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Lundberg G (1994). Practice parameter for the use of fresh-frozen plasma, cryoprecipitate, and platelets. Fresh-Frozen Plasma, Cryoprecipitate, and Platelets Administration Practice Guidelines Development Task Force of the College of American Pathologists. Journal of the American Medical Association 271, 777–81. Marcus DM (1969). The ABO and Lewis blood-group system. Immunochemistry, genetics and relation to human disease. New England Journal of Medicine 280, 994–1006. Murphy MF (1999). New variant Creutzfeldt-Jakob disease (nvCJD): the risk of transmission by blood transfusion and the potential benefit of leukocytereduction of blood components. Transfusion Medicine Review 13, 75–83. Murphy S, Heaton WA, Rebulla P (1996). Platelet production in the Old World and the New. Transfusion 36, 751–4. Novotny VM (1999). Prevention and management of platelet transfusion refractoriness. Vox Sanguinis 76, 1–13. Pehta JC (1996). Clinical studies with solvent detergent-treated products. Transfusion Medicine Review 10, 303–11. Popovsky MA, Moore SB (1985). Diagnostic and pathogenetic considerations in transfusion-related acute lung injury. Transfusion 25, 573–7. Przepiorka D, et al. (1996). Use of irradiated blood components: practice parameter. American Journal of Clinical Pathology 106, 6–11. Race R (1944). An ‘incomplete’ antibody in human serum. Nature 153, 771. Reid ME, Yazdanbakhsh K (1998). Molecular insights into blood groups and implications for blood transfusion. Current Opinions in Hematology 5, 93– 102. Schreiber GB, et al. (1996). The risk of transfusion-transmitted viral infections. The Retrovirus Epidemiology Donor Study. New England Journal of Medicine 334, 1685–90. Silliman C (1999). Transfusion-related acute lung injury. Transfusion Medicine Review 13, 177–86. Snyder EL (1995). The role of cytokines and adhesive molecules in febrile nonhemolytic transfusion reactions. Immunological Investigations 24, 333–9. Turner ML, Ironside JW (1998). New-variant Creutzfeldt-Jakob disease: the risk of transmission by blood transfusion. Blood Review 12, 255–68. Vengelen-Tyler V, ed (1996). Technical manual, 12th edn. American Association of Blood Banks, Bethesda. Vogelsang GB, Hess AD (1994). Graft-versus-host disease: new directions for a persistent problem. Blood 84, 2061–7. Wandt H, et al. (1998). Safety and cost effectiveness of a 10×10(9)/L trigger for prophylactic platelet transfusions compared with the traditional 20×10(9)/L trigger: a prospective comparative trial in 105 patients with acute myeloid leukemia. Blood 91, 3601–6. Wiener A (1943). Genetic theory of the Rh blood types. Procedings of the Society for Experimental Biology Medicine 54, 316. Williamson LM, Warwick RM (1995). Transfusion-associated graft-versus-host disease and its prevention. Blood Review 9, 251–61. Winslow RM (1999). New transfusion strategies: red cell substitutes. Annual Review of Medicine 50, 337–53. Yamamoto F, et al. (1990). Molecular genetic basis of the histo-blood group ABO system. Nature 345, 229.

