**2. Genetic resistance to malaria**

Population genetics studies suggest that a large proportion of the variability in malaria incidence among people residing in a malaria endemic area may be attributable to genetic factors (Mackinnon *et al* 2005). This indicates that in addition to the well known genetic variations in red cell components there are a number of other genetic variations, albeit with less obvious phenotype, that also affect susceptibility to malaria. The discovery of these variations has accelerated in the recent while thanks to advances in molecular biology techniques especially the capacity to do high throughput DNA sequencing (Williams 2009).

#### **2.1 Protection against malaria by haemoglobinopathies and other red cell mutations**

Haldane in 1949 was the first to hypothesize that certain red cell mutations reached unexpectedly high prevalence in malaria endemic areas because these mutations protect against malaria and hence confer survival advantage over non-carriers (Haldane 1949, Piel *et al* 2010, Weatherall 1997). This hypothesis has since been confirmed through a number of studies that have reported over 80% protection against severe malaria among sickle cell

The Immunology of Malaria 177

out some of the polymorphism identified so far, and illustrates that a large number of genes

Interferon alpha receptor 1 (IFNαR1); interferon gamma and interferon gamma receptor (IFN-γR & IFNγR); Tumor Necrotic Factor alpha (TNF-α); Interleukin 1α, 1β ,4

and 12b (IL-1α/β, 4, & 12b)

Binding Lectin 2 (MBL2)

Thrombospondin receptor (CD36),

Intercellular adhesion molecule1 (ICAM1); Platelet-endothelial cell adhesion molecule

are involved in determining susceptibility to malaria.

Pro-inflammatory cytokine and cytokine

Nitric oxide (NO) pathway

Cellular Adhesion Molecules

Chromosomal region with immune genes

**3. Innate and acquired immunity** 

2009, Marsh and Kinyanjui 2006).

receptors

**Functions Gene (Protein)** Major Histocompatibility complex antigens HLA-B53, HLA-DRB1

Anti-inflammatory cytokines Interleukin 10 (IL-10)

B-cell function regulation Interleukin 4 (IL-4), TNFSF5 (CD40L) Complement pathway components Complement receptor 1 (CR1); Mannose

(immunoregulatory and microbicidal) Nitric oxide synthase 2A (NOS2A)

Blood cells development Stem Cell Growth Factor (SCGF)

cluster Chromosome 5 region 5q31-q33

Decreasing frequency and severity of malaria episodes with age among endemic populations is the best indicator that people do acquire immunity to malaria following repeated exposure. However, because both the truly protective immune response to malaria and those that simply reflect exposure to malaria increase concurrently with age, many putative in vitro measure of "immunity" to malaria show no correlation with protection against malaria. As such disentangling protective responses from non-protective ones in the complex milieu of responses provoked by malaria parasites is a major objective of malaria immunity studies. Unfortunately, differences in study methodology, polymorphism of target antigens or epitopes and other factors, such as variation in transmission in different study settings and even microvariations in transmission within a given study setting makes it difficult to develop a consistent picture of the efficacy of a given natural or vaccineinduced immune response in protecting against malaria (Bejon *et al* 2009, Kinyanjui *et al*

Table 1. Genes Reported to Be Associated with Susceptibility to Malaria

Blood Group antigens Groups A, O, B Acute Phase proteins Haptoglobin

Components of innate immunity Toll-like receptors 1, 4, 9 (TLR1, TLR4,

Macrophage receptor for antibodies Fc gama receptor 2A and 3B (FcγRIIA &

TLR9)

FcγRIIIB)

(PECAM)

heterozygotes (Hill *et al* 1991, Williams *et al* 2005b) and haemoglobin C homozygotes (Modiano *et al* 2001) and between 40-60% protection among α+ thalassaemia heterozygotes (Allen *et al* 1997, Wambua *et al* 2006, Williams *et al* 2005d). Interestingly, when thalassaemia and sickle cell are co-inherited the protection provided by each trait separately was lost (Penman *et al* 2009, Williams *et al* 2005c). Mutations that affect other components of the red cell have also been shown to provide protection against malaria. Although some studies suggest that only hemizygote Glucose-6-Phosphate Dehydrogenase (G6PD) deficient male are protected against malaria (Guindo *et al* 2007) other studies found that female homozygotes also enjoy significant protection against malaria (Clark *et al* 2009, Ruwende *et al* 1995).

The mechanisms by which haemoglobinopathies protect against malaria are poorly understood. Decreased parasite invasion and growth, possibly due to altered membrane characteristics and physiology in abnormal cells has been reported (Senok *et al* 1997). The susceptibility of G6PD deficient and thalassaemic cells to oxidative damage which in turn kills the parasite inside has been cited as a possible explanation for their protection against malaria (Friedman 1978, Golenser and Chevion 1989, Mendez *et al* 2011) At the same time, infected abnormal red cells exhibit reduced cytoadherence and rosetting, two phenomena that have been implicated in pathogenesis of cerebral malaria (Carlson *et al* 1994, Cholera *et al* 2008, Fairhurst *et al* 2005).

