IMMUNE PRIVILEGE OF THE HEMATOPOIETIC STEM CELL NICHE IN THE BONE MARROW SHIELDS METASTATIC PROSTATE CANCER FROM IMMUNITY

Background, Hypothesis/Objective: Over 70% of patients with advanced prostate cancer display bone or bone marrow (BM) metastases that are strongly resistant to different treatment modalities, including immune therapy. For example, a new immune therapy, anti-PD-1 antibody treatment, exhibited a remarkable therapeutic efficacy against different solid cancers, but not against advanced prostate cancer. It remains unclear what causes this high therapeutic resistance of prostate cancer following bone metastases, as well as the bone tropism of prostate cancer. The goal of this proposed project is to test whether the BM microenvironment for hematopoietic stem cell (HSC) residence, or the HSC niche, is an immunological sanctuary for stem cells, termed an immune privileged (IP) site, where prostate cancer cells preferentially home and escape from immunity, leading to therapeutic resistance.IP was originally discovered more than 60 years ago in the placenta and the testis, the location of embryonic and germline stem cells. Rigorous immune suppression in these tissues is evidenced by the prolonged persistence of transplanted allogeneic or xenogeneic grafts in the absence of any immune suppressive therapy. There is little information about whether tissue-committed stem cell niches are IP sites.Our recent study demonstrated evidence suggesting that the HSC niche located on the inner bone surface is an IP site. Novel high-resolution, in vivo microscopy enabled us to detect rare allogeneic (allo-) HSCs that persist on the inner bone surface in immune-competent mice without immune suppressive therapy for over 60 days. Moreover, HSCs were adjacent to potent FoxP3+ regulatory T cell (Treg) populations. The specific reduction of these niche-associated Tregs, achieved by conditional deletion of CXCR4 in Tregs, led to loss of allo-HSCs. The data indicate that potent Tregs render the HSC niche an IP site.We further showed that various malignant cells, including prostate cancer cells, neuroblastoma cells, and BCR-ABL and MLL-AF9 leukemia cells home in close proximity to BM Treg, a hallmark of IP sites of the niche. These results prompted us to hypothesize that prostate cancer homes to the sanctuary for HSCs, where potent Tregs shield metastatic tumors from immunity, leading to therapeutic resistance.Specific Aim: Elucidate the role of IP within the BM in progression and therapeutic resistance of metastatic prostate cancer.Study Design: We will determine whether IP disruption alone or in combination with anti-PD1 antibody treatment inhibits tumor progression and overcomes therapeutic resistance to anti-PD1 antibody treatment.We will use a novel genetically engineered prostate cancer mouse model that frequently displays spontaneous BM metastases. In this cancer mouse model, we will disrupt IP by using genetic or pharmacological approaches to module (1) niche-associated Tregs and (2) CD11b+Gr1+ cells, another immune modulatory cell population within the BM. We will assess anti-tumor effects of IP disruption alone or in combination with anti-PD1 antibody treatment by using the survival curve analysis, histological analysis, and high-speed in vivo microscopy. This in vivo microscopy enables the repeated imaging of the same BM region of the same mouse over months (without euthanasia) to track the spatial distribution of early disseminated cancer cells, bulk tumor, and minimal residual disease with respect to the niche, as well as to monitor the tumor volume. Furthermore, using RNA sequencing of Tregs in the BM tumor microenvironment, we will seek to identify possible new therapeutic target genes to disrupt IP.Impact: Completion of the proposed study will create an unprecedented paradigm that metastatic tumor cells hijack an immunological sanctuary for normal stem cells, leading to therapeutic resistance of metastatic cancer. The use of different IP disruption approaches will allow us to dissect IP mechanisms and lead to development of novel therapeutic strategies to overcome resistance of advanced prostate cancer to anti-PD1 antibody treatment. Therefore, successful studies will open up novel avenues of the basic research in stem cell biology, oncology and immunology, as well as new therapeutic strategies in advanced prostate cancer.