Our analysis revealed three interrelated mechanistic insights: (1) a prominent myeloid cell‐dominated immune barrier driven by rapid influx of blood‐derived macrophages (Fig. 1C-E), (2) capacity of the PD‐1 blockade to activate previously latent anti-tumor T cell responses and redirect PCNA toward an apoptosis regulatory role (Figs. 2-4), and (3) synergistic remodeling of the tumor microenvironment through a combination of focal irradiation and PD‐1 inhibition (Fig. 5). Here, PCNA is linked to neoplasm.