Natural regulatory T cells mediate the development of cerebral malaria by modifying the pro-inflammatory response

Extracellular vesicles derived from plasmodium-infected red blood cells alleviate cerebral malaria in plasmodium berghei ANKA-infected C57BL/6J mice

RTS,S is the first malaria vaccine recommended for implementation among young children at risk. However, vaccine efficacy is modest and short-lived. To mitigate the risk of cerebral malaria (CM) among children under the age of 5, it is imperative to develop new vaccines. EVs are potential vaccine candidates as they obtain the ability of brain-targeted delivery and transfer plasmodium antigens and immunomodulators during infections. This study extracted EVs from BALB/c mice infected with Plasmodium yoelii 17XNL (P.y17XNL). C57BL/6J mice were intravenously immunized with EVs (EV-I.V. + CM group) or subcutaneously vaccinated with the combination of EVs and CpG ODN-1826 (EV + CPG ODN-S.C. + CM group) on days 0 and 20, followed by infection with Plasmodium berghei ANKA (P.bANKA) on day 20 post-second immunization. We monitored Parasitemia and survival rate. The integrity of the Blood-brain barrier (BBB) was examined using Evans blue staining.The levels of cytokines and adhesion molecules were evaluated using Luminex, RT-qPCR, and WB. Brain pathology was evaluated by hematoxylin and eosin and immunohistochemical staining. The serum levels of IgG, IgG1, and IgG2a were analyzed by enzyme-linked immunosorbent assay. Compared with those in the P.bANKA-infected group, parasitemia increased slowly, death was delayed (day 10 post-infection), and the survival rate reached 75 %-83.3 % in the EV-I.V. + ECM and EV + CPG ODN-S.C. + ECM groups. Meanwhile, compared with the EV + CPG ODN-S.C. + ECM group, although parasitemia was almost the same, the survival rate increased in the EV-I.V. + ECM group.Additionally, EVs immunization markedly downregulated inflammatory responses in the spleen and brain and ameliorated brain pathological changes, including BBB disruption and infected red blood cell (iRBC) sequestration. Furthermore, the EVs immunization group exhibited enhanced antibody responses (upregulation of IgG1 and IgG2a production) compared to the normal control group. EV immunization exerted protective effects, improving the integrity of the BBB, downregulating inflammation response of brain tissue, result in reduces the incidence of CM. The protective effects were determined by immunological pathways and brain targets elicited by EVs. Intravenous immunization exhibited better performance than subcutaneous immunization, which perhaps correlated with EVs, which can naturally cross BBB to play a better role in brain protection.

Oral administration of IPI549 protects mice from neuropathology and an overwhelming inflammatory response during experimental cerebral malaria

Infection with Plasmodium falciparum is often deadly when it results in cerebral malaria, which is associated with neuropathology described as an overwhelming inflammatory response and mechanical obstruction of cerebral microvascular. PI3Kγ is a critical component of intracellular signal transduction and plays a central role in regulating cell chemotaxis, migration, and activation. The purpose of this study was to examine the relationship between inhibiting the PI3Kγ pathway and the outcome of experimental cerebral malaria (ECM) in C57BL/6J mice infected with the mouse malaria parasite, Plasmodium berghei ANKA. We observed that oral administration of the PI3Kγ inhibitor IPI549 after infection completely protected mice from ECM. IPI549 treatment significantly dampened the magnitude of inflammatory responses, with reduced production of pro-inflammatory factors, decreased T cell activation, and altered differentiation of antigen-presenting cells. IPI549 treatment protected the infected mice from neuropathology, as assessed by an observed reduction of pathogenic T cells in the brain. Treating the infected mice with IPI549 three days after parasite inoculation improved the murine blood brain barrier (BBB) integrity and helped the mice pass the onset of ECM. Together, these data indicate that oral administration of the PI3Kγ inhibitor IPI549 has a suppressive role in host inflammation and alleviates cerebral pathology, which supports IPI549 as a new malaria treatment option with potential therapeutic implications for cerebral malaria.

CD4+ICOS+Foxp3+: a sub-population of regulatory T cells contribute to malaria pathogenesis

BACKGROUND

Regulatory T cells are known to play a key role to counter balance the protective immune response and immune mediated pathology. However, the role of naturally occurring regulatory cells CD4⁺CD25⁺Foxp3⁺ in malaria infection during the disease pathogenesis is controversial. Beside this, ICOS molecule has been shown to be involved in the development and function of regulatory T cell enhance IL-10 production. Therefore, possible involvement of the ICOS dependent regulatory CD4⁺ICOS⁺Foxp3⁺ T cells in resistance/susceptibility during malaria parasite is explored in this study.

METHODS

5 × 10⁵ red blood cells infected with non-lethal and lethal parasites were inoculated in female Balb/c mice by intra-peritoneal injection. Infected or uninfected mice were sacrificed at early (3rd day post infection) and later stage (10th day post infection) of infection. Harvested cells were analysed by using flow cytometer and serum cytokine by Bioplex assay.

