An in vitro assay for sequestration: binding of Plasmodium falciparum-infected erythrocytes to formalin-fixed endothelial cells and amelanotic melanoma cells.

Erythrocytes infected with Plasmodium falciparum bind specifically to cultured endothelial cells and to a line of amelanotic melanoma cells. We have fixed endothelial cells and amelanotic melanoma cells in various ways and determined whether the fixed cells were still able to bind infected erythrocytes. Only cells fixed with 1.0-2.5% formalin in phosphate-buffered saline continued to bind infected erythrocytes as well as unfixed cells. The mechanism of binding to fixed and unfixed cells appeared to be identical for the following reasons. First, erythrocytes infected by parasite strains that bound to unfixed cells also bound to fixed cells while those that did not bind to unfixed cells did not bind to fixed cells. Second, immune serum that inhibited binding to unfixed cells also inhibited binding to fixed cells. Third, electron microscopy showed that knobs were the points of attachment between infected erythrocytes and both fixed and unfixed melanoma cells. Fixed cells gave reproducible results over at least 2 months. Thus, we have developed a simplified, reproducible assay for measuring binding of P. falciparum-infected erythrocytes to target cells.