Clinical Lymphoma, Myeloma & Leukemia, Vol.22, Suppl.2 - October 2022

S78 E XABS-176-C LL Cli n ical Sig n ifica n ce of B-Cell Receptor Stereotype i n C LL Kostas Sta m atopoulos, M D , Ph D 1, * 1 Institute of Applied Biosciences, Centre for Research and Technology Hellas, Thessaloniki, Greece *Correspo n di n g author: kostas.stamatopoulos@certh.gr Keyword CLL In the early 1990s, it was first reported that the leukemic B cells of patients with CLL express a restricted repertoire of IG heavy variable (IGHV) genes. 1,2 By the end of the decade, it had become obvious that the IG gene repertoire of CLL is non-random and, moreover, that a significant fraction of all CLL carried somatic hypermutation (SHM) within their clonotypic rearranged IGHV genes. Notably, the SHM status of the rearranged IGHV genes can directly predict CLL patient survival, as first documented independently in 1999 in seminal publications by the Stevenson and Chiorazzi groups. 3,4 In more detail, patients with BcR IG carrying mutated IGHV genes (‘IG-mutated’ CLL, M-CLL) generally follow a more indolent course than those with unmutated IGHV genes (‘IG-unmutated’ CLL, U-CLL), who tend to show evidence of advanced, progressive disease, adverse genetic features, clonal evolution, and less benefit from chemoimmunotherapy. 5 In 2003, a large fraction of CLL cases with B-cell receptor immunoglobulin (BcR IG) encoded by the IGHV3-21 gene were found to carry highly homologous if not identical amino acid sequences within the variable heavy complementarity determining region 3 (VH CDR3), the main determinant for antigen specificity; moreover, these cases exhibited highly restricted usage of the IGLV3-21 gene. 6 Soon thereafter, additional subgroups of cases with homologous BcR IG were identified in both M-CLL and U-CLL. 7 ,8 Such (quasi)identical BcR IG were termed ‘stereotyped’ i.e. repeated with limited or even no variation in different patients. Since the original publications, it has been established that a large fraction of patients can be assigned to subgroups with distinct, stereotyped VH CDR3 sequences. In a recent study of ~30,000 patients with CLL, 41% of all cases expressed stereotyped BcR IG, which is truly remarkable if one considers that the probability of this occurring randomly is in the range of 1:10 -16 to 1:10 -18 . 9 Restricted VH CDR3 motifs define distinct groups, termed stereotyped subsets, some of which some are quite well populated (‘major’) since the public motif is shared by many different cases. Given the existence of stereotyped subsets, the obvious question was whether the molecular subtyping of CLL based on BcR IG stereotype could have biological and or clinical implications. Put differently, could the similarities of cases belonging to the same subset extend from primary amino acid IG sequences to other aspects of their biology and cellular dynamics which could be reflected in a similar clinical behavior and outcome? On the available evidence, the answer is in the affirmative. Indicatively: (i) particular genomic aberrations are strikingly enriched in certain subsets; 10-13 (ii) different subsets can display distinct DNA methylation profiles, even if their SHM status is concordant; 14 (iii) cell signaling via innate immunity receptors (e.g., Toll-like receptors, TLRs) as well as the BcR can be markedly different between subsets - this holds true even for the novel mode of cell activation reported for CLL whereby CLL cells can display cell autonomous signaling. 7 Notably, certain subsets have also been shown to display consistent clinical characteristics: the prime examples are subset #2 and #8. 15-1 7 Subset #2 represents the largest stereotyped subset in CLL, accounting for ~2.5–3% of all patients and ~5.5% of patients requiring treatment. 9 The particular BcR IG of subset #2 is composed of heavy and light chains encoded by the IGHV3-21 and the IGLV3-21 genes, respectively. The clonotypic IGHV3-21 genes bear a variable SHM load, with most cases(~60%–65%) classified as M-CLL. 6,9,18-20 It was recently shown that stereotyped subset #169, a minor CLL subset (~0.2% of all CLL), bears striking immunogenetic similarities to subset #2. 18 Independent of the SHM status, subset #2 cases have a particularly dismal clinical outcome similar to that of patients with TP53 aberrations, although they very rarely harbor such aberrations, as well as a poor response to chemoimmunotherapy, despite mostly representing M-CLL. This was shown in a meta-analysis of 3 prospective clinical trials conducted by the German CLL Study Group (CLL8:fludarabine- cyclophosphamide-rituximab versus fludarabine-cyclophosphamide; CLL10: fludarabine-cyclophosphamide-rituximab versus bendamustine-rituximab; CLL11: chlorambucil versus chlorambucil-rituximab versus chlorambucil-obinutuzumab). 16 Membership of subset #2 was found to be an independent prognostic marker for shorter TTFT, time-to-next-treatment (TTNT), and PFS, irrespective of the SHM status. This suggests that CIT may be less optimal for subset #2 patients, while also highlighting the usefulness of this information for risk stratification of patients, a practice already followed by different study groups worldwide. In that regard, it is relevant to mention the results of the NCRI FLAIR trial, where a hazard ratio for disease progression and