Cross-talk between myeloid and mesenchymal stem cells (MSC) in the aged human bone marrow

Bone Marrow undergoes structural and functional reshaping during aging. The bone trabecular structure filled with hematopoietic cells is gradually replaced by a rarefied trabecular net, often demineralized, filled with adipocytes at various stages of differentiation, from pre-adipocytes to mature coalescent bodies. In long bones such as femur, this diminishes the bone mechanical strength increasing the risk of bone necrosis and femur fracture. Moreover, it deeply affects the functional interplay between Hematopoietic Stem Cells (HSC), MSCs, ECM proteins and the variety of soluble interactors composing the BM microenvironment. As a results, the MSC skew their differentiation potential toward adipocytes and Hematopoietic Stem cells reduce their self-renewing meanwhile increasing the myeloid differentiation and decreasing the generation of Common Lymphoid Progenitors. In this study we have investigated some aspects of the cross-talk between Myeloid Derived Suppressor Cells (MDSC) and Mesenchimal Stem Cells (MSC) in Bone Marrow (BM) samples obtained from 57 subject ranging 18- 92 years. Results of hemocytometric analysis and multiparametric immuno-phenotyping demonstrated that: the number of WBC decrease significantly with age, with no substantial imbalance of the myeloid and lymphoid popula-tions and subpopulations with exception of B cells, greatly reduced in elders. Also the percentage of MDSC was stable during aging. However, MDSC from elders showed a strong basal immune-suppression of autologous or het-erologous T cell proliferation. As no differences between youngster and elders were observed when suppression was induced by G/GM treatment, we hypothesized that the basal suppression observed in elders depended on the progressive accumulation in BM of activating stimuli. These could include cell-cell contact with MSC and adipocytes or the activity of IL-6, IL-1β and TNF-α that were found dramatically increased in aged BM. We therefore de-rived MSC from youngsters, adults and elders BM, characterized and used them in co-culture experiments with youngsters MDSC. Results indicated that the contact between MDSC and MSC or adipocytes, but not the activity of plasma from elders or of supernatants of MSC/MDSC co-cultures, induced the suppression activity of MDSC. We next investigated whether the cross-talk between MSC and MDSC in-duced the activation of adipogenesis in BM-MSC. Results of co-culturing youngsters MSC with elders MDSC and viceversa demonstrated that this was the case, as assessed by detecting the activation of p38 MAPK, the expression of C/EBP beta transcription factor and the presence of the lipid drops in treat-ed MSC. In contrast with what observed when inducing the suppressive activ-ity in MDSC, the BM-plasma of elders containing high levels of IL-6 (500-1000 ng/ml), but not that of youngsters almost devoid of IL-6, could efficiently replace MDSC as inducers of adipogenesis. This was of particular interest in the light of the finding that MSC adipogenesis was associated with strong induction of IL-6 gene expression. The activity of IL-6 among the other cytokines was further confirmed by dose response experiments of induction of adipogenesis in MSC performed with recombinant IL-6. Collectively these results demonstrated that a detrimental cross-talk occurs in elders BM resulting in induction of suppression in MDSC and of adipogenesis in MSC most likely induced or maintained by both cell-cells contacts and Il-6. These result open up new possibilities to define markers and targets for preventive or therapeutic strategies in aging eventually regulating immunity, inflammation and bone resorption in elders.