Role of Toll-like receptors in the pathogenesis of COVID-19.

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has led to a pandemic since March 2020. The exact pathogenesis of SARS-CoV-2 and the role of each component of the innate and adaptive immune system is still unknown. However, available data from other coronavirus families, such as SARS-CoV and the Middle East respiratory syndrome and also new findings could be useful for a better understanding of SARS-CoV-2. Toll-like receptors (TLR) play an important role in recognition of viral particles and activation of the innate immune system. Activation of TLR pathways leads to secretion of pro-inflammatory cytokines, such as interleukin-1 (IL-1), IL-6, and tumor necrosis factor-α, as well as type 1 interferon. Different TLRs, like TLR2, TLR3, TLR4, TLR6, TLR7, TLR8, and TLR9 are potentially important in COVID-19 infection. It is also worth mentioning that we should bear in mind both the beneficial and harmful effects of TLR in confronting COVID-19 infection. TLRs could be a potential target in controlling the infection in the early stages of disease and production of vaccine against SARS-CoV-2.