The antibodies 8B8, 68, AP3, AP5, LIBS1, LIBS6 have been previously described6 (link),22 (link),23 (link). Human embryonic kidney 293 (HEK293) cells were obtained from the American Tissue Type Collection (ATCC, Manassus, VA). CHOLec 3.2.8.1 cells were provided by Pamela Stanley24 (link). Truncated recombinant human vitronectin N-terminal fragment (VTN-N) was purchased from Thermo Fisher scientific (Waltham, MA). Cell culture media and antibiotics were prepared by the University of California, San Francisco Cell Culture Facility using deionized water and analytical grade reagents. Fetal calf serum was obtained from Invitrogen (Carlsbad, CA). The CHOlec 3.2.8.1 cell line was functionally verified by production of hypoglycosylated recombinant protein by SDS-PAGE. The cell line was not tested for mycoplasma contamination.