Plasma cell differentiation in the B-cell malignancy Waldenström Macroglobulinemia

Waldenström macroglobulinemia (WM) is unique amongst B-cell malignancies in that the neoplastic cells retain the capacity to undergo plasma cell differentiation. This gives rise to a clonal neoplasm encompassing the spectrum of differentiation and as such presents a challenge to model and treat effectively. Therefore, the feasibility of using an in vitro culture system to model the differentiation of primary WM B-cells derived from patient samples was examined and their response to different stimuli characterised. Subsequent to T-dependent activation, WM B-cells successfully differentiate and generate plasma cells with an equivalent immunophenotype and lifespan to that of healthy B-cells, and do so more efficiently than B-cells derived from patients with splenic marginal zone lymphoma, which shares a related aetiology to WM. Unlike healthy cells or those derived from other B-cell neoplasms, however, WM B-cells generate a novel population of plasma cells that do not express CD38 and their appearance is strongly linked to the characteristic MYD88L265P mutation. The virtually ubiquitous presence of MYD88L265P, coupled with reports suggesting that TLR signalling is required for the survival of MYD88-mutated cells prompted investigation into the response of WM cells to stimuli mimicking a T-independent immune response. Whilst WM B-cells remain receptive to TLR stimulation, they fail to differentiate in response to a combination of TLR7 agonism and BCR ligation and demonstrate a profound apoptotic response that is in stark contrast to their healthy counterparts. This unexpected response is not as a result of a secreted factor acting in trans or due to the upregulation of Fas or its ligand within the WM population. Indeed, RNA sequencing identifies substantial disruption of multiple essential pathways within these cells subsequent to TLR stimulation, but no significant differences in the expression of either pro-apoptotic or survival genes. The data presented here suggest that WM cells rely on additional support from the bone marrow microenvironment for their survival and that ligation of TLR7 and the BCR without an additional signal such as CD40L are insufficient to sustain the WM population.