Conformational Change of β2-glycoprotein I : Evaluation of Difference in Binding Capacity of Autoantibodies to Open and Closed Forms of β2-glycoprotein I

Abstract: Antiphospolipidsyndrome (APS) is one of the most common autoimmune diseases characterized bythrombosis, fetal loss and presence of antiphospholipid antibodies. In APS research the antibodies of biggestinterest are anti-β2-glycoprotein I antibodies (Aβ2GPIA). β2-glycoprotein I (β2GPI)is a plasma protein which becomes activated and obtains a open structure incontact with negative charged surface molecules such as phospholipids. Inactiveβ2GPI has a closed, circular shape which can’t bind autoantibodies. Thereis no golden standard for APS diagnosing and the methods used often giveinconsistent results. The purpose of this examination project work was toconvert β2GPI into the open and closed forms, respectively, by dialyzing againsthigh ionic strength, low and high pH and determine if there is any differencein binding capacity between the two forms and Aβ2GPIAon a microtiter plate.                                                The binding capacity was tested inan ELISA (enzyme-linkedimmunosorbent assay) using purified IgG from patient sera and thedifferent conformational forms of β2GPI. An ELISA for measuring of Aβ2GPIAon several patient samples was also performed.               No difference in binding capacitycould be detected which might be explained by that the conversion of β2GPI was unsuccessful.Perhaps no difference can be measured between the structures because the closedform is expected to open on microtiter plates. An unexpected result was thepresence of immune complexes of β2GPI-Aβ2GPIA found in the serum of one of the patients. In theory an ELISA based on theopen form of β2GPI would provide more reliable diagnoses and furtherresearch is needed in this area.