Circulating Th1-Cell-type Tfh Cells that Exhibit Impaired B Cell Help Are Preferentially Activated during Acute Malaria in Children

The impact of Plasmodium-driven immunoregulatory networks on immunity to malaria

Malaria, caused by infection with Plasmodium parasites, drives multiple regulatory responses across the immune landscape. These regulatory responses help to protect against inflammatory disease but may in some situations hamper the acquisition of adaptive immune responses that clear parasites. In addition, the regulatory responses that occur during Plasmodium infection may negatively affect malaria vaccine efficacy in the most at-risk populations. Here, we discuss the specific cellular mechanisms of immunoregulatory networks that develop during malaria, with a focus on knowledge gained from human studies and studies that involve the main malaria parasite to affect humans, Plasmodium falciparum. Leveraging this knowledge may lead to the development of new therapeutic approaches to increase protective immunity to malaria during infection or after vaccination.

Mature beyond their years: young children who escape detection of parasitemia despite living in settings of intense malaria transmission

Despite having the highest risk of progressing to severe disease due to lack of acquired immunity, the youngest children living in areas of highly intense malaria transmission have long been observed to be infected at lower rates than older children. Whether this observation is due to reduced exposure to infectious mosquito bites from behavioral and biological factors, maternally transferred immunity, genetic factors, or enhanced innate immunity in the young child has intrigued malaria researchers for over half a century. Recent evidence suggests that maternally transferred immunity may be limited to early infancy and that the young child's own immune system may contribute to control of malarial symptoms early in life and prior to the development of more effective adaptive immunity. Prospective studies of active and passive detection of Plasmodium falciparum blood-stage infections have identified young children (<5 years old) who remain uninfected through a defined surveillance period despite living in settings of highly intense malaria transmission. Yet, little is known about the potential immunological basis for this 'aparasitemic' phenotype. In this review, we summarize the observational evidence for this phenotype in field studies and examine potential reasons why these children escape detection of parasitemia, covering factors that are either extrinsic or intrinsic to their developing immune system. We discuss the challenges of distinguishing malaria protection from lack of malaria exposure in field studies. We also identify gaps in our knowledge regarding cellular immunity in the youngest age group and propose directions that researchers may take to address these gaps.

Regulation of B-cell function and expression of CD11c, T-bet, and FcRL5 in response to different activation signals

CD11c, FcRL5, or T-bet are commonly expressed by B cells expanding during inflammation, where they can make up >30% of mature B cells. However, the association between the proteins and differentiation and function in the host response remains largely unclear. We have assessed the co-expression of CD11c, T-bet, and FcRL5 in an in vitro B-cell culture system to determine how stimulation via the BCR, toll-like receptor 9 (TLR9), and different cytokines influence CD11c, T-bet, and FcRL5 expression. We observed different expression dynamics for all markers, but a largely overlapping regulation of CD11c and FcRL5 in response to BCR and TLR9 activation, while T-bet was strongly dependent on IFN-γ signaling. Investigating plasma cell differentiation and APC functions, there was no association between marker expression and antibody secretion or T-cell help. Rather the functions were associated with TLR9-signalling and B-cell-derived IL-6 production, respectively. These results suggest that the expression of CD11c, FcRL5, and T-bet and plasma cell differentiation and improved APC functions occur in parallel and are regulated by similar activation signals, but they are not interdependent.

T follicular helper cell profiles differ by malaria antigen and for children compared to adults

Background

Circulating T-follicular helper (cT FH ) cells have the potential to provide an additional correlate of protection against Plasmodium falciparum ( Pf) as they are essential to promote B cell production of long-lasting antibodies. Assessing the specificity of cT FH subsets to individual malaria antigens is vital to understanding the variation observed in antibody responses and identifying promising malaria vaccine candidates.

Methods

Using spectral flow cytometry and unbiased clustering analysis we assessed antigen-specific cT FH cell recall responses in vitro to malaria vaccine candidates Pf SEA-1A and Pf GARP within a cross-section of children and adults living in a malaria holoendemic region of western Kenya.

Findings

In children, a broad array of cT FH subsets (defined by cytokine and transcription factor expression) were reactive to both malaria antigens, Pf SEA-1A and Pf GARP, while adults had a narrow profile centering on cT FH 17- and cT FH 1/17-like subsets following stimulation with Pf GARP only.

Interpretation

Because T FH 17 cells are involved in the maintenance of memory antibody responses within the context of parasitic infections, our results suggest that Pf GARP might generate longer lived antibody responses compared to Pf SEA-1A. These findings have intriguing implications for evaluating malaria vaccine candidates as they highlight the importance of including cT FH profiles when assessing interdependent correlates of protective immunity.

IL-15 Complex-Induced IL-10 Enhances Plasmodium-specific CD4+ T Follicular Helper Differentiation and Antibody Production

Malaria, which results from infection with Plasmodium parasites, remains a major public health problem. Although humans do not develop long-lived, sterilizing immunity, protection against symptomatic disease develops after repeated exposure to Plasmodium parasites and correlates with the acquisition of humoral immunity. Despite the established role Abs play in protection from malaria disease, dysregulated inflammation is thought to contribute to the suboptimal immune response to Plasmodium infection. Plasmodium berghei ANKA (PbA) infection results in a fatal severe malaria disease in mice. We previously demonstrated that treatment of mice with IL-15 complex (IL-15C; IL-15 bound to an IL-15Rα-Fc fusion protein) induces IL-10 expression in NK cells, which protects mice from PbA-induced death. Using a novel MHC class II tetramer to identify PbA-specific CD4+ T cells, in this study we demonstrate that IL-15C treatment enhances T follicular helper (Tfh) differentiation and modulates cytokine production by CD4+ T cells. Moreover, genetic deletion of NK cell-derived IL-10 or IL-10R expression on T cells prevents IL-15C-induced Tfh differentiation. Additionally, IL-15C treatment results in increased anti-PbA IgG Ab levels and improves survival following reinfection. Overall, these data demonstrate that IL-15C treatment, via its induction of IL-10 from NK cells, modulates the dysregulated inflammation during Plasmodium infection to promote Tfh differentiation and Ab generation, correlating with improved survival from reinfection. These findings will facilitate improved control of malaria infection and protection from disease by informing therapeutic strategies and vaccine design.

Integrating circulating T follicular memory cells and autoantibody repertoires for characterization of autoimmune disorders

Introduction

Autoimmune diseases are heterogeneous and often lack specific or sensitive diagnostic tests. Increased percentages of CD4+CXCR5+PD1+ circulating T follicular helper (cTfh) cells and skewed distributions of cTfh subtypes have been associated with autoimmunity. However, cTfh cell percentages can normalize with immunomodulatory treatment despite persistent disease activity, indicating the need for identifying additional cellular and/or serologic features correlating with autoimmunity.

Methods

The cohort included 50 controls and 56 patients with autoimmune cytopenias, gastrointestinal, pulmonary, and/or neurologic autoimmune disease. Flow cytometry was used to measure CD4+CXCR5+ T cell subsets expressing the chemokine receptors CXCR3 and/or CCR6: CXCR3+CCR6- Type 1, CXCR3-CCR6- Type 2, CXCR3+CCR6+ Type 1/17, and CXCR3- CCR6+ Type 17 T cells. IgG and IgA autoantibodies were quantified using a microarray featuring 1616 full-length, conformationally intact protein antigens. The 97.5th percentile in the control cohort defined normal limits for T cell subset percentages and total number (burden) of autoantibodies.

