Rapid recruitment of virus-specific CD8 T cells restructures immunodominance during protective secondary responses

T cell responses to SARS-CoV-2 in people with and without neurologic symptoms of long COVID

Many people experiencing long COVID syndrome, or post-acute sequelae of SARS-CoV-2 infection (PASC), suffer from debilitating neurologic symptoms (Neuro-PASC). However, whether virus-specific adaptive immunity is affected in Neuro-PASC patients remains poorly understood. We report that Neuro-PASC patients exhibit distinct immunological signatures composed of elevated humoral and cellular responses toward SARS-CoV-2 Nucleocapsid protein at an average of 6 months post-infection compared to healthy COVID convalescents. Neuro-PASC patients also had enhanced virus-specific production of IL-6 from and diminished activation of CD8+ T cells. Furthermore, the severity of cognitive deficits or quality of life disturbances in Neuro-PASC patients were associated with a reduced diversity of effector molecule expression in T cells but elevated IFN-γ production to the C-terminal domain of Nucleocapsid protein. Proteomics analysis showed enhanced plasma immunoregulatory proteins and reduced pro-inflammatory and antiviral response proteins in Neuro-PASC patients compared with healthy COVID convalescents, which were also correlated with worse neurocognitive dysfunction. These data provide new insight into the pathogenesis of long COVID syndrome and a framework for the rational design of predictive biomarkers and therapeutic interventions.

Intrinsic IL-2 production by effector CD8 T cells affects IL-2 signaling and promotes fate decisions, stemness, and protection

Here, we show that the capacity to manufacture IL-2 identifies constituents of the expanded CD8 T cell effector pool that display stem-like features, preferentially survive, rapidly attain memory traits, resist exhaustion, and control chronic viral challenges. The cell-intrinsic synthesis of IL-2 by CD8 T cells attenuates the ability to receive IL-2-dependent STAT5 signals, thereby limiting terminal effector formation, endowing the IL-2-producing effector subset with superior protective powers. In contrast, the non-IL-2-producing effector cells respond to IL-2 signals and gain effector traits at the expense of memory formation. Despite having distinct properties during the effector phase, IL-2-producing and nonproducing CD8 T cells appear to converge transcriptionally as memory matures to form populations with equal recall abilities. Therefore, the potential to produce IL-2 during the effector, but not memory stage, is a consequential feature that dictates the protective capabilities of the response.

Humanization of T cell-mediated immunity in mice

[Figure: see text].

Type I IFN Signaling Is Dispensable during Secondary Viral Infection

Innate immune responses in general, and type I interferons (T1IFNs) in particular, play an important and often essential role during primary viral infections, by directly combatting the virus and by maximizing the primary adaptive immune response. Several studies have suggested that T1IFNs also contribute very substantially to the secondary (recall) response; they are thought (i) to be required to drive the early attrition of memory T cells, (ii) to support the subsequent expansion of surviving virus-specific memory cells, and (iii) to assist in the suppression and clearance of the infectious agent. However, many of these observations were predicated upon models in which T1IFN signaling was interrupted prior to a primary immune response, raising the possibility that the resulting memory cells might be intrinsically abnormal. We have directly addressed this by using an inducible-Cre model system in which the host remains genetically-intact during the primary response to infection, and in which T1IFN signaling can be effectively ablated prior to secondary viral challenge. We report that, in stark contrast to primary infection, T1IFN signaling is not required during the recall response. IFNαβR-deficient memory CD8+ and CD4+ memory T cells undergo attrition and expansion with kinetics that are indistinguishable from those of receptor-sufficient cells. Moreover, even in the absence of functional T1IFN signaling, the host's immune capacity to rapidly suppress, and then to eradicate, a secondary infection remains intact. Thus, this study shows that T1IFN signaling is dispensable during the recall response to a virus infection. Moreover, two broader implications may be drawn. First, a T cell's requirement for a cytokine is highly dependent on the cell's maturation / differentiation status. Consequently, second, these data underscore the importance of evaluating a gene's impact by modulating its expression or function in a temporally-controllable manner.

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.