RESULTS

Thin blood films show that percentages of parasitaemia increases with disease progression in infections with the lethal malaria parasite and mice eventually die by day 14th post-infection. Whereas in case of non-lethal malaria parasite, parasitaemia goes down by 7th day post infection and gets cleared within 13th day. The number of CD4⁺ ICOS⁺ T cells increases in lethal infection with disease progression. Surprisingly, in non-lethal parasite, ICOS expression decreases after day 7th post infection as parasitaemia goes down. The frequency of CD4⁺ICOS⁺FoxP3⁺ Tregs was significantly higher in lethal parasitic infection as compared to the non-lethal parasite. The level of IL-12 cytokine was remarkably higher in non-lethal infection compared to the lethal infection. In contrast, the level of IL-10 cytokines was higher in lethal parasite infection compared to the non-lethal parasite.

CONCLUSION

Taken together, these data suggest that lethal parasite induce immunosuppressive environment, protecting from host immune responses and help the parasite to survive whereas non-lethal parasite leads to low frequencies of Treg cells seldom impede immune response that allow the parasite to get self-resolved.

Monocyte Locomotion Inhibitory Factor confers neuroprotection and prevents the development of murine cerebral malaria

Cerebral malaria (CM) is a neurological complication derived from the Plasmodium falciparum infection in humans. The mechanisms involved in the disease progression are still not fully understood, but both the sequestration of infected red blood cells (iRBC) and leukocytes and an exacerbated host inflammatory immune response are significant factors. In this study, we investigated the effect of Monocyte Locomotion Inhibitory Factor (MLIF), an anti-inflammatory peptide, in a well-characterized murine model of CM. Our data showed that the administration of MLIF increased the survival and avoided the neurological signs of CM in Plasmodium berghei ANKA (PbA) infected C57BL/6 mice. MLIF administration down-regulated systemic inflammatory mediators such as IFN-γ, TNF-α, IL-6, CXCL2, and CCL2, as well as the in situ expression of TNF-α in the brain. In the same way, MLIF reduced the expression of CD31, CD36, CD54, and CD106 in the cerebral endothelium of infected animals and prevented the sequestration of iRBC and leucocytes in the brain microvasculature. Furthermore, MLIF inhibited the activation of astrocytes and microglia and preserved the integrity of the blood-brain barrier (BBB). In conclusion, our results demonstrated that the administration of MLIF increased survival and conferred neuroprotection by decreasing neuroinflammation in murine CM.

Plasmodium yoelii 17XL infection modified maturation and function of dendritic cells by skewing Tregs and amplificating Th17

Background

Emerging data has suggested that Tregs, Th17, Th1 and Th2 are correlated with early immune mechanisms by controlling Plasmodium infection. Plasmodium infection appeared to impair the antigen presentation and maturation of DCs, leading to attenuation of specific cellular immune response ultimately. Hence, in this study, we aim to evaluate the relevance between DCs and Tregs/Th17 populations in the process and outcomes of infection with Plasmodium yoelii 17XL (P.y17XL).

Methods

DCs detection/analysis dynamically was performed by Tregs depletion or Th17 neutralization in P.y17XL infected BALB/c mice via flow cytometry. Then the levels of cytokines production were detected using enzyme-linked mmunosorbent assay (ELISA).

Results

Our results indicated that Tregs depletion or Th17 neutralization in BALB/c mice infected with P.y17XL significantly up-regulated the percentages of mDC and pDC, increased the expressions of major histocompatibility complex (MHC) class II, CD80, CD86 on DCs and the levels of IL-10/IL-12 secreted by DCs, indicating that abnormal amplification of Tregs or Th17 may damage the maturation and function of DCs during the early stage of malaria infection. Interestingly, we also found that the abnormal amplification of Th17, as well as Tregs, could inhibit the maturation of DCs.

Conclusions

Tregs skewing or Th17 amplification during the early stage of malaria infection may inhibit the maturation and function of DCs by modifying the subsets of DCs, expressions of surface molecules on DCs and secretion mode of cytokines.

Understanding host-parasite relationship: the immune central nervous system microenvironment and its effect on brain infections

The central nervous system (CNS) has been recognized as an immunologically specialized microenvironment, where immune surveillance takes a distinctive character, and where delicate neuronal networks are sustained by anti-inflammatory factors that maintain local homeostasis. However, when a foreign agent such as a parasite establishes in the CNS, a set of immune defences is mounted and several immune molecules are released to promote an array of responses, which ultimately would control the infection and associated damage. Instead, a host-parasite relationship is established, in the context of which a close biochemical coevolution and communication at all organization levels between two complex organisms have developed. The ability of the parasite to establish in its host is associated with several evasion mechanisms to the immune response and its capacity for exploiting host-derived molecules. In this context, the CNS is deeply involved in modulating immune functions, either protective or pathogenic, and possibly in parasitic activity as well, via interactions with evolutionarily conserved molecules such as growth factors, neuropeptides and hormones. This review presents available evidence on some examples of CNS parasitic infections inducing different morbi-mortality grades in low- or middle-income countries, to illustrate how the CNS microenvironment affect pathogen establishment, growth, survival and reproduction in immunocompetent hosts. A better understanding of the influence of the CNS microenvironment on neuroinfections may provide relevant insights into the mechanisms underlying these pathologies.

