Unique transcriptional profile of liver-resident memory CD8+ T cells induced by immunization with malaria sporozoites

Tissue adaptation of CD4 T lymphocytes in homeostasis and cancer

The immune system is traditionally classified as a defense system that can discriminate between self and non-self or dangerous and non-dangerous situations, unleashing a tolerogenic reaction or immune response. These activities are mainly coordinated by the interaction between innate and adaptive cells that act together to eliminate harmful stimuli and keep tissue healthy. However, healthy tissue is not always the end point of an immune response. Much evidence has been accumulated over the years, showing that the immune system has complex, diversified, and integrated functions that converge to maintaining tissue homeostasis, even in the absence of aggression, interacting with the tissue cells and allowing the functional maintenance of that tissue. One of the main cells known for their function in helping the immune response through the production of cytokines is CD4+ T lymphocytes. The cytokines produced by the different subtypes act not only on immune cells but also on tissue cells. Considering that tissues have specific mediators in their architecture, it is plausible that the presence and frequency of CD4+ T lymphocytes of specific subtypes (Th1, Th2, Th17, and others) maintain tissue homeostasis. In situations where homeostasis is disrupted, such as infections, allergies, inflammatory processes, and cancer, local CD4+ T lymphocytes respond to this disruption and, as in the healthy tissue, towards the equilibrium of tissue dynamics. CD4+ T lymphocytes can be manipulated by tumor cells to promote tumor development and metastasis, making them a prognostic factor in various types of cancer. Therefore, understanding the function of tissue-specific CD4+ T lymphocytes is essential in developing new strategies for treating tissue-specific diseases, as occurs in cancer. In this context, this article reviews the evidence for this hypothesis regarding the phenotypes and functions of CD4+ T lymphocytes and compares their contribution to maintaining tissue homeostasis in different organs in a steady state and during tumor progression.

Accelerated prime-and-trap vaccine regimen in mice using repRNA-based CSP malaria vaccine

Malaria, caused by Plasmodium parasites, remains one of the most devastating infectious diseases worldwide, despite control efforts to lower morbidity and mortality. Both advanced candidate vaccines, RTS,S and R21, are subunit (SU) vaccines that target a single Plasmodium falciparum (Pf) pre-erythrocytic (PE) sporozoite (spz) surface protein known as circumsporozoite (CS). These vaccines induce humoral immunity but fail to elicit CD8 + T-cell responses sufficient for long-term protection. In contrast, whole-organism (WO) vaccines, such as Radiation Attenuated Sporozoites (RAS), achieved sterile protection but require a series of intravenous doses administered in multiple clinic visits. Moreover, these WO vaccines must be produced in mosquitos, a burdensome process that severely limits their availability. To reduce reliance on WO while maintaining protection via both antibodies and Trm responses, we have developed an accelerated vaccination regimen that combines two distinct agents in a prime-and-trap strategy. The priming dose is a single dose of self-replicating RNA encoding the full-length P. yoelii CS protein, delivered via an advanced cationic nanocarrier (LIONTM). The trapping dose consists of one dose of WO RAS. Our vaccine induces a strong immune response when administered in an accelerated regimen, i.e., either 5-day or same-day immunization. Additionally, mice after same-day immunization showed a 2-day delay of blood patency with 90% sterile protection against a 3-week spz challenge. The same-day regimen also induced durable 70% sterile protection against a 2-month spz challenge. Our approach presents a clear path to late-stage preclinical and clinical testing of dose-sparing, same-day regimens that can confer sterilizing protection against malaria.

Memory CD8+ T cell-mediated protection against liver-stage malaria

Nearly half of the world's population is at risk of malaria, a disease caused by the protozoan parasite Plasmodium, which is estimated to cause more than 240,000,000 infections and kill more than 600,000 people annually. The emergence of Plasmodia resistant to chemoprophylactic treatment highlights the urgency to develop more effective vaccines. In this regard, whole sporozoite vaccination approaches in murine models and human challenge studies have provided substantial insight into the immune correlates of protection from malaria. From these studies, CD8+ T cells have come to the forefront, being identified as critical for vaccine-mediated liver-stage immunity that can prevent the establishment of the symptomatic blood stages and subsequent transmission of infection. However, the unique biological characteristics required for CD8+ T cell protection from liver-stage malaria dictate that more work must be done to design effective vaccines. In this review, we will highlight a subset of studies that reveal basic aspects of memory CD8+ T cell-mediated protection from liver-stage malaria infection.

Accelerated prime-and-trap vaccine regimen in mice using repRNA-based CSP malaria vaccine

Malaria, caused by Plasmodium parasites, remains one of the most devastating infectious diseases worldwide, despite control efforts that have lowered morbidity and mortality. The only P. falciparum vaccine candidates to show field efficacy are those targeting the asymptomatic pre-erythrocytic (PE) stages of infection. The subunit (SU) RTS,S/AS01 vaccine, the only licensed malaria vaccine to date, is only modestly effective against clinical malaria. Both RTS,S/AS01 and the SU R21 vaccine candidate target the PE sporozoite (spz) circumsporozoite (CS) protein. These candidates elicit high-titer antibodies that provide short-term protection from disease, but do not induce the liver-resident memory CD8+ T cells (Trm) that confer strong PE immunity and long-term protection. In contrast, whole-organism (WO) vaccines, employing for example radiation-attenuated spz (RAS), elicit both high antibody titers and Trm, and have achieved high levels of sterilizing protection. However, they require multiple intravenous (IV) doses, which must be administered at intervals of several weeks, complicating mass administration in the field. Moreover, the quantities of spz required present production difficulties. To reduce reliance on WO while maintaining protection via both antibodies and Trm responses, we have developed an accelerated vaccination regimen that combines two distinct agents in a prime-and-trap strategy. While the priming dose is a self-replicating RNA encoding P. yoelii CS protein, delivered via an advanced cationic nanocarrier (LION™), the trapping dose consists of WO RAS. This accelerated regime confers sterile protection in the P. yoelii mouse model of malaria. Our approach presents a clear path to late-stage preclinical and clinical testing of dose-sparing, same-day regimens that can confer sterilizing protection against malaria.

Accelerated prime-and-trap vaccine regimen in mice using repRNA-based CSP malaria vaccine

Malaria, caused by Plasmodium parasites, remains one of the most devastating infectious diseases worldwide, despite control efforts that have lowered morbidity and mortality. The only P. falciparum vaccine candidates to show field efficacy are those targeting the asymptomatic pre-erythrocytic (PE) stages of infection. The subunit (SU) RTS,S/AS01 vaccine, the only licensed malaria vaccine to date, is only modestly effective against clinical malaria. Both RTS,S/AS01 and the SU R21 vaccine candidate target the PE sporozoite (spz) circumsporozoite (CS) protein. These candidates elicit high-titer antibodies that provide short-term protection from disease, but do not induce the liver-resident memory CD8+ T cells (Trm) that confer strong PE immunity and long-term protection. In contrast, whole-organism (WO) vaccines, employing for example radiation-attenuated spz (RAS), elicit both high antibody titers and Trm, and have achieved high levels of sterilizing protection. However, they require multiple intravenous (IV) doses, which must be administered at intervals of several weeks, complicating mass administration in the field. Moreover, the quantities of spz required present production difficulties. To reduce reliance on WO while maintaining protection via both antibodies and Trm responses, we have developed an accelerated vaccination regimen that combines two distinct agents in a prime-and-trap strategy. While the priming dose is a self-replicating RNA encoding P. yoelii CS protein, delivered via an advanced cationic nanocarrier (LION™), the trapping dose consists of WO RAS. This accelerated regime confers sterile protection in the P. yoelii mouse model of malaria. Our approach presents a clear path to late-stage preclinical and clinical testing of dose-sparing, same-day regimens that can confer sterilizing protection against malaria.

