There is currently no vaccine available against group A Streptococcus (GAS) infections but a large number of virulence factors have been identified and M-protein based vaccines have reached clinical trials. To assess the effectiveness of antibodies raised from vaccination trials, an opsonophagocytic assay (OPA) can be used as a surrogate of protection. This assay determines whether antibodies raised are able to encourage uptake and killing of GAS by neutrophils, mimicking infection clearance by the adaptive immune system in the body. Currently the Lancefield whole blood assay is used as a surrogate of protection OPA, but this has a number of limitations including the need for fresh human whole blood as a source of phagocytes and complement, leading to high variability between experiments due to heterogeneity of the donors and the difficulty in controlling the pre-existing background level of anti-GAS antibodies prior to addition of test sera.
In our work we have been developing an OPA based on neutrophil-differentiated HL60 cells and baby rabbit complement, for potential use as a surrogate of protection in the evaluation of efficacy of GAS vaccine candidates, using a GAS serotype M1 strain. Pooled human intravenous immunoglobulin (IVIG), identified to have a high level of anti-GAS M1 antibodies, was used as a source of human anti-GAS antibodies.
The development of a cell culture based standardised OPA will be highly beneficial to the GAS community to ensure consistent assay results between research groups to measure the functional activity of immune sera during vaccine development.