Expansion of the CD4-, CD8- gamma delta T cell subset in the spleens of mice during non-lethal blood-stage malaria

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.

γδ T Cells Kill Plasmodium falciparum in a Granzyme- and Granulysin-Dependent Mechanism during the Late Blood Stage

Plasmodium spp., the causative agent of malaria, have a complex life cycle. The exponential growth of the parasites during the blood stage is responsible for almost all malaria-associated morbidity and mortality. Therefore, tight immune control of the intraerythrocytic replication of the parasite is essential to prevent clinical malaria. Despite evidence that the particular lymphocyte subset of γδ T cells contributes to protective immunity during the blood stage in naive hosts, their precise inhibitory mechanisms remain unclear. Using human PBMCs, we confirmed in this study that γδ T cells specifically and massively expanded upon activation with Plasmodium falciparum culture supernatant. We also demonstrate that these activated cells gain cytolytic potential by upregulating cytotoxic effector proteins and IFN-γ. The killer cells bound to infected RBCs and killed intracellular P. falciparum via the transfer of the granzymes, which was mediated by granulysin in a stage-specific manner. Several vital plasmodial proteins were efficiently destroyed by granzyme B, suggesting proteolytic degradation of these proteins as essential in the lymphocyte-mediated death pathway. Overall, these data establish a granzyme- and granulysin-mediated innate immune mechanism exerted by γδ T cells to kill late-stage blood-residing P. falciparum.

Metformin Promotes the Protection of Mice Infected With Plasmodium yoelii Independently of γδ T Cell Expansion

Adaptive immune responses are critical for protection against infection with Plasmodium parasites. The metabolic state dramatically changes in T cells during activation and the memory phase. Recent findings suggest that metformin, a medication for treating type-II diabetes, enhances T-cell immune responses by modulating lymphocyte metabolism. In this study, we investigated whether metformin could enhance anti-malaria immunity. Mice were infected with Plasmodium yoelii and administered metformin. Levels of parasitemia were reduced in treated mice compared with those in untreated mice, starting at ~2 weeks post-infection. The number of γδ T cells dramatically increased in the spleens of treated mice compared with that in untreated mice during the later phase of infection, while that of αβ T cells did not. The proportions of Vγ1⁺ and Vγ2⁺ γδ T cells increased, suggesting that activated cells were selectively expanded. However, these γδ T cells expressed inhibitory receptors and had severe defects in cytokine production, suggesting that they were in a state of exhaustion. Metformin was unable to rescue the cells from exhaustion at this stage. Depletion of γδ T cells with antibody treatment did not affect the reduction of parasitemia in metformin-treated mice, suggesting that the effect of metformin on the reduction of parasitemia was independent of γδ T cells.

γδ T Cells in Antimalarial Immunity: New Insights Into Their Diverse Functions in Protection and Tolerance

Uniquely expressing diverse innate-like and adaptive-like functions, γδ T cells exist as specialized subsets, but are also able to adapt in response to environmental cues. These cells have long been known to rapidly proliferate following primary malaria infection in humans and mice, but exciting new work is shedding light into their diverse functions in protection and following repeated malaria infection. In this review, we examine the current knowledge of functional specialization of γδ T cells in malaria, and the mechanisms dictating recognition of malaria parasites and resulting proliferation. We discuss γδ T cell plasticity, including changing interactions with other immune cells during recurrent infection and potential for immunological memory in response to repeated stimulation. Building on recent insights from human and murine experimental studies and vaccine trials, we propose areas for future research, as well as applications for therapeutic development.

A Macrophage Colony-Stimulating-Factor-Producing γδ T Cell Subset Prevents Malarial Parasitemic Recurrence

Despite evidence that γδ T cells play an important role during malaria, their precise role remains unclear. During murine malaria induced by Plasmodium chabaudi infection and in human P. falciparum infection, we found that γδ T cells expanded rapidly after resolution of acute parasitemia, in contrast to αβ T cells that expanded at the acute stage and then declined. Single-cell sequencing showed that TRAV15N-1 (Vδ6.3) γδ T cells were clonally expanded in mice and had convergent complementarity-determining region 3 sequences. These γδ T cells expressed specific cytokines, M-CSF, CCL5, CCL3, which are known to act on myeloid cells, indicating that this γδ T cell subset might have distinct functions. Both γδ T cells and M-CSF were necessary for preventing parasitemic recurrence. These findings point to an M-CSF-producing γδ T cell subset that fulfills a specialized protective role in the later stage of malaria infection when αβ T cells have declined.

