Platelets release mitochondrial antigens in systemic lupus erythematosus

The accumulation of DNA and nuclear components in blood and their recognition by autoantibodies play a central role in the pathophysiology of systemic lupus erythematosus (SLE). Despite the efforts, the sources of circulating autoantigens in SLE are still unclear. Here, we show that in SLE, platelets release mitochondrial DNA, the majority of which is associated with the extracellular mitochondrial organelle. Mitochondrial release in patients with SLE correlates with platelet degranulation. This process requires the stimulation of platelet FcγRIIA, a receptor for immune complexes. Because mice lack FcγRIIA and murine platelets are completely devoid of receptor capable of binding IgG-containing immune complexes, we used transgenic mice expressing FcγRIIA for our in vivo investigations. FcγRIIA expression in lupus-prone mice led to the recruitment of platelets in kidneys and to the release of mitochondria in vivo. Using a reporter mouse with red fluorescent protein targeted to the mitochondrion, we confirmed platelets as a source of extracellular mitochondria driven by FcγRIIA and its cosignaling by the fibrinogen receptor α2bβ3 in vivo. These findings suggest that platelets might be a key source of mitochondrial antigens in SLE and might be a therapeutic target for treating SLE.

The Pathogenesis of Primary Biliary Cholangitis: A Comprehensive Review

Primary biliary cholangitis (PBC) is a chronic cholestatic liver disease characterized by autoimmune destruction of small to medium size intrahepatic bile ducts. The etiology of PBC remains unknown and pathogenesis features immune-mediated biliary injury, alongside the consequences of chronic cholestasis. PBC is strongly associated with the loss of immune tolerance against mitochondrial antigens and the subsequent presence of an articulated immunologic response that involves both humoral and cellular responses. Both environmental factors and genetic variants increase PBC susceptibility. Biliary epithelial cells have often been considered a passive target of the immune attack in PBC; however, cholangiocyte dedifferentiation, senescence, stress, and deoxyribonucleic acid damage have been recognized to play an active role in the pathogenesis of PBC. This review highlights and discusses the most relevant pathogenetic mechanisms in PBC, focusing on the key factors that lead to the onset of cholestasis and immune activation.

Bile Acids and Deregulated Cholangiocyte Autophagy in Primary Biliary Cholangitis

Background: Primary biliary cholangitis (PBC) is characterized by a high prevalence of serum anti-mitochondrial antibodies against the E2 subunit of the pyruvate dehydrogenase complex and bile duct lesions called chronic non-suppurative destructive cholangitis (CNSDC) in small bile ducts, eventually followed by extensive bile duct loss and biliary cirrhosis. Macroautophagy (a major type of autophagy) is a process of cellular self-digestion that plays a critical role in energy homeostasis and in the cytoprotection to various stresses. Deregulated autophagy is thought to be associated with various human diseases. Key Messages: Accumulating evidences suggest that deregulated autophagy may be a central player in the pathogenesis of PBC. Damaged cholangiocytes involved in CNSDC show vesicular expression of autophagy marker LC3 and accumulation of p62/sequestosome-1, suggesting deregulated autophagy. Deregulated autophagy may be involved in the autoimmune process via the abnormal expression of mitochondrial antigens and also in cholangiocyte senescence in bile duct lesions in PBC. In vitro study showed that hydrophobic bile acids, such as glycochenodeoxycholic acid (GCDC), as well as serum deprivation and oxidative stress, cause autophagy, deregulated autophagy and abnormal expression of mitochondrial antigens followed by cellular senescence in cholangiocytes. Although exact mechanisms of deregulated autophagy remain to be clarified, endoplasmic reticulum (ER) stress may be a plausible cause of deregulated autophagy induced by GCDC in cholangiocytes. Impaired ‘biliary bicarbonate umbrella' may further exacerbate the toxicity of GCDC to cholangiocytes. Interestingly, pretreatment with ursodeoxycholic acid (UDCA) and tauro-UDCA, which is a chemical chaperone enhancing the adaptive capacity of the ER, significantly suppressed ER stress, deregulated autophagy and cellular senescence induced by GCDC and other stresses in cholangiocytes. Conclusions: GCDC may play a role in the occurrence of deregulated autophagy and cellular senescence at least partly through the induction of ER stress in PBC. Deregulated autophagy and cellular senescence can be a promising therapeutic target in PBC.