An In Vitro Model for Measuring Immune Responses to Malaria in the Context of HIV Co-infection

Malaria and HIV co-infection is a growing health priority. However, most research on malaria or HIV currently focuses on each infection individually. Although understanding the disease dynamics for each of these pathogens independently is vital, it is also important that the interactions between these pathogens are investigated and understood. We have developed a versatile in vitro model of HIV-malaria co-infection to study host immune responses to malaria in the context of HIV infection. Our model allows the study of secreted factors in cellular supernatants, cell surface and intracellular protein markers, as well as RNA expression levels. The experimental design and methods used limit variability and promote data reliability and reproducibility. All pathogens used in this model are natural human pathogens (Plasmodium falciparum and HIV-1), and all infected cells are naturally infected and used fresh. We use human erythrocytes parasitized with P. falciparum and maintained in continuous in vitro culture. We obtain freshly isolated peripheral blood mononuclear cells from chronically HIV-infected volunteers. Every condition used has an appropriate control (P. falciparum parasitized vs. normal erythrocytes), and every HIV-infected donor has an HIV uninfected control, from which cells are harvested on the same day. This model provides a realistic environment to study the interactions between malaria parasites and human immune cells in the context of HIV infection.     

Regulatory T cells: friend or foe in immunity to infection?

Homeostasis in the immune system depends on a balance between the responses that control infection and tumour growth and the reciprocal responses that prevent inflammation and autoimmune diseases. It is now recognized that regulatory T cells have a crucial role in suppressing immune responses to self-antigens and in preventing autoimmune diseases. Evidence is also emerging that regulatory T cells control immune responses to bacteria, viruses, parasites and fungi. This article explores the possibility that regulatory T cells can be both beneficial to the host, through limiting the immunopathology associated with anti-pathogen immune responses, and beneficial to the pathogen, through subversion of the protective immune responses of the host.     

Variant antigen gene expression in malaria

Pathogens of the genus Plasmodium are unicellular parasites that infect a variety of animals, including reptiles, birds and mammals. All Plasmodium species target host erythrocytes and replicate asexually within this niche. In humans, proliferation within erythrocytes causes disease symptoms ranging from asymtomatic infection to severe disease, including mild to severe febrile and respiratory symptoms, profound anaemia and obstruction of blood flow. The most serious form of human malaria is caused by Plasmodium falciparum, a pathogen that is responsible for several million deaths annually throughout the developing world. Malaria parasites succeed in evading the host immune response to establish long-term, persistent infections, thus increasing the efficiency by which they are transmitted to the mosquito vector. The ability to evade the host immune system, in particular the avoidance of antibody-mediated immunity against parasite-encoded surface proteins, is the result of amplification of extensive repertoires of multicopy, hypervariable gene families that encode infected erythrocyte or merozoite surface proteins. Via switching between antigenically diverse genes within these large families, populations of parasites have the capacity for rapid variation in antigenicity and virulence over the course of an infection. Here we review the amplification and generation of antigenic diversity within the Plasmodium variant gene families, as well as discuss the mechanisms underlying their tightly controlled gene expression and antigenic switching.     

The effects of symmetry on the dynamics of antigenic variation

In the studies of dynamics of pathogens and their interactions with a host immune system, an important role is played by the structure of antigenic variants associated with a pathogen. Using the example of a model of antigenic variation in malaria, we show how many of the observed dynamical regimes can be explained in terms of the symmetry of interactions between different antigenic variants. The results of this analysis are quite generic, and have wider implications for understanding the dynamics of immune escape of other parasites, as well as for the dynamics of multi-strain diseases.     

NK cells in HIV infection: paradigm for protection or targets for ambush

Natural killer cells are a crucial component of the innate immune response to certain tumours and to various viruses, fungi, parasites and bacteria. HIV has infected more than 60 million people worldwide and has led to more than 23 million deaths. At present, there are approximately 40 million people who are living with HIV infection, and there were 5 million new infections in 2004. As part of the innate immune system, natural killer cells might have an important role in host defence against HIV infection, as well as in the control of HIV replication in vivo. In this regard, it is important to understand how natural killer cells and HIV interact. This Review focuses on the role of natural killer cells in controlling HIV infection and on the impact of HIV and HIV-viraemia-induced immune activation on natural-killer-cell function.     

