Inhibiting TLR9 and other UNC93B1-dependent TLRs paradoxically increases accumulation of MYD88^L265P plasmablasts in vivo

The MYD88^L265P mutation is found in 2% to 10% of chronic lymphocytic leukemia, 29% of activated B-cell type diffuse large B-cell lymphoma and 90% of Waldenström macroglobulinemia, making it conceptually attractive to treat these malignancies with inhibitors of endosomal Toll-like receptors (TLR9, TLR7) that activate MYD88. Here we show that genetic inhibition of endosomal TLRs has the opposite effect on accumulation of MYD88^L265P B cells in vitro and in vivo. Activated mature B cells from wild-type, Unc93b1^3d/3d-mutant, or Tlr9-deficient mice were transduced with retrovirus encoding MYD88^L265P and analyzed either in vitro or after transplantation into Rag1^-/- recipient mice. Unc93b1^3d/3d mutation, which blocks TLR9 and TLR7 signaling, or Tlr9 deficiency suppressed MYD88^L265P B-cell growth in vitro but paradoxically increased in vivo accumulation of MYD88^L265P B cells as CD19^low plasmablasts by 10- to 100-fold. These results reveal an unexpected, powerful inhibitory effect of TLR9 on MYD88^L265P B-cell proliferation and differentiation that appears independent of TLR7, and they providea preclinical indicator for caution in clinical trials of TLR7/9 inhibitors for MYD88^L265P B-cell malignancies.