Radiation therapy (RT) can induce upregulation of programmed death ligand 1 (PD-L1) on tumor cells or myeloid cells, which may affect response to PD-1-based immunotherapy. PD-L1 upregulation during RT is a dynamic process that has been difficult to monitor during treatment. The aim of this study was to evaluate the RT-induced PD-L1 upregulation in the tumor and its microenvironment using immunoPET/CT imaging of two syngeneic murine tumor models (HPV+ head and neck squamous cell carcinoma (HNSCC) or B16F10 melanoma). Tumors were established in two locations per mouse (neck and flank), and fractionated RT (2 Gy × 4 or 2 Gy × 10) was delivered only to the neck tumor, alone or during anti-PD-1 mAb immunotherapy. PD-L1 expression was measured by PET/CT imaging using Zr-89 labeled anti-mouse PD-L1 mAb, and results were validated by flow cytometry. PET/CT imaging demonstrated significantly increased tracer uptake in irradiated neck tumors compared with non-irradiated flank tumors. Ex vivo analysis by biodistribution and flow cytometry validated PD-L1 upregulation specifically in irradiated tumors. In the HNSCC model, RT-induced PD-L1 upregulation was only observed after 2 Gy × 10 fractionated RT, while in the B16F10 model upregulation of PD-L1 occurred after 2 Gy × 4 fractionated RT. Fractionated RT, but not anti-PD-1 therapy, upregulated PD-L1 expression on tumor and infiltrating inflammatory cells in murine models, which could be non-invasively monitored by immunoPET/CT imaging using Zr-89 labeled anti-mouse PD-L1 mAb, and differentially identified anti-PD-1 responsive as well as selectively irradiated tumors in vivo.

Background. Dermatofibroma (DF) is a superficial form of benign fibrous histiocytoma, composed of a mixture of fibroblastic cells and histiocytic cells. The histogenesis of this lesion is a matter of controversy. Methods. Forty‐five cases of DF were investigated by single and multiple immunostaining techniques, using panel of 12 antibodies including proliferating cell nuclear antigen (PCNA), vimentin, and macrophage/histiocyte markers (HAM56 and CD68). Results. Double immunostaining demonstrated that 58% of the DF cells simultaneously expressed PCNA and vimentin, whereas only 5% were PCNA⁺/HAM56⁺, and 2% were PCNA⁺/CD68⁺. By triple‐stain for PCNA, vimentin, and HAM56, 56% of the DF cells were PCNA⁺/ vimentin⁺/HAM56, but only a few cells were PCNA⁺/HAM56⁺/vimentin⁺. Of the PCNA positive cells, 88% were vimentin⁺ HAM56‐ and 10% were HAM56⁺ vimentin‐/⁺. The cells positive for CD68 showed similar distribution to that of HAM 56⁺ cells, though there were fewer of the former than the latter in most cases. These results suggest that the majority of the proliferating cells in DF express vimentin but not histiocytic markers. Morphologically, the PCNA⁺/vimentin⁺/HAM56‐ (or CD68') cells exhibited a spindle‐shaped configuration resembling fibroblasts, whereas most of the HAM56⁺/CD68⁺ cells possessed abundant rounded cytoplasm and were similar to normal histiocytes. Conclusions. The present study suggests that the proliferative compartment of DF cells is composed chiefly of mesenchymal/fibroblastic lineage, accompanied by varying numbers of normal reactive histiocytes.