Escape of malaria parasites from host immunity requires CD4+ CD25+ regulatory T cells

Infection with malaria parasites frequently induces total immune suppression, which makes it difficult for the host to maintain long-lasting immunity. Here we show that depletion of CD4(+)CD25(+) regulatory T cells (T(reg)) protects mice from death when infected with a lethal strain of Plasmodium yoelii, and that this protection is associated with an increased T-cell responsiveness against parasite-derived antigens. These results suggest that activation of T(reg) cells contributes to immune suppression during malaria infection, and helps malaria parasites to escape from host immune responses.     

Disruption of Type I Interferon Induction by HIV Infection of T Cells

Our main objective of this study was to determine how Human Immunodeficiency Virus (HIV) avoids induction of the antiviral Type I Interferon (IFN) system. To limit viral infection, the innate immune system produces important antiviral cytokines such as the IFN. IFN set up a critical roadblock to virus infection by limiting further replication of a virus. Usually, IFN production is induced by the recognition of viral nucleic acids by innate immune receptors and subsequent downstream signaling. However, the importance of IFN in the defense against viruses has lead most pathogenic viruses to evolve strategies to inhibit host IFN induction or responses allowing for increased pathogenicity and persistence of the virus. While the adaptive immune responses to HIV infection have been extensively studied, less is known about the balance between induction and inhibition of innate immune defenses, including the antiviral IFN response, by HIV infection. Here we show that HIV infection of T cells does not induce significant IFN production even IFN I Interferon production. To explain this paradox, we screened HIV proteins and found that two HIV encoded proteins, Vpu and Nef, strongly antagonize IFN induction, with expression of these proteins leading to loss of expression of the innate immune viral RNA sensing adaptor protein, IPS-1 (IFN-β promoter stimulator-1). We hypothesize that with lower levels of IPS-1 present, infected cells are defective in mounting antiviral responses allowing HIV to replicate without the normal antiviral actions of the host IFN response. Using cell lines as well as primary human derived cells, we show that HIV targeting of IPS-1 is key to limiting IFN induction. These findings describe how HIV infection modulates IFN induction providing insight into the mechanisms by which HIV establishes infection and persistence in a host.     

A model for the coevolution of immunity and immune evasion in vector-borne diseases with implications for the epidemiology of malaria

We describe a model of host-parasite coevolution, where the interaction depends on the investments by the host in its immune response and by the parasite in its ability to suppress (or evade) its host's immune response. We base our model on the interaction between malaria parasites and their mosquito hosts and thus describe the epidemiological dynamics with the Macdonald-Ross equation of malaria epidemiology. The qualitative predictions of the model are most sensitive to the cost of the immune response and to the intensity of transmission. If transmission is weak or the cost of immunity is low, the system evolves to a coevolutionarily stable equilibrium at intermediate levels of investment (and, generally, at a low frequency of resistance). At a higher cost of immunity and as transmission intensifies, the system is not evolutionarily stable but rather cycles around intermediate levels of investment. At more intense transmission, neither host nor parasite invests any resources in dominating its partner so that no resistance is observed in the population. These results may help to explain the lack of encapsulated malaria parasites generally observed in natural populations of mosquito vectors, despite strong selection pressure for resistance in areas of very intense transmission.     

