Cytotoxic T cells generated against heteroclitic peptides kill primary tumor cells independent of the binding affinity of the native tumor antigen peptide

Abstract

Heteroclitic peptide modifications increase immunogenicity, allowing generation of cytotoxic T lymphocytes (CTLs) against weakly immunogenic tumor-associated antigens (TAAs). A critical issue is whether T cells generated against heteroclitic peptides retain the ability to recognize and kill tumor cells expressing the original weak TAAs, and whether there is a lower threshold of binding affinity of the native peptides, below which such CTLs can still kill primary tumor cells. To examine this we used a model examining the ability of native and heteroclitic immunoglobulin (Ig)-derived peptides to generate CTLs that can kill chronic lymphocytic leukemia (CLL) cells. We demonstrate that CTLs generated against heteroclitic peptides have enhanced killing of CD40-activated B cells pulsed with either heteroclitic (P < .001) or native peptide (P ‫؍‬ .04) and primary CLL cells (P ‫؍‬ .01). The novel finding reported here is that the rate-limiting factor appears to be the ability to generate CTLs and that once generated, CTL lysis of primary tumor cells is independent of the binding affinity of the native peptide. These findings have implications for vaccination strategies in malignancies and are currently being further examined in vivo in murine models. IntroductionA primary requirement for successful tumor vaccine development is the identification of tumor-associated antigens (TAAs). The first TAA identified was the idiotype (Id) of the immunoglobulin (Ig) expressed in malignant B cells, [1][2][3] and Id vaccination strategies induce specific humoral and cellular immune responses that can lead to tumor regression or rejection. [4][5][6][7][8][9][10] A bioinformatics approach has been applied to the identification of T-cell epitopes from a variety of candidate TAAs, including proteinase 3, 11 MUC-1, 12 melanoma antigen 3, 13 and telomerase, 14 as well as the Id of malignant B cells. 15,16 A conceptual drawback to targeting Id has been the need to create an individualized reagent for each patient. In chronic lymphocytic leukemia (CLL), subsets of patients have been identified with remarkably similar antigen receptors, [17][18][19][20][21][22] and shared Ig framework region (FR)-derived peptides represent important targets for cross-reactive Id therapy, offering the advantage of a less patient-specific immunotherapeutic strategy in B-cell malignancies. 16 A major limitation of this method is the generally low immunogenicity and low binding affinity of these peptides to major histocompatibility complex (MHC) class I and class II molecules. Heteroclitic peptide modifications can increase immunogenicity of low-binding peptides while leaving T-cell recognition residues intact, 23 and these "heteroclitic peptides" lead to improved ability to generate cytotoxic T lymphocyte (CTL) responses against primary tumors. 15,24,25 Heteroclitic modifications as a strategy to enhance immune responses have been tested in several tumor antigens, 26,27 and agonist analogs of subdominant epitopes, once optimized for binding class I molecules, c...

The only proposed T-cell epitope derived from the TEL-AML1 translocation is not naturally processed

“…On the other hand, it has been reported that for generating effector CD8 ϩ T cells using heteroclitic peptides, the rate limiting step was the priming step and once the effector cells were generated, the binding affinity of the native peptide to MHC class I was not limiting in the capacity of these cells to perform effector functions. 27 The same study reported increased killing of targets expressing native peptides with FI equaling zero. FI of the native TEL-AML1 peptide equals 0.27 ( Figure 1C), which might explain its specific recognition by anchor-modified peptide primed CD8 ϩ T cells.…”

Peptide-Based Anticancer Vaccines: Recent Advances and Future Perspectives

“…When these substitutions are made in residues that contact the TCR, the peptides are called "altered peptide ligands". Such peptides are of interest for use in anticancer vaccine development because they can strongly stimulate T-cells and therefore achieve more potent immune responses as compared with the native epitope [73][74][75]. They can act as superagonists and have been shown to break T-cell tolerance in a mouse viral hepatitis model [76], which might make them particularly useful for tumorspecific immune responses [77,78].…”

Peptides in Melanoma Therapy

“…Such peptides are of interest for use in anticancer vaccine development because they can strongly stimulate T-cells and therefore achieve more potent immune responses as compared with the native epitope [56,57]. They can act as superagonists and have been shown to break T-cell tolerance in a mouse viral hepatitis model [58].…”