Complexing CpG adjuvants with cationic liposomes enhances vaccine-induced formation of liver TRM cells

Tissue resident memory T cells (T_RM cells) can provide effective tissue surveillance and can respond rapidly to infection. Vaccination strategies aimed at generating T_RM cells have shown promise against a range of pathogens. We have previously shown that the choice of adjuvant critically influences CD8⁺ T_RM cell formation in the liver. However, the range of adjuvants tested was limited. Here, we assessed the ability of a broad range of adjuvants stimulating membrane (TLR4), endosomal (TLR3, TLR7 and TLR9) and cytosolic (cGAS, RIG-I) pathogen recognition receptors for their capacity to induce CD8⁺ T_RM formation in a subunit vaccination model. We show that CpG oligodeoxynucleotides (ODN) remain the most efficient inducers of liver T_RM cells among all adjuvants tested. Moreover, their combination with the cationic liposome DOTAP further enhances the potency, particularly of the class B ODN CpG 1668 and the human TLR9 ligand CpG 2006 (CpG 7909). This study informs the design of efficient liver T_RM-based vaccines for their potential translation.

Sporozoite immunization: innovative translational science to support the fight against malaria

INTRODUCTION

Malaria, a devastating febrile illness caused by protozoan parasites, sickened 247,000,000 people in 2021 and killed 619,000, mostly children and pregnant women in sub-Saharan Africa. A highly effective vaccine is urgently needed, especially for Plasmodium falciparum (Pf), the deadliest human malaria parasite.

AREAS COVERED

Sporozoites (SPZ), the parasite stage transmitted by Anopheles mosquitoes to humans, are the only vaccine immunogen achieving >90% efficacy against Pf infection. This review describes >30 clinical trials of PfSPZ vaccines in the U.S.A., Europe, Africa, and Asia, based on first-hand knowledge of the trials and PubMed searches of 'sporozoites,' 'malaria,' and 'vaccines.'

EXPERT OPINION

First generation (radiation-attenuated) PfSPZ vaccines are safe, well tolerated, 80-100% efficacious against homologous controlled human malaria infection (CHMI) and provide 18-19 months protection without boosting in Africa. Second generation chemo-attenuated PfSPZ are more potent, 100% efficacious against stringent heterologous (variant strain) CHMI, but require a co-administered drug, raising safety concerns. Third generation, late liver stage-arresting, replication competent (LARC), genetically-attenuated PfSPZ are expected to be both safe and highly efficacious. Overall, PfSPZ vaccines meet safety, tolerability, and efficacy requirements for protecting pregnant women and travelers exposed to Pf in Africa, with licensure for these populations possible within 5 years. Protecting children and mass vaccination programs to block transmission and eliminate malaria are long-term objectives.

Secondary infections rejuvenate the intestinal CD103 + tissue-resident memory T cell pool

Resident T lymphocytes (T RM ) protect tissues during pathogen reexposure. Although T RM phenotype and restricted migratory pattern are established, we have a limited understanding of their response kinetics, stability, and turnover during reinfections. Such characterizations have been restricted by the absence of in vivo fate-mapping systems. We generated two mouse models, one to stably mark CD103 + T cells (a marker of T RM cells) and the other to specifically deplete CD103 − T cells. Using these models, we observed that intestinal CD103 + T cells became activated during viral or bacterial reinfection, remained organ-confined, and retained their original phenotype but failed to reexpand. Instead, the population was largely rejuvenated by CD103 + T cells formed de novo during reinfections. This pattern remained unchanged upon deletion of antigen-specific circulating T cells, indicating that the lack of expansion was not due to competition with circulating subsets. Thus, although intestinal CD103 + resident T cells survived long term without antigen, they lacked the ability of classical memory T cells to reexpand. This indicated that CD103 + T cell populations could not autonomously maintain themselves. Instead, their numbers were sustained during reinfection via de novo formation from CD103 − precursors. Moreover, in contrast to CD103 - cells, which require antigen plus inflammation for their activation, CD103 + T RM became fully activated follwing exposure to inflammation alone. Together, our data indicate that primary CD103 + resident memory T cells lack secondary expansion potential and require CD103 − precursors for their long-term maintenance.

Tissue-resident memory T cells in chronic liver diseases: Phenotype, development and function

Tissue-resident memory (T_RM) T cells are a unique subset of memory T cells that are critical for the first line of defense against pathogens or antigens in peripheral non-lymphoid tissues such as liver, gut, and skin. Generally, T_RM cells are well adapted to the local environment in a tissue-specific manner and typically do not circulate but persist in tissues, distinguishing them from other memory T cell lineages. There is strong evidence that liver T_RM cells provide a robust adaptive immune response to potential threats. Indeed, the potent effector function of hepatic T_RM cells makes it essential for chronic liver diseases, including viral and parasite infection, autoimmune liver diseases (AILD), nonalcoholic fatty liver disease (NAFLD), hepatocellular carcinoma (HCC) and liver transplantation. Manipulation of hepatic T_RM cells might provide novel promising strategies for precision immunotherapy of chronic liver diseases. Here, we provide insights into the phenotype of hepatic T_RM cells through surface markers, transcriptional profiles and effector functions, discuss the development of hepatic T_RM cells in terms of cellular origin and factors affecting their development, analyze the role of hepatic T_RM cells in chronic liver diseases, as well as share our perspectives on the current status of hepatic T_RM cell research.

Tissue-specific differentiation of CD8+ resident memory T cells

CD8⁺ tissue-resident memory T (T_RM) cells play crucial roles in defense against infections and cancer and have been implicated in autoimmune diseases such as psoriasis. In mice and humans, they exist in all nonlymphoid organs and share key characteristics across all tissues, including downregulation of tissue egress and lymph node homing pathways. However, recent studies demonstrate considerable heterogeneity across T_RM cells lodged in different tissues - linked to the activity of tissue-specific molecules, including chemokines, cytokines, and transcription factors. Current work indicates that transforming growth factor (TGF)-β plays a major role in generating T_RM heterogeneity at phenotypic and functional levels. Here, we review common and unique features of T_RM cells in different tissues and discuss putative strategies aimed at harnessing T_RM cells for site-specific protection against infectious and malignant diseases.

Mapping the spatial distribution of T cells in repertoire dimension

T cells mediate adaptive immunity in diverse anatomic compartments through recognition of specific antigens via unique T cell receptor (TCR) structures. However, little is known about the spatial distribution of an organism's TCR repertoire. Here, using high-throughput TCR sequencing (TCRseq), we investigated the TCR repertoires of sixteen tissues in healthy C57B/L6 mice. We found that TCR repertoires generally classified into three categories (lymph nodes, non-lymph node tissues and small intestine) based on sequence similarity. Clonal distribution and diversity analyses showed that small intestine compartment had a more skewed repertoire as compared to lymph nodes and non-lymph node tissues. However, analysis of TRBV and TRBJ gene usage across tissue compartments, as well as comparison of CDR3 length distributions, showed no significant tissue-dependent differences. Interestingly, analysis of clonotype sharing between mice showed that although non-redundant public clonotypes were found more easily in lymph nodes, small intestinal CD4 + T cells harbored more abundant public clonotypes. These findings under healthy physiological conditions offer an important reference dataset, which may contribute to our ability to better manipulate T cell responses against infection and vaccination.

