Interleukin-6 (IL-6), a multifunctional cytokine, exists in several forms ranging from a low molecular weight (MW 20-30 kDa) non-complexed form to high MW (200-450 kDa), complexes. Accurate baseline IL-6 assessment is pivotal to understand clinical responses to IL-6-targeted treatments. Existing assays measure only the low MW, non-complexed IL-6 form. The present work aimed to develop a validated assay to measure accurately total IL-6 (complexed and non-complexed) in serum or plasma as matrix in a high throughput and easily standardized format for clinical testing.
Commercial capture and detection antibodies were screened against humanized IL-6 and evaluated in an enzyme-linked immunosorbent assay format. The best antibody combinations were screened to identify an antibody pair that gave minimum background and maximum recovery of IL-6 in the presence of 100% serum matrix. A plate-based total IL-6 assay was developed and transferred to the Meso Scale Discovery (MSD) platform for large scale clinical testing.
The top-performing antibody pair from 36 capture and four detection candidates was validated on the MSD platform. The lower limit of quantification in human serum samples (n = 6) was 9.77 pg l(-1) , recovery ranged from 93.13-113.27%, the overall pooled coefficients of variation were 20.12% (inter-assay) and 8.67% (intra-assay). High MW forms of IL-6, in size fractionated serum samples from myelodysplastic syndrome and rheumatoid arthritis patients, were detected by the assay but not by a commercial kit.
This novel panoptic (sees all forms) IL-6 MSD assay that measures both high and low MW forms may have clinical utility.