Proteinase 3 (PR3) is a neutrophil-derived serine protease involved in inflammatory and immune responses, as well as the regulation of apoptosis. It contributes to the pathogenesis of autoimmune diseases such as granulomatosis with polyangiitis (GPA), where anti-PR3 autoantibodies (ANCAs) induce neutrophil activation and vasculitis. PR3 modulates immune activity through proteolytic processing of extracellular matrix components and cytokines.Additionally,PR3 cleaves and inactivates XIAP, an anti-apoptotic protein, thereby promoting apoptosis and enhancing cellular sensitivity to death signals. This interaction links inflammation to programmed cell death regulation. Thus a functional ELISA assay was conducted to detect the interaction of recombinant human PR3 and recombinant human XIAP. Briefly, PR3 was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to XIAP-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-PR3 pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody for 1h at 37℃, wells were aspirated and washed 5 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37℃. Finally, add 50 μL stop solution to the wells and read at 450/630nm immediately. The binding activity of recombinant human PR3 and recombinant human XIAP was shown in Figure 1, and this effect was in a dose dependent manner.