Background: Streptococcal serological tests provide evidence of prior infection by Group A Streptococcus (GAS). These tests are crucial to the diagnosis of the post-infectious immune sequelae acute rheumatic fever (ARF) and glomerulonephritis since these syndromes develop several weeks after a GAS infection. Current tests measure anti-streptolysin-O (ASO) and anti-DNaseB (ADB) antibodies. Though widely used, these tests are not without limitations including high background (and associated risk of false positives) in settings where GAS is endemic and incompatible methodology that means the two assays must be run in parallel. The aims of this study were to i) assess the utility of a novel GAS antigen, SpnA, for streptococcal serology, and ii) to assess the feasibility of using a bead-based immunoassay platform to measure titers to ASO, ADB and SpnA simultaneously.
Method and Results: Recombinant streptolysin-O, DNAseB and SpnA were conjugated to functional microsphere beads that fluoresced in unique positions for measurement of serum antibody binding using a Cytometric Bead Array (CBA). Standard curves for each of the antigens were generated using antigen-specific IgG purified from pooled human immunoglobulin. Multiplex assay were run on sera samples collected from participants in three groups; ARF, ethnically matched healthy controls and healthy adults. The ability of the antigens to detect a previous GAS exposure for ARF diagnosis was assessed using the 80th centile of the healthy children group as cut-off (upper limit of normal). Using these experimentally determined cut-offs SpnA had the highest sensitivity at 88%, compared to 75% for SLO and 56% for DNaseB.
Conclusion: SpnA has favorable immunokinetics for streptococcal serology, and the combination of this new antigen with SLO and DNaseB in a multiplex immunoassay should improve the efficiency and accuracy of streptococcal serology.