Malaria-induced immunosuppression has been demonstrated in humans and experimental animals. The suppressed immune response has been suggested to be primarily humoral and not cellular in nature, since classical lymphocytic cell-mediated responses have been reported to be normal. Since previous results have demonstrated that an impairment in macrophage antigen processing may be a contributing factor in malaria-induced immunosuppression, the present studies were conducted to determine if the macrophage/reticuloendothelial system (RES) alteration occurs parallel to the course of the malarial infection and if the impairment in antibody formation is temporally related to the RES alteration. The present study has demonstrated that a profound impairment in splenic direct plaque forming cell (PFC) formation occurs in malaria-infected Balb/c mice which had been immunized with sheep erythrocytes (SRBC) either 2 or 4 days after inoculation with Plasmodium berghei, NYU-2 strain. Serum hemagglutinin titers were significantly depressed in mice which received the SRBC 4 days post-inoculation; however, no alterations in antibody titers were observed in mice immunized with SRBC 2 days post-inoculation. Coincident with the depression of serum antibody titers at the day 4 immunization period was a profound increase in the vascular clearance of 51Cr-SRBC with an enhanced hepatic uptake of the 51Cr-SRBC and a decreased splenic localization of the labelled erythrocytes. It is suggested that a direct vascular exposure of the splenic lymphoid-macrophage elements to the parasite may be responsible for the initial early alterations in the PFC response while the impairment in serum antibody titers and splenic phagocytic activity may be a result of the pathological alterations occurring later in the infection, e.g., tissue anoxia, anemia, and hemolysis.
Present address: Institute of Experimental Pathology and Toxicology, Albany Medical College, Albany, New York 12208.