Expansion and Activation of CD103(+) Dendritic Cell Progenitors at the Tumor Site Enhances Tumor Responses to Therapeutic PD-L1 and BRAF Inhibition.

Large numbers of melanoma lesions develop resistance to targeted inhibition of mutant BRAF or fail to respond to checkpoint blockade. We explored whether modulation of intratumoral antigen-presenting cells (APCs) could increase responses to these therapies. Using mouse melanoma models, we found that CD103(+) dendritic cells (DCs) were the only APCs transporting intact antigens to the lymph nodes and priming tumor-specific CD8(+) T cells. CD103(+) DCs were required to promote anti-tumoral effects upon blockade of the checkpoint ligand PD-L1; however, PD-L1 inhibition only led to partial responses. Systemic administration of the growth factor FLT3L followed by intratumoral poly I:C injections expanded and activated CD103(+) DC progenitors in the tumor, enhancing responses to BRAF and PD-L1 blockade and protecting mice from tumor rechallenge. Thus, the paucity of activated CD103(+) DCs in tumors limits checkpoint-blockade efficacy and combined FLT3L and poly I:C therapy can enhance tumor responses to checkpoint and BRAF blockade.

MEK inhibitors lead to PDGFR pathway upregulation and sensitize tumors to RAF dimer inhibitors in NF1-deficient malignant peripheral nerve sheath tumor (MPNST).

PURPOSE: Malignant peripheral nerve sheath tumor (MPNST) is a highly aggressive subtype of soft tissue sarcoma with a high propensity to metastasize and extremely limited treatment options. Loss of the RAS-GAP NF1 leads to sustained RAF/MEK/ERK signaling in MPNST. However, single-agent MEK inhibitors (MEKi) have failed to elicit a sustained inhibition of the MAPK signaling pathway in MPNST. EXPERIMENTAL DESIGN: We employed pharmacological, biochemical, and genetic perturbations of the receptor tyrosine kinase (RTK) and MAPK signaling pathway regulators to investigate the mechanisms of MEKi resistance and evaluated combination therapeutic strategies in various preclinical MPNST models in vitro and in vivo. RESULTS: Here, we report that MEKi treatment resistance in MPNST involves two adaptive pathways: direct transcriptional upregulation of the receptor tyrosine kinase (RTK) PDGFRß, and MEKi-induced increase in RAF dimer formation and activation of downstream signaling. While the pharmacological combination of MEKi with a PDGFRß specific inhibitor was more effective than treatment with MEKi alone, the combination of MEKi and RAF-dimer inhibitors led to a robust inhibition of the MAPK pathway signaling. This combination treatment was effective in vitro and in vivo, as demonstrated by the significant increase in drug synergism and its high effectiveness in decreasing MPNST viability. CONCLUSIONS: Our findings suggest that the combination of MEKi and PDGFR and/or RAF dimer inhibitors can overcome MEKi resistance and may serve as a novel targeted therapeutic strategy for NF1-deficient MPNST patients, which in turn could impact future clinical investigations for this patient population.

ETV4 mediates dosage-dependent prostate tumor initiation and cooperates with p53 loss to generate prostate cancer.

The mechanisms underlying ETS-driven prostate cancer initiation and progression remain poorly understood due to a lack of model systems that recapitulate this phenotype. We generated a genetically engineered mouse with prostate-specific expression of the ETS factor, ETV4, at lower and higher protein dosage through mutation of its degron. Lower-level expression of ETV4 caused mild luminal cell expansion without histologic abnormalities, and higher-level expression of stabilized ETV4 caused prostatic intraepithelial neoplasia (mPIN) with 100% penetrance within 1 week. Tumor progression was limited by p53-mediated senescence and Trp53 deletion cooperated with stabilized ETV4. The neoplastic cells expressed differentiation markers such as Nkx3.1 recapitulating luminal gene expression features of untreated human prostate cancer. Single-cell and bulk RNA sequencing showed that stabilized ETV4 induced a previously unidentified luminal-derived expression cluster with signatures of cell cycle, senescence, and epithelial-to-mesenchymal transition. These data suggest that ETS overexpression alone, at sufficient dosage, can initiate prostate neoplasia.