row was obtained from an identical twin, such attempts in humans universally failed until a clear understanding of the immune processes involved in tolerance and rejection became available. Much of the pioneering work in making possible human bone marrow transplantation was carried out by E. Donald Thomas and colleagues in the United States, work for which Thomas received the Nobel Prize jointly in 1990. In the post-Second World War era, experiments on inbred mice showed that lethally irradiated animals could be rescued by transfusion of bone marrow from unirradiated mice and that this protection was the result of engraftment of the normal marrow in the recipient. Successful engraftment depended upon the donor marrow being genetically acceptable by the recipient mouse or the recipient mouse being sufficiently immunosuppressed. Successful engraftment when there was immunological disparity between the donor and recipient was followed after a period of 2 weeks or so by a ‘secondary’ disease in which the recipient failed to thrive and developed gastrointestinal disorders and skin abnormalities manifest by poor further development and eventual death from infection. This so-called ‘runt disease’ is the murine equivalent of graft-versus-host disease (GVHD) in humans in which immunocompetent cells from the immunologically disparate donor mount an attack against recipient tissues. From these and other experiments in outbred animals it was recognized that transplantation of bone marrow would carry the special risk of GVHD and that histocompatibility would be a critical requirement for successful transplantation. Further work in animals demonstrated that certain treatments, in particular total body irradiation and cyclophosphamide, were sufficiently immunosuppressive to permit engraftment, and that GVHD could be controlled to some extent, where there was not great disparity between the histocompatibility antigens of donor and recipient, with methotrexate. The elucidation of the major histocompatibility locus on chromosome 6 in humans, with the identification of the histocompatibility antigens at the A, B, or C (class I) and DR (class II) loci of the HLA system, finally allowed the identification of appropriate donors for human transplantation. The paramount importance of histocompatibility in haemopoietic stem cell transplantation has been confirmed subsequently by extensive clinical practice. The first successful transplant from a non-identical, but HLA compatible, sibling was carried out in 1968 for a patient with severe combined immune deficiency where the underlying disease prevented rejection. Successful allogeneic transplantation from sibling donors in patients who required conditioning with total body irradiation and cyclophosphamide to permit engraftment was carried out in 1969 in Seattle by the group led by Thomas. Many thousands of such transplants have been carried out subsequently, though it would be fair to say that the indications for transplantation, particularly in malignant disease, are not always as clear as they might be and the problems of GVHD, graft failure, and infection remain hazards which contribute to transplant-related mortality. On the other hand, better support with blood products and antibiotics, improved tissue typing techniques, and the introduction of less toxic ways of controlling rejection and GVHD, as well as better selection of recipients, have improved outcomes steadily over the last 30 years.

22.8.2 Haemopoietic stem

Histocompatibility complex and haemopoietic stem cell transplantation

cell transplantation E. C. Gordon-Smith Introduction The idea that haemopoietic stem cells from the bone marrow could be transferred from a normal individual to a patient to replace defective bone marrow has a long history. With the exception of rare instances where mar-

The organization of the major histocompatibility complex (MHC) on chromosome 6, and its importance in transplantation, is described in detail in Chapter 5.7. The closeness of the relevant genes in the complex means that within families there is little crossing-over in germ line cells and inheritance more or less follows the autosomal pattern, so that the chances of a sibling having the same HLA type as a patient is about 1:4. This is genotypic identity, in which many unidentified sequences are identical by descent between siblings. At each HLA locus there are large numbers of possible alleles in humans leading to a potential of many millions of different histocompatibility profiles. However, within populations, certain HLA alleles

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tend to be associated and segregate together, ‘genetic disequilibrium’, so that it is theoretically and practically possible to find phenotypically identical pairs within an unrelated population. The identification of phenotypes was originally based upon serological testing for A, B, and DR antigens. The introduction of molecular techniques for identifying DNA sequences directly has shown that there may be a large number of HLA gene products whose cognate protein molecules are assigned to the same phenotype by serological methods. Some of these differences are moreover of considerable importance in terms of immunological incompatibility. These observations on the MHC within the population have made possible the establishment of large volunteer donor pools of individuals prepared to supply haemopoietic stem cells, but also highlight the difficulties of unrelated transplants from an immunological point of view. Indeed, even where there appears to be close identity in an unrelated pool, there are likely to be many fine genetic variations. Selection of donors by improved typing techniques has reduced the risks associated with unrelated transplants, but selection has also restricted the range of appropriate donors. It has also become clear that there are very wide variations in the linkage disequilibria at MHC loci between different populations of the world so that a donor pool of one ethnic type may have a much reduced chance of providing donors for another. Where there is a histocompatibility disparity between donor and recipient, haemopoietic stem cell transplants may be possible, but the incidence of complications rises steadily as the degree of disparity increases. It is also apparent that the antigens of the MHC are not the only antigens which are important in determining the presence or absence of GVHD. GVHD is mediated by CD4+ and CD8+ cytotoxic T lymphocytes, but the role of specific HLA antigens and minor antigens in determining the attack, and the part played by recipient antigens in susceptibility to the disease, have not been worked out in detail. As discussed later, the immunological attack on normal tissues which produces GVHD seems to be linked to an ability to attack abnormal tissues, particularly malignant, producing a graft-versusleukaemia (GVL) effect. Much effort has gone into trying to identify the cells which mediate GVL and to see if they can be separated from those that produce GVHD. So far the results are inconclusive. The problems and benefits of immunological disparity obviously only apply in the allogeneic transplantation procedures and are absent when autologous stem cells are used to restore haemopoiesis after intensive chemotherapy.

