From crucial to negligible: functional CD8⁺ T-cell responses and their dependence on CD4⁺ T-cell help

Functional impairment of "helpless" CD8 + memory T cells is transient and driven by prolonged but finite cognate antigen presentation

Generation of functional CD8 + T cell memory typically requires engagement of CD4 + T cells. However, in certain scenarios, such as acutely-resolving viral infections, effector (T E ) and subsequent memory (T M ) CD8 + T cell formation appear impervious to a lack of CD4 + T cell help during priming. Nonetheless, such "helpless" CD8 + T M respond poorly to pathogen rechallenge. At present, the origin and long-term evolution of helpless CD8 + T cell memory remain incompletely understood. Here, we demonstrate that helpless CD8 + T E differentiation is largely normal but a multiplicity of helpless CD8 T M defects, consistent with impaired memory maturation, emerge as a consequence of prolonged yet finite exposure to cognate antigen. Importantly, these defects resolve over time leading to full restoration of CD8 + T M potential and recall capacity. Our findings provide a unified explanation for helpless CD8 + T cell memory and emphasize an unexpected CD8 + T M plasticity with implications for vaccination strategies and beyond.

Conditional NKT Cell Depletion in Mice Reveals a Negative Feedback Loop That Regulates CTL Cross-Priming

NKT cells are unconventional T cells whose biological role is incompletely understood. Similar to TH cells, activated NKT cells can cause dendritic cell (DC) maturation, which is required for effective CTL responses. However, it is unclear whether and how NKT cells affect CTLs downstream of the DC maturation phase. This is partially due to the lack of techniques to conditionally deplete NKT cells in vivo. To overcome this problem, we have developed two approaches for this purpose in mice: the first is based on mixed bone marrow chimeras where Jα18 knockout and depletable CD90 congenic bone marrow is combined, and the second used PLZFCre × iDTR bone marrow chimeras, which target innate-like T cells. Using these tools, we found that NKT cell depletion at 20 h, that is, after initial DC activation, did not render CTLs helpless, as CD40L signaling by non-NKT cells sufficed. Instead, NKT cell depletion even augmented CD8 T cell expansion and cytotoxicity by mechanisms distinct from reduced STAT6 signaling. These findings revealed a negative feedback loop by which NKT cells control CTL cross-priming downstream of DC maturation. The techniques described in this study expand the toolbox to study NKT cells and other unconventional T cell subsets in vivo and uncovered a hidden immunoregulatory mechanism.

A synthetic biology approach to engineering circuits in immune cells

A synthetic circuit in a biological system involves the designed assembly of genetic elements, biomolecules, or cells to create a defined function. These circuits are central in synthetic biology, enabling the reprogramming of cellular behavior and the engineering of cells with customized responses. In cancer therapeutics, engineering T cells with circuits have the potential to overcome the challenges of current approaches, for example, by allowing specific recognition and killing of cancer cells. Recent advances also facilitate engineering integrated circuits for the controlled release of therapeutic molecules at specified locations, for example, in a solid tumor. In this review, we discuss recent strategies and applications of synthetic receptor circuits aimed at enhancing immune cell functions for cancer immunotherapy. We begin by introducing the concept of circuits in networks at the molecular and cellular scales and provide an analysis of the development and implementation of several synthetic circuits in T cells that have the goal to overcome current challenges in cancer immunotherapy. These include specific targeting of cancer cells, increased T-cell proliferation, and persistence in the tumor microenvironment. By harnessing the power of synthetic biology, and the characteristics of certain circuit architectures, it is now possible to engineer a new generation of immune cells that recognize cancer cells, while minimizing off-target toxicities. We specifically discuss T-cell circuits for antigen density sensing. These circuits allow targeting of solid tumors that share antigens with normal tissues. Additionally, we explore designs for synthetic circuits that could control T-cell differentiation or T-cell fate as well as the concept of synthetic multicellular circuits that leverage cellular communication and division of labor to achieve improved therapeutic efficacy. As our understanding of cell biology expands and novel tools for genome, protein, and cell engineering are developed, we anticipate further innovative approaches to emerge in the design and engineering of circuits in immune cells.

Tumor antigen-loaded AAV vaccine drives protective immunity in a melanoma animal model

We previously described therapeutic opportunities provided by capsid- and expression cassette-optimized adeno-associated virus serotype 6 (AAV6) vectors to suppress tumor growth in both solid and metastatic mouse models by using artificial ovalbumin (OVA) immunogen. In the current study, we further elucidated the mechanism of function of a novel AAV-based vaccine loaded with the melanoma tumor-associated antigens premelanosome protein gp100, tyrosinase (Tyr), tyrosinase-related protein 1 (TRP1), and dopachrome tautomerase (TRP2). We showed that the AAV6-based vaccine creates cellular and humoral antigen-specific responses, while antigen expression at the site of vaccine injection was temporal, and the clearance of antigen coincided with T cell infiltration. Our data revealed the superior protective immune response of optimized AAV6-TRP1 compared with other self-antigens in a disease-free mouse model. We further assessed the ability of AAV6-TRP1 to protect animals from metastatic spread in the lungs and to extend animal survival by inhibiting solid tumor growth. Flow cytometry-based analysis indicated significant infiltration of CD8⁺ T cells and natural killer (NK) cells in the tumor site, as well as changes in the polarization of intratumoral macrophages. Altogether, our data strongly support the use of optimized AAV vectors for cancer vaccine development.

Japanese Encephalitis Vaccine Generates Cross-Reactive Memory T Cell Responses to Zika Virus in Humans

Objective. Zika virus (ZIKV) and Japanese encephalitis virus (JEV) are mosquito-borne flaviviruses with sequence homology. ZIKV circulates in some regions where JEV also circulates, or where JE vaccination is used. Cross-immunity between flaviviruses exists, but the precise mechanisms remain unclear. We previously demonstrated that T cell immunity induced by the live-attenuated Japanese encephalitis (JE) SA14-14-2 vaccine conferred protective immunity against ZIKV infection in mice, which could even bypass antibody-dependent enhancement. However, the role of T cell immune, especially memory T cell subsets, in cross-reactive immune responses between JE vaccine and ZIKV in humans has not been reported. Methods. We examined central and effector memory CD4+ and CD8+ T cell (TCM and TEM) responses (including degranulation, cytokines, and chemokines) in the presence of JEV and ZIKV, respectively, by using qualified peripheral blood mononuclear cell samples from 18 children who had recently received a two-dose course of JE vaccine SA14-14-2 as well as seven children without JE vaccination. Results. Cross-reactive CD8+ TCM in response to ZIKV was characterized by secretion of IFN-γ, whereas CD8+ TEM did not show significant upregulation of functional factors. In the presence of ZIKV, IFN-γ and TNF-α expression was upregulated by CD4+ TEM, and the expression signature of CD4+ TCM is more cytotoxic potential. Conclusions. We profiled the cross-reactive memory T cell responses to ZIKV in JE vaccine recipients. These data will provide evidence for the mechanism of cross-reactive memory T cell immune responses between JEV and ZIKV and a more refined view of bivalent vaccine design strategy.

Autocrine and paracrine IL-2 signals collaborate to regulate distinct phases of CD8 T cell memory

Differential interleukin-2 (IL-2) signaling and production are associated with disparate effector and memory fates. Whether the IL-2 signals perceived by CD8 T cells come from autocrine or paracrine sources, the timing of IL-2 signaling and their differential impact on CD8 T cell responses remain unclear. Using distinct models of germline and conditional IL-2 ablation in post-thymic CD8 T cells, this study shows that paracrine IL-2 is sufficient to drive optimal primary expansion, effector and memory differentiation, and metabolic function. In contrast, autocrine IL-2 is uniquely required during primary expansion to program robust secondary expansion potential in memory-fated cells. This study further shows that IL-2 production by antigen-specific CD8 T cells is largely independent of CD4 licensing of dendritic cells (DCs) in inflammatory infections with robust DC activation. These findings bear implications for immunizations and adoptive T cell immunotherapies, where effector and memory functions may be commandeered through IL-2 programming.