Results

This study focused on CD4+CXCR5+ T cells because CXCR5 upregulation occurs after cognate T-B cell interactions characteristic of autoimmune diseases. We refer to these cells as circulating T follicular memory (cTfm) cells to acknowledge the dynamic nature of antigen-experienced CXCR5+ T cells, which encompass progenitors of cTfh or Tfh cells as well as early effector memory T cells that have not yet lost CXCR5. Compared to controls, 57.1% of patients had increased CXCR5+CXCR3+CCR6+ cTfm1/17 and 25% had increased CXCR5+CXCR3-CCR6+ cTfm17 cell percentages. Patients had significantly more diverse IgG and IgA autoantibodies than controls and 44.6% had an increased burden of autoantibodies of either isotype. Unsupervised autoantibody clustering identified three clusters of patients with IgG autoantibody profiles distinct from those of controls, enriched for patients with active autoimmunity and monogenic diseases. An increased percentage of cTfm17 cells was most closely associated with an increased burden of high-titer IgG and IgA autoantibodies. A composite measure integrating increased cTfm1/17, cTfm17, and high-titer IgG and/or IgA autoantibodies had 91.1% sensitivity and 90.9% specificity for identifying patients with autoimmunity. Percentages of cTfm1/17 and cTfm17 percentages and numbers of high-titer autoantibodies in patients receiving immunomodulatory treatment did not differ from those in untreated patients, thus suggesting that measurements of cTfm can complement measurements of other cellular markers affected by treatment.

Conclusions

This study highlights two new approaches for assessing autoimmunity: measuring CD4+CXCR5+ cTfm subsets as well as total burden of autoantibodies. Our findings suggest that these approaches are particularly relevant to patients with rare autoimmune disorders for whom target antigens and prognosis are often unknown.

Persistence of Anti-SE36 Antibodies Induced by the Malaria Vaccine Candidate BK-SE36/CpG in 5-10-Year-Old Burkinabe Children Naturally Exposed to Malaria

Information on the dynamics and decline/persistence of antibody titres is important in vaccine development. A recent vaccine trial in malaria-exposed, healthy African adults and children living in a malaria hyperendemic and seasonal area (Ouagadougou, Burkina Faso) was the first study in which BK-SE36/CpG was administered to different age groups. In 5- to 10-year-old children, the risk of malaria infection was markedly lower in the BK-SE36/CpG arm compared to the control arm. We report here data on antibody titres measured in this age-group after the high malaria transmission season of 2021 (three years after the first vaccine dose was administered). At Year 3, 83% of children had detectable anti-SE36 total IgG antibodies. Geometric mean antibody titres and the proportion of children with detectable anti-SE36 antibodies were markedly higher in the BK-SE36/CpG arm than the control (rabies) arm. The information obtained in this study will guide investigators on future vaccine/booster schedules for this promising blood-stage malaria vaccine candidate.

Distinct transcriptomic signatures define febrile malaria depending on initial infective states, asymptomatic or uninfected

BACKGROUND

Cumulative malaria parasite exposure in endemic regions often results in the acquisition of partial immunity and asymptomatic infections. There is limited information on how host-parasite interactions mediate the maintenance of chronic symptomless infections that sustain malaria transmission.

METHODS

Here, we determined the gene expression profiles of the parasite population and the corresponding host peripheral blood mononuclear cells (PBMCs) from 21 children (< 15 years). We compared children who were defined as uninfected, asymptomatic and those with febrile malaria.

RESULTS

Children with asymptomatic infections had a parasite transcriptional profile characterized by a bias toward trophozoite stage (~ 12 h-post invasion) parasites and low parasite levels, while early ring stage parasites were characteristic of febrile malaria. The host response of asymptomatic children was characterized by downregulated transcription of genes associated with inflammatory responses, compared with children with febrile malaria,. Interestingly, the host responses during febrile infections that followed an asymptomatic infection featured stronger inflammatory responses, whereas the febrile host responses from previously uninfected children featured increased humoral immune responses.

CONCLUSIONS

The priming effect of prior asymptomatic infection may explain the blunted acquisition of antibody responses seen to malaria antigens following natural exposure or vaccination in malaria endemic areas.

Cytolytic circumsporozoite-specific memory CD4+ T cell clones are expanded during Plasmodium falciparum infection

Clinical immunity against Plasmodium falciparum infection develops in residents of malaria endemic regions, manifesting in reduced clinical symptoms during infection and in protection against severe disease but the mechanisms are not fully understood. Here, we compare the cellular and humoral immune response of clinically immune (0-1 episode over 18 months) and susceptible (at least 3 episodes) during a mild episode of Pf malaria infection in a malaria endemic region of Malawi, by analysing peripheral blood samples using high dimensional mass cytometry (CyTOF), spectral flow cytometry and single-cell transcriptomic analyses. In the clinically immune, we find increased proportions of circulating follicular helper T cells and classical monocytes, while the humoral immune response shows characteristic age-related differences in the protected. Presence of memory CD4+ T cell clones with a strong cytolytic ZEB2+ T helper 1 effector signature, sharing identical T cell receptor clonotypes and recognizing the Pf-derived circumsporozoite protein (CSP) antigen are found in the blood of the Pf-infected participants gaining protection. Moreover, in clinically protected participants, ZEB2+ memory CD4+ T cells express lower level of inhibitory and chemotactic receptors. We thus propose that clonally expanded ZEB2+ CSP-specific cytolytic memory CD4+ Th1 cells may contribute to clinical immunity against the sporozoite and liver-stage Pf malaria.

Advancements and Challenges in Developing Malaria Vaccines: Targeting Multiple Stages of the Parasite Life Cycle

Malaria, caused by Plasmodium species, remains a major global health concern, causing millions of deaths annually. While the introduction of the RTS,S vaccine has shown promise, there is a pressing need for more effective vaccines due to the emergence of drug-resistant parasites and insecticide-resistant vectors. However, the complex life cycle and genetic diversity of the parasite, technical obstacles, limited funding, and the impact of the 2019 pandemic have hindered progress in malaria vaccine development. This review focuses on advancements in malaria vaccine development, particularly the ongoing clinical trials targeting antigens from different stages of the Plasmodium life cycle. Additionally, we discuss the rationale, strategies, and challenges associated with vaccine design, aiming to enhance the immune response and protective efficacy of vaccine candidates. A cost-effective and multistage vaccine could hold the key to controlling and eradicating malaria.

Spatiotemporal development of the human T follicular helper cell response to Influenza vaccination

We profiled blood and draining lymph node (LN) samples from human volunteers after influenza vaccination over two years to define evolution in the T follicular helper cell (TFH) response. We show LN TFH cells expanded in a clonal-manner during the first two weeks after vaccination and persisted within the LN for up to six months. LN and circulating TFH (cTFH) clonotypes overlapped but had distinct kinetics. LN TFH cell phenotypes were heterogeneous and mutable, first differentiating into pre-TFH during the month after vaccination before maturing into GC and IL-10+ TFH cells. TFH expansion, upregulation of glucose metabolism, and redifferentiation into GC TFH cells occurred with faster kinetics after re-vaccination in the second year. We identified several influenza-specific TFH clonal lineages, including multiple responses targeting internal influenza proteins, and show each TFH state is attainable within a lineage. This study demonstrates that human TFH cells form a durable and dynamic multi-tissue network.

Plasmodium vivax infection alters the peripheral immunoregulatory network of CD4 T follicular cells and B cells

Regulatory and effector cell responses to Plasmodium vivax, the most common human malaria parasite outside Africa, remain understudied in naturally infected populations. Here, we describe peripheral CD4+ T- and B-cell populations during and shortly after an uncomplicated P. vivax infection in 38 continuously exposed adult Amazonians. Consistent with previous observations, we found an increased frequency in CD4+ CD45RA- CD25+ FoxP3+ T regulatory cells that express the inhibitory molecule CTLA-4 during the acute infection, with a sustained expansion of CD21- CD27- atypical memory cells within the CD19+ B-cell compartment. Both Th1- and Th2-type subsets of CXCR5+ ICOShi PD-1+ circulating T follicular helper (cTfh) cells, which are thought to contribute to antibody production, were induced during P. vivax infection, with a positive correlation between overall cTfh cell frequency and IgG antibody titers to the P. vivax blood-stage antigen MSP119 . We identified significant changes in cell populations that had not been described in human malaria, such as an increased frequency of CTLA-4+ T follicular regulatory cells that antagonize Tfh cells, and a decreased frequency of circulating CD24hi CD27+ B regulatory cells in response to acute infection. In conclusion, we disclose a complex immunoregulatory network that is critical to understand how naturally acquired immunity develops in P. vivax malaria.

