Professor Satu Mustjoki has received honoraria and research funding from Novartis, Pfizer and Bristol-Myers Squibb (not related to this study). Professor Vincenzo Cerullo is cofounder and shareholder of the Valo Therapeutics LTD (not related to this study). All other named authors have no conflict of interest, financial or otherwise. Synopsis:Molecular mimicry can induce autoimmunity. By developing and using a bioinformatic tool to analyze molecular mimicry between tumor and viral antigens, the authors show this phenomenon can also play a role in antitumor immune responses.

Objective: Immune checkpoint inhibitors (ICI) can cause endocrine adverse events. However, endocrine AEs could be related to better treatment outcomes. Our aim was to investigate whether this holds true in a real-world setting of metastatic melanoma patients. Design: A retrospective single-institution study. Methods: We included 140 consecutive metastatic melanoma patients treated with ICI between January 2012 and May 2019. We assessed endocrine toxicity and best possible treatment outcomes from electronic patient records, including laboratory parameters, and radiological images. Results: Of the treated patients, 21 patients (15%) were treated with ipilimumab, 46 (33%) with nivolumab, 67 (48%) with pembrolizumab, and six (4%) with combination therapy (ipilimumab + nivolumab). Endocrine AEs appeared in 29% (41/140) patients. Three patients had two different endocrine AEs. Thyroid disorders were the most common: 26% (36/140), followed by hypophysitis: 4% (5/140). Three subjects (2%, 3/140) were diagnosed with autoimmune diabetes. Three patients had to terminate treatment due to endocrine toxicity. Radiological manifestations of endocrine AEs were found in 16 patients (39%, 16/41). Endocrine toxicity was associated with significantly better treatment outcomes. Median progression-free survival (8.1 months, range 5.1 – 11.1 months vs. 2.7 months, range 2.4 – 3.0 months, P < 0.001), and median overall survival (47.5 months, range 15.5 – 79.5 months vs. 23.7 months, range 15.3 – 32.1 months, P = 0.035) were longer for patients experiencing endocrine AEs. Conclusions: The higher number of endocrine AEs suggest regular laboratory monitoring aids in AE detection. Endocrine AEs in metastatic melanoma may correlate with better treatment outcomes.

Molecular mimicry is known to be one of the leading mechanisms by which infectious agents may induce autoimmunity. However, whether a similar mechanism triggers anti-tumor immune response is unexplored, and the role of anti-viral T-cells infiltrating the tumor has remained anecdotal. To address this question, we first developed a bioinformatic tool to identify tumor peptides with high similarity to viral epitopes. Using peptides identified by this tool, we showed that, in mice, viral pre-existing immunity enhanced the efficacy of cancer immunotherapy via molecular mimicry. Specifically, when treated with a cancer vaccine consisting of peptides with a high degree of homology with specific viral peptides, the mice with induced pre-existing immunity to these viral peptides showed significantly better anti-tumor response. To understand whether this mechanism could partly explain immunotherapy-response in humans, we analyzed a cohort of melanoma patients undergoing PD1 treatment with high IgG titer for Cytomegalovirus (CMV). In this cohort of patients, we showed that high level of CMV-antibodies was associated with a prolonged progression free survival, and found that in some cases PBMCs could cross-react with both melanoma and CMV homologous peptides. Finally, T cell TCR sequencing revealed expansion of the same CD8+ T-cell clones, when PBMCs were pulsed with tumor- or homologous viral peptides. In conclusion, we have demonstrated that pre-existing immunity and molecular mimicry could explain part of the response observed in immunotherapy. Most importantly, we have developed a tool able to identify tumor antigens and neoantigens based on their similarity to pathogen antigens, in order to exploit molecular mimicry and cross-reactive T-cells in cancer vaccine development.