Aspects of immunosuppression during Plasmodium berghei infection in rats.

Depression in immunological responsiveness was manifested in phase with parasitaemia in rats infected with Plasmodium berghei. The spleen was the most affected organ. The response of spleen cells to phytohaemagglutinin (PHA) and the number of plaque forming cells among spleen cells of rats injected with sheep red blood cells (SRBC), were reduced especially at peak parasitaemia. At the onset of the disease the spleen was activated and the responses were amplified. Antibody titres in the serum revealed basically the same picture. Malaria changed also the dose response to antigens so that an overdose of SRBC that normally causes 'immune paralysis' gave rise to significant numbers of plaque forming cells (PFC) in the spleen even in very sick rats. Infection with P. berghei influenced in different ways the two concurrent infections studied: Trypanosoma lewisi and Nipponstrongylus brasiliensis. The severity of trypanosomiasis was proportional to the P. berghei parasitaemia, while the number of the nematodes was not influenced by the malaria in any case. The immunity against T. lewisi depends on the activity of an intact spleen whereas the immunity against N. brasiliensis depends mainly on the mesenteric lymph nodes. The overall results suggest that in malaria the immunological functions of the spleen are severely impaired.