Date Published: July 10, 2007
Publisher: Public Library of Science
Author(s): Stanley R Riddell, Frederick R Appelbaum
Abstract: Stanley Riddell and Frederick Appelbaum review progress in preventing graft-versus-host disease following allogeneic hematopoietic cell transplantation for malignancies or other life-threatening blood diseases.
Partial Text: This year approximately 20,000 individuals will receive an allogeneic hematopoietic cell transplant (HCT) as treatment for a malignant, or life-threatening non-malignant, hematopoietic disease. The process of HCT generally begins with administration of a preparative regimen to eradicate the underlying disease and immunosuppress the patient in order to prevent rejection of the subsequently transfused hematopoietic stem cells. Following HCT, donor T cells transplanted with or developing from the hematopoietic stem cells react with cells of the human leukocyte antigen (HLA)-matched but genetically non-identical host, providing a beneficial graft-versus-tumor (GVT) response but also resulting in possibly life-threatening graft-versus-host disease (GVHD). The manifestations of GVHD vary over its course. Acute GVHD usually appears within several weeks of HCT and is characterized by a diffuse maculopapular rash, mucosal inflammation causing crampy abdominal pain and diarrhea, and elevated liver function tests (Figure 1). GVHD that first appears or persists more than three months after allogeneic HCT is termed chronic GVHD and resembles a chronic autoimmune disorder. Patients frequently develop lichen planus skin lesions, ocular and oral sicca, obliterative bronchiolitis, and hepatic abnormalities resembling primary biliary sclerosis.
One area of progress, as noted above, has been in HLA-typing technology and donor selection. Historically, HLA typing was conducted using serologic methods, but with the advent of the polymerase chain reaction assay in the 1980s, it became possible to perform molecular typing of donor and recipient. When patients previously transplanted from serologically matched donors were retrospectively analyzed using molecular typing, approximately 30% were found to be mismatched with the donor for one or more alleles, and such mismatching was shown to lead to more GVHD and poorer survival . Thus, molecular typing of the HLA locus has become the standard of care. Even with the use of molecular typing to identify fully HLA-matched donors, unrelated donor transplants continue to be associated with more GVHD than seen with HLA-matched sibling transplants. Using a novel technique that allows for the typing of individual DNA strands, Petersdorf et al. have now shown that among allele-matched unrelated donor–recipient pairs, those that shared the same physical linkage of HLA-A, -B, and -DRB1 were much less likely to develop severe GVHD (Figure 2) . These results imply that other unidentified transplantation antigens exist within the same genetic region as HLA, and offer a method for improved selection among HLA-matched unrelated donors.
The pathogenesis of GVHD involves the expansion and differentiation of donor T cells reacting in peripheral lymphoid tissues against host antigen-presenting cells that display disparate minor histocompatibility antigens. These antigen-presenting cells are activated as a consequence of tissue injury and the resulting release of proinflammatory cytokines . Polymorphisms in cytokine genes between donor and recipient may influence the host inflammatory response to tissue injury and the severity of GVHD. The interleukin-10 (IL-10) pathway is the most extensively studied, and specific polymorphisms in the recipient IL-10 promoter region as well as in the donor IL-10 receptor beta gene have each been found to be associated with a lower risk of GVHD and non-relapse mortality [8,9]. Polymorphisms in other immune regulatory genes, including those encoding interleukin-6, interferon gamma, and tumor necrosis factor-alpha, have also been suggested to influence the development of GVHD .
