LACK OF TISSUE TRANSGLUTAMINASE PROTEIN CROSS-LINKING LEADS TO LEAKAGE OF MACROMOLECULES FROM DYING CELLS - RELATIONSHIP TO DEVELOPMENT OF AUTOIMMUNITY IN MRLLPR LPR MICE/

Genetic defects of the CD95 (Fas/Apo-1) receptor/ligand system, has re cently been involved in the development of human and murine autoimmuni ty, We investigated whether a deregulation of the 'tissue' transglutam inase (tTG), a multifunctional enzyme which is part of the molecular p rogram of apoptosis, may act as a cofactor in the development of autoi mmunity. We found that MRLIpr/Ipr, which are characterized by a defect in the CD95 receptor and suffer of a severe systemic lupus erythemato sus-like disease, produce large amounts of circulating tTG autoantibod ies. This phenomenon is paralleled by an abnormal accumulation of an i nactive enzyme protein in the accessory cells of lymphoid organs. To i nvestigate the molecular mechanisms by which tTG inhibition may contri bute to the development of autoimmunity we generated a cell culture mo del system consisting of L929 cells stably transfected with a fu II le ngth tTG cDNA. When L929 eel Is were killed by Tumor Necrosis Factor a lpha (TNF alpha) a pronounced release of DNA and Lactate Dehydrogenase (LDH) was observed, Overexpression of tTG in these cells largely prev ented the leakage of macromolecules determined by TNF alpha treatment, an effect which is abolished by inactivating the enzyme cross-linking activity by a synthetic inhibitor. These in vitro observations provid ed the basis to explain the increased levels of plasmatic LDH we detec ted in MRLIpr/Ipr mice. These data suggest that lack of an active tTG may represent a cofactor in the development of autoimmunity.