Gammaherpesvirus latency differentially impacts the generation of primary versus secondary memory CD8+ T cells during subsequent infection

Unlike laboratory animals, humans are infected with multiple pathogens, including the highly prevalent herpesviruses. The purpose of these studies was to determine the effect of gammaherpesvirus latency on T cell number and differentiation during subsequent heterologous viral infections. Mice were first infected with murine gammaherpesvirus 68 (MHV68), a model of Epstein-Barr virus (EBV) infection, and then after latency was established, they were challenged with the Armstrong strain of lymphocytic choriomeningitis virus (LCMV). The initial replication of LCMV was lower in latently infected mice, and the maturation of dendritic cells was abated. Although the number of LCMV-specific effector CD8(+) T cells was not altered, they were skewed to a memory phenotype. In contrast, LCMV-specific effector CD4(+) T cells were increased in latently infected mice compared to those in mice infected solely with LCMV. When the memory phase was reached, latently infected mice had an LCMV-specific memory T cell pool that was increased relative to that found in singly infected mice. Importantly, LCMV-specific memory CD8(+) T cells had decreased CD27 and increased killer cell lectin-like receptor G1 (KLRG1) expression. Upon secondary challenge, LCMV-specific secondary effector CD8(+) T cells expanded and cleared the infection. However, the LCMV-specific secondary memory CD8(+) T cell pool was decreased in latently infected animals, abrogating the boosting effect normally observed following rechallenge. Taken together, these results demonstrate that ongoing gammaherpesvirus latency affects the number and phenotype of primary versus secondary memory CD8(+) T cells during acute infection.

IMPORTANCE

CD8(+) T cells are critical for the clearance of intracellular pathogens, including viruses, certain bacteria, and tumors. However, current models for memory CD8(+) T cell differentiation are derived from pathogen-free laboratory mice challenged with a single pathogen or vaccine vector. Unlike laboratory animals, all humans are infected with multiple acute and chronic pathogens, including the highly prevalent herpesviruses Epstein-Barr virus (EBV), cytomegalovirus (CMV), herpes simplex viruses (HSV), and varicella-zoster virus (VZV). The purpose of these studies was to determine the effect of gammaherpesvirus latency on T cell number and differentiation during subsequent heterologous viral infections. We observed that ongoing gammaherpesvirus latency affects the number and phenotype of primary versus secondary memory CD8(+) T cells during acute infection. These results suggest that unlike pathogen-free laboratory mice, infection or immunization of latently infected humans may result in the generation of T cells with limited potential for long-term protection.

Differential T cell responses to residual viral antigen prolong CD4+ T cell contraction following the resolution of infection

The contraction phase of the T cell response is a poorly understood period after the resolution of infection when virus-specific effector cells decline in number and memory cells emerge with increased frequencies. CD8(+) T cells plummet in number and quickly reach stable levels of memory following acute lymphocytic choriomeningitis virus infection in mice. In contrast, virus-specific CD4(+) T cells gradually decrease in number and reach homeostatic levels only after many weeks. In this study, we provide evidence that MHCII-restricted viral Ag persists during the contraction phase following this prototypical acute virus infection. We evaluated whether the residual Ag affected the cell division and number of virus-specific naive and memory CD4(+) T cells and CD8(+) T cells. We found that naive CD4(+) T cells underwent cell division and accumulated in response to residual viral Ag for >2 mo after the eradication of infectious virus. Surprisingly, memory CD4(+) T cells did not undergo cell division in response to the lingering Ag, despite their heightened capacity to recognize Ag and make cytokine. In contrast to CD4(+) T cells, CD8(+) T cells did not undergo cell division in response to the residual Ag. Thus, CD8(+) T cells ceased division within days after the infection was resolved, indicating that CD8(+) T cell responses are tightly linked to endogenous processing of de novo synthesized virus protein. Our data suggest that residual viral Ag delays the contraction of CD4(+) T cell responses by recruiting new populations of CD4(+) T cells.

