Plasmodium berghei: ectopic antibody synthesis in splenectomized rodents.

Jirds (Meriones unguiculatus) were able to maintain acquired antimalaria immunity independent of the spleen approximately 4 months after initial infection. The memory cells appeared to become peripheralized, and persist outside the spleen for +/- 10 months if no further antigenic stimulus is applied. With regular stimulation, immunity was maintained indefinitely. In immune splenectomized jirds, the secondary, splenic germinal center function appeared to be taken over by cellular infiltrates in the liver that are organized as "pseudofollicles." They were comprised of macrophages that contained very finely divided malaria pigment and functional B and T cells. These pseudofollicles were located at the vascular triangles of the hepatic lobules. Strings of plasma cells appeared to be differentiated at the edges of the pseudofollicles. Like the splenic germinal centers, the pseudofollicles appeared largest about 10 days after challenge and became completely resorbed after 3 weeks. Similar structures were observed in asplenic aged rats, whether the spleen was removed before or after initial infection. However, rats splenectomized prior to infection developed low-grade chronic parasitemias; rats splenectomized later remained solidly immune, confirming the view that the pseudofollicles replace only the secondary, humoral response of the splenic germinal centers. It is not known at which site the memory for sterilization locates after peripheralization. The liver also appears to assume the splenic function of storage, and possibly detoxification, of clumps of indigestible malaria pigment. The pigment is located in clusters of macrophages dispersed throughout the parenchyma.