Haematopoietic progenitor or stem cell transplantation (HSCT) refers to any process in which haemopoietic stem cells are given to a patient with the intention of repopulating the patients’ haemopoietic system either in total or in part. HSCT has been used ever since it was discovered that leukaemia could be treated with radiation and the patient could be rescued from the subsequent aplasia with the infusion of stem cells from a compatible donor.
The main indications for HSCT include the haematological malignancies, including amongst others, the Acute and Chronic Leukaemia’s, Myloproliferative disorders, Myelodysplastic syndrome, the Lymphomas (Hodgkin’s and non-Hodgkin’s), Multiple Myeloma, some non neoplastic disorders such as Aplastic Anaemia, some autoimmune diseases, immunodeficiency’s, haemoglobinopathies, inborn errors of metabolism as well as some solid tumours.
There are potentially three main sources of haematopoietic stem cells for transplantation – bone marrow (BM), peripheral blood stem cells (PBSC) and umbilical cord blood stem cells. Bone marrow (BM) is traditionally harvested from multiple well spaced sites on the iliac crests under general anaesthetic. Collection usually aims for a minimum CD34+ count of 2 x 106/kg patient weight. Peripheral blood stem cells are collected after being mobilised from the marrow with granulocyte colony stimulating factor (G-CSF). Collection usually aims for a minimum CD34+ count of 7 x 106/kg patient weight. Umbilical cord blood stem cells are collected after birth and banked in a state ready for issue. Collection usually aims for a minimum Total Nucleated Cell (TNC) count of 100 x 107. Cord blood can be sourced from cord blood banks in single or double doses.
The first successful cord blood transplant was performed in a patient with Fanconi Anaemia by Gluckman et al in 1988 using cord blood stem cells from a HLA matched sibling. The patient is still alive and well today. This success prompted the setting up of the first unrelated cord blood bank in New York by Rubinstein et al in 1991. The number of unrelated cord blood banks around the world has steadily expanded since then and today there are over 50 public cord blood banks worldwide. Cord blood banking involves the collection, processing, testing, banking, registration, selection and release of cord blood unit under strict quality controlled conditions for ultimate transplantation.
Organisations setting up Cord blood banks do so either in or associated with hospitals with large maternity units, often aiming for units with more than 5,000 births a year. A typical strategy in countries with a predominantly Caucasian population is to work with hospitals based in regions with a large percentage of minority ethnic mothers. This increases the genetic diversity of the Cord Blood bank over that typically seen in the adult stem cell donor registries. Collection of cord blood from mothers requires full informed consent. In European countries, the European Union Tissues and Cells Directive applies. This states under the heading of Consent that ‘The procurement of human tissues or cells shall be authorised only after all mandatory consent or authorisation requirements in force in the Member State concerned have been met’. Umbilical cord blood stem cells are collected post partum either in utero in the delivery room during the third stage of labour before the placenta is delivered or outside the delivery room ex utero from the freshly delivered placenta. Collection ex utero does have a higher risk of introducing microbial contamination. In general, the in utero collection method yields a larger volume and higher total nucleated cell count, though more recent studies have shown that with appropriate training it is possible to obtain high collection volumes and cell counts ex utero.
Processing mainly involves volume reduction to optimise freezer storage facilities. Volume reduction involves removal of red blood cells and plasma, leaving the stem cell in a buffy coat of around 21ml. Semi automated closed systems such as the Sepax system are now available for processing of cord blood units. Volume reduced units have the additional advantage over whole units of requiring much less DMSO for freezing and can, depending on the size of the recipient, be transfused without having to first wash the unit.