Protein Tyrosine Phosphatase Inhibition Prevents Experimental Cerebral Malaria by Precluding CXCR3 Expression on T Cells

Cerebral malaria induced by Plasmodium berghei ANKA infection is dependent on the sequestration of cytotoxic T cells within the brain and augmentation of the inflammatory response. Herein, we demonstrate that inhibition of protein tyrosine phosphatase (PTP) activity significantly attenuates T cell sequestration within the brain and prevents the development of neuropathology. Mechanistically, the initial upregulation of CXCR3 on splenic T cells upon T cell receptor stimulation was critically decreased through the reduction of T cell-intrinsic PTP activity. Furthermore, PTP inhibition markedly increased IL-10 production by splenic CD4⁺ T cells by enhancing the frequency of LAG3⁺CD49b⁺ type 1 regulatory cells. Overall, these findings demonstrate that modulation of PTP activity could possibly be utilized in the treatment of cerebral malaria and other CXCR3-mediated diseases.

Regulatory T Cell and Forkhead Box Protein 3 as Modulators of Immune Homeostasis

The transcription factor forkhead box protein 3 (FOXP3) is an essential molecular marker of regulatory T cell (Treg) development in different microenvironments. Tregs are cells specialized in the suppression of inadequate immune responses and the maintenance of homeostatic tolerance. Studies have addressed and elucidated the role played by FOXP3 and Treg in countless autoimmune and infectious diseases as well as in more specific cases, such as cancer. Within this context, the present article reviews aspects of the immunoregulatory profile of FOXP3 and Treg in the management of immune homeostasis, including issues relating to pathology as well as immune tolerance.

Bacillus Calmette-Guérin-inoculation at different time points influences the outcome of C57BL/6 mice infected with Plasmodium chabaudi chabaudi AS

Bacillus Calmette-Guérin (BCG) is an attenuated Mycobacterium tuberculosis vaccine. We performed a series of co-infection experiments with BCG-Plasmodium chabaudi chabaudi Landau, 1965 AS using C57BL/6 mice to analyse whether BCG can affect the development of protective immunity to infection with Plasmodium spp. and the mechanism of this protection. We divided mice into four groups: BCG-inoculation 4 weeks prior to P. c. chabaudi AS infection (B-4w-Pc); simultaneous BCG-inoculation and P. c. chabaudi AS infection (Pc+B); BCG-inoculation 3 days post P. c. chabaudi AS (Pc-3-B) infection; and mono-P. c. chabaudi AS infection as control (Pc). The parasitemia level in the B-4w-Pc group was noticeably higher than control group at 6-19 days post infection (dpi). Compared with the control group, the proportion of CD4(+)CD69(+) T cells was significantly reduced 5, 8 and 12 dpi, but the proportion of CD4(+)CD25(+)Foxp3(+) Tregs was significantly increased in the B-4w-Pc group on 5 and 8 dpi. The B-4w-Pc group also demonstrated reduced levels of IFN-γ and TNF-α on 5 and 8 dpi and significantly elevated level of IL-10 on 12 dpi. There were significantly fewer mDCs (CD11c(+)CD11b(+)) and pDCs (CD11c(+)B220(+)) in the B-4w-Pc group than the control group at all the time points post infection and the expression of MHC II was noticeably reduced on day 8 pi. Our findings confirmed that BCG inoculation prior to Plasmodium infection resulted in excessive activation and proliferation of Tregs and upregulation of anti-inflammatory mediators, which inhibited establishment of a Th1-dominant immune response during the early stages of Plasmodium infection by inhibiting dendritive cells response. BCG inoculation prior to P. c. chabaudi AS infection may contribute to overgrowth of parasites as well as mortality in mice.

Suppression of CD4+ Effector Responses by Naturally Occurring CD4+ CD25+ Foxp3+ Regulatory T Cells Contributes to Experimental Cerebral Malaria

The role of naturally occurring CD4(+) CD25(+) Foxp3(+) regulatory T cells (nTreg) in the pathogenesis of cerebral malaria (CM), which involves both pathogenic T cell responses and parasite sequestration in the brain, is still unclear. To assess the contribution and dynamics of nTreg during the neuropathogenesis, we unbalanced the ratio between nTreg and naive CD4(+) T cells in an attenuated model of Plasmodium berghei ANKA-induced experimental CM (ECM) by using a selective cell enrichment strategy. We found that nTreg adoptive transfer accelerated the onset and increased the severity of CM in syngeneic C57BL/6 (B6) P. berghei ANKA-infected mice without affecting the level of parasitemia. In contrast, naive CD4(+) T cell enrichment prevented CM and promoted parasite clearance. Furthermore, early during the infection nTreg expanded in the spleen but did not efficiently migrate to the site of neuroinflammation, suggesting that nTreg exert their pathogenic action early in the spleen by suppressing the protective naive CD4(+) T cell response to P. berghei ANKA infection in vivo in both CM-susceptible (B6) and CM-resistant (B6-CD4(-/-)) mice. However, their sole transfer was not sufficient to restore CM susceptibility in two CM-resistant congenic strains tested. Altogether, these results demonstrate that nTreg are activated and functional during P. berghei ANKA infection and that they contribute to the pathogenesis of CM. They further suggest that nTreg may represent an early target for the modulation of the immune response to malaria.

