Background: There is evidence that intratumoral myeloid cell infiltration can modulate response to immune checkpoint inhibitors in some tumor types, but its role in ccRCC remains unclear. Here, we investigated the role of tumor-infiltrating CD163-positive macrophages as a determinant of clinical outcome to anti-PD-1 therapy in patients (pts) with advanced ccRCC treated with first-line nivolumab therapy as part of the HCRN GU16-260 clinical trial. Methods: Primary tumor tissues from pts with ccRCC (n= 67) were analyzed by multiparametric immunofluorescence (IF). Image analysis algorithms were used to assess the density of CD163-positive cells (dCD163) in tumor areas with high CD8+ T cell-infiltrates. In addition, dCD163 was also assessed in randomly selected tumor areas. Log-transformed densities were correlated with objective response rate (ORR) and progression-free survival (PFS) using Cox proportional hazards and binary logistic (modeling odds of attaining objective response) regression analysis, respectively. Alpha level was set a priori at 5% (2-sided). The functional form of CD163 for the PFS endpoint was obtained using a smoothed martingale residual plot. Results: Analysis of dCD163, assessed in high-CD8+ cell tumor areas and measured as a continuous variable, showed a positive association with ORR (OR= 2.21, p= 0.002) and PFS (HR= 0.77, p= 0.028). However, visual inspection of the dCD163 functional form plot for PFS revealed a threshold effect at the raw (log-transformed) value of 262.5 (5.6) cell/mm². Similar to the entire cohort, in patients with dCD163 below this threshold (n= 54), dCD163 was positively associated with PFS (HR= 0.61, p<.001). In contrast, in patients with dCD163 above this threshold (n=13), results suggested the presence of a negative association between dCD163 and PFS (HR= 1.36, p= 0.726). Of note, similar findings were obtained by analyzing dCD163 assessed in random tumor areas. Conclusions: Levels of tumor-infiltrating CD163-positive macrophages are associated with improved clinical outcomes to first-line nivolumab in pts with advanced ccRCC up to a density threshold, above which an association with poor outcomes is noted. Further investigations into the role of tumor-infiltrating myeloid cells in predicting outcomes to anti-PD-1-based therapies are ongoing. Clinical trial information: NCT05403541.