Clonally Expanded Virus-Specific CD8 T Cells Acquire Diverse Transcriptional Phenotypes During Acute, Chronic, and Latent Infections

CD8+ T cells play a crucial role in the control and resolution of viral infections and can adopt a wide range of phenotypes and effector functions depending on the inflammatory context and the duration and extent of antigen exposure. Similarly, viral infections can exert diverse selective pressures on populations of clonally related T cells. Technical limitations have nevertheless made it challenging to investigate the relationship between clonal selection and transcriptional phenotypes of virus-specific T cells. We therefore performed single-cell T cell receptor (TCR) repertoire and transcriptome sequencing of virus-specific CD8 T cells in murine models of acute, chronic and latent infection. We observed clear infection-specific populations corresponding to memory, effector, exhausted, and inflationary phenotypes. We further uncovered a mouse-specific and polyclonal T cell response, despite all T cells sharing specificity to a single viral epitope, which was accompanied by stereotypic TCR germline gene usage in all three infection types. Persistent antigen exposure during chronic and latent viral infections resulted in a higher proportion of clonally expanded T cells relative to acute infection. We furthermore observed a relationship between transcriptional heterogeneity and clonal expansion for all three infections, with highly expanded clones having distinct transcriptional phenotypes relative to less expanded clones. Together our work relates clonal selection to gene expression in the context of viral infection and further provides a dataset and accompanying software for the immunological community.

TIM-3 as a potential exhaustion marker in CD4+ T cells of COVID-19 patients

Background

COVID‐19 causes a range of clinical symptoms from mild to critical and can be life‐threatening. Up to now, it has led to many deaths. We aimed to evaluate exhausted markers on CD4⁺ T cells of COVID‐19 patients.

Methods

In this study, we evaluated 44 patients with confirmed COVID‐19 disease and 16 healthy individuals. Patients were divided into moderate/severe and critical groups. Peripheral blood mononuclear cells (PBMCs) were isolated and stained by anti‐human CD39, PD‐1, TIM‐3, and anti‐human CD4. The percentage of each CD4⁺ subpopulation was calculated by flow cytometry. Furthermore, we collected clinical information and laboratory data of both control and patient groups.

Results

We detected overexpression of TIM‐3 on CD4⁺ T cells in both critical and moderate/severe patients than in healthy individuals (HIs; p < .01 and p < .0001, respectively). CD4⁺ TIM‐3⁺ CD39⁺ lymphocytes were significantly higher in the critical patients than in HI (p < .05). Both Patient groups showed lymphopenia in comparison with HI, but CD4⁺ lymphocytes did not show any significant difference between study subjects. The increased amount of C‐reactive protein, erythrocyte sedimentation rate, creatinine, blood urea nitrogen, and neutrophil count was observed in patients compared to HI.

Conclusion

T cell exhaustion occurs during COVID‐19 disease and TIM‐3 is the most important exhausted marker on CD4⁺ T cells.

B cell depletion impairs vaccination-induced CD8+ T cell responses in a type I interferon-dependent manner

OBJECTIVES

The monoclonal anti-CD20 antibody rituximab is frequently applied in the treatment of lymphoma as well as autoimmune diseases and confers efficient depletion of recirculating B cells. Correspondingly, B cell-depleted patients barely mount de novo antibody responses during infections or vaccinations. Therefore, efficient immune responses of B cell-depleted patients largely depend on protective T cell responses.

METHODS

CD8⁺ T cell expansion was studied in rituximab-treated rheumatoid arthritis (RA) patients and B cell-deficient mice on vaccination/infection with different vaccines/pathogens.

RESULTS

Rituximab-treated RA patients vaccinated with Influvac showed reduced expansion of influenza-specific CD8⁺ T cells when compared with healthy controls. Moreover, B cell-deficient JHT mice infected with mouse-adapted Influenza or modified vaccinia virus Ankara showed less vigorous expansion of virus-specific CD8⁺ T cells than wild type mice. Of note, JHT mice do not have an intrinsic impairment of CD8⁺ T cell expansion, since infection with vaccinia virus induced similar T cell expansion in JHT and wild type mice. Direct type I interferon receptor signalling of B cells was necessary to induce several chemokines in B cells and to support T cell help by enhancing the expression of MHC-I.

CONCLUSIONS

Depending on the stimulus, B cells can modulate CD8⁺ T cell responses. Thus, B cell depletion causes a deficiency of de novo antibody responses and affects the efficacy of cellular response including cytotoxic T cells. The choice of the appropriate vaccine to vaccinate B cell-depleted patients has to be re-evaluated in order to efficiently induce protective CD8⁺ T cell responses.

TLR9 Sensing of Self-DNA Controls Cell-Mediated Immunity to Listeria Infection via Rapid Conversion of Conventional CD4+ T Cells to Treg

CD4⁺ T lymphocytes are crucial for controlling a range of innate and adaptive immune effectors. For CD8⁺ cytotoxic T lymphocyte (CTL) responses, CD4⁺ T cells can function as helpers (T_H) to amplify magnitude and functionality or as regulatory cells (T_reg) capable of profound inhibition. It is unclear what determines differentiation to these phenotypes and whether pathogens provoke alternate programs. We find that, depending on the size of initial dose, Listeria infection drives CD4⁺ T cells to act as T_H or induces rapid polyclonal conversion to immunosuppressive T_reg. Conversion to T_reg depends on the TLR9 and IL-12 pathways elicited by CD8a⁺ dendritic cell (DC) sensing of danger-associated neutrophil self-DNA. These findings resolve long-standing questions regarding the conditional requirement for T_H amongst pathogens and reveal a remarkable degree of plasticity in the function of CD4⁺ T cells, which can be quickly converted to T_regin vivo by infection-mediated immune modulation.

Dolina et al. show that Listeria infectious dose drives conventional CD4⁺ T cells to act as T_H or mediates conversion to T_reg. Differentiation to T_reg dominates heightened doses and is promoted by CD8α⁺ DC TLR9 engagement of neutrophil self-DNA and IL-12 production, revealing plasticity in the function of CD4⁺ T cells.

Role of CD4+ T Cells in the Control of Viral Infections: Recent Advances and Open Questions

CD4+ T cells orchestrate adaptive immune responses through their capacity to recruit and provide help to multiple immune effectors, in addition to exerting direct effector functions. CD4+ T cells are increasingly recognized as playing an essential role in the control of chronic viral infections. In this review, we present recent advances in understanding the nature of CD4+ T cell help provided to antiviral effectors. Drawing from our studies of natural human immunodeficiency virus (HIV) control, we then focus on the role of high-affinity T cell receptor (TCR) clonotypes in mediating antiviral CD4+ T cell responses. Last, we discuss the role of TCR affinity in determining CD4+ T cell differentiation, reviewing the at times divergent studies associating TCR signal strength to the choice of a T helper 1 (Th1) or a T follicular helper (Tfh) cell fate.

