All implanted biomaterials are targets
of the host’s immune
system. While the host inflammatory response was once considered a
detrimental force to be blunted or avoided, in recent years, it has
become a powerful force to be leveraged to augment biomaterial–tissue
integration and tissue repair. In this review, we will discuss the
major immune cells that mediate the inflammatory response to biomaterials,
with a focus on how biomaterials can be designed to modulate immune
cell behavior to promote biomaterial–tissue integration. In
particular, the intentional activation of monocytes and macrophages
with controlled timing, and modulation of their interactions with
other cell types involved in wound healing, have emerged as key strategies
to improve biomaterial efficacy. To this end, careful design of biomaterial
structure and controlled release of immunomodulators can be employed
to manipulate macrophage phenotype for the maximization of the wound
healing response with enhanced tissue integration and repair, as opposed
to a typical foreign body response characterized by fibrous encapsulation
and implant isolation. We discuss current challenges in the clinical
translation of immunomodulatory biomaterials, such as limitations
in the use of in vitro studies and animal models to model the human
immune response. Finally, we describe future directions and opportunities
for understanding and controlling the biomaterial–immune system
interface, including the application of new imaging tools, new animal
models, the discovery of new cellular targets, and novel techniques
for in situ immune cell reprogramming.
Mesenchymal stromal cells (MSCs) have unique immunomodulatory capacities. We investigated hair follicle-derived MSCs (HF-MSCs) from the dermal sheath, which are advantageous as an alternative source because of their relatively painless and minimally risky extraction procedure. These cells expressed neural markers upon isolation and maintained stemness for a minimum of 10 passages. Furthermore, HF-MSCs showed responsiveness to pro-inflammatory environments by expressing type-II major histocompatibility complex antigens (MHC)-II to a lesser extent than adipose tissue-derived MSCs (AT-MSCs). HF-MSCs effectively inhibited the proliferation of peripheral blood mononuclear cells equivalently to AT-MSCs. Additionally, HF-MSCs promoted the induction of CD4+CD25+FOXP3+ regulatory T cells to the same extent as AT-MSCs. Finally, HF-MSCs, more so than AT-MSCs, skewed M0 and M1 macrophages towards M2 phenotypes, with upregulation of typical M2 markers CD163 and CD206 and downregulation of M1 markers such as CD64, CD86, and MHC-II. Thus, we conclude that HF-MSCs are a promising source for immunomodulation.
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