Somatic mutations of ras proto-oncogenes occur in approximately 20% of all human malignancies. The mutations involve a single nucleotide exchange within codons 12 or 61 which result in the expression of a p21ras protein with a single substituted amino acid at residues 12 or 61. Preliminary studies in mice have shown that p21ras can become immunogenic to T cells by virtue of the single substituted amino acid. Thus, p21ras protein represents a potential tumor-specific antigen related to the transforming events and shared by many individuals. In similar studies, we have shown that the joining region of a chimeric protein (p210bcr-abl) which is expressed in chronic myelogenous leukemia is likewise immunogenic. Thus, the immunogenicity of proteins expressed by aberrant oncogenes is not limited to p21ras. The major perceived problem of using p21ras, p21ras, p210bcr-abl or other similar proteins as targets for T cell attack is that they are internal cellular proteins. T cells do not respond to whole protein, but rather respond to processed peptides bound to MHC molecules. Preliminary data validated that the mutated segment of p21ras protein can be processed by antigen presenting cells (APC) and presented by class II MHC molecules and thereby is immunogenic to CD4+ T cells. In vivo therapeutic efficacy of CD4+ T cells requires that p21ras be taken up by APC in the environment of tumor, processed and presented in sufficient enough concentration to stimulate proximate immune T cells to secrete cytokines involved in direct and/or indirect cytolytic mechanisms. At present, these stringent conditions have been met in two murine models in which p21ras specific T cells have been shown to be effective at preventing the growth of ras-positive tumors. The observation that mutated p21ras is a tumor-specific antigen provides a strong impetus for initiating human studies to determine whether ras-specific responses can be generated and utilized therapeutically to treat or prevent the recurrence of ras-positive malignancies. The first objective is to determine whether patients bearing ras-positive malignancies have existent immune responses to aberrant p21ras. Ultimately, it might be necessary to immunize patients to elicit ras-specific T cells. The current grant proposes to perfom the studies necessary prior to embarking upon human immunization studies.
The specific aims of the current proposal are: (1) to determine whether patients with aberrant ras-positive tumors have existent CD8+ T cells primed to the aberrant p21ras protein; (2) to determine whether patients with aberrant ras-positive tumors have existent CD8+ T cells primed to the aberrant p21ras protein; (3) to determine whether T cells from normal individuals with potential reactivity to aberrant p21ras can be detected, activated and expanded in vitro; and (4) to determine whether patients with aberrant ras-positive tumors develop specific humoral responses to the aberrant p21ras protein.
| Pan, C; Xue, B H; Ellis, T M et al. (1997) Changes in telomerase activity and telomere length during human T lymphocyte senescence. Exp Cell Res 231:346-53 |
