The question of bone marrow stromal fibroblast traffic

The Efficiency of Bone Marrow Aspiration for the Harvest of Connective Tissue Progenitors from the Human Iliac Crest

BACKGROUND

The rational design and optimization of tissue engineering strategies for cell-based therapy requires a baseline understanding of the concentration and prevalence of osteogenic progenitor cell populations in the source tissues. The aim of this study was to (1) define the efficiency of, and variation among individuals in, bone marrow aspiration as a means of osteogenic connective tissue progenitor (CTP-O) harvest compared with harvest from iliac cancellous bone, and (2) determine the location of CTP-Os within native cancellous bone and their distribution between the marrow-space and trabecular-surface tissue compartments.

METHODS

Eight 2-mL bone marrow aspiration (BMA) samples and one 7-mm transcortical biopsy sample were obtained from the anterior iliac crest of 33 human subjects. Two cell populations were obtained from the iliac cancellous bone (ICB) sample. The ICB sample was placed into αMEM (alpha-minimal essential medium) with antibiotic-antimycotic and minced into small pieces (1 to 2 mm in diameter) with a sharp osteotome. Cells that could be mechanically disassociated from the ICB sample were defined as marrow-space (IC-MS) cells, and cells that were disassociated only after enzymatic digestion were defined as trabecular-surface (IC-TS) cells. The 3 sources of bone and marrow-derived cells were compared on the basis of cellularity and the concentration and prevalence of CTP-Os through colony-forming unit (CFU) analysis.

RESULTS

Large variation was seen among patients with respect to cell and CTP-O yield from the IC-MS, IC-TS, and BMA samples and in the relative distribution of CTP-Os between the IC-MS and IC-TS fractions. The CTP-O prevalence was highest in the IC-TS fraction, which was 11.4-fold greater than in the IC-MS fraction (p < 0.0001) and 1.7-fold greater than in the BMA fraction. However, the median concentration of CTP-Os in the ICB (combining MS and TS fractions) was only 3.04 ± 1.1-fold greater than that in BMA (4,265 compared with 1,402 CTP/mL; p = 0.00004).

CONCLUSIONS

Bone marrow aspiration of a 2-mL volume at a given needle site is an effective means of harvesting CTP-Os, albeit diluted with peripheral blood. However, the median concentration of CTP-Os is 3-fold less than from native iliac cancellous bone. The distribution of CTP-Os between the IC-MS and IC-TS fractions varies widely among patients.

CLINICAL RELEVANCE

Bone marrow aspiration is an effective means of harvesting CTP-Os but is associated with dilution with peripheral blood. Overall, we found that 63.5% of all CTP-Os within iliac cancellous bone resided on the trabecular surface; however, 48% of the patients had more CTP-Os contributed by the IC-MS than the IC-TS fraction.

Homing and Immunogenicity of Murine Stromal Cells Transfected with Xenogeneic Mhc Class II Genes

Syngeneic (murine) and xenogeneic (canine) marrow-derived stromal cells were injected intravenously into SCID and normal mice to examine the homing pattern and persistence of these cells in vivo. By in situ hybridization, these stromal cells were detectable in the bone marrow cavity and the spleen 21 days after injection. Xenogeneic cells did not persist in normal mice but persisted in SCID mice. Conditioning of the recipients with irradiation or S-fluorouracil (5-FU) treatment did not alter these results. In addition, syngeneic murine stromal cells were transfected with the genes for canine MHC class II (DRA + DRB) and transplanted into murine recipients to investigate their homing pattern and immunogenicity. These transfected syngeneic stromal cells did also home to marrow and spleen even in normal recipients. However, these cells led to sensitization of the host towards canine antigens as shown by accelerated skin graft rejection and delayed type hypersensitivity (DTH). Thus, immunodeficient (SCID) mice allow for the homing of xenogeneic stromal cells to hemopoietic organs and for prolonged persistence. In immunocompetent (normal) mice, no xenogeneic stromal cells were identified in spleen and marrow, either because of their inability to home or more likely because of immunological rejection. In contrast, syngeneic stromal cells expressing xenogeneic MHC class II genes did home to spleen and marrow and persisted even though the recipient had become sensitized. Their survival may be due to a loss of expression of the transfected gene. Alternatively, the presentation of these xenogeneic gene products in the hemopoietic organs was such that a cytotoxic response was not induced. These results also show that stromal cells can serve as a vehicle for gene delivery, conceivably with the possibility of organ targeting.

