Malaria protection in beta 2-microglobulin-deficient mice lacking major histocompatibility complex class I antigens: essential role of innate immunity, including gammadelta T cells

Aspecific binding of anti-NK1.1 antibodies on myeloid cells in an experimental model for malaria-associated acute respiratory distress syndrome

BACKGROUND

Conventional natural killer (cNK) cells play an important role in the innate immune response by directly killing infected and malignant cells and by producing pro- and anti-inflammatory cytokines. Studies on their role in malaria and its complications have resulted in conflicting results.

METHODS

Using the commonly used anti-NK1.1 depletion antibodies (PK136) in an in-house optimized experimental model for malaria-associated acute respiratory distress syndrome (MA-ARDS), the role of cNK cells was investigated. Moreover, flow cytometry was performed to characterize different NK cell populations.

RESULTS

While cNK cells were found to be dispensable in the development of MA-ARDS, the appearance of a NK1.1+ cell population was observed in the lungs upon infection despite depletion with anti-NK1.1. Detailed characterization of the unknown population revealed that this population consisted of a mixture of monocytes and macrophages that bind the anti-NK1.1 antibody in an aspecific way. This aspecific binding may occur via Fcγ receptors, such as FcγR4. In contrast, in vivo depletion using anti-NK1.1 antibodies was proved to be specific for cNK cells.

CONCLUSION

cNK cells are dispensable in the development of experimental MA-ARDS. Moreover, careful flow cytometric analysis, with a critical mindset in relation to potential aspecific binding despite the use of commercially available Fc blocking reagents, is critical to avoid misinterpretation of the results.

Characterization of γδT cells in lung of Plasmodium yoelii-infected C57BL/6 mice

Background

Malaria has high morbidity and mortality rates in some parts of tropical and subtropical countries. Besides respiratory and metabolic function, lung plays a role in immune system. γδT cells have multiple functions in producing cytokines and chemokines, regulating the immune response by interacting with other cells. It remains unclear about the role of γδT cells in the lung of mice infected by malaria parasites.

Methods

Flow cytometry (FCM) was used to evaluate the frequency of γδT cells and the effects of γδT cells on the phenotype and function of B and T cells in Plasmodium yoelii-infected wild-type (WT) or γδTCR knockout (γδT KO) mice. Haematoxylin-eosin (HE) staining was used to observe the pathological changes in the lungs.

Results

The percentage and absolute number of γδT cells in the lung increased after Plasmodium infection (p < 0.01). More γδT cells were expressing CD80, CD11b, or PD-1 post-infection (p < 0.05), while less γδT cells were expressing CD34, CD62L, and CD127 post-infection (p < 0.05). The percentages of IL-4⁺, IL-5⁺, IL-6⁺, IL-21⁺, IL-1α⁺, and IL-17⁺ γδT cells were increased (p < 0.05), but the percentage of IFN-γ-expressing γδT cells decreased (p < 0.05) post-infection. The pathological changes in the lungs of the infected γδT KO mice were not obvious compared with the infected WT mice. The proportion of CD3⁺ cells and absolute numbers of CD3⁺ cells, CD3⁺ CD4⁺ cells, CD3⁺ CD8⁺ cells decreased in γδT KO infected mice (p < 0.05). γδT KO infected mice exhibited no significant difference in the surface molecular expression of T cells compared with the WT infected mice (p > 0.05). While, the percentage of IFN-γ-expressing CD3⁺ and CD3⁺ CD8⁺ cells increased in γδT KO infected mice (p < 0.05). There was no significant difference in the absolute numbers of the total, CD69⁺, ICOS⁺, and CD80⁺ B cells between the WT infected and γδT KO infected mice (p > 0.05).

Conclusions

The content, phenotype, and function of γδT cells in the lung of C57BL/6 mice were changed after Plasmodium infection. γδT cells contribute to T cell immune response in the progress of Plasmodium infection.

