Impaired T cell function in argininosuccinate synthetase deficiency.

ASS1 is a cytosolic enzyme that plays a role in the conversion of citrulline to arginine. In human and mouse tissues, ASS1 protein is found in several components of the immune system, including the thymus and T cells. However, the role of ASS1 in these tissues remains to be defined. Considerable attention has been focused recently on the role of metabolism in T cell differentiation and function. Based on the expression of ASS1 in the immune system, we hypothesized that ASS1 deficiency would result in T cell defects. To evaluate this question, we characterized immune function in hypomorphic fold/fold mice. Analysis of splenic T cells by flow cytometry showed a marked reduction in T cell numbers with normal expression of activation surface markers. Gene therapy correction of liver ASS1 to enhance survival resulted in a partial recovery of splenic T cells for characterization. In vitro and in vivo studies demonstrated the persistence of the ASS1 enzyme defect in T cells and abnormal T cell differentiation and function. Overall, our work suggests that ASS1 plays a role in T cell function, and deficiency produces primary immune dysfunction. In addition, these data suggest that patients with ASS1 deficiency (citrullinemia type I) may have T cell dysfunction.

Autoimmune hepatitis: evolving concepts.

The liver is continuously exposed to a large antigenic load that includes pathogens, toxins, tumor cells and dietary antigens. A loss of tolerance against its own antigens may result in autoimmune hepatitis (AIH). The current paradigm holds that the disease is the result of self-perpetuating autoimmune process triggered by yet unknown factors (infections, chemicals, drugs) in a genetically susceptible host. To date, several putative hepatocellular surface antigens have been identified: P450-IID6 (recognized by the anti-LKM-1 autoantibodies) a membrane bound asialoglycoprotein receptor (a liver-specific membrane protein), a cytosolic UGA-suppressor tRNA associated protein (recognized by anti-SMA and anti-LP antibodies) and argininosuccinate lysate and formiminotransferase cyclodeaminase (recognized by ant-LC1 antibodies). In contrast to other chronic hepatitides patients with AIH display significant T cell hypereactivity to autologous liver antigens. Tissue injury seems to be mediated by CD4+ or CD8+ T cells and/or by antibody-dependent cell mediated cytotoxicity.

Argininosuccinate lyase: a new autoantigen in liver disease.

Anti-liver cytosol 1 autoantibody (LC1) characterizes a severe form of autoimmune hepatitis (AIH), staining the cytoplasm of periportal hepatocytes and targeting an unidentified 60-kD liver cytosolic antigen. To identify its target, we used high-titre anti-LCI+ sera from two patients with AIH to screen 18 cytoplasm enzymes with periportal location by double immunodiffusion (DDI). Both sera gave a broad precipitin line against human liver cytosol, suggesting that they may recognize two distinct antigens, a possibility confirmed by the appearance of two precipitin lines when DDI conditions were optimized (0.8% agarose and 3% polyethylene glycol (PEG)). Experiments by DDI and Western blot (WB) identified a liver cytosolic autoantigen of 50 kD, different from LC1, giving a line of identity with argininosuccinate lyase (ASL). Reactivity to ASL was then investigated by DDI and WB in 57 patients with AIH, 17 with primary biliary cirrhosis (PBC), 15 with chronic hepatitis B virus (HBV) infection, 13 with alphal-antitrypsin deficiency, 17 with Wilson's disease, 18 with extrahepatic autoimmune disorders, and in 48 healthy controls. Anti-ASL was found in 16% of AIH and 23% of PBC patients by DDI and in 14% of AIH, 23% of PBC and 20% of HBV patients by WB. No argininosuccinate was present in the urine of four anti-ASL+ patients tested, excluding an inhibition of enzymatic activity by anti-ASL. The addition of anti-ASL+ serum to human fibroblast cultures induced a significant increase in ASL activity. ASL is a new autoantigen in liver disease and its clinical relevance warrants further investigation.

Characterization of argininosuccinate lyase (EC 4.3.2.1) from Chlamydomonas reinhardtii.

We have isolated and characterized argininosuccinate lyase (ASL; EC 4.3.2.1) from the photosynthetic green alga, Chlamydomonas reinhardtii. The general properties of Chlamydomonas ASL are very similar to those described previously for ASLs from phylogenetically diverse organisms. The algal ASL has a native Mr, determined by gel-filtration chromatography, of 218,000 +/- 25,000, and a pI of 5.4-5.6. The Km for argininosuccinate at 37 degrees C and pH 7.5 is 0.26 mM. The subunit Mr of Chlamydomonas ASL is approx. 50,000, determined by SDS/polyacrylamide-gel electrophoresis, in contrast with a previously reported value of 39,000. Rabbit antisera prepared against the Mr-50,000 protein completely abolished ASL activity in vitro. In contrast, serum prepared against the Mr-39,000 protein was ineffective in inhibiting ASL activity. Despite the general similarity of the physical properties of Chlamydomonas ASL and those of other ASLs, antiserum raised against the algal ASL did not cross-react with ASL preparations from Escherichia coli, Saccharomyces cerevisiae or bovine liver.