Distinct Immunogenetic Signatures in IgA Versus IgG Multiple Myeloma

Immunogenetic analysis of MM has proven instrumental in elucidating disease ontogeny e.g. by revealing the clonal relationship between switch variants expressed by the bone marrow plasma cells and myeloma progenitors in the marrow and blood; demonstrating the marked under-representation of the inherently autoreactive IGHV4-34 gene; and, identifying patterns of somatic hypermutation (SHM) indicative of post-germinal center derivation. Yet, limited information exists about the composition of the immunoglobulin (IG) gene repertoire in MM cases expressing different heavy chain isotype, in particular A versus G. This is relevant in light of studies showing an overall higher SHM impact in CD27⁺IgA⁺ compared to CD27⁺IgG⁺ normal memory B cells, perhaps reflecting a distinct location of the immune response, especially considering that IgA class switching mostly occurs in mucosa-associated lymphoid tissues. From a clinical perspective, it is also relevant to note that IgA patients exhibit a higher incidence of the t(4;14) translocation, shorter progression-free survival and worse median overall survival compared to IgG patients. Here, we explored potential differences in the immunoprofiles of IgA versus IgG MM focusing on IG gene repertoire and SHM characteristics. In total, 428 patients with a diagnosis of MM following the IMWG criteria from collaborating institutions in Greece, Italy and Spain (n=355) or retrieved from the LIGM-DB (n=73) were included in the study. Of these, 135 and 293 belonged to IgA and IgG MM groups, respectively. Amongst the evaluated productive IG rearrangements, IGHV3 subgroup genes predominated in both groups (IgA: 58.5%; IgG: 52.2%). However, at the individual gene level, major asymmetries were noted, since only 7 IGHV genes accounted for 41.6% of the IgA and 46.7% of the IgG cases, respectively. Of these, 3 genes were shared between IgA and IgG MM cases: IGHV3-30 (IgA: 11.9% - IgG: 13.3%), IGHV3-23 (IgA: 5.2% - IgG: 6.8%) and IGHV3-9 (IgA: 6.7% - IgG: 4.4%), whereas the remaining 4 of the 7 most frequent genes were specific for each group with significant (p<0.05) differences regarding the IGHV3-7 (5.2% in IgA versus 1.7% in IgG) and IGHV3-21 gene (0.7% in IgA 4.1% in IgG). IGHD3 predominated in both groups (IgA: 37% - IgG: 39.6%) followed by IGHD2 in IgG MM (18.4%) and IGHD6 in IgA MM (20.7%). IGHJ4 and IGHJ6 were the most frequent IGHJ genes with no significant differences in relative frequency. Searching for restricted IGHV-IGHJ combinations, we noted that the IGHV3-9 gene preferentially paired with the IGHJ6 gene in IgA MM versus the IGHJ4 gene in IgG MM (66.7% and 46.2% of all IGHV3-9 rearrangements, respectively). The median complementarity-determining region 3 (CDR3) length was identical in both IgA and IgG MM (15 amino acids, aa), yet differences were identified for specific CDR3 lengths as in the case of 19 aa, concerning 10.4% of all IgA versus 4.8% of all IgG rearrangements (p<0.05). Turning to SHM, the vast majority of rearrangements (IgA: 90.4%, IgG: 85%) were heavily mutated (IGHV germline identity (GI) <95%) with median GI of 91.8% for IgA and 92.2% for IgG. To study the topology of SHM, we compared the ratios of replacement (R) to silent (S) mutations in the framework (FR) and complementarity determining regions (CDRs) in cases expressing common frequent IGHV genes, namely IGHV3-23, IGHV3-30 and IGHV3-9 and identified distinct SHM patterns in all 3 instances: (i) IGHV3-23: the highest R/S ratios in IgA versus IgG MM were observed in FR2 (3.88) and CDR1 (3.9), respectively; (ii) IGHV3-30: overall "normal" SHM topology with higher R/S in CDRs rather than FRs, however, compared to IgG, IgA cases also showed a very high R/S in FR2 (5.3 versus 1.4); and, (iii) IGHV3-9: significantly (p<0.05) higher R/S ratios in CDR1 and CDR2 in IgG versus IgA cases (10.2 and 8.3 versus 1 and 2.9, respectively). Overall, in-depth immunogenetic analysis in the largest to-date series of IgA MM and IgG MM patients reveals differences regarding IGH gene repertoires, CDR3 characteristics and the topology of SHM. These findings suggest distinct antigen exposure histories and/or affinity maturation processes for IgA versus IgG MM, further highlighting the importance of microenvironmental stimuli in disease pathogenesis.