Circulating Th2-biased T follicular helper cells impede antiviral humoral responses during chronic hepatitis B infection through upregulating CTLA4

Failure in curing chronic hepatitis B (CHB) caused by hepatitis B virus (HBV) can lead to functional impairment of B cells. Cytotoxic T-lymphocyte associated antigen 4 (CTLA4) regulates B cell and T follicular helper (Tfh) cell differentiation. In addition, Tfh cells play a critical role in helping B cells generate antibodies upon pathogen exposure. Here, we analyzed the global and HBsAg-specific B cells and circulating Tfh (cTfh) cells using samples from treatment-naïve and Peg-IFN-α-treated CHB patients and healthy subjects. Compared to healthy subjects, CTLA4 expression was significantly increased in cTfh cells, from CHB patients. The frequency of CTLA4⁺cTfh2 cells was negatively correlated with that of HBsAg-specific resting memory B cells. Importantly, inhibition of CTLA4 restored HBsAb secretion and promoted plasma cell differentiation. In addition, CTLA4⁺cTfh2 cells from CHB patients were ineffective in providing B cell help. Both expression of CTLA4 in cTfh and cTfh2 cells and ratios of CLTA4⁺cTfh and CTLA4⁺cTfh2 cells were significantly decreased in Peg-IFN-α-treated CHB patients who showed complete responses. Thus, our results highlighted that cTh2-biased T follicular helper cells could impede antiviral humoral responses during chronic HBV infection by upregulating CTLA4, suggesting that further optimizing potent Tfh cell responses may promote functional cure of CHB.

Tracking of activated cTfh cells following sequential influenza vaccinations reveals transcriptional profile of clonotypes driving a vaccine-induced immune response

Introduction

A vaccine against influenza is available seasonally but is not 100% effective. A predictor of successful seroconversion in adults is an increase in activated circulating T follicular helper (cTfh) cells after vaccination. However, the impact of repeated annual vaccinations on long-term protection and seasonal vaccine efficacy remains unclear.

Methods

In this study, we examined the T cell receptor (TCR) repertoire and transcriptional profile of vaccine-induced expanded cTfh cells in individuals who received sequential seasonal influenza vaccines. We measured the magnitude of cTfh and plasmablast cell activation from day 0 (d0) to d7 post-vaccination as an indicator of a vaccine response. To assess TCR diversity and T cell expansion we sorted activated and resting cTfh cells at d0 and d7 post-vaccination and performed TCR sequencing. We also single cell sorted activated and resting cTfh cells for TCR analysis and transcriptome sequencing.

Results and discussion

The percent of activated cTfh cells significantly increased from d0 to d7 in each of the 2016-17 (p < 0.0001) and 2017-18 (p = 0.015) vaccine seasons with the magnitude of cTfh activation increase positively correlated with the frequency of circulating plasmablast cells in the 2016-17 (p = 0.0001) and 2017-18 (p = 0.003) seasons. At d7 post-vaccination, higher magnitudes of cTfh activation were associated with increased clonality of cTfh TCR repertoire. The TCRs from vaccine-expanded clonotypes were identified and tracked longitudinally with several TCRs found to be present in both years. The transcriptomic profile of these expanded cTfh cells at the single cell level demonstrated overrepresentation of transcripts of genes involved in the type-I interferon pathway, pathways involved in gene expression, and antigen presentation and recognition. These results identify the expansion and transcriptomic profile of vaccine-induced cTfh cells important for B cell help.

The Complexity of Interferon Signaling in Host Defense against Protozoan Parasite Infection

Protozoan parasites, such as Plasmodium, Leishmania, Toxoplasma, Cryptosporidium, and Trypanosoma, are causative agents of health-threatening diseases in both humans and animals, leading to significant health risks and socioeconomic losses globally. The development of effective therapeutic and prevention strategies for protozoan-caused diseases requires a full understanding of the pathogenesis and protective events occurring in infected hosts. Interferons (IFNs) are a family of cytokines with diverse biological effects in host antimicrobial defense and disease pathogenesis, including protozoan parasite infection. Type II IFN (IFN-γ) has been widely recognized as the essential defense cytokine in intracellular protozoan parasite infection, whereas recent studies also revealed the production and distinct function of type I and III IFNs in host defense against these parasites. Decoding the complex network of the IFN family in host-parasite interaction is critical for exploring potential new therapeutic strategies against intracellular protozoan parasite infection. Here, we review the complex effects of IFNs on the host defense against intracellular protozoan parasites and the crosstalk between distinct types of IFN signaling during infections.

Clonal composition and persistence of antigen-specific circulating T follicular helper cells

T follicular helper (T_FH ) cells play an essential role in promoting B cell responses and antibody affinity maturation in germinal centers (GC). A subset of memory CD4⁺ T cells expressing the chemokine receptor CXCR5 has been described in human blood as phenotypically and clonally related to GC T_FH cells. However, the antigen specificity and relationship of these circulating T_FH (cT_FH ) cells with other memory CD4⁺ T cells remain poorly defined. Combining antigenic stimulation and T cell receptor (TCR) Vβ sequencing, we found T cells specific to tetanus toxoid (TT), influenza vaccine (Flu), or Candida albicans (C.alb) in both cT_FH and non-cT_FH subsets, although with different frequencies and effector functions. Interestingly, cT_FH and non-cT_FH cells specific for C.alb or TT had a largely overlapping TCR Vβ repertoire while the repertoire of Flu-specific cT_FH and non-cT_FH cells was distinct. Furthermore, Flu-specific but not C.alb-specific PD-1⁺ cT_FH cells had a "GC T_FH -like" phenotype, with overexpression of IL21, CXCL13, and BCL6. Longitudinal analysis of serial blood donations showed that Flu-specific cT_FH and non-cT_FH cells persisted as stable repertoires for years. Collectively, our study provides insights on the relationship of cT_FH with non-cT_FH cells and on the heterogeneity and persistence of antigen-specific human cT_FH cells.

T follicular helper 17 (Tfh17) cells are superior for immunological memory maintenance

A defining feature of successful vaccination is the ability to induce long-lived antigen-specific memory cells. T follicular helper (Tfh) cells specialize in providing help to B cells in mounting protective humoral immunity in infection and after vaccination. Memory Tfh cells that retain the CXCR5 expression can confer protection through enhancing humoral response upon antigen re-exposure but how they are maintained is poorly understood. CXCR5⁺ memory Tfh cells in human blood are divided into Tfh1, Tfh2, and Tfh17 cells by the expression of chemokine receptors CXCR3 and CCR6 associated with Th1 and Th17, respectively. Here, we developed a new method to induce Tfh1, Tfh2, and Tfh17-like (iTfh1, iTfh2, and iTfh17) mouse cells in vitro. Although all three iTfh subsets efficiently support antibody responses in recipient mice with immediate immunization, iTfh17 cells are superior to iTfh1 and iTfh2 cells in supporting antibody response to a later immunization after extended resting in vivo to mimic memory maintenance. Notably, the counterpart human Tfh17 cells are selectively enriched in CCR7⁺ central memory Tfh cells with survival and proliferative advantages. Furthermore, the analysis of multiple human cohorts that received different vaccines for HBV, influenza virus, tetanus toxin or measles revealed that vaccine-specific Tfh17 cells outcompete Tfh1 or Tfh2 cells for the persistence in memory phase. Therefore, the complementary mouse and human results showing the advantage of Tfh17 cells in maintenance and memory function supports the notion that Tfh17-induced immunization might be preferable in vaccine development to confer long-term protection.

