Systemic lupus erythematosus historically has been and still remains a poorly understood autoimmune disorder. Recent advances have been made in identifying lupus promoting single gene alterations that phenocopy many aspects of lupus in humans. Despite these advances there remains no unifying hypothesis and no new mechanism based, directed immunotherapy for this disease. In recent years several clinical and experimental studies have pointed to the impairment in the clearance of apoptotic cells as a common feature of lupus. As the molecular basis for the recognition, binding, and uptake of apoptotic cells by macrophages has become better defined, gene alterations associated with the impaired expression of phagocytic receptors for """"""""eat me"""""""" signals are associated with murine models of lupus. Examples include the milk fat globule-EGF factor 8 and the c-mer proto-oncogene tyrosine kinase. The phospholipid lyso- phosphatidylcholine has been identified as an important """"""""find me"""""""" signal for the recruitment of macrophages involved in the clearance of apoptotic cells. However, how lyso-phosphatidylcholine is generated as a find me signal is unknown and the identification of comparable alterations in the genes encoding the enzyme or enzymes involved in lyso-PC formation that would result in a lupus phenotype have not been reported. Our laboratory discovered, cloned, and characterized a novel phospholipase A2 named lysosomal phospholipase A2. This enzyme is distinct among the family of phospholipase A2s. Lysosomal phospholipase A2 is characterized by an acidic pH optimum, the ability to transacylate ceramide, and it secretion from macrophages in response to phagocytic stimuli such as zymosan. A knockout mouse, lacking the expression of lysosomal phospholipase A2 is characterized by a late onset autoimmune phenotype that mimics most aspects of lupus. These mice develop lymphoproliferation with hepatosplenomegaly, wasting, and renal failure. The mice exhibit markedly positive ANAs, anti-dsDNA titers, high circulating immunoglobulin levels, and glomerulonephritis. The spleens and lymph nodes are characterized by the persistence of high levels of apoptotic bodies in association with resident macrophages, consistent with a defect in the clearance of apoptotic cells. Based on these observations, the following primary hypothesis is proposed. Lysosomal phospholipase A2 is necessary for the digestion and clearance of apoptotic cells by macrophages. The following specific aims are proposed to test this hypothesis: 1) To immunologically phenotype LPLA2 null mice. 2) To determine the mechanism by which the absence of lysosomal phospholipase A2 inhibits or eliminates the clearance of apoptotic cells by macrophages. 3) To determine lysosomal phospholipase A2 is either sufficient or necessary to rescue the lupus phenotype in the LPLA2 null mice. 4) To determine whether inhibition of lysosomal phospholipase A2 by hydralazine, quinidine, and procainamide is the basis for drug induced lupus.
We have discovered and characterized a new lysosomal phospholipase A2. Knockout mice, deficient in the activity of this enzyme, develop a late onset autoimmune phenotype with features similar to systemic lupus erythematosus. Understanding the mechanisms whereby impaired degradation of phospholipids result in the autoimmune phenotype may provide new insights into the pathogenesis of lupus and potential new targets for therapy.
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