Phenylhydrazine administration accelerates the development of experimental cerebral malaria

Phenylhydrazine (PHZ) treatment is generally used to enhance parasitemia in infected mice models. Transient reticulocytosis is commonly observed in iron-deficient anemic hosts after treatment with iron supplementation, and is also associated with short-term hemolysis caused by PHZ treatment. In this study, we investigated the relationship between reticulocytosis and cerebral malaria (CM) in a murine model induced by PHZ administration before Plasmodium berghei ANKA (PbA) infection. Mortality and parasitemia were checked daily. Pro-inflammatory cytokines and IL-10 were quantified by ELISA. The expression of CXCL9, CXCL10, CCL5, and CXCR3 mRNAs was determined by real-time PCR. Brain sequestration of CD4(+) and CD8(+) T cells and populations of splenic Th1 CD4(+) T cells, dendritic cells (DCs), CD11b(+) Gr1(+) cells, and regulatory T cells (Tregs) were assessed by FACS. PHZ administration dramatically increased parasitemia from day 3 to day 5 post infection (p.i.) compared with the untreated control infected mice group; also, CM developed at day 5 p.i., compared with day 7 p.i. in untreated control infected mice, as well as significantly decreased blood-brain barrier function (P < 0.001). PHZ administration during PbA infection significantly increased the expression of CXCL9 (P <0.05) and VCAM-1 (P <0.001) in the brain, increased the expression of CXCL10, CCL5 and CXCR3, and significantly increased the recruitment of CD4(+) and CD8(+) T cells (P <0.001 and P <0.01, respectively) as well as CD11b(+) Gr1(+) cells to the brain. In addition, PHZ administration significantly increased the numbers of IL-12-secreting DCs at days 3 and 5 p.i. compared to those of untreated control infected mice (P <0.001 and P <0.01, respectively). Consequently, the activation of CD4(+) T cells, especially the expansion of the Th1 subset (P <0.05), was significantly and dramatically enhanced and was accompanied by marked increases in the production of protein and/or mRNA of the Th1-type pro-inflammatory mediators, IFN-γ and TNF-α (P <0.01 for both for protein; P <0.05 for TNF-α mRNA). Our results suggest that, compared to healthy individuals, people suffering from reticulocytosis may be more susceptible to severe malaria infection in malaria endemic areas. This has implications for the most appropriate selection of treatment, which may also cause reticulocytosis in patients living in such areas.

L-arginine exacerbates experimental cerebral malaria by enhancing pro-inflammatory responses

L-Arginine (L-Arg), the substrate for nitric oxide (NO) synthase, has been used to treat malaria to reverse endothelial dysfunction in adults. However, the safety and efficacy of L-Arg remains unknown in malaria patients under the age of five, who are at the greatest risk of developing cerebral malaria (CM), a severe malaria complication. Here, we tested effects of L-Arg treatment on the outcomes of CM using a mouse model. Experimental cerebral malaria (ECM) was induced in female C57BL/6 mice infected with Plasmodium berghei ANKA, and L-Arg was administrated either prophylactically or after parasite infection. Surprisingly, both types of L-Arg administration caused a decline in survival time and raised CM clinical scores. L-Arg treatment increased the population of CD4(+)T-bet(+)IFN-γ(+) Th1 cells and the activated macrophages (F4/80(+)CD36(+)) in the spleen. The levels of pro-inflammatory cytokines, IFN-γ and TNF-α, in splenocyte cultures were also increased by L-Arg treatment. The above changes were accompanied with a rise in the number of dendritic cells (DCs) and an increase in their maturation. However, L-Arg did not affect the population of regulatory T cells or the level of IL-10 in the spleen. Taken together, these data suggest that L-Arg may enhance the Th1 immune response, which is essential for a protective response in uncomplicated malaria but could be lethal in CM patients. Therefore, the prophylactic use of L-Arg to treat CM, based on the assumption that restoring the bioavailability of endothelial NO improves the outcome of CM, may need to be reconsidered especially for children.

Targeting regulatory T cells to improve vaccine immunogenicity in early life

Human newborns and infants are bombarded with multiple pathogens on leaving the sterile intra-uterine environment, and yet have suboptimal innate immunity and limited immunological memory, thus leading to increased susceptibility to infections in early life. They are thus the target age group for a host of vaccines against common bacterial and viral pathogens. They are also the target group for many vaccines in development, including those against tuberculosis (TB), malaria, and HIV infection. However, neonatal and infant responses too many vaccines are suboptimal, and in the case of the polysaccharide vaccines, it has been necessary to develop the alternative conjugated formulations in order to induce immunity in early life. Immunoregulatory factors are an intrinsic component of natural immunity necessary to dampen or control immune responses, with the caveat that they may also decrease immunity to infections or lead to chronic infection. This review explores the key immunoregulatory factors at play in early life, with a particular emphasis on regulatory T cells (Tregs). It goes on to explore the role that Tregs play in limiting vaccine immunogenicity, and describes animal and human studies in which Tregs have been depleted in order to enhance vaccine responses. A deeper understanding of the role that Tregs play in limiting or controlling vaccine-induced immunity would provide strategies to improve vaccine immunogenicity in this critical age group. New adjuvants and drugs are being developed that can transiently suppress Treg function, and their use as part of human vaccination strategies against infections is becoming a real prospect for the future.