Effects of schistosomes on host anti-viral immune response and the acquisition,virulence, and prevention of viral infections: A systematic review

Although a growing number of studies suggest interactions between Schistosoma parasites and viral infections, the effects of schistosome infections on the host response to viruses have not been evaluated comprehensively. In this systematic review, we investigated how schistosomes impact incidence, virulence, and prevention of viral infections in humans and animals. We also evaluated immune effects of schistosomes in those coinfected with viruses. We screened 4,730 studies and included 103. Schistosomes may increase susceptibility to some viruses, including HIV and Kaposi’s sarcoma-associated herpesvirus, and virulence of hepatitis B and C viruses. In contrast, schistosome infection may be protective in chronic HIV, Human T-cell Lymphotropic Virus-Type 1, and respiratory viruses, though further research is needed. Schistosome infections were consistently reported to impair immune responses to hepatitis B and possibly measles vaccines. Understanding the interplay between schistosomes and viruses has ramifications for anti-viral vaccination strategies and global control of viral infections.     

Antagonistic effects of a Mhc class I allele on malaria-infected house sparrows

Genes of the Major Histocompatibility Complex ( Mhc ) play a fundamental role during the immune response because MHC molecules expressed on cell surface allow the recognition and presentation of antigenic peptides to T-lymphocytes. Although Mhc alleles have been found to correlate with pathogen resistance in several host-parasite systems, several studies have also reported associations between Mhc alleles and an accrued infection risk or an accelerated disease progression. The existence of these susceptibility alleles is puzzling, as the cost generated by the infection should rapidly eliminate them from the population. Here, we show that susceptibility alleles may be maintained in a population of house sparrows ( Passer domesticus ) if they have antagonistic effects on different malaria parasites. We found that one Mhc class I allele was associated with a 2.5-fold increase in the risk to be infected with a Plasmodium strain, but with a 6.4-fold reduction in the risk to harbour a Haemoproteus strain. We suggest that this antagonistic effect might arise because Mhc genes can alter the competitive interactions between malaria parasites within the host.     

A retrospective evaluation of the role of T cells in the development of malaria vaccine

Due to the fact that the life cycle of malaria parasites is complex, undergoing both an extracellular and intracellular phases in its host, the human immune system has to mobilize both the humoral and cellular arms of immune responses to fight against this parasitic infection. Whereas humoral immunity is directed toward the extracellular stages which include sporozoites and merozoites, cell-mediated immunity (CMI), in which T cells play a major role, targets hepatic stages - liver stages - of the parasites. In this review, the role of T cells in protective immunity against liver stages of the malaria infection is being re-evaluated. Furthermore, this review intends to address how to translate the findings regarding the role of T cells obtained in experimental systems to actual development of malaria vaccine for humans.     

The impact of immunization on competition within Plasmodium infections

Evolutionary theory argues that ecological interactions between pathogens within an infection can be a potent source of selection shaping traits such as virulence, drug resistance, and infectiousness. In humans, malaria infections are frequently genetically diverse, with mixed genotype infections the norm. A wide variety of evidence shows that crowding occurs within infections, with the population densities of individual genotypes suppressed by the presence of others. Public health interventions are expected to impact on levels of immunity experienced by pathogens, indirectly by reducing the rate of acquisition of natural immunity by reducing the force of infection, and directly in the case of vaccination programs. Here we ask how enhanced host immunity affects competitive interactions between malaria parasites within hosts and thus the strength of in-host selection on traits such as virulence. We used a model malaria system, Plasmodium chabaudi in laboratory mice, where it has been previously shown that less virulent parasites are competitively suppressed by more virulent strains, generating within-host selection for increased virulence. We found that immunization with either a recombinant antigen or with live parasites suppressed parasite densities, but that there was no evidence that immunization relieved or exacerbated competitive suppression, or affected the relative frequency of clones within infections. There is thus no reason to think that immunization strengthens or alleviates the potentially very potent selection on parasite traits arising from interactions between pathogen genotypes within infections.     

