High efficiency of antiviral CD4(+) killer T cells

Assessment of the cytolytic potential of a multivirus-targeted T cell therapy using a vital dye-based, flow cytometric assay

Reliable and sensitive characterization assays are important determinants of the successful clinical translation of immunotherapies. For the assessment of cytolytic potential, the chromium 51 (51Cr) release assay has long been considered the gold standard for testing effector cells. However, attaining the approvals to access and use radioactive isotopes is becoming increasingly complex, while technical aspects [i.e. sensitivity, short (4-6 hours) assay duration] may lead to suboptimal performance. This has been the case with our ex vivo expanded, polyclonal (CD4+ and CD8+) multivirus-specific T cell (multiVST) lines, which recognize 5 difficult-to-treat viruses [Adenovirus (AdV), BK virus (BKV), cytomegalovirus (CMV), Epstein Barr virus (EBV), and human herpes virus 6 (HHV6)] and when administered to allogeneic hematopoietic stem cell (HCT) or solid organ transplant (SOT) recipients have been associated with clinical benefit. However, despite mediating potent antiviral effects in vivo, capturing in vitro cytotoxic potential has proven difficult in a traditional 51Cr release assay. Now, in addition to cytotoxicity surrogates, including CD107a and Granzyme B, we report on an alternative, vital dye -based, flow cytometric platform in which superior sensitivity and prolonged effector:target co-culture duration enabled the reliable detection of both CD4- and CD8-mediated in vitro cytolytic activity against viral targets without non-specific effects.

Cytotoxic CD4+ T cells in chronic viral infections and cancer

CD4+ T cells play an important role in immune responses against pathogens and cancer cells. Although their main task is to provide help to other effector immune cells, a growing number of infections and cancer entities have been described in which CD4+ T cells exhibit direct effector functions against infected or transformed cells. The most important cell type in this context are cytotoxic CD4+ T cells (CD4+ CTL). In infectious diseases anti-viral CD4+ CTL are mainly found in chronic viral infections. Here, they often compensate for incomplete or exhausted CD8+ CTL responses. The induction of CD4+ CTL is counter-regulated by Tregs, most likely because they can be dangerous inducers of immunopathology. In viral infections, CD4+ CTL often kill via the Fas/FasL pathway, but they can also facilitate the exocytosis pathway of killing. Thus, they are very important effectors to keep persistent virus in check and guarantee host survival. In contrast to viral infections CD4+ CTL attracted attention as direct anti-tumor effectors in solid cancers only recently. Anti-tumor CD4+ CTL are defined by the expression of cytolytic markers and have been detected within the lymphocyte infiltrates of different human cancers. They kill tumor cells in an antigen-specific MHC class II-restricted manner not only by cytolysis but also by release of IFNγ. Thus, CD4+ CTL are interesting tools for cure approaches in chronic viral infections and cancer, but their potential to induce immunopathology has to be carefully taken into consideration.

CTLs: Killers of intracellular bacteria

Many microbial pathogens have evolved a range of capabilities to evade host immune defense mechanisms and to survive and multiply in host cells. The presence of host intracellular bacteria makes it difficult for specific antibodies to function. After the intracellular bacteria escape the attack of the innate immune system, such as phagocytes, they survive in cells, and then adaptive immunity comes into play. Cytotoxic T lymphocytes (CTLs) play an important role in eliminating intracellular bacteria. The regulation of key transcription factors could promote CD4+/CD8+ T cells to acquire cytolytic ability. The TCR-CD3 complex transduces activation signals generated by TCR recognition of antigen and promotes CTLs to generate multiple pathways to kill intracellular bacteria. In this review, the mechanism of CD4/CD8 CTLs differentiation and how CD4/CD8 CTLs kill intracellular bacteria are introduced. In addition, their application and prospects in the treatment of bacterial infections are discussed.

Host immune response against DENV and ZIKV infections

Dengue is a major public health concern, affecting almost 400 million people worldwide, with about 70% of the global burden of disease in Asia. Despite revised clinical classifications of dengue infections by the World Health Organization, the wide spectrum of the manifestations of dengue illness continues to pose challenges in diagnosis and patient management for clinicians. When the Zika epidemic spread through the American continent and then later to Africa and Asia in 2015, researchers compared the characteristics of the Zika infection to Dengue, considering both these viruses were transmitted primarily through the same vector, the Aedes aegypti female mosquitoes. An important difference to note, however, was that the Zika epidemic diffused in a shorter time span compared to the persisting feature of Dengue infections, which is endemic in many Asian countries. As the pathogenesis of viral illnesses is affected by host immune responses, various immune modulators have been proposed as biomarkers to predict the risk of the disease progression to a severe form, at a much earlier stage of the illness. However, the findings for most biomarkers are highly discrepant between studies. Meanwhile, the cross-reactivity of CD8+ and CD4+ T cells response to Dengue and Zika viruses provide important clues for further development of potential treatments. This review discusses similarities between Dengue and Zika infections, comparing their disease transmissions and vectors involved, and both the innate and adaptive immune responses in these infections. Consideration of the genetic identity of both the Dengue and Zika flaviviruses as well as the cross-reactivity of relevant T cells along with the actions of CD4+ cytotoxic cells in these infections are also presented. Finally, a summary of the immune biomarkers that have been reported for dengue and Zika viral infections are discussed which may be useful indicators for future anti-viral targets or predictors for disease severity. Together, this information appraises the current understanding of both Zika and Dengue infections, providing insights for future vaccine design approaches against both viruses.