Impact of exposure to malaria and nutritional status on responses to the experimental malaria vaccine ChAd63 MVA ME-TRAP in 5-17 month-old children in Burkina Faso

The experimental malaria vaccine ChAd63 MVA ME-TRAP previously showed protective efficacy against Plasmodium falciparum infection in Phase IIa sporozoite challenge studies in adults in the United Kingdom and in a Phase IIb field efficacy trial in Kenyan adults. However, it failed to demonstrate efficacy in a phase IIb trial in 5-17 month-old children in an area of high malaria transmission in Burkina Faso. This secondary analysis investigated whether exposure to malaria or nutritional status might be associated with reduced responses to vaccination in this cohort. Parasite blood smears and anti-AMA-1 IgG titres were used to assess history of exposure to malaria and weight-for-length Z scores were calculated to assess nutritional status. Differences in vaccine-specific anti-TRAP IgG titre and ex vivo IFNγ ELISpot response were measured between groups. In total, n = 336 volunteers randomised to receive the experimental vaccine regimen were included in this analysis. A positive smear microscopy result was associated with reduced anti-TRAP IgG titre (geometric mean titre: 2775 (uninfected) vs 1968 (infected), p = 0.025), whilst anti-AMA-1 IgG titres were weakly negatively correlated with reduced ex vivo IFNγ ELISpot response (r = -0.18, p = 0.008). Nutritional status was not associated with either humoral or cellular immunogenicity. Vaccine efficacy was also measured separately for vaccinees with positive and negative blood smears. Although not significant in either group compared to controls, vaccine efficacy measured by Cox hazard ratio was higher in uninfected compared to infected individuals (19.8% [p = 0.50] vs 3.3% [p = 0.69]). Overall, this data suggests exposure to malaria may be associated with impaired vaccine immunogenicity. This may have consequences for the testing and eventual deployment of various vaccines, in areas with high endemicity for malaria.

Trial Registration

Pactr.org, identifier PACTR201208000404131; ClinicalTrials.gov, identifier NCT01635647.

The RTS,S malaria vaccine: Current impact and foundation for the future

The RTS,S vaccine has recently been recommended for implementation as a childhood vaccine in regions with moderate-to-high malaria transmission. We discuss mechanisms of vaccine protection and longevity, implementation considerations, and future research needed to increase the vaccine’s health impact, including vaccine modifications for higher efficacy and longevity of protection.

Deciphering imprints of impaired memory B-cell maturation in germinal centers of three patients with common variable immunodeficiency

Common variable immunodeficiency (CVID), characterized by recurrent infections, low serum class-switched immunoglobulin isotypes, and poor antigen-specific antibody responses, comprises a heterogeneous patient population in terms of clinical presentation and underlying etiology. The diagnosis is regularly associated with a severe decrease of germinal center (GC)-derived B-cell populations in peripheral blood. However, data from B-cell differentiation within GC is limited. We present a multiplex approach combining histology, flow cytometry, and B-cell receptor repertoire analysis of sorted GC B-cell populations allowing the modeling of distinct disturbances in GCs of three CVID patients. Our results reflect pathophysiological heterogeneity underlying the reduced circulating pool of post-GC memory B cells and plasmablasts in the three patients. In patient 1, quantitative and qualitative B-cell development in GCs is relatively normal. In patient 2, irregularly shaped GCs are associated with reduced somatic hypermutation (SHM), antigen selection, and class-switching, while in patient 3, high SHM, impaired antigen selection, and class-switching with large single clones imply increased re-cycling of cells within the irregularly shaped GCs. In the lymph nodes of patients 2 and 3, only limited numbers of memory B cells and plasma cells are formed. While reduced numbers of circulating post GC B cells are a general phenomenon in CVID, the integrated approach exemplified distinct defects during GC maturation ranging from near normal morphology and function to severe disturbances with different facets of impaired maturation of memory B cells and/or plasma cells. Integrated dissection of disturbed GC B-cell maturation by histology, flow cytometry, and BCR repertoire analysis contributes to unraveling defects in the essential steps during memory formation.

Mouse Models for Unravelling Immunology of Blood Stage Malaria

Malaria comprises a spectrum of disease syndromes and the immune system is a major participant in malarial disease. This is particularly true in relation to the immune responses elicited against blood stages of Plasmodium-parasites that are responsible for the pathogenesis of infection. Mouse models of malaria are commonly used to dissect the immune mechanisms underlying disease. While no single mouse model of Plasmodium infection completely recapitulates all the features of malaria in humans, collectively the existing models are invaluable for defining the events that lead to the immunopathogenesis of malaria. Here we review the different mouse models of Plasmodium infection that are available, and highlight some of the main contributions these models have made with regards to identifying immune mechanisms of parasite control and the immunopathogenesis of malaria.

Five decades of clinical assessment of whole-sporozoite malaria vaccines

In 1967, pioneering work by Ruth Nussenzweig demonstrated for the first time that irradiated sporozoites of the rodent malaria parasite Plasmodium berghei protected mice against a challenge with infectious parasites of the same species. This remarkable finding opened up entirely new prospects of effective vaccination against malaria using attenuated sporozoites as immunization agents. The potential for whole-sporozoite-based immunization in humans was established in a clinical study in 1973, when a volunteer exposed to X-irradiated P. falciparum sporozoites was found to be protected against malaria following challenge with a homologous strain of this parasite. Nearly five decades later, much has been achieved in the field of whole-sporozoite malaria vaccination, and multiple reports on the clinical evaluation of such candidates have emerged. However, this process has known different paces before and after the turn of the century. While only a few clinical studies were published in the 1970’s, 1980’s and 1990’s, remarkable progress was made in the 2000’s and beyond. This article reviews the history of the clinical assessment of whole-sporozoite malaria vaccines over the last forty-nine years, highlighting the impressive achievements made over the last few years, and discussing some of the challenges ahead.

PD-1 directed immunotherapy alters Tfh and humoral immune responses to seasonal influenza vaccine

Anti-programmed death-1 (anti-PD-1) immunotherapy reinvigorates CD8 T cell responses in patients with cancer but PD-1 is also expressed by other immune cells, including follicular helper CD4 T cells (Tfh) which are involved in germinal centre responses. Little is known, however, about the effects of anti-PD-1 immunotherapy on noncancer immune responses in humans. To investigate this question, we examined the impact of anti-PD-1 immunotherapy on the Tfh-B cell axis responding to unrelated viral antigens. Following influenza vaccination, a subset of adults receiving anti-PD-1 had more robust circulating Tfh responses than adults not receiving immunotherapy. PD-1 pathway blockade resulted in transcriptional signatures of increased cellular proliferation in circulating Tfh and responding B cells compared with controls. These latter observations suggest an underlying change in the Tfh-B cell and germinal centre axis in a subset of immunotherapy patients. Together, these results demonstrate dynamic effects of anti-PD-1 therapy on influenza vaccine responses and highlight analytical vaccination as an approach that may reveal underlying immune predisposition to adverse events.

Antibody dynamics in children with first or repeat Plasmodium falciparum infections

Immunoglobulin (Ig) production during and after infection with Plasmodium parasites is one of the greatest adaptive immune defenses the human host has against this parasite. Infection with P. falciparum has been shown to induce different B cell maturation responses dependent upon the age of the patient, number of previous exposures, and severity of the disease. Described here are dynamics of Ig responses to a panel of 32 P. falciparum antigens by patients followed for 42 days and classified individuals as showing characteristics of an apparent first P. falciparum infection (naïve) or a repeat exposure (non-naïve). Six parameters were modeled to characterize the dynamics of IgM, IgG1, IgG3, and IgA for these two exposure groups with differences assessed among Ig isotypes/subclasses and unique antigens. Naïve patients had significantly longer periods of time to reach peak Ig titer (range 4–7 days longer) and lower maximum Ig titers when compared with non-naïve patients. Modeled time to seronegativity was significantly higher in non-naïve patients for IgM and IgA, but not for the two IgG subclasses. IgG1 responses to Rh2030, HSP40, and PfAMA1 were at the highest levels for non-naïve participants and may be used to predict previous or nascent exposure by themselves. The analyses presented here demonstrate the differences in the development of the Ig response to P. falciparum if the infection represents a boosting response or a primary exposure. Consistency in Ig isotype/subclasses estimates and specific data for P. falciparum antigens can better guide interpretation of seroepidemiological data among symptomatic persons.