Filarial infection induces protection against P. berghei liver stages in mice

Chronic helminth infections such as filariasis in human hosts can be life long, since parasites are equipped with a repertoire of immune evasion strategies. In many areas where helminths are prevalent, other infections such as malaria are co-endemic. It is still an ongoing debate, how one parasite alters immune responses against another. To dissect the relationships between two different parasites residing in the same host, we established a murine model of co-infection with the filarial nematode Litomosoides sigmodontis and the malaria parasite Plasmodium berghei (ANKA strain). We found that filarial infection of BALB/c mice leads to protection against a subsequent P. berghei sporozoite infection in one-third of co-infected mice, which did not develop blood-stage malaria. This finding did not correlate with adult worm loads, however it did correlate with the presence of microfilariae in blood. Interestingly, protection was abrogated in IL-10-deficient mice. Thus, murine filariasis, in particular when it is a patent infection, is able to modify the immunological balance to induce protection against an otherwise deadly Plasmodium infection and is therefore able to influence the course of malaria in favour of the host.     

Fighting a virus with a virus: a dynamic model for HIV-1 therapy

A mathematical model examined a potential therapy for controlling viral infections using genetically modified viruses. The control of the infection is an indirect effect of the selective elimination by an engineered virus of infected cells that are the source of the pathogens. Therefore, this engineered virus could greatly compensate for a dysfunctional immune system compromised by AIDS. In vitro studies using engineered viruses have been shown to decrease the HIV-1 load about 1000-fold. However, the efficacy of this potential treatment for reducing the viral load in AIDS patients is unknown. The present model studied the interactions among the HIV-1 virus, its main host cell (activated CD4+ T cells), and a therapeutic engineered virus in an in vivo context; and it examined the conditions for controlling the pathogen. This model predicted a significant drop in the HIV-1 load, but the treatment does not eradicate HIV. A basic estimation using a currently engineered virus indicated an HIV-1 load reduction of 92% and a recovery of host cells to 17% of their normal level. Greater success (98% HIV reduction, 44% host cells recovery) is expected as more competent engineered viruses are designed. These results suggest that therapy using viruses could be an alternative to extend the survival of AIDS patients.     

Temperature-Dependent Pre-Bloodmeal Period and Temperature-Driven Asynchrony between Parasite Development and Mosquito Biting Rate Reduce Malaria Transmission Intensity

A mosquito needs to bite at least twice for malaria transmission to occur: once to acquire parasites and, after these parasites complete their development in their mosquito host, once to transmit the parasites to the next vertebrate host. Here we investigate the relationship between temperature, parasite development, and biting frequency in a mosquito-rodent malaria model system. We show that the pre-bloodmeal period (the time lag between mosquito emergence and first bloodmeal) increases at lower temperatures. In addition, parasite development time and feeding exhibit different thermal sensitivities such that mosquitoes might not be ready to feed at the point at which the parasite is ready to be transmitted. Exploring these effects using a simple theoretical model of human malaria shows that delays in infection and transmission can reduce the vectorial capacity of malaria mosquitoes by 20 to over 60%, depending on temperature. These delays have important implications for disease epidemiology and control, and should be considered in future transmission models.     

Host immunity to Plasmodium infection: Contribution of Plasmodium berghei to our understanding of T cell-related immune response to blood-stage malaria

Malaria is a life-threatening disease caused by infection with Plasmodium parasites. The goal of developing an effective malaria vaccine is yet to be reached despite decades of massive research efforts. CD4⁺ helper T cells, CD8⁺ cytotoxic T cells, and γδ T cells are associated with immune responses to both liver-stage and blood-stage Plasmodium infection. The immune responses of T cell-lineages to Plasmodium infection are associated with both protection and immunopathology. Studies with mouse model of malaria contribute to our understanding of host immune response. In this paper, we focus primarily on mouse malaria model with blood-stage Plasmodium berghei infection and review our knowledge of T cell immune responses against Plasmodium infection. Moreover, we also discuss findings of experimental human studies. Uncovering the precise mechanisms of T cell-mediated immunity to Plasmodium infection can be accomplished through further investigations using mouse models of malaria with rodent Plasmodium parasites. Those findings would be invaluable to advance the efforts for development of an effective malaria vaccine.     

