Importance of complement source in measuring meningococcal bactericidal titers

Abstract

Complement-mediated bactericidal antibodies in serum confer protection against meningococcal disease. The minimum protective titer is estimated to be between 1:4 and 1:8 when measured by the Goldschneider assay performed with human complement, the assay used in the 1960s to establish the correlation between bactericidal antibodies and protection. A more recently described bactericidal assay standardized by an international consortium uses rabbit complement, which is known to augment bactericidal titers. To define a protective titer measured by the standardized assay, we compared bactericidal titers against serogroup C strains measured by this assay to titers measured by the assay described by Goldschneider et al. A titer of > or =1:128 measured by the standardized assay was needed to predict with > or =80% certainty a positive titer of > or =1:4 as measured by the Goldschneider assay. However, the majority of samples with titers of 1:4 measured by the Goldschneider assay had titers of <1:128 when measured by the standardized assay. Therefore, by the results of the standardized assay such persons would be falsely categorized as being susceptible to disease. In conclusion, high bactericidal titers measured with the standardized assay performed with rabbit complement are predictive of protection, but no threshold titer is both sensitive and specific for predicting a positive titer measured by the Goldschneider assay using human complement. Up to 10% of the U.S. adult population lacks intrinsic bactericidal activity against serogroup C strains in serum and can serve as complement donors. Therefore, use of the Goldschneider assay or an equivalent assay performed with human complement is preferred over assays that use rabbit complement.

FIG. 1

(A) Percentages of subjects (± 95% CI) stratified by bactericidal titer to serogroup C measured by the standardized protocol (15) using rabbit complement that were positive at titers of ≥1:8 when retested by the Goldschneider assay performed with human complement. (B) The same analysis is shown as in panel A, but a positive titer with the Goldschneider assay was defined as ≥1:4.

FIG. 2

Percentages of samples that were positive when tested by the standardized assay with rabbit complement according to the bactericidal titer measured by the Goldschneider assay performed with human complement. (A) Positive by the standardized assay is defined as 1:128 or greater; (B) positive by the standardized assay is defined as 1:256 or greater. A titer of 1:4 to 1:7 by the Goldschneider assay is shown as 1:4; a titer of 1:8 to 1:15 is shown as 1:8, etc.

FIG. 3

Relationship between the magnitude of the bactericidal antibody response to serogroup C measured by the Goldschneider and standardized assays and the respective anticapsular IgG antibody concentration measured by ELISA. (A) Goldschneider assay; (B) standardized assay. C., complement; Rab., rabbit.

FIG. 4

Relationship between the magnitude of the bactericidal antibody response to serogroup C measured by the Goldschneider and standardized assays and the respective anticapsular IgM antibody concentration measured by ELISA. This analysis was performed on the subset of sera with low but comparable IgG antibody concentrations (0.6 to 2.0 EU/ml) to minimize the contribution of IgG antibody. (A) Goldschneider assay; (B) standardized assay. C., complement; Rab., rabbit.