IMRAS-Immunization with radiation-attenuated Plasmodium falciparum sporozoites by mosquito bite: Cellular immunity to sporozoites, CSP, AMA1, TRAP and CelTOS

BACKGROUND

Immunization with radiation-attenuated sporozoites (RAS) by mosquito bites provides >90% sterile protection against Plasmodium falciparum malaria in humans. We conducted a clinical trial based on data from previous RAS clinical trials that suggested that 800-1200 infected bites should induce ~50% protective vaccine efficacy (VE) against controlled human malaria infection (CHMI) administered three weeks after the final immunization. Two cohorts were immunized separately. VE was 55% in Cohort 1 but 90% in Cohort 2, the cohort that received a higher first dose and a reduced (fractional) fifth dose. Immune responses were better boosted by the fractional fifth dose in Cohort 2 and suggested the importance of the fractional fifth dose for increased protection in Cohort 2 responses. Three protected subjects were later boosted and were protected suggesting that protection could be extended to at least 67 weeks.

METHODS

The ex vivo FluoroSpot assay was used to measure peripheral IFN-γ, IL2, and IFN-γ+IL2 responses to PfNF54 sporozoites and malaria antigens CSP, AMA1, TRAP, and CelTOS using pools of synthetic overlapping 15mer peptides spanning each antigen.

RESULTS

There was no correlation between IFN-γ, IL2, and IFN-γ+IL2 responses to sporozoites and protection, but fold-increases between post-4th and post-5th responses greater than 1.0 occurred mostly in protected subjects. IFN-γ and IL2 responses to TRAP, CelTOS and CSP occurred only in protected subjects. Peripheral IFN-γ, IL2, and IFN-γ+IL2 responses were short-lived and low by 27 weeks post-CHMI but were restored by boosting.

CONCLUSIONS

These studies highlight the importance of vaccine dose and schedule for vaccine efficacy, and suggest that CSP, TRAP, AMA1 and CelTOS may be targets of protective immunity. The correlation between fold-increases in responses and protection should be explored in other vaccine trials.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01994525.

Chemoprophylaxis vaccination with a Plasmodium liver stage autophagy mutant affords enhanced and long-lasting protection

Genetically attenuated sporozoite vaccines can elicit long-lasting protection against malaria but pose risks of breakthrough infection. Chemoprophylaxis vaccination (CVac) has proven to be the most effective vaccine strategy against malaria. Here, we demonstrate that a liver stage-specific autophagy mutant of Plasmodium berghei (ATG8 overexpressor), when used as a live vaccine under a CVac regimen, provides superior long-lasting protection, in both inbred and outbred mice, as compared to WT-CVac. Uniquely, the protection elicited by this mutant is predominantly dependent on a CD8⁺ T-cell response through an IFN-γ-independent mechanism and is associated with a stable population of antigen-experienced CD8⁺ T cells. Jointly, our findings support the exploitation of liver-stage mutants as vaccines under a CVac protocol. This vaccination strategy is also a powerful model to study the mechanisms of protective immunity and discover new protective antigens.

Tissue-Resident Memory T Cells in Antifungal Immunity

Fungi are an integral part of the mammalian microbiota colonizing most if not all mucosal surfaces and the skin. Maintaining stable colonization on these surfaces is critical for preventing fungal dysbiosis and infection, which in some cases can lead to life threatening consequences. The epithelial barriers are protected by T cells and additional controlling immune mechanisms. Noncirculating memory T cells that reside stably in barrier tissues play an important role for host protection from commensals and recurrent pathogens due to their fast response and local activity, which provides them a strategic advantage. So far, only a few specific examples of tissue resident memory T cells (TRMs) that act against fungi have been reported. This review provides an overview of the characteristics and functional attributes of TRMs that have been established based on human and mouse studies with various microbes. It highlights what is currently known about fungi specific TRMs mediating immunosurveillance, how they have been targeted in preclinical vaccination approaches and how they can promote immunopathology, if not controlled. A better appreciation of the host protective and damaging roles of TRMs might accelerate the development of novel tissue specific preventive strategies against fungal infections and fungi-driven immunopathologies.

Glycolipid-peptide vaccination induces liver-resident memory CD8+ T cells that protect against rodent malaria

Liver resident-memory CD8⁺ T cells (T_RM cells) can kill liver-stage Plasmodium-infected cells and prevent malaria, but simple vaccines for generating this important immune population are lacking. Here, we report the development of a fully synthetic self-adjuvanting glycolipid-peptide conjugate vaccine designed to efficiently induce liver T_RM cells. Upon cleavage in vivo, the glycolipid-peptide conjugate vaccine releases an MHC I-restricted peptide epitope (to stimulate Plasmodium-specific CD8⁺ T cells) and an adjuvant component, the NKT cell agonist α-galactosylceramide (α-GalCer). A single dose of this vaccine in mice induced substantial numbers of intrahepatic malaria-specific CD8⁺ T cells expressing canonical markers of liver T_RM cells (CD69, CXCR6, and CD101), and these cells could be further increased in number upon vaccine boosting. We show that modifications to the peptide, such as addition of proteasomal-cleavage sequences or epitope-flanking sequences, or the use of alternative conjugation methods to link the peptide to the glycolipid improved liver T_RM cell generation and led to the development of a vaccine able to induce sterile protection in C57BL/6 mice against Plasmodium berghei sporozoite challenge after a single dose. Furthermore, this vaccine induced endogenous liver T_RM cells that were long-lived (half-life of ~425 days) and were able to maintain >90% sterile protection to day 200. Our findings describe an ideal synthetic vaccine platform for generating large numbers of liver T_RM cells for effective control of liver-stage malaria and, potentially, a variety of other hepatotropic infections.

Harnessing liver-resident memory T cells for protection against malaria

INTRODUCTION

Tissue-resident memory T cells (T_RM cells) are powerful mediators of protracted adaptive immunity to infection in peripheral organs. Harnessing T_RM cells through vaccination hence promises unprecedented potential for protection against infection. A paramount example of this is malaria, a major infectious disease for which immunity through traditional vaccination strategies remains challenging. Liver T_RM cells appear to be highly protective against malaria, and recent developments in our knowledge of the biology of these cells have defined promising, novel strategies for their induction.

AREAS COVERED

Here, we describe the path that led to the discovery of T_RM cells and discuss the importance of liver T_RM cells in immunity against Plasmodium spp. infection; we summarize current knowledge on T_RM cell biology and discuss the current state and potential of T_RM-based vaccination against malaria.

EXPERT OPINION

T_RM based vaccination has emerged as a promising means to achieve efficient protection against malaria. Recent advances provide a solid basis for continuing the development of this area of research. Deeper understanding of the mechanisms that mediate T_RM formation and maintenance and identification of immunogenic and protective target epitopes suitable for human vaccination remain the main challenges for translation of these discoveries.

