Precociously disseminated cancer cells may seed quiescent sites of future metastasis if they can protect themselves from immune surveillance. However, there is little knowledge about how such sites might be achieved. Here, we present evidence that prostate cancer stem-like cells (CSC) can be found in histopathologically negative prostate draining lymph nodes (PDLN) in mice harboring oncogene-driven prostate intraepithelial neoplasia (mPIN). PDLN-derived CSCs were phenotypically and functionally identical to CSC obtained from mPIN lesions, but distinct from CSCs obtained from frank prostate tumors. CSC derived from either PDLN or mPIN used the extracellular matrix protein Tenascin-C (TNC) to inhibit T-cell receptor-dependent T-cell activation, proliferation, and cytokine production. Mechanistically, TNC interacted with a5b1 integrin on the cell surface of T cells, inhibiting reorganization of the actin-based cytoskeleton therein required for proper T-cell activation. CSC from both PDLN and mPIN lesions also expressed CXCR4 and migrated in response to its ligand CXCL12, which was overexpressed in PDLN upon mPIN development. CXCR4 was critical for the development of PDLN-derived CSC, as in vivo administration of CXCR4 inhibitors prevented establishment in PDLN of an immunosuppressive microenvironment. Taken together, our work establishes a pivotal role for TNC in tuning the local immune response to establish equilibrium between disseminated nodal CSC and the immune system. Cancer Res; 75(10); 2095-108. Ó2015 AACR.

BRCA1-mutated breast cancer is primarily driven by DNA copy-number alterations (CNAs) containing large numbers of candidate driver genes. Validation of these candidates requires novel approaches for high-throughput in vivo perturbation of gene function. Here we develop genetically engineered mouse models (GEMMs) of BRCA1-deficient breast cancer that permit rapid introduction of putative drivers by either retargeting of GEMM-derived embryonic stem cells, lentivirus-mediated somatic overexpression or in situ CRISPR/Cas9-mediated gene disruption. We use these approaches to validate Myc, Met, Pten and Rb1 as bona fide drivers in BRCA1-associated mammary tumorigenesis. Iterative mouse modeling and comparative oncogenomics analysis show that MYC-overexpression strongly reshapes the CNA landscape of BRCA1-deficient mammary tumors and identify MCL1 as a collaborating driver in these tumors. Moreover, MCL1 inhibition potentiates the in vivo efficacy of PARP inhibition (PARPi), underscoring the therapeutic potential of this combination for treatment of BRCA1-mutated cancer patients with poor response to PARPi monotherapy.