P-ERK Total ERK Mel888 CHL1 WM BRAFi (nmol/L) Supplemental Figure 2: Other surface molecules unaffected ns PD-L1 ns MCSP DMSO BRAFi Supplemental Figure 1: Validation of MAPK pathway inhibition ns HLA-DR * HLA A,B,C MFI

Total ERK P-ERK WT BRAFi  Tail (nmol/L) Y320A Supplemental Figure 3. Phospho-ERK expression levels following inhibition treatment Transduced Mel888 cells S335A BRAFi (nmol/L) P-ERK Total ERK

*** ns HLA-A2 surface expression (MFI) Supplemental Figure 4: HLA-A2 surface expression following treatment with MEKi HLA-A2 variants  Tail WT S335A  Y320A DMSO MEKi DMSO MEKi WM793

Supplemental Figure 6. Intracellular IFN-  (  Tail and Y320A) IFN-  positive T cells (% of CD8) HLA-A2 (  Tail) HLA-A2 (Y320A) Mel888 WM793 Supplemental Figure 5. MART-1 TIL recognition of peptide-pulsed T2 cells ns

MART-1  -actin Supplemental Figure 7. MART-1 protein expression levels following treatment with BRAFi over 48hrs Mel888 Hrs. post-BRAFi treatment Supplemental Figure 8. Mel S pulse-chase time course and MHC-I IP BRAFi DMSO MEKi DMSO Hrs. post 35 S-Met labeling Mel MART-1 Relative Density