Signalling through the IL-2 receptor γ(c) peptide (CD132) is essential for the expression of immunity to Plasmodium chabaudi adami blood-stage malaria

A genetic dissection approach was employed to determine whether the IL-2 receptor complex (IL-2R) comprised of α, β and γ chains is required for the suppression of Plasmodium chabaudi adami parasitemia. Blood-stage infections in IL-2Rγ(c)(-/y) mice failed to cure with parasitemia remaining elevated for > 50 days indicating the IL-2Rγ(c) through which all members of the γ(c) family of cytokines signal has an essential role in protective immunity against blood-stage malarial parasites. In contrast, the curing of parasitemia in IL-2/15Rβ⁻/⁻ mice, deficient in both IL-2 and IL-15 signalling was significantly delayed but did occur, indicating that neither cytokine plays an essential role in parasite clearance. Moreover, the observation that the time course of parasitemia in IL-15⁻/⁻ mice was nearly identical to that seen in controls suggests that the parasitemia-suppressing role of stimulating through the IL-2/15Rβ chain is owing to IL-2 signalling and not a redundant function of IL-15.

Splenic gammadelta T cells regulated by CD4+ T cells are required to control chronic Plasmodium chabaudi malaria in the B-cell-deficient mouse

Little is known about the function and regulation of splenic gammadelta T cells during chronic Plasmodium chabaudi malaria. The splenic gammadelta T-cell population continues to expand, reaching levels equal to 4 times the number of splenocytes in an uninfected mouse. Splenic gammadelta T cells from J(H)-/- mice with chronic malaria expressed Vgamma1+ or Vdelta4+ in the same ratio as uninfected controls with Vgamma1 cells dominating, but the Vgamma2 ratio declined about twofold. Gammadelta T cells from G8 mice specific for the TL antigen increased only 2-fold in number, compared with 10-fold in BALB/c controls, but G8 gammadelta T cells failed to express the B220 activation marker. Elimination of the parasite by drug treatment caused a slow depletion in the number of splenic gammadelta, CD4+, and CD8+ T cells. Following challenge, drug-cured J(H)-/- mice exhibited nearly identical parasitemia time courses as naïve controls. Depletion of either CD4+ T cells or gammadelta T cells from chronically infected J(H)-/- mice by monoclonal antibody treatment resulted in an immediate and significant (P < 0.05) exacerbation of parasitemia coupled with a marked decrease in splenic gammadelta T-cell numbers. The number of CD4+ T cells, in contrast, did not decrease in mice after anti-T-cell receptor gammadelta treatment. The results indicate that cell-mediated immunity against blood-stage malarial parasites during chronic malaria (i) requires the continued presence of blood-stage parasites to remain functional, (ii) is dependent upon both gammadelta T cells and CD4+ T cells, and (iii) lacks immunological memory.

Plasmodium chabaudi chabaudi (AS): Differential cellular responses to infection in resistant and susceptible mice

The infection with blood stages of Plasmodium chabaudi chabaudi (AS) was followed in BALB/c and DBA/2 mice. Both strains show a peak parasitemia by 7-9 days after infection, display splenic hypercellularity of T and B cells, thymic atrophy, nearly complete depletion of B cells in the bone marrow, and mount comparable polyclonal IgM and IgG responses in the serum. In contrast, these strains diverge in some aspects of the immune response and susceptibility to infection: while BALB/c survive, 70-80% of DBA/2 die within 2 weeks; BALB/c but not DBA/2 show marked increases in the levels of splenic gamma/delta and regulatory T cells, dendritic cells and macrophages and parasite-specific IgM and IgG levels; however, lower levels of TNF-alpha and IL-12 were observed. These results suggest the relevance of different cell populations that are known to participate/regulate specific antibody responses and cytokine production in the susceptibility to infection.

