There are several well-characterized genetic mutations in erythrocyte membrane proteins that are associated with impaired merozoite invasion. For example, African populations carry a highly prevalent single nucleotide polymorphism (SNP) in the promoter region controlling expression of the Duffy antigen receptor for chemokines gene (DARC or Duffy; rs2814778). The SNP prevents erythrocytic expression and results in the Duffy-negative blood phenotype [
8]. Duffy is a ligand for a
P. vivax merozoite protein called the Duffy-binding protein (PvDBP) and the Duffy-PvDBP interaction is essential for merozoite invasion of the erythrocyte [
9,
10]. Therefore, Duffy-negative individuals are protected against
Plasmodium vivax infection and in areas of Africa with a high incidence of this SNP,
P. vivax infection is virtually non-existent [
11,
12]. In contrast
P. vivax is common in other malaria endemic areas where populations do not carry this polymorphism (Asia and South America) [
12,
13]. However, complete protection due to this polymorphism has been questioned recently by observations of
P. vivax infection in Duffy negative individuals in several studies [
14‐
16]. In contrast to
P. vivax,
P. falciparum merozoites utilize several redundant invasion pathways and different host ligands. In 1977, Miller et al. [
17] reported that erythrocytes from individuals with the rare En (a−) mutation were resistant to
P. falciparum merozoite invasion. En (a−) erythrocytes lack Glycophorin A (GYPA), highlighting a possible role for this protein as a parasite ligand. Indeed, GYPA is now known to be a binding partner of
P. falciparum erythrocyte binding antigen 175 (PfEBA-175) [
18,
19], and more recently, was found to bind to merozoite surface protein 1 (PfMSP1) [
20]. In the later study, the erythrocyte surface band 3-GYPA complex was shown to be essential for invasion by both
P. falciparum and the rodent malaria parasite,
Plasmodium yoelii. Mutations in other erythrocyte glycophorins (GYPB and GYPC) are common in endemic malaria populations, and recent studies have demonstrated comparable importance for these proteins in parasite invasion. The
P. falciparum protein erythrocyte-binding ligand-1 (PfEBL-1) binds to GYPB [
21,
22], while GYPC is a receptor for erythrocyte binding antigen 140 (PfEBA-140) [
23]. Antibodies against the binding domains of both PfEBL-1 and PfEBA-140 inhibit merozoite invasion, while erythrocytes deficient in either GYPB (blood group S-s-U-), or GYPC (Gerbich negativity) are resistant to invasion [
21‐
24]. Erythrocyte expressed complement receptor 1 (CR1) is also a demonstrated receptor for the
P. falciparum merozoite protein PfRh4, with antibodies against CR1, as well as soluble CR1 protein, preventing invasion [
25,
26]. This may partially explain the enhanced malaria resistance exhibited by individuals with inherited mutations resulting in CR1 deficiency [
27]. Recently, two high throughput screening approaches were developed to identify novel host binding partners of merozoite proteins involved in invasion. The first utilized the Avidity-based extracellular interaction screen (AVEXIS) to identify binding interactions between erythrocyte and merozoite proteins [
28]. This led to the discovery of a single definitive interaction between the host protein basigin and
P. falciparum reticulocyte-binding protein homologue 5 (PfRH5). Antibody blocking studies demonstrated that this interaction is essential for invasion by all
P. falciparum strains tested to date [
28,
29], and erythrocytes expressing naturally occurring mutations in basigin (known as the Ok blood group antigen) also prevented parasite invasion [
28]. This host-parasite receptor ligand interaction also appears to be important in clinical disease based on the findings that naturally acquired antibodies to PfRH5 are associated with improved outcomes to infection [
30]. Another screening approach has utilized in vitro differentiated human erythroid cells in conjunction with short hairpin RNA (shRNA) libraries to target and screen for erythroid-expressed proteins necessary for
P. falciparum merozoite invasion. As well as corroborating a requirement for basigin and CR1 in invasion, a novel interaction involving erythrocyte CD55, also known as Decay accelerating factor (DAF), was found to be essential for the invasion process. The requirement for this interaction was confirmed utilizing naturally-occurring CD55 null erythrocytes, which were refractory to invasion by several
P. falciparum strains, including clinical isolates [
31]. Interestingly, polymorphisms in CD55 are enriched in populations with historical exposure to malaria [
32,
33], indicating a possible selection pressure on this gene. Importantly, the identification of host cell receptor-parasite ligand interactions has produced several possible targets for vaccine or therapeutic development.
