Investigations On The Mechanism And Implications Of The Extracellular DNA-binding Protein Sap-mediated Regulation In DNA Immune Recognition.

The innate immune recognition of nucleic acids is crucial to protective and pathological immunities and is mediated by the endosomal Toll-like receptors (TLRs) and cytosolic receptors. Recent reports have identified a group of extracellular proteins to be sentinels for nucleic acids and facilitated innate immune responses. However, it remains unknown whether a mechanism exists that negatively regulates these nucleic-acid-sensing systems. Here we show that human serum amyloid P protein (hSAP) functions as a universal inhibitor for DNA sensing. Human SAP bind to all immunogenic nucleic acids examined with a correlation between affinity and immunogenic-inhibiting potential. Cells incubated with hSAP-DNA complex show a significant defect to secrete pro-inflammatory cytokines, accompanied by impaired activation of the transcription factors interferon regulatory factor 3 (IRF3) and nuclear factor (NF)-kB. Human SAP also significantly inhibited the functions of two identified DNA sentinels high-mobility group B protein (HMGB) 1 and antimicrobial peptide LL37, and redirected DNA update to Fc receptors leading to endocytosis and endosomal degradation. Interestingly, we also indicated SAP as a species-specific regulator in the nucleic-acid-mediated activation of immune responses, with the mouse SAP has a much weaker activity. These findings may be helpful for understanding the evolution of the innate immune system and have some implications for the development of DNA vaccine and treatment of autoimmune disorders such as liver hepatitis and HCC.For DNA vaccine studies, we hypothesize that serum amyloid P component (SAP) contributes to the differences between human and mice and then limits DNA vaccine's efficacy in vivo. In our study, DNA vaccine-induced adaptive immune responses were also significantly decreased in the human SAP transgenic mice. We also found that a chemical SAP inhibitor strongly recovered the suppressed innate immune responses to DNA in the presence of human serum and enhanced the immunogenicity of DNA vaccines in vivo. Our data indicated that SAP is a key negative regulator for innate immune responses to DNA and may be partly responsible for the insufficient immune responses following DNA vaccinations in humans. SAP suppression may be a novel strategy for improving efficacy of human DNA vaccines and requires further clinical investigations.For alcoholic liver injury, our studies have indicated that RIG-I pathway is involved in the DNA recognition by liver parenchymal cells and extracellular DNA could be greatly uptaked by liver parenchymal cells and subsequently activate RIG-I/IRF3 to induce the production of type-I IFNs. We also proved that SAP facilitated the uptake of extracellular DNA by liver cells. In this regard, we suggest that gut-derived bacterial DNA also contributes to IRF3 activation and may play a key role in the prevention of alcoholic liver injury. We also suggest that DNA-mediated hepatic IRF3 activation is of great significances in China because of the high rate of hepatitis B virus (HBV) infection. We also found that HBxAg could inhibit the activation of IRF3 and induce great changes of many genes in DNA-treated hepatocytes. We therefore hypothesize that HBV may also albeit the prevention functions of IRF3 and type-I IFNs via inhibiting the MAVS protein and HBV infection may increase the risk of ALD, which require further investigations.