β2M antibody decreased AR expression and induced the cell death of subcutaneous C4-2 and PC3 prostate tumor growth in athymic nu/nu mice. A, immunohistochemical analysis showed dramatic down-regulation of AR expression in β2M antibody – treated subcutaneous C4-2 tumor mouse xenografts (n = 4) but not in the control IgG-treated (n = 4) or saline-treated (n = 4) C4-2 tumor-bearing mice. The β2M antibody also markedly induced apoptotic death in both subcutaneous C4-2 (n = 4) and PC3 (n = 4) prostate tumors in xenograft mice assayed by M30 CytoDeath marker staining. Bar, 100 μm. B, quantification of the positive AR and M30 CytoDeath marker staining cells in C4-2 and PC3 tumor specimens from the immunohistochemical analysis (A). One hundred cells at five randomly selected areas were counted. **, P < 0.005, significant differences from the control IgG group. Columns, mean; bars, SD.