The current practice is to perform a number of tests prior to banking with further tests carried out if and when a unit is reserved for a potential patient. Pre storage tests include full cell counts, especially nucleated red cell counts, TNC and CD34+ counts, HLA typing, ABO Rh testing and bacteriology testing of the cord blood unit. Haemoglobinopathy tests may also be undertaken. Also pre storage, microbiology markers including HIV, HCV, HBV, HTLV and CMV testing is undertaken on the mother. At reservation, the additional tests carried out are in part driven by the requirements of the transplant centre making the reservation and the county that Centre is based in. Additional tests typically include maternal HLA type, confirmatory HLA type of the cord blood unit and microbiological tests on the cord blood unit.
Banking of a cord blood unit typically follows a full medical review of the history, including all processing data and all test results on the mother and the unit. Cleared units are control rate frozen and stored in liquid nitrogen. Units suitable for transplantation are registered with national and international stem cell or dedicated cord blood registries. In the UK, the NHS cord blood bank, which is part of NHSBT and the Anthony Nolan collect and bank cord blood in England. The NHS cord blood units are also registered with NETCORD and Bone Marrow Donors Worldwide (BMDW). NETCORD runs an accreditation program for cord blood banks which many banks around the world are either accredited to or are seeking accreditation to.
Cord blood banking has many advantages, one of the principle ones being the ready availability of cord blood units for stem cell transplantation. Sourcing of adult stem cells from registries can take anything from 3 – 6 months for the search, selection of potential donors, contact with those donors to confirm willingness to donate, performance of confirmatory typing and any additional tests and medical clearance of the donor. Cord blood stem cells have the advantage of being immediately available. Collection is easy and harmless to the donor compared to adult bone marrow or stem cell donation which involve either a general anaesthetic and the risks that posses or the use of GCSF and the unknown long term risks of that process.
The targeting of hospitals with a high percentage of minority ethnic mother means that cord blood banks typically have a higher representation of donations from minority ethnic donors and a higher proportion ‘rare’ HLA types compared to adult stem cell registries. The British Bone Marrow Registry (BBMR) for instance has around 5% of donors registered as being from minority ethnic backgrounds. For cord blood units this figure is 20%. In terms of HLA type, the NHS cord blood bank has a much higher HLA-A/B/DRB1 allele frequency of the rarer HLA types compared to the BBMR.
Transplantation with Cord Blood derived stem cells does lead to a reduced incidence and severity of GvHD due to the relative immaturity of the immune system at birth. This allows less stringent HLA matching criteria for cord blood transplantation with one or two HLA gene mismatches (i.e. 4/6 or 5/6 HLA-A, B, DRB1 mismatches) tolerated. Cell dose may be the more important factor for cord blood transplantation. An emerging consensus is to use cord with no more than 2 mismatches at HLA-A, B and DRB1 (4/6) provided cell dose is greater than 3 x 107/Kg of patient weight.
Cord blood units have a lower risk of transmitting infectious diseases compared to BM and PBSC due to the lack of exposure of the donor.
GvHD is the most frequent post allogeneic stem cell transplant complication and is a major cause of morbidity and mortality. GvHD is a consequence of activation of donor T lymphocytes by recipient antigen presenting cells. It requires three conditions, transplantation of immunocompetent donor cells, histo-incompatibility between recipient and donor and immunocompromised recipient such that the recipient cannot mount an adequate immune response against the donor cells. Transplantation with Cord Blood derived stem cells has a much lower incidence and severity of GvHD compared to BM and PBSC due to the relative immaturity of the immune system at birth. This can be explained by the lower cell numbers and the mostly naive repertoire of cord blood T cells. Lower GvHD is one of the main advantages of cord blood banking as the reduced incidence and severity of GvHD allows for a less stringent HLA match thereby increasing the pool of potential donations for a given patient. The Graft versus Leukemia (GvL) effect does appear to be preserved.
