Mechanical Resistance of Extracellular Matrix Directs Immune Fate of Bone Marrow Niche

Abstract

Artificial systems of extracellular matrix (ECM) hydrogels with human cells were developed to study how the mechanical resistance of ECM directs the immune fate of bone marrow niche cells - mesenchymal stromal cells (MSCs) and monocytes. An artificial fibrillar ECM was fabricated with interpenetrating networks of type-I collagen and chemically-modified polysaccharides. Viscoelasticity was specifically tuned independent of other material properties across a physiologic range of bone marrow stiffness. First, the effect of ECM stiffness on MSCs was examined. MSCs reside in the hematopoietic niche and can produce immunomodulatory factors to restrain inflammation. Stiffness of more viscous, but not elastic, ECM upregulated their expression of immunomodulatory markers. Next, it was determined how bone marrow monocytes respond to mechanical cues of viscoelasticity of ECM. Slower stress-relaxing, more elastic ECM promoted monocytes to secrete inflammatory cytokines and differentiate into antigen-presenting cells, whereas cells in viscous hydrogels remained immature. Mechanistically, monocytes in elastic ECM upregulated their cortical F-actin cytoskeleton, in a manner dependent on PI3K-gamma signaling, and inhibition of PI3K-gamma blocked antigen-presenting cell differentiation driven by mechanical cues. Further, the Jak-Stat signaling pathway that drives human diseases associated with myelofibrosis was upregulated by matrix elasticity. Overt fibrosis in biopsies from patients with primary myelofibrosis was associated with upregulated cytokines, such as IL8 and IL6, which were also upregulated in donor monocytes by matrix elasticity and sensitive to inhibition of PI3K-gamma. Together, these data suggest that viscoelasticity of ECM directs immune fates of the bone marrow niche. More fluid-like, viscous matrix regulates immunmodulatory expression of MSCs and maintenance of immature monocytes, which is consistent with homeostasis in healthy bone marrow. Conversely, increased solid-like, elasticity of fibrotic ECM polarizes inflamed monocytes and directs their differentiation into antigen-presenting cells in a PI3K-gamma-dependent manner. A major impact of this work is that PI3K-gamma was revealed as a promising therapeutic target in diseases associated with myelofibrosis, and potentially more broadly for fibrotic and inflammatory diseases in general.