25The immunosuppressive tumour microenvironment constitutes a significant hurdle to the 26 response to immune checkpoint inhibitors. Both soluble factors and specialised immune cells 27 such as regulatory T cells (TReg) are key components of active intratumoural 28 immunosuppression. Previous studies have shown that Inducible Co-Stimulatory receptor 29 (ICOS) is highly expressed in the tumour microenvironment, especially on TReg, suggesting that 30 it represents a relevant target for preferential depletion of these cells. Here, we used immune 31 profiling of samples from tumour bearing mice and cancer patients to characterise the 32 expression of ICOS in different tissues and solid tumours. By immunizing an Icos knockout 33 transgenic mouse line expressing antibodies with human variable domains, we selected a fully 34 human IgG1 antibody called KY1044 that binds ICOS from different species. Using KY1044, we 35 demonstrated that we can exploit the differential expression of ICOS on T cell subtypes to 36 modify the tumour microenvironment and thereby improve the anti-tumour immune 37 response. We showed that KY1044 induces sustained depletion of ICOS high TReg cells in mouse 38 tumours and depletion of ICOS high T cells in the blood of non-human primates, but was also 39 associated with secretion of pro-inflammatory cytokines from ICOS low TEFF cells. Altogether, 40 KY1044 improved the intratumoural TEFF:TReg ratio and increased activation of TEFF cells, 41 resulting in monotherapy efficacy or in synergistic combinatorial efficacy when administered 42 with the immune checkpoint blocker anti-PD-L1. In summary, our data demonstrate that 43 targeting ICOS with KY1044 can favourably alter the intratumoural immune contexture, 44 promoting an anti-tumour response. 45 46 48 targeting immune checkpoints (e.g. CTLA-4, PD-1 and PD-L1). These immune checkpoint 49 inhibitors (ICIs) are associated with strong and durable responses in patients suffering from 50 advanced malignancies, including but not limited to metastatic melanoma, non-small cell lung 51 cancer (NSCLC), head and neck cancer, renal and bladder cancer (1). However, within all these 52 indications, there are still a high proportion of patients who exhibit intrinsic or acquired 53 resistance to ICIs. These patients represent a population with high unmet medical needs who 54 may benefit from novel combinatory approaches with ICIs. 55 Multiple molecular and cellular mechanisms have been associated with the lack of response to 56 immunotherapies (2). Accumulating evidence has shown that a low incidence of cytotoxic T 57 cells and the presence of immunosuppressive cells represent major barriers to establishing a 58 response to ICIs. One such class of immunosuppressive cells are regulatory T cells (TReg), which 59 are characterised by the expression of the transcription factor FOXP3 and function by blocking 60 the activation and cytotoxic potential of effector T-cells (TEff) through multiple mechanisms (3, 61 4). In fact, numbers of intratumoural TReg cells neg...