Phospholipase A2 inhibitor and LY6/PLAUR domain-containing protein PINLYP regulates type I interferon innate immunity

Type I interferons (IFNs) are the first frontline of the host innate immune response against invading pathogens. Herein, we characterized an unknown protein encoded by phospholipase A2 inhibitor and LY6/PLAUR domain-containing (PINLYP) gene that interacted with TBK1 and induced type I IFN in a TBK1- and IRF3-dependent manner. Loss of PINLYP impaired the activation of IRF3 and production of IFN-β induced by DNA virus, RNA virus, and various Toll-like receptor ligands in multiple cell types. Because PINLYP deficiency in mice engendered an early embryonic lethality in mice, we generated a conditional mouse in which PINLYP was depleted in dendritic cells. Mice lacking PINLYP in dendritic cells were defective in type I IFN induction and more susceptible to lethal virus infection. Thus, PINLYP is a positive regulator of type I IFN innate immunity and important for effective host defense against viral infection.

Interferon (IFN)-mediated antiviral responses serve as the first line of the host innate immune defense against viral infection. IFNs are divided into three families based on sequence homology: type I, type II, and type III (1, 2). The type I IFN family encodes 13 subtypes of IFN-α in humans (14 in mice), a single IFN-β subtype, and several poorly defined subtypes (3, 4). Type I IFNs were originally identified based on their ability to interfere with viral replication, restrain virus dissemination, and activate adaptive immune responses (5–7). They can be induced in most cell types by microbial pathogen-associated and damage-associated molecular patterns recognized by pattern recognition receptors (PRRs) (3). By inducing the expression of IFN-stimulated genes (ISGs), type I IFNs elicit antiviral innate immunity and mediate adaptive immune responses (8, 9).The induction of antiviral type I IFN response is elicited in response to the stimulation of PRRs that detect pathogen-associated molecular patterns, such as viral nucleic acids, viral replicative intermediates, and surface glycoproteins (10, 11). There are four major subfamilies of PRRs: the Toll-like receptors (TLRs), nucleotide-binding oligomerization domain/leucine-rich repeat-containing receptors, RIG-1-like receptors (RLRs), and the C-type lectin receptors, which are located at the cell surface, in the cytosol, or endosomal compartments (11–14). Among the TLR family members, TLR3, TLR7, TLR8, and TLR9 are involved in the recognition of viral nucleotides. Viral DNA enriched in CpG-DNA motifs is recognized by TLR9, single-stranded RNA is recognized by TLR7 and TLR8, and double-stranded RNA and its synthetic analog polyinosinic-polycytidylic acid (poly I:C) are recognized by TLR3 (15, 16). Some viral envelope proteins can be recognized by TLR4 or TLR2 (16, 17).Following viral infection, cytosolic DNA can be sensed by cyclic guanosine monophosphate (GMP)–adenosine monophosphate (AMP) synthase (cGAS) that induces the production of cyclic GMP-AMP (cGAMP) (18, 19). cGAMP functions as a second messenger that binds and activates the endoplasmic reticulum (ER) adaptor STING (19–22). Translocation of activated STING from the ER to the Golgi apparatus leads to the activation of kinase TBK1, which subsequently phosphorylates IRF3 and triggers the production of type I IFN (22–24). Cytosolic RNA can be recognized by the RLRs like RIG-1 and MDA5, which signal via mitochondrial antiviral signaling protein (MAVS; also known as CARDIF, IPS1, and VISA) and subsequently activate TBK1 and IRF3–IRF7, leading to the induction of type I IFNs and other antiviral genes (25–27).The lymphocyte antigen-6 (Ly6)/urokinase-type plasminogen activator receptor (uPAR) superfamily is characterized by the LU domain and a domain containing 10 cysteines that form distinct disulfide bridges, which create the three-fingered structural motif. The Ly6/uPAR family members regulate a wide range of functions in various cell types (28). Here, we uncovered the previously uncharacterized role of the Ly6/uPAR family member PINLYP in the induction of type I IFNs in response to DNA virus, RNA virus, and other TLR ligands. This study further defined the pivotal function of PINLYP in the effective host defense against virus infection.