Noninfectious mixed cryoglobulinaemic glomerulonephritis associated with monoclonal gammopathy of undetermined significance

Abstract Background Cryoglobulins are cold-precipitable immunoglobulins that may be found in association with systemic vasculitis including cryoglobulinaemic glomerulonephritis (CGN). Type 1 cryoglobulins consist of isolated monoclonal immunoglobulin (mIg), whereas mixed cryoglobulins are typically immune complexes comprising either monoclonal (type 2) or polyclonal (type 3) Ig with rheumatoid activity against polyclonal IgG. Only CGN related to type 1 cryoglobulins has been clearly associated with monoclonal gammopathy of undetermined significance (MGUS) using the conventional serum-, urine- or tissue-based methods of paraprotein detection. Methods We retrospectively assessed our patient cohort of CGN related to mixed (type 2 or 3) cryoglobulins for those with a monoclonal band on serum protein electrophoresis (SPEP), with exclusion of infectious cases and those involving malignant clonal plasma cell or B cell disorders. Results We identified four patients with a median age of 54 years, including three women. Two patients had type 2 cryoglobulinaemia, one had type 3 cryoglobulinaemia, and one lacked definitive typing of the serum cryoprecipitate. The serum monoclonal band identified on SPEP was IgM-κ in all four cases. Treatments included corticosteroids, cyclophosphamide, plasma exchange, and rituximab. At median 3.5 years’ follow-up, no patient had developed a haematological malignancy or advanced chronic kidney disease. Other potential causes of mixed cryoglobulinaemia were also present in our cohort, notably primary Sjögren’s syndrome in three cases. Conclusions Our study raises questions regarding the attribution of causation to MGUS, and the role of clonally directed therapies outside the setting of haematological malignancy.