A Novel Plant Mannose-Binding Lectin, NTL, Preserves Cord Blood Hematopoietic Stem/Progenitor Cells in Long-Term Culture and Enhances Their Ex Vivo Expansion.

Ex vivo expansion of hematopoietic stem and progenitor cells in cytokine combinations is effective in promoting differentiation and proliferation of multilineage progenitor cells, but often results in reduction of self-renewable stem cells. In this study, we investigated the effect of a mannose-binding lectin, NTL, purified from Narcissus tazetta var. chinensis on prolonged maintenance and expansion of cord blood CD34⁺ cells. Enriched CD34⁺ cells (1 x 10⁵/mL, n=5) or mononuclear cells (1 x 10⁶/mL, n=8) were cultured in X-VIVO-10 medium for 14, 21, 28 and 35 days without supplementary cytokine or medium changing. Our results showed that the presence of NTL (200 ng/mL) or FL-3 ligand (FL, 40 ng/mL) significantly preserved populations of early stem/progenitor cells (total CFU, BFU/CFU-E, CFU-GM, CFU-GEMM) in these cultures, compared with respective controls at various time points. In the ex vivo expansion study (n=16), the presence of stem cell factor (S, 50 ng/mL), thrombopoietin (T, 50 ng/mL), FL (F, 80 ng/mL) effectively expanded total nucleated cells (TNC) at day 8 (116 ± 20.2 fold) and day 12 (424 ± 68.8 fold), as well as all subsets of progenitor cells as demonstrated by flow cytometry and CFU assays. The presence of NTL (200 ng/mL) significantly increased TNC (148 ± 24.5 fold at day 8; 572 ± 91.9 fold at day 12; P < 0.01) and expansion of early progenitor cells (CD34⁺, CD34⁺CD38⁻, CFU-GEMM) and committed CFU of the myeloid (CFU-GM), erythroid (BFU/CFU-E) and the megakaryocytic lineage (CFU-MK) (P < 0.01 compared with respective TSF cultures). There was also slight but consistent increase of CD61⁺CD41⁺ cells in the presence of NTL (8.58 ± 2.14 x 10⁵ vs. 7.30 ± 1.82 x 10⁵ cells/mL, P < 0.001). Significantly, the increased expansion was not only contributed by the higher TNC, but also by the increase in the proportion of CD34⁺ cells, CD34⁺CD38⁻ cells and the density of differential CFU. Six weeks after enriched CD34⁺ cells at day 0 or expanded cells at day 12 were infused into sub-lethally irradiated NOD/SCID mice, human CD45⁺ cells were detectable in the BM, spleen and PB of the mice. In the BM, there were engraftments of human hematopoietic cells of the early (CD34⁺), myeloid (CD33⁺, CD14⁺), B-lymphoid (CD19⁺) and megakaryocytic (CD61⁺) lineages. In animals that received day 12 expanded cells in the TSF + NTL group, there was a significant increase of human CD45⁺ cells in the BM (19.3% vs. 11.5%, P = 0.03, n = 15) when compared with those only exposed to TSF, and a trend of increased engraftment in their spleen (P = 0.07, n = 14). Comparison of the complete amino acid sequences of NTL and FRIL (a dicot mannose-binding lectin shown to preserve hematopoietic stem cells, PNAS, 96, 646–650, 1999) showed 10.2% identity and both peptides contain putative functional/structural sites such as those for N-myristoylation, casein kinase II phosphorylation, protein kinase C phosphorylation and N-glycosylation. The dual functions of NTL on long-term preservation and expansion of early stem/multilineage progenitor cells could be developed for applications in various cell therapy strategies, such as the clinical expansion of CD34⁺ cells for transplantation.