Toll-like receptors (TLRs) are key mediators of innate immunity, and their activation by microbial components leads to the production of cytokines and interferons. Recombinant alpha interferon has been used to treat several viral diseases and is the current standard of care for hepatitis C virus (HCV) infection. Recently, agonists of TLR7 and TLR9 have been shown to have clinical efficacy in HCV patients, and this is correlated with their ability to induce endogenous type I interferon production. We have carried out a comprehensive study of agonists of TLRs 1 to 9 to determine if any additional TLRs can induce antiviral molecules from human peripheral blood mononuclear cells (PBMCs). The agonists were incubated with PBMCs, and the supernatant was then removed and added to HCV replicon cells to assess antiviral activity. Agonists of TLRs 3, 4, 7, 8, and 9 were found to be potent inducers of antiviral activity in PBMC supernatants, and the activity correlated with the induction of alpha interferon and the interferon-induced antiviral biomarker 2,5-oligoadenylate synthase. Antiviral activity of TLR7 and TLR8 agonists was blocked by an antibody that binds to the type I interferon receptor, confirming that the antiviral activity results from type I interferon induction. TLR4 and TLR8 agonists were found to strongly induce the proinflammatory cytokines interleukin 1␤ and tumor necrosis factor alpha at concentrations similar to those inducing antiviral activity. This raises concerns about adverse side effects if these were to be used as antiviral agents. We therefore conclude that TLRs 3, 7, and 9 represent the most attractive targets for the development of new HCV therapies.The World Health Organization reports that ϳ170 million people worldwide are infected with hepatitis C virus (HCV) (29). The disease affects primarily the liver, with 10 to 20% of chronic infections leading to cirrhosis and 1 to 5% leading to liver cancer. The recommended standard therapy for HCV is a once-weekly injection of pegylated interferon alpha 2a (IFN␣2a) plus daily oral ribavirin. The sustained viral response is approximately 50% (23), and the therapy is associated with significant side effects including flu-like symptoms, depression, and injection site reactions (9). In addition, some patients develop neutralizing antibodies to recombinant IFN, which may contribute to treatment failures (2, 13). There is therefore a need to discover new drugs with improved efficacy and reduced side effects.Toll-like receptors (TLRs) are a family of receptors that play a key role in innate immunity. On binding microbial ligands, they induce a signaling cascade resulting in the induction of type I IFNs and other cytokines, which drive an inflammatory response and activate the adaptive immune system (for a review, see reference 27). Whereas current IFN therapy involves the administration of a single IFN-␣ subtype (2a or 2b), TLR activation induces a range of different IFN subtypes. For example, the TLR7 agonist imiquimod has been shown to induce IFN-␣1, -␣2...

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SummaryAdvanced therapy medicinal products (ATMPs) represent the current pinnacle of ‘patient‐specific medicines’ and will change the nature of medicine in the near future. They fall into three categories; somatic cell‐therapy products, gene therapy products and cells or tissues for regenerative medicine, which are termed ‘tissue engineered’ products. The term also incorporates ‘combination products’ where a human cell or tissue is combined with a medical device. Plainly, many of these new medicines share similarities with conventional haematological stem cell transplant products and donor lymphocyte infusions as well as solid organ grafts and yet ATMPs are regulated as medicines and their development has remained predominantly in academic settings and within specialist centres. However, with the advent of commercialisation of dendritic cell vaccines, chimeric antigen receptor (CAR)‐T cells and genetically modified autologous haematopoietic stem cells to cure single gene‐defects in β‐thalassaemia and haemophilia, the widespread availability of these therapies needs to be accommodated. Uniquely to ATMPs, the patient or an allogeneic donor is regularly part of the manufacturing process. All of the examples given above require procurement of blood, bone marrow or an apheresate from a patient as a starting material for manufacture. This can only occur in a clinical facility licensed for the procurement of human cells for therapeutic use and this is likely to fall to haematology departments, either as stem cell transplant programmes or as blood transfusion departments, to provide under a contract with the company that will manufacture and supply the final medicine. The resource implications associated with this can impact on all haematology departments, not just stem cell transplant units, and should not be under‐estimated.

Toll-like receptors recognize invading microorganisms and activate innate immune responses. Their discovery has opened up a range of therapeutic possibilities, in particular for infectious diseases. Responses to TLR agonists have been largely studied in mice and little information exists in other species. Given that rats are commonly used for pharmacokinetic and toxicology studies in drug development, we compared TLR7 responses in rat and human. Stimulation of rat and human peripheral blood mononuclear cells with the TLR7 agonist SM360320 showed that in rat cells, the interferon-induced gene, 2', 5' oligoadenylate synthase and tumor necrosis factor alpha were induced at lower concentrations and to a greater degree compared with human cells. Both human and rat cells demonstrated tolerance and could not be restimulated following initial treatment with high concentrations of SM360320. Reducing the concentration of the initial treatment allowed cells to be restimulated following a period of recovery. The initial treatment concentration had to be reduced to a greater extent to enable restimulation of rat cells compared with human cells. Dosing whole rats repeatedly with different concentrations of SM360320 confirmed the in vitro results. Treatment of human cells with high concentrations of interferon alpha did not induce tolerance to subsequent treatment with SM360320 indicating that tolerance occurs in the TLR7 signaling pathway, rather than the interferon signaling pathway. We conclude that rat and human cells respond differently to TLR7 activation and that these differences should be considered when using rat as a model to study TLR7 agonists.