Liver-Resident Memory CD8+ T Cells: Possible Roles in Chronic HBV Infection

Achieving a functional cure for chronic hepatitis B virus (HBV) infection or complete elimination of HBV covalently closed circular DNA (cccDNA) has been challenging in the treatment of patients with chronic HBV infection. Although novel antivirals are being investigated, improving HBV-specific adaptive immune responses is also important for durable viral clearance. Tissue-resident memory CD8⁺ T (T_RM) cells were recently reported as a T-cell population that resides in peripheral tissues and does not recirculate. T_RM cells have been studied in the livers of mice and humans. Liver T_RM cells have distinct characteristics compared to T cells in peripheral blood or other tissues, which may be associated with the unique microenvironment of the liver. In this review, we describe the characteristics of liver T_RM cells and their implications in chronic HBV infection. We emphasize that liver T_RM cells can be an immunotherapeutic target for the treatment of chronic HBV infection.

Targeting Skin-Resident Memory T Cells via Vaccination to Combat Staphylococcus aureus Infections

Tissue-resident memory T cells are important in adaptive immunity against many infections, rendering these cells attractive potential targets in vaccine development. Genetic and experimental evidence highlights the importance of cellular immunity in protection from Staphylococcus aureus skin infections, yet skin-resident memory T cells are, thus far, an untested component of immunity during such infections. Novel methods of generating and sampling vaccine-induced skin memory T cells are paralleled by discoveries of global, skin-wide immunosurveillance. We propose skin-resident memory CD4⁺ T cells as a potential missing link in the search for correlates of protection during S. aureus infections. A better appreciation of their phenotypes and functions could accelerate the development of preventive vaccines against this highly virulent and antibiotic-resistant pathogen.

Resident Memory T Cells and Their Role within the Liver

Immunological memory is fundamental to maintain immunity against re-invading pathogens. It is the basis for prolonged protection induced by vaccines and can be mediated by humoral or cellular responses-the latter largely mediated by T cells. Memory T cells belong to different subsets with specialized functions and distributions within the body. They can be broadly separated into circulating memory cells, which pace the entire body through the lymphatics and blood, and tissue-resident memory T (T_RM) cells, which are constrained to peripheral tissues. Retained in the tissues where they form, T_RM cells provide a frontline defense against reinfection. Here, we review this population of cells with specific attention to the liver, where T_RM cells have been found to protect against infections, in particular those by Plasmodium species that cause malaria.

Tissue-Resident Memory T Cells in the Liver-Unique Characteristics of Local Specialists

T cells play an important role to build up an effective immune response and are essential in the eradication of pathogens. To establish a long-lasting protection even after a re-challenge with the same pathogen, some T cells differentiate into memory T cells. Recently, a certain subpopulation of memory T cells at different tissue-sites of infection was detected-tissue-resident memory T cells (T_RM cells). These cells can patrol in the tissue in order to encounter their cognate antigen to establish an effective protection against secondary infection. The liver as an immunogenic organ is exposed to a variety of pathogens entering the liver through the systemic blood circulation or via the portal vein from the gut. It could be shown that intrahepatic T_RM cells can reside within the liver tissue for several years. Interestingly, hepatic T_RM cell differentiation requires a distinct cytokine milieu. In addition, T_RM cells express specific surface markers and transcription factors, which allow their identification delimited from their circulating counterparts. It could be demonstrated that liver T_RM cells play a particular role in many liver diseases such as hepatitis B and C infection, non-alcoholic fatty liver disease and even play a role in the development of hepatocellular carcinoma and in building long-lasting immune responses after vaccination. A better understanding of intrahepatic T_RM cells is critical to understand the pathophysiology of many liver diseases and to identify new potential drug targets for the development of novel treatment strategies.

Multidose Priming and Delayed Boosting Improve Plasmodium falciparum Sporozoite Vaccine Efficacy Against Heterologous P. falciparum Controlled Human Malaria Infection

Background

A live-attenuated Plasmodium falciparum sporozoite (SPZ) vaccine (PfSPZ Vaccine) has shown up to 100% protection against controlled human malaria infection (CHMI) using homologous parasites (same P. falciparum strain as in the vaccine). Using a more stringent CHMI, with heterologous parasites (different P. falciparum strain), we assessed the impact of higher PfSPZ doses, a novel multi-dose prime regimen, and a delayed vaccine boost upon vaccine efficacy (VE).

Methods

We immunized 4 groups that each contained 15 healthy, malaria-naive adults. Group 1 received 5 doses of 4.5 x 105 PfSPZ (Days 1, 3, 5, and 7; Week 16). Groups 2, 3, and 4 received 3 doses (Weeks 0, 8, and 16), with Group 2 receiving 9.0 × 105/doses; Group 3 receiving 18.0 × 105/doses; and Group 4 receiving 27.0 × 105 for dose 1 and 9.0 × 105 for doses 2 and 3. VE was assessed by heterologous CHMI after 12 or 24 weeks. Volunteers not protected at 12 weeks were boosted prior to repeat CHMI at 24 weeks.

Results

At 12-week CHMI, 6/15 (40%) participants in Group 1 (P = .04) and 3/15 (20%) participants in Group 2 remained aparasitemic, as compared to 0/8 controls. At 24-week CHMI, 3/13 (23%) participants in Group 3 and 3/14 (21%) participants in Group 4 remained aparasitemic, versus 0/8 controls (Groups 2–4, VE not significant). Postboost, 9/14 (64%) participants versus 0/8 controls remained aparasitemic (3/6 in Group 1, P = .025; 6/8 in Group 2, P = .002).

Conclusions

Administering 4 stacked priming injections (multi-dose priming) resulted in 40% VE against heterologous CHMI, while dose escalation of PfSPZ using single-dose priming was not significantly protective. Boosting unprotected subjects improved VE at 24 weeks, to 64%.

Clinical Trials Registration

NCT02601716.

A new era of rational malaria vaccine development

Holz et al. report a glycolipid-conjugate vaccine that provides sterile immunity in mice against Plasmodium berghei ANKA sporozoite challenge by inducing long-lasting tissue-resident memory (TRM) CD8+ T cells in the liver.

A Peptide-Based PD1 Antagonist Enhances T-Cell Priming and Efficacy of a Prophylactic Malaria Vaccine and Promotes Survival in a Lethal Malaria Model

The blockade of programmed cell death-1 (PD1) and its ligand PDL1 has been proven to be a successful immunotherapy against several cancers. Similar to cancer, PD1 contributes to the establishment of several chronic infectious diseases, including malaria. While monoclonal antibodies (mAbs) targeting checkpoint receptors are revolutionary in cancer treatment, the immune-related adverse events (irAEs) may prevent their utilization in prophylactic and therapeutic treatments of infectious diseases. The irAEs are, in part, due to the prolonged half-life of mAbs resulting in prolonged activation of the immune system. As an alternative modality to mAbs, peptides represent a viable option because they possess a shorter pharmacokinetic half-life and offer more formulation and delivery options. Here, we report on a 22-amino acid immunomodulatory peptide, LD01, derived from a Bacillus bacteria. When combined prophylactically with an adenovirus-based or irradiated sporozoite-based malaria vaccine, LD01 significantly enhanced antigen-specific CD8⁺ T cell expansion. Therapeutically, LD01 treatment of mice infected with a lethal malaria strain resulted in survival that was associated with lower numbers of FOXP3⁺Tbet⁺CD4⁺ regulatory T cells. Taken together, our results demonstrate that LD01 is a potent immunomodulator that acts upon the adaptive immune system to stimulate T cell responses both prophylactically and therapeutically.