Age-dependent changes in circulating Tfh cells influence development of functional malaria antibodies in children

T-follicular helper (Tfh) cells are key drivers of antibodies that protect from malaria. However, little is known regarding the host and parasite factors that influence Tfh and functional antibody development. Here, we use samples from a large cross-sectional study of children residing in an area of high malaria transmission in Uganda to characterize Tfh cells and functional antibodies to multiple parasites stages. We identify a dramatic re-distribution of the Tfh cell compartment with age that is independent of malaria exposure, with Th2-Tfh cells predominating in early childhood, while Th1-Tfh cell gradually increase to adult levels over the first decade of life. Functional antibody acquisition is age-dependent and hierarchical acquired based on parasite stage, with merozoite responses followed by sporozoite and gametocyte antibodies. Antibodies are boosted in children with current infection, and are higher in females. The children with the very highest antibody levels have increased Tfh cell activation and proliferation, consistent with a key role of Tfh cells in antibody development. Together, these data reveal a complex relationship between the circulating Tfh compartment, antibody development and protection from malaria.

Targeting TFH cells in human diseases and vaccination: rationale and practice

The identification of CD4⁺ T cells localizing to B cell follicles has revolutionized the knowledge of how humoral immunity is generated. Follicular helper T (T_FH) cells support germinal center (GC) formation and regulate clonal selection and differentiation of memory and antibody-secreting B cells, thus controlling antibody affinity maturation and memory. T_FH cells are essential in sustaining protective antibody responses necessary for pathogen clearance in infection and vaccine-mediated protection. Conversely, aberrant and excessive T_FH cell responses mediate and sustain pathogenic antibodies to autoantigens, alloantigens, and allergens, facilitate lymphomagenesis, and even harbor viral reservoirs. T_FH cell generation and function are determined by T cell antigen receptor (TCR), costimulation, and cytokine signals, together with specific metabolic and survival mechanisms. Such regulation is crucial to understanding disease pathogenesis and informing the development of emerging therapies for disease or novel approaches to boost vaccine efficacy.

Atypical B cells in chronic infectious diseases and systemic autoimmunity: puzzles with many missing pieces

The world’s struggle to contain the SARS-CoV-2 epidemic, primarily through vaccination, has highlighted the importance of better understanding the biology of B cells that participate in defense against infectious diseases, both acute and chronic. Here, we focus on a population of human B cells, termed atypical B cells (ABCs), that comprise a distinct B-cell lineage that differentiates from naive B cells in an interferon-γ-driven process, and are infrequent in healthy individuals but significantly expanded in chronic infectious diseases, including malaria, as well as in systemic autoimmune diseases such as systemic lupus erythematosus (SLE). Recent comparisons of ABCs by single-cell RNAseq provided evidence that ABCs in diverse chronic infectious diseases and in systemic autoimmune diseases are highly related and share common drivers of differentiation and expansion. However, ABCs in different diseases are not identical and also show discrete disease-specific features. Here, we compare and contrast key features of two ABC populations, namely those that are expanded in individuals living in malaria-endemic areas of the world versus those in SLE patients. This comparison is of interest as it appears that unique features of these two diseases result in participation of autoreactive ABCs in parasite-specific responses in malaria but in pathogenic autoimmune responses in SLE. A better understanding of the commonality and differences in the ABC responses in these two diseases may provide critical insights into the development of vaccines that drive pathogen-specific antibody responses and avoid autoimmunity.

Clonal evolution and TCR specificity of the human TFH cell response to Plasmodium falciparum CSP

T follicular helper (T_FH) cells play a crucial role in the development of long-lived, high-quality B cell responses after infection and vaccination. However, little is known about how antigen-specific T_FH cells clonally evolve in response to complex pathogens and what guides the targeting of different epitopes. Here, we assessed the cell phenotype, clonal dynamics, and T cell receptor (TCR) specificity of human circulating T_FH (cT_FH) cells during successive malaria immunizations with radiation-attenuated Plasmodium falciparum (Pf) sporozoites. Repeated parasite exposures induced a dynamic, polyclonal cT_FH response with high frequency of cells specific to a small number of epitopes in Pf circumsporozoite protein (PfCSP), the primary sporozoite surface protein and well-defined vaccine target. Human leukocyte antigen (HLA) restrictions and differences in TCR generation probability were associated with differences in the epitope targeting frequency and indicated the potential of amino acids 311 to 333 in the Th2R/T* region as a T cell supertope. But most of vaccine-induced anti-amino acid 311 to 333 TCRs, including convergent TCRs with high sequence similarity, failed to tolerate natural polymorphisms in their target peptide sequence, thus demonstrating that the T_FH cell response was limited to the vaccine strain. These data suggest that the high parasite diversity in endemic areas will limit boosting of the vaccine-induced T_FH cell response by natural infections. Our findings may guide the further design of PfCSP-based malaria vaccines able to induce potent T helper cell responses for broad, long-lasting antibody responses.

Atypical B cells up-regulate costimulatory molecules during malaria and secrete antibodies with T follicular helper cell support

Several infectious and autoimmune diseases are associated with an expansion of CD21-CD27- atypical B cells (atBCs) that up-regulate inhibitory receptors and exhibit altered B cell receptor (BCR) signaling. The function of atBCs remains unclear, and few studies have investigated the biology of pathogen-specific atBCs during acute infection. Here, we performed longitudinal flow cytometry analyses and RNA sequencing of Plasmodium falciparum (Pf)-specific B cells isolated from study participants before and shortly after febrile malaria, with simultaneous analysis of influenza hemagglutinin (HA)-specific B cells as a comparator. At the healthy baseline before the malaria season, individuals had similar frequencies of Pf- and HA-specific atBCs that did not differ proportionally from atBCs within the total B cell population. BCR sequencing identified clonal relationships between Pf-specific atBCs, activated B cells (actBCs), and classical memory B cells (MBCs) and revealed comparable degrees of somatic hypermutation. At the healthy baseline, Pf-specific atBCs were transcriptionally distinct from Pf-specific actBCs and classical MBCs. In response to acute febrile malaria, Pf-specific atBCs and actBCs up-regulated similar intracellular signaling cascades. Pf-specific atBCs showed activation of pathways involved in differentiation into antibody-secreting cells and up-regulation of molecules that mediate B-T cell interactions, suggesting that atBCs respond to T follicular helper (TFH) cells. In the presence of TFH cells and staphylococcal enterotoxin B, atBCs of malaria-exposed individuals differentiated into CD38+ antibody-secreting cells in vitro, suggesting that atBCs may actively contribute to humoral immunity to infectious pathogens.

Transcriptome and TCR Repertoire Measurements of CXCR3+ T Follicular Helper Cells Within HIV-Infected Human Lymph Nodes

Follicular-helper T cells (T_FH) are an essential arm of the adaptive immune system. Although T_FH were first discovered through their ability to contribute to antibody affinity maturation through co-stimulatory interactions with B cells, new light has been shed on their ability to remain a complex and functionally plastic cell type. Due to a lack sample availability, however, many studies have been limited to characterizing T_FH in mice or non-canonical tissue types, such as peripheral blood. Such constraints have resulted in a limited, and sometimes contradictory, understanding of this fundamental cell type. One subset of T_FH receiving attention in chronic infection are CXCR3-expressing T_FH cells (CXCR3⁺T_FH) due to their abnormal accumulation in secondary lymphoid tissues. Their function and clonal relationship with other T_FH subsets in lymphoid tissues during infection, however, remains largely unclear. We thus systematically investigated this and other subsets of T_FH within untreated HIV-infected human lymph nodes using Mass CyTOF and a combination of RNA and TCR repertoire sequencing. We show an inflation of the CXCR3⁺T_FH compartment during HIV infection that correlates with a lower HIV burden. Deeper analysis into this population revealed a functional shift of CXCR3⁺T_FH away from germinal center T_FH (GC-T_FH), including the altered expression of several important transcription factors and cytokines. CXCR3⁺T_FH also upregulated cell migration transcriptional programs and were clonally related to peripheral T_FH populations. In combination, these data suggest that CXCR3⁺T_FH have a greater tendency to enter circulation than their CXCR3^- counterparts, potentially functioning through distinct modalities that may lead to enhanced defense.

