PfEMP1s are concentrated in nanoscale, electron-dense protrusions of the plasma membrane of
PfIEs, the so-called knobs. They are formed in erythrocytes about 16 h after parasite invasion and reach their highest density 20 h after infection
[33][34]. Single knobs have a hemispherical ellipsoid shape with a minor axis of 20 nm and a major axis of 120 nm
[35]. Knobs are composed of various submembrane structural proteins, including the major protein of this structure, knob-associated histidine-rich protein (KAHRP). These consist of
PfEMP3, the ring-infected red cell antigen (RESA), the mature parasite-infected red cell surface antigen (MESA)/
PfEMP2, and
Pf332
[35][36]. The knobs consist of a highly organized skeleton made of a spiral structure located beneath specialized areas of the erythrocyte membrane
[37]. The arrangement of
PfEMP1s in a cluster near the top of the knobs is assumed to increase the binding capacity of
PfIEs, especially under flow conditions (see below)
[38][39][40][41].
4. P. falciparum and CD36
Looking at the
PfEMP1 family, the question arises why, depending on the parasite genome, between 75–85% of
var genes encode
PfEMP1s, which have a CIDRα2–6 domain for CD36 binding
[14][24][28]. Interestingly, the CIDRα domains were shown to be present only in the
P. falciparum-containing branch (clade B) of the
Laverania subgenus. This could indicate that the binding to CD36 provides a selective advantage for
P. falciparum [42]. What kind of selection advantage this was is yet unclear.
What advantage does the parasite have in retaining this large number of CD36-binding PfEMP1 variants in its genome? Additionally, what is the difference between the individual variants or, more generally, between CD36 binding mediated by group B or C PfEMP1s?
5. CD36
CD36 is a pattern recognition receptor (PRR) that belongs to the class B scavenger receptor family. It is a glycoprotein present in many tissues and involved in several key processes. These include lipid processing and uptake, thrombostasis, glucose metabolism, immune function, angiogenesis, and fat taste (for review
[43][44][45][46][47]. CD36 is found on platelets, mononuclear phagocytes, adipocytes, hepatocytes, myocytes, some epithelia and, as mentioned above, expressed on the endothelia of liver, spleen, skin, lung, muscle, and adipose tissue
[48][49][50]. On microvascular ECs, CD36 is a receptor for thrombospondin-1 and related proteins and functions as a negative regulator of angiogenesis. At least 60 variants have been described in the coding region of the
CD36 gene. The mutations of CD36 caused by gene variants can also influence the adhesion of
PfIEs and ECs. This could directly influence the severity of a malaria infection via the degree of cytoadhesion. There are several studies on this, but with contradictory results
[51]).
6. CD36 Binding PfEMP1 Variants—Benefits for Parasite and Host
Several observations may help explain why a large number of CD36-binding PfEMP1 variants is not only beneficial for parasite development, but may also be an advantage for the infected host.
-
The parasite targets a region of CD36 that is essential for its physiological role in fatty acid uptake because mutation of F153 disrupts the interaction of CD36 with CIDRα2–6 but also abolishes the binding of CD36 to oxidized LDL particles. This reduces the likelihood that the human host can escape from
PfEMP1 binding by altering its CD36
[25].
-
In contrast to the EPCR binding surface of CIDRα1 domains, which protrudes and is a structure that is likely to be well recognized by antibodies, the CD36 binding site is concave, and the conserved hydrophobic residues are hidden in a pocket, so maybe they are less easily recognized. In addition, the binding site is surrounded by a sequence-diverse protein surface containing a flexible loop that may make antibody recognition less likely. This unique interaction site of the parasite with CD36, which protects essential residues from exposure to the immune system, appears to allow the parasite to utilize an antigenically diverse set of CIDRα2–6 for cytoadhesion to CD36 to be protected from splenic clearance
[25].
-
CD36 is found in cells of the innate and adaptive immune system
[43][44][45][46][47]. It has been shown that
PfIEs can adhere to dendritic cells (DCs). This attachment inhibits maturation of these cells and their ability to stimulate T cells. Thus, the parasite can trigger dysregulation of the immune system. This favors the development of the parasite by impairing the host immune system’s ability to clear the infection
[47][52][53][54][55]. However, there is also an observation that the mechanism of DC inhibition by
PfIEs may be independent of
PfEMP1 and CD36
[56].
-
The previously determined hierarchy of
var expression upon parasite entry into human blood begins with group B and suggests that most parasites bind to CD36, as they all encode a CD36-binding phenotype. Most infected individuals, including those who are not immune, do not develop severe malaria, and cytoadhesion of
PfIEs occurs in extensive microvascular beds in tissues other than the brain (skin, muscle, adipose tissue). Therefore, cytoadhesion in such non-vital tissues could promote survival and transmission of the parasite while minimizing host damage and death
[57][58][59][60].
