Different strategies based on peptides are available for cancer treatment, in particular to counter-act the progression of tumor growth and disease relapse. In the last decade, in the context of therapeutic strategies against cancer, peptide-based vaccines have been evaluated in different tumor models. The peptides selected for cancer vaccine development can be classified in two main type: tumor-associated antigens (TAAs) and tumor-specific antigens (TSAs), which are captured, internalized, processed and presented by antigen-presenting cells (APCs) to cell-mediated immunity. Peptides loaded onto MHC class I are recognized by a specific TCR of CD8+ T cells, which are activated to exert their cytotoxic activity against tumor cells presenting the same peptide-MHC-I complex. This process is defined as active immunotherapy as the host’s immune system is either de novo activated or restimulated to mount an effective, tumor-specific immune reaction that may ultimately lead to tu-mor regression. However, while the preclinical data have frequently shown encouraging results, therapeutic cancer vaccines clinical trials, including those based on peptides have not provided satisfactory data to date. The limited efficacy of peptide-based cancer vaccines is the consequence of several factors, including the identification of specific target tumor antigens, the limited immunogenicity of peptides and the highly immunosuppressive tumor microenvironment (TME). An effective cancer vaccine can be developed only by addressing all such different aspects. The present review describes the state of the art for each of such factors.
In 1991, van der Bruggen et al, Ludwig’s researchers in Brussels, published in Science a groundbreaking study on tumor antigens. They described for the first time that cytotoxic T cells can selectively recognize a tumor antigen expressed by the human melanoma (MAGE Ag) (1). This article represents a pillar oftumor immunology introducing the concept that such specific antigens may be used to design and develop an effective active immunotherapy. Since the discovery of MAGE, the field has moved forward rapidly. Scientists have discovered many other tumor antigens, of which many are being tested as targets for immunotherapy and in particular for therapeutic peptide-based cancer vaccines (2, 3).
Active immunotherapy is aimed either at amplifying the existing antitumor immune response by nonspecific proinflammatory molecules/adjuvants or at eliciting a specific de novo host immune response against selected tumor antigens by cancer vaccines. In particular, peptide-based cancer vaccines are able to activate the effector adaptive immune response as well as to provide long-term acquired immunity against a “foreign” tumor antigen. Indeed, cancer cells can be distinguished from normal cells by either upregulation/overexpression of endogenous proteins or mutation of those proteins. As a result, any mutated or differentially expressed protein in cancer cells can potentially represent a vaccine target. In particular, the antigens derived from overexpressed self proteins in tumor cells are defined tumor-associated antigens (TAAs) and might be shared among patients with the same tumor (4) (5, 6). To this class of antigens belong cell lineage differentiation antigens, which are normally not expressed in adult tissue (7, 8); and cancer/germline antigens (also known as cancer/testis) (9). Since TAAs are also expressed by normal cells, they may be subject to immunological tolerance and therefore may be poorly immunogenic. In contrast, antigens arising from cancer-related nonsynonymous mutations or other genetic alterations in cancer cells, named tumor-specific antigens (TSAs), are not expressed on the surface of normal cells and are specific to each type of tumors (10–12). Consequently, TSAs are not subject to central and peripheral immune tolerance and are able to trigger a specific and effective T cell response against cancer cells (13).
The transformed cells are recognized by the immune system through immunosurveillance. Indeed, new antigens, including tumor antigens, are constantly presented by antigen-presenting cells (APC) to B and T lymphocytes of the adaptive immune response. The APCs can express the peptides through the MHC class I molecule for presentation to CD8+ T cells, or MHC class II molecule for presentation to CD4+ T helper cells. The latter may differentiate in two major subtypes, Th2 and Th1, involved in inflammatory response as well as in potentiating and sustaining the activity of CD8+ T cells (CTLs), respectively (14). Consequently, activated CD8+ T cells recognize cancer antigens expressed on the surface of tumor cells and initiate the release of apoptotic factors such as Perforin, Fas Ligand and Granzymes, leading to cell-mediated cytotoxicity (15). In this regard, the basic principles for a successful therapeutic vaccination against tumors include delivery of large amounts of high immunogenic antigens to APCs, induction of strong and sustained CD4+ T helper cell and CD8+ cytotoxic T lymphocyte (CTL) responses, infiltration of the TME as well as durability and maintenance of the immune response. In particular, strategies for improving activation and maturation of APCs, aiming at a more efficient antigen presentation to elicit an optimal T cell response, are actively pursued by several groups. Such strategies are mostly focused on the identification, selection and validation of novel adjuvants able to stimulate and enhance the magnitude and durability of antigen-specific T and B cell responses (16).Several therapeutic peptide-based vaccine strategies and formulations have been evaluated in different tumor types in the last 2 decades. However, only modest effects have been reported with an overall rate of clinical benefit of around 20% (17, 18). MUC1 represents a paradigmatic example of a TAA, highly expressed by adenocarcinomas, which has been evaluated in several human clinical trials with poor efficacy. In particular, MUC1 was targeted to dendritic cells, demonstrating the induction of highly specific immune response in preclinical and in human clinical trials (19). Subsequently, the ligand mannan used to target antigens in the mannose receptor lead to maturation of dendritic cells and activation via the Toll-Like Receptor 4 (20–24). The National Cancer Institute Translational Research Working Group has listed MUC1 as the second promising target in cancer research from 75 tumor-related antigens (25). The distinctive biological structure of MUC1 and its aberrant glycosylation in cancer cells make it a recognized tumor-specific antigen on epithelial tumor cells (26, 27). In particular, MUC1 consists of both a C-terminal fragment (MUC1-C), highly conserved region, and an N-terminal fragment (MUC1-N), which contains the variable number tandem repeat (VNTR) region. The latter has been described to play a critical role in the MUC1 immunogenicity.
