Reduction of the surface-volume ratio: a physical mechanism contributing to the loss of red cell deformability in malaria.

Plasmodia and other intraerythrocytic parasites reduce the deformability of the red cells they infect. One mechanism potentially responsible for this reduction in deformability is the decrease in the surface:volume (S/V) ratio of the red cell which occurs with parasite growth. To examine this hypothesis, normal red cells were allowed to phagocytize polylysine-coated latex spheres 1.0 to 2.9 microns in diameter. Deformability decreased progressively with spheres of increasing size, consistent with the decreasing S/V ratios of those cells (from an initial length:width [L/W] ratio of 2.398 +/- 0.549 for normal red cells to 1.559 +/- 0.249 for red cells containing 2.92 microns latex spheres at 40 dynes per cm2, p less than 0.001). Nevertheless, red cells containing latex spheres 2.0-2.9 microns in diameter remained deformable and continued to tank tread, in contrast to red cells containing Plasmodium falciparum parasites of that size, which are not deformable and do not tank tread. The progressive decrease in S/V produced by the latex spheres is consistent with their effect on the L/W ratio. However, the total loss of deformability observed with red cells containing parasites of similar or smaller size cannot be explained on these grounds alone. It suggests an additional mechanism, such as calcium-induced crosslinking of the red cell cytoskeleton.