Molecular profiling reveals features of clinical immunity and immunosuppression in asymptomatic P. falciparum malaria

Clinical immunity to P. falciparum malaria is non-sterilizing, with adults often experiencing asymptomatic infection. Historically, asymptomatic malaria has been viewed as beneficial and required to help maintain clinical immunity. Emerging views suggest that these infections are detrimental and constitute a parasite reservoir that perpetuates transmission. To define the impact of asymptomatic malaria, we pursued a systems approach integrating antibody responses, mass cytometry, and transcriptional profiling of individuals experiencing symptomatic and asymptomatic P. falciparum infection. Defined populations of classical and atypical memory B cells and a T_H2 cell bias were associated with reduced risk of clinical malaria. Despite these protective responses, asymptomatic malaria featured an immunosuppressive transcriptional signature with upregulation of pathways involved in the inhibition of T-cell function, and CTLA-4 as a predicted regulator in these processes. As proof of concept, we demonstrated a role for CTLA-4 in the development of asymptomatic parasitemia in infection models. The results suggest that asymptomatic malaria is not innocuous and might not support the induction of immune processes to fully control parasitemia or efficiently respond to malaria vaccines.

The Dynamic Change of Immune Responses Between Acute and Recurrence Stages of Rodent Malaria Infection

Malaria infections are persistent as frequent recrudescence of the disease may occur following the acute infection stage, but the different immune responses that control the acute and recrudescence stages are still largely unknown. Using single-cell RNA sequencing (scRNA-seq), we showed that the number of Th1 and plasma cells in the spleen was significantly reduced during the recurrence stage compared to the acute stage of Plasmodium chabaudi chabaudi AS (P. chabaudi) infection. Additionally, the ability of both CD4+ T cell responses and B cells to control P. chabaudi recurrence was significantly reduced compared to their roles in the control of acute infection. In contrast, the number of innate immune cells, including red pulp macrophages (RPMs), gamma delta (γδ) T cells, and Dendritic cells (DCs) were significantly increased during the recurrence stage and showed to be critical for P. chabaudi infection recurrence control. Thus, our data strongly suggest the complementary role of innate immune responses in controlling malaria recrudescence when adaptive immune responses are suppressed. These findings shed new light on the development of immune interventions against malaria.

B cell-intrinsic TBK1 is essential for germinal center formation during infection and vaccination in mice

The germinal center (GC) is a site where somatic hypermutation and clonal selection are coupled for antibody affinity maturation against infections. However, how GCs are formed and regulated is incompletely understood. Here, we identified an unexpected role of Tank-binding kinase-1 (TBK1) as a crucial B cell–intrinsic factor for GC formation. Using immunization and malaria infection models, we show that TBK1-deficient B cells failed to form GC despite normal Tfh cell differentiation, although some malaria-infected B cell–specific TBK1-deficient mice could survive by GC-independent mechanisms. Mechanistically, TBK1 phosphorylation elevates in B cells during GC differentiation and regulates the balance of IRF4/BCL6 expression by limiting CD40 and BCR activation through noncanonical NF-κB and AKT^T308 signaling. In the absence of TBK1, CD40 and BCR signaling synergistically enhanced IRF4 expression in Pre-GC, leading to BCL6 suppression, and therefore failed to form GCs. As a result, memory B cells generated from TBK1-deficient B cells fail to confer sterile immunity upon reinfection, suggesting that TBK1 determines B cell fate to promote long-lasting humoral immunity.

How to induce protective humoral immunity against Plasmodium falciparum circumsporozoite protein

The induction of protective humoral immune responses against sporozoite surface proteins of the human parasite Plasmodium falciparum (Pf) is a prime goal in the development of a preerythrocytic malaria vaccine. The most promising antibody target is circumsporozoite protein (CSP). Although PfCSP induces strong humoral immune responses upon vaccination, vaccine efficacy is overall limited and not durable. Here, we review recent efforts to gain a better molecular and cellular understanding of anti-PfCSP B cell responses in humans and discuss ways to overcome limitations in the induction of stable titers of high-affinity antibodies that might help to increase vaccine efficacy and promote long-lived protection.

Systems analysis of immune responses to attenuated P. falciparum malaria sporozoite vaccination reveals excessive inflammatory signatures correlating with impaired immunity

Immunization with radiation-attenuated sporozoites (RAS) can confer sterilizing protection against malaria, although the mechanisms behind this protection are incompletely understood. We performed a systems biology analysis of samples from the Immunization by Mosquito with Radiation Attenuated Sporozoites (IMRAS) trial, which comprised P. falciparum RAS-immunized (PfRAS), malaria-naive participants whose protection from malaria infection was subsequently assessed by controlled human malaria infection (CHMI). Blood samples collected after initial PfRAS immunization were analyzed to compare immune responses between protected and non-protected volunteers leveraging integrative analysis of whole blood RNA-seq, high parameter flow cytometry, and single cell CITEseq of PBMCs. This analysis revealed differences in early innate immune responses indicating divergent paths associated with protection. In particular, elevated levels of inflammatory responses early after the initial immunization were detrimental for the development of protective adaptive immunity. Specifically, non-classical monocytes and early type I interferon responses induced within 1 day of PfRAS vaccination correlated with impaired immunity. Non-protected individuals also showed an increase in Th2 polarized T cell responses whereas we observed a trend towards increased Th1 and T-bet+ CD8 T cell responses in protected individuals. Temporal differences in genes associated with natural killer cells suggest an important role in immune regulation by these cells. These findings give insight into the immune responses that confer protection against malaria and may guide further malaria vaccine development.

Trial registration: ClinicalTrials.gov NCT01994525.

Malaria remains a serious global health problem, causing hundreds of thousands of deaths every year. An effective malaria vaccine would be an important tool to fight this disease. Previous work has shown that irradiated sporozoites, the form of the malaria parasite injected into humans by mosquitos, are not capable of progressing to a symptomatic blood stage malaria infection, and act as a protective vaccine against future malaria exposure. However the mechanisms that produce this protection are unknown. In this work, we studied individuals vaccinated with irradiated sporozoites before being exposed to live malaria parasites. Roughly half of these individual were protected against malaria. By analyzing blood samples taken at multiple points after the first vaccination using RNA sequencing and flow cytometry we identified immune responses that differed between protected and non-protected study participants. Notably, we observed a rapid increase in inflammation and interferon-associated genes in non-protected individual. We also observed protection-associated changes in T cell and NK cell associated pathways. Our study provides novel insights into immune responses associated with effective malaria vaccination, and can point the way to improved design of whole-sporozoite malaria vaccine approaches.

The tangled web of autoreactive B cells in malaria immunity and autoimmune disease

Two seminal observations suggest that the African genome contains genes selected by malaria that protect against systemic lupus erythematosus (SLE) in individuals chronically exposed to malaria, but which in the absence of malaria, are risk factors for SLE. First, Brian Greenwood observed that SLE was rare in Africa and that malaria prevented SLE-like disease in susceptible mice. Second, African-Americans, as compared with individuals of European descent, are at higher risk of SLE. Understanding that antibodies play central roles in malaria immunity and SLE, we discuss how autoreactive B cells contribute to malaria immunity but promote SLE pathology in the absence of malaria. Testing this model may provide insights into the regulation of autoreactivity and identify new therapeutic targets for SLE.

Immunoprofiling Identifies Functional B and T Cell Subsets Induced by an Attenuated Whole Parasite Malaria Vaccine as Correlates of Sterile Immunity

Immune correlates of protection remain elusive for most vaccines. An identified immune correlate would accelerate the down-selection of vaccine formulations by reducing the need for human pathogen challenge studies that are currently required to determine vaccine efficacy. Immunization via mosquito-delivered, radiation-attenuated P. falciparum sporozoites (IMRAS) is a well-established model for efficacious malaria vaccines, inducing greater than 90% sterile immunity. The current immunoprofiling study utilized samples from a clinical trial in which vaccine dosing was adjusted to achieve only 50% protection, thus enabling a comparison between protective and non-protective immune signatures. In-depth immunoprofiling was conducted by assessing a wide range of antigen-specific serological and cellular parameters and applying our newly developed computational tools, including machine learning. The computational component of the study pinpointed previously un-identified cellular T cell subsets (namely, TNFα-secreting CD8⁺CXCR3⁻CCR6⁻ T cells, IFNγ-secreting CD8⁺CCR6⁺ T cells and TNFα/FNγ-secreting CD4⁺CXCR3⁻CCR6⁻ T cells) and B cell subsets (i.e., CD19⁺CD24^hiCD38^hiCD69⁺ transitional B cells) as important factors predictive of protection (92% accuracy). Our study emphasizes the need for in-depth immunoprofiling and subsequent data integration with computational tools to identify immune correlates of protection. The described process of computational data analysis is applicable to other disease and vaccine models.

