Interleukin-21 Induces the Differentiation of Human Umbilical Cord Blood CD34. ⁻lineage⁻ Cells Into Pseudomature Lytic NK Cells.

Umbilical cord blood (UCB) is increasingly used as an alternative source of transplantable CD34⁺ haematopoietic stem cells (HSC) for neoplastic and non-neoplastic diseases. In addition to CD34-expressing HSC, human UCB contains a rare population of CD34⁻lineage⁻ cells endowed with the ability to differentiate along the T/NK pathway in response to interleukin (IL)-15 and in the presence of a stromal cell support. IL-21 is a four-helix bundle cytokine released by activated CD4⁺ T cells and by NKT cells. IL-21 is a crucial regulator of NK cell function, whose influence on IL-15-induced differentiation of CD34⁻lineage⁻ cells has not been investigated previously. The present study was designed and conducted to address whether IL-21 might replace the stromal cell requirements and foster the IL-15-induced NK differentiation of human UCB CD34⁻lineage⁻ cells.

CD34⁻lineage⁻ cells were maintained in liquid culture with 10⁻⁶M hydrocortisone, 20 ng/ml Flt3-L and 20 ng/ml SCF, with the addition of 50 ng/ml IL-15 and 20 ng/ml IL-21, either alone or in combination. Cultures were established in the absence of feeder cells or serum supplementation. Cytokine-treated cells were used to evaluate the following parameters: a) cell surface phenotype; b) expression of molecular determinants of lymphoid/NK cell differentiation; c) secretion of IFN-γ, GM-CSF, TNF-α and CCL3/MIP-1α; d) cytolytic activity against NK-sensitive tumour cell targets and e) relative amount of Stat1 (Tyr701), Stat3 (Tyr705) and Stat5 (Tyr694) phosphorylation in response to IL-21. For all the above detailed experiments, control cultures were established with UCB-derived CD34⁺ HSC.

Freshly isolated CD34⁻lineage⁻ cells stained negatively for stem cell-associated (CD34, CD133) and NK/lymphoid surface antigens (CD7, CD56, CD16, CD3, TCRαβ), and comprised 0.22% on average of UCB mononuclear cells (samples analyzed = 8). CD34⁻lineage⁻ cells proliferated vigorously in response to IL-15 and IL-21 (average fold expansion at week +4 of culture = 42.5) but not to IL-21 alone, and up-regulated phosphorylated Stat1 and Stat3 proteins, in good agreement with previously published reports on the IL-21-induced activation of Stat signaling. CD34⁻lineage⁻ cells expanded by IL-21 in combination with IL-15 acquired a peculiar lymphoid morphology with heavy cytoplasmic granules. When compared with CD34-derived NK cells, CD34⁻lineage⁻ cells emerging from IL-15+IL-21-containing cultures expressed very low levels of CD16 and killer-cell immunoglobulin-like receptor (KIR), but high levels of CD56, NKG2D and IL-21 receptor, consistent with pseudo-mature NK cells. IL-21/IL-15-differentiated cells up-regulated mRNA signals for Bcl-2, GATA-3 and Id2, a master switch required for NK-cell development, and harboured un-rearranged TCRγ genes, suggesting that NK commitment under the experimental conditions here established occurs through a pathway that does not include TCR rearrangement. From a functional standpoint, IL-21/IL-15-treated cells secreted copious amounts of IFN-γ, GM-CSF and CCL3/MIP-1α, and expressed cell surface CD107a upon contact with NK-sensitive tumour targets, a measure of exocytosis of NK secretory granules. Specifically, an average 65±11% of CD56⁺ NK cells differentiated with IL-15+IL-21 stained positively for CD107a in co-cultures established with NK-sensitive K562 cells. NK cell degranulation occurred at significantly lower levels in co-cultures containing K562 cells and IL-15-differentiated CD34⁻lineage⁻ cells (mean percentage of CD107a⁺CD56⁺ NK cells equal to 35±6 at E:T ratio = 1; p < 0.01 compared with cultures containing IL-15+IL-21-matured NK cells), suggesting that IL-15 and IL-21 exerted synergistic effects on NK activity. Finally, NK cells differentiated from CD34⁺ HSC with either IL-15 alone or IL-15+IL-21 manifested a similar cytotoxic activity to that of cytokine-differentiated CD34⁻lineage⁻ cells.

This study suggests that considerable numbers of highly pure, lytic CD56⁺CD16^−/+ NK cells for adoptive immunotherapy can be obtained from UCB CD34⁻lineage⁻ cells using a serum-free, feeder cell-free culture system. The findings highlighted herein also shed some light into the developmental intermediates of NK cells that can be differentiated after the exposure of CD34⁻lineage⁻ cells to IL-21.

No relevant conflicts of interest to declare.