Regulation of immunity to malaria: valuable lessons learned from murine models

A major advance in immunology has been the establishment of a framework for analysing how certain immune responses dominate following exposure to a particular pathogen or antigen. CD4(+) T helper (Th) cells can be separated into two major subsets which mediate qualitatively distinct cell-mediated (Th1) and humoral (Th2) immune responses. Immunity to most pathogens can be broadly categorized into a predominant protective response of either type. A characteristic of murine malarias is that primary infections with asexual erythrocytic parasites (the pathogenic stage of the malaria life cycle) generate a host protective immune response with a broad spectrum of Th1- and Th2-type CD4(+) T-cell involvement and so can be examined as models of the interaction of Th1 and Th2 cells during an immune response to an infectious agent. Andrew Taylor-Robinson here describes recent events in the dissection of the mechanisms responsible for the generation of protective immunity to Plasmodium chabaudi chabaudi and other experimental malarias in mice.

Dendritic cells induce immunity and long-lasting protection against blood-stage malaria despite an in vitro parasite-induced maturation defect

Dendritic cells (DC) suffer a maturation defect following interaction with erythrocytes infected with malaria parasites and become unable to induce protective malaria liver-stage immunity. Here we show that, by contrast, maturation-arrested DC in vitro are capable of the successful induction of antigen-specific gamma interferon (IFN-gamma) and interleukin 4 (IL-4) T-cell responses, antibody responses, and potent protection against lethal blood-stage malaria challenge in vivo. Similar results were found with DC pulsed with intact parasitized Plasmodium yoelii or Plasmodium chabaudi erythrocytes. Cross-strain protection was also induced. High levels of protection (80 to 100%) against lethal challenge were evident from 10 days after a single immunization and maintained up to 120 days. Interestingly, correlation studies versus blood-stage protection at different time points suggest that the immune effector mechanisms associated with protection could change over time. Antibody-independent, T-cell- and IL-12-associated protection was observed early after immunization, followed by antibody and IL-4-associated, IFN-gamma-independent protection in long-term studies. These results indicate that DC, even when clearly susceptible to parasite-induced maturation defect effects in vitro, can be central to the induction of protection against blood-stage malaria in vivo.

The influence of gammadelta T cells on the CD4+ T cell and antibody response during a primary Plasmodium chabaudi chabaudi infection in mice

A primary infection with Plasmodium chabaudi chabaudi (AS) is characterized by an expansion of gammadelta cells after the acute phase of infection in mice. This is particularly marked during chronic infections in B cell-deficient mice. Infections in gammadelta T cell-deficient mice suggest that, although these cells play some role in the control of parasitaemia and can produce interferon-gamma, they do not appear to be involved in the development of hypoglycaemia, loss of weight and temperature during a P. c. chabaudi infection. However, gammadelta T cells do influence the nature of the CD4+ T cell response during infection since, in their absence, Th2-like responses, such as interleukin (IL)-4 production and help for malaria-specific antibody responses, are more pronounced. This alteration in CD4+ T cells is reflected in a more rapid and greater immunoglobulin (Ig)G1 and IgG3 antibody response to the parasite. The large gammadelta T cell expansion normally observed in infected B cell-deficient mice did not take place in the absence of IL-2, and double-knockout mice lacking both B cells and functional IL-2 were highly susceptible to lethal infection with P. c. chabaudi. The majority of the single IL-2 knockout mice, in contrast, were able to control and clear a primary infection, suggesting that for the CD4+ T cell and antibody response, IL-2 could be replaced by other cytokines.

Activation of T cells in the blood of patients with acute malaria: proliferative activity as indicated by Ki-67 expression

The expression of the proliferation-associated nuclear antigen Ki-67 in peripheral blood mononuclear cells was studied in 30 patients with acute malarial illness and 11 healthy controls from Addis Ababa or Nazareth in Ethiopia. Seventeen patients had Plasmodium falciparum infections and 13 had Plasmodium vivax. Two-colour immunoenzymatic staining was developed in order to simultaneously detect the expression of the nuclear antigen Ki-67 and determine the surface phenotype of the cell. The median percentage of proliferating, Ki-67 positive lymphocytes was significantly higher in patients with acute P. falciparum (11.8%) and P. vivax (15.6%) illnesses compared to the controls (4.3%). The majority of Ki-67 positive cells were T cells (CD3+) while the relative increase of Ki-67 expressing cells was similar for both the CD4+ and CD8+ T-cell subsets. Our data show an increased number of activated cells driven to proliferation in the peripheral blood of patients during acute malaria illness.