Pathological alteration and therapeutic implications of sepsis-induced immune cell apoptosis

Sepsis is a life-threatening organ dysfunction syndrome caused by dysregulated host response to infection that leads to uncontrolled inflammatory response followed by immunosuppression. However, despite the high mortality rate, no specific treatment modality or drugs with high efficacy is available for sepsis to date. Although improved treatment strategies have increased the survival rate during the initial state of excessive inflammatory response, recent trends in sepsis show that mortality occurs at a period of continuous immunosuppressive state in which patients succumb to secondary infections within a few weeks or months due to post-sepsis “immune paralysis.” Immune cell alteration induced by uncontrolled apoptosis has been considered a major cause of significant immunosuppression. Particularly, apoptosis of lymphocytes, including innate immune cells and adaptive immune cells, is associated with a higher risk of secondary infections and poor outcomes. Multiple postmortem studies have confirmed that sepsis-induced immune cell apoptosis occurs in all age groups, including neonates, pediatric, and adult patients, and it is considered to be a primary contributing factor to the immunosuppressive pathophysiology of sepsis. Therapeutic perspectives targeting apoptosis through various strategies could improve survival in sepsis. In this review article, we will focus on describing the major apoptosis process of immune cells with respect to physiologic and molecular mechanisms. Further, advances in apoptosis-targeted treatment modalities for sepsis will also be discussed.

Malaria vaccines in the eradication era: current status and future perspectives

INTRODUCTION

The challenge to eradicate malaria is an enormous task that will not be achieved by current control measures, thus an efficacious and long-lasting malaria vaccine is required. The licensing of RTS, S/AS01 is a step forward in providing some protection, but a malaria vaccine that protects across multiple transmission seasons is still needed. To achieve this, inducing beneficial immune responses while minimising deleterious non-targeted effects will be essential.

AREAS COVERED

This article discusses the current challenges and advances in malaria vaccine development and reviews recent human clinical trials for each stage of infection. Pubmed and ScienceDirect were searched, focusing on cell mediated immunity and how T cell subsets might be targeted in future vaccines using novel adjuvants and emerging vaccine technologies.

EXPERT COMMENTARY

Despite decades of research there is no highly effective licensed malaria vaccine. However, there is cause for optimism as new adjuvants and vaccine systems emerge, and our understanding of correlates of protection increases, especially regarding cellular immunity. The new field of heterologous (non-specific) effects of vaccines also highlights the broader consequences of immunization. Importantly, the WHO led Malaria Vaccine Technology Roadmap illustrates that there is a political will among the global health community to make it happen.

Innate Lymphocytes and Malaria - Players or Spectators?

Malaria remains an important global disease. Despite significant advances over the past decade in reducing disease morbidity and mortality, new measures are needed if malaria is to be eliminated. Significant advances in our understanding about host immune responses during malaria have been made, opening up opportunities to generate long-lasting antiparasitic immunity through vaccination or immune therapy. However, there is still much debate over which immune cell populations contribute to immunity to malaria, including innate lymphocytes that comprise recently identified innate lymphoid cells (ILCs) and better known innate-like T cell subsets. Here, we review research on these immune cell subsets and discuss whether they have any important roles in immunity to malaria or if they are redundant.

Gamma/Delta T Cells and Their Role in Protection Against Malaria

Whether and how γδT cells play a protective role in immunity against Plasmodium infection remain open questions. γδT cells expand in patients and mice infected with Plasmodium spp, and cytokine production and cytotoxic responses against blood-stage parasites are observed in vitro. Their expansion is associated with protective immunity induced by irradiated sporozoite immunization, and depletion of γδT cells in some mouse models of malaria excacerbates blood-stage infections. It is now clear that these cells can have many different functions, and data are emerging suggesting that in addition to having direct parasitocidal effects, they can regulate other immune cells during Plasmodium infections. Here we review some of the historic and more recent data on γδT cells, and in light of the new information on their potential protective roles we suggest that it is a good time to re-evaluate their activation requirements, specificity and function during malaria.

A Unique Subset of γδ T Cells Expands and Produces IL-10 in Patients with Naturally Acquired Immunity against Falciparum Malaria

Although expansions in γδ T cell populations are known to occur in the peripheral blood of patients infected with Plasmodium falciparum, the role of these cells in people with naturally acquired immunity against P. falciparum who live in malaria-endemic areas is poorly understood. We used a cross-sectional survey to investigate the role of peripheral blood γδ T cells in people living in Lao People's Democratic Republic, a malaria-endemic area. We found that the proportion of non-Vγ9 γδ T cells was higher in non-hospitalized uncomplicated falciparum malaria patients (UMPs) from this region. Notably, we found that the non-Vγ9 γδ T cells in the peripheral blood of UMPs and negative controls from this region had the potential to expand and produce IL-10 and interferon-γ when cultured in the presence of IL-2 and/or crude P. falciparum antigens for 10 days. Furthermore, these cells were associated with plasma interleukin 10 (IL-10), which was elevated in UMPs. This is the first report demonstrating that, in UMPs living in a malaria-endemic area, a γδ T cell subset, the non-Vγ9 γδT cells, expands and produces IL-10. These results contribute to understanding of the mechanisms of naturally acquired immunity against P. falciparum.