CD8+ memory T cells appear exhausted within hours of acute virus infection

CD8(+) memory T cells are abundant and are activated in a near-synchronous manner by infection, thereby providing a unique opportunity to evaluate the coordinate functional and phenotypic changes that occur in vivo within hours of viral challenge. Using two disparate virus challenges of mice, we show that splenic CD8(+) memory T cells rapidly produced IFN-γ in vivo; however, within 18-24 h, IFN-γ synthesis was terminated and remained undetectable for ≥ 48 h. A similar on/off response was observed in CD8(+) memory T cells in the peritoneal cavity. Cessation of IFN-γ production in vivo occurred despite the continued presence of immunostimulatory viral Ag, indicating that the initial IFN-γ response had been actively downregulated and that the cells had been rendered refractory to subsequent in vivo Ag contact. Downregulation of IFN-γ synthesis was accompanied by the upregulation of inhibitory receptor expression on the T cells, and ex vivo analyses using synthetic peptides revealed a concurrent hierarchical loss of cytokine responsiveness (IL-2, then TNF, then IFN-γ) taking place during the first 24 h following Ag contact. Thus, within hours of virus challenge, CD8(+) memory T cells display the standard hallmarks of T cell exhaustion, a phenotype that previously was associated only with chronic diseases and that is generally viewed as a gradually developing and pathological change in T cell function. Our data suggest that, instead, the "exhaustion" phenotype is a rapid and normal physiological T cell response.

ICAM-1-dependent tuning of memory CD8 T-cell responses following acute infection

CD8 T-cell responses are critical for protection against intracellular pathogens and tumors. The induction and properties of these responses are governed by a series of integrated processes that rely heavily on cell-cell interactions. Intercellular adhesion molecule (ICAM)-1 functions to enhance the strength of antigenic stimulation, extend the duration of contact with antigen-presenting cells, and augment cytokine signals, which are all factors that influence peripheral CD8 T-cell differentiation. Although previous studies suggest that ICAM-1 is essential for establishing memory T-cell populations following peptide immunization, the roles of ICAM-1 in antiviral cellular immunity are less well understood. Here we show that, following a prototypic acute viral infection, the formation and maintenance of memory-phenotype CD127(hi), KLRG-1(lo) CD8 T cells does not require ICAM-1. Nevertheless, ICAM-1 expression on nonlymphocytes dictates the phenotypic and functional attributes of the antiviral CD8 T-cell populations that develop and promotes the gradual attrition of residual effector-like CD127(lo), KLRG-1(hi) CD8 T cells during the memory phase of the response. Although memory T cells do emerge and are maintained if ICAM-1 expression is abolished, the secondary proliferative capacity of these T cells is severely curtailed. Collectively, these studies reveal potential dual roles for ICAM-1 in both promoting the decay of effector responses and programming the sensitivity of memory CD8 T cells to secondary stimuli.

Antioxidant treatment regulates the humoral immune response during acute viral infection

Generation of reactive oxygen intermediates (ROI) following antigen receptor ligation is critical to promote cellular responses. However, the effect of antioxidant treatment on humoral immunity during a viral infection was unknown. Mice were infected with lymphocytic choriomeningitis virus (LCMV) and treated with Mn(III)tetrakis(4-benzoic acid)porphyrin chloride (MnTBAP), a superoxide dismutase mimetic, from days 0 to 8 postinfection. On day 8, at the peak of the splenic response in vehicle-treated mice, virus-specific IgM and IgG antibody-secreting cells (ASC) were decreased 22- and 457-fold in MnTBAP-treated animals. By day 38, LCMV-specific IgG ASC were decreased 5-fold in the bone marrow of drug-treated mice, and virus-specific antibodies were of lower affinity. Interestingly, antioxidant treatment had no effect on the number of LCMV-specific IgG memory B cells. In addition to decreases in ASC, MnTBAP treatment decreased the number of functional virus-specific CD4(+) T cells. The decreased numbers of ASC observed on day 8 in drug-treated mice were due to a combination of Bim-mediated cell death and decreased proliferation. Together, these data demonstrate that ROI regulate antiviral ASC expansion and have important implications for understanding the effects of antioxidants on humoral immunity during infection and immunization.

