Keywords

5.1 Cell Source

Haematopoiesis (Fig. 5.1) refers to the production of all types of blood cells including the formation, development, and differentiation of these cells. In adults, haematopoiesis primarily occurs in bone marrow which is contained in the pelvis, sternum, vertebral column, and skull.

Fig. 5.1
A flow chart for multipotential hematopoietic stem cells leads to the common myeloid and lymphoid progenitor. It further lists various cells.

Hematopoiesis in bone marrow. (Adapted from Blood Cell—An Overview of Studies in Haematology Ed: T E Moschandreau 2012 Diagram.pgn A. Rad 2006)

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All blood cells are derived from progenitor stem cells—pluripotent stem cells.

These cells have the capacity for unlimited self-renewal and the ability to differentiate into all types of mature blood cells, starting from the common myeloid or the common lymphoid progenitor. This process occurs continually in order to maintain adequate concentrations of circulating components necessary for normal haematopoietic and immune system function.

Cells in the myeloid lineage, such as red blood cells, platelets and white blood cells, are responsible for haemopoiesis (tissue nourishment, oxygenation, coagulation) and immune functions such as innate and adaptive immunity. The lymphoid lineage components, namely, T cells and B cells, provide the foundation for the adaptive immune system.

Haematopoietic stem cell (HSC) products for autologous or allogeneic transplantation are available from bone marrow, peripheral blood, and umbilical cord blood (UCB) sources.

The role of UCB in allogeneic transplantation remains important due to the relative immunologic naiveté of the donor cells which allow for the use of multiple antigen-mismatched donors, particularly when there is no available HLA matched donor . The relatively low cell dose obtained from UCB limited the use in the adult recipient until relatively recently, with concerns primarily focused on the increased risk of delayed engraftment and a resulting increase in infectious complications. The introduction of ‘double cord transplants’ has to a certain extent ameliorated some of these difficulties by improving engraftment times. Nevertheless, late infections remain a concern, and lack of available donor lymphocytes means that in some settings, other cell sources are often favoured over UCB (Ballen 2013).

Bone marrow was the original source of cells for transplantation prior to the development of granulocyte colony stimulating factor (GCSF) and apheresis collection procedures . Today peripheral blood stem cells (PBSC) have largely replaced bone marrow in both autologous and allogeneic transplantation setting. The rapid engraftment kinetics of PBSC compared to the bone marrow is widely acknowledged. Median times to achieve an absolute neutrophil count greater than 500/μl after autologous PBSC transplantation are approximately 11–14 days in autologous setting (Klaus 2007).

The choice of cell source in allogeneic transplantation may be influenced by a variety of factors including donor availability, donor donation preference, donor and recipient body weight and recipient disease. In the absence of an HLA-identical sibling donor, the search and identification of a suitably matched unrelated donor may take several months. Depending on recipients underlying disease and required timescales for transplantation, related haploidentical donors or cord blood unit(s) might be selected (Ruggeri et al. 2015).

Donor general health and medical assessment are integral to donor selection and must be completed prior to commencement of any patient conditioning therapy to ensure donor and patient safety and a good transplant outcome. Clear exclusion and eligibility criteria are available for reference and guidance (WMDA, 2022) https://share.wmda.info/display/DMSR/WMDA+Donor+Medical+Suitability+Recommendations (accessed Oct 2022).

5.2 Cell Collection

5.2.1 Bone Marrow Collection

Liquid bone marrow is harvested from both posterior iliac crests under general anaesthetic by two operators, one on each side of the donor, who is placed in the prone position. The most significant risk associated with donating bone marrow is that associated with the general anaesthetic, which for the healthy donor, who has undergone a detailed medical assessment is likely to be minimal (Gottschalk et al. 2011). Other known procedure-related risks include pain, bleeding and injury to nerves, for which the donor is counselled as part of the consent process. A review of bone marrow donors enlisted by the National Marrow Donor Programme indicates that serious adverse events are rare 1.34% with a small number of donors experiencing long-term complications (Miller et al. 2008).

