MOLECULAR CHARACTERIZATION OF DEFECTIVE ANTIGEN-PROCESSING IN HUMAN PROSTATE-CANCER

Background: Gene-modified tumor cell vaccines have shown efficacy in a nimal models of malignancy, including prostate cancer. Class I major h istocompatibility complex (MHC) assembly and function in the cellular targets of such therapies is pivotal in determining the efficacy of sp ecific cytokine-secreting tumor vaccines. Purpose: To help guide devel opment of genetically engineered vaccine therapy for human prostate ca ncer, potential immune resistance pathways were evaluated by analysis of class I MHC assembly in prostate cancer cells. Method: Class I MHC assembly in metastasis-derived human prostate cancer cell lines (LNCaP , PPC-1, DU-145, PC-3, and TSU) and a normal prostate-derived cell lin e (TP-2) were characterized by phenotypic, molecular, and functional a ssays. Assembled class I MHC and antigen was measured by flow cytometr y; mRNA levels of assembly components (class I MHC heavy chain, beta(2 )-microglobulin, and the antigen transporter gene product TAP-2) were determined; and antigen processing was measured with a chimeric recons tituted system using vaccinia vectors. Restoration of antigen processi ng was attempted by interferon gamma stimulation and by transfection w ith mouse class I MHC heavy-chain cDNA. Results: Assembled class I MHC was underexpressed in two (LNCaP and PPC-1) of five prostate cancer c ell lines compared with normal prostate-derived controls. PPC-1 cells underexpressed TAP-2 mRNA despite abundant class I MHC and beta(2)-mic roglobulin message. Induction of TAP-2 by interferon gamma indicated t hat coding sequences for TAP-2 message were present in PPC-1. Resistan ce to cytotoxic T lymphocytes (CTL) lysis showed a functional defect i n antigen transport by PPC-1 cells; reversal of the molecular defect w ith interferon gamma led to restoration of functional antigen processi ng. In contrast, LNCaP cells had competent antigen transport but defic ient class I MHC heavy-chain function despite abundant class I MHC RNA ; though refractory to stimulation by interferon gamma, this defect re sponded to transfection of class I MHC heavy-chain cDNA. Conclusions: Metastatic prostate cancer cells can escape T-cell recognition via div ergent mechanisms of defective class I MHC assembly. The specific unde rexpression of TAP-2 gene product in PPC-1 cells contrasts with prior studies of TAP gene underexpression in lung cancer (which concurrently underexpressed class I MHC heavy chain) and provides evidence for a r egulatory pathway controlling TAP-2 gene expression in human cancers t hat may not affect class I MHC heavy-chain expression. Implications: I n clinical application of gene therapy for prostate cancer, these find ings provide a rationale for focusing on strategies that can circumven t sole reliance on class I MHC-mediated tumor cell recognition by CTL.