Nodal and extranodal plasmacytomas expressing immunoglobulin a: an indolent lymphoproliferative disorder with a low risk of clinical progression.

Plasmacytomas expressing immunoglobulin A are rare and not well characterized. In this study, 9 cases of IgA-positive plasmacytoma presenting in lymph node and 3 in extranodal sites were analyzed by morphology, immunohistochemistry, and polymerase chain reaction examination of immunoglobulin heavy and κ light chain genes. Laboratory features were correlated with clinical findings. There were 7 males and 5 females; age range was 10 to 66 years (median, 32 y). Six of the patients were younger than 30 years of age, 5 of whom had nodal disease. About 67% (6 of 9) of the patients with nodal disease had evidence of immune system dysfunction, including human immunodeficiency virus infection, T-cell deficiency, autoantibodies, arthritis, Sjögren syndrome, and decreased B cells. An IgA M-spike was detected in 6 of 11 cases, and the M-protein was nearly always less than 30 g/L. All patients had an indolent clinical course without progression to plasma cell myeloma. Histologically, nodal IgA plasmacytomas showed an interfollicular or diffuse pattern of plasma cell infiltration. The plasma cells were generally of mature Marschalko type with little or mild pleomorphism and exclusive expression of monotypic IgA. There was an equal expression of κ and λ light chains (ratio 6:6). Clonality was showed in 9 of 12 cases: by polymerase chain reaction in 7 cases, by cytogenetic analysis in 1 case, and by immunofixation in 1 case. Clonality did not correlate with pattern of lymph node infiltration. Our results suggest that IgA plasmacytomas may represent a distinct form of extramedullary plasmacytoma characterized by younger age at presentation, frequent lymph node involvement, and low risk of progression to plasma cell myeloma.