BackgroundMyeloid cells represent an abundant yet heterogeneous cell population in the colorectal cancer microenvironment, and their roles remain poorly understood.MethodsWe used multiplexed immunofluorescence combined with digital image analysis to identify CD14+monocytic and CD15+granulocytic cells and to evaluate their maturity (HLA-DR and CD33), immunosuppressive potential (ARG1) and proximity to cytokeratin (KRT)-positive tumor cells in 913 colorectal carcinomas. Using covariate data of 4465 incident colorectal cancers in two prospective cohort studies, the inverse probability weighting method was used with multivariable-adjusted Cox proportional hazards models to assess cancer-specific mortality according to ordinal quartiles (Q1–Q4) of myeloid cell densities. Immune cell–tumor cell proximity was measured with the nearest neighbor method and the G-cross function, which determines the likelihood of any tumor cell having at least one immune cell of the specified type within a certain radius.ResultsHigher intraepithelial (Ptrend=0.0002; HR for Q4 (vs Q1), 0.48, 95% CI 0.31 to 0.76) and stromal (Ptrend<0.0001; HR for Q4 (vs Q1), 0.42, 95% CI 0.29 to 0.63) densities of CD14+HLA-DR+cells were associated with lower colorectal cancer-specific mortality while, conversely, higher intraepithelial densities of CD14+HLA-DR−cells were associated with higher colorectal cancer-specific mortality (Ptrend=0.0003; HR for Q4 (vs Q1), 1.78, 95% CI 1.25 to 2.55). Spatial analyses indicated that CD15+cells were located closer to tumor cells than CD14+cells, and CD14+HLA-DR+cells were closer to tumor than CD14+HLA-DR−cells (p<0.0001). The G-cross proximity measurement, evaluating the difference in the likelihood of any tumor cell being colocated with at least one CD14+HLA-DR+cell versus CD14+HLA-DR−cell within a 20 µm radius, was associated with lower colorectal cancer-specific mortality (Ptrend<0.0001; HR for Q4 (vs Q1), 0.37, 95% CI 0.24 to 0.57).ConclusionsMyeloid cell populations occur in spatially distinct distributions and exhibit divergent, subset-specific prognostic significance in colorectal cancer, with mature CD14+HLA-DR+and immature CD14+HLA-DR−monocytic phenotypes most notably showing opposite associations. These results highlight the prognostic utility of multimarker evaluation of myeloid cell infiltrates and reveal a previously unrecognized degree of spatial organization for myeloid cells in the immune microenvironment.