Allicin enhances host pro-inflammatory immune responses and protects against acute murine malaria infection

ABSTRACT: BACKGROUND: During malaria infection, multiple pro-inflammatory mediators including IFN-gamma, TNF and nitric oxide (NO) play a crucial role in the protection against the parasites. Modulation of host immunity is an important strategy to improve the outcome of malaria infection. Allicin is the major biologically active component of garlic and shows anti-microbial activity. Allicin is also active against protozoan parasites including Plasmodium, which is thought to be mediated by inhibiting cysteine proteases. In this study, the immunomodulatory activities of allicin were assessed during acute malaria infection using a rodent malaria model Plasmodium yoelii 17XL. METHODS: To determine whether allicin modulates host immune responses against malaria infection, mice were treated with allicin after infection with P. yoelii 17XL. Mortality was checked daily and parasitaemia was determined every other day. Pro-inflammatory mediators and IL-4 were quantified by ELISA, while NO level was determined by the Griess method. The populations of dendritic cells (DCs), macrophages, CD4+ T and regulatory T cells (Treg) were assessed by FACS. RESULTS: Allicin reduced parasitaemia and prolonged survival of the host in a dose-dependent manner. This effect is at least partially due to improved host immune responses. Results showed that allicin treatment enhanced the production of pro-inflammatory mediators such as IFN-gamma, TNF, IL-12 and NO. The absolute numbers of CD4+ T cells, DCs and macrophages were significantly higher in allicin-treated mice. In addition, allicin promoted the maturation of CD11c+ DCs, whereas it did not cause major changes in IL-4 and the level of anti-inflammatory cytokine IL-10. CONCLUSIONS: Allicin could partially protect host against P. yoelii 17XL through enhancement of the host innate and adaptive immune responses.     

Modelling contact spread of infection in host-parasitoid systems: Vertical transmission of pathogens can cause chaos

All animals and plants are, to some extent, susceptible to disease caused by varying combinations of parasites, viruses and bacteria. In this paper, we develop a mathematical model of contact spread infection to investigate the effect of introducing a parasitoid-vectored infection into a one-host-two-parasitoid competition model. We use a system of ordinary differential equations to investigate the separate influences of horizontal and vertical pathogen transmission on a model system appropriate for a variety of competitive situations. Computational simulations and steady-state analysis show that the transient and long-term dynamics exhibited under contact spread infection are highly complex. Horizontal pathogen transmission has a stabilising effect on the system whilst vertical transmission can destabilise it to the point of chaotic fluctuations in population levels. This has implications when considering the introduction of host pathogens for the control of insect vectored diseases such as bovine tuberculosis or yellow fever.     

Malaria: immune evasion by parasites

Malaria is one of the most life-threatening infectious diseases worldwide. Specific immunity to natural infection is acquired slowly despite a high degree of repeated exposure and rarely continues for a long time even in endemic areas. Malaria parasites have evolved to acquire diverse immune evasion mechanisms that evoke poor immune responses and allow infection of individuals previously exposed. The shrewd schema of malaria parasites also hampers the development of effective vaccines. Furthermore, some of those mechanisms are essential for malaria pathogenesis. In this article, an outline of protective immunity to malaria is given, then strategies used by malaria parasites to evade host immunity, including antigen diversity/polymorphism, antigen variation and total immune suppression, are reviewed. Finally, trials to control malaria based on accumulating insights into the host-parasite relationship are discussed.     

Genetics and evolution of parasites and hosts: some comments

We provide a brief commentary on aspects of the analysis of the genetics and evolution of malaria parasites. Any attempt to understand the nature and manifestations of an infectious disease requires an understanding of the genetics of both pathogen and host. The outcome of a malaria infection, i.e. whether it is asymptomatic, mild, severe or causes cerebral malaria, is due to a complex interaction between the products of parasite and host genes. In general terms, genes in the parasite determine its ability to infect the host, its virulence, etc., while host genes will determine resistance or susceptibility to infection. More than this, however, genetics is about the spread of genes in populations, how they mutate and recombine to produce novel genotypes, and how the parasite and its hosts co-evolve with changing environments. This is a complex subject, and we present some discussion of a few aspects of its analysis.