Interferon-Gamma Impairs Expansion and Hematopoietic Support of Bone Marrow Mesenchymal Stromal Cells

Maintenance of hematopoietic stem cells (HSCs) and regulation of their quiescence and self-renewal is critical for maintaining a lifelong supply of blood cells. The ability of HSCs to stay quiescent is thought to depend on their specific niche in the bone marrow (BM). Mesenchymal stromal cells (MSC) in the BM are multipotent stem cells that form part of the vascular HSC niche and provide micro-environmental support to HSCs both in vivo and upon expansion ex vivo. Culture-expanded MSCs also exhibit immunomodulatory properties that can be enhanced by pre-treatment with interferon-gamma (IFN-γ). BM MSC are thus attractive candidates for cellular therapy after hematopoietic stem cell transplantation, for promoting rapid hematopoietic recovery and reducing the incidence or severity of graft versus host disease. Although IFN-γ pre-treatment can improve the immunomodulatory properties of MSCs, elevated IFN-γ levels have also been associated with anemia and BM failure in multiple chronic inflammatory diseases. While the impact of IFN-γ on HSC has been elucidated in recent years, it remains largely unknown whether IFN-γ can also influence hematopoietic support by BM stromal cells.

In this study, we aim to elucidate the impact of IFN-γ on hematopoietic support of BM MSC.

We show that in vitro expansion of primary BM MSC cultures from healthy donors was significantly reduced in the presence of IFN-γ, and this effect could be reproduced in the BM stromal cell line MS-5. Concurrently, IFN-γ diminished the clonal capacity of BM MSC, as measured by CFU-F assays. In addition, BM MSC that were pre-stimulated with IFN-γ produced significantly lower levels of CXCL12, suggesting a loss of hematopoietic support potential. Indeed, support of CD34⁺ hematopoietic stem and progenitor cells (HSPC) in a co-culture assay was greatly reduced in when MSC were pre-treated with IFN-γ.

To determine the impact of IFN-γ on BM MSC in vivo, we investigated the BM stromal compartment of IFN-γ AU-rich element deleted (ARE-Del) mice, which constitutively express IFN-γ in steady state conditions. FACS analysis revealed a remodeling of the BM stromal compartment in ARE-Del mice compared to littermate controls, with significantly fewer MSCs, identified as CD45^-Ter119^-CD31^-CD51⁺PDGFRa⁺ cells. Numbers of other stromal cell subsets, such as osteoblasts and fibroblasts, were not altered.

The reduction of BM MSC in ARE-Del mice coincided with a loss of quiescence in HSCs; only 35% of long term HSC (LT-HSC) in ARE-Del mice were quiescent, compared to 70% in WT mice, as determined by Ki-67 staining. Loss of quiescence in LT-HSC did not lead to increased self-renewal, but rather induced increased differentiation towards short-term HSC and multi-potent progenitors.

We then sorted LT-HSC from WT and ARE-Del mice and performed in vitro HSC culture assays in the absence of IFN-γ. Absolute numbers of LT-HSC were rapidly decreased in ARE-Del compared to WT cultures after 3 and 7 days of HSC culture, while numbers of more differentiated progenitors were increased. These data indicate that an IFN-γ-mediated loss of BM MSC in ARE-Del mice leads to loss of quiescent LT-HSCs and induces a tendency towards HSC differentiation over self-renewal.

In conclusion, we have shown that IFN-γ has a negative impact on expansion and hematopoietic support of BM MSC in vitro and in vivo across species. Although IFN-γ treatment enhances the immunomodulatory function of MSCs in a clinical setting, it is obvious from our data that IFN-γ impairs their HSC supporting function. These data also provide more insight in the underlying mechanism by which IFN-γ contributes to the pathogenesis of anemia and BM failure.