Stereotyped B Cell Receptors in Chronic Lymphocytic Leukaemia Implications for Antigen Selection in Leukemogenesis

Sammanfattning: Biased immunoglobulin heavy variable (IGHV) gene usage and distinctive B-cell receptor (BCR) features have been reported in chronic lymphocytic leukaemia (CLL), which may reflect clonal selection by antigens during disease development. Furthermore, the IGHV gene mutation status distinguishes two clinical entities of CLL, where patients with unmutated IGHV genes have an inferior prognosis compared to those with mutated IGHV genes. Recently, one subgroup of CLL patients expressing the IGHV3-21 gene was found to display highly similar immunoglobulin (IG) gene features, even within the heavy chain complementarity-determining region 3 (HCDR3). Patients in this subgroup typically had a poor prognosis.In paper I, we aimed to identify further subgroups with restricted BCR features among 346 CLL cases. Six subsets were defined which carried virtually identical BCRs in terms of rearranged heavy and light chain (LC) IG genes and CDR3 length and composition. In paper II, we investigated 90 IGHV3-21 cases from diverse geographical locations. We confirmed the highly restricted HCDR3 characteristics in 56% of patients and a biased usage of the IGLV3-21 gene in 72% of cases. Survival analysis also confirmed the poor outcome of this group, irrespective of IGHV gene mutation status and geographical origin.Papers III and IV involved a large-scale analysis of IGH and IG kappa and lambda (IGK/L) gene rearrangements, to define subsets with ‘stereotyped’ BCRs and also to systematically examine the somatic hypermutation (SHM) features of the IG genes in CLL. We studied a cohort of 1967 IGH and 891 IGK/L gene sequences from 1939 patients from 6 European institutions. Over 5300 IGH and ~4700 IGK/L sequences from non-CLL B cells were used as a control data set. In total, 110 CLL stereotyped subsets were defined according to HCDR3 homology. Striking IGK/L gene biases were also evident within subsets, along with distinctive K/LCDR3 features, such as length and amino acid composition. At cohort level, the patterns of mutation appeared to be consistent with that of a canonical SHM mechanism. However, at a subgroup level, certain stereotyped subsets, e.g. IGHV3-21/IGLV3-21 and IGHV4-34/IGKV2-30 CLL, deviated from this pattern. Furthermore, recurrent ‘stereotyped’ mutations occurred in cases belonging to subsets with restricted HCDR3s, in both IGHV and IGK/LV genes, which were subset- and CLL-biased when compared to non-CLL B cells.In conclusion, our findings implicate antigen selection as a significant factor in the pathogenesis of CLL, particularly in cases carrying stereotyped BCRs. The presence of stereotyped mutations throughout the VH and VL domain also indicates involvement of IG regions other than the CDR3 in antigen recognition. Finally, biased IGK/L gene usage and specific K/LCDR3 features are strong indications that LCs are crucial in shaping the specificity of leukemic BCRs, in association with defined heavy chains.