Mantle cell lymphoma with t(11;14) and unmutated or mutated VH genes expresses AID and undergoes isotype switch events.

Isotype switch commonly follows onset of somatic hypermutation in the germinal center (GC), with activation-induced cytidine deaminase (AID) as a prerequisite. Mantle cell lymphoma (MCL) with t(11;14) includes a subset with unmutated (UM) and a minor subset with mutated (MUT) VH genes. Here, we investigated whether switch events and AID expression occur in MCL. In 4 of 6 UM and 4 of 7 MUT MCLs, alternative tumor-derived Cgamma,alpha,epsilon transcripts were identified. AID transcripts, including a splice variant, were common to both subsets. AID expression correlated with switch in 8 of 8 cases, but in 3 of 5 cases it occurred with switch absent. Circle transcripts (Igamma-Cmicro/Ialpha-Cmu) were identified in 5 of 7 evaluated cases. In 1 of 12 cases, 12% of tumor cells expressed immunoglobulin L-restricted surface IgA. Ongoing switch recombination events appear to be a feature of MCL, likely restricted to a minor tumor subpopulation, with occasional variant sIg expression. UM MCLs implicate origins from pre-GC B cells and reveal switch events at ectopic sites.