Neutrophils, the most abundant granulocytes in circulation, play a very important role in nonspecific immunity against bacteria and fungi. BM niche damage and neutropenia caused by chemical drugs, chemoradiotherapy, or severe infection could be fatal, especially during the treatment of cancer. The therapy of neutropenia includes granulocyte mobilization from BM with cytokine G-CSF or PGE2 (prostaglandin E2) promoted HSC propagation and granulocyte development; HSCT with neutrophil differentiation bias; or direct neutrophil transfusion.
The granulocyte implantation through HSCT is time cost, hence at high risk of infection. Even in the quickest implantation, the reconstruction of neutrophils and platelets will lag for 8–12 days. It was later suggested that transfusion of ex vivo expanded HSPCs to chemotherapy-treated patients significantly shortened neutropenic days because the ex vivo expansion of HSPCs amplified the granulocytes or their precursors as well. The correlative products include the serum-free medium containing SCF, TPO, and G-CSF made by Amgen Inc., which can expand ex vivo large-scale HSPCs with bias toward neutrophil differentiation; and a cellular product named CLT-008 from Cellerant Therapeutics Inc. CLT-008 is neutrophils amplified from mobilized HSPCs, requires no HLA matching, and can be used directly for transplantation. However, due to insufficient cell number, multiple donors are required for one patient therapy.
Ex vivo generated neutrophils were also explored for therapeutic usage. Based on clinic observations, a dose of 2–10 × 1010 neutrophil transplantation could only maintain physiologic neutrophil level for 24 h, so the cell transplantation should be carried out every day until the infection disappeared or the level of neutrophils reached 0.5 × 109/L. A pioneering clinical trial of granulocyte transfusion (GTx) infused granulocytes harvested from CML patients and showed good effect. The first clinical application of HSPC-derived granulocytes or the precursors was reported in 1999; the transplantation of ex vivo generated granulocytes shortened granulocyte recovery from 9 to 6 days in average. The manufacture of granulocytes from hESCs and iPSCs was also reported in 2009 and 2011, respectively, despite of the low induction yield. To date, artificial neutrophils which have the highest clinical potential were produced by Nielsen team from Australian. By utilizing the WAVE bioreactor, a large-scale neutrophil generation from UCB HSPCs was achieved, and 1010 neutrophils were produced from 1 unit cord blood, which just meets the dosage required for a single transplant. These neutrophils were named eNeut (ex vivo manufactured neutrophils). Further consideration for eNeut clinical application includes the reduction of immunogenicity (by irradiation) and the improvement of cell function (using small molecular agonist).55