Evaluation of Human Circulating T Follicular Helper Cells in Influenza- and SARS-CoV-2-Specific B Cell Immunity

Generation of effective immune protection against viral infection and vaccination depends greatly on a successful engagement and stimulation of adaptive immune B cells and a specialized CD4⁺ T cell subset called T follicular helper cells (T_FH cells). Since T_FH cells primarily reside in lymphoid tissues, they can be challenging to study in human settings. However, a counterpart of these cells, circulating T_FH (cT_FH) cells, can be detected in peripheral blood. Assessment of cT_FH cells serves as an informative marker of humoral responses following viral infection and vaccination and can be predictive of antibody titers. Here, we describe a comprehensive flow cytometry detection method for dissecting cT_FH subsets and activation, together with the assessment of antibody-secreting cells (ASCs), from a small volume of human whole blood. This approach allows the investigation of cellular events that underpin successful immune responses following influenza and SARS-CoV-2 infection/vaccination in humans and is applicable to other viral disease settings.

Adults with Plasmodium falciparum malaria have higher magnitude and quality of circulating T-follicular helper cells compared to children

BACKGROUND

Protective malarial antibodies are acquired more rapidly in adults than children, independently of cumulative exposure, however the cellular responses mediating these differences are unknown. CD4 T-follicular helper (Tfh) cells have key roles in inducing antibodies, with Th2-Tfh cell activation associated with antibody development in malaria. Whether Tfh cell activation in malaria is age dependent is unknown and no studies have compared Tfh cell activation in children and adults with malaria.

METHODS

We undertook a comprehensive study of Tfh cells, along with B cells and antibody induction in children and adults with malaria. Activation and proliferation of circulating Tfh (cTfh) cell subsets was measured ex vivo and parasite-specific Tfh cell frequencies and functions studied with Activation Induced Marker (AIM) assays and intracellular cytokine staining.

FINDINGS

During acute malaria, the magnitude of cTfh cell activation was higher in adults than in children and occurred across all cTfh cell subsets in adults but was restricted only to the Th1-cTfh subset in children. Further, adults had higher levels of parasite-specific cTfh cells, and cTfh cells which produced more Th2-Tfh associated cytokine IL-4. Consistent with a role of higher Tfh cell activation in rapid immune development in adults, adults had higher activation of B cells during infection and higher induction of antibodies 7 and 28 days after malaria compared to children.

INTERPRETATION

Our data provide evidence that age impacts Tfh cell activation during malaria, and that these differences may influence antibody induction after treatment. Findings have important implications for vaccine development in children.

FUNDING

This word was supported by the National Health and Medical Research Council of Australia, Wellcome Trust, Charles Darwin University Menzies School of Health Research, Channel 7 Children's Research Foundation, and National Health Institute.

Circulatory follicular helper T lymphocytes associate with lower incidence of CMV infection in kidney transplant recipients

Primary infection and/or reactivation of cytomegalovirus (CMV) in kidney transplant recipients (KTR) favor rejection and mortality. T follicular helper cells (TFH) could contribute to protection against CMV. Circulatory TFH (cTFH) were studied pretransplant and early posttransplant in 90 CMV seropositive KTR not receiving antithymocyte globulin or antiviral prophylaxis, followed-up for 1 year. Patients who presented CMV infection had significantly lower cTFH and activated cTFH pretransplant and early posttransplant. Pretransplant activated cTFH were also lower within patients who developed CMV disease. Pre- and 14 days posttransplant activated cTFH were an independent protective factor for CMV infection (HR 0.41, p = .01; and 0.52, p = .02, respectively). KTR with low cTFH 7 days posttransplant (<11.9%) had lower CMV infection-free survival than patients with high cTFH (28.2% vs. 67.6%, p = .002). cTFH were associated with CMV-specific neutralizing antibodies (Nabs). In addition, IL-21 increased interferon-γ secretion by CMV-specific CD8⁺ T cells in healthy controls. Thus, we show an association between cTFH and lower incidence of CMV infection, probably through their cooperation in CMV-specific Nab production and IL-21-mediated enhancement of CD8⁺ T cell activity. Moreover, monitoring cTFH pre- and early posttransplant could improve CMV risk stratification and help select KTR catalogued at low/intermediate risk who could benefit from prophylaxis.

Recent Advances in Understanding the Inflammatory Response in Malaria: A Review of the Dual Role of Cytokines

Malaria is a serious and, in some unfortunate cases, fatal disease caused by a parasite of the Plasmodium genus. It predominantly occurs in tropical areas where it is transmitted through the bite of an infected Anopheles mosquito. The pathogenesis of malaria is complex and incompletely elucidated. During blood-stage infection, in response to the presence of the parasite, the host's immune system produces proinflammatory cytokines including IL-6, IL-8, IFN-γ, and TNF, cytokines which play a pivotal role in controlling the growth of the parasite and its elimination. Regulatory cytokines such as transforming growth factor- (TGF-) β and IL-10 maintain the balance between the proinflammatory and anti-inflammatory responses. However, in many cases, cytokines have a double role. On the one hand, they contribute to parasitic clearance, and on the other, they are responsible for pathological changes encountered in malaria. Cytokine-modulating strategies may represent a promising modern approach in disease management. In this review, we discuss the host immune response in malaria, analyzing the latest studies on the roles of pro- and anti-inflammatory cytokines.

T follicular helper cells in the humoral immune response to SARS-CoV-2 infection and vaccination

Vaccination remains the most effective mechanism to reduce the impact of COVID‐19. Induction of neutralizing antibodies is a strong correlate of protection from infection and severe disease. An understanding of the cellular events that underpin the generation of effective neutralizing antibodies is therefore key to the development of efficacious vaccines that target emerging variants of concern. Analysis of the immune response to Severe Acute Respiratory Syndrome Coronavirus 2 (SARS‐CoV‐2) infection and vaccination has identified circulating T follicular helper cells (cT_FH) as a robust correlate of the neutralizing antibody response. Here, we discuss the analysis of cT_FH cells and their lymphoid counterparts in human humoral immune responses during COVID‐19, and in response to vaccination with SARS‐CoV‐2 spike. We discuss the phenotypic heterogeneity of cT_FH cells and the utility of cT_FH subsets as informative biomarkers for development of humoral immunity. We posit that the analysis of the most effective cT_FH will be critical to inducing durable immunity to new variants of SARS‐CoV‐2.

Safety, Tolerability, Pharmacokinetics and Pharmacodynamics of Co-administered Ruxolitinib and Artemether-Lumefantrine in Healthy Adults

Despite repeated malaria infection, individuals living in malaria endemic areas remain vulnerable to re-infection. The Janus kinase (JAK1/2) inhibitor ruxolitinib could potentially disrupt the parasite-induced dysfunctional immune response when administered with anti-malarial therapy. This randomized, single-blind, placebo-controlled, single center phase 1 trial investigated the safety, tolerability, pharmacokinetic and pharmacodynamic profile of ruxolitinib and the approved antimalarial artemether-lumefantrine in combination. Ruxolitinib pharmacodynamics were assessed by inhibition of phosphorylation of signal transducer and activator of transcription 3 (pSTAT3). Eight healthy male and female participants aged 18-55 years were randomized to either ruxolitinib (20 mg) (n = 6) or placebo (n = 2) administered 2 h after artemether-lumefantrine (80/480 mg) twice daily for three days. Mild adverse events occurred in six participants (four ruxolitinib; two placebo). The combination of artemether-lumefantrine and ruxolitinib was well tolerated, with adverse events and pharmacokinetics consistent with the known profiles of both drugs. The incidence of adverse events and artemether, dihydroartemisinin (the major active metabolite of artemether) and lumefantrine exposure were not affected by ruxolitinib co-administration. Ruxolitinib co-administration resulted in a 3-fold greater pSTAT3 inhibition compared to placebo (geometric mean ratio: 3.01 [90%CI 2.14, 4.24]), with a direct and predictable relationship between ruxolitinib plasma concentrations and %pSTAT3 inhibition. This study supports the investigation of the combination of artemether-lumefantrine and ruxolitinib in healthy volunteers infected with Plasmodium falciparum malaria. (This study has been registered at ClinicalTrials.gov under registration no. NCT04456634).

