‘Arid5a expression is induced through MYD88-independent pathway in response to TLR4 stimulation in IFNγ-sensitized human macrophages.’

Abstract

Abstract Sepsis is caused by infection, especially gram-negative bacteria, followed by an inflammatory cascade leading to cytokine storm. High IL-6 serum level in patients with sepsis is correlated with severity and high mortality. However, the exact pathological mechanism(s) in sepsis regulating high IL-6 expression and how Arid5a, a post-transcriptional regulator of IL-6, is related to exaggerated IL-6 expression are not fully understood yet. We found that Arid5a and IL-6 are more markedly expressed in response to Toll-like receptor 4 (TLR4) than TLR2 stimulation in both human primary monocyte-derived macrophages (MDM) as well as human macrophage cell line (THP-1 cells). TLR4 signals through the cell surface as well as the endosomal compartment activating MYD88-indpendent pathway. Pre-treatment of THP-1 cells with endocytosis inhibitor (MiTMAB) significantly decreases the levels of Arid5a, IL-6 and IFNβ, whereas TNFα and IL-8 are marginally affected. Moreover, MYD88 gene-knockout THP-1 cells showed similar expression level of Arid5a compared to the wild type cells. Furthermore, IFNγ pre-treatment of THP-1 cells as well as MDM results in enhancement of TLR4 endocytosis, hence higher expression of Arid5a, IL-6 and IFNβ upon LPS stimulation, while IFNγ pre-treatment has little effect on Arid5a and IL-6 expression upon TLR2 stimulation. On the contrary, IL-4 pre-treatment results in an opposite effect on TLR4 endocytosis. These results taken together indicate that priming of macrophages with IFNγ or IL-4 determines the responsiveness to LPS and that Arid5a expression is regulated by MYD88-independent pathway.