and protective short-lived effector CTL and enhanced Tfh cell and memory precursor CTL formation in vivo. This pooled shRNA screening approach should have utility in numerous immunological studies.

Analysis of relationships between peptide/MHC structural features and naive T cell frequency in humans

The structural rules governing peptide/MHC (pMHC) recognition by T cells remain unclear. To address this question, we performed a structural characterization of several HLA-A2/peptide complexes and assessed in parallel their antigenicity, by analyzing the frequency of the corresponding Ag-specific naive T cells in A2(+) and A2(-) individuals, as well as within CD4(+) and CD8(+) subsets. We were able to find a correlation between specific naive T cell frequency and peptide solvent accessibility and/or mobility for a subset of moderately prominent peptides. However, one single structural parameter of the pMHC complexes could not be identified to explain each peptide antigenicity. Enhanced pMHC antigenicity was associated with both highly biased TRAV usage, possibly reflecting favored interaction between particular pMHC complexes and germline TRAV loops, and peptide structural features allowing interactions with a broad range of permissive CDR3 loops. In this context of constrained TCR docking mode, an optimal peptide solvent exposed surface leading to an optimal complementarity with TCR interface may constitute one of the key features leading to high frequency of specific T cells. Altogether our results suggest that frequency of specific T cells depends on the fine-tuning of several parameters, the structural determinants governing TCR-pMHC interaction being just one of them.

Epitope specificity delimits the functional capabilities of vaccine-induced CD8 T cell populations

Despite progress toward understanding the correlates of protective T cell immunity in HIV infection, the optimal approach to Ag delivery by vaccination remains uncertain. We characterized two immunodominant CD8 T cell populations generated in response to immunization of BALB/c mice with a replication-deficient adenovirus serotype 5 vector expressing the HIV-derived Gag and Pol proteins at equivalent levels. The Gag-AI9/H-2K(d) epitope elicited high-avidity CD8 T cell populations with architecturally diverse clonotypic repertoires that displayed potent lytic activity in vivo. In contrast, the Pol-LI9/H-2D(d) epitope elicited motif-constrained CD8 T cell repertoires that displayed lower levels of physical avidity and lytic activity despite equivalent measures of overall clonality. Although low-dose vaccination enhanced the functional profiles of both epitope-specific CD8 T cell populations, greater polyfunctionality was apparent within the Pol-LI9/H-2D(d) specificity. Higher proportions of central memory-like cells were present after low-dose vaccination and at later time points. However, there were no noteworthy phenotypic differences between epitope-specific CD8 T cell populations across vaccine doses or time points. Collectively, these data indicate that the functional and phenotypic properties of vaccine-induced CD8 T cell populations are sensitive to dose manipulation, yet constrained by epitope specificity in a clonotype-dependent manner.

TCR signal quantity and quality in CD4+ T cell differentiation

The adaptive immune system protects its host from a myriad of pathogens. This ability stems from a vast set of lymphocytes, each with a different antigen receptor, a small number of which will bind to antigens derived from a given pathogen. Although the cells within any antigen-specific population appear to be relatively homogenous before antigenic encounter, recent work on T cells indicates that individual cells within the population differentiate in very different ways after exposure to the antigen. We focus here on studies of CD4⁺ T cells and review evidence indicating that variable differentiation of effector cells from single naïve cells is caused by both cell-extrinsic stochastic factors and cell-intrinsic factors related to T cell antigen receptor (TCR) signal quantity and quality.

CD8+ T cell immunity against human respiratory syncytial virus

Human respiratory syncytial virus (HRSV) was first discovered in the 1950s, but despite decades of research, a licensed vaccine against it is not available. Epidemiological studies indicate that antibodies directed against the fusion protein (F) partially correlate with protection. In addition, an F-specific monoclonal antibody is licensed as a prophylactic treatment in children who are at high risk of developing complications following HRSV infection. Therefore, most HRSV-oriented vaccination strategies focus on inducing a humoral immune response against F. In the quest for the development of a safe HRSV vaccine, the induction of a T cell immune response has received a lot less attention. T cell immunity directed against HRSV has not been associated unequivocally with protection against HRSV and CD4(+) T helper cell responses may even worsen disease due to HRSV. However, many studies support a protective role for CD8(+) T cells in clearance of HRSV from the lungs. In this review we highlight the clinical and experimental evidence in favor of a CD8(+) T lymphocyte-based vaccination strategy to protect against HRSV. First, we describe how T cell responses and T cell memory are induced in the lungs upon respiratory viral infection. HRSV has evolved mechanisms that hamper CD8(+) T cell priming and effector functions. We appraise the information on HRSV-specific CD8(+) T cell immunity gained from laboratory mouse studies, taking into account the advantages and limitations of this animal model and, where possible, the accordance with clinical evidence. Finally, we focus on recent efforts to develop T cell based vaccines against HRSV.