Neutralizing antibody and T cell responses against SARS-CoV-2 variants of concern following ChAdOx-1 or BNT162b2 boosting in the elderly previously immunized with CoronaVac vaccine

Background

The existence of SARS-CoV-2 variants of concern (VOCs) in association with evidence of breakthrough infections despite vaccination resulted in the need for vaccine boosting. In elderly individuals, information on the immunogenicity of booster vaccinations is limited. In countries where the CoronaVac inactivated vaccine is the primary vaccine, the appropriate boosting regimen is not clear. Immunologic studies of the effects of booster vaccination against VOCs, particularly Delta and Omicron, following CoronaVac in elderly individuals are helpful for policy makers. In this study, we determined the immune responses against VOCs following ChAdOx-1 or BNT162b2 boosting in elderly individuals previously immunized with CoronaVac.

Results

Before boosting, the median % inhibition of neutralizing antibodies (NAbs) against the wild-type (WT), Alpha, Beta, Delta and Omicron variants in the ChAdOx-1 and BNT162b2 groups was 52.8% vs. 53.4, 36.6% vs. 39.9, 5.2% vs. 13.7, 34.3% vs. 44.9, and 20.8% vs. 18.8%, respectively. After boosting with ChAdOx-1 or BNT162b2, the % inhibition of NAbs were increased to 97.3% vs. 97.4, 94.3% vs. 97.3%, 79.9 vs. 93.7, 95.5% vs. 97.5, and 26.9% vs. 31.9% for WT, Alpha, Beta, Delta and Omicron variants, respectively. Boosting with BNT162b2 induced significantly higher NAb levels than boosting with ChAdOx-1 against the Alpha, Beta and Delta variants but not the WT and Omicron variants. NAb levels against Omicron variant were not significantly different before and after boosting with ChAdOx-1 or BNT162b2. To evaluate T-cell responses, S peptides of the WT, Alpha, Beta and Delta variants were used to stimulate T cells. Upon stimulation, the expression of IL-17A in CD8 T cells was higher in the BNT162b2 group than in the ChAdOx-1 boosting group. However, IFN-γ production in CD4 and CD8 T cells did not significantly differ under all vaccination regimens. The expression of FasL in CD4 T cells, but not CD8 T cells, was higher in the BNT162b2-boosted group.

Conclusion

Boosting with either ChAdOx-1 or BNT162b2 in CoronaVac-primed healthy elderly individuals induced high NAb production against all examined VOCs except Omicron. BNT162b2 stimulated higher NAb and some T-cell responses than ChAdOx-1. Vaccine boosting is, therefore, recommended for elderly individuals previously immunized with CoronaVac.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12979-022-00279-8.

CD4+ Cytotoxic T cells - Phenotype, Function and Transcriptional Networks Controlling Their Differentiation Pathways

The two major subsets of peripheral T cells are classically divided into the CD4⁺ T helper cells and the cytotoxic CD8⁺ T cell lineage. However, the appearance of some effector CD4⁺ T cell populations displaying cytotoxic activity, in particular during viral infections, has been observed, thus breaking the functional dichotomy of CD4⁺ and CD8⁺ T lymphocytes. The strong association of the appearance of CD4⁺ cytotoxic T lymphocytes (CD4 CTLs) with viral infections suggests an important role of this subset in antiviral immunity by controlling viral replication and infection. Moreover, CD4 CTLs have been linked with anti-tumor activity and might also cause immunopathology in autoimmune diseases. This raises interest into the molecular mechanisms regulating CD4 CTL differentiation, which are poorly understood in comparison to differentiation pathways of other Th subsets. In this review, we provide a brief overview about key features of CD4 CTLs, including their role in viral infections and cancer immunity, and about the link between CD4 CTLs and immune-mediated diseases. Subsequently, we will discuss the current knowledge about transcriptional and epigenetic networks controlling CD4 CTL differentiation and highlight recent data suggesting a role for histone deacetylases in the generation of CD4 CTLs.

Next-generation T cell-activating vaccination increases influenza virus mutation prevalence

To determine the potential for viral adaptation to T cell responses, we probed the full influenza virus genome by next-generation sequencing directly ex vivo from infected mice, in the context of an experimental T cell-based vaccine, an H5N1-based viral vectored vaccinia vaccine Wyeth/IL-15/5Flu, versus the current standard-of-care, seasonal inactivated influenza vaccine (IIV) and unvaccinated conditions. Wyeth/IL-15/5Flu vaccination was coincident with increased mutation incidence and frequency across the influenza genome; however, mutations were not enriched within T cell epitope regions, but high allele frequency mutations within conserved hemagglutinin stem regions and PB2 mammalian adaptive mutations arose. Depletion of CD4⁺ and CD8⁺ T cell subsets led to reduced frequency of mutants in vaccinated mice; therefore, vaccine-mediated T cell responses were important drivers of virus diversification. Our findings suggest that Wyeth/IL-15/5Flu does not generate T cell escape mutants but increases stochastic events for virus adaptation by stringent bottlenecks.