Designer Parasites: Genetically Engineered Plasmodium as Vaccines To Prevent Malaria Infection

A highly efficacious malaria vaccine that prevents disease and breaks the cycle of infection remains an aspirational goal of medicine. Whole parasite vaccines based on the sporozoite forms of the parasite that target the clinically silent pre-erythrocytic stages of infection have emerged as one of the leading candidates. In animal models of malaria, these vaccines elicit potent neutralizing Ab responses against the sporozoite stage and cytotoxic T cells that eliminate parasite-infected hepatocytes. Among whole-sporozoite vaccines, immunization with live, replication-competent whole parasites engenders superior immunity and protection when compared with live replication-deficient sporozoites. As such, the genetic design of replication-competent vaccine strains holds the promise for a potent, broadly protective malaria vaccine. In this report, we will review the advances in whole-sporozoite vaccine development with a particular focus on genetically attenuated parasites both as malaria vaccine candidates and also as valuable tools to interrogate protective immunity against Plasmodium infection.

Innate immunity limits protective adaptive immune responses against pre-erythrocytic malaria parasites

Immunization with attenuated whole Plasmodium sporozoites constitutes a promising vaccination strategy. Compared to replication-deficient parasites, immunization with replication-competent parasites confers better protection and also induces a type I IFN (IFN-1) response, but whether this IFN-1 response has beneficial or adverse effects on vaccine-induced adaptive immunity is not known. Here, we show that IFN-1 signaling-deficient mice immunized with replication-competent sporozoites exhibit superior protection against infection. This correlates with superior CD8 T cell memory including reduced expression of the exhaustion markers PD-1 and LAG-3 on these cells and increased numbers of memory CD8 T cells in the liver. Moreover, the adoptive transfer of memory CD8 T cells from the livers of previously immunized IFN-1 signaling-deficient mice confers greater protection against liver stage parasites. However, the detrimental role of IFN-1 signaling is not CD8 T cell intrinsic. Together, our data demonstrate that liver stage-engendered IFN-1 signaling impairs hepatic CD8 T cell memory via a CD8 T cell-extrinsic mechanism.

Here, Minkah et al. show that, while immunization with replication-competent Plasmodium parasites can confer sterile protection against infection, it also induces a type I interferon response that adversely affects anti-malaria immunity by affecting numbers of protective hepatic CD8 T cells and CD8 T cell function.

The Roles of Liver-Resident Lymphocytes in Liver Diseases

Tissue-resident lymphocytes usually reside in barrier sites and are involved in innate and adaptive immunity. In recent years, many studies have shown that multiple types of lymphocytes are resident in the liver, including memory CD8⁺ T (T_RM) cells; "unconventional" T cells, such as invariant natural killer T (iNKT) cells, mucosal associated invariant T (MAIT) cells, and γδT cells; innate lymphoid cells (ILCs) such as natural killer (NK) cells and other ILCs. Although diverse types of tissue-resident lymphocytes share similar phenotypes, functional properties, and transcriptional regulation, the unique microenvironment of the liver can reshape their phenotypic and functional characteristics. Liver-resident lymphocytes serve as sentinels and perform immunosurveillance in response to infection and non-infectious insults, and are involved in the maintenance of liver homeostasis. Under the pathological conditions, distinct liver-resident lymphocytes exert protective or pathological effects in the process of various liver diseases. In this review, we highlight the unique properties of liver-resident lymphocytes, and discuss their functional characteristics in different liver diseases.

Parasite load stemming from immunization route determines the duration of liver-stage immunity

Immunization with radiation-attenuated Plasmodium sporozoites (RAS) induces sterile and long-lasting protective immunity. Although intravenous (IV) route of RAS immunization is reported to induce superior immunity compared to intradermal (ID) injection, its role in the maintenance of sterile immunity is yet to be understood. We investigated whether the route of homologous sporozoite challenge of Plasmodium berghei (Pb) RAS-immunized mice would influence the longevity of protection. C57BL/6 mice immunized with Pb-RAS by IV were 100% protected upon primary IV/ID sporozoite challenge. In contrast, ID immunization resulted in 80% protection, regardless of primary challenge route. Interestingly, the route of primary challenge was found to bring difference in the maintenance of sterile protection. While IV Pb RAS-immunized mice remained protected at all challenges regardless of the route of primary challenge, ID Pb-RAS-immunized mice receiving ID primary challenge became parasitaemic upon secondary IV challenge. Significantly, primary IV challenge of Pb RAS ID-immunized mice resulted in 80% and 50% survival at secondary and tertiary challenges, respectively. According to phenotypically diverse liver CD8⁺ T cells, the percentages and the numbers of both CD8⁺ T effector memory and resident memory cells were significantly higher in IV than in ID Pb RAS-immunized mice. IFN-γ-producing CD8⁺ T cells specific to Pb TRAP₁₃₀ and MIP-4-Kb-17 were also found significantly higher in IV mice than in ID mice. The enhanced T-cell generation and the longevity of protection appear to be dependent on the parasite load during challenge when infection is tolerated under suboptimal CD8⁺ T-cell response.

Vaccination With Sporozoites: Models and Correlates of Protection

Despite continuous efforts, the century-old goal of eradicating malaria still remains. Multiple control interventions need to be in place simultaneously to achieve this goal. In addition to effective control measures, drug therapies and insecticides, vaccines are critical to reduce mortality and morbidity. Hence, there are numerous studies investigating various malaria vaccine candidates. Most of the malaria vaccine candidates are subunit vaccines. However, they have shown limited efficacy in Phase II and III studies. To date, only whole parasite formulations have been shown to induce sterile immunity in human. In this article, we review and discuss the recent developments in vaccination with sporozoites and the mechanisms of protection involved.

Can Patrolling Liver-Resident T Cells Control Human Malaria Parasite Development?

Recently, a population of non-recirculating, tissue-resident memory CD8⁺ T cells has been identified; cells that seems to act as key sentinels for invading microorganisms with enhanced effector functions. In malaria, the liver represents the first site for parasite development before a definite infection is established in circulating red blood cells. Here, we discuss the evidence obtained from animal models on several diseases and hypothesize that liver-resident memory CD8⁺ T cells (hepatic T_RM) play a critical role in providing protective liver-stage immunity against Plasmodium malaria parasites. Although observations in human malaria trials are limited to peripheral blood, we propose recommendations for the translation of some of these findings to human malaria research.