COVID-19: The Emerging Immunopathological Determinants for Recovery or Death

Hyperactivation of the host immune system during infection by SARS-CoV-2 is the leading cause of death in COVID-19 patients. It is also evident that patients who develop mild/moderate symptoms and successfully recover display functional and well-regulated immune response. Whereas a delayed initial interferon response is associated with severe disease outcome and can be the tipping point towards immunopathological deterioration, often preceding death in COVID-19 patients. Further, adaptive immune response during COVID-19 is heterogeneous and poorly understood. At the same time, some studies suggest activated T and B cell response in severe and critically ill patients and the presence of SARS-CoV2-specific antibodies. Thus, understanding this problem and the underlying molecular pathways implicated in host immune function/dysfunction is imperative to devise effective therapeutic interventions. In this comprehensive review, we discuss the emerging immunopathological determinants and the mechanism of virus evasion by the host cell immune system. Using the knowledge gained from previous respiratory viruses and the emerging clinical and molecular findings on SARS-CoV-2, we have tried to provide a holistic understanding of the host innate and adaptive immune response that may determine disease outcome. Considering the critical role of the adaptive immune system during the viral clearance, we have presented the molecular insights of the plausible mechanisms involved in impaired T cell function/dysfunction during various stages of COVID-19.

Type I IFN Sensing by cDCs and CD4+ T Cell Help Are Both Requisite for Cross-Priming of AAV Capsid-Specific CD8+ T Cells

Adeno-associated virus (AAV) vectors are widely used in clinical gene therapy to correct genetic disease by in vivo gene transfer. Although the vectors are useful, in part because of their limited immunogenicity, immune responses directed at vector components have complicated applications in humans. These include, for instance, innate immune sensing of vector components by plasmacytoid dendritic cells (pDCs), which sense the vector DNA genome via Toll-like receptor 9. Adaptive immune responses employ antigen presentation by conventional dendritic cells (cDCs), which leads to cross-priming of capsid-specific CD8+ T cells. In this study, we sought to determine the mechanisms that promote licensing of cDCs, which is requisite for CD8+ T cell activation. Blockage of type 1 interferon (T1 IFN) signaling by monoclonal antibody therapy prevented cross-priming. Furthermore, experiments in cell-type-restricted knockout mice showed a specific requirement for the receptor for T1 IFN (IFNaR) in cDCs. In contrast, natural killer (NK) cells are not needed, indicating a direct rather than indirect effect of T1 IFN on cDCs. In addition, co-stimulation by CD4+ T cells via CD40-CD40L was required for cross-priming, and blockage of co-stimulation but not of T1 IFN additionally reduced antibody formation against capsid. These mechanistic insights inform the development of targeted immune interventions.

Functional MAIT Cells Are Associated With Reduced Simian-Human Immunodeficiency Virus Infection

Mucosa-associated invariant T (MAIT) cells are recently characterized as a novel subset of innate-like T cells that recognize microbial metabolites as presented by the MHC-1b-related protein MR1. The significance of MAIT cells in anti-bacterial defense is well-understood but not clear in viral infections such as SIV/HIV infection. Here we studied the phenotype, distribution, and function of MAIT cells and their association with plasma viral levels during chronic SHIV infection in rhesus macaques (RM). Two groups of healthy and chronic SHIV-infected macaques were characterized for MAIT cells in blood and mucosal tissues. Similar to human, we found a significant fraction of macaque T cells co-expressing MAIT cell markers CD161 and TCRVα-7.2 that correlated directly with macaque MR1 tetramer. These cells displayed memory phenotype and expressed high levels of IL-18R, CCR6, CD28, and CD95. During chronic infection, the frequency of MAIT cells are enriched in the blood but unaltered in the rectum; both blood and rectal MAIT cells displayed higher proliferative and cytotoxic phenotype post-SHIV infection. The frequency of MAIT cells in blood and rectum correlated inversely with plasma viral RNA levels and correlated directly with total CD4 T cells. MAIT cells respond to microbial products during chronic SHIV infection and correlated positively with serum immunoreactivity to flagellin levels. Tissue distribution analysis of MAIT cells during chronic infection showed significant enrichment in the non-lymphoid tissues (lung, rectum, and liver) compared to lymphoid tissues (spleen and LN), with higher levels of tissue-resident markers CD69 and CD103. Exogenous in vitro cytokine treatments during chronic SHIV infection revealed that IL-7 is important for the proliferation of MAIT cells, but IL-12 and IL-18 are important for their cytolytic function. Overall our results demonstrated that MAIT cells are enriched in blood but unaltered in the rectum during chronic SHIV infection, which displayed proliferative and functional phenotype that inversely correlated with SHIV plasma viral RNA levels. Treatment such as combined cytokine treatments could be beneficial for enhancing functional MAIT cells during chronic HIV infection in vivo.

WASp Is Essential for Effector-to-Memory conversion and for Maintenance of CD8+T Cell Memory

Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency characterized by recurrent infections, micro thrombocytopenia, eczema, and a high incidence of autoimmunity and malignancy. A defect in the T cell compartment is thought to be a major cause of immunodeficiency in patients with WAS; However, whether the antigen specific T memory cell is altered has not been extensively studied. Here, we examined the expansion/contraction kinetics of CD8⁺ memory T cells and their maintenance in WASp^−/− mice. The results showed that WAS protein (WASp) is not required for differentiation of CD8⁺ effector T cells; however, CD8⁺ T cells from WASp^−/− mice were hyperactive, resulting in increased cytokine production. The number of CD8⁺ T memory cells decreased as mice aged, and CD8⁺ T cell recall responses and protective immunity were impaired. WASp-deficient CD8⁺ T cells in bone marrow chimeric mice underwent clonal expansion, but the resulting effector cells failed to survive and differentiate into CD8⁺ memory T cells. Taken together, these findings indicate that WASp plays an intrinsic role in differentiation of CD8⁺ memory T cells.

CD4+ T cell help in cancer immunology and immunotherapy

Cancer immunotherapy aims to promote the activity of cytotoxic T lymphocytes (CTLs) within a tumour, assist the priming of tumour-specific CTLs in lymphoid organs and establish efficient and durable antitumour immunity. During priming, help signals are relayed from CD4⁺ T cells to CD8⁺ T cells by specific dendritic cells to optimize the magnitude and quality of the CTL response. In this Review, we highlight the cellular dynamics and membrane receptors that mediate CD4⁺ T cell help and the molecular mechanisms of the enhanced antitumour activity of CTLs. We outline how deficient CD4⁺ T cell help reduces the response of CTLs and how maximizing CD4⁺ T cell help can improve outcomes in cancer immunotherapy strategies.

Chimeric antigen receptor T-cell approaches to HIV cure

PURPOSE OF REVIEW

Combination antiretroviral therapy (ART) has enabled tremendous progress in suppressing HIV replication in infected patients. However, ART alone cannot eradicate HIV and its latent, persisting reservoirs. Novel approaches are needed to eradicate the virus or achieve functional cure in the absence of ART.

RECENT FINDINGS

Adoptive T-cell therapies were initially tested in HIV-infected individuals with limited efficiency. Benefiting from new and improved methodologies, an increasing array of CAR T-cell therapies has been successfully developed in the cancer immunotherapy field, demonstrating promising new avenues that could be applied to HIV. Numerous studies have characterized various HIV-specific CAR constructs, types of cytolytic effector cells, and CAR-expressing cells' trafficking to the reservoir compartments, warranting further in-vivo efforts. Notably, the ability of CAR cells to persist and function in low-antigen environments in vivo, that is, in ART-suppressed patients, remains unclear.

SUMMARY

Despite promising results in preclinical studies, only a handful of clinical trials have been initiated worldwide. Several obstacles remain prior to successful application of HIV-specific CAR T-cell therapies in patients. In this review, we survey the current state of the field, and address paths towards realizing the goal of an efficacious HIV CAR T-cell product.