Multiple aspirations are performed by each operator on either side of the pelvis with approximately 5–10 ml of liquid marrow blood obtained via each aspiration. The usual total volume harvested should not exceed 20 ml/kg of donor weight or 1600 ml in order to achieve the desired CD34+ cell doses and avoid complications.

Red cell transfusions are rarely indicated in donors undergoing bone marrow harvest. In the event of a significant fall in haemoglobin, iron supplementation may be considered, although this is not required in the majority of cases. Pain at the site of marrow aspiration may last for several days to weeks following bone marrow donation usually requiring simple analgesia (Miller et al. 2008).

5.2.2 Cord Blood Collection

Umbilical cord blood (UCB) is collected from the placental vein after infant delivery. The umbilical cord is cut and clamped; blood is drawn from the cord with a needle and attached bag (sterile venesection kit). The timing of cord clamping after delivery of the infant correlates with the volume of cord blood collected, with earlier clamping relating to greater collection volumes. Cell dose is an important predictor of outcome after UCB transplantation, and many cord blood units are discarded because of small cell doses. Greater cell quantities are obtained from infants with higher birth weight, independent of gender and gestational age. Many cord blood banks reduce the volume of the product by depleting red cells and plasma in order to minimize storage space and reduce possible infusion-related toxicities from mature blood cells contained in unfractioned cord blood units. UCB will maintain viability for a period of at least 15 years if appropriately cryopreserved (Schoemans et al. 2006).

Finding a suitably HLA matched adult donor for patients from diverse or mixed ethnic heritage can prove difficult.

Of the 525 patients undergoing combined searches, 10/10 HLA-matched unrelated donors were identified in 53% of those with European ancestry but only 21% of patients with non-European origins. Of note, UCB searches are able to identify 5–6/6 UCB units for the majority of patients in both groups (Barker et al. 2010).

Availability of UCB as a transplant cell source can significantly improve transplant options in designated groups of patients whose HLA type is uncommon or where potential donors are under-represented on the donor panels. The immature nature of cord blood stem cells allows a greater degree of flexibility when looking at HLA matching and thus the ability to offer a transplant to many individuals who would previously not have been treated.

5.2.3 Mononeuclear Cell Collections

These cells are collected via the process of apheresis – this is a broad term covering the withdrawal and separation of blood into its component layers, allowing some portion to be retained and the remainder returned to the patient or donor. Leukapheresis specifically refers to the separation of the white blood cells layers from circulating blood using the above process .

Peripheral blood stem cells express a CD34 positive (CD34+) marker on the cell surface, CD34 positive cells can be found in the umbilical cord and bone marrow and are also evident on other cells such as mesenchymal stem cells and endothelial progenitor cells to name but a few.

The development of a rapid laboratory test to measure circulating levels of CD34+ cells has been instrumental in the ability to monitor cellular collections and enhance the efficiency of peripheral blood stem cell harvesting in the transplant setting. CD34+ cells account for 1–2% of all bone marrow cells. The concentration in the bone marrow being significantly greater than that in the peripheral blood by approximately 18-fold (Korbling et al. 2001).

Therefore to collect sufficient cells to facilitate a transplant procedure it is necessary to ‘move’ the CD34+ stem cells out of the bone marrow to increase the circulating concentrations in the blood. The movement or mobilisation of haematopoietic stem cells into the peripheral blood can be stimulated by different disease-specific and relatively predictable mobilisation regimens in combination with granulocyte colony stimulating factor (G-CSF) to produce a relatively predictable rise in white cell counts and CD34+ cells for collection.

The collection and separation of peripheral blood mononuclear cells via leukapheresis is the first step in the process for many novel immunotherapies. These cells once collected from the patient then be genetically modified expanded, and activated ex vivo to facilitate an anti-tumour effect once reinfused into the patient (Zhang 2017). This is a complex therapy but essentially involves the reprogramming of the individual’s immune system which can then be used to target their cancer in a personalised manner.