Stem cell grafts contain distinct functional and phenotypic subsets of T cells, including antigen-inexperienced naïve T cells (TN), antigen-experienced memory T cells, (TM), and regulatory T cells (TREG). Recent studies have begun to dissect the contribution of these individual T cell subsets to GVHD and have identified opportunities for more refined manipulation of the T cell content of stem cell grafts that may reduce GVHD without the severe T cell deficiency associated with complete depletion (Figure 3). For example, the selective depletion of TN from allogeneic stem cell grafts abrogated GVHD in both CD4- and CD8-dependent multiple minor histocompatibility antigen–mismatched mouse models, and the remaining TM provided reconstitution of immunity to pathogens [12,13]. Human TN and TM can also be distinguished based on phenotype— TN are CD45RA+ and CD62L+, while TM are CD45RO+ and either CD62L+ or CD62L-, and emerging data suggest that alloreactivity for minor histocompatibility antigens is predominantly contained in the TN repertoire subset . The human TM comprises less than 1% of the overall T cell receptor diversity and consists of large numbers of T cells specific for cytomegalovirus, Epstein Barr virus, and other pathogens that cause opportunistic infections in HCT recipients . Thus, unless the donor has been previously sensitized to recipient minor histocompatibility antigens (which would convert alloreactive naïve T cells to the memory pool), transplants using stem cells depleted of naïve T cells could reduce or eliminate GVHD while preserving the transfer of memory T cells to common infectious agents. Such transplants would overcome a major limitation of transplantation using complete T cell depletion. The recognition that donor CD4+CD25+Foxp3+ TREG cells suppress T cell responses in vitro and in vivo suggests another attractive approach to donor graft manipulation for preventing GVHD. The importance of TREG in GVHD is supported by murine studies showing that their depletion from stem cell grafts exacerbates GVHD and that the infusion of additional TREG at the time of HCT reduces lethal GVHD, apparently by limiting the initial activation of alloreactive T cells in lymph nodes [16,17]. Clinical studies have suggested that stem cell grafts from donors with higher numbers of TREG confer a lower risk of GVHD , and efforts are in progress to isolate and expand populations of human TREG that might be used to supplement stem cell grafts and abrogate GVHD .
Although these new approaches to allogeneic HCT are likely to reduce the severity of GVHD, an important concern for patients undergoing HCT for a malignant disease is whether reducing GVHD might increase the risk of tumor recurrence. Like GVHD, GVT is the result of donor T cells reacting with disparate minor histocompatibility antigens, and elimination of GVHD would seem almost certain to diminish the GVT effect. Elucidation of the molecular structure, HLA restriction, and tissue expression of human minor histocompatibility antigens, and the identification of non-polymorphic leukemia-associated antigens that can be recognized by T lymphocytes offers the exciting prospect that targeted T cell therapy after HCT might selectively augment GVT activity . An increasing number of minor histocompatibility antigens have now been molecularly characterized, and novel mechanisms of polypeptide processing have been uncovered . Several minor histocompatibility antigens are expressed in both normal and malignant hematopoietic cells of the recipient, but not in epithelium . Thus, donor T cells reactive with such tissue-restricted antigens will target recipient hematopoietic and leukemic cells without damaging non-hematopoietic tissues or engrafting donor hematopoietic progenitors. Techniques for cloning minor histocompatibility antigen–specific T cells have been developed, and these T cell clones can eliminate primary human leukemias in immunodeficient mice . Pilot studies in which cloned donor T cells reactive with minor histocompatibility antigens have been adoptively transferred to treat patients with post-transplant disease recurrence have demonstrated the principle that GVT and GVHD can be segregated based on the tissue expression of the target antigen . The feasibility of this strategy is advancing with refinements in the culture methods used for isolating effector T cells from naïve T cell precursors and programming these effector cells for GVT activity. The identification of candidate leukemia antigens that are not derived from polymorphic proteins provides additional targets for T cell therapy or vaccination that may be broadly applied . Thus, strategies to abrogate GVHD and its complications need not be associated with loss of the GVT effect, but may instead employ targeted immunotherapy to reduce the intensity and duration of post-grafting immunosuppression while augmenting GVT activity.
The human graft-versus-host reaction continues to both fascinate and frustrate clinical investigation with its lack of predictability, possibly lethal toxicity, but potentially lifesaving anti-tumor effects. The ability to select optimal donors based on improved HLA-typing technologies and better understanding of non-HLA contributions to immune reactivity seems at hand, and should substantially reduce the risk of developing severe GVHD. Clinical studies of donor graft manipulation removing TN subsets, retaining TM subsets, and possibly supplementing TREG subsets are just being initiated. In the long term, perhaps the most exciting potential lies in the segregation of GVHD from the GVT reaction based on increased recognition of minor histocompatibility antigens exclusively expressed on normal versus malignant hematopoietic tissues. Whether these new advances will lead to the hoped-for clinical victories will become apparent in the next few years.