Malaria Disease Manifestations and Asymptomatic Malaria
Malaria Disease Manifestations and Asymptomatic Malaria
Studies on the association of the roles of host factors such as genes of the immune system, RBC polymorphisms or disorders, circulating levels of immunoglobulins and cytokines, and pregnancy should not be limited only to symptomatic malaria, but also include asymptomatic malaria. To date this has usually not been the case; the role of host factors in asymptomatic malaria largely remains a field ripe for inquiry.
Significant correlations between SNPs (single nucleotide polymorphisms) in regulatory or coding regions and severe malaria have been reported for several immunologically relevant genes, including cluster for differentiation-40 ligand (CD40L), Fc gamma receptor II (FcγRII), complement receptor 1 (CR1), tumor necrosis factor α (TNF-α), interleukins-4, -12, and -13 (IL-4, IL-12, IL-13), intracellular cell adhesion molecule-1 (ICAM-1), CD-36, platelet endothelial cell adhesion molecule-1 (PECAM-1), toll-like receptor (TLR), and mannose binding lectin 2 (MBL2). However, the association between a particular SNP and response to infection is highly dependent on ethnic background, i.e., an association demonstrated for one ethnic group may not be the same for a population that differs both in genetic background as well as frequency of exposure to infection. Thus, disease correlation with certain SNPs inferred from studies of African or Caucasian populations may or may not yield the same results in ethnically divergent or admixed populations. For example, allelic and genotypic frequencies of interleukin-10 (IL-10–1087 A/G) and in IL-4–590 C/T polymorphisms have been reported to vary based on inter-ethnic differences. A study conducted in Gabon reported no statistically significant association between MBL, TNFα-308, or NOS2 polymorphisms and asymptomatic malaria. However, this may not be true for other geographical regions of the world. Studies on the genetic differences between populations may provide a better understanding of the role of these genes. A summary of the variants and polymorphisms of the erythrocytic and immunologically relevant genes of the human system involved in malaria susceptibility and resistance is given in Additional file.
Several RBC polymorphisms, like G6PD deficiency, haemoglobin variants, ABO blood group antigen, ovalocytosis, and polymorphisms in complement receptor 1, have been shown to provide at least some protection against severe malaria, suggesting the possibility of coevolution between parasite and host. Such protective roles are much less clear for mild malaria and for asymptomatic malaria.
However, research on G6PD polymorphisms has expanded in the last decade to include a range of disease outcomes. For example, G6PD A heterozygosity in females confers protection against all forms of malaria, including the asymptomatic form. The mechanism of this protection may be because the parasite in G6PD A heterozygous female host must cycle between G6PD A and G6PD wild type erythrocytes and may fail to adapt to the G6PD A environment.
The global distribution of ABO blood group antigens reflects natural selection by various pathogens. There is strong evidence that O blood group provides protection against malaria by a mechanism of reduced rosetting and sequestration, while other reports have found an association between presence of blood group A with higher incidence of severe malaria. A study that associated the blood groups of children with malaria disease outcome, found a high prevalence of asymptomatic malaria in children with the blood group O antigen, when compared with those without this antigen. This study suggested that the blood group O provides protection against clinical forms of malaria. However, the association in this study was weak (p = 0.05). Further evidence for such protective effects must not only consider the distribution of blood groups, but also other explanations, such as the anti-rosette formation effect associated with blood group antigens.
Different clinical outcomes during malaria infection may be due to differences in the host immunity level. Acquisition of natural immunity to malaria has been observed in high and stable (i.e., intense and constant over months and years) malaria transmission areas. This immunity acquired due to repeated exposure reduces the risk of both severe and mild malaria. Therefore, asymptomatic malaria could be a consequence of natural immunity. However, in low transmission areas in South America clinical immunity is found to develop in underexposed individuals, resulting in asymptomatic malaria. Hence, it could be possible that a person in a low transmission area develops immunity faster because: (1) there is less antigenic diversity circulating in the regions and/or (2) there are less infections to overwhelm the immune response.