These new treatment modalities include Chimeric Antigen Receptor T -cell therapies (CART-T), Natural Killer cell therapies and cancer vaccine development using dendritic cells.

The demand for apheresis procedures is rapidly increasing in line with increasing demands for these novel targeted therapies. Typically, these cells are collected through the leukapheresis of unmobilised donor or patients for a variety of clinical indications and diseases, depending on specifications, the number of cells required to produce the end product can vary significantly, as can the parameters required from the procedure, such as the amount of blood needed to be processed, product volume requirement and the ability to undertake one or two day collections to achieve a defined end point. Ensuring a good product has been collected can be challenging (Korell et al. 2020).

As we are currently in the early days of establishing collection protocols, the recommendations for achieving optimum efficiency and best practice are yet to reach a consensus. Many groups are working towards the development of guidelines for standardisation in the procurement of cellular starting materials. The collection process, as indicated, requires a steady state unstimulated apheresis procedure which can present its own challenges. Patients often present for collection with advanced and progressive disease, poor blood counts from previous treatments and poor venous access (Qayed 2022).

5.3 Mobilization of Stem Cells and Apheresis

5.3.1 The Role of CD34+ Cells

CD34+ is the cell surface marker most frequently used in clinical practice to determine the extent and efficiency of peripheral blood stem cell collections (Brando et al. 2000). Target collection endpoints can vary between treating centre though, broadly, these are based on the underlying disease, source of stem cells, and the type of planned transplant. In general, a target level of 2 × 106 CD34+ cells/kg recipient body weight is considered the minimum for transplant with optimal levels being >5 × 106 CD34+ cells/kg for a single transplant and > 6 × 106 CD34+ cells/kg for a tandem transplant (Pierelli et al. 2012).

Pre-collection analysis of CD34 levels in the peripheral blood is a good correlator for end target yields.

5.3.2 Cytokines and Mobilisation Regimes

Several cytokines have been identified to play an important role in haematopoiesis. When progenitor cells are exposed to these cytokines, the maturation cascade producing committed mature blood cell components can occur. These cytokines are administered to patients and donors in an effort to enhance the availability of circulating CD34+ stem cells for collection.

Because of its efficacy compared to other cytokines and its low toxicity profile, G-CSF is the cytokine most commonly used to increase the level of myeloid progenitor cells in the blood. Recombinant methionyl human G-CSF (filgrastim) and recombinant human G-CSF (lenograstim) are the two forms of this cytokine available for clinical use. The end objective of any mobilisation regime is to collect sufficient stem cells to allow the patient to procced to transplantation in a manner minimising risk and optimising outcome (Giralt et al. 2014).In clinical practice, for autologous PBSC, the most frequent mobilization procedure is the administration of filgrastim in combination with chemotherapy for those requiring autologous collections. Alternatively high dose GCSF over 4–5 days is used for donor stem cell collections. A variety of chemotherapy regimes have been utilised to mobilise stem cells from patients, some more efficiently than others, it is not uncommon for disease – directed chemotherapy regimes to be used such as ESHAP (Etoposide, methylprednisolone, cytarabine, cisplatin) or DHAP (Dexamethasone, cytarabine, cisplatin)or mobilisation-specific regimes such as high dose cyclophosphamide to be used both in combination with GCSF. The timing of regime commencement and commencement of CD34 monitoring is crucial to achieve a successful collection with white cell count recovery/rebound varying significantly (Pierelli et al. 2012). For growth factor mobilization alone, the first collection procedure is calculated on days 4–5 when the peak of CD34+ cell count is expected to be achieved. After mobilization with chemotherapy regimens and growth factor, the expected day can vary between days 12 and 15 (Pierelli et al. 2012).

