Fig. 4: Increase in mast cells after anti-PD-1 therapy.

a CIBERSORT analysis of the RNA-seq data set (GSE161353) showed a higher abundance of mast related genes in tumors (WM3629 and A375) obtained from Hu-mice after anti-PD-1 treatment and increased expression of CXCL10, a chemoattractant for mast cells (b). The presence of mast cells was further confirmed by mast cell tryptase IHC staining. c, d Stitched image (top left panels) and three different fields from an untreated tumor (WM3629) had negligible staining for mast cells (top right panel and bottom two panels; scale bar: 100 μm), whereas stitched image (top right middle panel) and three different fields from anti-PD-1 treated tumor had robust staining for mast cells, which was highly significant compared to control-treated mice (p = 0.005407; top right panel and bottom 2 panels; scale bar: 100 μm). e Co-localization of FOXP3⁺ Treg and mast cells after anti-PD-1 therapy. Co-localization of these cells as determined by IHC staining was significant (p = 0.000047; scale bar: 200 μm) suggesting crosstalk. f Downmodulation of HLA class I. HLA class I molecules as determined by staining with anti-HLA class I antibody (red) were downmodulated in tumor (WM3629) areas (black arrows) that were infiltrated by mast cells (blue). Scale bar represents 50 μm. g Mast cells co-express CXCR3. Mast cells were co-stained with anti-MCT (blue) and anti-human CXCR3 (red; white arrows) antibodies (see Supplementary Fig. 11 for digital quantification). Scale bar represents 50 μm. h Melanoma (WM3629) cells co-express CXCL10. Tumor cells were co-stained with anti-melanoma (HMB45 [blue]) and anti-human CXCL10 (red; white arrows) antibodies. Scale bar represents 100 μm. Histology staining (c–h) was confirmed in repeat experiments (2×). Source data are provided as a Source Data file (data are included as part of Supplementary Fig. 10).