Cord blood comprises antigen-experienced T cells specific for maternal minor histocompatibility antigen HA-1

Beyond HLA: the significance of genomic variation for allogeneic hematopoietic stem cell transplantation

The last 2 years have seen much excitement in the field of genetics with the identification of a formerly unappreciated level of “structural variation” within the normal human genome. Genetic structural variants include deletions, duplications, and inversions in addition to the recently discovered, copy number variants. Single nucleotide polymorphisms are the most extensively evaluated variant within the genome to date. Combining our knowledge from these studies with our rapidly accumulating understanding of structural variants, it is apparent that the extent of genetic dissimilarity between any 2 individuals is considerable and much greater than that which was previously recognized. Clearly, this more diverse view of the genome has significant implications for allogeneic hematopoietic stem cell transplantation, not least in the generation of transplant antigens but also in terms of individual susceptibility to transplant-related toxicities. With advances in DNA sequencing technology we now have the capacity to perform genome-wide analysis in a high throughput fashion, permitting a detailed genetic analysis of patient and donor prior to transplantation. Understanding the significance of this additional genetic information and applying it in a clinically meaningful way will be one of the challenges faced by transplant clinicians in the future.

Exploiting beneficial alloreactive T cells

Although the T-cell response to allogeneic cells is typically regarded as a detrimental phenomenon responsible for rejection of transplanted allografts and graft-vs.-host disease following haematopoietic stem cell transplantation, beneficial components also exist within the alloreactive population. Alloreactive T cells specific for tumour antigens can contribute to the elimination of malignant cells, and alloantigen-specific regulatory T cells can promote transplant tolerance. The challenge is to separate the good from the bad. We review how the identification, isolation and manipulation of beneficial alloreactive T cells has grown from a greater understanding of the molecular basis of the T-cell alloresponse and how alloaggression could be exploited for immunotherapy.

Minor histocompatibility antigens: from transplantation problems to therapy of cancer

The idea of transferring healthy marrow for the therapeutic treatment of the various diseases of the blood and of the immune system made a significant contribution to controlling diseases and to advancing modern clinical sciences. The first series of bone marrow transplantations in the 1960s were confronted with severe complications. It became clear that matching for the human leukocyte antigen (HLA) system between donor and recipient significantly improved the clinical results. Nonetheless, an unacceptable percentage of severe complications remained that is mainly attributable to non-HLA histocompatibility systems, i.e., minor histocompatibility antigens. Observations in the 1970s that minor histocompatibility antigens cause serious problems in human bone marrow transplantation laid the basis for their use as curative antigens in stem cell transplantation to date. Thus, the allo-immune T cell activities caused by minor histocompatibililty antigen disparities between HLA-matched donor and recipient can now be applied for the benefit of the transplant patient.

Non-HLA immunogenetics in hematopoietic stem cell transplantation

Hematopoietic stem cell transplantation (HSCT) provides a unique environment in which to evaluate the role of immunogenetics of both the donor and the recipient to success of the procedure. The central role of HLA matching in HSCT has been established; however, recipients of allogeneic HSCT incur the risk of graft versus host disease (GVHD) even when the donor is a sibling who shares the major histocompatibility genes. Therefore, the perfect HLA match does not represent the optimal genetic make up. Other genetic systems operate and affect the various outcomes of HSCT, including GVHD, infections, transplant-related mortality, and overall survival. Minor histocompatibility antigens contribute to the control of GVHD as well as graft versus leukaemia reactions. In addition, genes controlling inflammatory processes, including cytokines, chemokines and their receptors, can modulate GVHD, and genes from both arms of the immune response (innate and adaptive) are strong candidates for susceptibility factors to infections in allogenic transplantation.

The effect of noninherited maternal antigens in allogeneic transplantation

Confrontation of the unborn child immune system with the noninherited maternal antigens (NIMAs) has a lifelong modulating impact on the immune response of the child against the NIMAs. In this review we summarize the clinical evidence for the existence of the NIMA effect, discuss the possible cellular and molecular basis of the phenomenon, and outline the necessity of further clinical research.

