Heterodimer-specific TLR2 stimulation results in divergent functional outcomes in B-cell precursor acute lymphoblastic leukemia

Abstract

Reports of spontaneous acute lymphoblastic leukemia (ALL) remissions following severe bacterial infections suggest that bacterial components may trigger elimination of ALL. To date, TLR2, which recognizes a broad range of bacterial pathogens through TLR1 or TLR6 heterodimerization, has not been fully evaluated for direct effects on ALL. Studies investigating TLR2 signaling in other tumor cell types utilizing single ligands have yielded contradictory results, and comparative, heterodimer-specific analyses of TLR2 stimulation are lacking. In this study, we report that two well-characterized heterodimer-specific TLR2 ligands, Pam3 CSK4 (TLR2/1), and Pam2 CSK4 (TLR2/6), induce ALL cell lines and primary ALL samples to upregulate CD40 expression. However, only Pam3 CSK4 triggers Caspase-8-mediated apoptosis and sensitizes cells to vincristine-mediated cytotoxicity. Consistent with this result, stimulation of ALL cells through TLR2/1 or TLR2/6 activates Mal, p38 and the NF-κB and PI3K signaling pathways with divergent kinetics that may underlie their distinct downstream effects. Our results reveal a novel branching in downstream responses to heterodimer-specific TLR2 stimulation in ALL cells and emphasize the need for comparative studies to determine differential biological effects observed in specific tumor cells. Based on our results, TLR2/1 ligand Pam3 CSK4 possesses potential for generating anti-ALL activity through its direct effects on leukemic blasts.

Keywords: CD40; Caspase-8-mediated apoptotic cell death; Heterodimer-specific TLR2 stimulation; Pediatric acute lymphoblastic leukemia; Signaling kinetics.