6). This implies that TAMs in colorectal cancer possess a greater capacity to present antigen and co-stimulate T cells than TAMs in other cancers. To assess the functional capacity of colorectal TAMs in co-stimulating T cells, we performed an MLR assay. TAMs were sorted from colorectal co-culture spheroids and incubated

with allogeneic T cells for 4 days, after which T-cell proliferation was measured by tritiated-thymidine R788 solubility dmso incorporation. Indeed, the TAMs were highly competent at stimulating T-cell proliferation (Fig. 4B). Tumour cells sorted from the co-cultures were unable to stimulate T-cell proliferation, indicating that tumour cells per se do not possess T-cell co-stimulatory properties, and in vitro differentiated macrophages were poor stimulators. Together, these observations indicated that TAMs acquired T-cell co-stimulation capabilities during the co-culture with colorectal tumour cells. Of the T cells that proliferated upon incubation

with TAMs, 71% expressed PI3K inhibitor CD25, an activation marker, and 62% produced IFN-γ, a type-1 inflammatory cytokine (Fig. 4C), indicating that TAMs were able to activate type-1 T cells. There was no activation of type-2, type-17 or regulatory-T cells, indicated by the lack of IL-4, IL-17A or FoxP3 (Fig. 4C and D). Together, these results illustrated that TAMs in the colorectal cancer model were capable of stimulating T-cell proliferation and promoting type-1 PIK3C2G T-cell responses. To confirm the in vitro findings on colorectal TAMs, we studied primary tumour tissues from five colorectal cancer patients (Table 1). Pro-inflammatory TAMs were detected in the colorectal tumour sections, as they stained positive for IFN-γ (Fig. 5A, white arrows). The percentage of TAMs that were IFN-γ+ in each tumour sample was quantified using the software TissueQuest, on five images (each ∼350×250 μm) randomly taken from each tumour tissue section. The images

were analysed together to give a representative plot for every tumour sample (Supporting Information Fig. 7). This approach takes into account variations from different parts of the tissue section. The percentage of macrophages that were IFN-γ+ in the tumour samples varied from 6.6 to 50% (Fig. 5B and Table 1). To confirm the in vitro findings that TAMs in colorectal cancers could attract T cells, we quantified the numbers of tumour-infiltrating T cells and TAMs. Indeed, the numbers of tumour-infiltrating T cells and TAMs were highly correlated (r2=0.66, Fig. 5C). Furthermore, the TAMs and T cells were often observed to be in close contact (Fig. 5D, black arrows), suggesting direct interaction of the two cell types, such as antigen presentation to and co-stimulation of T cells by TAMs.