Therapeutic MUC1−based vaccines have been tested in numerous early stage clinical trials (28); ClinicalTrials.gov https://www.frankenthalerfoundation.org but none of them has shown the expected anti-tumor effect in patients. In particular, these studies indicate that both the VNTR (MUC1-N) and non-VNTR (MUC1-C) regions contribute to immune evasion by cancer cells, which needs to be taken into consideration and addressed during the further development of MUC1-based cancer vaccines (29–31).
To date, the only FDA-approved therapeutic cancer vaccine is Provenge® (Sipucleucel-T) for patients with castration-resistant prostate cancer (32). However, a modest increase in overall survival is observed together with a partial tumor regression. In addition to Sipucleucel-T vaccine, other peptide-based cancer vaccines are developed for prostate cancer, such as Cancer-associated membrane carbohydrates, including ganglioside (GM2), mucin 1 (MUC1), globo H and Thompson–Friedenreich antigen. Overall, five approaches - GVAX, DCVAC/PCa, a multi-epitope peptide vaccine, sipuleucel-T and PROSTVAC (33) - have been investigated in randomized phase III trials. They all showed safety and partial immunological activity but no effective clinical efficacy (34).
Such a poor efficacy of the therapeutic cancer vaccines developed for prostate cancer is common to most of the vaccines developed for other cancers. This is the consequence of a series of events that facilitate tumor development. Among these, the immunosuppressive TME plays a central role in cancer cell immune escape, inhibiting the activation of a specific T cell immune response against tumor cells (35). Moreover, during the tumor growth, the constant pressure from the adaptive immune system, coupled with the genetic instability of cancer cells, can select cellular sub-clones with reduced immunogenicity able to evade the immune recognition and destruction. Such low immunogenic sub-clones are often affected by loss of antigen presentation, due to a defective antigen- presenting machinery (36), hampering the recognition of tumor antigens by conventional (CD4+ and CD8+) and unconventional T cells (37). Moreover, chronically stimulated CD8+ T cells infiltrating the TME may acquire an ‘exhausted’ state characterized by loss of cytolytic activity, reduced cytokine production and reduced proliferation capacity.
To circumvent such adverse components of TME and improve their efficacy, cancer vaccines should be combined with immunomodulatory drugs (namely immunocheckpoint inhibitors, ICIs) to counterbalance the intra-tumor immune suppressive factors (38–40).
In the present review, we provide an overview of biological mechanisms underlying peptide-based vaccine platform developed in the last years for cancer therapy. We also describe the identification, characterization and design of such antigens, their formulation, as well as different combinatorial strategies to optimize their antigenicity and overcome the immune evasion for more specific and effective immune response.
The immunodominance is a key aspect to consider when planning a vaccination strategy, in particular when the vaccine formulation is based on few and short specific epitopes. In particular, the immunodominance of T cell determinants results from several factors: 1) intrinsic characteristics of the epitope, e.g. the binding affinity to MHC molecules; 2) the presence of appropriate MHC molecules and T cell receptors at the individual level; and 3) competition between different epitopes of a given protein antigen for the available MHC binding sites. The combination of all such factors will give rise to distinct immunodominant epitopes selectively driving the T cell response.
Thus, to maximize the efficacy of a vaccine based on a mix of peptides, it is relevant to ensure a balanced T cell response for all the peptides included in the mix. Otherwise, the less immunogenic epitopes would elicit an inefficient anti-cancer T cell response, with a resulting immunological escape of tumor cells.
Epitope specificity for T cells is mediated by the T-cell receptor (TCR), which binds peptides presented in the “peptide binding groove” of class I or class II major histocompatibility complexes (MHCs, also known as human leukocyte antigen, HLA, for humans). Whole proteins are in
