Neisseria gonorrhoeae has high infection rates and growing antibiotic resistance, and despite decades of effort there is still no vaccine. One reason is that the field has lacked a reliable in vitro correlate of protection: a standard way to measure whether antibodies in a serum sample actually kill the bacterium.
Researchers at the FDA's Center for Biologics Evaluation and Research (CBER) worked with our team at Pel-Freez to build one, a standardized, high-throughput human complement serum bactericidal activity (SBA) assay for N. gonorrhoeae. The paper is open access: Development and validation of a standardized human complement serum bactericidal activity assay to measure functional antibody responses to Neisseria gonorrhoeae. What follows is a summary of the methods and the main results.
How the assay was designed
The complement source was the first decision. Pel-Freez supplied IgG/IgM-depleted human complement serum, which removes the donors' own anti-gonococcal antibodies so that killing in the assay reflects the antibody under test and not the complement's background.
To make the bacteria behave as they do during infection, N. gonorrhoeae was grown with CMP-NANA, which sialylates its lipooligosaccharide and reproduces the serum resistance the organism acquires in the host.
The assay was set up in 96-well microtiter plates so many serum samples can be run side by side, and it was validated for precision (reproducibility across analysts and experiments), specificity (anti-gonococcal killing with no interference from non-specific complement activation) and linearity (consistent results across a range of serum dilutions).
What the validation showed
The assay was reproducible: more than 85% of samples met the reproducibility criteria across analysts and experimental conditions. It distinguished serum-sensitive from serum-resistant N. gonorrhoeae strains and measured antibody responses without interference from non-specific complement activation. The Pel-Freez IgG/IgM-depleted human complement performed consistently across multiple lots. The assay worked on sera from rabbits, mice and humans, and the CMP-NANA sialylation step let bactericidal activity be measured under conditions that resemble infection.
Why it matters
A standardized SBA gives gonococcal vaccine developers a shared framework: immune responses can be compared across studies, and any strain of interest can be tested with minimal complement screening. Because the assay reproduces physiologically relevant conditions such as sialylation, it is also a tool for studying how antibodies and complement defend against the organism, and it is adaptable to other hosts and strains for broader infectious disease work. Consistent methods across laboratories make results comparable, which is what collaborative vaccine research needs.
Disclaimer. The views and opinions presented in this blog article should not be considered to represent an endorsement or guidance on behalf of the U.S. Food and Drug Administration or study authors.