CD4+ T Cells Control Murine Cytomegalovirus Infection Indirectly

CD4⁺ T cells are key to controlling cytomegalovirus infections. Salivary gland infection by murine cytomegalovirus (MCMV) provides a way to identify mechanisms. CD11c⁺ dendritic cells (DC) disseminate MCMV to the salivary glands, where they transfer infection to acinar cells. Antiviral CD4⁺ T cells are often considered to be directly cytotoxic for cells expressing major histocompatibility complex class II (MHCII). However, persistently infected salivary gland acinar cells are MHCII^- and are presumably inaccessible to direct CD4 T cell recognition. Here, we show that CD4⁺ T cell depletion amplified infection of MHCII^- acinar cells but not MHCII⁺ cells. MCMV-infected mice with disrupted MHCII on CD11c⁺ cells showed increased MHCII^- acinar infection; antiviral CD4⁺ T cells were still primed, but their recruitment to the salivary glands was reduced, suggesting that engagement with local MHCII⁺ DC is important for antiviral protection. As MCMV downregulates MHCII on infected DC, the DC participating in CD4 protection may thus be uninfected. NK cells and gamma interferon (IFN-γ) may also contribute to CD4⁺ T cell-dependent virus control: CD4 T cell depletion reduced NK cell recruitment to the salivary glands, and both NK cell and IFN-γ depletion equalized infection between MHCII-disrupted and control mice. Taken together, these results suggest that CD4⁺ T cells protect indirectly against infected acinar cells in the salivary gland via DC engagement, requiring the recruitment of NK cells and the action of IFN-γ. Congruence of these results with an established CD4⁺ T cell/NK cell axis of gammaherpesvirus infection control suggests a common mode of defense against evasive viruses. IMPORTANCE Cytomegalovirus infections commonly cause problems in immunocompromised patients and in pregnancy. We lack effective vaccines. CD4⁺ T cells play an important role in normal infection control, yet how they act has been unknown. Using murine cytomegalovirus as an accessible model, we show that CD4⁺ T cells are unlikely to recognize infected cells directly. We propose that CD4⁺ T cells interact with uninfected cells that present viral antigens and recruit other immune cells to attack infected targets. These data present a new outlook on understanding how CD4⁺ T cell-directed control protects against persistent cytomegalovirus infection.

Treg-Resistant Cytotoxic CD4+ T Cells Dictate T Helper Cells in Their Vicinity: TH17 Skewing and Modulation of Proliferation

Cytotoxic CD4⁺ T cells (CD4 CTL) are terminally differentiated T helper cells that contribute to autoimmune diseases, such as multiple sclerosis. We developed a novel triple co-culture transwell assay to study mutual interactions between CD4 CTL, conventional TH cells, and regulatory T cells (Tregs) simultaneously. We show that, while CD4 CTL are resistant to suppression by Tregs in vitro, the conditioned medium of CD4 CTL accentuates the suppressive phenotype of Tregs by upregulating IL-10, Granzyme B, CTLA-4, and PD-1. We demonstrate that CD4 CTL conditioned medium skews memory TH cells to a TH17 phenotype, suggesting that the CD4 CTL induce bystander polarization. In our triple co-culture assay, the CD4 CTL secretome promotes the proliferation of TH cells, even in the presence of Tregs. However, when cell−cell contact is established between CD4 CTL and TH cells, the proliferation of TH cells is no longer increased and Treg-mediated suppression is restored. Taken together, our results suggest that when TH cells acquire cytotoxic properties, these Treg-resistant CD4 CTL affect the proliferation and phenotype of conventional TH cells in their vicinity. By creating such a pro-inflammatory microenvironment, CD4 CTL may favor their own persistence and expansion, and that of other potentially pathogenic TH cells, thereby contributing to pathogenic responses in autoimmune disorders.

Immunity to Influenza Infection in Humans

This review discusses the human immune responses to influenza infection with some insights from studies using animal models, such as experimental infection of mice. Recent technological advances in the study of human immune responses have greatly added to our knowledge of the infection and immune responses, and therefore much of the focus is on recent studies that have moved the field forward. We consider the complexity of the adaptive response generated by many sequential encounters through infection and vaccination.

Mucosal IL-4R antagonist HIV vaccination with SOSIP-gp140 booster can induce high-quality cytotoxic CD4+/CD8+ T cells and humoral responses in macaques

Inducing humoral, cellular and mucosal immunity is likely to improve the effectiveness of HIV-1 vaccine strategies. Here, we tested a vaccine regimen in pigtail macaques using an intranasal (i.n.) recombinant Fowl Pox Virus (FPV)-gag pol env-IL-4R antagonist prime, intramuscular (i.m.) recombinant Modified Vaccinia Ankara Virus (MVA)-gag pol-IL-4R antagonist boost followed by an i.m SOSIP-gp140 boost. The viral vector-expressed IL-4R antagonist transiently inhibited IL-4/IL-13 signalling at the vaccination site. The SOSIP booster not only induced gp140-specific IgG, ADCC (antibody-dependent cellular cytotoxicity) and some neutralisation activity, but also bolstered the HIV-specific cellular and humoral responses. Specifically, superior sustained systemic and mucosal HIV Gag-specific poly-functional/cytotoxic CD4⁺ and CD8⁺ T cells were detected with the IL-4R antagonist adjuvanted strategy compared to the unadjuvanted control. In the systemic compartment elevated Granzyme K expression was linked to CD4⁺ T cells, whilst Granzyme B/TIA-1 to CD8⁺ T cells. In contrast, the cytotoxic marker expression by mucosal CD4⁺ and CD8⁺ T cells differed according to the mucosal compartment. This vector-based mucosal IL-4R antagonist/SOSIP booster strategy, which promotes cytotoxic mucosal CD4⁺ T cells at the first line of defence, and cytotoxic CD4⁺ and CD8⁺ T cells plus functional antibodies in the blood, may prove valuable in combating mucosal infection with HIV-1 and warrants further investigation.

