Requirement for an additional serum factor essential for the antibody-independent activation of the classical complement sequence by Gram-negative bacteria.

Killing of Salmonella minnesota and Salmonella typhimurium S and R strains in serum of nonimmune humans and guinea pigs was drastically reduced in the selective absence of C1q, C1r, Ca2+, C4, or C2, the components of the classical complement pathway. Binding of C1 and C1q to the S form and six different core-deficient R mutant strains became stronger the shorter the lipopolysaccharide molecule. C1 and C1q had, under physiological conditions, no affinity to the serum-resistant S forms, whereas these components were bound by the serum-sensitive R forms with high affinity. However, a mixture of the individual complement components C1-C9, which rapidly lysed sensitized erythrocytes, did not kill the serum-sensitive bacteria. Isolated C1 bound to these bacteria cleaved fluid-phase C4 but did not convert C2. C2 turnover could be detected only when serum was used as a source of C1 or C4, indicating that an additional serum component is necessary for the antibody-independent bactericidal effect. Functional tests indicated that this factor is a euglobulin which mediates binding of C4 to the bacteria even in the absence of C1 or after treatment with EDTA. Binding of C4 followed by the generation of C4b sites as acceptors for C2 was a prerequisite for the killing of the bacteria. The factor could not be replaced by immunoglobulin G or immunoglobulin M, nor was it blocked by preincubation with anti-immunoglobulin G or anti-immunoglobulin M.