Tissue-resident lymphocytes: from adaptive to innate immunity

Efficient immune responses against invading pathogens often involve coordination between cells from both the innate and adaptive immune systems. For multiple decades, it has been believed that CD8⁺ memory T cells and natural killer (NK) cells constantly and uniformly recirculate. Only recently was the existence of noncirculating memory T and NK cells that remain resident in the peripheral tissues, termed tissue-resident memory T (T_RM) cells and tissue-resident NK (trNK) cells, observed in various organs owing to improved techniques. T_RM cells populate a wide range of peripheral organs, including the skin, sensory ganglia, gut, lungs, brain, salivary glands, female reproductive tract, and others. Recent findings have demonstrated the existence of T_RM in the secondary lymphoid organs (SLOs) as well, leading to revision of the classic theory that they exist only in peripheral organs. trNK cells have been identified in the uterus, skin, kidney, adipose tissue, and salivary glands. These tissue-resident lymphocytes do not recirculate in the blood or lymphatic system and often adopt a unique phenotype that is distinct from those of circulating immune cells. In this review, we will discuss the recent findings on the tissue residency of both innate and adaptive lymphocytes, with a particular focus on CD8⁺ memory T cells, and describe some advances regarding unconventional T cells (invariant NKT cells, mucosal-associated invariant T cells (MAIT), and γδ T cells) and the emerging family of trNK cells. Specifically, we will focus on the phenotypes and functions of these subsets and discuss their implications in anti-viral and anti-tumor immunity.

Prime and target immunization protects against liver-stage malaria in mice

Despite recent advances in treatment and vector control, malaria is still a leading cause of death, emphasizing the need for an effective vaccine. The malaria life cycle can be subdivided into three stages: the invasion and growth within liver hepatocytes (pre-erythrocytic stage), the blood stage (erythrocytic stage), and, finally, the sexual stage (occurring within the mosquito vector). Antigen (Ag)-specific CD8+ T cells are effectively induced by heterologous prime-boost viral vector immunization and known to correlate with liver-stage protection. However, liver-stage malaria vaccines have struggled to generate and maintain the high numbers of Plasmodium-specific circulating T cells necessary to confer sterile protection. We describe an alternative “prime and target” vaccination strategy aimed specifically at inducing high numbers of tissue-resident memory T cells present in the liver at the time of hepatic infection. This approach bypasses the need for very high numbers of circulating T cells and markedly increases the efficacy of subunit immunization against liver-stage malaria with clinically relevant Ags and clinically tested viral vectors in murine challenge models. Translation to clinical use has begun, with encouraging results from a pilot safety and feasibility trial of intravenous chimpanzee adenovirus vaccination in humans. This work highlights the value of a prime-target approach for immunization against malaria and suggests that this strategy may represent a more general approach for prophylaxis or immunotherapy of other liver infections and diseases.

Prime-and-Trap Malaria Vaccination To Generate Protective CD8+ Liver-Resident Memory T Cells

Tissue-resident memory CD8⁺ T (Trm) cells in the liver are critical for long-term protection against pre-erythrocytic Plasmodium infection. Such protection can usually be induced with three to five doses of i.v. administered radiation-attenuated sporozoites (RAS). To simplify and accelerate vaccination, we tested a DNA vaccine designed to induce potent T cell responses against the SYVPSAEQI epitope of Plasmodium yoelii circumsporozoite protein. In a heterologous "prime-and-trap" regimen, priming using gene gun-administered DNA and boosting with one dose of RAS attracted expanding Ag-specific CD8⁺ T cell populations to the liver, where they became Trm cells. Vaccinated in this manner, BALB/c mice were completely protected against challenge, an outcome not reliably achieved following one dose of RAS or following DNA-only vaccination. This study demonstrates that the combination of CD8⁺ T cell priming by DNA and boosting with liver-homing RAS enhances formation of a completely protective liver Trm cell response and suggests novel approaches for enhancing T cell-based pre-erythrocytic malaria vaccines.

A Systematic Review: The Role of Resident Memory T Cells in Infectious Diseases and Their Relevance for Vaccine Development

Background

Resident memory T cells have emerged as key players in the immune response generated against a number of pathogens. Their ability to take residence in non-lymphoid peripheral tissues allows for the rapid deployment of secondary effector responses at the site of pathogen entry. This ability to provide enhanced regional immunity has gathered much attention, with the generation of resident memory T cells being the goal of many novel vaccines.

Objectives

This review aimed to systematically analyze published literature investigating the role of resident memory T cells in human infectious diseases. Known effector responses mounted by these cells are summarized and key strategies that are potentially influential in the rational design of resident memory T cell inducing vaccines have also been highlighted.

Methods

A Boolean search was applied to Medline, SCOPUS, and Web of Science. Studies that investigated the effector response generated by resident memory T cells and/or evaluated strategies for inducing these cells were included irrespective of published date. Studies must have utilized an established technique for identifying resident memory T cells such as T cell phenotyping.

Results

While over 600 publications were revealed by the search, 147 articles were eligible for inclusion. The reference lists of included articles were also screened for other eligible publications. This resulted in the inclusion of publications that studied resident memory T cells in the context of over 25 human pathogens. The vast majority of studies were conducted in mouse models and demonstrated that resident memory T cells mount protective immune responses.

Conclusion

Although the role resident memory T cells play in providing immunity varies depending on the pathogen and anatomical location they resided in, the evidence overall suggests that these cells are vital for the timely and optimal protection against a number of infectious diseases. The induction of resident memory T cells should be further investigated and seriously considered when designing new vaccines.

Varying Immunizations With Plasmodium Radiation-Attenuated Sporozoites Alter Tissue-Specific CD8+ T Cell Dynamics

Whole sporozoite vaccines represent one of the most promising strategies to induce protection against malaria. However, the development of efficient vaccination protocols still remains a major challenge. To understand how the generation of immunity is affected by variations in vaccination dosage and frequency, we systematically analyzed intrasplenic and intrahepatic CD8⁺ T cell responses following varied immunizations of mice with radiation-attenuated sporozoites. By combining experimental data and mathematical modeling, our analysis indicates a reversing role of spleen and liver in the generation of protective liver-resident CD8⁺ T cells during priming and booster injections: While the spleen acts as a critical source compartment during priming, the increase in vaccine-induced hepatic T cell levels is likely due to local reactivation in the liver in response to subsequent booster injections. Higher dosing accelerates the efficient generation of liver-resident CD8⁺ T cells by especially affecting their local reactivation. In addition, we determine the differentiation and migration pathway from splenic precursors toward hepatic memory cells thereby presenting a mechanistic framework for the impact of various vaccination protocols on these dynamics. Thus, our work provides important insights into organ-specific CD8⁺ T cell dynamics and their role and interplay in the formation of protective immunity against malaria.

Humanized Mouse Models for the Study of Human Malaria Parasite Biology, Pathogenesis, and Immunity

Malaria parasite infection continues to inflict extensive morbidity and mortality in resource-poor countries. The insufficiently understood parasite biology, continuously evolving drug resistance and the lack of an effective vaccine necessitate intensive research on human malaria parasites that can inform the development of new intervention tools. Humanized mouse models have been greatly improved over the last decade and enable the direct study of human malaria parasites in vivo in the laboratory. Nevertheless, no small animal model developed so far is capable of maintaining the complete life cycle of Plasmodium parasites that infect humans. The ultimate goal is to develop humanized mouse systems in which a Plasmodium infection closely reproduces all stages of a parasite infection in humans, including pre-erythrocytic infection, blood stage infection and its associated pathology, transmission as well as the human immune response to infection. Here, we discuss current humanized mouse models and the future directions that should be taken to develop next-generation models for human malaria parasite research.