Chemokine Signatures of Pathogen-Specific T Cells I: Effector T Cells

The choreography of complex immune responses, including the priming, differentiation, and modulation of specific effector T cell populations generated in the immediate wake of an acute pathogen challenge, is in part controlled by chemokines, a large family of mostly secreted molecules involved in chemotaxis and other patho/physiological processes. T cells are both responsive to various chemokine cues and a relevant source for certain chemokines themselves; yet, the actual range, regulation, and role of effector T cell-derived chemokines remains incompletely understood. In this study, using different in vivo mouse models of viral and bacterial infection as well as protective vaccination, we have defined the entire spectrum of chemokines produced by pathogen-specific CD8⁺ and CD4⁺T effector cells and delineated several unique properties pertaining to the temporospatial organization of chemokine expression patterns, synthesis and secretion kinetics, and cooperative regulation. Collectively, our results position the "T cell chemokine response" as a notably prominent, largely invariant, yet distinctive force at the forefront of pathogen-specific effector T cell activities and establish novel practical and conceptual approaches that may serve as a foundation for future investigations into the role of T cell-produced chemokines in infectious and other diseases.

Identification of protective T-cell antigens for smallpox vaccines

Background aims

E3L is an immediate-early protein of vaccinia virus (VV) that is detected within 0.5 h of infection, potentially before the many immune evasion genes of vaccinia can exert their protective effects. E3L is highly conserved among orthopoxviruses and hence could provide important protective T-cell epitopes that should be retained in any subunit or attenuated vaccine. We have therefore evaluated the immunogenicity of E3L in healthy VV-vaccinated donors.

Methods

Peripheral blood mononuclear cells from healthy volunteers (n = 13) who had previously received a smallpox vaccine (Dryvax) were activated and expanded using overlapping E3L peptides and their function, specificity and antiviral activity was analyzed. E3L-specific T cells were expanded from 7 of 12 (58.3%) vaccinated healthy donors. Twenty-five percent of these produced CD8+ T-cell responses and 87.5% produced CD4+ T cells. We identified epitopes restricted by HLA-B35 and HLA-DR15.

Results

E3L-specific T cells killed peptide-loaded target cells as well as vaccinia-infected cells, but only CD8+ T cells could prevent the spread of infectious virus in virus inhibition assays. The epitopes recognized by E3L-specific T cells were shared with monkeypox, and although there was a single amino acid change in the variola epitope homolog, it was recognized by vaccinia-specific T-cells.

Conclusions

It might be important to include E3L in any deletion mutant or subunit vaccine and E3L could provide a useful antigen to monitor protective immunity in humans.

Cytomegalovirus Coinfection Is Associated with Increased Vascular-Homing CD57+ CD4 T Cells in HIV Infection

Cytotoxic CD4 T cells are linked to cardiovascular morbidities and accumulate in both HIV and CMV infections, both of which are associated with increased risk of cardiovascular disease (CVD). In this study, we identify CMV coinfection as a major driver of the cytotoxic phenotype, characterized by elevated CD57 expression and reduced CD28 expression, in circulating CD4 T cells from people living with HIV infection, and investigate potential mechanisms linking this cell population to CVD. We find that human CD57+ CD4 T cells express high levels of the costimulatory receptor CD2 and that CD2/LFA-3 costimulation results in a more robust and polyfunctional effector response to TCR signals, compared with CD28-mediated costimulation. CD57+ CD4 T cells also express the vascular endothelium-homing receptor CX3CR1 and migrate toward CX3CL1-expressing endothelial cells in vitro. IL-15 promotes the cytotoxic phenotype, elevates CX3CR1 expression, and enhances the trafficking of CD57+ CD4 T cells to endothelium and may therefore be important in linking these cells to cardiovascular complications. Finally, we demonstrate the presence of activated CD57+ CD4 T cells and expression of CX3CL1 and LFA-3 in atherosclerotic plaque tissues from HIV-uninfected donors. Our findings are consistent with a model in which cytotoxic CD4 T cells contribute to CVD in HIV/CMV coinfection and in atherosclerosis via CX3CR1-mediated trafficking and CD2/LFA-3-mediated costimulation. This study identifies several targets for therapeutic interventions and may help bridge the gap in understanding how CMV infection and immunity are linked to increased cardiovascular risk in people living with HIV infection.