Real-time imaging reveals the dynamics of leukocyte behaviour during experimental cerebral malaria pathogenesis

During experimental cerebral malaria (ECM) mice develop a lethal neuropathological syndrome associated with microcirculatory dysfunction and intravascular leukocyte sequestration. The precise spatio-temporal context in which the intravascular immune response unfolds is incompletely understood. We developed a 2-photon intravital microscopy (2P-IVM)-based brain-imaging model to monitor the real-time behaviour of leukocytes directly within the brain vasculature during ECM. Ly6C^hi monocytes, but not neutrophils, started to accumulate in the blood vessels of Plasmodium berghei ANKA (PbA)-infected MacGreen mice, in which myeloid cells express GFP, one to two days prior to the onset of the neurological signs (NS). A decrease in the rolling speed of monocytes, a measure of endothelial cell activation, was associated with progressive worsening of clinical symptoms. Adoptive transfer experiments with defined immune cell subsets in recombinase activating gene (RAG)-1-deficient mice showed that these changes were mediated by Plasmodium-specific CD8⁺ T lymphocytes. A critical number of CD8⁺ T effectors was required to induce disease and monocyte adherence to the vasculature. Depletion of monocytes at the onset of disease symptoms resulted in decreased lymphocyte accumulation, suggesting reciprocal effects of monocytes and T cells on their recruitment within the brain. Together, our studies define the real-time kinetics of leukocyte behaviour in the central nervous system during ECM, and reveal a significant role for Plasmodium-specific CD8⁺ T lymphocytes in regulating vascular pathology in this disease.

Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection that takes a significant toll on human life. Blockage of the brain blood vessels contributes to the clinical signs of CM, however we know little about the precise pathological events that lead to this disease. To this end, studies in Plasmodium-infected mice, that also develop a similar fatal disease, have proven useful. These studies have revealed an important role for leukocytes not so much in protecting but rather promoting pathology in the brain. To better understand leukocyte behaviour during experimental CM, we established a brain-imaging model that allows us to ‘peek’ into the brain of living mice and watch immunological events as they unfold. We found that worsening of disease was accompanied by an accumulation of monocytes in the blood vessels. Monocyte accumulation was regulated by activated CD8⁺ T cells but only when present in critical numbers. Monocyte depletion resulted in reduced T cell trafficking to the brain, but this did not result in improved disease outcome. Our studies reveal the orchestration of leukocyte accumulation in real time during CM, and demonstrate that CD8⁺ T cells play a crucial role in promoting clinical signs in this disease.

The spleen CD4+ T cell response to blood-stage Plasmodium chabaudi malaria develops in two phases characterized by different properties

The pivotal role of spleen CD4⁺ T cells in the development of both malaria pathogenesis and protective immunity makes necessary a profound comprehension of the mechanisms involved in their activation and regulation during Plasmodium infection. Herein, we examined in detail the behaviour of non-conventional and conventional splenic CD4⁺ T cells during P. chabaudi malaria. We took advantage of the fact that a great proportion of CD4⁺ T cells generated in CD1d^-/- mice are I-A^b-restricted (conventional cells), while their counterparts in I-A^b-/- mice are restricted by CD1d and other class IB major histocompatibility complex (MHC) molecules (non-conventional cells). We found that conventional CD4⁺ T cells are the main protagonists of the immune response to infection, which develops in two consecutive phases concomitant with acute and chronic parasitaemias. The early phase of the conventional CD4⁺ T cell response is intense and short lasting, rapidly providing large amounts of proinflammatory cytokines and helping follicular and marginal zone B cells to secrete polyclonal immunoglobulin. Both TNF-α and IFN-γ production depend mostly on conventional CD4⁺ T cells. IFN-γ is produced simultaneously by non-conventional and conventional CD4⁺ T cells. The early phase of the response finishes after a week of infection, with the elimination of a large proportion of CD4⁺ T cells, which then gives opportunity to the development of acquired immunity. Unexpectedly, the major contribution of CD1d-restricted CD4⁺ T cells occurs at the beginning of the second phase of the response, but not earlier, helping both IFN-γ and parasite-specific antibody production. We concluded that conventional CD4⁺ T cells have a central role from the onset of P. chabaudi malaria, acting in parallel with non-conventional CD4⁺ T cells as a link between innate and acquired immunity. This study contributes to the understanding of malaria immunology and opens a perspective for future studies designed to decipher the molecular mechanisms behind immune responses to Plasmodium infection.

