The Role of the Capsular Polysaccharide in the Activation of the Alternative Pathway by the Pneumococcus

Abstract

Abstract The alternative pathway plays a significant role in the non-immune host's defense against the pneumococcus. The following studies concern the role of the capsular polysaccharide in the activation of the alternative pathway by the pneumococcus. Some pneumococcal capsular polysaccharides, but not all, are able to activate the alternative pathway. Soluble purified capsular polysaccharide types 1, 4, and 25 activate C3 in C4-deficient guinea pig serum (C4D GPS), where types 2, 3, 14, and 19 do not. Since capsular polysaccharides exist in their native form attached to the pneumococcal surface, selected capsular polysaccharides were also tested when coupled by CrCl3 to a particulate carrier, sheep erythrocytes. When attached to sheep erythrocytes, capsular polysaccharide types 2 and 3 failed to consume C3 in C4D GPS, paralleling the results obtained when they were in solution. In contrast, type 25 capsular polysaccharide coupled to erythrocytes not only consumed C3 in C4D GPS, as it had when in solution, but also initiated alternative pathway lysis of the erythrocytes.