Tumor Formation of Adult Stem Cell Transplants in Rodent Arthritic Joints

Abstract

Purpose: While imaging matrix-associated stem cell transplants aimed for cartilage repair in a rodent arthritis model, we noticed that some transplants formed locally destructive tumors. The purpose of this study was to determine the cause for this tumor formation in order to avoid this complication for future transplants.

Procedures: Adipose-derived stem cells (ADSC) isolated from subcutaneous adipose tissue were implanted into 24 osteochondral defects of the distal femur in ten athymic rats and two immunocompetent control rats. All transplants underwent serial magnetic resonance imaging (MRI) up to 6 weeks post-transplantation to monitor joint defect repair. Nine transplants showed an increasing size over time that caused local bone destruction (group 1), while 11 transplants in athymic rats (group 2) and 4 transplants in immunocompetent rats did not. We compared the ADSC implant size and growth rate on MR images, macroscopic features, histopathologic features, surface markers, and karyotypes of these presumed neoplastic transplants with non-neoplastic ADSC transplants.

Results: Implants in group 1 showed a significantly increased two-dimensional area at week 2 (p = 0.0092), 4 (p = 0.003), and 6 (p = 0.0205) compared to week 0, as determined by MRI. Histopathological correlations confirmed neoplastic features in group 1 with significantly increased size, cellularity, mitoses, and cytological atypia compared to group 2. Six transplants in group 1 were identified as malignant chondrosarcomas and three transplants as fibromyxoid sarcomas. Transplants in group 2 and immunocompetent controls exhibited normal cartilage features. Both groups showed a normal ADSC phenotype; however, neoplastic ADSC demonstrated a mixed population of diploid and tetraploid cells without genetic imbalance.

Conclusions: ADSC transplants can form tumors in vivo. Preventive actions to avoid in vivo tumor formations may include karyotyping of culture-expanded ADSC before transplantation. In addition, serial imaging of ADSC transplants in vivo may enable early detection of abnormally proliferating cell transplants.

Keywords: Chondrosarcomas; Fibromyxoid sarcomas; Magnetic resonance imaging; Malignant tumors in vivo; Osteochondral transplants; Stem cell therapy.

Fig. 1

Magnetic resonance findings for neoplastic and non-neoplastic transplants. a Sagittal T2-weighted FSE images (TR/TE = 3000 ms/30 ms) of ADSC transplants in cartilage defects of distal femurs (arrows): neoplastic ADSC implants show significant increase in size over 6 weeks, whereas non-neoplastic ADSC implants did not expand. b Corresponding size of neoplastic and non-neoplastic ADSC transplants at different time points after implantation, displayed as mean transplant area with standard errors. A single asterisk indicates significant differences between neoplastic and non-neoplastic ADSC transplants (p < 0.05).

Fig. 2

Macroscopic and microscopic features of neoplastic and non-neoplastic ADSC transplants. a Neoplastic ADSC transplants present as an expansile and locally destructive mass (arrow). b Histopathological features of the malignant chondrosarcomas at × 100 and c × 400 include myxoid production (closed arrowhead) and hyaline cartilage (open arrowheads). d The malignant cells infiltrate adjacent skeletal muscle (× 200), and e adipose tissue with encroachment upon a nerve (× 200). f Non-neoplastic ADSC transplants do not extend beyond the transplant site. H&E stains of non-neoplastic transplants at g × 40 and h × 400 exhibit a well-defined cell implant with chondrogenic cells surrounded by unremarkable native bone.

Fig. 3

Histopathological classification of neoplastic and non-neoplastic transplants and MRI correlation. Normal and malignant ADSC implants were quantitatively compared based on a cellularity (1+ = paucicellular, 2+ = moderately cellular, 3+ = highly cellular) and b the proliferation rate per high power field, hpf (× 400 magnification). Malignant transplants were defined based on high cellularity and high mitosis rate. Non-neoplastic transplants exhibited low mitosis rates that were the result of cartilaginous proliferation.

Fig. 4

Neoplastic and non-neoplastic cells’ CD marker expression. Dual axis dot plots show similar expression for neoplastic and non-neoplastic transplants: CD29 and CD44H positive and CD31 negative. Phenotypic markers are not sufficient to discriminate the presence of neoplastic cells in a future transplant.

Fig. 5

Chromosome analysis of a neoplastic and b non-neoplastic ADSC. Neoplastic cells show a mixed population of normal male diploid (42, XY) and aberrant tetraploid (84, XXYY) cells without obvious structural rearrangement or genetic imbalance while non-neoplastic ADSC show a normal karyotype.