Malaria infection induces polyclonal immunoglobulin production the proportion of which determines protection against the blood stages of P. falciparum. Evidence suggests that antibody-dependent mechanisms play an important role in the reduction of parasitaemia and this alleviates clinical symptoms, as demonstrated by the passive transfer of hyper-immune immunoglobulin G (IgG). Among the various IgG isotypes, cytophilic antibodies IgG1 and IgG3 have been consistently correlated with uncomplicated malaria and even offer protection, while IgG4 does not protect against malaria. Infections have also been associated with elevations in total IgE, with higher levels detected in cerebral P. falciparum malaria than in uncomplicated mild malaria. The role of anti-parasite specific IgE is, however, controversial. Some studies have observed higher antimalarial IgE levels and functional activity in asymptomatic and uncomplicated malaria groups than in severe or cerebral malaria groups. Additionally, high anti-P. falciparum IgE levels have been associated with a reduced risk of developing clinical malaria. This supports a theory of IgE antibodies having a role in protection against disease. Results from others studies suggest that high levels of parasite-specific IgE are observed in patients with severe malaria, which indicates a putative role in pathogenesis.
One of the most challenging issues with studying malaria immunology is that individuals at risk for malaria infections are typically at risk for other parasitic or non-parasitic diseases. Distilling the immunological response induced by malaria apart from other infectious diseases is not straightforward. For example, co-infection of Plasmodium parasites with soil transmitted helminthes or Hepatitis B Virus (HBV) has been reported in several studies. Helminth infections may alter susceptibility to clinical malaria by changing the T-helper1/T-helper2 (Th1/Th2) balance, thereby affecting immunoglobulin levels. These findings are based on the assumption that helminth infections induce a strong and highly polarized immune response, which has been suggested to help facilitate the acquisition of immunity to malaria. In contrast, HBV when co-infected with Plasmodium influences the malaria burden by stimulating an increased inflammatory response. Plasmodium-infected individuals with HBV infection were more likely to be asymptomatic with lower levels of parasitaemia and a decreased inflammatory cytokine profile. Thus, the host's immune system is modified by other co-infecting organisms thereby complicating our understanding of malaria clinical outcome.
Age is considered one of the most important factors that correlate with protective immunity in malaria endemic areas. Infections among non-immune individuals invariably result in clinical symptoms, and often lead to death in young children if untreated. Young children are most susceptible to malaria infections and disease onset as they have not yet acquired clinical immunity. In endemic areas, adults and older children have a lower prevalence of malaria infection and lower incidence of clinical malaria. These individuals acquire immunity from severe malaria episodes during childhood. and therefore as an adolescent or adult they are more likely to develop uncomplicated or asymptomatic malaria than severe malaria. Severe cases are typically found in adults if they are non-immune or have not encountered malaria before.
In adult individuals, the acquired immunity or protection which is thought to keep their malaria asymptomatic or uncomplicated could be due to increased frequency of both type 1 and type 2 cytokine-producing T cells. Cytokines are involved in both protection from, as well as the pathogenesis of, malaria infections. Elevated levels of circulating cytokines such as interleukin-6 (IL-6), IL-12, IL-1, and IL-10, as well as high circulating levels of TNF-α, all appear to have some correlation with the severity of the disease. In falciparum malaria, cytokines may stimulate beneficial immunological responses by inducing acute phase responses, inhibiting parasite growth, and clearing vascular parasites and debris. Inflammatory responses mediated by interferon gamma (IFN-γ) in the interleukin 12 (IL-12) and 18 (IL-18) dependent manner, seems to be crucial for the control of parasitaemia through the induction of tumor necrosis factor (TNF) and the enhanced release of anti-parasitic reactive nitrogen and oxygen radicals. TNF and INF-γ stimulate neutrophils in order to increase parasite destruction.
Pregnant women are more susceptible to malaria infection, a phenomenon especially apparent during the first pregnancy. In successive pregnancies, a decrease in intensity of infection has been observed and attributed to the acquisition of antibodies against variant surface antigens (VSA) such as PfEMP-1 (P. falciparum Erythrocyte Membrane Protein-1), rifins, and stevors. These antigens are expressed on parasitized RBCs infecting the placenta and are most commonly referred to as Variant Surface Antigens of Pregnancy Associated Malaria (VSA-PAM). The immunological relevance of these biological factors associated with asymptomatic malaria needs to be established. Studies on the evaluation of cytokine and growth factor in asymptomatic pregnant women showed an association between increased plasma concentrations of IL-10 and G-CSF.