Haemopoietic stem cells The idea that there was a cell in the haemopoietic system which was capable of giving rise to all lineages of the haemopoietic system for life through a process of self-renewal, proliferation, and differentiation of progeny became current in the early part of the twentieth century. Experiments by Till and McCulloch in mice demonstrated that there were individual cells which could give rise to colonies of different haemopoietic lineages in the spleen of irradiated and transplanted mice. Subsequently it was shown that the passage of small numbers of early precursor cells could repopulate the haemopoietic system serially in lethally irradiated mice. It seems probable that a single stem cell can repopulate an entire animal in terms of haemopoiesis and the immune system. In animals, stem cells can be identified by immunophenotyping, purifying this population of cells, and showing that they are capable of haemopoietic reconstitution in a series of lethally irradiated animals. Such experiments in humans are impossible, but the best in vitro techniques have suggested that the human haemopoietic stem cell is closely related to precursors that carry an antigen designated CD34, lack other haemopoietic markers including CD33, and have no lineage-specific markers. Whether such cells are truly the most primitive cells that are capable of giving rise to both haemopoetic and immunological precursors is not of practical importance since successful haemopoietic reconstitution, both in allogeneic and autologous transplants, is closely related to the number of such cells present in the donation. The CD34+, CD33- cells represent

some 1 × 10–3 to 10–4 of the cells of normal human haemopoietic marrow.

Sources of haemopoietic stem cells In the first 20 years or so of haemopoietic stem cell transplantation virtually all donations were collected from the bone marrow. Animal experiments had demonstrated that marrow infused intravenously into a recipient was capable of repopulating the marrow and this method of delivery was practised from the beginning in human transplantation. Within normal marrow, haemopoietic stem cells are located in specific areas, usually close to the bony trabeculas in the haemopoietic spaces. The observation that marrow infused into the circulation could find its way to the marrow cavity indicated that haemopoietic stem cells were capable of trafficking through the circulation and homing to the appropriate part of the marrow microenvironment. It was also recognized that there were small numbers of stem cells in normal circulating blood, and that this number was increased during the marrow recovery following cytotoxic chemotherapy. The discovery of haemopoietic growth factors and their subsequent production by recombinant technology led to their use in clinical practice. Administration of many of these cytokines, particularly granulocyte colony-stimulating factor (G-CSF), granulocyte–macrophage colony-stimulating factor (GM-CSF), and stem cell factor increases the number of circulating colony-forming cells and CD34+ cells enormously, such that for a period of a few days following treatment there would be more than adequate numbers in the circulation to use as a source of transplant cells. Homing and mobilization of haemopoietic stem cells seems to be a continuous, dynamic process— even under normal conditions. In the early development of the fetus, haemopoiesis takes place in the liver, and fetal liver cells have been used as a source of haemopoietic stem cells, mainly for the treatment of inherited disorders characterized by severe combined immune deficiency. The logistics of such transplants, which require 11-week-old fetal livers, make this an impractical approach. However, research on embryonic stem cells suggests that there may be other important sources of stem cells, not only for haemopoiesis but for other types of tissue replacement. Of more immediate practical importance was the finding that cord blood contained large numbers of haemopoietic cells with high proliferative potential and characteristics of stem cells. Cord blood has become a third practical source of donor cells. Each of these sources—bone marrow, peripheral blood, and cord blood—have advantages and disadvantages that impinge on clinical management. A critical requirement for successful transplantation is that there should be a sufficient number of stem cells—the ability to expand stem cells ex vivo would solve this and other requirements, but so far this has not proved to be practical for clinical use.

