Follicular lymphoma (FL) is characterized by the apoptosis-suppressing overexpression of BCL2 protein in mature B cells, driven in most cases by the t(14;18)(q32;q21) translocation. FL usually follows an indolent course and may be preceded by a slow accumulation of precursor cells in the lymph nodes and peripheral blood, so called follicular lymphoma in situ (FLIS) and follicular lymphoma-like cells (FLLC), respectively. The presence of the t(14;18) in apparently healthy individuals indicates that other molecular events are required for the development of lymphoma, but until now it has been difficult to map the process by which these precursor cells might progress to malignancy. This series of experiments from the group of Dr. Bertrand Nadel and Dr. Sandrine Roulland at the Aix-Marseille Université in Marseille, France, builds upon their previous work on FLIS by using an elegant mouse model of early FL to track memory B cell behavior and relating this to human t(14;18) cells.The model used in this study was the BCL2tracer mouse, which has a human BCL2 transgene that is only activated upon inversion during V(D)J recombination, creating a mosaic that mimics the sporadic occurrence of the t(14;18) translocation. These mice carry BCL2 rearranged cells at a low frequency, much as in the human condition, and a single round of immunization using sheep red blood cells did not substantially alter this. However, in the presence of long-term antigenic stimulation by repeated challenges over nine months, there was a marked accumulation of BCL2 rearranged cells in the memory and germinal center compartments, which expressed IgM on their surface. This finding was reproduced following adoptive transfer of transgenic B cells from chronically stimulated animals into wild-type mice, where they accumulated in germinal centers with features reminiscent of FLIS. This points to the re-circulation of memory B cells, with the BCL2 overexpressing clones having a survival advantage over normal B cells. The BCL2 rearranged cells were analyzed by exome sequencing, which showed a high frequency of mutations (G > A and C > T transitions) characteristic of activation-induced cytidine deaminase (AID) activity, a feature of germinal center processing.Parallel studies in human subjects were carried out by testing healthy organ donors for the presence of t(14;18) cells in the blood, bone marrow, and spleen. They found that most of the t(14;18) cells resided in the lymphatic tissues or bone marrow, that they co-expressed BCL2 and BCL6, and that the translocated alleles showed somatic hypermutation and class switch recombination, all in keeping with an arrested germinal center–like phenotype. Analysis of the level of clonal divergence in these cells by fluctuation long-range polymerase chain reaction (to cover both the switch region and the BCL2/Jh junction) revealed an extraordinarily high level of intraclonal variation, with multiple subclones detected in different tissues by comparing somatic hypermutation and class switch events. This level of variation is not seen in normal memory B cells, reinforcing the idea that t(14;18) cells enjoy a survival advantage that leads to their repeated passage through germinal center reactions, accumulating mutations that predispose them to malignant transformation.
This study fits with other work1 examining the effect of repeated immune stimulation on B-cell dynamics, which has shown that IgM+ memory cells can re-enter germinal centers, possibly as a means of evolving and updating the repertoire of responses. This normal process may be subject to a takeover by long-lived and maturation-arrested t(14;18)–bearing cells, allowing the accumulation of highly mutated cells with a high risk of further transformative events. This would suggest that recurrent or chronic immune stimulation could provide an important predisposing factor for FL. It also highlights the role of AID activity in driving intraclonal variation, and another recent article2 highlighted the poor prognostic implications of mutations within the BCL2 coding region.
Dr. Peter Johnson indicated no relevant conflicts of interest.