A proportion of patients fail to collect enough stem cells to proceed to autologous transplantation using a combination of G-CSF +/− mobilising chemotherapy (Pusic et al., 2008). Poor stem cell yields after mobilization might occur. Inadequate stem cell yields or poor mobilisation in patients can be related to previous exposure to myelosuppressive chemotherapy; agents which are toxic to stem cells such as cyclophosphamide (doses >7.5 g), melphalan, carmustine, procarbazine, fludarabine, nitrogen mustard and chlorambucil are particularly detrimental to stem cell collection yields. Other risk factors associated with low CD34+ cell collections include advanced age (>60 years), previous radiation therapy, short time interval between chemotherapy and mobilization, extensive disease burden and tumour infiltration of the bone marrow (Olivieri et al. 2012).

Those groups of patients are defined ‘poor mobilizers’. In this case the use of plerixafor, a CXCR4 antagonist used in combination with G-CSF has been shown to improve CD34+ cell collections in lymphoma and multiple myeloma patients (Olivieri et al. 2012). Collection end targets may be multifactorially infucenced by factors such as the mobilisation strategy employed, patient specifics, operator and equipment variables, and procedural complications.

5.4 Leukapheresis Collections

The optimum day for the collection of stem cells is determined by WBC count and predictive CD34+ predictive cell count done on a peripheral blood sample. These thresholds may vary across collection facilities but typically range from 10 to 20 CD34+ cells/ml on the background of a rapidly rising WBC count will trigger a collection procedure. Identifying the correct day for collection should avoid unnecessary procedures for the patient, limit the impact on the processing facility and storage facility and limit unnecessary costs. Leukapheresis collection dates when collecting for immune effector cell therapies are in general defined by the availability of manufacturing slots and if the product is to be shipped fresh or cryopreserved .

When collecting stem cells the objective is to collect a product with the prescribed stem cell dose, which has low cross cellular contamination,in the smallest possible collect volume (approx. 100 ml to minimise DMSO toxicity) and in as few procedures as possible. This will ensure cost optimization of the end product and enhance patient comfort and safety.

The role of the clinical apheresis nurse varies from institution to institution but must include the close monitoring of the collection process and the patients for any adverse reactions, an awareness of the regulatory requirements and standards of practice that should be adhered to in the collection facility.

Patients are connected to the apheresis cell separator machine by their centrally or peripherally inserted venous catheters. One lumen is used to withdraw blood out of the patient and into the machine where the blood is centrifuged in a bowl housed within the machines body. The desired cells are then siphoned off before the remaining blood components are returned to the patient through the second lumen of their catheter. This second lumen can be used to administer intravenous fluids, electrolyte supplements and medications to the patient if necessary. Each apheresis session lasts on average 4–6 h, but this can vary significantly and is influenced by patient size, venous access, procedural complications such as citrate toxicity and the required procedural end points that need to be met. During the procedure an average of 7–12 l of blood, or twice the average total blood volume (as calculated by height weight and gender), is processed.

5.5 Complications and Challenges

5.5.1 Adverse Reactions

Apheresis procedures are relatively safe procedures for the patient and used for a variety of indications, complications are in general classed as mild to moderate, with severe adverse events being rare (Henriksson et al. 2016). In their review of the World Apheresis Registry data they outline clearly the extent of side effects which may occur during apheresis procedures so that appropriate risks can be assessed and precautions taken. The most common of which are discussed below:

Table 5.1 also outlines some of the advantages and disadvantages of both collection methods.

Table 5.1 Advantages and disadvantages of haematopoietic stem cell collection methods
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The most common of which are discussed below:

5.5.2 Vascular Access

Appropriate catheter selection and placement should be scheduled prior to the first stem cell collection (Toro et al. 2007). Good venous access is essential to the success of a procedure facilitating a good steady blood flow through the cell separator.

Catheters used for apheresis procedures must be able to tolerate large fluctuations in circulating blood volume, Peripheral access is desirable where possible to minimise the need for invasive procedures for the patients. Two separate and distances points of access are required to conduct cellular collections. One to remove blood and a second to return the blood to the patient simultaneously to maintain a continuous flow through the machine. A variety of peripheral devices are available for use, however a wider short gauge needle appropriate to the vein size is preferable for drawing blood from the patient eg fixed back-eyed 16–17 g dialysis needle sited in a large vein such as the ante cubital fossae. Large gauge peripheral cannulas sited ideally in the other arm or central venous devices can be used to return blood to the patient.

