Overcoming doubts and other obstacles in the development of effective peptide-based therapeutic vaccines against cancer

There is considerable skepticism regarding whether therapeutic vaccines against established/advanced tumors will ever exhibit any therapeutic benefit in the clinic [1–3]. Although many promising observations have been made in preclinical animal tumor models, results in clinical studies have been less convincing, engendering wide disbelief that the immune system, and in particular cytotoxic T lymphocytes (CTLs), will be capable of eliminating established tumors. Most immunologists agree that the innate and adaptive arms of the immune system have evolved in mammals mainly to fight off infectious pathogens. The ultimate goal of cancer immunotherapy is to manipulate the immune system to recognize molecular alterations in the tumor cells (i.e., tumor antigens) and mount effector responses that would be strong enough to reject tumors in a similar manner as pathogens (and allogeneic transplants) are eliminated. Regrettably, the magnitude of the immune responses generated towards tumor antigens are not remotely close to those observed against antigens expressed by pathogenic microorganisms during acute infections, which lead to the pathogen’s eradication. The majority of tumor antigens selected for developing immunotherapy are also expressed to some extent by normal cells and are referred to as tumor-associated antigens (TAAs). It is clear that the immune system is not always eager to mount responses against ‘self’ antigens and that it is necessary to override immune tolerance to generate immune responses against TAAs. Nevertheless, the existence of autoimmune disorders constitutes clear evidence that the barriers imposed by immune tolerance are not insurmountable.

For a long time, it was believed that distinction of ‘nonself’ from ‘self’ was solely the function of the adaptive immune system through the recognition of the molecular structure of the antigens by binding to B- or T-cell receptors. However, it has recently become clear that the innate immune system plays an equal or perhaps more important role in the assessment of self from nonself for the generation of successful adaptive immune responses that lead to the elimination of many biological threats. Without the active participation of the innate immune system, which is activated by so-called ‘danger signals’, the adaptive immune system will not respond even to strong foreign antigens (i.e., immunological ignorance). The danger signals that awaken the innate immune system are conserved molecular patterns either derived from pathogens (e.g., prokaryotic DNA, dsRNA, lipopeptides or bacterial lipopolysaccharides) or from damaged tissues (e.g., uric acid, S100 proteins or heat-shock proteins). Pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns are recognized by various receptors (e.g., Toll-like receptors [TLRs] and helicases) expressed by cells from both the innate and adaptive immune systems. During acute infections, the intensity and duration of immune responses will largely depend on the sustained presence of antigens, and PAMPs or damage-associated molecular patterns.

We believe that in order to design a successful immunotherapy strategy against tumors, it is critical not to ignore the important discoveries made by our immunologist colleges working in the areas of infection, immunity and autoimmunity. For example, successful immunological resolutions of bacterial and viral infections are usually accompanied by massive expansions of pathogen-reactive CTLs, whereas in some instances close to 50% of all CD8⁺ T cells are antigen specific, which only subside once the infection has been cleared. On the other hand, most cancer vaccines generate rather modest responses (<1% of CD8⁺ T cells are antigen-specific CTLs) that disappear soon after immunization. Thus, it should not be that surprising that few, if any, tumor responses or regressions are observed. Therefore, we propose that an effective antitumor vaccine should attempt to mimic an acute infection providing not only relevant TAAs but also potent immunological adjuvants and immune costimulatory signals to generate CTL responses that approximate the magnitude of those responses observed during infections. The use of recombinant viruses or bacteria expressing TAAs has attempted to adopt this strategy but, unfortunately, the immune system focuses mostly on generating CTL responses against the microbial foreign antigens, where no immune tolerance exists, paying less attention to the TAAs. To circumvent these issues, our laboratory has developed a strategy that combines the use of TAAs in the form of synthetic peptides representing CTL epitopes, with synthetic compounds representing PAMPs, which stimulate various members of the TLR family [4–6]. More recently we have included a third component to these subunit vaccines, corresponding to immune costimulatory monoclonal antibodies. These subunit vaccines, which we call TriVax (consisting of three components: peptide antigen, TLR ligand and costimulatory anti-CD40 antibody) are effective in generating massive and sustained CTL responses in mice, which in some instances account for 50–80% of all CD8⁺ T cells being antigen-specific CTLs [7,8]. To our best knowledge, this is the first report of CTL responses of this magnitude against TAAs using any kind of vaccine, without the use of adoptive T-cell transfers. More importantly, significant antitumor effects (e.g., reduced tumor growth and increased survival) were observed in mice bearing established B16 melanoma tumors. These results indicate that it is possible to overcome the barriers imposed by immune ignorance and tolerance to generate very large CTL responses using subunit vaccines. Despite this accomplishment, it may be a little premature to raise a ‘mission accomplished’ banner. It is well known that tumors develop evasion strategies to circumvent the effector function of CTLs, such as loss of TAA expression and the production of numerous immune inhibitory activities. Using the B16 murine melanoma system, we have made the unexpected observation that the antitumor effectiveness of TriVax is substantially better if the effects of IFN-γ are blocked. IFN-γ has long been considered a beneficial cytokine against tumor cells [9], mostly due to its antiproliferative and immune-enhancing effects. Nevertheless, we have made the paradoxical observation that TriVax therapeutic immunization is successful in completely eliminating large established tumors only when the effects of IFN-γ are blocked (in IFN-γ-deficient animals or using B16 cells that are unresponsive to IFN-γ). Thus, it appears that tumor cells can detect the initiation of an immune response via IFN-γ signals and respond accordingly by becoming resistant to CTLs. Although the exact mechanism(s) involved in the generation of a CTL-resistant state of the tumor cells induced by IFN-γ has not been elucidated, we believe that increased expression of the immune inhibitory molecule PD-L1 (also known as B7-H1 or CD274) on the tumor cell surface, which is induced by IFN-γ [10,11], is largely responsible for this effect. Future work will be required to confirm this hypothesis and to assess if this phenomenon occurs in other tumor types.

In summary, the early demise of therapeutic cancer vaccines has been the result of prematurely performed clinical studies that showed suboptimal CTL responses and disappointing beneficial antitumor effects. The generation of large immune responses against TAAs by a new generation of optimized vaccines brings us closer to achieving success against established malignancies. However, it will be necessary to find ways to deal with the immune evasion strategies that many tumors utilize to avoid complete rejection.

Financial & competing interests disclosure

Supported in part by grant R01CA136828 from the NIH to Esteban Celis. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.