CD31+ Cells From Peripheral Blood Facilitate Bone Regeneration in Biologically Impaired Conditions Through Combined Effects on Immunomodulation and Angiogenesis

Abstract

Controlled revascularization and inflammation are key elements regulating endogenous regeneration after (bone) tissue trauma. Peripheral blood-derived cell subsets, such as regulatory T-helper cells and circulating (endothelial) progenitor cells, respectively, can support endogenous tissue healing, whereas effector T cells that are associated with an aged immune system can hinder bone regeneration. CD31 is expressed by diverse leukocytes and is well recognized as a marker of circulating endothelial (precursor) cells; however, CD31 is absent from the surface of differentiated effector T cells. Thus, we hypothesized that by separating the inhibitory fractions from the supportive fractions of circulating cells within the peripheral blood (PB) using the CD31 marker, bone regeneration in biologically compromised conditions, such as those observed in aged patients, could be improved. In support of our hypothesis, we detected an inverse correlation between CD31+ cells and effector T cells in the hematomas of human fracture patients, dependent on the age of the patient. Furthermore, we demonstrated the regenerative capacity of human PB-CD31+ cells in vitro. These findings were translated to a clinically relevant rat model of impaired bone healing. The transplantation of rat PB-CD31+ cells advanced bone tissue restoration in vivo and was associated with an early anti-inflammatory response, the stimulation of (re)vascularization, and reduced fibrosis. Interestingly, the depletion or enrichment of the highly abundant CD31+/14+ monocytes from the mixed CD31+ cell population diminished tissue regeneration at different levels, suggesting combined effects within the PB-CD31+ subsets. In summary, an intraoperative enrichment of PB-CD31+ cells might be a novel option to facilitate endogenous regeneration under biologically impaired situations by supporting immunomodulation and vascularization. © 2016 American Society for Bone and Mineral Research.

Biophysical induction of cell release for minimally manipulative cell enrichment strategies

“…However, an increase in naive T cells also took place. In a previous study, a similar concept of selective enrichment was reported by Sass et al but using the magnetic activated cell sorting [13]. The conditioned medium from the cell population enriched with this improved system led to an increase in the total tube network length in an angiogenesis assay, and enhanced the osteodifferentiation of MSCs.…”

“…To confirm the previously reported regenerative potential of enriched CD31+ cells [13] when obtained using the optimal column conditions, both an osteogenic and angiogenic in vitro assay were utilized. An in vitro tube formation assay validated the increased angiogenic potential of the enriched cell population; the total length of capillary-like tubes was increased 1.9±0.2 fold, as compared to the control condition (Fig 5a/5b, S3 Fig).…”

“…The osteogenic potential was also confirmed, as conditioned medium from enriched cells increased calcification by a factor of 1.8±0.3 in MSCs, as compared to the control condition (Fig 5c/5d, S3 Fig). Further data of control groups and analyses of the subpopulations have previously proven the high regenerative potential of the total CD31+ population in vitro and in vivo [13], based on which a simplified in vitro assessment was performed here.…”

“…Whole blood was examined as the cell source, since no additional invasive procedure would be necessary for the patient and it could be done in parallel with a surgery. The initial target for this work is CD31, also known as Platelet/Endothelial Cell Adhesion Molecule 1 (PECAM-1), cells, which have angiogenic, osteogenic and immunosuppressive properties [13]. These cells have been used successfully in vivo for treating ischemic vascular disease [14–16].…”

“…These cells have been used successfully in vivo for treating ischemic vascular disease [14–16]. These cells are widely available, as 70 to 78% of viable lymphocytes and monocytes express CD31 and their properties appear to be independent of gender, and preserved with aging [13]. To achieve a positive selection approach that yields a virtually "untouched" cell population (bead free, ligand free), we exploited reversible binding of cells to aptamers.…”