206P Characterisation of tumour-infiltrating lymphocytes (TILs) in liver metastases (LM) and primary tumour (PT) of microsatellite stable (MSS) colorectal cancers

Background: The immunosuppressive tumor microenvironment (TME) appears to be the main challenge against cancer patients receiving benefits from PD-1/PD-L1targeting immune checkpoint inhibitors (ICIs), and IL-33/ST2 signaling pathway fulfills critical roles in the TME.However, it remains uncertain whether IL-33 limits the therapeutic efficacy of anti-PD-L1 treatment.Methods: Cellular and molecular mechanisms of the IL-33/ST2 on anti-PD-L1 treatment in lung cancer were assessed using RNA-seq, scRNA-seq, IB, and IF.A sST2-Fc fusion protein was constructed to target IL-33 and combined with an anti-PD-L1 antibody atezolizumab in lung tumor models.Based on this, bifunctional fusion proteins were generated to block IL-33 in tumors specifically targeting PD-L1.The underlying mechanisms of dual targeting of IL-33 and PD-L1 were revealed using RNAseq, scRNA-seq, FACS, and IF.Results: After anti-PD-L1 administration, tumor-infiltrating ST2 + regulatory T cells (Tregs) were elevated.Blocking IL-33/ST2 signaling with sST2-Fc fusion protein potentiated the antitumor efficacy of the PD-L1 antibody by boosting CD8 + T cell responses.Bifunctional fusion protein anti-PD-L1-sST2 demonstrated superior antitumor efficacy compared to combination therapy, not only inhibited tumor progression and extended survival, but also provided long-term protective antitumor immunity.Mechanistically, the superior antitumor activity of targeting both IL-33 and PD-L1 was due to a reduction in immunosuppressive factors, such as Tregs and exhausted CD8+ T cells, while increasing tumor-infiltrating cytotoxic T lymphocytes.Conclusions: This study demonstrated that IL-33/ST2 is involved in the immunosuppression mechanism of PD-L1 antibody therapy.Blockade by sST2-Fc or anti-PD-L1-sST2 could remodel the inflammatory TME and induce a potent antitumor effect.These findings highlight the potential therapeutic strategies for tumor treatment by simultaneously targeting IL-33 and PD-L1.