Photo by Clay Eals
Dr. Brenda Sandmaier's research in the Clinical Research Division takes on one of the most daunting hurdles of bone- marrow transplantation - the challenge of outwitting the immune system.
Her laboratory's research is crucial, since a patient's immune system can perceive incoming marrow cells as invaders and prevent them from grafting, thereby blocking a cure.
Sandmaier is studying new ways to suppress the immune system by targeting molecules on immune cells.
The goal is to achieve engraftment, the process by which the graft takes hold, while reducing the need to give patients massive doses of total-body irradiation prior to transplantation, part of the current practice for immunosuppression.
An option for more people
High doses of total-body irradiation currently limit stem-cell transplantation to younger patients with more aggressive diseases - leukemias, for example. Achieving immunosuppression with lower radiation doses would make stem-cell transplantation an option for more people who would otherwise not be eligible because of age or other medical problems.
"The aim," she said, "is to reduce the graft-rejection response in patients, not only with malignant disorders but also with nonmalignant indications such as genetic disorders."
Using lower radiation levels decreases nonspecific toxicity in patients. However, this increases the risk of graft rejection.
In ongoing trials, center investigators including Sandmaier and Drs. Rainer Storb and David Maloney and Dr. Peter McSweeney, now of the University of Colorado, are using a "mini-transplant" regimen in cases with matched tissue types by combining low levels of total-body irradiation with an intensive, immunosuppressive drug regimen.
Sandmaier hopes to further reduce radiation toxicity - or perhaps eliminate it - by targeting radiation to the immune cells in the bloodstream that hamper engraftment. A radioactive element (radioisotope) is fused to an antibody that seeks out and attaches to a protein called CD45 that is found on certain blood and immune cells.
In effect, the antibodies turn the radiation "hit" provided by the radioisotopes into a molecular smart bomb targeted directly to blood and immune cells without indiscriminately blasting other vital organs.
"The radioisotope has enough energy to kill cells already," Sandmaier said. "The antibody delivers the specificity."
The research involving the radiolabeled CD45 antibody will be published in Blood this July and was carried out in collaboration with postdoctoral fellow Dr. Wolfgang Bethge of the Sandmaier lab, University of Washington research scientist Donald Hamlin and Dr. Scott Wilbur of the UW Radiation Oncology Department. The foundation for the CD45 antibody work was based on research by the center's Dr. Dana Matthews, who used a similar approach with a different radioisotope in patients with malignant disorders.
Sandmaier and colleagues also are using an approach similar to the CD45 antibody to fuse radioisotopes to antibodies that target receptor proteins on the surface of immune cells known as T cells.
T cells are the primary culprits in graft rejection when the donor's marrow is matched by major criteria but mismatched for minor antigens. By targeting T cells directly, the radioisotope facilitates immunosuppression.
Family of proteins
Tissue type is determined by a family of proteins on the surface of cells called MHC (major histocompatibility complex) molecules. The incoming marrow is seen as "self" if MHC molecules match, or "non-self" if mismatched. An MHC mismatch triggers a different type of immune response: both T cells and so-called natural killer cells contribute to graft rejection.
To extend stem-cell transplantation to more patients, one area of Sandmaier's research for more than a decade has been bone-marrow transplantation from donors who have different tissue types.
"Not everyone has an MHC-matched brother or sister," she said, "so alternative donors need to be used."
However, the more the disparity in MHC between donor and recipient, the more risk of graft rejection.
To counter this, Sandmaier and coworkers have determined that an antibody called S5, which has been shown to improve the success of mismatched animal transplants, recognizes another protein on immune cells, CD44.
By isolating the gene for CD44, Sandmaier is working out the molecular mechanics of how S5 inhibits graft rejection. Current studies aim to demonstrate one way that S5 could help the graft take hold - by triggering the death (or apoptosis) of both T cells and natural-killer cells.
"The introductory work is done," said Sandmaier. "We are refining it in the pre-clinical model."
Visiting fellows Drs. Jens Panse and Takahiro Fukuda and research technician Erlinda Santos are collaborators in the S5 work.
Sandmaier's team found that adding S5 to a low-dose, total-body irradiation regimen enhanced engraftment, even when grafts came from only partially matched donors. Without S5, the majority of test animals rejected their grafts.
Further studies are in progress to create an S5 that is compatible with human CD44. Sandmaier's research aims to preserve the unique binding properties of S5 in human transplants so that S5 can remain effective in enhancing engraftment.
When will these antibodies be tested in clinical trials involving stem-cell transplantation? The answer is several years down the road.
"The pre-clinical studies will be used to design the clinical trials," said Sandmaier, who is optimistic that lowering the toxicity associated with traditional stem-cell transplantation will make the therapy an option for more people. [Danielle Ippolito is a graduate student in the University of Washington Department of Pharmacology.]