Differential Expression Patterns of Toll-like Receptors in COVID-19 Patients

Since Toll-like receptors (TLRs) recognize the earliest signs of infection or cell damage, they play fundamental roles in innate immunity. This review summarizes the numerous studies on the expression of TLRs in patients with Coronavirus disease 2019 (COVID-19). We show that infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can stimulate at least six of the ten TLRs in humans and that this can shape the severity of COVID-19. Specifically, TLR2, TLR4, and TLR9 appear to play pathogenic roles while TLR3, TLR7, and TLR8 may be protective. Most have mutations that could partly explain the susceptibility phenotypes of COVID-19. Further understanding the roles of TLRs in COVID-19 immunopathogenesis could reveal prognostic biomarkers and help drive the development of novel and effective therapeutics for COVID-19.