In the absence of adequate peripheral veins, a large-bore, double lumen device can be inserted for apheresis. These can be placed in the femoral vein or internal jugular by experienced practitioners and may be inserted temporarily for collection only or placed and used for the transplant process.

5.5.3 Citrate Toxicity

One of the most common adverse effects seen during all apheresis procedures is citrate toxicity, frequently manifested by hypocalcaemia.

Sodium citrate is used during apheresis to prevent blood from clotting while it is being processed by the machine. Citrate is known to bind to ionized serum calcium leading to hypocalcaemia. Signs and symptoms of this complication can include:

  • Burning sensations.

  • Numbness and tingling in the extremities and/or the area around the mouth.

  • Muscle twitching, tetany and generalised vibrations.

  • Abdominal cramps and nausea.

  • Shivering and rigors.

  • In severe cases cardiac arrhythmia/arrest and chest pain.

Citrate toxicity can be managed by slowing the apheresis flow rate and providing patients with oral calcium supplements. In severe cases, intravenous supplementation of calcium may be given to the patient in order to prevent severe reactions such as tetany, seizure, and cardiac arrhythmia. Serum monitoring of calcium levels prior to each apheresis session is often helpful in decreasing the likelihood of hypocalcaemia.

Other effects stemming from citrate toxicity include hypomagnesemia, hypokalaemia, and metabolic alkalosis. Magnesium, like calcium, is a bivalent ion that is bound by citrate. Declines in serum magnesium levels often are more pronounced and take longer to normalize compared to aberrations in calcium levels. Signs and symptoms of hypomagnesemia are muscle weakness or spasm, decreased vascular tone, and abnormal cardiac contractility. Oral and intravenous supplementation with magnesium and potassium is often effective.

5.5.4 Hypovolemia

There is a low extra corporeal volume involved in cellular collections of generally less than 200 ml, some patients do experience symptoms of hypovolemia. Due to fluctuations in blood volume prior to starting a procedure, baseline pulse and blood pressure should be measured and continually rechecked at designated intervals. It is also recommended that haemoglobin and haematocrit be monitored after the procedure as well. Patients at risk for developing hypovolemia include those with anaemia, a previous history of cardiovascular compromise and children or adults with a small frame. Preventative measures are aimed at minimizing the extracorporeal volume shift by priming the apheresis machine with red blood cells and fresh frozen plasma in place of normal saline, Clinical manifestations of hypovolemia can include dizziness, light-headedness, tachycardia, hypotension and diaphoresis. Most concerning is the development of a cardiac dysrhythmia which can be life-threatening.

Sessions should be interrupted and symptoms should subside before proceeding with collections. Hypovolemia may also be managed with providing intravenous fluid boluses and slowing the rate of flow on the apheresis machine.

5.5.5 Thrombocytopenia

Thrombocytopenia is a potential complication encountered during cell collections. Platelets can stick to the bowl used during the centrifugation process or aggregate in the circuit during collection The necessity of pre-procedural blood counts can minimise the likelihood of bleeding related to platelet loss during apheresis. Loss can be assessed and audited by post -procedural full blood count assessment. The need for pre- or post-procedural transfusion should be guided by local policy.

5.6 Apheresis Collection Facility Standards and Quality Management

The Joint Accreditation Committee ISCT Europe and EBMT (http://www.ebmt.org) provides guidance and accreditation for best practice and quality for institutions providing cellular therapy procedures using haematopoietically derived cellular product including the collection of immune effector cells (IEC and genetically modified cellular products). Accreditation requires that the clinical program has access to personnel who are formally trained, experienced and competent in the management of patients undergoing cellular therapy procedures. Apheresis Collection Facility shall be licensed, registered or accredited as required by the appropriate governmental authorities for the activities performed and incorporate a quality management plan which should include and reference, policies and standard operating procedures addressing personnel training requirements for each key position in the Apheresis Collection Facility. It should also include policies to address appropriate allogeneic and Autologous selection, eligibility and management pre, during and post collection procedures.