CD4 T cells in protection from influenza virus: Viral antigen specificity and functional potential

CD4 T cells convey a number of discrete functions to protective immunity to influenza, a complexity that distinguishes this arm of adaptive immunity from B cells and CD8 T cells. Although the most well recognized function of CD4 T cells is provision of help for antibody production, CD4 T cells are important in many aspects of protective immunity. Our studies have revealed that viral antigen specificity is a key determinant of CD4 T cell function, as illustrated both by mouse models of infection and human vaccine responses, a factor whose importance is due at least in part to events in viral antigen handling. We discuss research that has provided insight into the diverse viral epitope specificity of CD4 T cells elicited after infection, how this primary response is modified as CD4 T cells home to the lung, establish memory, and after challenge with a secondary and distinct influenza virus strain. Our studies in human subjects point out the challenges facing vaccine efforts to facilitate responses to novel and avian strains of influenza, as well as strategies that enhance the ability of CD4 T cells to promote protective antibody responses to both seasonal and potentially pandemic strains of influenza.

Immune Exhaustion: Past Lessons and New Insights from Lymphocytic Choriomeningitis Virus

Lymphocytic choriomeningitis virus (LCMV) is a paradigm-forming experimental system with a remarkable track record of contributing to the discovery of many of the fundamental concepts of modern immunology. The ability of LCMV to establish a chronic infection in immunocompetent adult mice was instrumental for identifying T cell exhaustion and this system has been invaluable for uncovering the complexity, regulators, and consequences of this state. These findings have been directly relevant for understanding why ineffective T cell responses commonly arise during many chronic infections including HIV and HCV, as well as during tumor outgrowth. The principal feature of exhausted T cells is the inability to elaborate the array of effector functions necessary to contain the underlying infection or tumor. Using LCMV to determine how to prevent and reverse T cell exhaustion has highlighted the potential of checkpoint blockade therapies, most notably PD-1 inhibition strategies, for improving cellular immunity under conditions of antigen persistence. Here, we discuss the discovery, properties, and regulators of exhausted T cells and highlight how LCMV has been at the forefront of advancing our understanding of these ineffective responses.

IL-33 Augments Virus-Specific Memory T Cell Inflation and Potentiates the Efficacy of an Attenuated Cytomegalovirus-Based Vaccine

Candidate vaccines designed to generate T cell-based immunity are typically vectored by nonpersistent viruses, which largely fail to elicit durable effector memory T cell responses. This limitation can be overcome using recombinant strains of CMV. Proof-of-principle studies have demonstrated the potential benefits of this approach, most notably in the SIV model, but safety concerns require the development of nonreplicating alternatives with comparable immunogenicity. In this study, we show that IL-33 promotes the accumulation and recall kinetics of circulating and tissue-resident memory T cells in mice infected with murine CMV. Using a replication-deficient murine CMV vector, we further show that exogenous IL-33 boosts vaccine-induced memory T cell responses, which protect against subsequent heterologous viral challenge. These data suggest that IL-33 could serve as a useful adjuvant to improve the efficacy of vaccines based on attenuated derivatives of CMV.

CD4 T cell responses to flaviviruses

Flaviviruses pose an increasing threat to global health with their potential to cause severe disease in millions of people. Protective and long-lived immunity is closely linked to the generation of CD4 T cells, which provide B cell help and support high affinity neutralizing antibody responses. Research performed during the last years revealed important new insights into the antigen specificities and diverse effector functions of CD4 T cell responses to flaviviruses. Moreover, the identification of mechanisms involved in the regulation of T cell specificity and function provides significant advances in our understanding of how durable protective immunity is established. Here, we summarize what is known about human CD4 T cell responses to flaviviruses, with a special emphasis on CD4 T cells that provide direct help to B cells producing neutralizing and protective antibodies. We review recent progress in the identification of epitope sites in the context of the atomic structures of flavivirus proteins and highlight specific influences that shape the human CD4 T cell response in the context of infection or vaccination. Finally, we discuss challenges facing vaccine efforts to generate appropriate CD4 T cell responses, as well as recent strategies to enhance T cell-mediated antibody responses.

Legacy of the influenza pandemic 1918: The host T cell response

The influenza virus was instrumental in unravelling critical aspects of the antiviral T lymphocyte mediated immune response. A major finding was the demonstration that CD8 T lymphocytes recognize short viral peptides presented by class I molecules of the major histocompatibility complex. Studies of influenza specific T cells have also led to an understanding of their important role in recovery from influenza virus infection in humans.

Cerebral Malaria in Mouse and Man

Cerebral malaria (CM) is an acute encephalopathy caused by the malaria parasite Plasmodium falciparum, which develops in a small minority of infected patients and is responsible for the majority of deaths in African children. Despite decades of research on CM, the pathogenic mechanisms are still relatively poorly defined. Nevertheless, many studies in recent years, using a combination of animal models, in vitro cell culture work, and human patients, provide significant insight into the pathologic mechanisms leading to CM. In this review, we summarize recent findings from mouse models and human studies on the pathogenesis of CM, understanding of which may enable development of novel therapeutic approaches.

Listeria Monocytogenes: A Model Pathogen Continues to Refine Our Knowledge of the CD8 T Cell Response

Listeria monocytogenes (Lm) infection induces robust CD8 T cell responses, which play a critical role in resolving Lm during primary infection and provide protective immunity to re-infections. Comprehensive studies have been conducted to delineate the CD8 T cell response after Lm infection. In this review, the generation of the CD8 T cell response to Lm infection will be discussed. The role of dendritic cell subsets in acquiring and presenting Lm antigens to CD8 T cells and the events that occur during T cell priming and activation will be addressed. CD8 T cell expansion, differentiation and contraction as well as the signals that regulate these processes during Lm infection will be explored. Finally, the formation of memory CD8 T cell subsets in the circulation and in the intestine will be analyzed. Recently, the study of CD8 T cell responses to Lm infection has begun to shift focus from the intravenous infection model to a natural oral infection model as the humanized mouse and murinized Lm have become readily available. Recent findings in the generation of CD8 T cell responses to oral infection using murinized Lm will be explored throughout the review. Finally, CD8 T cell-mediated protective immunity against Lm infection and the use of Lm as a vaccine vector for cancer immunotherapy will be highlighted. Overall, this review will provide detailed knowledge on the biology of CD8 T cell responses after Lm infection that may shed light on improving rational vaccine design.

Generation of TCR-Expressing Innate Lymphoid-like Helper Cells that Induce Cytotoxic T Cell-Mediated Anti-leukemic Cell Response

CD4⁺ T helper (Th) cell activation is essential for inducing cytotoxic T lymphocyte (CTL) responses against malignancy. We reprogrammed a Th clone specific for chronic myelogenous leukemia (CML)-derived b3a2 peptide to pluripotency and re-differentiated the cells into original TCR-expressing T-lineage cells (iPS-T cells) with gene expression patterns resembling those of group 1 innate lymphoid cells. CD4 gene transduction into iPS-T cells enhanced b3a2 peptide-specific responses via b3a2 peptide-specific TCR. iPS-T cells upregulated CD40 ligand (CD40L) expression in response to interleukin-2 and interleukin-15. In the presence of Wilms tumor 1 (WT1) peptide, antigen-specific dendritic cells (DCs) conditioned by CD4-modified CD40L^high iPS-T cells stimulated WT1-specific CTL priming, which eliminated WT1 peptide-expressing CML cells in vitro and in vivo. Thus, CD4 modification of CD40L^high iPS-T cells generates innate lymphoid helper-like cells inducing bcr-abl-specific TCR signaling that mediates effectiveanti-leukemic CTL responses via DC maturation, showing potential for adjuvant immunotherapy against leukemia.

