Optimization of a self antigen for presentation of multiple epitopes in cancer immunity

Abstract

T cells recognizing self antigens expressed by cancer cells are prevalent in the immune repertoire. However, activation of these autoreactive T cells is limited by weak signals that are incapable of fully priming naive T cells, creating a state of tolerance or ignorance. Even if T cell activation occurs, immunity can be further restricted by a dominant response directed at only a single epitope. Enhanced antigen presentation of multiple epitopes was investigated as a strategy to overcome these barriers. Specific point mutations that create altered peptide ligands were introduced into the gene encoding a nonimmunogenic tissue self antigen expressed by melanoma, tyrosinase-related protein-1 (Tyrp1). Deficient asparagine-linked glycosylation, which was caused by additional mutations, produced altered protein trafficking and fate that increased antigen processing. Immunization of mice with mutated Tyrp1 DNA elicited cross-reactive CD8 + T cell responses against multiple nonmutated epitopes of syngeneic Tyrp1 and against melanoma cells. These multispecific anti-Tyrp1 CD8 + T cell responses led to rejection of poorly immunogenic melanoma and prolonged survival when immunization was started after tumor challenge. These studies demonstrate how rationally designed DNA vaccines directed against self antigens for enhanced antigen processing and presentation reveal novel self epitopes and elicit multispecific T cell responses to nonimmunogenic, nonmutated self antigens, enhancing immunity against cancer self antigens. IntroductionT cells play a central role in immunity against cancer. Cancer antigens recognized by T cells are encoded by 2 broad categories of genes: those expressed by normal somatic and germ cells and those expressed only by cancer cells (e.g., due to mutations acquired during or after malignant transformation) (1, 2).A question had been whether T cells capable of recognizing self antigens on cancer cells are present in the repertoire. Evidence shows that T cells are positively selected for survival during development in the thymus by signals elicited by self peptides complexed with MHC molecules (pMHCs) and maintained in the periphery by self pMHCs (3-7). Thus the repertoire of T cells develops and is maintained through self reactivity. To avoid autoimmunity, T cells with high avidity for self pMHCs are deleted, but more weakly self-reactive T cells survive.This pool of T cells discriminates nonself antigens of pathogens by cross-reactivity to foreign peptides in the context of proinflammatory and costimulatory signals elicited by pathogens through activation of APCs. On the other hand, cognate self peptides are too weak to activate T cells, particularly in the absence of sufficient costimulation (8). A corollary to these observations is that, despite abundant self-reactive T cells in the peripheral immune system, initiation of T cell responses against self antigens on cancer are restricted by insufficient signals.

An Effective Vaccine Strategy Protective against Antigenically Distinct Tumor Variants

“…Evidence suggests that the breadth and diversity of the T cell response is important for cancer immunity (5) and monospecific T cell responses may not be effective (6). Vaccination strategies have been employed that elicit immune responses against multiple epitopes (7). Because these vaccines have shown increased potential, it will be important to consider alternative sources of antigen that can elicit tumor-specific responses against multiple antigens.…”

DNA Immunization against Tissue-Restricted Antigens Enhances Tumor Immunity after Allogeneic Hemopoietic Stem Cell Transplantation

“…In draining lymph nodes of nontransplanted B6 mice immunized with human TRP-2 DNA, we detected 0.6% CD8 ϩ T cells that recognized the TRP-2 181-188 peptide using an intracellular cytokine assay. In nontransplanted B6 mice immunized with VP22-Opt-TYRP1 DNA, we detected responses to three CD8 ϩ epitopes in the native Tyrp1 sequence (32). The precursor frequency in transplanted mice that were immunized (HSCT plus vaccine) was Ͼ2-fold higher in the spleen and almost 3-fold higher in draining lymph nodes compared with nontransplanted immunized mice (vaccine) for both human TRP-2 plus GM-CSF (Fig.…”

Impact of Sequence Variation in a Dominant HLA-A*02-Restricted Epitope in Hepatitis C Virus on Priming and Cross-Reactivity of CD8 + T Cells

“…For vaccine design, this implies that the optimal antigen sequence for immunization is not necessarily the most frequent epitope sequence in circulating isolates in a population. The concept of using altered peptide ligands for immunization and priming of cross-reactive CD8 ϩ T cells is well described in cancer immunology (27,31,32). Such altered peptide ligands, associated with better priming potential and broader cross-reactivity, are potentially also beneficial for immunization against highly variable pathogens.…”