Granulysin- and granzyme-dependent elimination of myeloid cells by therapeutic ova-specific type 1 regulatory T cells

The intrinsic immunosuppressive properties of regulatory T (Treg) cells can be harnessed for therapeutic approaches aiming at down-modulating harmful immune reactions. In this context, expanded type 1 Treg cells (Tr1 cells) specific for ovalbumin (ova-Tr1 cells) have been tested for clinical efficacy in the treatment of autoimmune disorders such as refractory Crohn's disease (CD). The clinical use of these therapeutic products warrants exploration of their mechanism of action. Here, we identified a relationship between the CD activity index and the expression of lytic molecules by the ova-Tr1 cells administered in the previously reported First-in-Man study [Crohn's And Treg cells Study 1 (CATS1) study]. Accordingly, ova-Tr1 cells were found to carry granules containing high levels of lytic molecules, including multiple granzymes and granulysin. These cells displayed a T-cell receptor (TCR)-independent cytotoxic activity, which was preferentially directed toward myeloid cell lines and monocyte-derived dendritic cells. Upon contact with myeloid cells, ova-Tr1 cells induced their apoptosis via a perforin-independent and a granulysin/granzyme-dependent mechanism. As compared to CD8+ cytotoxic T cells, ova-Tr1 cells required more time to lyse target cells and displayed a more gradual lytic activity over time. Notably, this activity was sustained over days resulting in the control of myeloid cell populations at a relatively low ratio. Our study reveals that ova-Tr1 cells are endowed with a sustained cytotoxic activity that relies on a unique combination of granulysin and granzymes and that preferentially eliminates myeloid target cells in a TCR-independent manner.

Producing Soluble Human Programmed Cell Death Protein-1: A Natural Supporter for CD4+T Cell Cytotoxicity and Tumor Cells Apoptosis

BACKGROUND

Programmed cell death protein-1 (PD-1)/PD-L1 pathway is one of the immune checkpoint pathways involved in the regulation of the immune responses and the suppression of anti-tumor defense. PD-1/PD-L1 blocking antibodies improve immune responses such as cytotoxic activity of CD8+/CD4+T cells and increase mortality of tumor cells as well; however, their use is accompanied by adverse side effects.

OBJECTIVES

We aimed to produce a native blocker of human PD-1/PD-L1, for developing T cells cytotoxicity and tumor cells apoptosis.

MATERIALS AND METHODS

We designed and cloned soluble human PD-1-GFP-pcDNA3.1/hygro construct in Escherichia coli strain TOP10 cells and then transfected this construct into the HEK cells. The concentration of the secreted shPD-1 in the supernatant was measured and the supernatant was used for blocking PD-L1 on the MDA-MB-231 cells. The cytotoxicity of CD8+/CD4+T cells and the apoptosis of MDA-MB-231 cells, under the influence of shPD-1 in the co-culture of T cells with the MDA-MB-231 cells, were evaluated using flow cytometry technique.

RESULTS

The GFP expression in the transfected cells illustrated the successful designing, transfection, and production of shPD-1. Soluble human PD-1 concentration in the supernatant of the transfected HEK cells was significantly higher than the untransfected cells. In addition, shPD-1 significantly blocked PD-L1 on the MDA- MB-231 cells, improved the cytotoxicity of CD4+T cells, and increased the apoptosis of MDA-MB-231 cells.

CONCLUSION

Overall, increased CD4+T cell cytotoxicity and tumor cells apoptosis under the influence of shPD-1, confirmed the effectiveness of shPD-1 as a natural blocker of PD-L1and as an augmenter of the anti-tumorimmune responses.

Aging boosts antiviral CD8+T cell memory through improved engagement of diversified recall response determinants

The determinants of protective CD8+ memory T cell (CD8+TM) immunity remain incompletely defined and may in fact constitute an evolving agency as aging CD8+TM progressively acquire enhanced rather than impaired recall capacities. Here, we show that old as compared to young antiviral CD8+TM more effectively harness disparate molecular processes (cytokine signaling, trafficking, effector functions, and co-stimulation/inhibition) that in concert confer greater secondary reactivity. The relative reliance on these pathways is contingent on the nature of the secondary challenge (greater for chronic than acute viral infections) and over time, aging CD8+TM re-establish a dependence on the same accessory signals required for effective priming of naïve CD8+T cells in the first place. Thus, our findings reveal a temporal regulation of complementary recall response determinants that is consistent with the recently proposed "rebound model" according to which aging CD8+TM properties are gradually aligned with those of naïve CD8+T cells; our identification of a broadly diversified collection of immunomodulatory targets may further provide a foundation for the potential therapeutic "tuning" of CD8+TM immunity.

Distinct and complementary roles of CD4 T cells in protective immunity to influenza virus

CD4 T cells play a multiplicity of roles in protective immunity to influenza. Included in these functions are help for high affinity antibody production, enhancement of CD8 T cell expansion, function and memory, acceleration of the early innate response to infection and direct cytotoxicity. The influenza-specific CD4 T cell repertoire in humans established through exposures to infection and vaccination has been found to be highly variable in abundance, specificity and functionality. Deficits in particular subsets of CD4 T cells recruited into the response result in diminished antibody responses and protection from infection. Therefore, improved strategies for vaccination should include better methods to identify deficiencies in the circulating CD4 T cell repertoire, and vaccine constructs that increase the representation of CD4 T cells of the correct specificity and functionality.