Haemopoietic stem cells from bone marrow Until about 1993 most transplants were conducted using bone marrow stem cells. Much of the data concerning the success and problems of stem cell transplantation are derived from the use of bone marrow and this remains the principal source of stem cells in allogeneic transplants. Bone marrow is harvested with the patient under general anaesthetic by aspiration from the posterior, superior iliac crests, and if necessary the sternum. Experience showed that some 3 × 108 nucleated cells/kg from the bone marrow were required for successful engraftment and this usually involved collecting 1 to 1.5 litres of bone marrow mixed, of course, with blood. Donors usually have a unit of blood collected before harvesting, which is returned at the end of the procedure to ameliorate the anaemia. The procedure takes 1 to 2 h and the donor usually requires brief admission to hospital to recover. Serious complications are very rare and are those associated with the general anaesthetic or local complications such as osteomyelitis or abscess formation. The advantage of this source of stem cells from the donor’s viewpoint is that collection is rapid with a maximum of

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48 h involvement. The disadvantage is the need to have an anaesthetic and the pain or discomfort that follows the procedure.

Haemopoietic stem cells from peripheral blood Haemopoietic stem cells may be mobilized into the peripheral blood following exposure to granulocyte colony-stimulating factor. For allogeneic transplantation, donors receive G-CSF (filgrastim or lenograstim) at a dose of 10 µg/kg subcutaneously daily for 5 days. The peripheral granulocyte count rises to 30 × 109/l or higher and CD34+ cells appear in the peripheral blood reaching a maximum 5 to 6 days after the start of treatment. Leucocytes are collected by cytopheresis with the objective of reaching more than 2 × 106 CD34+ cells/kg body weight of the recipient. Sufficient cells can usually be collected in one procedure. Attempts to increase the circulating stem cell concentration still further using additional cytokines, such as stem cell factor, have not proved to be sufficiently safe for general use. The main disadvantages for donors of this type of stem cell collection is that of bone pain or ache following the injections of G-CSF and the procedure of cytopheresis. The advantage is the avoidance of admission to hospital and an anaesthetic. When autologous collection of stem cells is required, the concentration of CD34+ cells may be increased further by giving cyclophosphamide (or some other chemotherapeutic agents, such as etoposide) before starting the G-CSF. The recovery from the marrow suppression so produced leads to mobilization of stem cells even without G-CSF. This procedure is used mainly for patients with malignant disease for whom the stem cells can be used to rescue them from the effects of further chemotherapy. The use of peripheral blood for harvesting stem cells for allogeneic transplants provides high numbers of CD34+ cells and more rapid engraftment than that seen with bone marrow-derived stem cells. On the other hand, peripheral blood contains more T cells than bone marrow and, whilst original concerns that acute GVHD would be unacceptably severe unless T cells were removed has not proved to be the case, chronic GVHD does seem to be more prevalent. Nevertheless, the ease of collection and advantages of rapid engraftment have meant that most autologous transplants, and an increasing proportion of allogeneic, are sourced from the peripheral blood.

Haemopoietic cells from umbilical cord blood Sourcing haemopoietic stem cells from umbilical cord blood has several theoretical and practical advantages. Umbilical cord blood is widely available with no risk to mother or infant, there is low viral contamination, the immaturity of the immune cells reduces the risk of GVHD, and the cells may readily be stored frozen. Furthermore, a balance of umbilical cord blood stem cells from different ethnic groups to take advantage of genetic disequilibrium can be achieved and specific HLA types can be targeted. A disadvantage is the relatively small numbers of haemopoietic stem cells that are present, so that cells derived from umbilical cord blood are mainly suitable for child recipients rather than adults; a further difficulty is the lack of any back-up source of cells should the transplant fail or relapse occur. There is also the theoretical risk that the umbilical cord blood stem cells carry some latent genetic defect which might appear years after the transplant.

