INVASION OF HEREDITARY OVALOCYTES BY PLASMODIUM-FALCIPARUM INVITRO AND ITS RELATION TO INTRACELLULAR ATP CONCENTRATION

Hereditary ovalocytes (stomatocytic ovalocytes), when examined within 1-2 days from the time that the blood sample is drawn, are invaded by Plasmodium falciparum in culture to the extent of at least 55% of normal control cells. The ovalocytes have extremely rigid membranes, characterised by a shear elastic modulus some 3-4 times greater than that of normal cells. The extent of invasion falls off very much more rapidly than that into normal cells on storage, and we surmise that this is the reason for earlier reports of resistance of ovalocytes to malarial invasion in vitro. The initial loss of susceptibility to invasion with time is not accompanied by any change in membrane rigidity, but is primarily a consequence of a rapid decline in intracellular ATP concentration: this falls to below the threshold level required for invasion (approx. 0.1 mM) over a period in which the ATP in normal cells remains almost constant. Incubation in a metabolic regenerating medium leads to a rise in the intracellular ATP concentration and invasion by P. falciparum is recovered, though to a much lower extent than in normal cells. The resistance of ovalocytes to invasion becomes irreversible, due possibly to degradative processes in the membrane, on further storage. The developing parasites in ovalocytes have a reduced number of merozoites and show distinct morphological abnormalities.