5.6.1 Training and Competencies

Core competencies are specified within the JACIE standards, and evidence of training in these competencies must be documented. This may be achieved by evidence of in-service training, attendance at conferences etc. While initial supervised training is easily documented, annual competency maintenance can be difficult to demonstrate. Ongoing training for clinical personnel should reflect their experience, individual competencies and proficiencies, orientation for new staff and necessary training. Training also needs to be undertaken in a timely manner to demonstrate continued competency in practice.

5.6.2 Labelling and Chain of Identity

All cellular therapy products should be labelled at the source of collection to prevent misidentification and according to ISBT 128 standard terminology following a defined and validated process by qualified and competent staff, this includes the application of warning labels as appropriate. Each cellular therapy product is assigned a unique identifier to enable it to be traced back to its donor, relevant documentation and final end point.

5.7 Cell Source and Apheresis in the Paediatric Population

Abstract

Haematopoietic stem cell transplantation (HSCT)has become a well-established treatment for many malignant and non-malignant disorders in children. Small body weight, venous access and ethical dilemmas in minors represent a challenge in the paediatric population.

Keywords

Apheresis • Cell source • Children • Paediatric population • HSCT

5.7.1 Introduction

Indications for paediatric HSCT have expanded considerably and these changes have informed decision-making in health-care planning and counselling (Miano et al. 2007; Merli et al. 2019). HSCT, the oldest immunotherapy used in clinical practice, still represents the gold standard consolidation treatment for a number of paediatric diseases including high-risk/relapsed acute leukaemia (Merli et al., 2019). However There is now also a growing body of evidence for the role of HSCT in non-haematological disorders such as autoimmune diseases (Sureda et al. 2015).

Some other common non-malignant diseases in paediatrics treated with haematopoietic stem cell transplant are identified below (Nuss et al. 2011):

  • Haematologic (severe aplastic anaemia, Fanconi anaemia, thalassemia, sickle-cell disease, Diamond-Blackfan anaemia, Chédiak-Higashi syndrome, chronic granulomatous disease, congenital neutropenia).

  • Solid tumours (Ewing’s sarcoma, soft tissue sarcoma, neuroblastoma and Wilms’ tumour, where there is high risk or 4CR1, osteogenic sarcoma, and brain tumours).

  • Immunodeficiency (severe combined immunodeficiency disease, Wiskott-Aldrich syndrome, functional T-cell deficiency).

  • Genetic (adrenoleukodystrophy, metachromatic leukodystrophy, Hurler syndrome, Hunter disease, Gaucher syndrome).

5.7.2 Cell Sources in the Paediatric Population

The proportion of autologous to allogeneic HSCT is different in the paediatric population (29% autologous) compared with adults (62% autologous). Autologous cell sources are the primary cell source in the treatment of solid tumours (Passweg et al. 2013).

Allo-HSCT in children and adolescents represents over 20% of overall allo-HSCT activity, with a particular use in congenital and non-malignant diseases, many of which are rare (Snowden et al. 2022).

Improvements in high-resolution HLA matching in unrelated donor identification, conditioning regimens and supportive care for infectious and non-infectious complications have progressively reduced mortality and influenced the preference of allogenic transplantation in all settings. There has been a move towards allogenic transplantation at an earlier stage in the course of diseases - where patients exhibit a better performance status rather than as a ‘last chance for cure’. Graft versus host disease (GVHD) remains the major risk factor for patients without optimally matched donors. New allo-HSCT strategies should improve outcomes for mismatched alternative donors (MMAD) (Snowden et al. 2022).

