Abstract IA02: Selection of cancer antigens and conditions for success of immunotherapy of cancer involving vaccination
Authors: Melief Cornelis J. M. 
Published: November 2016

Abstract
Cancer antigens of choice include mutant DNA sequences encoding neo-antigens, selected cancer testis antigens, and differentiation antigens, in addition to viral antigens. Neo-antigens and viral antigens are the most attractive, because the T cell repertoire against these antigens has not been blunted by central thymic tolerance. Optimal vaccine platforms include DNA and RNA vaccines as well as synthetic long peptides and selected prime-boost protocols involving recombinant viruses encoding cancer antigens. The most effective cancer vaccines deliver concentrated antigen to both HLA class I and II molecules of DCs, promoting both CD4 and CD8 T cell responses. Suboptimal antigen selection, suboptimal vaccine design, and an immunosuppressive cancer microenvironment are the root causes of the lack of clinical efficacy of cancer vaccines in many clinical trials. Drugs or physical treatments can mitigate the immunosuppressive cancer microenvironment and include chemotherapeutics, radiation, inhibitors of T cell checkpoints, indoleamine 2,3-dioxygenase (IDO) inhibitors, agonists of selected TNF receptor family members, and inhibitors of undesirable cytokines. The specificity of therapeutic vaccination combined with such immunomodulation offers an attractive avenue for the development of future cancer therapies. Clinical benefit of certain therapeutic cancer vaccines has been established, usually by noting prolonged progression free or overall survival. Reviews of the properties and prospects of cancer vaccines have been listed below (1-3). The addition of one or more immunomodulation strategies may further enhance the potential of therapeutic cancer vaccines, including of those that have been only marginally active due to the immunosuppressive cancer microenvironment.
We have established clinical benefit of a cancer vaccine as monotherapy in patients with premalignant disease induced by high risk human papilloma virus. Regression of lesions was associated with a better vaccine-induced immune response against the target antigens of the vaccine. However in patients with cervical cancer the vaccine monotherapy failed to install clinically effective immunity. Recently we showed in a pilot trial that vaccine-induced immunity could be markedly enhanced by combination with standard of care chemotherapy for late stage recurrent or metastatic cervical cancer. Increased levels of myeloid-derived suppressor cells were shown to be depleted by the carboplatin and paclitaxel chemotherapy to the low levels of healthy donors, while T cell numbers in PBMC remained intact. Timed vaccination with a single dose of vaccine at the MDSC nadir during chemotherapy led to a robust vaccine-induced T cell response (4). In a subsequent chemo-immunotherapy trial with three vaccine doses, spaced by three weeks, and in cohorts of patients at four different vaccine doses, the initiation and maintenance of robust tumor-specific T cell responses was confirmed and a broad dose response relationship of tumor-specific T cell responses was noted. Preliminary data on clinical outcomes relative to HPV-specific immune responses will be discussed.
In summary, therapeutic benefit of cancer vaccines can be maximized in patients with established cancers by improving vaccine design and by using vaccines in combination with standard chemotherapies. In addition cancer vaccines can establish and/or maintain tumor-specific T cells in combination with checkpoint blockade or other immunomodulatory therapies. Finally, cancer vaccines can also help to sustain and expand adoptively transferred tumor-specific T cells.
References
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2. Melief CJ,.van Hall T, Arens R, Ossendorp F, van der Burg SH. Therapeutic cancer vaccines. J Clin Invest. 2015 125(9):3401-3412.
3. van der Burg SH, Arens R, Ossendorp F, van Hall T, Melief CJ. Vaccines for established cancer: overcoming the challenges posed by immune evasion. Nat Rev.Cancer. 2016 Apr;16(4):219-33.
4. Welters MJ, van der Sluis TC, van Meir H, Loof NM, van Ham VJ, van Duikeren S, Santegoets SJ, Arens R, de Kam ML, Cohen AF, van Poelgeest MI, Kenter GG, Kroep JR, Burggraaf J, Melief CJ, van der Burg SH. Vaccination during myeloid cell depletion by cancer chemotherapy fosters robust T cell responses. Sci Transl Med. 2016 Apr 8(334):334ra52.
Citation Format: Cornelis J. M. Melief. Selection of cancer antigens and conditions for success of immunotherapy of cancer involving vaccination [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr IA02.