Isotype-switched immunoglobulin genes with a high load of somatic hypermutation and lack of ongoing mutational activity are prevalent in mediastinal B-cell lymphoma

Primary mediastinal B-cell lymphoma (PMBL) is a subentity of diffuse large B-cell lymphoma with characteristic clinical, histomorphologic, immunopheno typical, and genetic features. Unlike other B-cell lymphomas, PMBL has not yet been the subject of comprehensive molecular studies on the rearranged i mmunoglobulin (1g) gene. Such investigations have proved essential to obtai ning information about the differentiation stage of the lymphomagenic B cel l. In the present study, the clonally rearranged immunoglobulin heavy-chain gene of 13 PMBL cases is analyzed by polymerase chain reaction (PCR) in co njunction with cloning and DNA sequencing. Twelve of 13 rearrangements were potentially functional. All clonally rearranged immunoglobulin genes bore a high load of somatic mutations (average, 13.0%), which appeared to be sel ected for a functional antibody in the majority of cases. The comparison of cloned PCR products revealed no evidence of ongoing mutation of the immuno globulin variable gene. By means of reverse-transcriptase PCR, lymphoma-spe cific immunoglobulin transcripts were detected in 8 of 13 cases, all of whi ch were of the postswitched type, whereas immunoglobulin protein expression was undetectable except for 1 case. A PMBL cell line, MedB-1, generated fr om an IgG(-) parental tumor, constitutively expressed IgG protein in a subs et of cells, which was moderately suppressed by interleukin-4 and upregulat ed in the presence of dexamethasone. PMBL is thus characterized by a heavil y mutated, class-switched immunoglobulin gene without evidence of ongoing m utational activity. Moreover, our data indirectly suggest that regulation b y extrinsic signals contributes to the immunoglobulin-negative phenotype of PMBL.