Protection by universal influenza vaccine is mediated by memory CD4 T cells

There is a diverse array of influenza viruses which circulate between different species, reassort and drift over time. Current seasonal influenza vaccines are ineffective in controlling these viruses. We have developed a novel universal vaccine which elicits robust T cell responses and protection against diverse influenza viruses in mouse and human models. Vaccine mediated protection was dependent on influenza-specific CD4⁺ T cells, whereby depletion of CD4⁺ T cells at either vaccination or challenge time points significantly reduced survival in mice. Vaccine memory CD4⁺ T cells were needed for early antibody production and CD8⁺ T cell recall responses. Furthermore, influenza-specific CD4⁺ T cells from vaccination manifested primarily Tfh and Th1 profiles with anti-viral cytokine production. The vaccine boosted H5-specific T cells from human PBMCs, specifically CD4⁺ and CD8⁺ T effector memory type, ensuring the vaccine was truly universal for its future application. These findings have implications for the development and optimization of T cell activating vaccines for universal immunity against influenza.

Many Th Cell Subsets Have Fas Ligand-Dependent Cytotoxic Potential

CD4⁺ Th cells can have cytotoxic activity against cells displaying relevant peptide-MHC class II (p:MHCII) ligands. Cytotoxicity may be a property of Th1 cells and depends on perforin and the Eomes transcription factor. We assessed these assertions for polyclonal p:MHCII-specific CD4⁺ T cells activated in vivo in different contexts. Mice immunized with an immunogenic peptide in adjuvant or infected with lymphocytic choriomeningitis virus or Listeria monocytogenes bacteria induced cytotoxic Th cells that killed B cells displaying relevant p:MHCII complexes. Cytotoxicity was dependent on Fas expression by target cells but was independent of Eomes or perforin expression by T cells. Although the priming regimens induced different proportions of Th1, Th17, regulatory T cells, and T follicular helper cells, the T cells expressed Fas ligand in all cases. Reciprocally, Fas was upregulated on target cells in a p:MHCII-specific manner. These results indicate that many Th subsets have cytotoxic potential that is enhanced by cognate induction of Fas on target cells.

Fighting Viral Infections and Virus-Driven Tumors with Cytotoxic CD4+ T Cells

CD4+ T cells have been and are still largely regarded as the orchestrators of immune responses, being able to differentiate into distinct T helper cell populations based on differentiation signals, transcription factor expression, cytokine secretion, and specific functions. Nonetheless, a growing body of evidence indicates that CD4+ T cells can also exert a direct effector activity, which depends on intrinsic cytotoxic properties acquired and carried out along with the evolution of several pathogenic infections. The relevant role of CD4+ T cell lytic features in the control of such infectious conditions also leads to their exploitation as a new immunotherapeutic approach. This review aims at summarizing currently available data about functional and therapeutic relevance of cytotoxic CD4+ T cells in the context of viral infections and virus-driven tumors.

Cytotoxic CD4 T Cells-Friend or Foe during Viral Infection?

CD4 T cells with cytotoxic function were once thought to be an artifact due to long-term in vitro cultures but have in more recent years become accepted and reported in the literature in response to a number of viral infections. In this review, we focus on cytotoxic CD4 T cells in the context of human viral infections and in some infections that affect mice and non-human primates. We examine the effector mechanisms used by cytotoxic CD4 cells, the phenotypes that describe this population, and the transcription factors and pathways that lead to their induction following infection. We further consider the cells that are the predominant targets of this effector subset and describe the viral infections in which CD4 cytotoxic T lymphocytes have been shown to play a protective or pathologic role. Cytotoxic CD4 T cells are detected in the circulation at much higher levels than previously realized and are now recognized to have an important role in the immune response to viral infections.

Aging promotes acquisition of naive-like CD8+ memory T cell traits and enhanced functionalities

Protective T cell memory is an acquired trait that is contingent upon the preservation of its constituents and therefore vulnerable to the potentially deleterious effects of organismal aging. Here, however, we have found that long-term T cell memory in a natural murine host-pathogen system can substantially improve over time. Comprehensive molecular, phenotypic, and functional profiling of aging antiviral CD8+ memory T cells (CD8+ TM) revealed a pervasive remodeling process that promotes the gradual acquisition of distinct molecular signatures, of increasingly homogeneous phenotypes, and of diversified functionalities that combine to confer a CD8+ TM-autonomous capacity for enhanced recall responses and immune protection. Notably, the process of CD8+ TM aging is characterized by a progressive harmonization of memory and naive T cell traits, is broadly amenable to experimental acceleration or retardation, and serves as a constitutional component for the "rebound model" of memory T cell maturation. By casting CD8+ TM populations within the temporal framework of their slowly evolving properties, this model establishes a simple ontogenetic perspective on the principal organization of CD8+ T cell memory that may directly inform the development of improved diagnostic, prophylactic, and therapeutic modalities.