Peroxiredoxin II regulates effector and secondary memory CD8+ T cell responses

Reactive oxygen intermediates (ROI) generated in response to receptor stimulation play an important role in cellular responses. However, the effect of increased H(2)O(2) on an antigen-specific CD8(+) T cell response was unknown. Following T cell receptor (TCR) stimulation, the expression and oxidation of peroxiredoxin II (PrdxII), a critical antioxidant enzyme, increased in CD8(+) T cells. Deletion of PrdxII increased ROI, S phase entry, division, and death during in vitro division. During primary acute viral and bacterial infection, the number of effector CD8(+) T cells in PrdxII-deficient mice was increased, while the number of memory cells were similar to those of the wild-type cells. Adoptive transfer of P14 TCR transgenic cells demonstrated that the increased expansion of effector cells was T cell autonomous. After rechallenge, effector CD8(+) T cells in mutant animals were more skewed to memory phenotype than cells from wild-type mice, resulting in a larger secondary memory CD8(+) T cell pool. During chronic viral infection, increased antigen-specific CD8(+) T cells accumulated in the spleens of PrdxII mutant mice, causing mortality. These results demonstrate that PrdxII controls effector CD8(+) T cell expansion, secondary memory generation, and immunopathology.

Characterization of CD8+ T cell function and immunodominance generated with an H2O2-inactivated whole-virus vaccine

CD8(+) T cells play an important role in protection against both acute and persistent viral infections, and new vaccines that induce CD8(+) T cell immunity are currently needed. Here, we show that lymphocytic choriomeningitis virus (LCMV)-specific CD8(+) T cells can be generated in response to a nonreplicating H(2)O(2)-inactivated whole-virus vaccine (H(2)O(2)-LCMV). Vaccine-induced CD8(+) T cell responses exhibited an increased ability to produce multiple cytokines at early time points following immunization compared to infection-induced responses. Vaccination with H(2)O(2)-LCMV induced the expansion of a narrow subset of the antigen-specific CD8(+) T cells induced by LCMV strain Arm infection, resulting in a distinct immunodominance hierarchy. Acute LCMV infection stimulated immunodominance patterns that shifted over time or after secondary infection, whereas vaccine-generated immunodominance profiles remained remarkably stable even following subsequent viral infection. Vaccine-induced CD8(+) T cell populations expanded sharply in response to challenge and were then maintained at high levels, with responses to individual epitopes occupying up to 40% of the CD8(+) T cell compartment at 35 days after challenge. H(2)O(2)-LCMV vaccination protected animals against challenge with chronic LCMV clone 13, and protection was mediated by CD8(+) T cells. These results indicate that vaccination with an H(2)O(2)-inactivated whole-virus vaccine induces LCMV-specific CD8(+) T cells with unique functional characteristics and provides a useful model for studying CD8(+) T cells elicited in the absence of active viral infection.

Defects in apoptosis increase memory CD8+ T cells following infection of Bim-/-Faslpr/lpr mice

During many infections, large numbers of effector CD8(+) T cells are generated. After pathogen clearance, the majority of these cells undergo apoptosis, while the survivors differentiate into memory CD8(+) T cells. Although loss of both Bim and Fas function dramatically increased antigen-specific CD8(+) T cells in the lymph nodes following acute lymphocytic choriomeningitis virus (LCMV) infection, it was unclear whether they were pardoned effector or true memory CD8(+) T cells. In this study, we demonstrate they are bona fide memory T cells as characterized by surface marker expression, cytokine production, homeostatic proliferation, and ability to clear a secondary challenge of pathogen. Loss of both Bim and Fas also increased the number of virus-specific CD4(+) T cells found in the lymph nodes compared to the parental genotypes or wildtype mice. These studies illustrate that decreasing apoptosis increases the number of memory T cells and therefore could increase the efficacy of vaccines.

