A unique Thy-1-negative immuno-fibroblast population emerges as a key determinant of fibrotic outcomes to biomaterials

Abstract

Microporous annealed particle (MAP) hydrogels are an exciting new development in biomaterial design. They regulate innate and acquired immunity which has been linked to their ability to evade normal host-material fibrosis. Yet, resident stromal fibroblasts, not immune cells, are the arbiters of the extracellular matrix assembly that characterizes fibrosis. In other idiopathic fibrotic disorders, a fibroblast subpopulation defined by its loss of cell surface Thy-1 expression is strongly correlated with degree of fibrosis. We have previously shown that Thy-1 is a critical v{beta}3 integrin regulator that enables normal fibroblast mechanosensing and here, leveraging non-fibrosing MAP gels, we demonstrate that Thy-1-/- mice mount a robust response to MAP gels that remarkably resembles a classical foreign body response. We further find that within the naive, Thy-1+ fibroblast population exists a distinct and cryptic SMA+ Thy-1- population that emerges in response to IL-1{beta} and TNF. Employing single-cell RNA sequencing, we find that IL-1{beta}/TNF-induced Thy-1- fibroblasts actually consist of two distinct subpopulations, both of which are strongly pro-inflammatory. These findings illustrate the emergence of a unique pro-inflammatory, pro-fibrotic fibroblast subpopulation that is central to material-associated fibrosis likely through amplifying local inflammatory signaling. Significance StatementDespite decades of research, implanted biomaterials are still significantly hampered by the foreign body response and fibrotic encapsulation. Advancements in material design and immunomodulation have made positive impacts, yet, a fuller mechanistic understanding of aberrant ECM remodeling in the biomaterial microenvironment could improve approaches in biomaterial-host interactions. Here, we leverage anti-fibrotic MAP hydrogels and demonstrate that their ability to evade fibrosis is linked to fibroblast Thy-1 (CD90) surface expression. Thy-1-/- mice exhibit elevated NF{kappa}B signaling and elevated fibrosis in response to MAP gel implantation. Interestingly, pro-inflammatory cytokines elicit a Thy-1-/SMA+ myofibroblast subpopulation. Single-cell RNA-Seq more fully identifies an immunofibroblast subpopulation defined by Thy-1 loss and a pro-inflammatory/fibrotic cytokine, chemokine, and cytokine receptor expression profile, suggesting a self-perpetuating pro-fibrotic subpopulation.