Diffuse Large B-Cell Lymphoma Lacking Surface B-Cell Receptor Expression Are Germinal Center B-Cell (GCB)-Type and Lack Evidence of Known Downstream Activating Mutations

Background: Diffuse large B-cell lymphoma (DLBCL) typically express monotypic surface immunoglobulin (sIg) light chain as part of the B-cell receptor (BCR) complex. Occasional cases are, however, sIg-negative and the clinical significance of this remains unclear. Distinct subtypes of DLBCL, including germinal center B-cell-like (GCB) and activated B-cell (ABC) type, exhibit differential dependence upon BCR-mediated survival signals. The role of activating mutations downstream of the BCR signaling complex (i.e. CD79A/B or CARD11), or in parallel costimulatory pathways (i.e. MYD88 mutations), are now well recognized in promoting such survival mechanisms. Surprisingly, however, less is known about the importance of sIg expression, or lack thereof, on various subsets of DLBCL or other aggressive large B cell lymphomas.

Design: We sought to further characterize the clinicopathologic and genetic features of primary human surface Ig-negative DLBCL-not otherwise specified (NOS). All cases of large B-cell lymphoma that had flow cytometry performed at the University of Rochester from 2010-2013 were screened for evidence of a surface Ig-negative B-cell population. Additional available immunophenotypic data and/or clinicopathologic features were recorded. PCR heteroduplex analysis and sequencing of fresh frozen tumor samples was performed to identify putative compensatory activating mutations in MYD88 and CD79A/B, respectively. In addition, using multiplexed PCR and sequencing analysis we attempted to identify the presence or absence of functional immunoglobulin heavy chain (IGH) gene rearrangements.

Results: Among 209 diagnostic specimens analyzed, 13 cases (6.2%) were found to contain a discrete population of CD19/CD20-positive B-cells lacking sIg light chain expression. These included 8 DLBCL-NOS, nearly all of which were GCB-type, including 6 out of 7 cases, or 83%, which were CD10-positive. This proportion is significantly greater than the 42% of all DLBCL-NOS that express CD10 (based on data from a large publicly available on-line database; p < 0.025, based on binomial probability). Furthermore, of the other high grade sIg-negative B cell lymphomas identified, 4 out of 5 were also CD10-positive. These included 2 cases of B-cell lymphoma, unclassifiable (BCL-U), intermediate between DLBCL and Burkitt lymphoma, and 2 cases of transformed follicular lymphoma (FL) - therefore also histologically and immunophenotypically consistent with derivation from a germinal center B-cell. The only exception was a non-GCB primary CNS lymphoma. Among 5 cases of sIg- DLBCL available for analysis, all were negative for the MYD88 L265P activating mutation, while only a single case contained an activating mutation in CD79A/B (CD79B Y196 ITAM mutation). In the same set, only 2 (of 5) cases produced amplicons by IGH PCR, and sequencing revealed only one of these had an intact reading frame.

Conclusion: DLBCL-NOS lacking definitive sIg light chain expression are infrequent (less than 10%), but these tumors are predominantly GCB subtype. This observation lends support to the model that these tumors rely less on BCR-mediated survival signals, which may have implications for choosing targeted therapy. The mechanism by which these lymphoma survive despite the apparent lack of functional BCR is unknown, but compensatory activating mutations known to be frequent among ABC-type DLBCL appear to be lacking.