Antiviral memory CD8 T-cell differentiation, maintenance, and secondary expansion occur independently of MyD88

Inflammatory signals induced during infection regulate T-cell expansion, differentiation, and memory formation. Toll-like receptors (TLRs) are inflammatory mediators that allow innate immune cells to recognize and respond to invading pathogens. In addition to their role in innate immune cells, we have found that signals delivered through the TLR adapter protein myeloid differentiation protein 88 (MyD88) play a critical, T cell-intrinsic role in supporting the survival and accumulation of antigen-specific effector cells after acute viral infection. However, the importance of MyD88-dependent signals in regulating the generation and maintenance of memory T cells remained unclear. To address this, we used a novel, inducible knockout system to examine whether MyD88 is required for optimal memory CD8 T-cell generation and responses after lymphocytic choriomeningitis virus infection. We show that whereas MyD88 is critical for initial T-cell expansion, it is not required for the subsequent differentiation and stable maintenance of a memory T-cell population. Furthermore, in contrast to naive CD8 T cells, memory CD8 T cells do not depend on MyD88 for their secondary expansion. Our findings clarify the importance of MyD88 during distinct phases of the antiviral T-cell response and establish differential dependence on MyD88 signaling as a novel characteristic that distinguishes naive from memory CD8 T cells.

IL-21 deficiency influences CD8 T cell quality and recall responses following an acute viral infection

CD4 T cells are principal producers of IL-21 and are often required for optimal CD8 T cell responses. Therefore, we investigated the importance of IL-21 in determining the phenotypic attributes, functional quality, and maintenance of antiviral CD8 T cells following acute infection with the prototypic mouse pathogen lymphocytic choriomeningitis virus. Previous reports have documented an obligatory role for IL-21 in sustaining CD8 T cell responses during chronic infections. Here we show that the requirements for IL-21 are less stringent following acute infections; however, in the absence of IL-21, the capacity of CD8 T cells to attain the polyfunctional trait of IL-2 production is consistently reduced during both the effector and memory phases. This is further supported by in vitro studies showing that the addition of IL-21 promotes the differentiation of IL-2-producing CD8 T cells. Although the generation of memory CD8 T cells, which are capable of mounting protective recall responses, proceeds independently of IL-21, we demonstrate that IL-21 does function to support secondary responses, especially under competitive conditions. Collectively, these studies highlight the potential roles of IL-21 in determining the quality of CD8 T cell responses postinfection.

Altered immunodominance hierarchies of CD8+ T cells in the spleen after infection at different sites is contingent on high virus inoculum

Activated epitope-specific CD8+ T cells after virus infection can be organized into hierarchies (immunodominance), based on their ability to focus the response on few viral determinants. The mechanisms responsible for immunodominance can be multifactorial, with CD8+ T cells precursor frequencies recently highlighted as a key regulator. Employing the LCMV infection model, we demonstrate that the hierarchies were altered when comparing different sites of infection but only at high viral doses. These findings have significant implications when investigating immunity to viruses with different replication abilities that may override the influence of T cell precursor frequencies.

CD8+ T cell immunodominance shifts during the early stages of acute LCMV infection independently from functional avidity maturation

Virus-specific T cell responses are often directed to a small subset of possible epitopes and their relative magnitude defines their hierarchy. We determined the size and functional avidity of 4 representative peptide-specific CD8(+) T cell populations in C57BL/6 mice at different time points after lymphocytic choriomeningitis virus (LCMV) infection. We found that the frequency of different peptide-specific T cell populations in the spleen changed independently over the first 8 days after infection. These changes were not associated with a larger or more rapid increase in functional avidity and yet still resulted in a shift in the final immunodominance hierarchy. Thus, the immunodominance observed at the peak of an antiviral T cell response is not necessarily determined by the initial size or rate of functional avidity maturation of peptide-specific T cell populations.

CD8+ T lymphocytes control murine cytomegalovirus replication in the central nervous system of newborn animals

Human CMV infection of the neonatal CNS results in long-term neurologic sequelae. To define the pathogenesis of fetal human CMV CNS infections, we investigated mechanisms of virus clearance from the CNS of neonatal BALB/c mice infected with murine CMV (MCMV). Virus titers peaked in the CNS between postnatal days 10-14 and infectious virus was undetectable by postnatal day 21. Congruent with virus clearance was the recruitment of CD8(+) T cells into the CNS. Depletion of CD8(+) T cells resulted in death by postnatal day 15 in MCMV-infected animals and increased viral loads in the liver, spleen, and the CNS, suggesting an important role for these cells in the control of MCMV replication in the newborn brain. Examination of brain mononuclear cells revealed that CD8(+) T cell infiltrates expressed high levels of CD69, CD44, and CD49d. IE1(168)-specific CD8(+) T cells accumulated in the CNS and produced IFN-gamma and TNF-alpha but not IL-2 following peptide stimulation. Moreover, adoptive transfer of brain mononuclear cells resulted in decreased virus burden in immunodepleted MCMV-infected syngeneic mice. Depletion of the CD8(+) cell population following transfer eliminated control of virus replication. In summary, these results show that functionally mature virus-specific CD8(+) T cells are recruited to the CNS in mice infected with MCMV as neonates.

