Influence of the spleen on the expression of surface antigens on parasitized erythrocytes.

Two malaria parasites, Plasmodium knowlesi and P. falciparum, when passaged in splenectomized hosts alter or fail to express parasite-dependent antigens on the surface membrane of erythrocytes infected with mature parasites. Experiments with cloned populations of P.knowlesi show this change to be phenotypic and to be modulated by the spleen of the host. In addition, the induction of antigen variation in P. knowlesi malaria apparently requires two factors: specific antibody and the spleen. Along with the altered expression of P. knowlesi variant antigen on the infected erythrocyte surface, there is a decrease in parasite virulence in non-splenectomized monkeys. It is suggested that the spleen-dependent expression of malarial antigens on the parasitized erythrocyte may be an adaptation of the malaria parasite for survival in the presence of a potentially destructive spleen-mediated host immunity.