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iPSC-derived T cells have molecular similarity to group 1 innate lymphoid cells

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iPSC-derived CD40L^high T cell-adjuvants induce leukemia-specific CTLs via DCs

STING agonists enable antiviral cross-talk between human cells and confer protection against genital herpes in mice

In recent years, there has been an increasing interest in immunomodulatory therapy as a means to treat various conditions, including infectious diseases. For instance, Toll-like receptor (TLR) agonists have been evaluated for treatment of genital herpes. However, although the TLR7 agonist imiquimod was shown to have antiviral activity in individual patients, no significant effects were observed in clinical trials, and the compound also exhibited significant side effects, including local inflammation. Cytosolic DNA is detected by the enzyme cyclic GMP-AMP (2’3’-cGAMP) synthase (cGAS) to stimulate antiviral pathways, mainly through induction of type I interferon (IFN)s. cGAS is activated upon DNA binding to produce the cyclic dinucleotide (CDN) 2’3’-cGAMP, which in turn binds and activates the adaptor protein Stimulator of interferon genes (STING), thus triggering type I IFN expression. In contrast to TLRs, STING is expressed broadly, including in epithelial cells. Here we report that natural and non-natural STING agonists strongly induce type I IFNs in human cells and in mice in vivo, without stimulating significant inflammatory gene expression. Systemic treatment with 2’3’-cGAMP reduced genital herpes simplex virus (HSV) 2 replication and improved the clinical outcome of infection. More importantly, local application of CDNs at the genital epithelial surface gave rise to local IFN activity, but only limited systemic responses, and this treatment conferred total protection against disease in both immunocompetent and immunocompromised mice. In direct comparison between CDNs and TLR agonists, only CDNs acted directly on epithelial cells, hence allowing a more rapid and IFN-focused immune response in the vaginal epithelium. Thus, specific activation of the STING pathway in the vagina evokes induction of the IFN system but limited inflammatory responses to allow control of HSV2 infections in vivo.

Herpes simplex virus (HSV)-2 is the leading cause of genital ulcers, and HSV-2 infection has also been reported to amplify HIV-transmission. So far, all attempts at making an effective anti-HSV2 vaccine have failed. In recent years, there has been an increasing interest in immunomodulatory therapy as a means to treat infections. Although the TLR7 agonist imiquimod has been shown to have antiviral activity in individual patients, no significant effects were observed in clinical trials, and the compound also exhibited significant side effects including local inflammation. Type I interferon (IFN)s are key players in antiviral defense, and it is now known that the DNA sensor cyclic GMP-AMP synthase produces the cyclic di-nucleotide (CDN) 2’3’-cyclic GMP-AMP (cGAMP), which activates the adaptor protein STING to induce IFN expression. In this work we show that natural and non-natural CDNs activate strong type I IFN responses in vivo without stimulating significant expression of genes driven by the transcription factor NF-κB, which induces inflammation. Application of CDNs at epithelial surfaces gave rise to local IFN activity, but only limited systemic responses. Importantly, all tested treatment regimens, strongly reduced replication of HSV-2 in a model for genital herpes, and significantly reduced development of disease. Finally, when comparing to TLR agonists, CDNs showed the best profile with strong IFN response specifically in the epithelial cells and limited induction of inflammation.

Whole-Inactivated Influenza Virus Is a Potent Adjuvant for Influenza Peptides Containing CD8+ T Cell Epitopes

Influenza peptide antigens coding for conserved T cell epitopes have the capacity to induce cross-protective influenza-specific immunity. Short peptide antigens used as a vaccine, however, often show poor immunogenicity. In this study, we demonstrate that whole-inactivated influenza virus (WIV) acts as an adjuvant for influenza peptide antigens, as shown by the induction of peptide-specific CD8⁺ T cells in HLA-A2.1 transgenic mice upon vaccination with the influenza-M1-derived GILGFVFTL peptide (GIL), formulated with WIV. By screening various concentrations of GIL and WIV, we found that both components contributed to the GIL-specific T cell response. Whereas co-localization of the peptide antigen and WIV adjuvant was found to be important, neither physical association between peptide and WIV nor fusogenic activity of WIV were relevant for the adjuvant effect of WIV. We furthermore show that WIV may adjuvate T cell responses to a variety of peptides, using pools of either conserved wild-type influenza peptides or chemically altered peptide ligands. This study shows the potential of WIV as an adjuvant for influenza peptides. The simple formulation process and the solid safety record of WIV make this an attractive adjuvant for T cell peptides, and may also be used for non-influenza antigens.

CD70 encoded by modified vaccinia virus Ankara enhances CD8 T-cell-dependent protective immunity in MHC class II-deficient mice

The immunological outcome of infections and vaccinations is largely determined during the initial first days in which antigen-presenting cells instruct T cells to expand and differentiate into effector and memory cells. Besides the essential stimulation of the T-cell receptor complex a plethora of co-stimulatory signals not only ensures a proper T-cell activation but also instils phenotypic and functional characteristics in the T cells appropriate to fight off the invading pathogen. The tumour necrosis factor receptor/ligand pair CD27/CD70 gained a lot of attention because of its key role in regulating T-cell activation, survival, differentiation and maintenance, especially in the course of viral infections and cancer. We sought to investigate the role of CD70 co-stimulation for immune responses induced by the vaccine vector modified vaccinia virus Ankara-Bavarian Nordic^® (MVA-BN^® ). Short-term blockade of CD70 diminished systemic CD8 T-cell effector and memory responses in mice. The dependence on CD70 became even more apparent in the lungs of MHC class II-deficient mice. Importantly, genetically encoded CD70 in MVA-BN^® not only increased CD8 T-cell responses in wild-type mice but also substituted for CD4 T-cell help. MHC class II-deficient mice that were immunized with recombinant MVA-CD70 were fully protected against a lethal virus infection, whereas MVA-BN^® -immunized mice failed to control the virus. These data are in line with CD70 playing an important role for vaccine-induced CD8 T-cell responses and prove the potency of integrating co-stimulatory molecules into the MVA-BN^® backbone.

Co-delivery of the NKT agonist α-galactosylceramide and tumor antigens to cross-priming dendritic cells breaks tolerance to self-antigens and promotes antitumor responses

Vaccines designed to abrogate the tolerance of tumor self-antigens and amplify cytotoxic CD8⁺ T cells (CTLs) have promise for the treatment of cancer. Type I natural killer (NKT) cells have attracted considerable interest in the cancer therapy field. In the current study, we have exploited the unique ability of NKT cells to serve as T-helper cells to license dendritic cells (DCs) for cross priming with the aim to generate efficient CTL antitumor responses. To this end, we designed a nanoparticle-based vaccine to target cross-priming DCs via the Clec9a endocytic pathway. Our results showed for the first time that simultaneous co-delivery of the NKT agonist α-galactosylceramide and tumor self-antigens (Trp2 and gp100) to CD8α⁺ DCs promotes strong antitumor responses in prophylactic and therapeutic settings (advanced solid tumor model in the mouse). We attributed the vaccine's therapeutic effects to NKT cells (but not to T-helper lymphocytes) and CD8⁺ T cells. Efficacy was correlated with an elevated ratio between tumor antigen-specific CD8⁺ T cells and regulatory CD4⁺ T lymphocytes within the tumor. The nanoparticle-based vaccine actively targeted human CLEC9A-expressing BDCA3⁺ DCs - the equivalent of murine cross-priming CD8α⁺ DCs - and induced a strong expansion of effector memory tumor self-antigen (Melan -A)-specific CD8⁺ T cells from peripheral blood mononuclear cells sourced from healthy donors and melanoma patients. Together, our result shed light on novel therapeutic approaches for controlling tumor development.