Stem cells for use in paediatric transplantation may be collected from the bone marrow (BM), peripheral blood (PBSC), or umbilical cord blood (UCB) as with the adult patient population. Each of these sources has their own advantages and disadvantages some of which are noted above .Despite the increased use of peripheral and umbilical cord blood, bone marrow remains a preferred graft source in the paediatric setting, with unrelated donors accounting for 49% of the cell source used in 2013 (Sureda et al. 2015). This can partly be explained by the higher incidence of non-malignant conditions transplanted in this group and the higher risk of chronic GvHD seen with peripheral blood as a stem cell source (Passweg et al. 2013).

The clinical opinion surrounding the optimum cell source for allogeneic transplantation in the paediatric population appears mixed. For children and adolescents aged 8–20, allogeneic transplantation from HLA identical sibling using peripheral blood stem cells was associated with higher mortality, than where bone marrow was used as the cell source (Eapen et al. 2004). Angelucci et al. (2014); evidence indicated that peripheral blood stem cells should be avoided because of the increased risk of chronic GVHD. In contrast, previous work showed that peripheral blood was superior to the bone marrow as a stem cell source for adults and adolescents (aged 12–55) (Bensinger et al. 2001).

However, Anasetti’s 2012 NMDP/CIBMTR randomized study comparing the use of unrelated marrow versus PBSC, which included paediatric patients, found that there were no significant differences in mortality between recipients of PBSC compared to the bone marrow. But according to Angelucci (2014), peripheral blood stem cells should be avoided because of the increased risk of cGVHD.

Most recently there has been an increase and improved outcome in the numbers of transplants using haploidentical family donors as opposed to HLA identical siblings. This has been influenced by the successful strategy of administering post cell infusion cyclophosphamide in haploidentical conditioning regimes. In 2014, the numbers of transplants in the USA using Haplo-Identical family donors surpassed the total numbers of umbilical cord transplant performed, accounting for 11% of all US allogeneic transplants (Pasquini and Zhu 2022).

Umbilical cord blood (UCB) characteristically differs from the marrow in a number of ways. The median doses of total nucleated cells (TNC), CD34+ cells and CD3+ cells in UCB unit are approximately ten times lower than that of a bone marrow graft (Moscardo et al. 2004; Barker and Wagner 2003).The indications for the use of UCB as a source for stem cells in children are identical to the indications for matched unrelated donor transplants (Sureda et al. 2015).

However the use of umbilical cord blood now appears to be steadily declining after a peak in 2009, down from 46% to 32% of all unrelated donor transplants performed in this age group (Sureda et al. 2015, Merli et al. 2019).

5.7.3 Apheresis in Paediatric Population

Experience with paediatric peripheral blood stem cell collections is limited. Challenges of apheresis in small children (<20 kg) include:

  • Small total blood volume.

  • Vascular access issues.

  • Concerns about tolerable anticoagulant doses.

  • Limitations in product volumes that can be safely collected.

In many countries worldwide, children under the age of 18 years are not permitted to donate haematopoietic stem cells (HSCs) for unrelated recipients (Sörensen et al. 2013).

Adequate peripheral vascular access is challenging to establish in young children, and often a central venous apheresis catheter (5–7 Fr with double offset lumens) is required with its attendant risks including pain and bleeding. This age group will also often require, general anaesthesia, or conscious sedation for catheter placement which brings with it additional risks. Central line placement should be done by an expert team using ultrasound guidance or interventional radiology.

The apheresis team must consider the size and type of catheter that will yield the highest flow rate during apheresis, as well as patient or donor comfort. Often the catheter used for apheresis may then be used for venous access during high-dose therapy/transplantation, reinfusion of stem cells and recovery phases. A trained expert team in paediatric apheresis is mandatory for a successful and safe procedure.

5.7.4 Key Differences: Paediatric Vs. Adult Apheresis

5.7.4.1 Red Cell Prime

Priming of the apheresis collection tubing with heterologous packed red blood cells is widely undertaken where donors weigh less than 20 kg to avoid the comparative extracorporeal volume shift risks as compared inherently related to the small total blood volume of a paediatric patient or donor. Priming in this manner helps to avoid hypovolemic shock when blood is initially drawn from the patient into the machine.