The main disadvantage of cord blood banking is that cord blood units have a much lower Total Nucleated Cell (TNC) count than adult stem cells from either bone marrow or peripheral blood stem cells. Total nucleated cell counts have typical median of 0.3 x 108/kg patient weight and CD34+ cell counts have a typical median of 0.2 x 106/kg patient weight. This is tenfold lower than is typically collected from adult stem cells. Peripheral blood stem cells also have the advantage of being able to collect from the donor over several days until an adequate volume is collected. This option is not available to cord blood banks. This disadvantage is now overcome to a large extent by the use of double cord blood transplants. Double cord blood transplantation involves the transplantation of two cord units together or in series to a patient. Data to date shows no increase in GvHD as a result of using two cord units rather than one. Interestingly, haematopoiesis after double cord blood transplantation is usually sustained by a single cord unit though the criteria that make one unit prevail over the other remains to be clarified.
The other disadvantage of the using a cord unit is that the donor is not available for Donor Lymphocyte Infusion (DLI) should one be required to rescue the patient from a failing graft. In addition, there is a lack of follow up of the donor so the cord blood bank does not know if the donor later develops congenital disorder. The haemoglobinopathy tests obtained at banking do help in this regard.
Engraftment with cord blood haematopoietic stem cells is also slower than with either BM or PBSC, taking up to 35 days to neutrophil engraftment and platelet engraftment can take even longer. During this time the patient has a delayed immune reconstitution with associated risk of infectious morbidity and mortality. Cord blood grafts also have a higher incidence of graft failure compared to BM and PBSC.
The reduced GvHD of cord blood may be accompanied by a reduced Graft versus Leukemia (GvL) effect which would be a disadvantage for cord blood use.
Cord blood transplantation was originally performed as a last resort if adult stem cells were not available and this practice continues though it may be changing. Transplant units now often ask for a cord blood unit search at the same time they are undertaking an adult stem cell donor search. The indications for cord blood transplant are the same as those for adult stem cell transplant provided cell counts are adequate, though results with bone marrow failure syndromes are less satisfactory.
Gluckman et al have recently published criteria on selecting cord blood units for transplantation. In general, patient and cord blood unit should be at least 4/6 HLA matching at -A, -B and -DRB1 loci, with a minimal cell dose infused being > 3 x 107/kg patient weight. Their recommendations for selecting the best unit are, in order of preference, 6/6 HLA-A, B, DRB1 match and > 3 x 107/Kg patient weight; 5/6 HLA-A, B, DRB1 match and > 4 x 107/Kg patient weight; 4/6 HLA-A, B, DRB1 match and > 5 x 107/Kg patient weight. Use of units < 3 x 107/kg patient weight and 3 – 4 HLA mismatches at HLA-A, B, DRB1 is not recommended. If several units with the same degree of HLA match are available, the one matched in DRB1 and higher cell dose should be chosen. No advantage of high resolution class I matching has so far been demonstrated. The patient should be HLA antibody screened if mismatched cord units are being selected to assess the presence of donor specific antibodies. A further consideration when selecting double cord blood units is to keeping the number of mismatches between the patient and each cord unit and between the cord units themselves to a minimum. Finally cell viability and microbiology results must be taken into account. In the UK a graft advisory panel is available to help with selection of cord units.
A number of recent studies comparing cord blood and adult stem cell transplant in children with AML showed similar 5 year disease free survival. In adults transplanted for malignant diseases, the mortality due to delayed engraftment appears to be counterbalanced by a reduction in acute GvHD so that the overall incidence of transplant related mortality, treatment failure and overall mortality are similar in patients receiving cord blood and adult stem cells.
Future challenges for cord blood banking and cord blood transplantation are to develop strategies to reduce the time to engraftment. Use of double cords is helping in that regard. Other strategies under investigation include the co- transplantation of stem cells from a haploidentical sibling and the ex vivo expansion of CD34+ cells. Aggressive early and pre-emptive therapy has been suggested as a means of overcoming the infection rates prior to engraftment.
- You get all the content of this website in one easy to read pdf format
- You get a list of over 300 references in H&I
- You have access to all the articles even when offline
- You can print at your leisure
- You get pull out tables summarising disease associations
- Finally you will be making a valuable contribution to the upkeep of this site.
Please follow me on Twitter @delordson