Effect of entinostat on NK cell-mediated cytotoxicity against osteosarcoma cells and osteosarcoma lung metastasis

There is a crucial need for a new therapeutic approach for osteosarcoma (OS) lung metastasis since this disease remains the main cause of mortality in OS. We previously demonstrated that natural killer (NK) cell therapy has minimal efficacy against OS metastasis. This study determined whether the histone deacetylase inhibitor entinostat could immunosensitize OS cells to NK cell lysis and increases the efficacy of NK cell therapy for OS lung metastasis. Entinostat upregulated ligands for NK cell-activating receptors (major histocompatibility complex [MHC] class I polypeptide-related chain A [MICA] and B [MICB]; UL16 binding proteins 1, 2, 5, and 6; and CD155) on OS cells both in vitro and in vivo and led to more susceptibility to NK cell-mediated cytotoxicity in vitro. Importantly, entinostat did not change NK cell viability, receptor expression, or function within the 24-h treatment. We also demonstrated two potential mechanisms by which entinostat enhanced expression of MICA and MICB on OS cells. Although entinostat upregulated ligands for the NK cell activating receptor on OS lung metastasis, it failed to augment the efficacy of NK cell therapy in our nude mouse human OS lung metastasis model. This can be partly explained by our finding that although the infused NK cells were active and functional and could penetrate into the lungs, they failed to infiltrate into the lung nodules. These challenges regarding cellular immunotherapy against solid tumors may be overcome by combination therapy, such as adding a NK cell-activating cytokine (IL-2 or IL-21).

Lymph node - an organ for T-cell activation and pathogen defense

The immune system is a multicentered organ that is characterized by intimate interactions between its cellular components to efficiently ward off invading pathogens. A key constituent of this organ system is the distinct migratory activity of its cellular elements. The lymph node represents a pivotal meeting point of immune cells where adaptive immunity is induced and regulated. Additionally, besides barrier tissues, the lymph node is a critical organ where invading pathogens need to be eliminated in order to prevent systemic distribution of virulent microbes. Here, we explain how the lymph node is structurally and functionally organized to fulfill these two critical functions - pathogen defense and orchestration of adaptive immunity. We will discuss spatio-temporal aspects of cellular immune responses focusing on CD8 T cells and review how and where these cells are activated in the context of viral infections, as well as how viral antigen expression kinetics and different antigen presentation pathways are involved. Finally, we will describe how such responses are regulated and 'helped', and discuss how this relates to intranodal positioning and cellular migration of the various cellular components that are involved in these processes.

Characteristics of CD4+ and CD8+ T cells in peripheral blood of patients with HBV-related primary liver cancer of Qi-Yin deficiency type

AIM

To observe the traditional Chinese medicine (TCM) syndrome distribution in the occurrence and progression of hepatitis B virus-related primary liver cancer (HBV-PLC) and compare the changes of T and B cells among different HBV-PLC syndromes.

METHODS

The laboratory characteristics and TCM syndromes of 67 HBV-PLV patients, 55 liver cirrhosis (LC) patients and 40 chronic hepatitis B (CHB) patients were collected at Beijing Ditan Hospital Affiliated to Capital Medical University. The distributions of CD3⁺ T, CD4⁺ T, CD8⁺ T and CD3^-CD19⁺ B cells in peripheral blood of all patients were detected by flow cytometry.

RESULTS

Compared with CHB and LC patients, the percentage of HBV-PLC patients of Qi and Yin deficiency type increased, but that of liver depression and spleen deficiency type declined. In terms of Child, Okuda, and BCLC stages of liver cancer, the early stage was mostly associated with liver depression and spleen deficiency, while the advanced stage was mainly associated with Qi and Yin deficiency. Compared with HBV-PLC patients with liver depression and spleen deficiency type, TBIL, DBIL, PT, PTA and the percentage of B cells increased in patients with Qi and Yin deficiency (P < 0.001), while A/G, CHE, the numbers and percentages of T and CD8⁺ cells, and the number of CD4⁺ cells significantly decreased (P < 0.05) in patients with Qi and Yin deficiency. The numbers of T, CD4⁺ T, CD8⁺ T and B cells were less in the LC and HBV-PLC patients of Qi and Yin deficiency type than in CHB patients of the same type (P < 0.05) .

CONCLUSION

The TCM syndrome changes from liver depression and spleen deficiency to Qi and Yin deficiency in the occurrence and progression of HBV-PLC. HBV-PLC patients with Qi and Yin deficiency have worse liver function, lower anti-tumor immune response and poorer prognosis compared with patients with other types.

Blimp-1-mediated CD4 T cell exhaustion causes CD8 T cell dysfunction during chronic toxoplasmosis

CD8, but not CD4, T cells are considered critical for control of chronic toxoplasmosis. Although CD8 exhaustion has been previously reported in Toxoplasma encephalitis (TE)-susceptible model, our current work demonstrates that CD4 not only become exhausted during chronic toxoplasmosis but this dysfunction is more pronounced than CD8 T cells. Exhausted CD4 population expressed elevated levels of multiple inhibitory receptors concomitant with the reduced functionality and up-regulation of Blimp-1, a transcription factor. Our data demonstrates for the first time that Blimp-1 is a critical regulator for CD4 T cell exhaustion especially in the CD4 central memory cell subset. Using a tamoxifen-dependent conditional Blimp-1 knockout mixed bone marrow chimera as well as an adoptive transfer approach, we show that CD4 T cell-intrinsic deletion of Blimp-1 reversed CD8 T cell dysfunction and resulted in improved pathogen control. To the best of our knowledge, this is a novel finding, which demonstrates the role of Blimp-1 as a critical regulator of CD4 dysfunction and links it to the CD8 T cell dysfunctionality observed in infected mice. The critical role of CD4-intrinsic Blimp-1 expression in mediating CD4 and CD8 T cell exhaustion may provide a rational basis for designing novel therapeutic approaches.

Immediate Dysfunction of Vaccine-Elicited CD8+ T Cells Primed in the Absence of CD4+ T Cells

CD4(+) T cell help is critical for optimal CD8(+) T cell memory differentiation and maintenance in many experimental systems. In addition, many reports have identified reduced primary CD8(+) T cell responses in the absence of CD4(+) T cell help, which often coincides with reduced Ag or pathogen clearance. In this study, we demonstrate that absence of CD4(+) T cells at the time of adenovirus vector immunization of mice led to immediate impairments in early CD8(+) T cell functionality and differentiation. Unhelped CD8(+) T cells exhibited a reduced effector phenotype, decreased ex vivo cytotoxicity, and decreased capacity to produce cytokines. This dysfunctional state was imprinted within 3 d of immunization. Unhelped CD8(+) T cells expressed elevated levels of inhibitory receptors and exhibited transcriptomic exhaustion and anergy profiles by gene set enrichment analysis. Dysfunctional, impaired effector differentiation also occurred following immunization of CD4(+) T cell-deficient mice with a poxvirus vector. This study demonstrates that following priming with viral vectors, CD4(+) T cell help is required to promote both the expansion and acquisition of effector functions by CD8(+) T cells, which is accomplished by preventing immediate dysfunction.

Viral vectors as vaccine carriers

This chapter reviews the performance of viral vectors based on adenoviruses or adeno-associated virus as vaccine carriers for infectious diseases. Replication-defective adenovirus vectors based on multiple human or non-human serotypes have consistently induced potent transgene product-specific B and T cell responses and are increasingly being explored in human clinical trials. The immunogenicity of most vectors based on adeno-associated virus vectors has been poor with the exception of a recently described hybrid vector from rhesus macaques that due to its ability to induce potent responses in mice warrant further investigation.

