TLR7 Ligand Potentiates GM-CSF-Initiated Antitumor Immunity Through Activation Of Plasmacytoid Dendritic Cells

Vaccination with irradiated granulocyte macrophage-colony stimulating factor (GM-CSF)-transduced autologous tumor cells (GVAX) has been shown to induce therapeutic antitumor immunity presumably through the activated maturation of myeloid dendritic cells (DCs). However, its effectiveness is limited, and little is known about the biological properties related to GM-CSF-sensitized DCs (GM-DCs) in tumor-draining lymph nodes (TDLNs). We therefore attempted to enhance the antitumor effect of GVAX therapy by identifying the key pathways in GM-DCs in TDLNs. We initially confirmed that syngeneic mice subcutaneously (s.c.) injected with poorly immunogenic Lewis lung carcinoma (LLC) cells transduced with Sendai virus encoding GM-CSF (LLC/SeV/GM) significantly rejected the tumor growth. Using cDNA microarrays, we obtained a large number of gene expression data from CD86⁺ GM-DCs and control DCs in TDLNs, and found that the expressions of type I interferons (IFNs)-related genes, including IRF7 and TLR7, known to be abundantly expressed in plasmacytoid DCs (pDCs), were upregulated in GM-DCs. Indeed, to further activate pDCs, mice s.c. challenged with LLC/SeV/GM cells in combination with a TLR7 ligand, imiquimod, but not a TLR4 ligand, LPS, significantly suppressed the tumor growth compared with mice treated with LLC/SeV/GM cells alone. In contrast, pDCs-depleted mice challenged with LLC/SeV/GM cells facilitated the tumor growth, strongly suggesting that pDCs are essential immune subpopulation in exerting GM-CSF-initiated antitumor effects.

Furthermore, the additional use of imiquimod overcame the refractoriness of therapeutic vaccines with irradiated LLC/SeV/GM cells in mice with pre-established LLC tumors. Moreover, similar improvement of GVAX therapy was also observed in a mouse model of CT26 colorectal cancer. Mechanistically, mice treated with the combined vaccination displayed increased cell ratio of PDCA-1⁺ pDCs and expression levels of CD86, CD9, which correlate with an antitumor phonetype, and Siglec-H, which promote CD8⁺ T cell proliferation, in TDLNs. Indeed, allogeneic MLR test showed that bone marrow-derived pDCs matured by in vitro culture with GM-CSF plus imiquimod elicited a superior capacity of CD8⁺ T cell proliferation compared with those with GM-CSF only. On the other hand, the ratio of CD4⁺CD25⁺FoxP3⁺ regulatory T cells (Tregs) was decreased in TDLNs mice treated with the combined vaccination.

These findings collectively elucidated that pDCs play positive roles in GM-CSF-initiated antitumor immunity and that further activation of pDC by imiquimod targeting TLR7-IRF7 dependent type I IFNs pathway enhance the antitumor effects of GM-CSF-based tumor vaccination.