Plasticity of stem cells It has become apparent that there are present in the bone marrow, and in other tissues, cells which are totipotent in their capacity to develop into differentiated cells depending upon the molecular and cellular microenvironment to which they are exposed. Thus bone marrow-derived cells may differentiate to cardiac muscle cells, nerve cells, striated muscle fibres, and many other tissues, whether they be ectodermal, mesodermal, or endodermal in origin. This potential is also present in embryonic stem cells. The

reconstitution of a whole animal from a single somatic nucleus reinserted into an enucleated oocyte (cloning) is the ultimate indication of plasticity. In the future, haemopoietic stem cells may be used to repair neurological or muscle defects and other sources of stem cells used to prepare haemopoietic deficiencies.

Donors for allogeneic stem cell transplantation Problems of transplant-related morbidity and mortality, graft rejection, GVDH, and infection increase with increasing donor disparity. HLAmatched sibling donors are not only phenotypically matched for the MHC, but have genotypic identity throughout most of the MHC. This does not eliminate transplant-related morbidity and mortality, but reduces the incidence and severity of the problems compared with unrelated volunteer donors matched phenotypically for the MHC. Sibling donors are therefore preferred. Same sex donors are more successful than mismatched, and transplantation from male donors is more successful than female. HLAmatched sibling donors are only available for about 1 in 3 recipients in populations with an average of two or three children per family. To overcome this shortfall, volunteer donor banks have been established, now including some 3 million typed donors worldwide. This pool can provide HLA-suitable matches for about 80 per cent of recipients with the same genetic disequilibrium as the donor pool, though finding the right match may take several weeks. Even with fully matched donors, either sibling or volunteer, extensive immunosuppression of the recipient is required pretransplant to prevent graft rejection and post-transplant to control GVHD. New methods of immunosuppression which allow the stepwise development of donor marrow may produce a greater degree of tolerance and permit successful engraftment of haemopoietic stem cells with some degree of HLA disparity. Volunteer donor stem cells were the source of about a quarter of all allogeneic transplants in 1999 and this proportion is increasing. Stem cells from umbilical cord blood banks have been used successfully in transplants for genetic abnormalities, particularly Fanconi anaemia, and also for children with malignant disease. However, this source has proved difficult for adults mainly because of the low numbers of stem cells in the cord blood.

Management of recipients for haemopoietic stem cell transplantation The treatment of recipients pretransplant includes measures to induce immunosuppression and irradication of diseased bone marrow. This was the theory behind the so-called conditioning regimens used during the first 30 years of stem cell transplantation. For haemopoietic stem cell transplantation for malignant disease, most protocols contained cyclophosphamide combined either with total body irradiation (single dose or fractionated) or with busulphan. For non-malignant conditions, particularly acquired aplastic anaemia, cyclophosphamide in higher dosage, either alone or combined with antilymphocyte globulin (ALG), was the major immunosuppressive agent. Some of the more widely used regimens are indicated in Table 1. The incidence and severity of GVHD was reduced by giving methotrexate intermittently post-transplant. The introduction of cyclosporin to reduce graft failure and ameliorate GVHD greatly improved the results of transplantation. Such conditioning regimens, particularly for malignant and genetic disorders, carry considerable delayed as well as acute toxicity, particularly for children. Where radiation is used and to a lesser extent busulphan, infertility is usual, growth is retarded, and other endocrine functions may be impaired. Late onset of solid tumours also occurs.