The risk of heterologous blood product administration in healthy donors, such as transfusion reaction and the risk of system overload if primed blood for some reason is reinfused, must always be taken into consideration. The apheresis circuit is usually primed with red blood cells which are cross-matched, irradiated and leukocyte-depleted.

In the paediatric setting, the most common apheresis-related adverse event is pain, observed after placing a central venous catheter (CVC). Pain at the site of puncture occurs more frequently in donors requiring a central line (58%) than those where peripheral vein access is used (38%) (Hequet, 2015).

Other reported side effects after paediatric apheresis are:

  • Haematoma formation.

  • Hypotension and cyanosis.

  • Allergic reaction to red blood cells.

  • Thrombocytopenia.

Rarer side effects reported are:

  • Low-grade fever during the mobilization.

  • Hypovolemic signs: tachycardia >120 (most cases), hypotension, systolic blood pressure <80 mmHg, pallor and diaphoresis.

  • Nausea related to citrate effects during the apheresis procedure.

In the absence of consensus and in order to prevent signs and symptoms of hypocalcaemia, some paediatric centers administrate orally calcium gluconate or replace with a continuous infusion of calcium intravenously during the apheresis procedure. Nurses who perform paediatric procedures need to achieve competence in machine settings to ensure the safe and effective anticoagulation of the patient during the procedure. The nurse must be competent in blood priming and in use of diluted or undiluted packed red blood cells, and in prevention of hypocalcaemia and maintenance of fluid balance, etc.

5.7.5 Ethical Considerations

The approach to minor donors is different in many countries. A donor is a person, no matter how small (Styczynski et al. 2012).

Styczynski et al. (2012) compared donor and recipient children’s age, donors of smaller body weight than the recipient and thus at higher risk of requiring a blood transfusion, additional apheresis procedures, pain and cardiovascular complications after anaesthesia. Most paediatric physicians who perform transplants believe it is acceptable to expose minors to the risks of a stem cell donation when donation offers a substantial prospect of benefit to a close family member and when proper consent is obtained (often from parents of both, donor and recipient).

The key issue that must be addressed with childhood procedures is the donor’s ability to understand and to voluntarily consent the procedure. Understanding increases with age into an ability to assent and then finally to legally consent. Because their stem cell donation may benefit the recipient more than any other cell source and because the procedure can be performed with limited risk, paediatric sibling donation under parental consent has been considered appropriate to date (Bitan et al. 2016).

Summarizing:

  • Advocacy and medical review of donors by a clinician independently of the recipient is highly recommended.

  • The recommendation is to focus on avoiding psychological harm to the donor rather than predicting whether donation will result in a psychological benefit to the donor.

  • Paediatric donors may be considered for research that carries minimal risk above the standard procedure or studies aimed at improving the safety and efficacy of the donation process.

  • Donors with medical conditions that may increase the risk of complications associated with donation should not ever be considered fit for donation.

  • Human leukocyte antigen tissue typing should not be undertaken in the first instance on potential donors with medical/psychological reasons not to donate (Bitan et al. 2016).

5.7.6 Psychosocial Risks and Benefits

The primary benefit to the donor is the psychosocial value of helping a sibling or other close family members. This benefit may accrue even if the transplant is unsuccessful, because the donor and family can at least be reassured that they have tried ‘their best’. There is a small but growing literature on the psychosocial risks and harms caused by haematopoietic stem cell donation by children. Data show that many children experience distress related to their role as a donor. Many paediatric donors believe that they did not have a choice and felt poorly prepared for the procedures, describing feeling responsible for the recipient’s transplant outcome (Weisz 1996). The safety and welfare of the donor are major concerns for the transplantation community, especially for related sibling donors of young recipients who are children and, thus, not able to fully consent (Bitan 2016).