BRAF and MEK Inhibitors Increase PD-1-Positive Melanoma Cells Leading to a Potential Lymphocyte-Independent Synergism with Anti–PD-1 Antibody

Abstract

BRAF and MEK inhibitors (BRAF/MEKi) favor melanoma-infiltrating lymphocytes, providing the rationale for current combinatorial trials with anti-PD-1 antibody. A portion of melanoma cells may express PD-1, and anti-PD-1 antibody could have a direct antitumor effect. Here, we explore whether BRAF/MEKi modulate rates of PD-1 melanoma cells, supporting an additional-lymphocyte-independent-basis for their therapeutic combination with anti-PD-1 antibody. With data mining and flow cytometry, we assessed PD-1, PD-L1/2 expression on melanoma cell lines (CCLE, = 61; validation cell lines, = 7) and melanoma tumors (TCGA, = 214). We explored how BRAF/MEKi affect rates of PD-1, PD-L1/2 melanoma cells, and characterized the proliferative and putative stemness features of PD-1 melanoma cells. We tested the functional lymphocyte-independent effect of anti-PD-1 antibody alone and in combination with BRAF/MEKi and in an immunodeficient murine model. PD-1 is consistently expressed on a small subset of melanoma cells, but PD-1 cells increase to relevant rates during BRAF/MEKi treatment [7.3% (5.6-14.2) vs. 1.5% (0.7-3.2), = 0.0156; = 7], together with PD-L2 melanoma cells [8.5% (0.0-63.0) vs. 1.5% (0.2-43.3), = 0.0312; = 7]. PD-1 cells proliferate less than PD-1 cells (avg. 65% less; = 7 days) and are preferentially endowed with stemness features. , the direct anti-melanoma activity of PD-1 blockage as monotherapy was negligible, but its association with BRAF/MEKi significantly delayed the development of drug resistance and tumor relapse. BRAF/MEKi increase the rates of PD-1 melanoma cells that may sustain tumor relapse, providing a lymphocyte-independent rationale to explore combinatory strategies with anti-PD-1 antibody. .

Mechanisms of resistance to BRAF and MEK inhibitors and clinical update of US Food and Drug Administration-approved targeted therapy in advanced melanoma

“… 52 In melanoma cell lines, BRAFi/MEKi increase the rates of PD1 + melanoma cells that may sustain tumor relapse. 48 These findings are intriguing, as immunocheckpoint blockade may be critical if combined with BRAF, in enhancing antitumor immunity and augmenting therapeutic responses. 48 , 52 This proimmunotherapy microenvironment is lost upon melanoma progression on BRAFi.…”

“… 48 These findings are intriguing, as immunocheckpoint blockade may be critical if combined with BRAF, in enhancing antitumor immunity and augmenting therapeutic responses. 48 , 52 This proimmunotherapy microenvironment is lost upon melanoma progression on BRAFi. Several studies have revealed a decrease in melanoma antigen expression, an increase in T-cell exhaustion, and a decrease in CD8 + T-cell infiltrates in melanoma tumor specimens obtained at time of progression on BRAFi.…”

“…Subsequent MAPK-pathway inhibition by an MEK inhibitor has restored melanoma antigen expression and promoted infiltration of CD8 + T cells. 48 , 69 , 70 Adding MEK inhibitors to BRAFi can overcome resistance to BRAFi and enhance immunosurveillance. In a mouse model of BRAF V600E melanoma, improved antitumor activity, in vivo cytotoxic activity, and intratumoral cytokine secretion have been reported after combining adoptive T-cell transfer with BRAFi.…”

Targeting the “undruggable” RAS - new strategies - new hope?

“…Recent evidence suggests that oncogenic RAS signaling can directly increase PD-L1 mRNA stability, which leads to enhanced PD-L1 surface expression and impaired tumor immune surveillance [82] . Whereas PD-L1 protein level were significantly increased in BRAF V600E mut melanoma cell lines resistant to BRAF inhibition, this effect was decreased both after using small interfering RNA (siRNA) mediated knockdown of ERK1/2, and after pharmacologic inhibition of MEK [83,84] . Jiang et al [83] further showed that depletion of c-Jun (transcription factor downstream of ERK) and STAT3 resulted in a synergistic decrease of PD-L1 surface expression, indicating a cooperation of RAS and STAT3 signaling in controlling PD-L1 expression in melanoma.…”

Immune Checkpoint-Mediated Interactions Between Cancer and Immune Cells in Prostate Adenocarcinoma and Melanoma

“…Programmed death-1 can be found expressed also on tumor cells, and PD-1 triggering on melanoma cells increases three-dimensional growth capability with concomitant activation of the mTOR pathway ( 3 ). Interestingly, treatment with BRAF and MEK inhibitors associated with increased frequency of PD-1 + tumor cells in melanoma patients, and PD-1 expression sensitized melanoma to PD-1 blockade in immunodeficient mice ( 46 ). The same authors also noticed a correlative expression of PD-1 and the stem cell marker Oct-4, thus linking PD-1 to cancer stem cells ( 46 ).…”