Lymphocyte fate specification as a deterministic but highly plastic process

The cellular progeny of a clonally selected lymphocyte must execute function. However, their function must often occur in more than one way, in more than one place and at more than one time. Experimental evidence supports the view that a single activated lymphocyte can produce a variety of cellular descendants. The mechanisms that are responsible for generating diversity among the progeny of a single lymphocyte remain a subject of lively controversy. Some groups have suggested stochastic mechanisms that are analogous to the diversification of the antigen receptor repertoire. We suggest that the complexity of lymphocyte fates in space and time can be derived from a single naive lymphocyte using the principles of cell diversification that are common in developmental and regenerative biology, including (but not limited to) asymmetric cell division.

CALGB 150905 (Alliance): rituximab broadens the antilymphoma response by activating unlicensed NK cells

Natural killer (NK) cells contribute to clinical responses in patients treated with rituximab, but the rules determining NK-cell responsiveness to mAb therapies are poorly defined. A deeper understanding of the mechanisms responsible for antibody-dependent cellular cytotoxicity (ADCC) could yield useful biomarkers for predicting clinical responses in patients. Unlicensed NK cells, defined as NK cells lacking expression of an inhibitory KIR for self-HLA class I ligands, are hyporesponsive in steady state, but are potent effectors in inflammatory conditions. We hypothesized that antitumor antibodies such as rituximab can overcome NK-cell dependence on licensing, making unlicensed NK cells important for clinical responses. Here, we examined the influences of variations in KIR and HLA class I alleles on in vitro responses to rituximab. We tested the clinical significance in a cohort of patients with follicular lymphoma treated with rituximab-containing mAb combinations, and show that rituximab triggers responses from all NK-cell populations regardless of licensing. Neither IL2 nor accessory cells are required for activating unlicensed NK cells, but both can augment rituximab-mediated ADCC. Moreover, in 101 patients with follicular lymphoma treated with rituximab-containing mAb combinations, a "missing ligand" genotype (predictive of unlicensed NK cells) is associated with a higher rate of progression-free survival. Our data suggest that the clinical efficacy of rituximab may be driven, in part, by its ability to broaden the NK-cell repertoire to include previously hyporesponsive, unlicensed NK cells. A "missing ligand" KIR and HLA class I genotype may be predictive of this benefit and useful for personalizing treatment decisions in lymphomas and other tumors.

Relationship between Poor Immunogenicity of HLA-A2-Restricted Peptide Epitopes and Paucity of Naïve CD8(+) T-Cell Precursors in HLA-A2-Transgenic Mice

We examined the immunogenicity of H-2 class I-restricted and HLA-A2-restricted epitopes through peptide immunization of HLA-A2-transgenic mice that also express mouse H-2 class I molecules. All four of the tested epitopes restricted by H-2 class I robustly elicited T-cell responses, but four of seven epitopes restricted by HLA-A2 did not induce T-cell responses, showing that HLA-A2-restricted peptide epitopes tend to be poorly immunogenic in HLA-A2-transgenic mice. This finding was confirmed in HLA-A2-transgenic mice infected with a recombinant vaccinia virus expressing hepatitis C virus proteins. We examined the precursor frequency of epitope-specific naïve CD8(+) T cells in HLA-A2-transgenic and conventional C57BL/6 mice and found that the poor immunogenicity of HLA-A2-restricted peptide epitopes is related to the paucity of naïve CD8(+) T-cell precursors in HLA-A2-transgenic mice. These results provide direction for the improvement of mouse models to study epitope repertoires and the immunodominance of human T-cell responses.

Efficiency of dendritic cell vaccination against B16 melanoma depends on the immunization route

Dendritic cells (DC) presenting tumor antigens are crucial to induce potent T cell-mediated anti-tumor immune responses. Therefore DC-based cancer vaccines have been established for therapy, however clinical outcomes are often poor and need improvement. Using a mouse model of B16 melanoma, we found that the route of preventive DC vaccination critically determined tumor control. While repeated DC vaccination did not show an impact of the route of DC application on the prevention of tumor growth, a single DC vaccination revealed that both the imprinting of skin homing receptors and an enhanced proliferation state of effector T cells was seen only upon intracutaneous but not intravenous or intraperitoneal immunization. Tumor growth was prevented only by intracutaneous DC vaccination. Our results indicate that under suboptimal conditions the route of DC vaccination crucially determines the efficiency of tumor defense. DC-based strategies for immunotherapy of cancer should take into account the immunization route in order to optimize tissue targeting of tumor antigen specific T cells.

