Home » Making Cancer Visible: The Center for Cell and Gene Therapy
August 2003 – Baylor College of Medicine
By Ruth SoRelle, MPH
Malignant cells run amok in the body because they are invisible to the normal processes that keep such anomalies in check.
Finding a way to “light up” such cells in order to let the immune system know they are there and enable it to eliminate them is one tactic currently being investigated by many scientists, including those at the Center for Cell and Gene Therapy (CAGT) at Baylor College of Medicine. The process is not as easy as it would seem.
Recently, a vaccine used to alert the immune system to the presence of leukemia cells has had some preliminary success in patients at Texas Children’s Cancer Center and The Methodist Hospital. Building that vaccine has required that scientists first understand what is going wrong and then to create an ingenious solution.
Raphael Rousseau, M.D., Ph.D., a post-doctoral fellow under the mentorship of Malcolm Brenner, M.D., Ph.D. in the Center for Cell and Gene Therapy and the Texas Children’s Cancer Center, sees the vaccine as a proof of principle.
“There’s been growing evidence that one of the main reasons the immune system doesn’t see leukemic cells is because most of them lack specific costimulatory molecules called CD80 and CD86 that make that possible,” said Rousseau. “If you can restore those molecules in a timely fashion to enable the T-cells (the assassins of the immune system) to recognize them, you can eventually generate an antileukemia immune response against a particular protein called an antigen that is found on the surface of the leukemia cells.”
Transferring to leukemia cells the genes that cause a cell to make those costimulatory molecules (CD80 and CD86) has been considered, but it does not work because of the limitations of current gene transfer techniques, said Rousseau.
“Once the T-cell has seen the blast (leukemia cell) without the molecules, it has decided it is not dangerous and shuts itself off,” he said. The time factor is crucial and not likely to be solved with current gene therapy techniques.
Instead, Rousseau and his colleagues looked for help from other molecules and decided on the CD40 and CD40 ligand molecules that had shown during in vitro experiments potency in stimulating T-cells to attack leukemia invaders.
“The CD40 ligand is a crucial molecule for cells of the immune system to see the antigen (a protein that evokes an immune response) and induce the T cells to attack the tumor,” he said.
“Stimulation of immune cells by CD40 ligand is not only capable of restoring CD80 and CD86 molecules, he said, “it is also capable of restoring the immune function.”
Previously, work conducted in Brenner’s lab showed that when mice with a form of lymphoma that expressed the CD40 molecule were injected with the lymphoma cells and the CD40L (ligand) cells, their immune system began to fight the tumor as the invader it is. When interleukin-2, another protein that stimulates T-cell proliferation, was combined with CD40L, the immune system became even more aggressive in fighting the tumor in mice.
Translating the success in mice to people was difficult. Choosing a viral vector (a form of virus inactivated to prevent reproduction that takes genes into cells) and which cells to use took time.
Rousseau and Brenner decided on a clinical grade adenoviral vector available at Baylor that is well known and safe when used to modify cells ex vivo before reinjecting them in patients. The vaccine would involve using the adenoviral vector encoding the genes for CD40L (ligand) and interleukin-2 to infect accessory skin cells susceptible to adenoviral infection.
Rousseau and his colleagues collected skin fibroblasts (cells that form part of the skin) from the patients and grew them in laboratory culture, increasing their numbers and insuring that the cells were of good enough quality to use in patient treatment. The adenoviral vectors encoding the CD40L and interleukin-2 genes were used to transfer those therapeutic genes into the skin fibroblasts. Those genetically modified skin fibroblasts were then injected back to the patients along with their own irradiated leukemic blast cells.
Only patients with cured, high-risk acute leukemia whose risk of relapse was greater than 50 percent were admitted into the study. They would receive six subcutaneous injections (shots given just under the skin) every one to two weeks. The dosage of IL-2 and leukemic blasts remained the same, but the dose of CD40L increased with each injection.
So far, 10 patients have received the vaccines. Three had acute myelogenous leukemia and seven had acute lymphocytic leukemia. All but one had undergone a bone marrow transplant and were in partial or complete remission from their disease when they received the vaccine. However, doctors had concern their remissions may not last forever, and that is why the vaccine was tried.
The results from eight patients could be evaluated and were recently presented at the Annual Meeting of the American Association for Cancer Research in Washington DC. No serious side effects from the vaccine were found. That was positive, said Rousseau, because determining the safety of the vaccine was the major reason for doing the study.
One patient who was in incomplete remission at the time of enrollment died because of progressive leukemia. Another had a relapse that was successfully treated with chemotherapy. All the others are still alive and maintain the remissions with which they started the study. Patients experienced only mild and reversible side effects.
After more than three injections, two patients have developed antibodies against their own leukemic blasts, said Rousseau. In five of the eight evaluable patients, he could also find evidence of a cellular, leukemia-specific immune response targeted at their own blasts.
Since very few patients have been enrolled in this phase I study, he said, it is too early to claim that the patients remain in remission solely because of the vaccine. Nor does he think that the vaccine will be something routinely used in the treatment of patients. It is a difficult vaccine to manufacture and must be tailored specifically for each patient.
“We have accumulated enough data on the phase I study that we feel confident that we should do a phase II study” that will determine the effectiveness of the vaccine.
“We are not saying that we have cured these patients or even prevented their relapse,” said Rousseau. The study does provide enough information about the effect of the CD40L gene given by the subcutaneous approach that he feels confident that it should be tried in more patients.”
Rather than using whole tumor cells, if scientists could identify antigens or proteins that provoke an immune response that is specific to the leukemias in these patients, building a vaccine would be easier, he said. From these patients who developed an immune response to their vaccines, he hopes to identify new leukemia-specific targets that could be then used to design new therapeutic approaches, hopefully more suitable for a large number of patients. Alternatively, he also hopes to combine this vaccine with other treatments, such as bone marrow transplantation, to try to improve the overall antileukemia treatment effect.
The vaccine was developed because the cells or proteins that would be best to target are still unknown, he said.
“That is true for most leukemias and neuroblastomas,” (another disease for which the Center for Cell and Gene Therapy and Texas Children’s Cancer Center are developing antitumor vaccines) said Rousseau. “The idea is that if you don’t know the antigen (protein that evokes an immune response), enhancing the immune system’s global response may not only be beneficial to the patient, but can also help researchers to identify new targets that fail to be recognized by more conventional screening methods in animals.”
Brenner, director of the Center for Cell and Gene Therapy at Baylor College of Medicine, was principal investigator on the study. Others who participated include Ettore Biagi, MD, Eric S. Yvon, MS, Zhuyong Mei, MD, Donna R. Rill, Bambi Grilley, RPh, Helen Heslop, MD, Uday Popat, MD, Adrian Gee, PhD, Robert Krance, MD, George Carrum, MD, Ingrid Kuehnle, MD and Judith Margolin, MD. The study was conducted at both The Methodist Hospital and Texas Children’s Cancer Center.