Sixty-two strains ofStreptococcus pneumoniae were studied for their abilities to consume selected components of classical and alternative complement pathways in human sera. The classical pathway was blocked by chelating calcium with ethyleneglycol-bis(,8-aminoethyl ether)-N,N-tetraacetic acid and by removing C4. The alternative pathway was blocked by removing factor B. Each strain's activation of the two pathways was compared with its nonimmune reactivity with the Fc region of immunoglobulin G (IgG). Activation of the classical complement pathway appeared to be independent of such Fc reactivity. Highly Fc-reactive strains, however, were shown to activate the alternative pathway more effectively than did less Fc-reactive strains. Since pneumococcal activation of the alternative pathway requires non-immunospecific IgG, these findings suggest that nonimmune binding of IgG on the pneumococcal surface endows it with complement-activating properties.

The properdin or alternate complement pathway may function as a heat-labile opsonin for pneumococci, and evidence has been sought for its activation in pneumococcal infections. Twenty-two patients had determinations of Clq, C4, properdin factor B, C3, and hemolytic complement during hospitalization for pneumococcal infection. Measurements were made during the first 36 h after admission on 16 patients and later during recovery on 16. The admission and recovery values were compared statistically with each other and with the levels of 15 normal individuals. The admission and recovery mean values were normal and nearly identical for Clq and C4, which are two early components of the classical pathway. The mean level of factor B, a properdin pathway component, was significantly depressed on admission, but the mean recovery value was normal. Admission levels for C3, a component ofthe late common pathway, were depressed, and recovery values were normal. Total hemolytic complement was decreased on admission, although the decrease was not significant for the patients with both admission and recovery determinations. The findings are consistent with the hypothesis that factor B is turned over rapidly, or consumed, early in pneumococcal infections; alternatively, persons with low baseline factor B levels may be particularly susceptible to pneumococcal infection.

Pneumoccal cell wall and capsular products from eight serotypes were tested for their ability to inhibit polymorphonuclear neutrophil killing of the same eight pneumococcal strains. Crude pneumococcal cell wall preparations from all serotypes inhibited phagocytic killing of several pneumococcal serotypes, and were just as effective with heterologous as with homologous strains. Phagocytosis dependent on heat-labile serum factors was inhibited, whereas phagocytosis not dependent on heat-labile factors was not significantly affected. These findings were compatible with inhibition of complement consumption. The inhibitory activity was found in a purified cell wall mucopeptide, whereas purified capsular polysaccharides failed to inhibit phagocytosis.

Bacterial killing by serum alone and by polymorphonuclear (PMN) leukocytes was studied at 370C and compared with killing at 39 and 410C. The test organisms for serum killing were Staphylococcus aureus 502A (serum resistant) and Escherichia coli 014 (serum sensitive). The organisms used in PMN killing tests were Streptococcus pneumoniae type 29 and E. coli 086. S. aureus was not killed by serum alone at any temperature. Changes in temperature did not affect the rate of serum killing of E. coli 014 for the first 60 min, but by 90 and 120 min there was a discrepancy with continued killing at 370C, but no further killing at 39 and 410C. PMN phagocytic killing of the pneumococcus was enhanced at 390C compared with 370C, and phagocytic killing ofE. coli 086 was decreased at 410C when compared with 370C. Therefore, it appears that under certain circumstances fever may aid the host PMNs in destroying organisms, whereas under other circumstances it may interfere with such destruction.

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