Role of the aryl hydrocarbon receptor in the immune response profile and development of pathology during Plasmodium berghei Anka infection

Infection with Plasmodium falciparum may result in severe disease affecting various organs, including liver, spleen, and brain, resulting in high morbidity and mortality. Plasmodium berghei Anka infection of mice recapitulates many features of severe human malaria. The aryl hydrocarbon receptor (AhR) is an intracellular receptor activated by ligands important in the modulation of the inflammatory response. We found that AhR-knockout (KO) mice infected with P. berghei Anka displayed increased parasitemia, earlier mortality, enhanced leukocyte-endothelial cell interactions in the brain microvasculature, and increased inflammation in brain (interleukin-17 [IL-17] and IL-6) and liver (gamma interferon [IFN-γ] and tumor necrosis factor alpha [TNF-α]) compared to infected wild-type (WT) mice. Infected AhR-KO mice also displayed a reduction in cytokines required for host resistance, including TNF-α, IL-1β, and IFN-γ, in the brain and spleen. Infection of AhR-KO mice resulted in an increase in T regulatory cells and transforming growth factor β, IL-6, and IL-17 in the brain. AhR modulated the basal expression of SOCS3 in spleen and brain, and P. berghei Anka infection resulted in enhanced expression of SOCS3 in brain, which was absent in infected AhR-KO mice. These data suggest that AhR-mediated control of SOCS3 expression is probably involved in the phenotype seen in infected AhR-KO mice. This is, to our knowledge, the first demonstration of a role for AhR in the pathogenesis of malaria.

Parasite densities modulate susceptibility of mice to cerebral malaria during co-infection with Schistosoma japonicum and Plasmodium berghei

Background

Malaria and schistosomiasis are endemic and co-exist in the same geographic areas, even co-infecting the same host. Previous studies have reported that concomitant infection with Schistosoma japonicum could offer protection against experimental cerebral malaria (ECM) in mice. This study was performed to evaluate whether alterations in parasite density could alter this protective effect.

Methods

Mice were inoculated with 100 or 200 S. japonicum cercariae followed by infection with high or low density of Plasmodium berghei ANKA strain eight weeks after the first infection. Then, parasitaemia, survival rate and blood–brain-barrier (BBB) damage were assessed. Interferon-gamma (IFN-γ), interleukin (IL)-4, IL-5, IL-13, IL-10, and TGF-β levels were determined in splenocyte supernatants using enzyme-linked immunosorbent assay (ELISA). Cell surface/intracellular staining and flow cytometry were used to analyse the level of CD4⁺/CD8⁺ T cells, CD4⁺CD25⁺Foxp3⁺ Tregs, IL-10-secreting Tregs, and IL-10⁺Foxp3^-CD4⁺ T cells in the spleen, and CD4⁺/CD8⁺ T cells infiltrating the brain.

Results

Co-infection with low density P. berghei and increased S. japonicum cercariae significantly increased the levels of IL-4, IL-5, IL-13, TGF-β and Tregs, but significantly decreased the levels of IFN-γ and the percentage of CD4⁺ T cells and CD8⁺ T cells in the spleen and CD8⁺ T cell infiltration in the brain. Increased worm loads also significantly decreased mortality and BBB impairment during ECM. When challenged with higher numbers of P. berghei and increased cercariae, the observed cytokine changes were not statistically significant. The corresponding ECM mortality and BBB impairment also remained unchanged.

Conclusions

This study demonstrates that protection for ECM depends on the numbers of the parasites, S. japonicum and P. berghei, during co-infection. Alterations in the regulatory response appear to play a key role in this adaptation.

Host matrix metalloproteinases in cerebral malaria: new kids on the block against blood-brain barrier integrity?

Cerebral malaria (CM) is a life-threatening complication of falciparum malaria, associated with high mortality rates, as well as neurological impairment in surviving patients. Despite disease severity, the etiology of CM remains elusive. Interestingly, although the Plasmodium parasite is sequestered in cerebral microvessels, it does not enter the brain parenchyma: so how does Plasmodium induce neuronal dysfunction? Several independent research groups have suggested a mechanism in which increased blood–brain barrier (BBB) permeability might allow toxic molecules from the parasite or the host to enter the brain. However, the reported severity of BBB damage in CM is variable depending on the model system, ranging from mild impairment to full BBB breakdown. Moreover, the factors responsible for increased BBB permeability are still unknown. Here we review the prevailing theories on CM pathophysiology and discuss new evidence from animal and human CM models implicating BBB damage. Finally, we will review the newly-described role of matrix metalloproteinases (MMPs) and BBB integrity. MMPs comprise a family of proteolytic enzymes involved in modulating inflammatory response, disrupting tight junctions, and degrading sub-endothelial basal lamina. As such, MMPs represent potential innovative drug targets for CM.

Regulation of immunopathogenesis during Plasmodium and Toxoplasma infections: more parallels than distinctions?

