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This document is a special issue of the International Journal of Molecular Sciences that focuses on the role of Toll-like receptors (TLRs) in infection and inflammation. It explores the involvement of TLRs in viral infections, including HIV-1, SARS-CoV-2, and co-infections with Hepatitis B or C Virus. The articles discuss the importance of TLRs in immune surveillance, disease development, and potential therapeutic applications. They also highlight the connection between acute viral infections and long-term effects, such as neuroinflammation and neurodegeneration. The document provides a comprehensive overview of TLRs and their significance in various infectious and inflammatory diseases, offering insights into potential treatment strategies and future research directions. [Extracted from the article]

Regulatory T cells (Tregs) play a crucial and complex role in balancing the immune response to viral infection. Primarily, they serve to regulate the immune response by limiting the expression of proinflammatory cytokines, reducing inflammation in infected tissue, and limiting virus-specific T cell responses. But excessive activity of Tregs can also be detrimental and hinder the ability to effectively clear viral infection, leading to prolonged disease and potential worsening of disease severity. Not much is known about the impact of Tregs during severe influenza. In the present study, we show that CD4+/CD25+FoxP3+ Tregs are strongly involved in disease progression during influenza A virus (IAV) infection in mice. By comparing sublethal with lethal dose infection in vivo, we found that not the viral load but an increased number of CD4+/CD25+FoxP3+ Tregs may impair the immune response by suppressing virus specific CD8+ T cells and favors disease progression. Moreover, the transfer of induced Tregs into mice with mild disease symptoms had a negative and prolonged effect on disease outcome, emphasizing their importance for pathogenesis. Furthermore, treatment with MEK-inhibitors resulted in a significant reduction of induced Tregs in vitro and in vivo and positively influenced the progression of the disease. Our results demonstrate that CD4+/CD25+FoxP3+ Tregs are involved in the pathogenesis of severe influenza and indicate the potential of the MEK-inhibitor zapnometinib to modulate CD4+/CD25+FoxP3+ Tregs. Thus, making MEK-inhibitors even more promising for the treatment of severe influenza virus infections. [ABSTRACT FROM AUTHOR]

Influenza causes substantial morbidity and mortality, and highly pathogenic and drug-resistant strains are likely to emerge in the future. Protease-activated receptor 1 (PAR1) is a thrombin-activated receptor that contributes to inflammatory responses at mucosal surfaces. The role of PAR1 in pathogenesis of virus infections is unknown. Here, we demonstrate that PAR1 contributed to the deleterious inflammatory response after influenza virus infection in mice. Activating PAR1 by administering the agonist TFLLR-NH2 decreased survival and increased lung inflammation after influenza infection. Importantly, both administration of a PAR1 antagonist and PAR1 deficiency protected mice from infection with influenza A viruses (IAVs). Treatment with the PAR1 agonist did not alter survival of mice deficient in plasminogen (PLG), which suggests that PLG permits and/or interacts with a PAR1 function in this model. PAR1 antagonists are in human trials for other indications. Our findings suggest that PAR1 antagonism might be explored as a treatment for influenza, including that caused by highly pathogenic H5N1 and oseltamivir-resistant H1N1 viruses.

The recent COVID-19 pandemic again highlighted the urgent need for broad-spectrum antivirals, both for therapeutic use in acute viral infection and for pandemic preparedness in general. The targeting of host cell factors hijacked by viruses during their replication cycle presents one possible strategy for development of broad-spectrum antivirals. By inhibiting the Raf/MEK/ERK signaling pathway, a central kinase cascade of eukaryotic cells, which is being exploited by numerous viruses of different virus phyla, the small-molecule MEK inhibitor zapnometinib has the potential to address this need. We here performed a side-by-side comparison of the antiviral efficacy of zapnometinib against IAV and SARS-CoV-2 to determine the concentration leading to 50% of its effect on the virus (EC50) and the concentration leading to 50% reduction of ERK phosphorylation (IC50) in a comparable manner, using the same experimental conditions. Our results show that the EC50 value and IC50 value of zapnometinib are indeed lower for IAV compared to SARS-CoV-2 using one representative strain for each. The results suggest that IAV's replication has a stronger dependency on an active Raf/MEK/ERK pathway and, thus, that IAV is more susceptible to treatment with zapnometinib than SARS-CoV-2. With zapnometinib's favorable outcome in a recent phase II clinical trial in hospitalized COVID-19 patients, the present results are even more promising for an upcoming phase II clinical trial in severe influenza virus infection. [ABSTRACT FROM AUTHOR]

Expression of the antiviral cytokines IFN-[alpha]/[beta] is among the most potent innate defenses of higher vertebrates to virus infections, which is controlled by the inducible transcription factor IFN regulatory factor (IRF)3. Borna disease virus (BDV) establishes persistent noncytolytic infections in animals and tissue culture cells, indicating that it can circumvent this antiviral reaction by an unexplained activity. In this study, we identify the BDV P protein as microbial gene product that associates with and inhibits the principal regulatory kinase of IRF3, Traf family member-associated NF-[kappa]B activator (TANK)-binding kinase 1 (TBK-1). We demonstrate that the P protein counteracts TBK-1-dependent IFN-[beta] expression in cells and, hence, the establishment of an antiviral state. Furthermore, our data show that the BDV P protein itself is phosphorylated by TBK-1, suggesting that P functions as a viral decoy substrate that prevents activation of cellular target proteins of TBK-1. Thus, our findings provide evidence for a previously undescribed mechanism by which a viral protein interferes with the induction of the antiviral IFN cascade. Boma disease virus | innate immunity