Quantitative assessment of soluble B7RP-1 in bound and unbound state in normal and Systemic Lupus Erythematosus patients’ plasma (130.4)

Abstract

Membrane associated receptors can undergo shedding following cross-linking with ligand. B7RP-1 receptor is present on B-cells and macrophages and binds to the inducible costimulatory (ICOS) molecule on activated and memory T cells. ICOS binding can induce B7RP-1 receptor shedding. An electrochemiluminescence assay (ECLA) on the Meso Scale Discovery Sector Imager 6000 was validated to quantitate the soluble B7RP1 (sB7RP-1) levels in plasma from normal and Systemic Lupus Erythematosus (SLE) subjects. Comparing results of untreated samples or those pretreated with acetic acid enabled the detection of free sB7RP-1 and complexed sB7RP-1 (bound with ICOS or anti-B7RP-1 antibody). Using a bivalent Fc fused B7RP-1 receptor as the standard, the assay range was validated from 0.55 to 40 ng/mL with less than 30% total bias (%relative error plus %CV) at each level. Repeat measures of 51 subjects (normal and SLE) over day 1, 8, and 15 were analyzed. Utilizing the ratio of the sB7RP-1 levels quantified at day 8 or 15 compared to day 1 (post to pre), the natural variability between subjects and over time was statistically determined. No significant differences were noted between SLE and normal subjects, hence, the post to pre ratios were pooled and the range was set between 0.63 to 1.47, representing the 2.5% and 97.5% distributions, respectively. Hence, this assay can help monitor the extent of B7RP-1 shedding following ligand binding and it's potential to form an immunocomplex.