Frameshift peptide-derived T-cell epitopes: A source of novel tumor-specific antigens

Abstract

Microsatellite instability (MSI) caused by defective DNA mismatch repair (MMR) is a hallmark of hereditary nonpolyposis colorectal cancers (HNPCC) but also occurs in about 15% of sporadic tumors. If instability affects microsatellites in coding regions, translational frameshifts lead to truncated proteins often marked by unique frameshift peptide sequences at their C-terminus. Since MSI tumors show enhanced lymphocytic infiltration and our previous analysis identified numerous coding mono-and dinucleotide repeatbearing candidate genes as targets of genetic instability, we examined the role of frameshift peptides in triggering cellular immune responses. Using peptide pulsed autologous CD40-activated B cells, we have generated cytotoxic T lymphocytes (CTL) that specifically recognize HLA-A2.1-restricted peptides derived from frameshift sequences. Among 16 frameshift peptides predicted from mutations in 8 different genes, 3 peptides conferred specific lysis of target cells exogenously loaded with cognate peptide. One peptide derived from a (؊1) frameshift mutation in the TGF␤IIR gene gave rise to a CTL bulk culture capable of lysing the MSI colorectal cancer cell line HCT116 carrying this frameshift mutation. Given the huge number of human coding microsatellites and assuming only a fraction being mutated and encoding immunologically relevant peptides in MSI tumors, frameshift protein sequences represent a novel subclass of tumor-specific antigens. It is tempting to speculate that a frameshift peptide-directed vaccination approach not only could offer new treatment modalities for existing MSI tumors but also might benefit asymptomatic at-risk individuals in HNPCC families by a prophylactic vaccination strategy. © 2001 Wiley-Liss, Inc. Key words: DNA mismatch repair; microsatellite instability; frameshift peptides; tumor antigens; T-cell epitopesThe accumulation of genetic alterations and resulting mutant proteins represents a major obstacle for tumor cells to escape immune surveillance. It has thus been hypothesized that mutant proteins or derived peptides must exist that are capable of eliciting specific cellular immune responses. In fact, CTL have been reported that recognize peptides of mutant or aberrantly expressed proteins such as p21/ras, MAGE-1, oncogenic fusion proteins, [1][2][3] products from alternative open reading frames 4 -6 and also frameshift mutated APC. 7 These tumor antigens originally have been identified in tumors showing chromosomal instability. 8 Much less is known about the immunogenicity of tumor cells that show more subtle genetic alterations such as small deletions and insertions in repetitive DNA sequences, termed microsatellites. 9,10 Microsatellite instability in these tumor cells is due to deficient DNA mismatch repair caused by germline and/or sporadic mutations in at least 5 different MMR genes, 11 leading to high spontaneous mutation rates. More than 90% of HNPCC and about 15% of sporadic cancers of different organs show MSI. 9,12 If instability affects microsatellites in coding re...

Microsatellite instability in colorectal cancer is associated with local lymphocyte infiltration and low frequency of distant metastases

“…The strong antitumoural lymphoid reaction, which frequently occurs in MSI-H colorectal cancers, is commonly attributed to tumour-specific neopeptides generated during MSI-H carcinogenesis. Functional in vitro assays proved the immunogenicity of certain MSI-associated frameshift peptides (Linnebacher et al, 2001;Saeterdal et al, 2001;Ripberger et al, 2003). Both features, MSI-H status and CLR, have been reported to be associated with longer survival times in previous studies.…”

Immune system and prognosis in colorectal cancer: a detailed immunohistochemical analysis

“…MSI þ tumors harbor numerous mutations that lead to the production of altered, potentially immunogenic epitopes that can induce CTL. 35 However, the induction of CTL responses takes time, leaving time for tumor cells to escape the immune system. Therefore, NK cells may play an important role, as they do not require prior sensitization and therefore are able to eliminate metastasizing HLA-nonexpressing cells instantaneously.…”

Immune Cells in Colorectal Cancer: Prognostic Relevance and Role of MSI

“…In contrast, pronounced lymphocytic infiltration is marked in high-graded microsatellite instable (MSI-H) CRC and might explain the better clinical outcome in these patients [64]. It has been postulated that MSI-H CRC are more immunogenic than microsatellite stable (MSS) tumors because of the generation of a large number of abnormal peptides by frameshift mutations [49,[65][66][67][68][69][70]. However, in the analysis of Chiba et al [52] there was no significant increase in the number of CD8 + IEL in cases with MSI.…”