The role of different components of the immune system against Plasmodium falciparum malaria: Possible contribution towards malaria vaccine development

Plasmodium falciparum malaria still remains a major global public health challenge with over 220 million new cases and well over 400,000 deaths annually. Most of the deaths occur in sub-Saharan Africa which bears 90 % of the malaria cases. Such high P. falciparum malaria-related morbidity and mortality rates pose a huge burden on the health and economic wellbeing of the countries affected. Lately, substantial gains have been made in reducing malaria morbidity and mortality through intense malaria control initiatives such as use of effective antimalarials, intensive distribution and use of insecticide-treated nets (ITNs), and implementation of massive indoor residual spraying (IRS) campaigns. However, these gains are being threatened by widespread resistance of the parasite to antimalarials, and the vector to insecticides. Over the years the use of vaccines has proven to be the most reliable, cost-effective and efficient method for controlling the burden and spread of many infectious diseases, especially in resource poor settings with limited public health infrastructure. Nonetheless, this had not been the case with malaria until the most promising malaria vaccine candidate, RTS,S/AS01, was approved for pilot implementation programme in three African countries in 2015. This was regarded as the most important breakthrough in the fight against malaria. However, RTS,S/AS01 has been found to have some limitations, the main ones being low efficacy in certain age groups, poor immunogenicity and need for almost three boosters to attain a reasonable efficacy. Thus, the search for a more robust and effective malaria vaccine still continues and a better understanding of naturally acquired immune responses to the various stages, including the transmissible stages of the parasite, could be crucial in rational vaccine design. This review therefore compiles what is currently known about the basic biology of P. falciparum and the natural malaria immune response against malaria and progress made towards vaccine development.

The expansion of human T-bethighCD21low B cells is T cell dependent

Accumulation of human CD21low B cells in peripheral blood is a hallmark of chronic activation of the adaptive immune system in certain infections and autoimmune disorders. The molecular pathways underpinning the development, function, and fate of these CD21low B cells remain incompletely characterized. Here, combined transcriptomic and chromatin accessibility analyses supported a prominent role for the transcription factor T-bet in the transcriptional regulation of these T-bethighCD21low B cells. Investigating essential signals for generating these cells in vitro established that B cell receptor (BCR)/interferon-γ receptor (IFNγR) costimulation induced the highest levels of T-bet expression and enabled their differentiation during cell cultures with Toll-like receptor (TLR) ligand or CD40L/interleukin-21 (IL-21) stimulation. Low proportions of CD21low B cells in peripheral blood from patients with defined inborn errors of immunity (IEI), because of mutations affecting canonical NF-κB, CD40, and IL-21 receptor or IL-12/IFNγ/IFNγ receptor/signal transducer and activator of transcription 1 (STAT1) signaling, substantiated the essential roles of BCR- and certain T cell–derived signals in the in vivo expansion of T-bethighCD21low B cells. Disturbed TLR signaling due to MyD88 or IRAK4 deficiency was not associated with reduced CD21low B cell proportions. The expansion of human T-bethighCD21low B cells correlated with an expansion of circulating T follicular helper 1 (cTfh1) and T peripheral helper (Tph) cells, identifying potential sources of CD40L, IL-21, and IFNγ signals. Thus, we identified important pathways to target autoreactive T-bethighCD21low B cells in human autoimmune conditions, where these cells are linked to pathogenesis and disease progression.

Type I interferon production elicits differential CD4 + T-cell responses in mice infected with Plasmodium berghei ANKA and P. chabaudi

Plasmodium parasites that infect humans are highly polymorphic, and induce various infections ranging from asymptomatic state to life-threatening diseases. However, how the differences between the parasites affect host immune responses during blood-stage infection remains largely unknown. We investigated the CD4 + T-cell immune responses in mice infected with P. berghei ANKA (PbA) or P. chabaudi chabaudi AS (Pcc) using PbT-II cells, which recognize a common epitope of these parasites. In the acute phase of infection, CD4 + T-cell responses in PbA-infected mice showed a lower involvement of Th1 cells and a lower proportion of Ly6C lo effector CD4 + T cells than those in Pcc-infected mice. Transcriptome analysis of PbT-II cells indicated that type I interferon (IFN)-regulated genes were expressed at higher levels in both Th1- and Tfh-type PbT-II cells from PbA-infected mice than those from Pcc-infected mice. Moreover, IFN-α levels were considerably higher in PbA-infected mice than in Pcc-infected mice. Inhibition of type I IFN signaling increased PbT-II and partially reversed the Th1 over Tfh bias of the PbT-II cells in both PbA- and Pcc-infected mice. In the memory phase, PbT-II cells in PbA-primed mice maintained higher numbers and exhibited better recall response to the antigen. However, recall responses were not significantly different between the infection groups after re-challenge with PbA, suggesting the effect of inflammatory environment by the infection. These observations suggest that the differences in Plasmodium-specific CD4 + T-cell responses between PbA- and Pcc-infected mice were associated with the difference in type I IFN production during the early phase of the infection.

Immunosuppression in Malaria: Do Plasmodium falciparum Parasites Hijack the Host?

Malaria reflects not only a state of immune activation, but also a state of general immune defect or immunosuppression, of complex etiology that can last longer than the actual episode. Inhabitants of malaria-endemic regions with lifelong exposure to the parasite show an exhausted or immune regulatory profile compared to non- or minimally exposed subjects. Several studies and experiments to identify and characterize the cause of this malaria-related immunosuppression have shown that malaria suppresses humoral and cellular responses to both homologous (Plasmodium) and heterologous antigens (e.g., vaccines). However, neither the underlying mechanisms nor the relative involvement of different types of immune cells in immunosuppression during malaria is well understood. Moreover, the implication of the parasite during the different stages of the modulation of immunity has not been addressed in detail. There is growing evidence of a role of immune regulators and cellular components in malaria that may lead to immunosuppression that needs further research. In this review, we summarize the current evidence on how malaria parasites may directly and indirectly induce immunosuppression and investigate the potential role of specific cell types, effector molecules and other immunoregulatory factors.

T Cell/B Cell Interactions in the Establishment of Protective Immunity

Follicular helper T cells (Tfh) are the T cell subset providing help to B cells for the generation of high-affinity antibodies and are therefore of key interest for the development of vaccination strategies against infectious diseases. In this review, we will discuss how the generation of Tfh cells and their interaction with B cells in secondary lymphoid organs can be optimized for therapeutic purposes. We will summarize different T cell subsets including Tfh-like peripheral helper T cells (Tph) capable of providing B cell help. In particular, we will highlight the novel concept of T cell/B cell interaction in non-lymphoid tissues as an important element for the generation of protective antibodies directly at the site of pathogen invasion.

Understanding the pathogenesis of infectious diseases by single-cell RNA sequencing

Infections are highly orchestrated and dynamic processes, which involve both pathogen and host. Transcriptional profiling at the single-cell level enables the analysis of cell diversity, heterogeneity of the immune response, and detailed molecular mechanisms underlying infectious diseases caused by bacteria, viruses, fungi, and parasites. Herein, we highlight recent remarkable advances in single-cell RNA sequencing (scRNA-seq) technologies and their applications in the investigation of host-pathogen interactions, current challenges and potential prospects for disease treatment are discussed as well. We propose that with the aid of scRNA-seq, the mechanism of infectious diseases will be further revealed thus inspiring the development of novel interventions and therapies.