Long-term culture of hematopoietic stem cells - validating the stromal component of the CAFC assay

The paradoxical dynamism of marrow stem cells: considerations of stem cells, niches, and microvesicles

Marrow stem cell regulation represents a complex and flexible system. It has been assumed that the system was intrinsically hierarchical in nature, but recent data has indicated that at the progenitor/stem cell level the system may represent a continuum with reversible alterations in phenotype occurring as the stem cells transit cell cycle. Short and long-term engraftment, in vivo and in vitro differentiation, gene expression, and progenitor numbers have all been found to vary reversibly with cell cycle. In essence, the stem cells appear to show variable potential, probably based on transcription factor access, as they proceed through cell cycle. Another critical component of the stem cell regulation is the microenvironment, so-called niches. We propose that there are not just several unique niche cells, but a wide variety of niche cells which continually change phenotype to appropriately interact with the continuum of stem cell phenotypes. A third component of the regulatory system is microvesicle transfer of genetic information between cells. We have shown that marrow cells can express the genetic phenotype of pulmonary epithelial cells after microvesicle transfer from lung to marrow cells. Similar transfers of tissue specific mRNA occur between liver, brain, and heart to marrow cells. Thus, there would appear to be a continuous genetic modulation of cells through microvesicle transfer between cells. We propose that there is an interactive triangulated Venn diagram with continuously changing stem cells interacting with continuously changing areas of influence, both being modulated by transfer of genetic information by microvesicles.

The stem cell continuum: considerations on the heterogeneity and plasticity of marrow stem cells

Traditional models of hematopoiesis have been hierarchical. Recent evidence showing that marrow stem cells are a cycling population and that the hematopoietic phenotype of these cells reversibly changes with cycle transit have suggested a continuum model of stem cell regulators. Studies on marrow cell conversion to lung cells have extended this continuum to cycle-related differentiation into nonhematopoietic stem cells. We postulate that stem cells transiting cell cycle continually change their chromatin structure, thus providing different windows of transcriptional opportunity and a continually changing phenotype. Final outcomes with this continuum model would be determined by the specific chromatin state of the cell and the presence of specific differentiation inducers.

In vitro functional defects of bone marrow-derived CD34+ progenitors from newly diagnosed mature B-cell malignancies with bone marrow tumor involvement

OBJECTIVE

We hypothesized that the presence of tumor cells in bone marrow (BM) could alter hematopoietic progenitor cell functions. Therefore, we evaluated phenotypic and in vitro functional properties of BM-derived CD34+ progenitors issued from untreated and newly diagnosed patients presenting a mature B-lymphoproliferative disorder (LPD) involving the BM (Inv+).

PATIENTS AND METHODS

In vitro proliferation and differentiation capacities of primitive and committed progenitors were evaluated by cobblestone area-forming cell (CAFC) and colony-forming cell (CFC) assays, and ex vivo cell expansion. Migratory capacities of CD34+ cells were explored by chemotaxis assays using a CXCL12alpha gradient.

RESULTS

Our results showed that CD34+ cells from Inv+ patients overexpressed CD117 and had a significant decrease of week-3 and -6 CAFC, and CFC frequencies, compared to cells obtained from healthy volunteers and LPD patients without BM involvement (Inv-). In addition, progenitors from Inv+ patients maintained a significantly decreased CFC capacity after ex vivo cell expansion, compared to healthy volunteers. However, the former cells held their migratory capacity in response to CXCL12alpha.

CONCLUSION

Functional defects of primitive and committed CD34+ progenitors detected among LPD patients with BM tumor involvement suggest either that tumor cells may induced bystander effects on progenitors or that "unusual" CD34+ cells may exist in the BM that could belong to the proliferating tumor tissue.

The symmetry of initial divisions of human hematopoietic progenitors is altered only by the cellular microenvironment

OBJECTIVE

We examined if cellular elements or adhesive ligands were able to alter asymmetric divisions of CD34(+)/CD38(-) cells in contrast to soluble factors at a single cell level.

MATERIALS AND METHODS

After single cell deposition onto 96-well plates, cells were cocultured for 10 days with the stem cell supporting cell line AFT024, fibronectin (FN), or bovine serum albumin (BSA). The divisional history was monitored with time-lapse microscopy. Subsequent function for the most primitive cells was assessed using the myeloid-lymphoid-initiating cell (ML-IC) assay. Committed progenitors were measured using colony-forming cells (CFC).

RESULTS

Only contact with AFT024 recruited significant numbers of CD34(+)/CD38(-) cells into cell cycle and increased asymmetric divisions. Although most ML-IC were still identified among cells that have divided fewer than 3 times, a significant number of ML-IC shifted into the fast-dividing fraction after exposure to AFT024. The increase in ML-IC frequency was predominantly due to recruitment of quiescent and slow-dividing cells from the starting population. Increase in CFC activity induced by AFT024 was found only among rapidly dividing cells.

CONCLUSIONS

For the first time, we have demonstrated that asymmetric divisions can be altered upon exposure with a stem cell-supporting microenvironment. For the primitive subset of cells (ML-IC), this was predominantly due to recruitment into cell cycle and increased rounds of cycling without loss of function. Exposure to AFT024 cells also increased proliferation and asymmetric divisions of committed CFC. Hence direct communication between hematopoietic progenitors with stroma cells is required for maintaining self-renewal potential.

Peripheral blood accessory cells modulate committed colony-forming units but not 5-week cobblestone-area-forming cell outgrowth from CD34+ cells

While cellular modulation in vitro of committed hematopoietic stem cell (HSC) growth has been known for some time, less is known about the effect of accessory cells (AC) on the growth of more immature HSC. We have examined the effect of peripheral blood (PB) AC on hematopoiesis by coculturing enriched PB CD34+ cells (>96% pure) with different quantities of CD34 cells (<1% contamination) harvested from 10 breast cancer patients. As expected colony growth was predominantly present in the CD34+ fractions, in which colony forming units granulocyte-macrophage (CFU-GM) varied between 89-3289/10(5) (median 1422/10(5) seeded cells) and week 5 cobblestone area forming cells (CAFC) between 64-1330/10(5) (median 427/10(5) seeded cells). Few CFU-GM (0-27/10(5) seeded cells) and no week 5 CAFC (0-1/10(5) seeded cells) were present in the CD34 fractions. The addition of PB CD34 cells to cultures of CD34+ cells resulted in a considerable variation in the cloning efficiency at the CFU-GM level, and the extent of modulation within the single patient was inconsistent between the different CD34+/CD34 cell mixtures. Overall the stimulatory effect was more pronounced than inhibition and on average the CFU-GM formation per CD34+ cell seeded increased 3 fold (stimulatory effect ranged between 3-17 fold and decreases between 2-10 fold). In contrast, the cloning efficiency at the week 5 CAFC level of differentiation remained unaffected by the addition of different amounts of CD34 cells (the stimulatory effect was maximally 3-fold and inhibition 3-fold). We conclude that while the CFU assay is modulated by the presence of AC, the CAFC assay is more robust and can be employed as a reliable and reproducible tool for HSC measurement.