Endogenous Il10 alleviates the systemic antiviral cellular immune response and T cell-mediated immunopathology in select organs of acutely LCMV-infected mice

The immunoregulatory cytokine IL-10 suppresses T-cell immunity. The complementary question, whether IL-10 is also involved in limiting the collateral damage of vigorous T cell responses, has not been addressed in detail. Here, we report that the particularly strong virus-specific immune response during acute primary infection with the lymphocytic choriomeningitis virus (LCMV) in mice is significantly further increased in Il10-deficient mice, particularly regarding frequencies and cytotoxic activity of CD8(+) T cells. This increase results in exacerbating immunopathology in select organs, ranging from transient local swelling to an increased risk for mortality. Remarkably, LCMV-induced, T cell-mediated hepatitis is not affected by endogenous Il10. The alleviating effect of Il10 on LCMV-induced immunopathology was found to be operative in delayed-type hypersensitivity footpad-swelling reaction and in debilitating meningitis in mice of both the C57BL/6 and BALB/c strains. These strains are prototypic counterpoles for genetically imprinted type 1-biased versus type 2-biased T cell-mediated immune responses against various infectious pathogens. However, during acute LCMV infection, neither systemic cytokine patterns nor the impact of Il10 on LCMV-induced immunopathology differed conspicuously between these two strains of mice. This study documents a physiological role of Il10 in the regulation of a balanced T-cell response limiting immunopathological damage.

T-cell therapies for HIV: Preclinical successes and current clinical strategies

Although antiretroviral therapy (ART) has been successful in controlling HIV infection, it does not provide a permanent cure, requires lifelong treatment, and HIV-positive individuals are left with social concerns such as stigma. The recent application of T cells to treat cancer and viral reactivations post-transplant offers a potential strategy to control HIV infection. It is known that naturally occurring HIV-specific T cells can inhibit HIV initially, but this response is not sustained in the majority of people living with HIV. Genetically modifying T cells to target HIV, resist infection, and persist in the immunosuppressive environment found in chronically infected HIV-positive individuals might provide a therapeutic solution for HIV. This review focuses on successful preclinical studies and current clinical strategies using T-cell therapy to control HIV infection and mediate a functional cure solution.

BCR-ABL-specific CD4+ T-helper cells promote the priming of antigen-specific cytotoxic T cells via dendritic cells

The advent of tyrosine kinase inhibitor (TKI) therapy markedly improved the outcome of patients with chronic-phase chronic myeloid leukemia (CML). However, the poor prognosis of patients with advanced-phase CML and the lifelong dependency on TKIs are remaining challenges; therefore, an effective therapeutic has been sought. The BCR-ABL p210 fusion protein's junction region represents a leukemia-specific neoantigen and is thus an attractive target for antigen-specific T-cell immunotherapy. BCR-ABL p210 fusion-region-specific CD4⁺ T-helper (Th) cells possess antileukemic potential, but their function remains unclear. In this study, we established a BCR-ABL p210 b3a2 fusion-region-specific CD4⁺ Th-cell clone (b3a2-specific Th clone) and examined its dendritic cell (DC)-mediated antileukemic potential. The b3a2-specific Th clone recognized the b3a2 peptide in the context of HLA-DRB1*09:01 and exhibited a Th1 profile. Activation of this clone through T-cell antigen receptor stimulation triggered DC maturation, as indicated by upregulated production of CD86 and IL-12p70 by DCs, which depended on CD40 ligation by CD40L expressed on b3a2-specific Th cells. Moreover, in the presence of HLA-A*24:02-restricted Wilms tumor 1 (WT1)_235-243 peptide, DCs conditioned by b3a2-specific Th cells efficiently stimulated the primary expansion of WTI-specific cytotoxic T lymphocytes (CTLs). The expanded CTLs were cytotoxic toward WT1_235-243-peptide-loaded HLA-A*24:02-positive cell lines and exerted a potent antileukemic effect in vivo. However, the b3a2-specific Th-clone-mediated antileukemic CTL responses were strongly inhibited by both TKIs and interferon-α. Our findings indicate a crucial role of b3a2-specific Th cells in leukemia antigen-specific CTL-mediated immunity and provide an experimental basis for establishing novel CML immunotherapies.

Expansion of Inefficient HIV-Specific CD8 T Cells during Acute Infection

Attrition within the CD4(+)T cell compartment, high viremia, and a cytokine storm characterize the early days after HIV infection. When the first emerging HIV-specific CD8(+)T cell responses gain control over viral replication it is incomplete, and clearance of HIV infection is not achieved even in the rare cases of individuals who spontaneously control viral replication to nearly immeasurably low levels. Thus, despite their partial ability to control viremia, HIV-specific CD8(+)T cell responses are insufficient to clear HIV infection. Studying individuals in the first few days of acute HIV infection, we detected the emergence of a unique population of CD38(+)CD27(-)CD8(+)T cells characterized by the low expression of the CD8 receptor (CD8(dim)). Interestingly, while high frequencies of HIV-specific CD8(+)T cell responses occur within the CD38(+)CD27(-)CD8(dim)T cell population, the minority populations of CD8(bright)T cells are significantly more effective in inhibiting HIV replication. Furthermore, the frequency of CD8(dim)T cells directly correlates with viral load and clinical predictors of more rapid disease progression. We found that a canonical burst of proliferative cytokines coincides with the emergence of CD8(dim)T cells, and the size of this population inversely correlates with the acute loss of CD4(+)T cells. These data indicate, for the first time, that early CD4(+)T cell loss coincides with the expansion of a functionally impaired HIV-specific CD8(dim)T cell population less efficient in controlling HIV viremia.

IMPORTANCE

A distinct population of activated CD8(+)T cells appears during acute HIV infection with diminished capacity to inhibit HIV replication and is predictive of viral set point, offering the first immunologic evidence of CD8(+)T cell dysfunction during acute infection.

B-Cell-Deficient and CD8 T-Cell-Depleted Gnotobiotic Pigs for the Study of Human Rotavirus Vaccine-Induced Protective Immune Responses

Genetically modified pigs have become available recently. In this study, we established the gnotobiotic pig model of human rotavirus (HRV) infection using cloned pigs with homozygous disruption in the gene encoding immunoglobulin heavy chain (HCKO), which totally impairs B-cell development. To clarify importance of B cells and cytotoxic T cells in rotavirus immunity, CD8 cells in a subset of the pigs were depleted by injecting antipig CD8 antibodies and the immune phenotypes of all pigs were examined. HCKO pigs, CD8 cell-depleted HCKO pigs, and wild-type (WT) pigs were vaccinated with an attenuated HRV vaccine and challenged with virulent HRV. Protection against HRV infection and diarrhea was assessed postchallenge and detailed T-cell subset responses were determined pre- and postchallenge. Significantly longer duration of virus shedding was seen in vaccinated HCKO pigs than in WT pigs, indicating the importance of B cells in vaccine-induced protective immunity. Vaccinated HCKO/CD8(-) pigs shed significantly higher number of infectious virus than WT pigs and non-CD8-depleted HCKO pigs, indicating the importance of CD8 T cells in controlling virus replication. Therefore, both B cells and CD8 T cells play an important role in the protection against rotavirus infection. HCKO and HCKO/CD8(-) pigs did not differ significantly in diarrhea and virus shedding postchallenge; increased CD4 and CD8(-) γδ T-cell responses probably compensated partially for the lack of CD8 T cells. This study demonstrated that HCKO pigs can serve as a valuable model for dissection of protective immune responses against viral infections and diseases.

