Antibody binding to different target cells lines was assessed using a modified version of our previously established immunofluorescence assay (28 (link)). Briefly, plasma samples were diluted 1:200 and incubated with fixed, lytically reactivated target cells. Mouse monoclonal anti-human IgG (American Type Culture Collection CRL-1786) was used as a secondary antibody. Consistent with previous methods, Cy-2 conjugated donkey anti-mouse IgG (Jackson ImmunoResearch 715-225-150) was the detection antibody for L1T2 cells, with 0.004% Evan’s Blue (Sigma Aldrich E2129) serving as a cellular counterstain. As iSLKBAC16 cells express GFP, donkey anti-mouse IgG AlexaFluor647 (Thermofisher Scientific A31571) was used to detect antibody binding at a dilution of 1:300. 300nM DAPI (Thermofisher Scientific D1306) was used to stain nuclei.