IgM memory and Waldenström macroglobulinemia’s cell of origin

Waldenström macroglobulinemia (WM) is a neoplasm of mature IgM-expressing B-lymphocytes that is characterized by the occurrence of a monoclonal IgM (mIgM) paraprotein in blood serum and the infiltration of hematopoietic bone marrow with malignant lymphoplasmacytic cells. WM remains incurable despite the development of new therapeutic options. Owing in large measure to having a low incidence, indolent clinical course and good long-term control with proper clinical management, WM has not been investigated as extensively as other B-lineage neoplasms. Major knowledge gaps in our understanding of the natural history of WM include the cell of origin. With that shortcoming in mind, here we discuss the significance of a specific gain-of-function mutation in the adapter protein, myeloid differentiation primary response 88 (MYD88), that occurs with near-complete penetrance in WM and suggests that tumor development is under strong selective pressure for elevated MYD88 signaling. This provides an intriguing link to IgM memory B-cells, which comprise two types of B-lymphocytes ( natural effector IgM⁺IgD⁺ cells and IgM^-only IgM⁺IgD^- cells ) that depend, in part, on MYD88 signaling and constitute intriguing candidates for WM’s cell of origin. We review the features and developmental history of IgM memory in greater depth and propose that WM may be derived from primitive innate-like B-cells ( marginal zone B-cells and B1 B-cells ) that feed the IgM memory compartment. We conclude with a model of MYD88-dependent tumor development in the mature B-cell lineage that considers two different ( convergent or divergent) oncogenesis pathways with respect to the cells of origin.