Peracetic Acid Treatment Generates Potent Inactivated Oral Vaccines from a Broad Range of Culturable Bacterial Species

Our mucosal surfaces are the main sites of non-vector-borne pathogen entry, as well as the main interface with our commensal microbiota. We are still only beginning to understand how mucosal adaptive immunity interacts with commensal and pathogenic microbes to influence factors such as infectivity, phenotypic diversity, and within-host evolution. This is in part due to difficulties in generating specific mucosal adaptive immune responses without disrupting the mucosal microbial ecosystem itself. Here, we present a very simple tool to generate inactivated mucosal vaccines from a broad range of culturable bacteria. Oral gavage of 10¹⁰ peracetic acid-inactivated bacteria induces high-titer-specific intestinal IgA in the absence of any measurable inflammation or species invasion. As a proof of principle, we demonstrate that this technique is sufficient to provide fully protective immunity in the murine model of invasive non-typhoidal Salmonellosis, even in the face of severe innate immune deficiency.

The multifaceted role of CD4(+) T cells in CD8(+) T cell memory

Following infection, T cells differentiate into a heterogeneous population of effector T cells that can mediate pathogen clearance. A subset of these effector T cells possesses the ability to survive long term and mature into memory T cells that can provide long-term immunity. Understanding the signals that regulate the development of memory T cells is crucial to efforts to design vaccines capable of eliciting T cell-based immunity. CD4(+) T cells are essential in the formation of protective memory CD8(+) T cells following infection or immunization. However, until recently, the mechanisms by which CD4(+) T cells act to support memory CD8(+) T cell development following infection were unclear. Here, we discuss recent studies that provide insight into the multifaceted role of CD4(+) T cells in the regulation of memory CD8(+) T cell differentiation.

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.

Induction of WT1-specific human CD8+ T cells from human HSCs in HLA class I Tg NOD/SCID/IL2rgKO mice

Induction of specific immune response against therapy-resistant tumor cells can potentially improve clinical outcomes in malignancies. To optimize immunotherapy in the clinic, we aimed to create an in vivo model enabling us to analyze human cytotoxic T-lymphocyte (CTL) responses against human malignancies. To this end, we developed NOD/SCID/IL2rgKO (NSG) mice expressing the HLA class I molecules HLA-A*0201 and A*2402. In the bone marrow (BM) and spleen of HLA class I transgenic (Tg) NSG mice transplanted with cord blood hematopoietic stem cells (HSCs), we found human memory CD8(+) T cells and antigen-presenting cells. To evaluate antigen-specific human CTL responses, we immunized HLA class I Tg NSG mice using polyinosinic:polycytidylic acid mixed Wilms tumor 1 (WT1) peptides, with or without WT1 peptide-loaded autologous dendritic cells. After immunization, the frequencies of HLA-restricted WT1-specific CTLs increased significantly in the spleen. Next, we transplanted the WT1-specific T-cell receptor (WT1-TCR) gene-transduced human HSCs into HLA class I Tg NSG newborn mice. WT1 tetramer-positive CD8(+) T cells differentiated from WT1-TCR-transduced HSCs in the recipients' BM, spleen, and thymus. Upon stimulation with WT1 peptide in vitro, these CTLs produced interferon-γ and showed lytic activity against leukemia cells in an antigen-specific, HLA-restricted manner. HLA class I Tg NSG xenografts may serve as a preclinical model to develop effective immunotherapy against human malignancies.

A systemic defect in Toll-like receptor 4 signaling increases lipopolysaccharide-induced suppression of IL-2-dependent T-cell proliferation in COPD

The susceptibility to bacterial infections is increased in chronic obstructive pulmonary disease (COPD). This promotes exacerbations. IL-2 triggers CD4(+)/Th1-cell proliferation, which is important for infection defense. Bacterial endotoxin (LPS) activates MyD88/IRAK and TRIF/IKKε/TBK1 pathways via Toll-like receptor-4 (TLR4) in Th1 cells. Systemic defects in TLR pathways in CD4(+)/Th1 cells cause an impairment of IL-2-dependent immune responses to bacterial infections in COPD. Peripheral blood CD4(+) T cells of never smokers, smokers without COPD, and smokers with COPD (each n = 10) were ex vivo activated towards Th1 and stimulated with LPS. IL-2, MyD88, and TRIF expression, and cell proliferation was analyzed by ELISA, quantitative RT-PCR, and bromodeoxyuridine (BrdU) and trypan blue staining comparative among the cohorts. IL-2 release from activated T cells was increased in COPD vs. smokers and never smokers. LPS reduced IL-2 expression and T-cell proliferation. These effects were increased in COPD vs. never smokers and inversely correlated with FEV1 (%predicted). The MyD88/TRIF ratio was decreased in Th1 cells of COPD. The suppression of IL-2 by LPS was abolished by MyD88/IRAK blockade in never smokers but by TRIF/IKKε/TBK1 blockade in COPD. Moxifloxacin restored IL-2 expression and T-cell proliferation in the presence of LPS by blocking p38 MAPK. The increased IL-2 release from Th1 cells in COPD might contribute to airway inflammation in disease exacerbations. A switch from MyD88/IRAK to TRIF/IKKε/TBK1 signaling amplifies the suppression of IL-2-dependent proliferation of CD4(+) T cells by LPS in COPD. This molecular pathology is of systemic origin, might impair adaptive immune responses, and could explain the increased susceptibility to bacterial infections in COPD. Targeting TLR4-downstream signaling, for example, with moxifloxacin, might reduce exacerbation rates.

Spatiotemporally Distinct Interactions with Dendritic Cell Subsets Facilitates CD4+ and CD8+ T Cell Activation to Localized Viral Infection

The dynamics of when and where CD4(+) T cells provide help for CD8(+) T cell priming and which dendritic cells (DCs) activate CD4(+) T cells in vivo after localized infection are poorly understood. By using a cutaneous herpes simplex virus infection model combined with intravital 2-photon imaging of the draining lymph node (LN) to concurrently visualize pathogen-specific CD4(+) and CD8(+) T cells, we found that early priming of CD4(+) T cells involved clustering with migratory skin DCs. CD8(+) T cells did not interact with migratory DCs and their activation was delayed, requiring later clustering interactions with LN-resident XCR1(+) DCs. CD4(+) T cells interacted with these late CD8(+) T cell clusters on resident XCR1(+) DCs. Together, these data reveal asynchronous T cell activation by distinct DC subsets and highlight the key role of XCR1(+) DCs as the central platform for cytotoxic T lymphocyte activation and the delivery of CD4(+) T cell help.

Robust Anti-viral Immunity Requires Multiple Distinct T Cell-Dendritic Cell Interactions

Host defense against viruses and intracellular parasites depends on effector CD8(+) T cells, whose optimal clonal expansion, differentiation, and memory properties require signals from CD4(+) T cells. Here, we addressed the role of dendritic cell (DC) subsets in initial activation of the two T cell types and their co-operation. Surprisingly, initial priming of CD4(+) and CD8(+) T cells was spatially segregated within the lymph node and occurred on different DCs with temporally distinct patterns of antigen presentation via MHCI versus MHCII molecules. DCs that co-present antigen via both MHC molecules were detected at a later stage; these XCR1(+) DCs are the critical platform involved in CD4(+) T cell augmentation of CD8(+) T cell responses. These findings delineate the complex choreography of cellular interactions underlying effective cell-mediated anti-viral responses, with implications for basic DC subset biology, as well as for translational application to the development of vaccines that evoke optimal T cell immunity.