Science Now – A single drug can shrink or cure human breast, ovary, colon, bladder, brain, liver, and prostate tumors that have been transplanted into mice, researchers have found. The treatment, an antibody that blocks a “do not eat” signal normally displayed on tumor cells, coaxes the immune system to destroy the cancer cells.
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.
“What we’ve shown is that CD47 isn’t just important on leukemias and lymphomas,” says Weissman. “It’s on every single human primary tumor that we tested.” Moreover, Weissman’s lab found that cancer cells always had higher levels of CD47 than did healthy cells. How much CD47 a tumor made could predict the survival odds of a patient.
To determine whether blocking CD47 was beneficial, the scientists exposed tumor cells to macrophages, a type of immune cell, and anti-CD47 molecules in petri dishes. Without the drug, the macrophages ignored the cancerous cells. But when the CD47 was present, the macrophages engulfed and destroyed cancer cells from all tumor types.
Next, the team transplanted human tumors into the feet of mice, where tumors can be easily monitored. When they treated the rodents with anti-CD47, the tumors shrank and did not spread to the rest of the body. In mice given human bladder cancer tumors, for example, 10 of 10 untreated mice had cancer that spread to their lymph nodes. Only one of 10 mice treated with anti-CD47 had a lymph node with signs of cancer. Moreover, the implanted tumor often got smaller after treatment — colon cancers transplanted into the mice shrank to less than one-third of their original size, on average. And in five mice with breast cancer tumors, anti-CD47 eliminated all signs of the cancer cells, and the animals remained cancer-free 4 months after the treatment stopped.
“We showed that even after the tumor has taken hold, the antibody can either cure the tumor or slow its growth and prevent metastasis,” says Weissman.