Role of Human Host Factors in Clinical Outcomes
Studies on the association of the roles of host factors such as genes of the immune system, RBC polymorphisms or disorders, circulating levels of immunoglobulins and cytokines, and pregnancy should not be limited only to symptomatic malaria, but also include asymptomatic malaria. To date this has usually not been the case; the role of host factors in asymptomatic malaria largely remains a field ripe for inquiry.
Polymorphisms of Immunologically Relevant Genes
Significant correlations between SNPs (single nucleotide polymorphisms) in regulatory or coding regions and severe malaria have been reported for several immunologically relevant genes, including cluster for differentiation-40 ligand (CD40L), Fc gamma receptor II (FcγRII), complement receptor 1 (CR1), tumor necrosis factor α (TNF-α), interleukins-4, -12, and -13 (IL-4, IL-12, IL-13), intracellular cell adhesion molecule-1 (ICAM-1), CD-36, platelet endothelial cell adhesion molecule-1 (PECAM-1), toll-like receptor (TLR), and mannose binding lectin 2 (MBL2). However, the association between a particular SNP and response to infection is highly dependent on ethnic background, i.e., an association demonstrated for one ethnic group may not be the same for a population that differs both in genetic background as well as frequency of exposure to infection. Thus, disease correlation with certain SNPs inferred from studies of African or Caucasian populations may or may not yield the same results in ethnically divergent or admixed populations. For example, allelic and genotypic frequencies of interleukin-10 (IL-10–1087 A/G) and in IL-4–590 C/T polymorphisms have been reported to vary based on inter-ethnic differences. A study conducted in Gabon reported no statistically significant association between MBL, TNFα-308, or NOS2 polymorphisms and asymptomatic malaria. However, this may not be true for other geographical regions of the world. Studies on the genetic differences between populations may provide a better understanding of the role of these genes. A summary of the variants and polymorphisms of the erythrocytic and immunologically relevant genes of the human system involved in malaria susceptibility and resistance is given in Additional file.
RBC Polymorphisms or Disorders
Several RBC polymorphisms, like G6PD deficiency, haemoglobin variants, ABO blood group antigen, ovalocytosis, and polymorphisms in complement receptor 1, have been shown to provide at least some protection against severe malaria, suggesting the possibility of coevolution between parasite and host. Such protective roles are much less clear for mild malaria and for asymptomatic malaria.
However, research on G6PD polymorphisms has expanded in the last decade to include a range of disease outcomes. For example, G6PD A heterozygosity in females confers protection against all forms of malaria, including the asymptomatic form. The mechanism of this protection may be because the parasite in G6PD A heterozygous female host must cycle between G6PD A and G6PD wild type erythrocytes and may fail to adapt to the G6PD A environment.
The global distribution of ABO blood group antigens reflects natural selection by various pathogens. There is strong evidence that O blood group provides protection against malaria by a mechanism of reduced rosetting and sequestration, while other reports have found an association between presence of blood group A with higher incidence of severe malaria. A study that associated the blood groups of children with malaria disease outcome, found a high prevalence of asymptomatic malaria in children with the blood group O antigen, when compared with those without this antigen. This study suggested that the blood group O provides protection against clinical forms of malaria. However, the association in this study was weak (p = 0.05). Further evidence for such protective effects must not only consider the distribution of blood groups, but also other explanations, such as the anti-rosette formation effect associated with blood group antigens.
Immunity Due to Circulating Levels of Immunoglobulins and Cytokines
Different clinical outcomes during malaria infection may be due to differences in the host immunity level. Acquisition of natural immunity to malaria has been observed in high and stable (i.e., intense and constant over months and years) malaria transmission areas. This immunity acquired due to repeated exposure reduces the risk of both severe and mild malaria. Therefore, asymptomatic malaria could be a consequence of natural immunity. However, in low transmission areas in South America clinical immunity is found to develop in underexposed individuals, resulting in asymptomatic malaria. Hence, it could be possible that a person in a low transmission area develops immunity faster because: (1) there is less antigenic diversity circulating in the regions and/or (2) there are less infections to overwhelm the immune response.
