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Predicting the quality of transplantable cord blood collections through prefreeze and postthaw Apgar scoring

2012; Wiley; Volume: 52; Issue: 2 Linguagem: Inglês

10.1111/j.1537-2995.2011.03501.x

1537-2995

Hal E. Broxmeyer,

Corneal Surgery and Treatments

The field of cord blood (CB) transplantation (T) has progressed rapidly since two reports published in 1989.1, 2 These papers suggested, based on laboratory efforts,1 and proved through a clinical study2 and many follow-up clinical reports,3 that CB collected from a single donor at birth contained transplantable hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) in such numbers that they could engraft an adequately conditioned recipient for clinical efficacy. The past and current status, as well as future prospects for CB biology, banking, transplantation, and regulation, has recently been reviewed in a multiauthored book.4 It is estimated that more than 25,000 recipients have been treated by CBT for the same variety of malignant and nonmalignant diseases previously and currently treated by HSCs and HPCs in bone marrow (BM).5 However, it is very clear that more laboratory and clinical efforts are required to truly optimize the applicability and efficiency of the life-saving treatments provided by CBT. Numerous efforts have gone into determining what constitutes an adequate or optimized engrafting CB collection, especially for CBT in which the recipients are older and/or of a heavier weight and who thus require a higher number and/or quality of HSCs and HPCs.3 In essence, the question being asked since the report of the first CBT2 is how does one determine the potency for engraftment of a CB unit (U; from one collection). Page and colleagues6 now provide a methodologic approach to the design of a scoring system based on a precryopreservation score (PCS), a postthaw score (PTS), and a combined score that takes into account both PCS and PTS, to predict the potency of a CBU. The authors term this scoring system: “the Cord Blood Apgar (CBA)”; it was created from results obtained in 435 consecutive single CB transplants performed under myeloablative conditions at the same transplant center from the years 2000 to 2008, with the study population divided randomly into training and testing sets, respectively, consisting of 299 and 136 CB transplants. It was found that the CBA was strongly predictive of engraftment after CBT, offering the hope that this will translate into a means for widespread usage of a system that will optimize the screening and detection of the most potent CBUs. To place this newly designed CBA scoring system into context of other efforts to determine and pick the best banked CBUs for transplantation, previous efforts have focused on total nucleated cells (TNCs), CD34+ cells, or HPCs (as determined by colony-forming cell [CFC] assays) to determine the potency of CBUs for engraftment.6 However, as correctly noted by Page and colleagues,6“Current selection criteria using TNC and/or CD34+ do not adequately assess potency and fail to discriminate CBUs at increased risk for graft failure.” While CFC assays have shown some evidence for predictive value,6 CFC, as well as CD34+ numbers are highly variable between different centers assessing these criteria, the first which is a functional endpoint and the second which is a cell surface phenotype; this variance is also especially noted when different centers have assessed the exact same CB samples. The investigators assessed PCS and PTS values involving TNCs, CD34+ cells, CFCs, mononuclear cell content, and sample volume, correlated these measurements with neutrophil engraftment and then, based on a magnitude of hazard ratios (that could have been better defined), using univariate analysis, developed a weighted CBA scoring system. Now that this system has been identified using CBUs obtained from different CB banks, but with the transplants with these CBUs being performed in one transplant center, a number of questions need to be addressed. These questions include: 1) how well the CBA scoring system will translate into a potency assessment of CBUs for other CBT centers worldwide; 2) will this system be as valuable for adult CB transplant recipients, as for children or lower weight recipients, as the majority of recipients in the present study were pediatric patients; 3) will the system be equally applicable for all diseases (malignant and nonmalignant) being treated; 4) can this system be of value once current efforts to enhance the engraftment capability of limiting numbers of cells in CB collections become a reality (see below for information on such ongoing efforts); 5) will this scoring system be of value in predicting the winning unit in a double CB transplant; and 6) can other measures be added or used in place of current endpoints to prepare an even more accurate means to assess the potency of cryopreserved CBUs. Since it is