Cytotoxic minor histocompatibility antigen HA-1–specific CD8+ effector memory T cells: artificial APCs pave the way for clinical application by potent primary in vitro induction

Induction of cytotoxic T lymphocytes (CTLs) for treatment of relapsed leukemia after allogeneic stem-cell transplantation is hindered by the laborious and time-consuming procedure of generating dendritic cells for antigen presentation. Artificial antigen-presenting cells (aAPCs) offer the advantage of being readily available in sufficient numbers, thus allowing for a highly standardized in vitro induction of CTLs. We generated aAPCs coated with anti-CD28 antibody (Ab) and either high-density (HD) or low-density (LD) major histocompatibility complex (MHC) class I molecules loaded with HA-1H, a nonapeptide derived from the hematopoiesis-restricted minor histocompatibility antigen HA-1. HD- and LD-aAPCs potently induced HA-1H–specific CD8+ CTLs from untouched CD8+ T cells of healthy donors. CTLs were subsequently purified by magnetic-activated cell sorting. HD- as well as LD-aAPC–induced CTLs exerted high HA-1H–specific cytotoxicity, resembled Tc1 effector memory cells, survived a long time in vitro, and were expanded by a factor varying between 8.2 × 104 and 51 × 104. The T-cell receptor (TCR) repertoire of HA-1H tetramer–positive CTLs was oligoclonal with a prominent usage of Vβ6. The TCR repertoire of tetramer-positive CTLs was distinct from and more restricted than that of tetramer-negative cells. These findings indicate that aAPCs are attractive tools for the ex vivo generation of HA-1H–specific CTLs suitable for immunotherapy of relapsed leukemia.

Detection of Microchimerism by Minor Histocompatibility Antigen HA-1 Allele-Specific Nested Polymerase Chain Reaction

Minor histocompatibility antigens (mHags) can induce T-cell reactivities with important consequences for the graft-versus-leukemia effect and the development of graft-versus-host disease in HLA-matched stem cell transplantation settings. Recently, mHag-specific T cells were also demonstrated in multiparous woman and in solid organ transplant recipients. Microchimeric cells have been detected in the latter settings. To study whether microchimerism is instrumental in the induction and/or maintenance of mHag T cells, we developed an HA-1 allele-specific nested polymerase chain reaction. To optimize and validate the reliability of this method at different levels of microchimerism, serial dilutions of HA-1(H) cells titrated into HA-1(R) cells were tested. We demonstrated that the HA-1(H) allele can be reliably and consistently detected at concentrations as low as 1:10(5) without losing specificity. The developed HA-1-specific nested polymerase chain reaction is an important tool that facilitates the detection of HA-1 microchimerism in various clinical specimens and that promotes investigation of the effects of microchimerism on induction of mHag-specific T cells in the various settings of immunization.

Immunotherapy of cancer through targeting of minor histocompatibility antigens

Minor histocompatibility antigens are allogeneic targets of T-cell mediated graft-versus-tumour effects following allogeneic stem cell transplantation. Recent research has identified several minor histocompatibility antigens as tumour proteins and has also disclosed their unique properties in both the induction and the effector phase of graft-versus-tumour effects. Targeting tumour-specific minor histocompatibility antigens by adoptive immunotherapy will battle against tumour tolerance and evoke allo-immune responses, thereby enhancing graft-versus-tumour effects against leukaemia and solid tumours. Recently acquired knowledge of the role of donor immunisation status, new techniques in the generation of minor histocompatibility antigen-specific cytotoxic T lymphocytes in vitro, and innovative principles in vaccination will help to design clinical trials that exploit minor histocompatibility antigens in the immunotherapy of cancer.

Umbilical Cord Blood Transplantation: A New Alternative Option

Allogeneic hematopoietic stem cell transplantation is a life-saving procedure for hematopoietic malignancies, marrow failure syndromes, and hereditary immunodeficiency disorders. However, wide application of this procedure is limited by availability of suitably HLA-matched adult donors. Umbilical cord blood (UCB) has being increasingly used as an alternative hematopoietic stem cell source for these patients. To date, over 6000 UCB transplant procedures in children and adults have been performed worldwide using UCB donors. Broader use of UCB for adult patients is however limited by the available infused cell dose. This has prompted intensive research on ex vivo expansion of UCB stem cells and UCB graft-engineering including accessory cells able to improve UCB engraftment and reconstitution and for tissue regenerative potential. Recently, two large European and North American retrospective studies demonstrated that UCB is an acceptable alternative source of hematopoietic stem cells for adult recipients who lack HLA-matched adult donors. UCB is anticipated to address needs in both transplantation and regenerative medicine fields. It has advantages of easy procurement, no risk to donors, low risk of transmitting infections, immediate availability and immune tolerance allowing successful transplantation despite HLA disparity.