Review
Blood cell manufacture: current methods and future challenges

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Blood transfusion depends on availability of donor material, and concerns over supply and safety have spurred development of methods to manufacture blood from stem cells. Current methods could theoretically yield therapeutic doses of red blood cells (RBCs) and platelets. However, due to the very large number of cells required to have any impact on supply (currently 1019 RBC/year in the US), realization of routine manufacture faces significant challenges. Current yields are orders of magnitude too low for production of meaningful quantities, and the physical scale of the problem is a challenge in itself. We discuss these challenges in relation to current methods and how it might be possible to realize limited ‘blood pharming’ of neutrophils in the near future.

Introduction

Blood cell transfusion is a mainstay of modern medicine, from red blood cell (RBC) and platelet transfusion through to bone marrow transplantation. Such practices are, however, entirely dependent on the availability of donor material, leading to supply constraints and safety concerns.

Although 80 million units of donor whole blood are collected globally every year [1], shortages still occur. Supply constraints are of particular concern in developing nations; only 39% of worldwide blood donations are collected in these regions, which account for 82% of the world population [1]. Even in the US, as many as 7% of hospitals report postponement of elective surgeries due to blood inventory shortages [2]. In the period from 2001 to 2002, at least six million units of donated blood went unscreened for HIV, hepatitis B and C, and syphilis [1]. Add to this list cytomegalovirus, variant Creutzfeld-Jacob disease, human T lymphotrophic virus, malaria, Chagas disease and parvovirus, as well as new or emerging diseases, and it is clear that the safety of donor blood products cannot be guaranteed even in the most advanced and wealthy nations.

In response to these concerns, ex vivo manufacture of blood cells from haematopoietic stem and progenitor cells (HPCs, Box 1) has been proposed as a means to ensure an adequate and safe supply of blood cell products. Aiming to harness the inherent capacity of HPCs to produce large numbers of differentiated progeny (Figure 1), recent research efforts have focused on maximizing the numerical expansion of RBCs and platelets in a laboratory setting. It is now possible, in theory at least, to generate several transfusion units of these blood products 3, 4 from a single umbilical cord blood (UCB) donation.

Routine ex vivo manufacture of blood products is crucially dependent on the availability of technologies that address not only the biological requirements for high yields of defined cell populations but also the physical challenges of producing vast quantities of cells. Absence of the latter means that routine clinical use of ex vivo manufactured blood products currently remains some way from realization.

Section snippets

Manufacture of RBCs

Perhaps the most obvious goal for ex vivo manufacture of a mature blood cell product is that of producing RBCs. However, any such process would face significant challenges with respect to scale. A single unit of RBCs contains 2 × 1012 cells, and assuming a density of 3 × 106 cells/mL [3], manufacture of this amount of cells would require around 660 L of medium and 9500 laboratory scale 175 cm2 tissue culture flasks (70 mL per flask). In 2006, 15.7 million units of whole blood and RBCs were collected

Bioreactors for blood cell manufacture

Several types of bioreactors have been used for cultivation of haematopoietic cells (for reviews, see 12, 13, 14). The typical approach to scale-up of a cell culture bioprocess is to move from a 2D surface culture into stirred 3D tank reactors, which enable efficient mass transport even at very large scales. HPC cultures have been scaled to 1 L with no indication that agitation has a detrimental effect 15, 16. In the biopharmaceutical industry, the use of stirred tank reactors for mammalian cell

Scale-up of RBC manufacture

Although bioreactors have been successfully employed for the cultivation of HPCs, they have not yet been applied in manufacturing standard donor products, such as RBCs. Due to the very large number of cells that are required, existing bioreactor technologies are currently insufficient to meet the demands of routine RBC manufacture, and new breakthroughs in related technologies are required.

The US Defence Advanced Research Programs Agency (USDARPA)’s ‘Blood Pharming’ program (//www.darpa.mil/dso/solicitations/baa07-21mod6.html

Manufacture of patelets

As for RBCs, the quantities of ex vivo manufactured platelets that would be required to have a significant impact on donor supply are extremely large. A single unit of donor platelets contains ∼5.5 × 1010 platelets, and 10.3 million units were transfused in the US alone in 2006 [2]. In contrast to RBCs however, there have been no reports of extensive and specific numerical expansion of platelets from HPCs ex vivo, and functionality of the product has not yet been thoroughly investigated.

In vivo,

Manufacture of neutrophils

In contrast to RBCs and platelets, routine collection of donor neutrophils is currently not practiced. Despite their potential use in providing transfusion support for neutropaenic patients [48], the complicated logistics of identifying and screening donors, the need to mobilize HPCs in these donors with haematopoietic cytokines, and difficulties in obtaining sufficient numbers of neutrophils even after mobilization [49] have limited the practice of neutrophil transfusion to only a few

Concluding remarks

Despite over two decades of research, and claims to the contrary, large-scale ex vivo manufacture of blood cells from HPCs has yet to be demonstrated. Although the clinical demand for manufactured RBCs and platelets is clear, current cultivation methods are not capable of generating the vast quantities of cells that would be required to make a significant contribution to the existing, but limited, donor supply.

Although several biological challenges remain, particularly with regards to

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