The Differentiation and Protective Function of Cytolytic CD4 T Cells in Influenza Infection

CD4 T cells that recognize peptide antigen in the context of class II MHC can differentiate into various subsets that are characterized by their helper functions. However, increasing evidence indicates that CD4 cells with direct cytolytic activity (CD4 CTL) play a role in chronic as well as acute infections, such as influenza A virus (IAV) infection. In the last couple of decades, techniques to measure the frequency and activity of these cytolytic cells has demonstrated their abundance in infections, such as human immunodeficiency virus, mouse pox, murine gamma herpes virus, cytomegalovirus, Epstein-Barr virus, and influenza among others. We now appreciate a greater role for CD4 CTL as direct effectors in viral infections and antitumor immunity through their ability to acquire perforin-mediated cytolytic activity and contribution to lysis of virally infected targets or tumors. As early as the 1980s, CD4 T cell clones with cytolytic potential were identified after influenza virus infection, yet much of this early work was dependent on in vitro culture and little was known about the physiological relevance of CD4 CTL. Here, we discuss the direct role CD4 CTL play in protection against lethal IAV infection and the factors that drive the generation of perforin-mediated lytic activity in CD4 cells in vivo during IAV infection. While focusing on CD4 CTL generated during IAV infection, we pull comparisons from the literature in other antiviral and antitumor systems. Further, we highlight what is currently known about CD4 CTL secondary and memory responses, as well as vaccination strategies to induce these potent killer cells that provide an extra layer of cell-mediated immune protection against heterosubtypic IAV infection.

Cytomegalovirus-Specific CD4 T Cells Are Cytolytic and Mediate Vaccine Protection

CD4 T cells provide protection against cytomegalovirus (CMV) and other persistent viruses, and the ability to quantify and characterize epitope-specific responses is essential to gain a more precise understanding of their effector roles in this regard. Here, we report the first two I-A(d)-restricted CD4 T cell responses specific for mouse CMV (MCMV) epitopes and use a major histocompatibility complex class II (MHC-II) tetramer to characterize their phenotypes and functions. We demonstrate that MCMV-specific CD4 T cells can express high levels of granzyme B and kill target cells in an epitope- and organ-specific manner. In addition, CD4 T cell epitope vaccination of immunocompetent mice reduced MCMV replication in the same organs where CD4 cytotoxic T lymphocyte (CTL) activity was observed. Together, our studies show that MCMV epitope-specific CD4 T cells have the potential to mediate antiviral defense by multiple effector mechanisms in vivo.

IMPORTANCE

CD4 T cells mediate immune protection by using their T cell receptors to recognize specific portions of viral proteins, called epitopes, that are presented by major histocompatibility complex class II (MHC-II) molecules on the surfaces of professional antigen-presenting cells (APCs). In this study, we discovered the first two epitopes derived from mouse cytomegalovirus (MCMV) that are recognized by CD4 T cells in BALB/c mice, a mouse strain commonly used to study the pathogenesis of this virus infection. Here, we report the sequences of these epitopes, characterize the CD4 T cells that recognize them to fight off MCMV infection, and show that we can use the epitopes to vaccinate mice and protect against MCMV.

Vaccination Produces CD4 T Cells with a Novel CD154-CD40-Dependent Cytolytic Mechanism

The discovery of new vaccines against infectious diseases and cancer requires the development of novel adjuvants with well-defined activities. The TLR4 agonist adjuvant GLA-SE elicits robust Th1 responses to a variety of vaccine Ags and is in clinical development for both infectious diseases and cancer. We demonstrate that immunization with a recombinant protein Ag and GLA-SE also induces granzyme A expression in CD4 T cells and produces cytolytic cells that can be detected in vivo. Surprisingly, these in vivo CTLs were CD4 T cells, not CD8 T cells, and this cytolytic activity was not dependent on granzyme A/B or perforin. Unlike previously reported CD4 CTLs, the transcription factors Tbet and Eomes were not necessary for their development. CTL activity was also independent of the Fas ligand-Fas, TRAIL-DR5, and canonical death pathways, indicating a novel mechanism of CTL activity. Rather, the in vivo CD4 CTL activity induced by vaccination required T cell expression of CD154 (CD40L) and target cell expression of CD40. Thus, vaccination with a TLR4 agonist adjuvant induces CD4 CTLs, which kill through a previously unknown CD154-dependent mechanism.

CD8⁺ T Cell-Independent Immune-Mediated Mechanisms of Anti-Tumor Activity

Despite the growing number of preclinical and clinical trials focused on immunotherapy for the treatment of malignant gliomas, the prognosis for this disease remains grim. Cancer immunotherapy seeks to recruit an effective immune response to eliminate tumor cells. To date, cancer vaccines have shown only limited effectiveness because of our incomplete understanding of the necessary effector cells and mechanisms that yield efficient tumor clearance. CD8+ T cell cytotoxic activity has long been proposed as the primary effector function necessary for tumor regression. However, there is increasing evidence that indicates that components of the immune system other than CD8+ T cells play important roles in tumor eradication and control. The following review should provide an understanding of the mechanisms involved in an effective antitumor response to guide future therapeutic designs. The information provided suggests an alternate means of effective tumor clearance in malignant glioma to the canonical CD8+ cytotoxic T cell mechanism.

Granzyme B secretion by human memory CD4 T cells is less strictly regulated compared to memory CD8 T cells

Background

Granzyme B (GrzB) is a serine proteinase expressed by memory T cells and NK cells. Methods to measure GrzB protein usually involve intracellular (flow cytometry) and extracellular (ELISA and ELISpot) assays. CD8 T cells are the main source of GrzB during immunological reactions, but activated CD4 T cells deploy GrzB as well. Because GrzB is an important mediator of cell death, tissue pathology and disease, clarification of differences of GrzB expression and secretion between CD4 and CD8 T cells is important for understanding effector functions of these cells.

