Expression of activation-induced cytidine deaminase (AID) in Burkitt lymphoma cells: rare AID-negative cell lines with the unmutated rearranged VH gene.

Activation-induced cytidine deaminase (AID) is an enzyme that catalyzes somatic hypermutation (SHM) and class switch recombination (CSR) of the immunoglobulin (Ig) gene. The expression of AID was reported to be confined to the germinal center (GC). Burkitt lymphoma (BL) cells are regarded as being derived from GC cells. BL cells have been reported to have SHM in the Ig gene with variety in terms of degree, antigen selection and ongoing mutation characteristic. We investigated the expression of AID in 15 BL cell lines by RT-PCR. In only 2 BL cell lines, Tree92 and Black93, AID expression was not detected, and the 1gVH gene of these 2 cell lines was not mutated. Tree92 expresses terminal deoxy-nucleotidyl transferase (TdT) and recombination activating gene (RAG)1, but Black93 does not, as is typical of the BL phenotype. BL cells are generally derived from GC B-cell expressing AID, but are rarely derived from the stage of B-lineage differentiation in which AID is not yet expressed, causing the absence of mutation in the IgVH.