Grancalcin (GCA) modulates Toll‐like receptor 9 (TLR9) mediated signaling through its direct interaction with TLR9

Toll‐like receptors (TLRs) are playing important roles in stimulating the innate immune response and intensifying adaptive immune response against invading pathogens. Appropriate regulation of TLR activation is important to maintain a balance between preventing tumor activation and inhibiting autoimmunity. Toll‐like receptor 9 (TLR9) senses microbial DNA in the endosomes of plasmacytoid dendritic cells and triggers myeloid differentiation primary response gene 88 (MyD88) dependent nuclear factor kappa B (NF‐κB) pathways and type I interferon (IFN) responses. However, mechanisms of how TLR9 signals are mediated and which molecules are involved in controlling TLR9 functions remain poorly understood. Here, we report that penta EF‐hand protein grancalcin (GCA) interacts and binds with TLR9 in a yeast two‐hybrid system and an overexpression system. Using siRNA‐mediated knockdown experiments, we also revealed that GCA positively regulates type I IFN production, cytokine/chemokine production through nuclear localization of interferon regulatory factor 7 (IRF7), NF‐κB activation, and mitogen‐activated protein kinase (MAPK) activation in plasmacytoid dendritic cells. Our results indicate that heterodimerization of GCA and TLR9 is important for TLR9‐mediated downstream signaling and might serve to fine tune processes against viral infection.