CD20 and CD19 promote proliferation driven by the IgM-TLR9-L265P MyD88 complex

The cancer driver mutation L265P MyD88 is found in approximately 30% of cases in the activated B cell-like subgroup of diffuse large B cell-like lymphoma (ABC DLBCL). L265P MyD88 forms a complex with TLR9 and IgM, referred to as the My-T-BCR complex, to drive proliferation. We here show that the B cell surface molecules CD19 and CD20 enhance proliferation mediated by the My-T-BCR complex. Using the interleukin 3 (IL-3)-dependent Ba/F3 line transduced to express the IgM complex (IgM, CD79a, and CD79b) and TLR9, we observed proliferation in the presence of anti-IgM antibody and the TLR9 ligand CpG-B. TLR9 was constitutively associated with IgM and L252P MyD88. CD19 promoted proliferation with anti-IgM and CpG-B specifically in L252P MyD88-expressing Ba/F3 cells, while CD20 enhanced the proliferation in both wild-type- and L252P MyD88-expressing Ba/F3 cells. Additionally, CD20 uniquely enabled IgM-mediated proliferation in L252P MyD88-expressing Ba/F3 cells. Although CpG-B was not required for this proliferation, TLR9 expression remained indispensable. In the ABC DLBCL line TMD8, anti-IgM antibody-mediated growth was impaired by the lack of CD20 and CD19 or of TLR9. Mechanistically, CD19 promoted IgM-dependent AKT phosphorylation, whereas CD20 increased expression of cell surface IgM, thereby enhancing the formation of the IgM-TLR9 complex. These findings suggest that CD19 and CD20 differentially contribute to the proliferation driven by the My-T-BCR complex.