22.8.2

haemop oiet ic stem ce ll t r anspl antat ion

Table: 1 Outline of examples of conditioning regimens for allogeneic haematopoietic stem cell transplantation Indications Myeloablative Cyclophosphamide at 120 mg/kg + TBI of 750 to 1400 cGy

Acute leukaemia Chronic myeloid leukaemia Relapsed lymphoma

Cyclophosphamide at 120 mg/kg + busulphan at 16 mg/kg

As above Thalassaemia major Other congenital bone marrow disorders

Cyclophosphamide at 200 mg/kg ± antilymphocyte globulin (ALG)

Acquired aplastic anaemia

Cyclophosphamide at 25 to 100 mg/kg + TBI of 200 cGY

Fanconi anaemia

Non-myeloablative Fludarabine at 30 mg/m2 + ALG or Campath 1H + low-dose cyclophosphamide or melphalan

Fanconi anaemia Congenital disorders of haemopoiesis or immune system Acquired aplastic anaemia Malignant disorders

With DLI

TBI, total body irradiation. Lower doses given in single fraction, higher doses fractionated. DLI, donor lymphocyte infusions. Given 6 weeks or longer postinfusion to provide GVL or graft-versus-tumour effect.

Where transplantation was used for patients who had already received irradiation or chemotherapy to the central nervous system, for example patients with a relapsed acute lymphoblastic leukaemia, intellectual impairment as well as the above problems are common. Subsequently it has been recognized that much of the success of stem cell transplantation in certain malignant conditions, most notably chronic myeloid leukaemia but also acute myeloid leukaemia, is related to the

13

immunosuppressive attack (GVL), provided by donor lymphocytes. Likewise the repopulation of marrow by donor haemopoietic stem cells does not require the immediate abolition of recipient marrow. Conditioning regimens have been introduced which do not rely on cytotoxic measures to obliterate recipient marrow and immune system, but which have increasing immunosuppressive effects to allow the gradual reintroduction of donor marrow. Such regimens include fludarabine, a highly immunosuppressive drug that is not very cytotoxic, together with antilymphocyte globulin or monoclonal antibodies that have a specific immunosuppressive effect. Depletion of T cells in the donor preparation, with subsequent later addback of donor lymphocytes, is also employed. Some examples of these so-called non-myeloablative regimens are included in Table 1. Results using this approach have been encouraging, but long-term follow-up will be necessary to confirm these advantages. Removal of T cells from donor preparations has long been used as a way of preventing GVHD. Unfortunately, survival rates are not generally improved by T-cell depletion. The benefit of reducing GVHD is balanced by an increasing graft failure and, in malignant disease, by an increase in cancer relapse.

Graft-versus-host disease (GVHD) Acute GVHD may develop at any time within the first 6 weeks post-transplant. The typical features and classification of severity are shown in Table 2. Grades III and IV of GVHD are an important cause of transplantrelated morbidity and mortality. The immunosuppressive effect of GVHD may lead to reactivation of latent viruses, particularly cytomegalovirus, as well as death from fungal or bacterial infections. Liver failure, catastrophic diarrhoea, and gastrointestinal haemorrhage are other direct causes of death from GVHD. Chronic GVHD mainly affects the skin. It may follow acute GVHD or arise de novo 6 weeks or so post-transplant. The rash may vary from a mild dryness of the skin in localized areas to a major extensive scleroderma-like illness with progressive ulceration and scarring. Extensive and chronic GVHD is associated with a poor outcome. Examples of acute and chronic GVHD are shown in Fig. 1.

Table: 2 Clinical analysis of acute graft-versus-host disease Clinical staging: Stage

Organ involvement* Skin

Liver

Gut

Bilirubin 30 to 50 µmol/l

Diarrhoea 0.5 to 1 litre/day and/or persistent nausea Diarrhoea 1 to 1.5 litre/day

+++

Maculopapular rash < 25 per cent of body surface Maculopapular rash 25 to 50 per cent of body surface Generalized erythroderma

++++

Desquamation and bullae

+

++

Clinical grade: Grade 0 (none) I (mild) II (moderate) III (severe) IV (life threatening) *Confirmation may require biopsy.