Toxoplasma and Plasmodium parasites exact a significant toll on public health. Host immunity required for efficient control of infection by these Apicomplexans involves the induction of potent T cell responses, which sometimes results in immunopathological damage. Thus, protective immune responses must be balanced by regulatory networks that limit immunopathology. We review several key cellular and molecular immunoregulatory networks operational during Toxoplasma and Plasmodium infections. Accumulating data show that despite differences in how the immune response controls these parasites, many host immunoregulatory pathways and cellular networks are common to both. Thus, understanding the cellular and molecular circuits that prevent or regulate immunopathological responses against one parasite is likely to inform our understanding of the host response to the other parasite.

The transcription factor T-bet regulates parasitemia and promotes pathogenesis during Plasmodium berghei ANKA murine malaria

The pathogenesis of experimental cerebral malaria (ECM) is an immunologic process, mediated in part by Th1 CD4(+) T cells. However, the role of the Th1 CD4(+) T cell differentiation program on the ability to control parasitemia and susceptibility to ECM disease during blood stage malaria has never been assessed directly. Using the Plasmodium berghei ANKA murine model of ECM and mice deficient for the transcription factor T-bet (the master regulator of Th1 cells) on the susceptible C57BL/6 background, we demonstrate that although T-bet plays a role in the regulation of parasite burden, it also promotes the pathogenesis of ECM. T-bet-deficient (Tbx21(-/-)) mice had higher parasitemia than wild type controls did during the ECM phase of disease (17.7 ± 3.1% versus 10.9 ± 1.5%). In addition, although 100% (10/10) of wild type mice developed ECM by day 9 after infection, only 30% (3/10) of Tbx21(-/-) mice succumbed to disease during the cerebral phase of infection. Resistance to ECM in Tbx21(-/-) mice was associated with diminished numbers of IFN-γ-producing CD4(+) T cells in the spleen and a lower accumulation of CD4(+) and CD8(+) T cells in the brain. An augmented Th2 immune response characterized by enhanced production of activated GATA-3(+) CD4(+) T cells and elevated levels of the eotaxin, MCP-1, and G-CSF cytokines was observed in the absence of T-bet. Our results suggest that in virulent malarias, immune modulation or therapy resulting in an early shift toward a Th2 response may help to ameliorate the most severe consequences of malaria immunopathogenesis and the prospect of host survival.

Pre-existing Schistosoma japonicum infection alters the immune response to Plasmodium berghei infection in C57BL/6 mice

BACKGROUND

Since helminths and malaria parasites are often co-endemic, it is important to clarify the immunoregulatory mechanism that occurs during the process of co-infection. A previous study confirmed that dendritic cells (DCs) are involved in the establishment and regulation of the T-cell-mediated immune response to malaria infection. In the current study, distinct response profiles for splenic DCs and regulatory T cell (Treg) responses were assessed to evaluate the effects of a pre-existing Schistosoma japonicum infection on malaria infection.

METHODS

Malaria parasitaemia, survival rate, brain histopathology and clinical experimental cerebral malaria (ECM) were assessed in both Plasmodium berghei ANKA-mono-infected and S. japonicum-P. berghei ANKA-co-infected mice. Cell surface/intracellular staining and flow cytometry were used to analyse the level of splenic DC subpopulations, toll-like receptors (TLRs), DC surface molecules, Tregs (CD4⁺CD25⁺Foxp3⁺), IFN-γ/IL-10-secreting Tregs, and IFN-γ⁺/IL-10⁺-Foxp3⁻CD4⁺ T cells. IFN-γ, IL-4, IL-5, IL-10 and IL-13 levels were determined in splenocyte supernatants using enzyme-linked immunosorbent assay (ELISA).

RESULTS

The co-infected mice had significantly higher malaria parasitaemia, compared with the mono-infected mice, on days 2, 3, 7 and 8 after P. berghei ANKA infection. Mono-infected mice had a slightly lower survival rate, while clinical ECM symptoms, and brain pathology, were significantly more severe during the period of susceptibility to ECM. On days 5 and 8 post P. berghei ANKA infection, co-infected mice had significantly lower levels of CD11c⁺CD11b⁺, CD11c⁺CD45R/B220⁺, CD11c⁺TLR4⁺, CD11c⁺TLR9⁺, CD11c⁺MHCII⁺, CD11c⁺CD86⁺, IFN-γ-secreting Tregs, and IFN-γ⁺Foxp3⁻CD4⁺ T cells in single-cell suspensions of splenocytes when compared with P. berghei ANKA-mono-infected mice. Co-infected mice also had significantly lower levels of IFN-γ and higher levels of IL-4, IL-5, and IL-13 in splenocyte supernatants compared to mono-infected mice. There were no differences in the levels of IL-10-secreting Tregs or IL-10⁺Foxp3⁻CD4⁺ T cells between co-infected and mono-infected mice.

CONCLUSIONS

A Tregs-associated Th2 response plays an important role in protecting against ECM pathology. Pre-existing S. japonicum infection suppressed TLR ligand-induced DC maturation and had an anti-inflammatory effect during malaria infection not only by virtue of its ability to induce Th2 responses, but also by directly suppressing the ability of DC to produce pro-inflammatory mediators.