CD8+ T-cell mediated anti-malaria protection induced by malaria vaccines; assessment of hepatic CD8+ T cells by SCBC assay

Malaria is a severe infectious disease with relatively high mortality, thus having been a scourge of humanity. There are a few candidate malaria vaccines that have shown a protective efficacy in humans against malaria. One of the candidate human malaria vaccines, which is based on human malaria sporozoites and called PfSPZ Vaccine, has been shown to protect a significant proportion of vaccine recipients from getting malaria. PfSPZ Vaccine elicits a potent response of hepatic CD8+ T cells that are specific for malaria antigens in non-human primates. To further characterize hepatic CD8+ T cells induced by the sporozoite-based malaria vaccine in a mouse model, we have used a cutting-edge Single-cell Barcode (SCBC) assay, a recently emerged approach/method for investigating the nature of T-cells responses during infection or cancer. Using the SCBC technology, we have identified a population of hepatic CD8+ T cells that are polyfunctional at a single cell level only in a group of vaccinated mice upon malaria challenge. The cytokines/chemokines secreted by these polyfunctional CD8+ T-cell subsets include MIP-1α, RANTES, IFN-γ, and/or IL-17A, which have shown to be associated with protective T-cell responses against certain pathogens. Therefore, a successful induction of such polyfunctional hepatic CD8+ T cells may be a key to the development of effective human malaria vaccine. In addition, the SCBC technology could provide a new level of diagnostic that will allow for a more accurate determination of vaccine efficacy.

Modeling the effect of boost timing in murine irradiated sporozoite prime-boost vaccines

Vaccination with radiation-attenuated sporozoites has been shown to induce CD8+ T cell-mediated protection against pre-erythrocytic stages of malaria. Empirical evidence suggests that successive inoculations often improve the efficacy of this type of vaccines. An initial dose (prime) triggers a specific cellular response, and subsequent inoculations (boost) amplify this response to create a robust CD8+ T cell memory. In this work we propose a model to analyze the effect of T cell dynamics on the performance of prime-boost vaccines. This model suggests that boost doses and timings should be selected according to the T cell response elicited by priming. Specifically, boosting during late stages of clonal contraction would maximize T cell memory production for vaccines using lower doses of irradiated sporozoites. In contrast, single-dose inoculations would be indicated for higher vaccine doses. Experimental data have been obtained that support theoretical predictions of the model.

Tissue-specific cellular immune responses to malaria pre-erythrocytic stages

Complete and long-lasting protective immunity against malaria can be achieved through vaccination with invasive live attenuated Plasmodium sporozoites, the motile stage inoculated in the host skin during a mosquito bite. Protective immunity relies primarily on effector CD8⁺ T cells targeting the parasite in the liver. Understanding the tissue-specific features of the immune response is emerging as a vital requirement for understanding protective immunity. The small parasite inoculum, the scarcity of infected cells and the tolerogenic properties of the liver represent hurdles for the establishment of protective immunity in endemic areas. In this review, we discuss recent advances on liver-specific features of immunity including innate recognition of malaria pre-erythrocytic stages, CD8⁺ T cell interactions with infected hepatocytes, antigen presentation for effective CD8⁺ T cell responses and generation of liver-resident memory CD8⁺ T cells. A better understanding of the factors involved in the induction and maintenance of effector CD8⁺ T cell immunity against malaria pre-erythrocytic stages is crucial for the development of an effective vaccine targeting the initial phase of malaria infection.

TCR-pMHC encounter differentially regulates transcriptomes of tissue-resident CD8 T cells

To investigate the role of TCR-pMHC interaction in regulating lung CD8 tissue-resident T cell (T_R ) differentiation, polyclonal responses were compared against NP_366-374 /D^b and PA_224-233 /D^b , two immunodominant epitopes that arise during influenza A infection in mice. Memory niches distinct from iBALTs develop within the lamina propria, supporting CD103⁺ and CD103^- CD8 T_R generation and intraepithelial translocation. Gene set enrichment analysis (GSEA) and weighted gene co-expression network analysis (WGCNA) identify dominant TCR, adherens junction, RIG-I-like and NOD-like pattern recognition receptor as well as TGF-β signaling pathways and memory signatures among PA_224-233 /D^b T cells consistent with T resident memory (T_RM ) status. In contrast, NP_366-374 /D^b T cells exhibit enrichment of effector signatures, upregulating pro-inflammatory mediators even among T_RM . While NP_366-374 /D^b T cells manifest transcripts linked to canonical exhaustion pathways, PA_224-233 /D^b T cells exploit P2rx7 purinoreceptor attenuation. The NP_366-374 /D^b CD103⁺ subset expresses the antimicrobial lactotransferrin whereas PA_224-233 /D^b CD103⁺ utilizes pore-forming mpeg-1, with <22% of genes correspondingly upregulated in CD103⁺ (or CD103^- ) subsets of both specificities. Thus, TCR-pMHC interactions among T_R and antigen presenting cells in a tissue milieu strongly impact CD8 T cell biology.

Human intrahepatic CD69 + CD8+ T cells have a tissue resident memory T cell phenotype with reduced cytolytic capacity

Tissue resident memory T cells (T_RM) have been identified in various tissues, however human liver T_RM to date remain unidentified. T_RM can be recognized by CD69 and/or CD103 expression and may play a role in the pathology of chronic hepatitis B (CHB) and hepatitis C virus infection (CHC). Liver and paired blood mononuclear cells from 17 patients (including 4 CHB and 6 CHC patients) were isolated and CD8+ T cells were comprehensively analysed by flowcytometry, immunohistochemistry and qPCR. The majority of intrahepatic CD8+ T cells expressed CD69, a marker used to identify T_RM, of which a subset co-expressed CD103. CD69 + CD8+ T cells expressed low levels of S1PR1 and KLF2 and a large proportion (>90%) was CXCR6+, resembling liver T_RM in mice and liver resident NK cells in human. Cytotoxic proteins were only expressed in a small fraction of liver CD69 + CD8+ T cells in patients without viral hepatitis, however, in livers from CHB patients more CD69 + CD8+ T cells were granzyme B+. In CHC patients, less intrahepatic CD69 + CD8+ T cells were Hobit+ as compared to CHB and control patients. Intrahepatic CD69 + CD8+ T cells likely T_RM which have a reduced cytolytic potential. In patients with chronic viral hepatitis T_RM have a distinct phenotype.

Cellular immune response to DNA and vaccinia prime-boost immunization kills Plasmodium yoelii-infected hepatocytes in vitro

Background

Plasmid DNA encoding Plasmodium yoelii circumsporozoite protein (PyCSP) followed by boosting with recombinant vaccinia virus containing the PyCSP elicited significant protective immunity in mice that was primarily mediated by CD8+ T-cell responses directed to P. yoelii -infected hepatocytes. This study was to further explore protection using in vitro cultures of P. yoelii parasites in mouse hepatocytes. Spleen cells from DNA/vaccinia virus-immunized mice were co-cultured in vitro with mouse hepatocytes containing developing P. yoelii liver stage parasites. A semipermeable membrane separating spleen cells and hepatocytes was used to demonstrate if cell-to-cell contact was required. Inhibitors of mediators likely involved in spleen cell killing were added to these co-cultures. Spleen cells from immunized mice inhibited in vitro P. yoelii parasite development, and inhibition was eliminated by separating effectors and targets with the semipermeable membrane. Additionally, inhibitors of inducible nitric oxide synthase, caspase activation, NF-κB activation as well as antibodies against interferon-gamma (IFN-γ) and ICAM-1 reduced parasite inhibition. These findings suggest that direct contact between spleen cells from immunized mice and P. yoelii-infected hepatocytes is required for eliminating liver stage parasites and provide more insight into CD8+ T-cell-mediated inhibition of malaria liver stage development.

