Opsonic requirements for phagocytosis of Streptococcus pneumoniae types VI, XVIII, XXIII, and XXV.

An assay system employing radiolabeled, heat-killed Streptococcus pneumoniae and human polymorphonuclear leukocytes was utilized to study serum pneumococcal opsonic requirements. Comparing the kinetics of phagocytosis in normal serum, heat-inactivated serum, immunoglobulin G (IgG)-deficient serum, C2-deficient serum, and magnesium dichloride ethyleneglycol-tetraacetic acid (MgEGTA)-chelated serum allowed definition of the opsonic requirements for four pneumococcal serotypes: VI XVIII, XXIII, and XXV. All four serotypes were efficiently opsonized in 10% normal serum. Only type XVIII was opsonized in heat-inactivated serum. All four were also opsonized in IgG-deficient serum but not as efficiently as in normal serum. Opsonization via the alternative pathway was diminished for all four serotypes in 10% MgEGTA-chelated and C2-deficient serum. Furthermore, by varying the concentration of MgEGTA-chelated serum, it was found that type XXV was least efficiently opsonized via the alternative pathway. The quantitative nature of this assay system will permit measurement of bacterial and host factors that may contribute to host susceptibility to pneumococcal infection.