CD8(+) T cell knockout mice are less susceptible to Cowdria ruminantium infection than athymic, CD4(+) T cell knockout, and normal C57BL/6 mice

The role of T cells in immunity to Cowdria ruminantium was investigated by studying the responses to infection of normal, athymic, CD4(+) T cell knock out (KO) and CD8(+) T cell KO C57BL/6 mice. Normal C57BL/6 mice could be immunized by infection and treatment, and immunity was adoptively transferable from immune to naive mice by splenocytes. Following infection, athymic mice died sooner than normal mice (P=0.0017), and could not be immunized by infection and treatment. CD4(+) T cell KO mice were as susceptible to infection as normal mice and could be immunized by infection and treatment. In contrast, CD8(+) T cell KO mice were less susceptible than normal and CD4(+) T cell KO mice and 43% self-cured, while those that died did so after a prolonged incubation period. Antibody responses to C. ruminantium were CD4(+) T cell dependent, because responses were detected in immune normal and CD8(+) T cell KO mice but not in immune CD4(+) KO mice (P=0.005). Since CD8(+) T cell KO mice were less susceptible to infection, and since CD4(+) T cell KO mice could be immunized, it can be concluded that immunity to C. ruminantium can be mediated by both CD4(+) and CD8(+) T cells.

T cell response in malaria pathogenesis: selective increase in T cells carrying the TCR V(beta)8 during experimental cerebral malaria

To characterize the T cells involved in the pathogenesis of cerebral malaria (CM) induced by infection with Plasmodium berghei ANKA clone 1.49L (PbA 1.49L), the occurrence of the disease was assessed in mice lacking T cells of either the alphabeta or gammadelta lineage (TCRalphabeta(-/-) or TCRgammadelta(-/-)). TCRgammadelta(-/-) mice were susceptible to CM, whereas all TCRalphabeta(-/-) mice were resistant, suggesting that T cells of the alphabeta lineage are important in the genesis of CM. The repertoire of TCR V(beta) segment gene expression was examined by flow cytometry in B10.D2 mice, a strain highly susceptible to CM induced by infection with PbA 1.49L. In these mice, CM was associated with an increase of T cells bearing the V(beta)8.1, 2 segments in the peripheral blood lymphocytes. Most V(beta)8.1, 2(+) T cells from peripheral blood lymphocytes of the mice that developed CM belonged to the CD8 subset, and exhibited the CD69(+), CD44(high) and CD62L(low) phenotype surface markers. The link between the increase in V(beta)8.1, 2(+) T cells and the neuropathological consequences of PbA infection was strengthened by the observation that the occurrence of CM was significantly reduced in mice treated with KJ16 antibodies against the V(beta)8.1 and V(beta)8.2 chains, and in mice rendered deficient in V(beta)8.1(+) T cells by a mouse mammary tumor virus superantigen.

Plasticity of Immune Responses Suppressing Parasitemia During Acute Plasmodium chabaudi Malaria

γδ T cells have a crucial role in cell-mediated immunity (CMI) against P. chabaudi malaria, but δ-chain knockout (KO) (δo/o) mice and mice depleted of γδ T cells with mAb cure this infection. To address the question of why mice deficient in γδ T cells resolve P. chabaudi infections, we immunized δo/o mice by infection with viable blood-stage parasites. Sera from infection-immunized mice were tested for their ability to protect JHo/o, δo/o double KO mice passively against P. chabaudi challenge infection. The onset of parasitemia was significantly delayed in mice receiving immune sera, compared with saline or uninfected serum controls. Immune sera were then fractionated into Ig-rich and Ig-depleted fractions by HPLC on a protein G column. Double KO mice were passively immunized with either fraction and challenged with P. chabaudi. The onset of parasitemia was significantly delayed in recipients of the Ig-rich fraction compared with recipients of the Ig-poor fraction of immune sera. We conclude that δo/o mice, which are unable to activate CMI against the parasite, suppress P. chabaudi infection by a redundant Ab-mediated process.