The erythrocyte membrane is supported by an underlying network of proteins called the cytoskeleton, which provides the cell with structure and deformability, and serves as an anchoring point for membrane proteins. Hereditary elliptocytosis and hereditary spherocytosis are conditions caused by mutations in genes encoding cytoskeletal proteins, including alpha spectrin, beta spectrin, ankyrin, band 3 and protein 4.1, which affect molecule conformation or abundance. Mutations in genes encoding some of these proteins have been associated with increased protection against infection or severe forms of malaria. A well-known example is Southeast Asian ovalocytosis (SAO), which is caused by mutations in the gene encoding band 3. Band 3 is a transmembrane protein that normally exists as a dimer or tetramer and interacts with both the cytoskeleton and cell membrane; it also functions as an anion transporter. In SAO, mutations disrupt the anion transporter activity and result in the formation of large-sized aggregates [
34‐
36], resulting in increased cell membrane rigidity [
37‐
39], oval shaped cell morphology, reduced band 3 mobility in the membrane [
40‐
42], and decreased expression of surface antigens [
43,
44]. Homozygous inheritance of SAO-causing band 3 mutations results in embryonic lethality, however heterozygosity is associated with a marked protection against severe
P. falciparum malaria, particularly cerebral malaria [
45,
46]. Despite the fitness cost, frequencies of these mutations are remarkably high some populations, particularly those residing in Papua New Guinea [
47]. There is strong evidence that
P. falciparum merozoite invasion of SAO erythrocytes is impaired, which could plausibly explain the reduced malaria susceptibility conveyed by this condition [
37,
39,
48,
49]. Mechanistic explanations variously include increased membrane rigidity, direct interference with parasite/band 3 binding [
37‐
39,
50,
51], a loss or reduction of merozoite ligands, such as GYPA (which forms a complex with band 3) [
20,
43,
44], and reduced band 3 mobility [
52,
53]. Interestingly, cells with artificially inhibited band 3 mobility also exhibit reduced rates of merozoite invasion [
53]. Erythrocytes from patients with other forms of hereditary elliptocytosis and spherocytosis have shown more variable, and sometimes inconsistent effects on parasite invasion. Erythrocytes deficient in protein 4.1 are reported to be less vulnerable to parasite invasion [
54], which could possibly be explained by a secondary deficiency of GYPC (90% less in protein 4.1 deficient cells), although the role of GYPC as a parasite ligand was not investigated in this study. Utilizing a recently developed method for measuring merozoite invasion in vivo [
55,
56], a study in mice carrying a mutation in ankyrin-1 (
Ank1) reported reduced rates of erythrocyte invasion by
Plasmodium chabaudi merozoites; this coincides with increased resistance and reduced parasitaemia reported in these mice [
57]. Investigations of hereditary spherocytosis caused by mutations in alpha spectrin have also reported reduced merozoite invasion [
58], however these studies are contradictory and the exact effects of these conditions remain inconclusive [
59].
Overall, the disruption of parasite invasion is an obvious and testable mechanism by which erythrocyte disorders can protect the host against malaria infection. This is most evident for mutations that disrupt a specific parasite-erythrocyte binding interaction. Erythrocytes with structural abnormalities, such as spherocytosis and ovalocytosis, may also present the merozoite with a less than ideal substrate for invasion, however the mechanisms are difficult to establish, and findings are less consistent.