MSCs: Delivery Routes and Engraftment, Cell-Targeting Strategies, and Immune Modulation

Mesenchymal stem cells (MSCs) are currently being widely investigated both in the lab and in clinical trials for multiple disease states. The differentiation, trophic, and immunomodulatory characteristics of MSCs contribute to their therapeutic effects. Another often overlooked factor related to efficacy is the degree of engraftment. When reported, engraftment is generally low and transient in nature. MSC delivery methods should be tailored to the lesion being treated, which may be local or systemic, and customized to the mechanism of action of the MSCs, which can also be local or systemic. Engraftment efficiency is enhanced by using intra-arterial delivery instead of intravenous delivery, thus avoiding the "first-pass" accumulation of MSCs in the lung. Several methodologies to target MSCs to specific organs are being developed. These cell targeting methodologies focus on the modification of cell surface molecules through chemical, genetic, and coating techniques to promote selective adherence to particular organs or tissues. Future improvements in targeting and delivery methodologies to improve engraftment are expected to improve therapeutic results, extend the duration of efficacy, and reduce the effective (MSC) therapeutic dose.

G-CSF increases mesenchymal precursor cell numbers in the bone marrow via an indirect mechanism involving osteoclast-mediated bone resorption

During the course of studies to investigate whether MPC circulate in response to G-CSF, the agent most frequently used to induce mobilization of hematopoietic progenitors, we observed that while G-CSF failed to increase the number of MPC in circulation (assayed in vitro as fibroblast colony-forming cells, CFU-F), G-CSF administration nevertheless resulted in a time-dependent increase in the absolute number of CFU-F within the BM, peaking at Day 7. Treatment of BM cells from G-CSF-treated mice with hydroxyurea did not alter CFU-F numbers, suggesting that the increase in their numbers in response to G-CSF administration is not due to proliferation of existing CFU-F. Given previous studies demonstrating that G-CSF potently induces bone turnover in mice, we hypothesized that the increase in CFU-F may be triggered by the bone resorption that occurs following G-CSF administration. In accord with this hypothesis, administration of an inhibitor of osteoclast differentiation, osteoprotegerin (OPG), prevented the increase of CFU-F numbers induced by G-CSF. In conclusion, these data indicate that the cytokine treatment routinely used to mobilize hematopoietic stem cells could provide a readily applicable method to induce in vivo expansion of MPC for clinical applications.

Promises and pitfalls of stem cell therapy for promotion of bone healing

There is promise in combining stem cells with allogeneic bone matrix to promote bone healing. Murine bone marrow, peripheral blood, and compact bone cells were transplanted ectopically under the kidney capsule in mice, alone or in combination with allogeneic matrix products: powder and putty to determine their bone forming potential in comparison to transplanted femoral bone fragments and long-term cultured bone marrow cells. The end point was the amount of bone formed as determined by quantitative histology. Mononuclear cells from marrow, peripheral blood, or bone alone transplanted under the kidney capsule did not form bone. Mononuclear cell populations did not combine readily with matrix products and there was in vivo migration of the transplanted combinations. Kidney subcapsular transplanted cultured bone marrow cells formed bone in proportion to the culture period, but after 9 weeks, the extent was only 20% by area of that of similarly transplanted femoral bone fragments. An inductive stimulus for bone formation seemed necessary. Osteoprogenitor cells were not detected in significant numbers in blood unless high doses of cytokines were administered. A better definition of the optimal cell populations and manipulations required for promotion of bone healing is needed along with new (transplant) models that allow for cell tracking. Much work remains to overcome current pitfalls in the use of stem cells to promote allograft integration and bone healing.

LEVEL OF EVIDENCE

Therapeutic study, Level V (expert opinion). See the Guidelines for Authors for a complete description of levels of evidence.