(2 to 3 mg/dl) Bilirubin 50 to 100 µmol/l (3 to 6 mg/dl) Bilirubin 100 to 250 µmol/l (6 to 15 mg/dl) Bilirubin > 250 µmol/l (> 15 mg/dl)

Diarrhoea > 1.5 litre/day Pain +/– ileus

Stage Skin

Liver

Gut

Functional impairment

0 + to 2+ + to 3+ 2+ to 3+ 2+ to 4+

0 0 + 2+ to 3+ 2+ to 4+

0 0 + 2+ to 3+ 2+ to 4+

0 0 + 2+ 3+

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Amelioration of GVHD with cyclosporin plus or minus methotrexate has already been discussed and the role of T-cell depletion mentioned. The

optimal regime for prevention has yet to be elucidated. Treatment depends on the use of corticosteroids together with specific anti-T-cell monoclonal

(b) Acute

(a) Acute

(a) Chronic

(b) Chronic

(c) Acute

(c) Chronic

Fig. 1 Skin manifestations of acute and chronic graft-versus-host disease. Acute GVHD: (a) Grade I, skin +, showing typical palmer maculopapular rash (recovered); (b) Grade IV, skin 4+, generalized erythroderma with early exfoliation; liver 3+, bilirubin > 250 µmol/l (fatal); (c) Grade III, skin 4+, bullous desquamation (recovered). Chronic GVHD: (a) Scleroderma-like plaques on hands; (b) Sclerotic scarring on back; (c) Severe ulceration and contracting scleroderma-like skin involvement.

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haemop oiet ic stem ce ll t r anspl antat ion

antibodies. Management of chronic GVHD with thalidomide has also been tried.

The observation that patients with leukaemia, particularly chronic myeloid leukaemia, who had allogeneic transplants and developed acute and/or chronic GVHD had considerably less relapse, though not better survival, than patients without GVHD led to the idea that there was a specific GVL effect (Fig. 2). This was confirmed when it was found that patients with chronic myeloid leukaemia who relapsed post-transplant could be put back into cytogenetic and molecular remission by giving them donor lymphocytes in increasing dosage. Sometimes this was associated with an increase of GVHD, but by no means in every case. There seems to be a hierarchy of susceptibility to GVL effect: chronic myeloid leukaemia being the most clear-cut, some effect in acute myeloid leukaemia, less in acute lymphoblastic leukaemia, and uncertain in lymphoma and myeloma. It is not yet clear whether the cells responsible for GVL are identical to those which produce GVHD or whether it is a separate population. Donor lymphocyte infusions now form part of the management plan post-transplant both for the management of relapse and for some of the non-myeloablative regimes.

Indications for haemopoietic stem cell transplantation The indications for haemopoietic stem cell transplantation fall broadly into two groups. In the first, donor stem cells are used for replacement therapy—a rather crude form of gene therapy for inherited disorders and the re-establishment of marrow function in non-malignant bone marrow failure syndromes. The main indications in this group are shown in Table 3. In the second group, donor stem cells are used as an adjunct to chemotherapy, both through additional cytotoxicity and biological modification through the GVL effect, in malignant disease. It is in this group that uncertainties remain as to the most appropriate timing as well as effectiveness of allogeneic transplantation. Randomized controlled trials have proved difficult to mount and much of the evidence is placed upon registry data or historical controls. At the same time that the results of haemopoietic stem cell transplantations have improved, the results of chemotherapy have also become better. Nevertheless, particularly in children and younger adults, allogeneic transplantation is widely used with some success, particularly for

Probability of survival %

100 HLA-identical sibling, ≥20y (N = 239)

80 60

Solid tumours

Bone marrow failure syndromes

Congenital disorders: Haematological

Immunological Metabolic

Acute leukaemias Chronic myeloid leukaemia Non-Hodgkin’s lymphoma Hodgkin’s lymphoma Myeloma and other plasma cell dyscrasias Chronic lymphocytic leukaemia Malignant teratoma Ewing’s sarcoma Renal cell carcinoma Myelodysplasias Myeloproliferative disease Aplastic anaemia Paroxysmal nocturnal haemoglobinaemia Fanconi anaemia Diamond Blackfan anaemia Kostmann’s syndrome Severe combined immune deficiency Malignant osteopetrosis Lysosomal diseases

