Determination of Ab1 epitope sequence and peptide competition study. (A) Ab1 immunoblot on a display of overlapping 8 amino acid synthetic peptides derived from the CD148 ectodomain sequence used for immunization. Ab1 specifically reacted to the sequence QSRDTEVL. (B-C) Stable CHO cell line was plated, serum-depleted, and cultured in growth medium supplemented with either no addition (□) or with Ab1 (67 nM, •) in combination with the indicated molar ratios of peptide, biotin-SGSGQSRDTEVL (synthesized by Chiron Technologies, Clayton, Australia) (B) or Ab1.Fab (C). Cell numbers were assessed at day 2 and day 4 as described in “Materials and methods.” Cell numbers at day 4 are shown. Both peptide and Ab1.Fab effectively antagonized growth inhibitory activity of Ab1 at molar ratios exceeding 3-fold that of Ab1. The data in panels B and C are means ± SEM of quadruplicate determinations.