Role of IL-10-producing regulatory B cells in control of cerebral malaria in Plasmodium berghei infected mice

Cerebral malaria (CM) is a neurological syndrome often occurring in severe malaria. Although CM is known as an immunopathology in brain tissue mediated by excessive proinflammatory cytokines, the immunoregulatory mechanism is poorly understood. Here, we investigated the role of IL-10-producing regulatory B (Breg) cells in modulating CM development in a murine model of Plasmodium berghei ANKA infection. We observed that blood-stage P. berghei induced expansion of IL-10-producing Breg cells in C57BL/6 mice. Adoptive transfer of IL-10(+) Breg cells to P. berghei infected mice significantly reduced the accumulation of NK and CD8(+) T cells and hemorrhage in brain tissue, and improved the survival of the mice compared with control groups, although parasitemia levels were not altered. Treatment of Breg-cell recipient mice with anti-IL-10 receptor mAb blocked the protective effect of Breg cells. Adoptive transfer of CD4(+) CD25(+) Treg cells failed to prevent CM in infected mice. Spleen cells from Breg-cell recipient mice produced increased levels of IL-10 in vitro. Cell co-culture showed that purified IL-10(+) B cells, but not IL-10(-) B cells, promoted IL-10 production by CD4(+) T cells. These results demonstrate that IL-10-producing Breg cells may represent an important mechanism for controlling the immunopathology and prevention of CM associated with P. berghei infection.

Age-related CD4(+)CD25(+)Foxp3(+) regulatory T-cell responses during Plasmodium berghei ANKA infection in mice susceptible or resistant to cerebral malaria

Different functions have been attributed to CD4⁺CD25⁺Foxp3⁺ regulatory T-cells (Tregs) during malaria infection. Herein, we describe the disparity in Treg response and pro- and anti-inflammatory cytokines during infection with Plasmodium berghei ANKA between young (3-week-old) and middle-aged (8-month-old) C57BL/6 mice. Young mice were susceptible to cerebral malaria (CM), while the middle-aged mice were resistant to CM and succumbed to hyperparasitemia and severe anemia. The levels of pro-inflammatory cytokines, such as TNF-α, in young CM-susceptible mice were markedly higher than in middle-aged CM-resistant mice. An increased absolute number of Tregs 3-5 days post-inoculation, co-occurring with elevated IL-10 levels, was observed in middle-aged CM-resistant mice but not in young CM-susceptible mice. Our findings suggest that Treg proliferation might be associated with the suppression of excessive pro-inflammatory Th1 response during early malaria infection, leading to resistance to CM in the middle-aged mice, possibly in an IL-10-dependent manner.

Sequential Plasmodium chabaudi and Plasmodium berghei infections provide a novel model of severe malarial anemia

Lack of an adequate animal model of Plasmodium falciparum severe malarial anemia (SMA) has hampered the understanding of this highly lethal condition. We developed a model of SMA by infecting C57BL/6 mice with P. chabaudi followed after recovery by P. berghei infection. P. chabaudi/P. berghei-infected mice had an initial 9- to 10-day phase of relatively low parasitemia and severe anemia, followed by a second phase of hyperparasitemia, more profound anemia, reticulocytosis, and death 14 to 21 days after infection. P. chabaudi/P. berghei-infected animals had more intense splenic hematopoiesis, higher interleukin-10 (IL-10)/tumor necrosis factor alpha and IL-12/gamma interferon (IFN-γ) ratios, and higher antibody levels against P. berghei and P. chabaudi antigens than P. berghei-infected or P. chabaudi-recovered animals. Early treatment with chloroquine or artesunate did not prevent the anemia, suggesting that the bulk of red cell destruction was not due to the parasite. Red cells from P. chabaudi/P. berghei-infected animals had increased surface IgG and C3 by flow cytometry. However, C3(-/-) mice still developed anemia. Tracking of red cells labeled ex vivo and in vivo and analysis of frozen tissue sections by immunofluorescence microscopy showed that red cells from P. chabaudi/P. berghei-infected animals were removed at an accelerated rate in the liver by erythrophagocytosis. This model is practical and reproducible, and its similarities with P. falciparum SMA in humans makes it an appealing system with which to study the pathogenesis of this condition and explore potential immunomodulatory interventions.

Targeting Toll-like receptors by chloroquine protects mice from experimental cerebral malaria

Excessive production of proinflammatory cytokines, elicited mostly by Th1 cells, is an important cause of cerebral malaria (CM). Dendritic cells (DCs), a critical link between innate and adaptive immune responses, rely heavily on Toll-like receptor (TLR) signaling. Using C57BL/6 mice infected with Plasmodium berghei ANKA (PbA) as an experimental CM model, we first confirmed that inhibition of TLR9 by suppressive oligodeoxynucleotides protected mice from CM. In addition to being a well-known antimalarial, chloroquine (CQ) has been used as an immunomodulator of endocytic TLRs because it inhibits endosomal acidification. We found that immediately before and shortly after infection by PbA, treatment with a single dose of 50 mg/kg of CQ protected mice from experimental CM. Both CQ treatments significantly inhibited expression of TLR9 and MHC-II on DCs, and reduced the number of myeloid and plasmatocytoid DCs at 3 and 5 days after infection. Consequently, activation of CD4+ T cells, especially the expansion of the Th1 subsets, was dramatically inhibited in CQ treated groups, which was accompanied by a remarkable decline in the production of Th1 type proinflammatory mediators IFN-γ, TNF-α, and nitric oxide. Taken together, these results corroborated the involvement of TLR9 in CM pathogenesis and suggest that interference with the activation of this receptor is a promising strategy to prevent deleterious inflammatory response mediating pathogenesis and severity of malaria.

