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Toll-like receptor (TLR) agonism in combination with the activation of type I NKT (iNKT) cells through systemic administration of their respective agonists has been shown to have a cooperative effect on activating antigen-presenting cells, stimulating cytokine production, and inducing adaptive immune responses to co-administered antigens. Here, it was hypothesised that it might be possible to harness these activities to treat solid tumours locally via intratumoural treatment to combat tumour growth while reducing toxicity to other organs.  An intratumoural treatment model combining the stimulatory activity of unmethylated DNA oligonucleotides consisting of synthetic cytosine-guanine motifs (CpG), a TLR9 agonist, with activation of iNKT cells through administration of the CD1d-binding iNKT agonist α-galactosylceramide (α-GalCer) intratumourally was shown to have significant anti-tumour activity. The treatment regimen showed superior efficacy to that achieved with either agent alone in several in vivo models representing different types of cancer. In some models, the combination of α-GalCer and CpG was effective at inducing the complete rejection of both treated and untreated tumours through the induction of a systemic adaptive immune response. Post tumour rejection, a memory response protected against rechallenge with the same, or similar, tumours. Intratumoural administration of the agents was associated with increases in IFN-α in the tumour (rather than the serum), and blockade or removal of the IFN-α receptor abrogated the anti-tumour response.  The importance of the draining lymph node and spleen in anti-tumour activity (as shown by the excision of these organs), and liver enzyme responses, suggested that some of the agonists/antigens may have dispersed into the lymphoid organs and liver to support the response. Nonetheless, the anti-tumour effect was dependent on local effects of the intratumoural administration on the tumour microenvironment, as subcutaneous and peritumoural routes of administration only minimally affected tumour growth despite the reagents potentially having greater exposure to lymphoid organs.  Through the use of various techniques including knockout mice, neutralising monoclonal antibodies, confocal microscopy and flow cytometry, it was shown that the combination of α-GalCer and CpG was dependent on the effector activity of CD8+ cells. However, optimal activity was associated with changes in other immune cell types, notably recruitment of iNKT cells into the tumour bed, and was also associated with induction of serum antibodies that could transfer some protection to naïve hosts. Induction of a successful response was dependent on conventional dendritic cells (DCs) of the “cDC1” phenotype, which are known to be effective at antigen cross-presentation to CD8+ T cells, while full tumour rejection also required the activity of plasmacytoid DCs, which are significant producers of IFN-α. In less immunogenic tumour models, the addition of relevant tumour associated antigens (TAAs) improved the anti-tumour response. The TAAs could be added as part of an admix, but improved responses were obtained when TAAs were chemically conjugated to α-GalCer via an enzymatically cleavable linker. Alternatively, intratumoural administration of α-GalCer and CpG as free agents could be combined effectively with low dose systemic chemotherapy to induce curative responses, potentially through a mechanism involving immunogenic cell death to improve the immunogenicity of TAAs in situ.

Harnessing iNKT cells to improve in situ vaccination for cancer therapy