Tentative T cells: memory cells are quick to respond, but slow to divide

T cell memory is a cornerstone of protective immunity, and is the key element in successful vaccination. Upon encountering the relevant pathogen, memory T cells are thought to initiate cell division much more rapidly than their naïve counterparts, and this is thought to confer a significant biological advantage upon an immune host. Here, we use traceable TCR-transgenic T cells to evaluate this proposed characteristic in CD4+ and CD8+ memory T cells. We find that, even in the presence of abundant antigen that was sufficient to induce in vivo IFNgamma production by memory T cells, both memory and naïve T cells show an extended, and indistinguishable, delay in the onset of proliferation. Although memory cells can detect, and respond to, virus infection within a few hours, their proliferation did not begin until approximately 3 days after infection, and occurred simultaneously in all anatomical compartments. Thereafter, cell division was extraordinarily rapid for both naïve and memory cells, with the latter showing a somewhat accelerated accumulation. We propose that, by permitting memory T cells to rapidly exert their effector functions while delaying the onset of their proliferation, evolution has provided a safeguard that balances the risk of infection against the consequences of severe T cell-mediated immunopathology.

Apoptosis regulators Bim and Fas function concurrently to control autoimmunity and CD8+ T cell contraction

Throughout most of adult life, lymphocyte number remains constant because of a balance of proliferation and apoptosis. Mutation of Bim, a proapoptotic protein in the intrinsic death pathway, or Fas, a tumor necrosis factor receptor (TNFR) superfamily member of the extrinsic pathway, results in late-onset autoimmunity and increased antigen-specific CD8(+) T cell responses during viral infection. However, virus-specific immune responses eventually return to amounts comparable to those for nonmutant mice. Here, we show that loss of both Bim and Fas function resulted in a synergistic disruption of lymphoid homeostasis, rapid-onset autoimmunity, and organ-specific blocks on contraction of antiviral immune responses. When lymphocytic choriomeningitis virus (LCMV)-specific immune responses were quantitated, double-mutant mice had 100-fold more antigen-specific memory CD8(+) T cells in their lymph nodes than wild-type mice. Our results demonstrate that multiple death pathways function concurrently to prevent autoimmunity and downsize T cell responses.

The requirement of reversible cysteine sulfenic acid formation for T cell activation and function

Reactive oxygen intermediates (ROI) generated in response to receptor stimulation play an important role in mediating cellular responses. We have examined the importance of reversible cysteine sulfenic acid formation in naive CD8(+) T cell activation and proliferation. We observed that, within minutes of T cell activation, naive CD8(+) T cells increased ROI levels in a manner dependent upon Ag concentration. Increased ROI resulted in elevated levels of cysteine sulfenic acid in the total proteome. Analysis of specific proteins revealed that the protein tyrosine phosphatases SHP-1 and SHP-2, as well as actin, underwent increased sulfenic acid modification following stimulation. To examine the contribution of reversible cysteine sulfenic acid formation to T cell activation, increasing concentrations of 5,5-dimethyl-1,3-cyclohexanedione (dimedone), which covalently binds to cysteine sulfenic acid, were added to cultures. Subsequent experiments demonstrated that the reversible formation of cysteine sulfenic acid was critical for ERK1/2 phosphorylation, calcium flux, cell growth, and proliferation of naive CD8(+) and CD4(+) T cells. We also found that TNF-alpha production by effector and memory CD8(+) T cells was more sensitive to the inhibition of reversible cysteine sulfenic acid formation than IFN-gamma. Together, these results demonstrate that reversible cysteine sulfenic acid formation is an important regulatory mechanism by which CD8(+) T cells are able to modulate signaling, proliferation, and function.