γδ T Cells Contribute to Control of Chronic Parasitemia in Plasmodium chabaudi Infections in Mice

During a primary infection of mice with Plasmodium chabaudi, γδ T cells are stimulated and their expansion coincides with recovery from the acute phase of infection in normal mice or with chronic infections in B cell-deficient mice (μ-MT). To determine whether the large γδ T cell pool observed in female B cell-deficient mice is responsible for controlling the chronic infection, studies were done using double-knockout mice deficient in both B and γδ cells (μ-MT × δ−/−TCR) and in γδ T cell-depleted μ-MT mice. In both types of γδ T cell-deficient mice, the early parasitemia following the peak of infection was exacerbated, and the chronic parasitemia was maintained at significantly higher levels in the absence of γδ T cells. The majority of γδ T cells in C57BL/6 and μ-MT mice responding to infection belonged predominantly to a single family of γδ T cells with TCR composed of Vγ2Vδ4 chains and which produced IFN-γ rather than IL-4.

Murine gamma delta T lymphocytes elicited during Plasmodium yoelii infection respond to Plasmodium heat shock proteins

gamma delta T cells accumulate during Plasmodium infections in both murine and human malarias. The biological role of these cells and the antigens that they recognize are not clearly understood, although recent findings indicate that gamma delta T cells in general influence both innate and antigen-specific adaptive host responses. We examined the accumulation of gamma delta T cells elicited during infection with virulent and avirulent Plasmodium yoelii parasites in relatively susceptible and resistant strains of mice. Our results indicated that in nonlethal malaria infections, gamma delta T cells comprise a larger proportion of splenic T cells than in lethal infections and that only a live infection is capable of inducing an increase in the percentage of gamma delta T cells in vivo. Furthermore, we demonstrate that gamma delta T cells elicited during a P. yoelii infection respond by proliferation in vitro to P. falciparum heat shock proteins (HSPs) of 60 and 70 kDa, suggesting a possible immunological involvement of parasite HSPs in this arm of the cellular immune response during malarial infection in mice.

Gamma delta T cells: their immunobiology and role in malaria infections

The status of research on gamma delta T cells is reviewed. Recent research shows that gamma delta T cells may see antigens in an immunoglobulin-like manner and that non-peptidic substance can be antigens for these cells. Considerable advances have been made in defining the immunobiology of gamma delta T cells, with evidence for sentinel, protective and immunoregulatory roles. Research on gamma delta T cells in malaria infections suggests that gamma delta T cells are mediators of protective immunity, most probably through the production of Th1 cytokines such as TNF alpha, TNF delta and IFN gamma and that excessive production of such cytokines may contribute to pathology. Our data on the features of the peripheral blood gamma delta T cells response in humans infected with Plasmodium falciparum show that there is considerable variation between individuals in the relative expansion of gamma delta T lymphocytes following primary or secondary infection. They confirm that activation of gamma delta T cells occurs during P. falciparum infection and that activated cells can persist for many weeks after treatment. The possibility that gamma delta T cells have an immunoregulatory function in malaria infections is proposed.

γδ T cells in malaria infections

The association of a pronounced gammadelta T-cell response with Plasmodium infections is intriguing. The ability of parasite material to activate gammadelta T cells in vitro, and the localization of these cells in vivo in the red pulp of the spleen, suggests that these cells could play a role in the killing of bloodstage malaria parasites. However, the magnitude, the response and the predominance of inflammatory cytokines secreted by these cells may also indicate a role in the pathology of malaria infections. In this article, Jean Langhorne reveiws the current status of gammadelta T cells in malaria in the context of what is known about the function and specificity of gammadelta T cells in general.

gamma/delta and other unconventional T lymphocytes: what do they see and what do they do?