the HSC compartment that allows for long-term repopulation, it would be best if scoring systems could better incorporate HSC numbers and their functional capacity into the equation. Currently, some systems incorporate HPCs as assessed by CFC assay. However, HPCs as offspring of HSCs are at best a surrogate assay for HSCs, although this assay has been useful in the past as it is the only functional assessment of cells used thus far to predict CBU quality.1, 3, 5, 6 Unfortunately, the best assay to determine numbers and engrafting activity of human HSCs requires the use of immune-deficient mice, an involved assay in which CD34+ cells or their subsets are injected intravenously into sublethally irradiated mice with a nonobese diabetic-severe combined immunodeficiency (NOD-SCID) genotype (with latest generations of such used mice having an interleukin [IL]-2 receptor gamma chain null phenotype) and the mice assessed for human cell chimerism many months after transplant.7 The self-renewal capacity of the injected cells in primary mouse recipients is tested by taking the engrafted BM and injecting these cells into secondary sublethally irradiated mouse recipients.7 Hence, this assay requires anywhere from approximately 6 months to a year for adequate readout of a long-term marrow engrafting and self-renewing HSCs.4, 7 While CD34+ cells have been used as a clinical marker, it has two distinct disadvantages for optimal assessment of human HSCs. First, CD34+ cells are not a pure population of HSCs, and in fact only a very small percentage of human CD34+ cells are HSCs. The majority of CD34+ cells are HPCs. There is recent encouraging news that the human HSC has been more definitively phenotyped (Lineage−CD34+CD38−CD45RA−Thy1+RholoCD49f+) so that one such phenotyped cell can engraft a NOD-SCID IL-2 receptor gamma chainnull mouse.8 Such phenotyping of human HSCs is too complicated for use in routine clinical assessment, and it may be many years before such a phenotype can be considered for use as a clinical marker. Second, phenotype does not always predict the functional capacity of a cell, although use of a more definitive phenotype of the human HSCs in CB may go a long way to assessing the quantity, if not necessarily the quality, of the HSC. There are other surrogate assays for human HSCs, but they are also based on long cultures in vitro up to 5 or more weeks, and these assays, such as for the long-term culture-initiating cell, most likely do not recognize the long-term repopulating HSC. While other possibilities to quickly identify the quantity and or quality of HSCs are being considered,6 only time will tell if these markers are useful for predicting CBU quality. One of the disadvantages of CB as a source of engrafting HSCs is the delayed time to engraftment of neutrophils and platelets (PLTs) and also immune cells compared to BM, especially compared to that of mobilized peripheral blood. A number of efforts are currently in progress to enhance the time to engraftment of CB cells. These activities include intrabone transplantation (to bypass problems in homing), ex vivo expansion (to increase numbers of HSCs), addition of accessory cells to the CBU (to facilitate engraftment), treatments to fucosylate the donor cells (to enhance homing capabilities of HSCs), inhibition of CD26/dipeptidylpeptidase IV on donor cells or within recipients (to enhance homing to and expansion of HSCs within the BM), and use of prostaglandin E on donor cells (to enhance homing to, and the expansion capacity of HSCs in BM), among other procedures.4, 5 It may be that combinations of procedures that modulate the donor cell population and/or treat the recipient will result in best enhancement of engraftment and recovery of neutrophils, PLTs, and immune cells. How such anticipated results will influence efforts to define potency of CBUs in mathematical derived model systems remains to be determined. The issue of potency of CBUs and the capability to predict those CBUs with the greatest potential for enhanced engraftment is of paramount importance to the future development and success of the field of CBT. The article by Page and coworkers6 has provided us with an interesting scoring system to test and if necessary to use as a basis to measure future improvements. The future of CBT will be bright as long as we do not become complacent and satisfied with the current status of the field and what we already know. Continuing efforts are necessary to improve our understanding of the biology of CB and the functional activities of HSCs and HPCs in CB, including their properties of self-renewal, proliferation, survival, differentiation, and migration (homing). Refinements in future scoring systems that incorporate such new information into enhanced predictive scoring and that also take into account advancements in clinical CBT itself (through donor cell and/or recipient modifying maneuvers) should bode well for the future of CBT. None.