Cross-competition of CD8+ T cells shapes the immunodominance hierarchy during boost vaccination

CD8⁺ T cell responses directed against multiple pathogen-derived epitopes are characterized by defined immunodominance hierarchy patterns. A possible explanation for this phenomenon is that CD8⁺ T cells of different specificities compete for access to epitopes on antigen-presenting cells, and that the outcome of this so-called cross-competition reflects the number of induced T cells. In our study using a vaccinia virus infection model, we found that T cell cross-competition is highly relevant during boost vaccination, thereby shaping the immunodominance hierarchy in the recall. We demonstrate that competition was of no importance during priming and was unaffected by the applied route of immunization. It strongly depended on the timing of viral antigen expression in infected APCs, and it was characterized by poor proliferation of T cells recognizing epitopes derived from late viral proteins. To our knowledge, this is the first demonstration of the functional importance of T cell cross-competition during a viral infection. Our findings provide a basis for novel strategies for how boost vaccination to defined antigens can be selectively improved. They give important new insights into the design of more efficient poxviral vectors for immunotherapy.

Role of Bim in regulating CD8+ T-cell responses during chronic viral infection

Apoptosis is critical for the development and maintenance of the immune system. The proapoptotic Bcl-2 family member Bim is important for normal immune system homeostasis. Although previous experiments have shown that Bim is critical for the apoptosis of antigen-specific CD8(+) T cells during acute viral infection, the role of Bim during chronic viral infection is unclear. Using lymphocytic choriomeningitis virus clone 13 infection of mice, we demonstrate a role for Bim in CD8(+) T-cell apoptosis during chronic viral infection. Enumeration of antigen-specific CD8(+) T cells by major histocompatibility complex class I tetramer staining revealed that CD8(+) D(b)NP396-404(+) T cells, which undergo extensive deletion in wild-type mice, exhibited almost no decrease in Bim mutant mice. This contrasts with CD8(+) D(b)GP33-41(+) and CD8(+) D(b)GP276-286(+) T cells that underwent similar decreases in numbers in both Bim mutant and wild-type mice. Increased numbers of CD8(+) D(b)NP396-404(+) T cells in Bim mutant mice were due to lack of apoptosis and could not be explained by altered proliferation, differential homing to tissues, or increased help from CD4(+) T cells. When viral titers were examined, high levels were initially observed in both groups, but in Bim mutant mice, clearance from the spleen and sera was slightly accelerated. These experiments demonstrate the critical role of Bim during chronic viral infection to down-regulate CD8(+) T-cell responses and have implications for designing strategies for optimizing immunotherapies during situations where antigen persists, such as chronic infection, autoimmune syndromes, and cancer.

Complex T cell memory repertoires participate in recall responses at extremes of antigenic load

The CD8 T cell memory response to the HLA-A2-restricted influenza epitope M1(58-66) can be an instructive model of immune memory to a nonevolving epitope of a frequently encountered pathogen that undergoes clearance. This memory repertoire can be complex, composed of a large number of clonotypes represented at low copy numbers, while maintaining a focus on the use of VB17 T cell receptors with identified Ag recognition motifs. Such a repertoire structure might provide a panoply of clonotypes whose differential avidity for the epitope would allow responses under varying antigenic loads. This possibility was tested experimentally by characterizing the responding repertoire in vitro while varying influenza Ag concentration over five orders of magnitude. At higher and lower Ag concentrations there was increased cell death, yet a focused but diverse response could still be observed. Thus, one of the characteristics of complex memory repertoires is to provide effector function at extremes of Ag load, a characteristic that is not generally considered in vaccination development but may be important in measuring its efficacy.

Enhanced immunogenicity and protective efficacy with the use of interleukin-12-encapsulated microspheres plus AS01B in tuberculosis subunit vaccination

Tuberculosis subunit vaccines codelivered with interleukin-12 (IL-12)-encapsulated microspheres (IL-12EM) are designed for a sustained release of IL-12 and could induce strong Th1 immune responses specific to Ag85A and ESAT-6. The adjuvant combination of IL-12EM plus AS01B was a more efficient way to induce a sustained Th1 immunity and protection against Mycobacterium tuberculosis.