TLR9 and osteosarcoma: In light of this finding and of several reports showing a higher therapeutic efficacy of TLR9 agonists when combined with immune checkpoint blocking antibodies [22, 26, 27], we tested whether this additive/synergistic effect occurs in our osteosarcoma models but we did not observe any difference in the therapeutic efficacy of SD101 administered alone or in combination with systemic anti PD-1 antibody, the latter also ineffective as monotherapy [16].