Mechanisms underlying the induction of regulatory T cells and its relevance in the adaptive immune response in parasitic infections

To fulfill its function, the immune system must detect and interpret a wide variety of signals and adjust the magnitude, duration, and specific traits of each response during the complex host-parasite relationships in parasitic infections. Inflammation must be tightly regulated since uncontrolled inflammation may be as destructive as the triggering stimulus and leads to immune-mediated tissue injury.

During recent years, increasing evidence points to regulatory T cells (Tregs) as key anti-inflammatory cells, critically involved in limiting the inflammatory response. Herein, we review the published information on the induction of Tregs and summarize the most recent findings on Treg generation in parasitic diseases.

Regulatory T cells in infection

Infectious agents have intimately co-evolved with the host immune system, acquiring a portfolio of highly sophisticated mechanisms to modulate immunity. Among the common strategies developed by viruses, bacteria, protozoa, helminths, and fungi is the manipulation of the regulatory T cell network in order to favor pathogen survival and transmission. Treg activity also benefits the host in many circumstances by controlling immunopathogenic reactions to infection. Interestingly, some pathogens are able to directly induce the conversion of naive T cells into suppressive Foxp3-expressing Tregs, while others activate pre-existing natural Tregs, in both cases repressing pathogen-specific effector responses. However, Tregs can also act to promote immunity in certain settings, such as in initial stages of infection when effector cells must access the site of infection, and subsequently in ensuring generation of effector memory. Notably, there is little current information on whether infections selectively drive pathogen-specific Tregs, and if so whether these cells are also reactive to self-antigens. Further analysis of specificity, together with a clearer picture of the relative dynamics of Treg subsets over the course of disease, should lead to rational strategies for immune intervention to optimize immunity and eliminate infection.

The effect of vitamin A supplementation and diphtheria-tetanus-pertussis vaccination on parasitaemia in an experimental murine malaria model

BACKGROUND

Vitamin A supplementation (VAS) decreases overall child mortality in low-income countries. For logistical reasons, VAS has been linked to routine childhood immunizations. However, several recent studies have indicated that VAS may increase mortality and morbidity from infectious diseases when given with the diphtheria-tetanus-pertussis (DTP) vaccine. The immunological effects of combining the 2 treatments are unknown.

METHODS

We studied the effect of treating C57BL/6 mice with VAS and DTP, 1 week prior to infection with Plasmodium berghei ANKA. The progression of disease was monitored through parasite load and time to death.

RESULTS

We found significantly higher levels of parasitaemia in VAS/DTP-treated mice than in control mice (crude geometric mean parasitaemia ratio 2.02 (1.08-3.76), p = 0.03). There was no effect of administering either VAS or DTP alone, indicating that the increase in parasitaemia was due to a synergistic effect of VAS and DTP (p for interaction = 0.02). The effect of VAS/DTP on levels of parasitaemia was modified by the specific parasite variant used. No effect was observed on time to death.

CONCLUSION

Our results indicate that VAS/DTP can negatively influence the outcome of malaria infection in mice, adding to the concerns about simultaneous VAS and DTP administration to children in low-income, malaria endemic countries.

Characterization of immune responses to single or mixed infections with P. yoelii 17XL and P. chabaudi AS in different strains of mice

The outcome of Plasmodium yoelii 17XL (P.y17XL)-infected BALB/c and DBA/2 mice, ranging from death to spontaneous cure, depends largely on the establishment of effective Th1 and Th2 responses and a successful switch between Th1 and Th2 responses, as well as appropriate functioning of CD4(+)CD25(+)Foxp3(+)regulatory T cells (Tregs). The infection with another malaria-causing parasite, Plasmodium chabaudi AS (P.cAS), leads to a different outcome in BALB/c and DBA/2 mice compared to mice infected with P.y17XL alone. To understand the consequence of co-infection with P.y17XL and P.cAS, we determined the proliferation curve of parasites, pro-inflammatory/anti-inflammatory cytokine profiles, and the dynamic changes of the number of Tregs in DBA/2 and BALB/c mice with single or mixed-species infections. The infective mode in mixed-species infections was the same as single P.y17XL infections. The multiplication of P.y17XL parasites prevailed in BALB/c and DBA/2 mice with early mixed infections, as detected by RTQ-PCR. Subsequently, the multiplication of P.cAS parasites dominated in DBA/2 mice with mixed infections, while BALB/c mice succumbed to infection. In addition, the dynamic changes in IFN-gamma and IL-4 production in mice with mixed infections, used as a measure of Th1 and Th2 responsiveness, were consistent with P.y17XL-infected mice. Treg activation and the IL-10 level were also closely related to susceptibility to infection. Our findings demonstrate that the characteristics of the immune response during infections with mixed species are dependent on the mode of proliferation of different species of Plasmodium. Indeed, different species of Plasmodium can influence each other in the same host.