Exploring antiviral strategies: From innate immunity modulation to viral proteins targeting

The search for new targets and antiviral agents is continuously growing, especially for agents with broad-spectrum activity. In fact, even though vaccines are the elective way to limit diseases and possibly eradicate pathogens, they are not always effective, particularly in people with compromised immune defenses, and they can also lose activity against rapidly evolving pathogens. Hence the identification of novel antiviral agents is a priority for the health systems, and the number of approved antiviral drugs is increasing yearly: until today over one hundred antivirals have been approved. The most common strategies to develop antiviral drugs are based on the identification of molecules targeting viral proteins and blocking viral replication as direct acting agents. Although successful, this strategy must consider that most viruses are easily capable to select drug resistant strains. A novel approach for the identification of potential broad-spectrum antivirals is to target cellular proteins inducing innate immune response, so avoiding the high mutagenesis rate occurring for viral proteins. An important help in fighting viral spread can arrive from the Traditional Chinese Medicine (TCM), widely used in China to treat infectious diseases both alone and in cooperation with western medicines. This thesis will investigate three possible paths for the identification of potential antivirals i) the identification of cellular targets to develop broad-spectrum antivirals, ii) target viral proteins in order to impair viral replication selectively, iii) study the effect of immunomodulators with unknown targets in order to identify potential broad-spectrum antiviral. Among the cellular proteins possibly used as drug target, a recently discovered one is STING: a downstream actor in the detection of non-self cytosolic nucleic acids related to viral infections and tumor conditions. The Stimulator of Interferon Genes (STING) plays a pivotal role in counteracting viral infections, independently from whether the viral genome is DNA or RNA, mounting a strong innate immune response driven principally by type I Interferon (IFN-I). On the one side, when cytosolic DNA is detected cGAS produces cyclic GMP-AMP (2’3’cGAMP) that directly binds STING determining TBK1 phosphorylation (pTBK1) and hence transcription of IFN-I. On the other side, when the RIG-I pathway is activated in response to viral RNA detection, STING interacts with activated Mitochondrial Antiviral Signaling protein (MAVS) determining pTBK1 mediated IFN-I production. The SARS-CoV-2 pandemic have highlighted the need of specific as well as potent antivirals; among SARS-CoV-2 proteins the Papain Like protease (PL-pro) is an important protein involved in viral life cycle and in the innate immune evasion, hence it will be investigated as potential viral target for the development of selective antiviral drugs. Finally, we will investigate the mechanism of action of a series of compounds derived from one molecule isolated from herbal extracts used as immune stimulator in the TCM, in order to verify their potency in inducing the innate immune response.