B-Cell Receptor Selection Pattern and Natural History Of Follicular Lymphoma

Follicular lymphoma (FL) is initiated by the t(14:18) that places the anti-apoptotic proto-oncogene BCL2 under transcriptional control of the immunoglobulin heavy chain (IgH) locus during primary VDJ recombination. Neoplastic FL B cells are arrested at the germinal center stage with ongoing somatic hypermutation of B-cell receptor (BCR) genes by activation-induced deaminase (AID). Antigen recognition by the clonal BCR may facilitate malignant growth upon entry of t(14;18)-immortalized naïve B cells into secondary lymphoid organs. We have reported at ASH 2012 that FL may be classified according to the AID-mediated BCR evolution pattern into two distinct subgroups: A predominantly IgM-expressing subgroup with evidence for antigen-driven affinity maturation, and a subgroup selected for BCR sequence preservation that has often undergone class switch recombination.

To investigate whether these BCR selection patterns were correlated to clinical outcome, we retrospectively analyzed somatic hypermutation of multiple BCR sequences in 66 patients (median age: 49; range: 29-75 years) with advanced-stage FL that had undergone a research tumor rebiopsy after inclusion into our prospective idiotype vaccination program (Blood, 2011) with informed consent. Biopsies were performed at a median interval of 20.1 months from diagnosis (range: 0-171 months). At rebiopsy, 43 patients had not yet received systemic cytoreductive therapy (chemotherapy, antibody therapy, or radiation therapy); 14 and 9 patients had been treated with one or two prior regimens, respectively.

IgH transcripts were cloned with an unbiased anchored PCR strategy with nested constant region-specific primers. A median of 8 clonal IgH sequences was subjected to bioinformatic analysis for BCR selection patterns by the focused test (Hershberg et al., 2008). 22 patients (33.3%) were classified as having evidence (focused test: p>0) for positive BCR selection through antigen-driven affinity maturation; 44 patients (66.6%) belonged to the BCR preservation category defined by p<0. Clinical risk according to the FLIPI was evenly distributed among both groups (BCR selection: 5 cases low FLIPI, 15 intermediate, 2 high; BCR preservation: 15, 22, 7). The BCR category distribution of app. 1:2 did not change with increasing time from diagnosis to biopsy or after prior therapy.

Among 39 patients who were managed initially with a “watch and wait” policy (36 of these underwent rebiopsy prior to initiation of therapy), BCR selection was associated with superior progression-free survival with a median of 87.7 months versus 16.9 months in the BCR preservation group (log-rank test: p=0.024). The treatment-free interval during which the rebiopsy was taken was longer in the BCR selection category for all patients (median 102.0 versus 31.0 months; p=0.030) and for patients with initial “watch and wait” policy (median 103.4 versus 21.0 months; p=0.049). Transformations to aggressive lymphoma occurred exclusively in the BCR preservation group (n=4). At a median total follow-up from diagnosis of 101.6 months, 8 patients (36,4%) in the BCR selection group and 7 patients (15,9%) of the BCR preservation group have not yet received any cytoreductive therapy. When death or transformation were defined as competing events, median event-free survival was not reached in either group with 2 events in the BCR selection group and 8 events in the BCR preservation group (p=0.10).

The BCR selection pattern as defined by the focused test through analysis of multiple BCR sequences may represent a novel prognostic factor for the natural history of FL in the pre-treatment phase. A focused test-based categorization could be readily incorporated into the diagnostic work-up of FL and could potentially complement clinical prognostication by means of the FLIPI score. Since the retrospective design of this study cannot entirely exclude possible selection bias, the hypothesis that antigen-driven BCR affinity maturation defines a favorable FL subgroup calls for validation in a prospective study. The relationship of the BCR selection pattern with the established prognostic role of the tumor microenvironment warrants investigation. In summary, we propose that BCR selection may govern the natural history of follicular lymphoma in treatment-free periods, thereby lending further support to the concept that BCR signaling plays an important causal role in FL lymphomagenesis.