Gammadelta T cells but not NK cells are essential for cell-mediated immunity against Plasmodium chabaudi malaria

Blood-stage Plasmodium chabaudi infections are suppressed by antibody-mediated immunity and/or cell-mediated immunity (CMI). To determine the contributions of NK cells and γδ T cells to protective immunity, C57BL/6 (wild-type [WT]) mice and B-cell-deficient (J(H(-/-))) mice were infected with P. chabaudi and depleted of NK cells or γδ T cells with monoclonal antibody. The time courses of parasitemia in NK-cell-depleted WT mice and J(H(-/-)) mice were similar to those of control mice, indicating that deficiencies in NK cells, NKT cells, or CD8(+) T cells had little effect on parasitemia. In contrast, high levels of noncuring parasitemia occurred in J(H(-/-)) mice depleted of γδ T cells. Depletion of γδ T cells during chronic parasitemia in B-cell-deficient J(H(-/-)) mice resulted in an immediate and marked exacerbation of parasitemia, suggesting that γδ T cells have a direct killing effect in vivo on blood-stage parasites. Cytokine analyses revealed that levels of interleukin-10, gamma interferon (IFN-γ), and macrophage chemoattractant protein 1 (MCP-1) in the sera of γδ T-cell-depleted mice were significantly (P < 0.05) decreased compared to hamster immunoglobulin-injected controls, but these cytokine levels were similar in NK-cell-depleted mice and their controls. The time courses of parasitemia in CCR2(-/-) and J(H(-/-)) × CCR2(-/-) mice and in their controls were nearly identical, indicating that MCP-1 is not required for the control of parasitemia. Collectively, these data indicate that the suppression of acute P. chabaudi infection by CMI is γδ T cell dependent, is independent of NK cells, and may be attributed to the deficient IFN-γ response seen early in γδ T-cell-depleted mice.

A double-edged sword: the role of NKT cells in malaria and HIV infection and immunity

NKT cells are known to play a role against certain microbial infections, including malaria and HIV, two major global infectious diseases. NKT cells exhibit either protective or pathogenic role against malaria. They are depleted by HIV infection and have a direct pathogenic role against many opportunistic infections common in end-stage AIDS. This review discusses the various features of the interaction between NKT cells and malaria parasites and HIV, and the potential to harness this interaction for therapeutic and vaccine strategies.

Correlation of memory T cell responses against TRAP with protection from clinical malaria, and CD4 CD25 high T cells with susceptibility in Kenyans

Background

Immunity to malaria develops naturally in endemic regions, but the protective immune mechanisms are poorly understood. Many vaccination strategies aim to induce T cells against diverse pre-erythrocytic antigens, but correlates of protection in the field have been limited. The objective of this study was to investigate cell-mediated immune correlates of protection in natural malaria. Memory T cells reactive against thrombospondin-related adhesive protein (TRAP) and circumsporozoite (CS) protein, major vaccine candidate antigens, were measured, as were frequencies of CD4⁺ CD25^high T cells, which may suppress immunity, and CD56⁺ NK cells and γδ T cells, which may be effectors or may modulate immunity.

Methodology and Principal Findings

112 healthy volunteers living in rural Kenya were entered in the study. Memory T cells reactive against TRAP and CS were measured using a cultured IFNγ ELISPOT approach, whilst CD4⁺ CD25^high T cells, CD56⁺ NK cells, and γδ T cells were measured by flow cytometry. We found that T cell responses against TRAP were established early in life (<5 years) in contrast to CS, and cultured ELISPOT memory T cell responses did not correlate with ex-vivo IFNγ ELISPOT effector responses. Data was examined for associations with risk of clinical malaria for a period of 300 days. Multivariate logistic analysis incorporating age and CS response showed that cultured memory T cell responses against TRAP were associated with a significantly reduced incidence of malaria (p = 0.028). This was not seen for CS responses. Higher numbers of CD4⁺ CD25^high T cells, potentially regulatory T cells, were associated with a significantly increased risk of clinical malaria (p = 0.039).

Conclusions

These data demonstrate a role for central memory T cells in natural malarial immunity and support current vaccination strategies aimed at inducing durable protective T cell responses against the TRAP antigen. They also suggest that CD4⁺ CD25^high T cells may negatively affect naturally acquired malarial immunity.