However, protection by haemoglobinopathies might not be entirely passive; a study by Williams et al (2005) found that protection by sickle cell trait against all forms of clinical malaria increased with age over the first ten years of life suggesting that the mechanisms cited above may interact with age-acquired immunity to enhance protection against malaria (Williams *et al* 2005a). Indeed, increased phagocytosis of infected mutant cells has been observed in the presence of otherwise normal parasite growth (Ayi *et al* 2004, Gallo *et al* 2009, Yuthavong *et al* 1990) further supporting the idea of synergy between natural and active immunity.

### **2.2 Other genetic polymorphisms that influence susceptibility to malaria**

In addition to red cell polymorphism, several other genetic polymorphisms have also been implicated in natural resistance to malaria. The majority of these are in DNA regions that encode or control the encoding of components of the immune system and cellular adhesion proteins (Lopez *et al* 2010). The latter are important with regard to malaria as they have been implicated in the adherence of malaria infected red cells in the microvasculature of organs such as the brain; a process that contributes to the pathology of malaria. While some of these polymorphisms such as class 1 HLA-Bw53 allele have large effects; about 40% protection against severe malarial anaemia and cerebral malaria (Hill *et al* 1991), the others have subtle effect and are difficult to detect except in very large studies. The relationship between genetic polymorphisms and susceptibility to malaria is complex. Different polymorphism have varying influence on different syndromes of malaria some affect susceptibility to severe but not mild malaria or asymptomatic infection. Furthermore, the association with susceptibility to malaria for some polymorphism is only evident in one geographic region (West Africa only in the case of HLABw53 allele) but absent in other regions (Hill *et al* 1991). Although this could reflect some methodological difference in studies done in different regions, it also suggests that other unidentified genetic and environmental factors may modify the association between a known polymorphism and malaria outcome. Table 1 lists

heterozygotes (Hill *et al* 1991, Williams *et al* 2005b) and haemoglobin C homozygotes (Modiano *et al* 2001) and between 40-60% protection among α+ thalassaemia heterozygotes (Allen *et al* 1997, Wambua *et al* 2006, Williams *et al* 2005d). Interestingly, when thalassaemia and sickle cell are co-inherited the protection provided by each trait separately was lost (Penman *et al* 2009, Williams *et al* 2005c). Mutations that affect other components of the red cell have also been shown to provide protection against malaria. Although some studies suggest that only hemizygote Glucose-6-Phosphate Dehydrogenase (G6PD) deficient male are protected against malaria (Guindo *et al* 2007) other studies found that female homozygotes

also enjoy significant protection against malaria (Clark *et al* 2009, Ruwende *et al* 1995).

*al* 2008, Fairhurst *et al* 2005).

active immunity.

The mechanisms by which haemoglobinopathies protect against malaria are poorly understood. Decreased parasite invasion and growth, possibly due to altered membrane characteristics and physiology in abnormal cells has been reported (Senok *et al* 1997). The susceptibility of G6PD deficient and thalassaemic cells to oxidative damage which in turn kills the parasite inside has been cited as a possible explanation for their protection against malaria (Friedman 1978, Golenser and Chevion 1989, Mendez *et al* 2011) At the same time, infected abnormal red cells exhibit reduced cytoadherence and rosetting, two phenomena that have been implicated in pathogenesis of cerebral malaria (Carlson *et al* 1994, Cholera *et* 

However, protection by haemoglobinopathies might not be entirely passive; a study by Williams et al (2005) found that protection by sickle cell trait against all forms of clinical malaria increased with age over the first ten years of life suggesting that the mechanisms cited above may interact with age-acquired immunity to enhance protection against malaria (Williams *et al* 2005a). Indeed, increased phagocytosis of infected mutant cells has been observed in the presence of otherwise normal parasite growth (Ayi *et al* 2004, Gallo *et al* 2009, Yuthavong *et al* 1990) further supporting the idea of synergy between natural and

In addition to red cell polymorphism, several other genetic polymorphisms have also been implicated in natural resistance to malaria. The majority of these are in DNA regions that encode or control the encoding of components of the immune system and cellular adhesion proteins (Lopez *et al* 2010). The latter are important with regard to malaria as they have been implicated in the adherence of malaria infected red cells in the microvasculature of organs such as the brain; a process that contributes to the pathology of malaria. While some of these polymorphisms such as class 1 HLA-Bw53 allele have large effects; about 40% protection against severe malarial anaemia and cerebral malaria (Hill *et al* 1991), the others have subtle effect and are difficult to detect except in very large studies. The relationship between genetic polymorphisms and susceptibility to malaria is complex. Different polymorphism have varying influence on different syndromes of malaria some affect susceptibility to severe but not mild malaria or asymptomatic infection. Furthermore, the association with susceptibility to malaria for some polymorphism is only evident in one geographic region (West Africa only in the case of HLABw53 allele) but absent in other regions (Hill *et al* 1991). Although this could reflect some methodological difference in studies done in different regions, it also suggests that other unidentified genetic and environmental factors may modify the association between a known polymorphism and malaria outcome. Table 1 lists

**2.2 Other genetic polymorphisms that influence susceptibility to malaria** 

out some of the polymorphism identified so far, and illustrates that a large number of genes are involved in determining susceptibility to malaria.


Table 1. Genes Reported to Be Associated with Susceptibility to Malaria
