In vivo anti- and pro-tumour activities of the TLR2 ligand FSL-1

TLR ligands as Th1 inducers have been investigated as potential anti-tumour agents. However, few attempts have been made to investigate the anti-tumour activity of TLR ligands as Th2 inducers. This study, therefore, was carried out to determine whether the TLR2 ligand FSL-1 as a Th2 inducers affects the growth of a QRsP tumour, a fibrosarcoma derived from the C57BL/6 (TLR2^+/+) mouse in vivo. Tumour volumes in TLR2^+/+ mice immunized with both FSL-1 and tumour-associated antigens were significantly smaller than those in control mice. Immunization with both FSL-1 and tumour-associated antigens increased the survival rate of TLR2^+/+ mice. However, surprisingly, immunization with FSL-1 alone significantly enhanced the growth of tumour. Both anti- and pro-tumour activities of FSL-1 were not observed in TLR2^−/− mice. Immunization of both FSL-1 and tumour-associated antigens induced tumour-associated antigen-specific cytolytic T cells, antibody-dependent cell-mediated cytotoxicity of natural killer cells by production of the tumour-specific antibodies, tumour lysis by complement activation and reduction of the number of regulatory T cells in the draining lymph node. Immunization with FSL-1 alone increased the number of regulatory T cells in the draining lymph node, and in vivo administration of anti-CD25 antibody into mice abrogated the pro-tumour activity of FSL-1, suggesting that regulatory T cells are involved in the pro-tumour activity.This study demonstrated that FSL-1 exhibited TLR2-mediated anti- and pro-tumour activities when immunized with and without tumour-associated antigens, respectively.