Synergism between the spleen and serum complement in experimental pneumococcemia.

To examine the interplay between the spleen and complement activation in host defense against pneumococcemia, serial colony counts in blood and survival rates were studied after high-dose (10(5)) and low-dose (10(2)) intravenous challenge with growth-phase Streptococcus pneumoniae type 3 in rats with splenic remnants of varying size (partial, total, or no splenic resection) and with or without complement depletion (by pretreatment with cobra venom factor [CVF]). After high-dose challenge, the combination of total splenectomy and CVF produced greater pneumococcemia and higher mortality than either factor alone. After low-dose challenge, survival was uniform except when CVF was coupled with total splenectomy or splenic remnants less than one third of normal size, combinations which resulted in extremely high early mortality. This synergistic interaction between total splenectomy and CVF reveals a serious postsplenectomy immune defect and provides a highly sensitive assay of residual protective function in the spleen.