T and B cell population changes in young and in adult rats infected with Plasmodium berghei.

Malaria infection in young rats is characterized by high parasitemia, severe anemia, and death. Parasitemia is lower in older rats, and the rats usually survive. This study was designed to investigate the immunological basis of this difference. T cell numbers in the thymuses and spleens of young (4 weeks old) and in adult (18 weeks old) infected and control rats were determined by killing with anti-theta serum and complement. The number of complement receptor lymphocytes (B cells) in spleens was determined after these cells had formed rosettes with sensitized, complement-coated sheep erythrocytes. Infection in young rats was characterized by progressive and severe thymic involution and by decreasing numbers of T and B cells in the spleen. In 18-week-old rats, T cell numbers in the spleen were slightly below those of controls early in infection but exceeded normal values by day 15. Progressive thymic involution was not a feature of infection in adult rats. The number of complement receptor lymphocytes in the spleens of adult rats decreased dramatically early in infection but were nearly normal by day 15. Severity of malarial infection in young rats is related to the inability of their lymphocytes to respond to Plasmodium berghei antigens early in infection in a way that leads to immunity.