*Stem cell transplantation may be considered an option according to availability of a suitable donor, the stage or severity of the disease, and the availability and effectiveness of other forms of management.

relapsed conditions. There is a very marked inverse relationship between success of transplantation and age, children having much less transplantrelated morbidity and mortality due to reduction in infection and GVHD. Children also tolerate a higher degree of HLA mismatching than adults. The upper age limit for allogeneic transplant has continued to rise as results improve and in some conditions where transplantation is the only hope of cure, for example chronic myeloid leukaemia, patients aged more than 60 years have been successfully transplanted. However, the transplantrelated morbidity and mortality at this age is very marked. As would be expected, results of allogeneic transplantation are best in low-risk groups, in first complete remission or with chemosensitive disease, and are worst in relapsed and resistant disease. However, it was in this last group that the potential benefits of allogeneic transplantation were first clearly demonstrated by Thomas and his group in Seattle. In most protocols for the management of leukaemias the inclusion of allogeneic transplantation, where a suitable sibling donor is available, is considered either up-front or as a form of rescue in younger patients. The results of unrelated donor transplants consistently lag behind those of matched sibling donors and whilst HLA antigen-mismatched stem cells are used in desperate situations, success rates decline as transplant-related morbidity and mortality increases.

HLA-identical sibling, ≥20y (N = 239)

Indications for autologous transplantation

Unrelated, ≥20y (N = 239)

40

Unrelated, ≥20 (N = 71) 20 P = 0.0001 0 0

Table: 3 Main disorders for which haematopoietic stem cell transplantation may be appropriate* Malignant disorders: Haematological malignancies

Graft versus leukaemia (GVL)

15

1

2

3 Years

4

5

6

Fig. 2 Probability of survival after allogeneic bone marrow transplantation for severe aplastic anaemia by donor type and recipient age (1991 to 1997). Data from 2064 transplants from the International Bone Marrow Transplant Registry, reproduced with permission.

The use of autologous haemopoietic stem cells for treatment of malignant disease can only be considered a form of rescue from increased chemotherapy since the allogeneic effects which produce GVL do not exist. Where there may be tumour antigens that are amenable to immune suppression, attempts have been made to induce specific immunotoxicity, so far without clear-cut benefit. On the other hand, autologous stem cell rescue does allow greatly increased chemotherapy regimens for lymphoma, myeloma, and a variety of solid tumours with shortening of hospital stay—indeed in some cases treatment can be managed in an outpatient setting—and a prolonged course of therapy with repeated rescue from stored cells. Autologous stem

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cells will also provide the vehicle for gene therapy once techniques for gene insertion and long-term expression become practical.

Future directions for haemopoietic stem cell transplantation Transplant-related morbidity and mortality should continue to decline as management of infections, particularly viral and fungal infections, improve. Undoubtedly the plasticity of totipotent stem cells will be explored to treat non-haemological or oncological conditions and both autologous and allogeneic stem cells will be used for specific gene therapy for both acquired and inherited disorders.

Further reading Laughlin MJ (2001). Mini-Review. Umbilical cord blood for allogeneic transplantation in children and adults. Bone Marrow Transplantation 27, 1–6. Przepiorka D et al. (1995). 1994 consensus conference on acute GVHD grading. Bone Marrow Transplantation 15, 825–8. Rubinstein P et al. (1998). Outcomes among 562 recipients of placental blood transplants from unrelated donors. New England Journal of Medicine 339, 1565–77. Thomas ED, Blume KG, Forman SJ, eds. (1999). Haematopoietic cell transplantation, 2nd edn. Blackwell Scientific Inc., Malden MA.