CD103+ CD8 T Cells in the Toxoplasma-Infected Brain Exhibit a Tissue-Resident Memory Transcriptional Profile

During chronic infection, memory T cells acquire a unique phenotype and become dependent on different survival signals than those needed for memory T cells generated during an acute infection. The distinction between the role of effector and memory T cells in an environment of persistent antigen remains unclear. Here, in the context of chronic Toxoplasma gondii infection, we demonstrate that a population of CD8 T cells exhibiting a tissue-resident memory (T_RM) phenotype accumulates within the brain. We show that this population is distributed throughout the brain in both parenchymal and extraparenchymal spaces. Furthermore, this population is transcriptionally distinct and exhibits a transcriptional signature consistent with the T_RM observed in acute viral infections. Finally, we establish that the CD103⁺ T_RM population has an intrinsic capacity to produce both IFN-γ and TNF-α, cytokines critical for parasite control within the central nervous system (CNS). The contribution of this population to pro-inflammatory cytokine production suggests an important role for T_RM in protective and ongoing immune responses in the infected CNS.

Accession number: GSE95105

Resident memory T cells in mouse and human

Recent researches are revealing the importance of a new subset of memory T cells called resident memory T cells (T_RM^s). Once they enter the tissues according to their tissue-homing receptors, TRMs do not go back to circulation and stay in the same tissues for a long time. These T cells are defined as expressing CD69 and/or CD103, and are known to show strong effector functions. It is considered that T_RM^s have an important role against infection in barrier tissues such as GI tract, skin, respiratory system and reproductive tract. Furthermore, recent reports indicate their roles in organ-specific chronic inflammatory disorders, autoimmune disorders and tumor immunology even in non-barrier tissues such as central nerve system, lymphatic tissue, liver, kidney, pancreas and joint. Here in this session, the author organized what have been known about T_RM both in mouse and human, including the development, functional activities and relation of T_RM to disease manifestation, for the detailed understanding of this fraction.

Chemoprophylaxis with sporozoite immunization in P. knowlesi rhesus monkeys confers protection and elicits sporozoite-specific memory T cells in the liver

Whole malaria sporozoite vaccine regimens are promising new strategies, and some candidates have demonstrated high rates of durable clinical protection associated with memory T cell responses. Little is known about the anatomical distribution of memory T cells following whole sporozoite vaccines, and immunization of nonhuman primates can be used as a relevant model for humans. We conducted a chemoprophylaxis with sporozoite (CPS) immunization in P. knowlesi rhesus monkeys and challenged via mosquito bites. Half of CPS immunized animals developed complete protection, with a marked delay in parasitemia demonstrated in the other half. Antibody responses to whole sporozoites, CSP, and AMA1, but not CelTOS were detected. Peripheral blood T cell responses to whole sporozoites, but not CSP and AMA1 peptides were observed. Unlike peripheral blood, there was a high frequency of sporozoite-specific memory T cells observed in the liver and bone marrow. Interestingly, sporozoite-specific CD4+ and CD8+ memory T cells in the liver highly expressed chemokine receptors CCR5 and CXCR6, both of which are known for liver sinusoid homing. The majority of liver sporozoite-specific memory T cells expressed CD69, a phenotypic marker of tissue-resident memory (TRM) cells, which are well positioned to rapidly control liver-stage infection. Vaccine strategies that aim to elicit large number of liver TRM cells may efficiently increase the efficacy and durability of response against pre-erythrocytic parasites.

The three Rs: Recruitment, Retention and Residence of leukocytes in the liver

The composition of leukocytes in the liver is highly distinct from that of the blood and lymphoid organs. In particular, the liver is highly enriched in non-conventional T cells such as natural killer T (NKT) cells, γδ T cells and mucosal-associated invariant T cells. In addition, there are significant populations of tissue-resident NK cells (or innate lymphoid cells (ILC1)) and memory CD8+ T cells. These cells are joined in conditions of inflammation by neutrophils, monocytes and macrophages. In recent years a multitude of studies have generated insights into how these cells arrest, move and remain resident in the liver. This new understanding has largely been due to the use of intra-vital microscopy to track immune cells in the liver, coupled with gene expression profiling and parabiosis techniques. These studies have revealed that leukocyte recruitment in the liver does not correspond to the classical paradigm of the leukocyte adhesion cascade. Rather, both lymphoid and myeloid cells have been found to adhere in the liver sinusoids in a platelet-dependent manner. Leukocytes have also been observed to patrol the hepatic sinusoids using a characteristic crawling motility. Moreover, T cells have been observed surveying hepatocytes for antigen through the unique fenestrated endothelium of the liver sinusoids, potentially negating the need for extravasation. In this review we highlight some of these recent discoveries and examine the different molecular interactions required for the recruitment, retention and-in some cases-residence of diverse leukocyte populations within the liver.

Angiotensin II type-1 receptor (AT1R) regulates expansion, differentiation, and functional capacity of antigen-specific CD8+ T cells

Angiotensin II (Ang II) and its receptor AT₁ (AT₁R), an important effector axis of renin-angiotensin system (RAS), have been demonstrated to regulate T-cell responses. However, these studies characterized Ang II and AT₁R effects using pharmacological tools, which do not target only Ang II/AT₁R axis. The specific role of AT₁R expressed by antigen-specific CD8⁺ T cells is unknown. Then we immunized transgenic mice expressing a T-cell receptor specific for SIINFEKL epitope (OT-I mice) with sporozoites of the rodent malaria parasite Plasmodium berghei expressing the cytotoxic epitope SIINFEKL. Early priming events after immunization were not affected but the expansion and contraction of AT₁R-deficient (AT₁R^-/-) OT-I cells was decreased. Moreover, they seemed more activated, express higher levels of CTLA-4, PD-1, LAG-3, and have decreased functional capacity during the effector phase. Memory AT₁R^-/- OT-I cells exhibited higher IL-7Rα expression, activation, and exhaustion phenotypes but less cytotoxic capacity. Importantly, AT₁R^-/- OT-I cells show better control of blood parasitemia burden and ameliorate mice survival during lethal disease induced by blood-stage malaria. Our study reveals that AT₁R in antigen-specific CD8⁺ T cells regulates expansion, differentiation, and function during effector and memory phases of the response against Plasmodium, which could apply to different infectious agents.

Protective immunity to liver-stage malaria

Despite decades of research and recent clinical trials, an efficacious long-lasting preventative vaccine for malaria remains elusive. This parasite infects mammals via mosquito bites, progressing through several stages including the relatively short asymptomatic liver stage followed by the more persistent cyclic blood stage, the latter of which is responsible for all disease symptoms. As the liver acts as a bottleneck to blood-stage infection, it represents a potential site for parasite and disease control. In this review, we discuss immunity to liver-stage malaria. It is hoped that the knowledge gained from animal models of malaria immunity will translate into a more powerful and effective vaccine to reduce this global health problem.