-
Antibody-induced selective binding and internalization of CD36 do not result in proinflammatory cytokine production by human macrophages. Interestingly, CD36-mediated phagocytosis of
PfIEs also did not result in cytokine secretion by primary macrophages
[61]. However, CD36-mediated binding of
PfIEs increases the likelihood of phagocytosis by macrophages. This can lead to a reduction in parasitemia, but also allows the parasite to maintain a viable infection without causing too much damage to the host through high parasitemia
[47][55][62][63].
-
DCs react to
P. falciparum very early during infection and can, thus, influence the development of immunity. Internalization of
PfIEs by DCs and subsequent pro-inflammatory cytokine production of DCs, NK, and T cells depends on CD36. Notably, plasmacytoid DCs regulate innate and adaptive immunity to malaria via the production of proinflammatory cytokines. As this effect is particularly evident at low levels of parasitemia, the role of CD36 for malaria immunity appears to take place early during infection and to promote the development of protective immunity against malaria
[63][64].
7. Binding Phenotypes of PfIEs
Cytoadhesion of
PfIEs is divided into the three phases: “tethering”, “rolling”, and “immobilization”, comparable to leukocyte diapedesis
[65][66][67]. However, the dynamics of cytoadhesion of
PfIEs to the vascular endothelium is controversial. For example, some authors describe cytoadhesion to ICAM-1 as rolling, and to CD36 as stationary, or vice versa
[68][69][70][71][72]. However, there is increasing evidence that
PfIEs are very likely to roll over CD36
[71][72][73][74][75][76]. Recently, the binding phenotype for different ECRs was investigated using a laminar flow system with transgenic Chinese hamster ovary (CHO) cells carrying different ECRs on their surface
[72]. Rolling was observed upon interaction with CD36, and the rolling behavior of disc-shaped
PfIEs at the trophozoite stage (flipping) differed from the rolling behavior of round-shaped
PfIEs at the schizont stage (continuous rolling). Moreover,
PfIEs in the schizont stage roll more stably than
PfIEs in the trophozoite stage at different shear stresses
[72]. The rolling motion of
PfIEs was also seen on transgenic mouse fibroblasts presenting CD36
[73] and on recombinant CD36 instead of transgenic eukaryotic cells
[74]. As described above, the dermal endothelium has large amounts of CD36. Rolling movements of
PfIEs have also been found on dermal ECs, as well as on human skin grafts, on which large amounts of CD36 are found
[71][73][75][76]. Additionally, last but not least, the rolling CD36 binding phenotype was also confirmed by in silico modeling
[77][78]. However, depending on the experimental setup, the parasite isolates used, and the parasite stage, different velocities were measured at similar shear forces. For trophozoite-stage parasites confronted with recombinant CD36, average velocities between 140 µm/min to 680 µm/min were measured at a shear force of 1.6 Pa, depending on the isolate
[74]. When transgenic CHO cells presenting CD36 on the surface were used instead of recombinant CD36 in a similar experimental setup, average velocities ranging from 11 µm/min to 33 µm/min, i.e., a 12–20 fold lower value, were measured, also depending on the parasite stage and isolate
[72]. If
PfIEs cytoadhere for approximately 30 h during their intraerythrocytic development, they travel distances between 25–122 cm or 2–6 cm, respectively, depending on the experimental setup
[72][74]. In both cases, however, the probability of passing over the spleen and being removed accordingly is low.
Further studies showed that initial contact of
PfIEs to CD36 under flow conditions activates Scr-family kinases, leading to dephosphorylation of CD36 via p130CAS signaling. This increases the binding affinity of
PfIEs to CD36 and, thus, leads to increased adhesion of the
PfIEs. This mechanism also leads to actin cytoskeletal remodeling and subsequent CD36 clustering, which further increases
PfIE adhesion
[73][76][79]. It is postulated that a small number of strongly adherent
PfIEs activate the endothelium, and thus enhance the cytoadhesion of most parasites
[76]. However, the binding mode of
PfIEs also seems to be strongly dependent on the respective ECR. For ICAM-1, CD9, P-selectin, as well as CSA, stationary binding, instead of rolling, was observed under flow conditions
[72]. Stationary binding to ICAM-1 was also demonstrated in an earlier study
[71]. However, while binding to CD36 occurred at shear forces below 4 dyn/cm
2, binding to ICAM-1, CD9, P-selectin, and CSA occurred mostly at lower shear forces (from 2 dyn/cm
2)
[72].
Of note, the origin and environment of the ECR studied (recombinant or presented on eukaryotic cells) also seems to be important for characterising the binding phenotype. Antia and colleagues observed a rolling binding type for
PfIEs, with an average rolling velocity of about 10 µm/s at 1–2 kPa and of 1–3 µm/s when recombinant ICAM-1 or CD36 was used, respectively
[68]. Interestingly, in the same study, stationary binding of
PfIEs, as also described by Lubiana and colleagues
[72], was observed on transgenic CHO cells presenting ICAM-1
[68]. However, the binding showed large variations. Thus, the
PfIEs came to a standstill for a few seconds, but were then also able to detach from the CHO cells again
[68].