Visualizing T Cell Migration in situ

Mounting a protective immune response is critically dependent on the orchestrated movement of cells within lymphoid tissues. The structure of secondary lymphoid organs regulates immune responses by promoting optimal cell-cell and cell-extracellular matrix interactions. Naïve T cells are initially activated by antigen presenting cells in secondary lymphoid organs. Following priming, effector T cells migrate to the site of infection to exert their functions. Majority of the effector cells die while a small population of antigen-specific T cells persists as memory cells in distinct anatomical locations. The persistence and location of memory cells in lymphoid and non-lymphoid tissues is critical to protect the host from re-infection. The localization of memory T cells is carefully regulated by several factors including the highly organized secondary lymphoid structure, the cellular expression of chemokine receptors and compartmentalized secretion of their cognate ligands. This balance between the anatomy and the ordered expression of cell surface and soluble proteins regulates the subtle choreography of T cell migration. In recent years, our understanding of cellular dynamics of T cells has been advanced by the development of new imaging techniques allowing in situ visualization of T cell responses. Here, we review the past and more recent studies that have utilized sophisticated imaging technologies to investigate the migration dynamics of naïve, effector, and memory T cells.

Impact of regulated secretion on antiparasitic CD8 T cell responses

CD8 T cells play a key role in defense against the intracellular parasite Toxoplasma, but why certain CD8 responses are more potent than others is not well understood. Here, we describe a parasite antigen, ROP5, that elicits a CD8 T cell response in genetically susceptible mice. ROP5 is secreted via parasite organelles termed rhoptries that are injected directly into host cells during invasion, whereas the protective, dense-granule antigen GRA6 is constitutively secreted into the parasitophorous vacuole. Transgenic parasites in which the ROP5 antigenic epitope was targeted for secretion through dense granules led to enhanced CD8 T cell responses, whereas targeting the GRA6 epitope to rhoptries led to reduced CD8 responses. CD8 T cell responses to the dense-granule-targeted ROP5 epitope resulted in reduced parasite load in the brain. These data suggest that the mode of secretion affects the efficacy of parasite-specific CD8 T cell responses.

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.

A novel T-cell receptor mimic defines dendritic cells that present an immunodominant West Nile virus epitope in mice

We used a newly generated T-cell receptor mimic monoclonal antibody (TCRm MAb) that recognizes a known nonself immunodominant peptide epitope from West Nile virus (WNV) NS4B protein to investigate epitope presentation after virus infection in C57BL/6 mice. Previous studies suggested that peptides of different length, either SSVWNATTAI (10-mer) or SSVWNATTA (9-mer) in complex with class I MHC antigen H-2D(b) , were immunodominant after WNV infection. Our data establish that both peptides are presented on the cell surface after WNV infection and that CD8(+) T cells can detect 10- and 9-mer length variants similarly. This result varies from the idea that a given T-cell receptor (TCR) prefers a single peptide length bound to its cognate class I MHC. In separate WNV infection studies with the TCRm MAb, we show that in vivo the 10-mer was presented on the surface of uninfected and infected CD8α(+) CD11c(+) dendritic cells, which suggests the use of direct and cross-presentation pathways. In contrast, CD11b(+) CD11c(-) cells bound the TCRm MAb only when they were infected. Our study demonstrates that TCR recognition of peptides is not limited to certain peptide lengths and that TCRm MAbs can be used to dissect the cell-type specific mechanisms of antigen presentation in vivo.

Oral infection drives a distinct population of intestinal resident memory CD8(+) T cells with enhanced protective function

The intestinal mucosa promotes T cell responses that might be beneficial for effective mucosal vaccines. However, intestinal resident memory T (Trm) cell formation and function are poorly understood. We found that oral infection with Listeria monocytogenes induced a robust intestinal CD8 T cell response and blocking effector T cell migration showed that intestinal Trm cells were critical for secondary protection. Intestinal effector CD8 T cells were predominately composed of memory precursor effector cells (MPECs) that rapidly upregulated CD103, which was needed for T cell accumulation in the intestinal epithelium. CD103 expression, rapid MPEC formation, and maintenance in intestinal tissues were dependent on T cell intrinsic transforming growth factor β signals. Moreover, intestinal Trm cells generated after intranasal or intravenous infection were less robust and phenotypically distinct from Trm cells generated after oral infection, demonstrating the critical contribution of infection route for directing the generation of protective intestinal Trm cells.