Inhibition of Type 4 Cyclic Nucleotide Phosphodiesterase Blocks Intracellular TLR Signaling in Chronic Lymphocytic Leukemia and Normal Hematopoietic Cells

A subset of chronic lymphocytic leukemia (CLL) BCRs interacts with Ags expressed on apoptotic cells, suggesting that CLL BCRs have the potential to internalize apoptotic cell RNA- or DNA-containing fragments with resultant activation of TLR7 or TLR9, respectively. By blocking cAMP degradation, type 4 cAMP phosphodiesterase (PDE4) inhibitors activate cAMP-mediated signaling and induce apoptosis in CLL cells. In this study, we show that autologous irradiated leukemic cells induce proliferation in CLL cells and that such proliferation is blocked by a TLR7/8/9 inhibitor, by DNase, and by the PDE4 inhibitor rolipram. Rolipram also inhibited CLL cell proliferation induced by synthetic TLR7 and TLR9 agonists, as well as TLR agonist-induced costimulatory molecule expression and TNF-a (but not IL-6 or IL-10) production. Whereas treatment with a TLR9 agonist protected IgH V region unmutated, but not mutated, CLL cells from apoptosis, PDE4 inhibitors augmented apoptosis in both subtypes, suggesting that cAMP-mediated signaling may abrogate a TLR9-mediated survival signal in prognostically unfavorable IGHV unmutated CLL cells. Rolipram inhibited both TLR7/8- and TLR9-induced IFN regulatory factor 5 and NF-kB p65 nuclear translocation. PDE4 inhibitors also blocked TLR signaling in normal human immune cells. In PBMC and CD14-positive monocytes, PDE4 inhibitors blocked IFN-a or TNF-a (but not IL-6) production, respectively, following stimulation with synthetic TLR agonists or RNA-containing immune complexes. These results suggest that PDE4 inhibitors may be of clinical utility in CLL or autoimmune diseases that are driven by TLR-mediated signaling.