CD47, a "don't eat me" signal for phagocytic cells, is expressed on the surface of all human solid tumor cells. Analysis of patient tumor and matched adjacent normal (nontumor) tissue revealed that CD47 is overexpressed on cancer cells. CD47 mRNA expression levels correlated with a decreased probability of survival for multiple types of cancer. CD47 is a ligand for SIRPα, a protein expressed on macrophages and dendritic cells. In vitro, blockade of CD47 signaling using targeted monoclonal antibodies enabled macrophage phagocytosis of tumor cells that were otherwise protected. Administration of anti-CD47 antibodies inhibited tumor growth in orthotopic immunodeficient mouse xenotransplantation models established with patient tumor cells and increased the survival of the mice over time. Anti-CD47 antibody therapy initiated on larger tumors inhibited tumor growth and prevented or treated metastasis, but initiation of the therapy on smaller tumors was potentially curative. The safety and efficacy of targeting CD47 was further tested and validated in immune competent hosts using an orthotopic mouse breast cancer model. These results suggest all human solid tumor cells require CD47 expression to suppress phagocytic innate immune surveillance and elimination. These data, taken together with similar findings with other human neoplasms, show that CD47 is a commonly expressed molecule on all cancers, its function to block phagocytosis is known, and blockade of its function leads to tumor cell phagocytosis and elimination. CD47 is therefore a validated target for cancer therapies.

CD133 is a cell surface marker expressed on progenitors of haematopoietic and endothelial cell lineages. Moreover, several studies have identified CD133 as a marker of brain tumor-initiating cells. In this study, human glioblastoma multiforme biopsies were engrafted intracerebrally into nude rats. The resulting tumors were serially passaged in vivo, and monitored by magnetic resonance imaging. CD133 expression was analyzed at various passages. Tumors initiated directly from the biopsies expressed little or no CD133, and showed no contrast enhancement suggesting an intact blood-brain barrier. During passaging, the tumors gradually displayed more contrast enhancement, increased angiogenesis and a shorter survival. Real-time qPCR and immunoblots showed that this was accompanied by increased CD133 expression. Primary biopsy spheroids and xenograft tumors were subsequently dissociated and flow sorted into CD133 negative and CD133 positive cell populations. Both populations incorporated BrdU in cell culture, and expressed the neural precursor marker nestin. Notably, CD133 negative cells derived from 6 different patients were tumorgenic when implanted into the rat brains. For 3 of these patients, analysis showed that the resulting tumors contained CD133 positive cells. In conclusion, we show that CD133 negative glioma cells are tumorgenic in nude rats, and that CD133 positive cells can be obtained from these tumors. Upon passaging of the tumors in vivo, CD133 expression is upregulated, coinciding with the onset of angiogenesis and a shorter survival. Thus, our findings do not suggest that CD133 expression is required for brain tumor initiation, but that it may be involved during brain tumor progression. ' 2007 Wiley-Liss, Inc.Key words: CD133; brain cancer; angiogenesis; cancer stem cell; xenograft At present, there is a search for tumor cell subpopulations that may be responsible for tumor initiation and progression. Such cells have been termed cancer stem cells and are defined by their capacity to self-renew, express stem cell markers and to initiate tumors in vivo. 1,2 Potential cancer stem cells have been identified in leukaemias, 3-5 breast, 6 prostate, 7 bone, 8 colon and brain cancer. [9][10][11][12][13] In some cases, these tumor-initiating cells have been distinguished from the non-tumor-initiating ones based on expression of cell surface markers. For instance, it has been shown that only CD44 1 / CD24 2 /Lineage 2 breast cancer cells are tumorgenic in animals. 6 In malignant brain tumors, CD133 has been suggested to be a cancer stem cell marker 11,14 since only CD133 positive cells from brain tumor biopsy material were able to initiate brain cancer in a mouse model. 14 Prominin-1 (PROM-1), also called CD133, is a protein with several isoforms of unknown physiological or pathological function, and is localized both in the cytoplasm and at the cell surface. 15,16 It is expressed by human neural stem cells and has been proposed to have a function in central nervous system (CNS) development. 17 It is also express...