Hypocomplementemia of unknown etiology: an opportunity to find cases of IgG4-positive multi-organ lymphoproliferative syndrome.

Recently, a new clinical entity, IgG4-positive multi-organ lymphoproliferative syndrome (IgG4+ MOLPS), characterized by hyper-IgG4 gammaglobulinemia and IgG4-positive plasma cell tissue infiltration, has been proposed. It includes autoimmune pancreatitis (AIP), Mikulicz's disease, and many other inflammatory conditions affecting multiple organs. However, diagnosis is difficult if the disease is not suspected because serum IgG subclasses are not measured routinely and the affected organs vary. Because hypocomplementemia is often observed in this condition, we investigated the serum subclasses of IgG in patients with hypocomplementemia, especially of unknown etiology. We found 6 patients with high serum IgG4 levels among 10 patients with hypocomplementemia of unknown etiology who visited our hospital between December 2004 and September 2007. The results of additional pathological and imaging examinations in the 6 patients with high serum IgG4 levels were compatible with IgG4+ MOLPS. Our results suggest that hypocomplementemia of unknown etiology offers an opportunity to find cases of IgG4+ MOLPS.