Prostate-specific antigen (PSA) constructs are immunogenic and induce antibody responses to multiple surface antigens on prostate tumour cell lines.
These responses occur "by epitope or determinant spreading after stimulation of the immune system by PSA immunization," according to investigators from Beth Israel Deaconess Medical Center, Dana-Farber Cancer Institute and Harvard University, all located in Boston, Massachusetts, United States.
The investigators assessed sera from patients with advanced prostate cancer who were receiving vaccinia- or fowlpox- expressed PSA. Flow cytometry was used to determine the serum IgG response to cell surface antigens on two prostate cancer cell lines, PSA-positive LNCAP and PSA-negative PC-3.
Results showed that sera from all seven patients in a Phase I study of vaccinia-PSA reacted with both tumour cell lines. The sera had been collected before the third immunization.
Also, in a Phase II trial of vaccinia-PSA versus fowlpox-PSA, the majority of patients showed sustainable antibody responses to cell surface antigens on the prostate tumour cell lines. Magnitudes and kinetics of the antibody responses were dependent on the schedule of immunization.