Two weeks afterwards, single cell subclones were genotyped by PCR and validated Western blotting for B7-H1 protein depletion. activation of p38 MAPK. B7-H1 was found to associate with the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) and this association promoted or maintained the activation of ERK or p38 MAPK in cancer cells. Importantly, we found that targeting B7-H1 by anti-B7-H1 monoclonal antibody (H1A) increased the sensitivity of human triple negative breast cancer cells to cisplatin therapy in?vivo. Our results suggest that targeting B7-H1 by an antibody capable of disrupting B7-H1 signals may be a new approach to sensitize cancer cells to chemotherapy. that leads to a high activation of ERK  although MDA-MB-231 cells constitutively express high levels of B7-H1. Accordingly, although MDA-MB-231 cells express higher levels of B7-H1 than MDA-MB-157 cells (a human triple negative breast cancer cell line) , both of them demonstrated similar sensitivity to cisplatin in?vitro. Besides their different B7-H1 expression, these two cell lines have multiple different gene mutations in p53 and RB pathways that also regulate sensitivity to chemotherapy [33, 34]. In this regard, B7-H1 expression alone may not be able to predict chemoresistance as multiple factors are responsive for drug resistance in cancer cells. However, our results support the therapeutic potential of targeting B7-H1 to promote the efficacy of chemotherapy in cancer cells that express B7-H1. In fact, we found that B7-H1 antibody (H1A) sensitized human breast cancer cells to cisplatin in?vivo, suggesting B7-H1 antibody may disrupt B7-H1 signals in cancer cells along with HI TOPK 032 blocking B7-H1 and PD-1 interaction that suppress antitumor immunity. In summary, our studies identify a pro-survival function of B7-H1 in cancer cells. B7-H1 may promote cancer cell survival by activation of ERK and p38 MAPK pathways through the association with DNA-PKcs. The pro-survival function of B7-H1 could be used by apoptosis-primed cancer cells to counteract the cytotoxicity of chemotherapy. To that end, we propose that targeting B7-H1 by monoclonal antibody to B7-H1 capable of disrupting B7-H1 signals may be a new approach to promote the efficacy of cancer chemotherapy. Recent clinical trials that have demonstrated the superiority of adding B7-H1 or PD-1 inhibitors to chemotherapy compared to chemotherapy alone further support our findings [10, 11, 12, 35]. 4.?Methods and materials 4.1. Cell lines and reagents Human cancer cell lines (MDA-MB-231, MDA-MB-157, 786-0, A549) were purchased from ATCC (Manassas, VA). Tumor cells were cultured and maintained in medium indicated by ATCC. B7-H1 or OVA (mock) transfected 624mel cells were cultured in RPMI 1640 medium (Cellgro) and supplemented as described previously . Cells were cultured in a 37 C humidified chamber at 5% CO2. Chemotherapy drugs were purchased form Mayo Pharmacy or Sigma. 4.2. B7-H1 transfection and knockout Human B7-H1 was knocked-out by CRISPR/Cas9 technology. The guide sequence HI TOPK 032 (5-ATTTACTGTCACGGTTCCCA-3) specific to human B7-H1 exon 3 (second coding exon), designed using CRISPR DESIGN tool (http://crispr.mit.edu) and cloned into px458 plasmid coexpressing GFP (Addgene, #52961). Thirty-six hours after transfection, cells were sorted for GFP and sub-cloned using flow cytometry. Two weeks later, single cell subclones were genotyped Flrt2 by PCR and validated Western blotting for B7-H1 protein depletion. B7-H1 expression level was determined by flow cytometry and Western blotting. 4.3. Immunofluorescence staining Following growth on PBS and medium pre-rinsed coverslips, cells were fixed with 4% formalin or paraformaldehyde for 15 min., washed 4x with PBS, and permeabilized for 10 min. with 0.2% Triton X-100 or 0.5% IGEPAL ca-360. After washing with PBS, cells were blocked with 3% milk/PBS, then incubated at 4?C overnight with primary antibodies (1:100 anti-DNA-PKcs and 1:300 anti-B7-H1 antibody 5H1) diluted in blocking solution. Six 3% milk/PBS washes were performed prior to 1-hour incubation with secondary antibody (Life Technologies Fluorescein-conjugated goat anti-mouse and HI TOPK 032 Alexa 594-conjugated goat anti-rabbit IgG) diluted HI TOPK 032 1:100 in blocking solution. Following five PBS washes, re-fixation for 10 min. with 4% paraformaldehyde, and two dH2O washes, coverslips were mounted with SlowFade Gold antifade reagent with DAPI (Invitrogen) and cured for 24 hrs in dark at RT. Nail-polish sealed coverslips were visualized using a Zeiss LSM 510 confocal microscope. The two-dimensional Z-section images were acquired and performed using a Zeiss ELYRA super-resolution structured illumination microscopy. 4.4. MTS cytotoxicity assay 1 104 cells were seeded into 96-well plates and chemo-drug was applied. Following 72-hour incubation, 20 l/well CellTiter 96 Aqueous One Solution Reagent (Promega) was added. After 2 hours of incubation, absorbance at 490 nm was recorded using an ELISA plate reader. Control and all concentrations of drug were assayed in triplicate, and the absorbance at each drug concentration was normalized relative to that of untreated controls. 4.5. Flow cytometry analysis Fluorochrome-conjugated Abs against human B7-H1 (MIH1), PD-1 (EH12.2H7) and CD80 (L307.4).