David J. Peace

T-cell immunity to oncogenic proteins including mutated RAS and chimeric BCR-ABL

The process of malignant transformation can be ascribed to a series of characteristics and definable mutations of genes which encode proteins that control cell growth and differentiation. During the course of malignant transformation the cancer-related genes are altered by a variety of mechanisms including translocations, deletions, and point mutations which commonly result in the expression of aberrant proteins. Our laboratory has focused on determining the extent to which cancer-specific proteins expressed by aberrant cancer-related genes can function as tumor-specific antigens. The current paper reviews our studies with two prototype cancer-specific proteins, mutated p21ras protein and chimeric p210bcr-abl protein. Ras protooncogenes are activated by point mutation in approximately 20% of human malignancies. The mutations occur primarily at codons 12 or 61 and result in the expression of p21ras proteins with single substituted amino acids. Only a limited number of amino acid substitutions occur. Murine studies demonstrate that immunization with synthetic peptides corresponding to the mutated segment can elicit both class II restricted CD4+ helper/inducer T-cell responses and class I restricted CD8+ cytotoxic T-cell responses specific for mutated p21ras protein. In addition, the existence in vivo of tumors expressing mutated ras proteins can be detected by assaying for T-cell immunity to the mutated segment of ras protein. Preliminary human studies show that some patients with colon cancer have existent antibody responses to p21ras protein, implying the possible existence of autochthonous T-cell immunity to mutated ras proteins in those patients. In chronic myelogenous leukemia the human c-abl protooncogene from chromosome 9 is translocated to the specific breakpoint cluster (bcr) region on chromosome 22. The translocation results in the formation of a bcr-abl fusion gene that encodes at 210-kD chimeric protein. The joining region segment of chimeric bcr-abl protein is composed of a unique combination of c-abl and bcr amino acids and is expressed only by malignant cells. Studies demonstrate that immunization of mice with synthetic peptides corresponding to the joining region segment can elicit class II restricted CD4+ T-cell responses to p210bcr-abl proteins. Preliminary studies show that bcr-abl peptides can bind in the groove of both murine and human class I MHC molecules and can elicit bcr-abl peptide-specific cytotoxic T lymphocytes (CTL). Whether bcr-abl peptide-specific CTL can lyse cells expressing bcr-abl protein is a yet unknown. In summary, the results of the studies reviewed confirm that cancer-specific oncogenic proteins can serve as tumor-specific antigens.

Induction of T cells specific for the mutated segment of oncogenic P21ras protein by immunization in vivo with the oncogenic protein

Many malignancies harbor mutated ras proto-oncogenes encoding 21 kDa proteins with single amino acid substitutions. Previous studies have shown that the aberrant p21ras proteins are potential tumor-specific antigens in that CD4+ class II major histocompatibility complex-restricted T cells specific for the mutated segment of various oncogenic p21ras proteins can be elicited by immunization in vivo with synthetic peptides corresponding to the mutated segment. T-cell recognition of an antigenic peptide within a protein may be influenced substantially, either positively or negatively, by flanking amino acid sequences as well as by more distal immunogenic or tolerogenic epitopes within the same protein. This study examined whether T cells specific for the mutated segment of an oncogenic p21ras protein can be elicited by immunization in vivo with the protein. The results showed that p21ras protein bearing the transforming single amino acid substitution of leucine for glutamine at residue 61 could elicit T cells specifically reactive to the mutated region of the protein in C3H/HeN mice. Thus, an abnormal p21ras protein specifically associated with malignant transformation can be immunogenic in vivo. These results predict that in some circumstances, mutated p21ras proteins expressed by malignancies might elicit detectable mutation-specific T-cell responses.

Long-Term Follow-Up of HLA-A2+ Patients with High-Risk, Hormone-Sensitive Prostate Cancer Vaccinated with the Prostate Specific Antigen Peptide Homologue (PSA146-154)

Twenty-eight HLA-A2+ patients with high-risk, locally advanced or metastatic, hormone-sensitive prostate cancer were immunized with a peptide homologue of prostate-specific antigen, PSA146-154, between July 2002 and September 2004 and monitored for clinical and immune responses. Fifty percent of the patients developed strong PSA146-154-peptide-specific delayed-type hypersensitivity skin responses, tetramer and/or IFN-γ responses within one year. Thirteen patients had stable or declining serum levels of PSA one year post-vaccination. A decreased risk of biochemical progression was observed in patients who developed augmented tetramer responses at six months compared to pre-vaccination levels (P = .02). Thirteen patients have died while 15 patients remain alive with a mean overall survival of 60 months (95% CI, 51 to 68 months) per Kaplan-Meier analysis. A trend towards greater overall survival was detected in men with high-risk, hormone-sensitive CaP who developed specific T-cell immunity following vaccination with PSA146-154 peptide.