The success of a stem-cell or bone-marrow transplant for leukemia or other blood cancers relies in large part on a careful tissue-type match between donor and patient. Yet research has shown that transplants between identical twins, who share the same genetic blueprint, are much less likely to be successful than those between tissue-matched non-twin siblings or unrelated individuals.
It's now clear that subtle differences exist even between tissue-matched donors and recipients, which can endow a graft with powerful cancer-fighting properties. These differences also can cause unwanted side effects, such as graft rejection or a complication known as graft-vs.-host disease (GVHD). Finding the factors responsible for both positive and negative effects — and putting them to work to improve transplant outcome — is the research focus of Dr. Hootie Warren and his colleagues in the Clinical Research Division's Immunology Program.
Minor histocompatibility antigens
Their research centers on tiny snippets of proteins known as minor histocompatibility antigens, which differ somewhat among all individuals except identical twins. Despite their inferior-sounding name, some minor antigens — when mismatched between donor and recipient — seem to have major impacts on multiple aspects of transplant outcome.
In recent studies, Warren, Dr. Stanley Riddell and others in the program have discovered certain minor-antigen mismatches that they suspect boost the cancer-fighting properties of a graft, while others may contribute to GVHD. Their work also explores whether minor antigens may hold the key to extending the curative powers of stem-cell transplants beyond blood cancers to solid tumors.
"Between non-twin siblings who have identical major determinants of tissue type, there can be hundreds, perhaps thousands of differences in minor antigens, although not all of them necessarily impact transplant outcome," Warren said. "Our work is aimed at finding the ones that do."
Cellular self and non-self
Minor antigens are distinct from the major determinants of tissue type, a set of six different proteins known as human leukocyte antigens, or HLA proteins. HLA proteins sit on the surfaces of all cells in the body and provide a molecular identification badge that enables the immune system to distinguish "self" from "non-self." This is the basis for the body's ability to react to and destroy foreign cells such as pathogenic bacteria and viruses or reject incompatible transplanted tissues. In the case of a bone-marrow or stem-cell transplant, a reverse rejection phenomenon — GVHD — also can occur because the graft contains donor immune-system cells that have the potential to recognize the host tissue as foreign.
To minimize the likelihood of either complication, HLA proteins from donor and recipient are analyzed and compared to find a donor who possesses variants of each HLA protein that are identical, or nearly identical, to those of the patient.
Warren said that the existence of additional factors that contribute to tissue-type incompatibility was inferred once researchers had accumulated enough data on the outcome of transplants between HLA-identical non-twin patients and donors.
"These patients still could develop GVHD or reject their graft," he said. "We now know that's because the HLA proteins work together with what we call minor antigens to help the immune system discriminate between self and non-self."
Minor antigens turn out to be nothing more than bits of the thousands of proteins produced by cells throughout the body as part of a normal breakdown process. These fragments end up on the surface of cell, held in place by large, catcher's-mitt-like grooves in the HLA proteins. Once there, the minor antigens are visible to circulating immune-system cells known as T cells, which recognize these fragments as "self" and therefore, do not provoke an immune response.
As T cells circulate through the body, they look for cells that display unfamiliar proteins bits — such as a fragment of a viral protein, a protein produced only by cancer cells or proteins derived from transplanted tissue. Such sightings trigger a protective response that results in destruction of the cell displaying non-self proteins.
"The system didn't evolve to protect the body from bone-marrow transplantation," Warren said. "But proteins from graft can appear as foreign to the patient's immune system, causing rejection."
Likewise, T cells from the donor can recognize the host's minor antigens as foreign — an event that can have both good and bad consequences. If the donor cells recognize minor antigens unique to cancer cells, the graft actually fights the cancer through what researchers call the graft-vs.-tumor effect. If the donor cells react against normal cells, the result can be GVHD.
Warren and colleagues have set out to identify the minor antigens unique to cancer cells as well as those that fall under attack in GVHD. By then finding the T cells that recognize each of these types of minor antigens, the researchers hope to someday engineer the graft so as to minimize the GVHD response and to boost the graft-vs.-tumor response.
The work is challenging because despite thousands of minor antigen differences among individuals, many are too slight to be noticed by the immune system.
"The vast majority of these protein fragments are pretty similar among individuals," Warren said. "The ones we want are those that are different enough to be seen as foreign and that elicit the desired immune response. "In addition, it is likely that GVHD is caused by multiple minor antigen mismatches between donor and recipient."
In his studies on the graft-vs.-tumor effect, Warren has identified T cells that selectively recognize a minor antigen that is produced exclusively on B cells, a type of white blood cell that can give rise to leukemia. If patient and donor are mismatched for this minor antigen, it's possible that the researchers may be able to boost the graft-vs.-tumor effect by giving patients infusions of the T cells that recognize this target.
In another study led by research fellow Dr. Sophia Randolph and published in January, the team analyzed male transplant recipients who received stem-cell transplants from female donors. The analysis revealed that such patients have a lower relapse rate than other sex combinations, suggesting that minor antigens that exist only in males provoke a selective graft-vs.-tumor from female donor cells. The researchers next hope to identify the gender-specific minor antigens that may contribute to this effect.
The work holds the potential to improve the effectiveness and safety of transplantation for blood cancers. The team also has found that minor antigens produced by solid tumors may provoke a cancer-fighting T-cell response from a stem-cell graft. Preliminary studies in conjunction with senior fellow Dr. Scott Tykodi have shown that stem-cell transplantation could be used to treat metastatic renal-cell carcinoma, an advanced form of kidney cancer, through the graft-vs.-tumor effect. The researchers now hope to identify the unique minor antigens present on the tumor cells (and not present on normal cells) that enable the donor-T cells to recognize the cancer. Once found, the target proteins may be used to augment the graft-vs.-tumor response after transplantation.