Results

Memory CD4 and memory CD8 T cells were purified from human peripheral blood of healthy donors, and production of GrzB was directly compared between memory CD4 and memory CD8 T cells from the same donors using parallel measurements of flow cytometry (intracellular GrzB), ELISpot (single cell secretion of GrzB), and ELISA (bulk extracellular GrzB). Memory CD8 T cells constitutively stored significantly more GrzB protein (~25%) compared to memory CD4 T cells as determined by flow cytometry (~3%), and this difference remained stable after 24 hrs of activation. However, measurement of extracellular GrzB by ELISA revealed that activated memory CD4 T cells secrete similar amounts of GrzB (~1,000 pg/ml by 1x10⁵ cells/200 μl medium) compared to memory CD8 T cells (~600 pg/ml). Measurement of individual GrzB-secreting cells by ELISpot also indicated that similar numbers of activated memory CD4 (~170/1x10⁵) and memory CD8 (~200/1x10⁵) T cells secreted GrzB. Expression of CD107a further indicated that Grzb is secreted similarly by activated CD4 and CD8 T cells, consistent with the ELISA and ELISpot results. However, memory CD8 T cells expressed and secreted more perforin compared to memory CD4 T cells, suggesting that perforin may be less associated with GrzB function for memory CD4 T cells.

Conclusions

Although measurement of intracellular GrzB by flow cytometry suggests that a larger proportion of CD8 T cells have higher capacity for GrzB production compared to CD4 T cells, ELISpot and ELISA show that similar numbers of activated CD4 and CD8 T cells secrete similar amounts of GrzB. Secretion of GrzB by activated CD8 T cells may be more tightly controlled compared to CD4 T cells.

HIV replication in conjunction with granzyme B production by CCR5+ memory CD4 T cells: Implications for bystander cell and tissue pathologies

Granzyme B (GrzB) is expressed by activated T cells and mediates cellular apoptosis. GrzB also acts as an extracellular protease involved in tissue degradation. We hypothesized that GrzB production from activated memory CD4 T cells may be associated with HIV pathogenesis. We found that stimulated memory CD4 T cells (via costimulation, cytokines, and TLR ligands) concomitantly produced GrzB and HIV. Both GrzB and HIV expression were mainly restricted to CCR5-expressing memory CD4+CD45RO+ T cells, including Th1 and Th17 subsets. Activated memory CD4 T cells also mediated tissue damage, such as disruption of intestinal epithelial monolayers. In non-human primates, CD4 T cells of rhesus macaques (pathogenic SIV hosts) expressed higher GrzB compared to African green monkeys (non-pathogenic SIV hosts). These results suggest that GrzB from CCR5+ memory CD4 T cells may have a role in cellular and tissue pathologies during HIV infection.

Inflammation enhances IL-2 driven differentiation of cytolytic CD4 T cells

Cytolytic CD4 T cells (CD4 CTL) have been identified in vivo in response to viral infections; however, the factors necessary for driving the cytolytic phenotype have not been fully elucidated. Our previously published work suggests IL-2 may be the master regulator of perforin-mediated cytotoxicity in CD4 effectors. To further dissect the role of IL-2 in CD4 CTL generation, T cell receptor transgenic mice deficient in the ability to produce IL-2 or the high affinity IL-2 receptor (IL-2Rα, CD25) were used. Increasing concentrations of IL-2 were necessary to drive perforin (Prf) expression and maximal cytotoxicity. Granzyme B (GrB) expression and killing correlated with STAT5 activation and CD25 expression in vitro, suggesting that signaling through the high affinity IL-2R is critical for full cytotoxicity. IL-2 signaling was also necessary in vivo for inducing the Th1 phenotype and IFN-γ expression in CD4 T cells during influenza A (IAV) infection. In addition, GrB expression, as measured by mean fluorescent intensity, was decreased in CD25 deficient cells; however, the frequency of CD4 cells expressing GrB was unchanged. Similarly, analysis of cytolytic markers such as CD107a/b and Eomesodermin indicate high IL-2Rα expression is not necessary to drive the CD4 CTL phenotype during IAV infection. Thus, inflammatory signals induced by viral infection may overcome the need for strong IL-2 signals in driving cytotoxicity in CD4 cells.

Cell-based flow cytometry assay to measure cytotoxic activity

Cytolytic activity of CD8+ T cells is rarely evaluated. We describe here a new cell-based assay to measure the capacity of antigen-specific CD8+ T cells to kill CD4+ T cells loaded with their cognate peptide. Target CD4+ T cells are divided into two populations, labeled with two different concentrations of CFSE. One population is pulsed with the peptide of interest (CFSE-low) while the other remains un-pulsed (CFSE-high). Pulsed and un-pulsed CD4+ T cells are mixed at an equal ratio and incubated with an increasing number of purified CD8+ T cells. The specific killing of autologous target CD4+ T cells is analyzed by flow cytometry after coculture with CD8+ T cells containing the antigen-specific effector CD8+ T cells detected by peptide/MHCI tetramer staining. The specific lysis of target CD4+ T cells measured at different effector versus target ratios, allows for the calculation of lytic units, LU₃₀/10(6) cells. This simple and straightforward assay allows for the accurate measurement of the intrinsic capacity of CD8+ T cells to kill target CD4+ T cells.