T lymphocytes recognize specific ligands by clonally distributed T-cell receptors (TCR). In humans and most animals, the vast majority of T cells express a TCR composed of an alpha chain and a beta chain, whereas a minor T-cell population is characterized by the TCR gamma/delta. Almost all of our knowledge about T cells stems from alpha/beta T cells and only now are we beginning to understand gamma/delta T cells. In contrast to conventional alpha/beta T cells, which are specific for antigenic peptides presented by gene products of the major histocompatibility complex, gamma/delta T cells directly recognize proteins and even nonproteinacious phospholigands. These findings reveal that gamma/delta T cells and alpha/beta T cells recognize antigen in a fundamentally different way and hence mitigate the dogma of exclusive peptide-major histocompatibility complex recognition by T cells. A role for gamma/delta T cells in antimicrobial immunity has been firmly established. Although some gamma/delta T cells perform effector functions, regulation of the professional and the nonprofessional immune system seems to be of at least equal importance. The prominent residence of gamma/delta T cells in epithelial tissues and the rapid mobilization of gamma/delta T cells in response to infection are consistent with such regulatory activities under physiological and pathologic conditions. Thus, although gamma/delta T cells are a minor fraction of all T cells, they are not just uninfluential kin of alpha/beta T cells but have their unique raison d'être.

Cytokine profiles for human V gamma 9+ T cells stimulated by Plasmodium falciparum

V gamma 9+ T cells from malaria non-exposed donors make proliferative responses to Plasmodium falciparum on in vitro stimulation. V gamma 9+ cells are strongly activated by components of the schizont stage of the parasite and by antigens released into the culture upon schizogony, while CD4+V gamma 9- cells are stimulated by the earlier stages of the parasite. Using reverse transcriptase-polymerase chain reaction (RT-PCR) we determined mRNA expression for 14 cytokines in highly purified V gamma 9+ cells enriched by positive selection after in vitro stimulation with P. falciparum schizont antigens. Interferon-gamma (IFN-gamma) and Tumor Necrosis Factor-alpha (TNF-alpha) were detected in all samples tested. The majority of samples also expressed TNF-beta, transforming growth factor-beta (TGF-beta) and Interleukin-8 (IL-8). Only occasional samples expressed IL-2, IL-5 and IL-10. Using the ELISPOT assay we found that a large fraction of the reactive V gamma 9+ cells produced IFN-gamma and that gamma delta T cells are the major producers of IFN-gamma in cultures stimulated with schizont antigens. The majority of V gamma 9+ cells in these cultures also express the membrane-bound form of TNF-alpha. Expression of these cytokines speaks for a cytolytic and/or inflammatory role of gamma delta cells in the response to malaria in non-exposed individuals.

Gamma delta T cell-induced nitric oxide production enhances resistance to mucosal candidiasis

Despite the prevalence of gamma delta T cells in mucosae that are typically colonized by Candida albicans, little is known of the possible role of these cells in resistance to candidiasis. A sharp increase in the number of gamma delta T cells and macrophages following intraperitoneal inoculation of mice with C. albicans led us to examine the role of these cells in the immune response to C. albicans. We show that the gamma delta T cells enhance macrophage nitric oxide (NO) production and anti-candida activity, in vitro. We also propose that the gamma delta T cells regulate macrophage function during candidiasis in vivo as well, because depletion of these cells abrogated inducible NO synthase expression in mucosae and enhanced murine susceptibility to candidiasis.

Activation and function of gamma delta T cells

T-cell receptor gene rearrangements in gamma delta T cells have been the subject of intense molecular investigations. This year, much has been learned about the mechanisms controlling this process. However, the specificity and function of gamma delta T cells still remains enigmatic. The application of molecular technology including the availability of mutant mice lacking defined T-cell populations and immunologically relevant surface proteins is beginning to provide answers as well as some surprises.

Antigens recognized by gamma delta T cells

Recent evidence demonstrates the important role played by gamma delta T cells in resistance to infections. Despite this, in most cases the antigens recognized by the responding gamma delta T cells are unknown. Antigen recognition by some gamma delta T-cell populations may not require the participation of either MHC class I or class II molecules. In other cases, evidence exists for the participation of MHC-encoded molecules, particularly non-classical class I molecules.