Immunohistochemical characterization of two monoclonal antibodies, P25.48 and P25.91, which define a new prostate-specific antigen.

We have generated two new mouse monoclonal antibodies against prostate cancer. P25.48 (IgG3) and P25.91 (IgG2a) were derived from a fusion using fresh prostate cancer cells as the immunogen. Initial screening was performed by indirect immunofluorescence on frozen tissue sections of prostate cancer specimens. The specificity analysis was performed by indirect immunoperoxidase on frozen sections of normal tissues and benign and malignant prostate tissues. P25.48 and P25.91 did not react with any benign prostatic tissues (0 of 17), but reacted with a subset of the malignant prostatic tissues. Five specimens of well-differentiated carcinoma were tested and did not react with P25.48 or with P25.91. Of 16 higher grade specimens, nine reacted with both P25.48 and P25.91, one reacted with P25.48 only, and one reacted with P25.91 only. In most positive cases, the reactivity was heterogenous. P25.48 and P25.91 showed a very restricted pattern of reactivity in nonprostatic tissues. Of 50 normal specimens, only one breast specimen showed some reactivity. None of the nine fetal tissues or of the 15 malignant tissues tested reacted with these monoclonal antibodies. The pattern of reactivity of P25.48 and P25.91 suggests that they recognize the same antigen. This antigen is selectively expressed by malignant prostatic epithelium. In addition, it appears to be distinct from all other previously described prostate cancer-associated antigens.