Locked and loaded: engineering and arming oncolytic adenoviruses to enhance anti-tumor immune responses

Introduction: Oncolytic adenoviruses (Ads) are promising therapeutics to enhance anti-tumor immune responses and modulate the immune-suppressive tumor microenvironment (TME). Due to their potent ability to deliver genes in vivo, oncolytic Ads have been armed with a variety of immune stimulatory payloads to boost tumor immunotherapy. Areas covered: We describe current knowledge about engineering oncolytic Ads to insert transgene payloads, including methods to increase its genome capacity for transgene insertion. We also review several categories of immune stimulatory payloads that have been used in oncolytic Ads to combat different barriers to effective immunotherapy. Expert opinion: We anticipate that multi-armed oncolytic Ads alone or in combination with other types of immunotherapies will greatly improve the efficacy of oncolytic virotherapy in the future by targeting several immune barriers. However, the production and testing of multiple payload-armed Ads can be complex and time-consuming due to the limitations of current tools. Given this, we should develop new tools for rapid construction of armed Ads, improve animal models or new systems to compare the efficacy of multiple payloads, and carefully monitor the immunological toxicity induced by payloads or by systemic delivery. Most importantly, we should pursue payload-arming approaches with great care to ensure safety.