Quartz crystal microbalances (QCM) are highly sensitive sensors that can reliably weigh material in the nanogram range and are highly sensitive to changes in biomechanical properties of coupled biomaterial. They consist of a thin, usually round slice of crystalline quartz with a gold electrode on each side. Oscillating crystals are composed of α-Quartz (SiO
2) that is known to be piezoelectric material, characterized by: (a) appearance of electrical potential when it is subjected to mechanical stress (piezoelectric effect); and, (b) deformation of the material when it is subjected to electrical potential (inverse piezoelectric effect). Due to the piezoelectric character the quartz is stimulated to oscillations when connected to AC voltage [
1]. Material attaching to the quartz’s surface reduces the frequency. Thus, an increase in weight leads to an adequate decrease in frequency. In biosciences, mass-sensitive QCM are already well accepted, while applications for clinical questions, in particular with whole blood samples, are novel and have not yet been exploited. Lately, this technology has been applied to the typing of blood groups, detection of bacteria, bacterial toxins, viruses, and measuring affinity of antibodies. Further studies on adherent cell types have been performed but the measurement of whole cell interactions of primarily non-adherent cells has not been addressed thoroughly [
1‐
6].
Here a novel QCM-based assay is presented to evaluate whole cell interactions by measuring cytoadhesion of erythrocytes infected with the malaria parasite
Plasmodium falciparum to endothelial cell receptors. Malaria is the most important parasitic infection and accounted for more than 200 million episodes and 438,000 deaths in 2015 [
7]. The disease is caused as a result of the infection by protozoan parasites of the genus
Plasmodium. Symptoms only occur during the blood stage of the infection when parasites multiply in red blood cells. Among five different species known to infect humans,
P. falciparum causes the most severe form of the disease and is responsible for the vast majority of deaths. In contrast to the other species,
P. falciparum-infected red blood cells (iRBCs) adhere to vascular endothelium of postcapillary venules (sequestration) and to non-infected erythrocytes (rosette formation) in the second half of their asexual replication, cycle via
P. falciparum erythrocyte membrane protein 1 (PfEMP1), which is expressed on the surface of iRBCs [
8,
9]. Through cytoadhesion the parasite avoids clearance by the spleen; this is an immune evasion mechanism that can lead to excessive parasite multiplication and may result in complications such as cerebral and placental malaria [
10‐
13]. Important receptors involved in this process are CD36, ICAM1 in cerebral malaria and chondroitin sulfate A (CSA) in placental malaria [
14‐
16]. This work focuses on the binding of iRBC to CD36 and CSA. CD36 is expected to be the main receptor of iRBC adhesion as nearly all laboratory isolates of
P. falciparum, as well as clinical isolates from patients attach to this receptor [
15‐
17]. CSA on the other hand, is well described to be the main receptor of parasite binding in placental malaria and parasites expressing the highly conserved PfEMP1 gene
var2csa are known to bind to this polysaccharide [
14,
18,
19]. Placental malaria can cause low birth weight of newborns, premature delivery, abortion, stillbirth, maternal anaemia, as well as infant and maternal morbidity and mortality [
20,
21]. CSA is also present on other endothelial tissue where parasites might bind to it [
22].
Different methods have been developed to measure PfEMP1-mediated cytoadhesion. Up until now most of the established cytoadhesion assays have been performed under static conditions [
23‐
26], thus lacking shear stress which is assumed to be important for the parasites to get in contact with the appropriate receptors. However, flow-based assays exist as well and some include static periods at the beginning of the measurement to allow settling of parasites, so that they are able to make contact with the immobilized receptors [
27‐
32]. The disadvantage of some of these systems is that handling of the constructions is cumbersome; for example, the read-out is microscopy-based and therefore analysis of results is less standardized [
33‐
36]. Hence, a real-time detection of cytoadhesion is not possible and an adequate evaluation of the experiments performed with these microscope-based assays is time consuming and often reader biased. Another limiting aspect is that measuring of binding times and kinetics as well as discrimination between loosely adsorbed and truly adhered cells are very difficult.
The analysis of this complex interaction would benefit from other highly sensitive and reproducible methods based on a different technology for validation. Therefore, QCM is adapted in this proof-of-concept experiment for the analysis of PfEMP1-mediated iRBC binding to the cell surface of CD36-expressing melanoma cells as well as to CSA. It is the first time that QCM under flow conditions has been used with viable iRBCs and these experiments show the great potential of this approach for future studies in cell–cell as well as cell-receptor studies.