Malaria infection induces polyclonal immunoglobulin production the proportion of which determines protection against the blood stages of P. falciparum. Evidence suggests that antibody-dependent mechanisms play an important role in the reduction of parasitaemia and this alleviates clinical symptoms, as demonstrated by the passive transfer of hyper-immune immunoglobulin G (IgG). Among the various IgG isotypes, cytophilic antibodies IgG1 and IgG3 have been consistently correlated with uncomplicated malaria and even offer protection, while IgG4 does not protect against malaria. Infections have also been associated with elevations in total IgE, with higher levels detected in cerebral P. falciparum malaria than in uncomplicated mild malaria. The role of anti-parasite specific IgE is, however, controversial. Some studies have observed higher antimalarial IgE levels and functional activity in asymptomatic and uncomplicated malaria groups than in severe or cerebral malaria groups. Additionally, high anti-P. falciparum IgE levels have been associated with a reduced risk of developing clinical malaria. This supports a theory of IgE antibodies having a role in protection against disease. Results from others studies suggest that high levels of parasite-specific IgE are observed in patients with severe malaria, which indicates a putative role in pathogenesis.
One of the most challenging issues with studying malaria immunology is that individuals at risk for malaria infections are typically at risk for other parasitic or non-parasitic diseases. Distilling the immunological response induced by malaria apart from other infectious diseases is not straightforward. For example, co-infection of Plasmodium parasites with soil transmitted helminthes or Hepatitis B Virus (HBV) has been reported in several studies. Helminth infections may alter susceptibility to clinical malaria by changing the T-helper1/T-helper2 (Th1/Th2) balance, thereby affecting immunoglobulin levels. These findings are based on the assumption that helminth infections induce a strong and highly polarized immune response, which has been suggested to help facilitate the acquisition of immunity to malaria. In contrast, HBV when co-infected with Plasmodium influences the malaria burden by stimulating an increased inflammatory response. Plasmodium-infected individuals with HBV infection were more likely to be asymptomatic with lower levels of parasitaemia and a decreased inflammatory cytokine profile. Thus, the host's immune system is modified by other co-infecting organisms thereby complicating our understanding of malaria clinical outcome.
Age is considered one of the most important factors that correlate with protective immunity in malaria endemic areas. Infections among non-immune individuals invariably result in clinical symptoms, and often lead to death in young children if untreated. Young children are most susceptible to malaria infections and disease onset as they have not yet acquired clinical immunity. In endemic areas, adults and older children have a lower prevalence of malaria infection and lower incidence of clinical malaria. These individuals acquire immunity from severe malaria episodes during childhood. and therefore as an adolescent or adult they are more likely to develop uncomplicated or asymptomatic malaria than severe malaria. Severe cases are typically found in adults if they are non-immune or have not encountered malaria before.
In adult individuals, the acquired immunity or protection which is thought to keep their malaria asymptomatic or uncomplicated could be due to increased frequency of both type 1 and type 2 cytokine-producing T cells. Cytokines are involved in both protection from, as well as the pathogenesis of, malaria infections. Elevated levels of circulating cytokines such as interleukin-6 (IL-6), IL-12, IL-1, and IL-10, as well as high circulating levels of TNF-α, all appear to have some correlation with the severity of the disease. In falciparum malaria, cytokines may stimulate beneficial immunological responses by inducing acute phase responses, inhibiting parasite growth, and clearing vascular parasites and debris. Inflammatory responses mediated by interferon gamma (IFN-γ) in the interleukin 12 (IL-12) and 18 (IL-18) dependent manner, seems to be crucial for the control of parasitaemia through the induction of tumor necrosis factor (TNF) and the enhanced release of anti-parasitic reactive nitrogen and oxygen radicals. TNF and INF-γ stimulate neutrophils in order to increase parasite destruction.
Pregnancy
Pregnant women are more susceptible to malaria infection, a phenomenon especially apparent during the first pregnancy. In successive pregnancies, a decrease in intensity of infection has been observed and attributed to the acquisition of antibodies against variant surface antigens (VSA) such as PfEMP-1 (P. falciparum Erythrocyte Membrane Protein-1), rifins, and stevors. These antigens are expressed on parasitized RBCs infecting the placenta and are most commonly referred to as Variant Surface Antigens of Pregnancy Associated Malaria (VSA-PAM). The immunological relevance of these biological factors associated with asymptomatic malaria needs to be established. Studies on the evaluation of cytokine and growth factor in asymptomatic pregnant women showed an association between increased plasma concentrations of IL-10 and G-CSF.
