Monocytes and macrophages in tissue repair: Implications for immunoregenerative biomaterial design

Abstract

Monocytes and macrophages play a critical role in tissue development, homeostasis, and injury repair. These innate immune cells participate in guiding vascular remodeling, stimulation of local stem and progenitor cells, and structural repair of tissues such as muscle and bone. Therefore, there is a great interest in harnessing this powerful endogenous cell source for therapeutic regeneration through immunoregenerative biomaterial engineering. These materials seek to harness specific subpopulations of monocytes/macrophages to promote repair by influencing their recruitment, positioning, differentiation, and function within a damaged tissue. Monocyte and macrophage phenotypes span a continuum of inflammatory (M1) to anti-inflammatory or pro-regenerative cells (M2), and their heterogeneous functions are highly dependent on microenvironmental cues within the injury niche. Increasing evidence suggests that division of labor among subpopulations of monocytes and macrophages could allow for harnessing regenerative functions over inflammatory functions of myeloid cells; however, the complex balance between necessary functions of inflammatory versus regenerative myeloid cells remains to be fully elucidated. Historically, biomaterial-based therapies for promoting tissue regeneration were designed to minimize the host inflammatory response; although, recent appreciation for the roles that innate immune cells play in tissue repair and material integration has shifted this paradigm. A number of opportunities exist to exploit known signaling systems of specific populations of monocytes/macrophages to promote repair and to better understand the biological and pathological roles of myeloid cells. This review seeks to outline the characteristics of distinct populations of monocytes and macrophages, identify the role of these cells within diverse tissue injury niches, and offer design criteria for immunoregenerative biomaterials given the intrinsic inflammatory response to their implantation.

Keywords: Monocyte; biomaterial; inflammation; macrophage; regeneration; wound healing.

Figure 1

During injury, heterogeneous populations of monocytes (IM and AM) are recruited from circulation, where they can persist transiently as monocytes or rapidly differentiate into macrophages ranging in phenotype from M1-type to M2-type. Within the specific injury niche, monocytes and macrophages can guide tissue repair by promoting angiogenesis and arteriogenesis, secreting growth factors that guide progenitor cell differentiation and proliferation, regulating the secretome of parenchymal cells, and releasing inflammatory mediators that guide other immune cells. (A color version of this figure is available in the online journal.)

Figure 2

The fate of monocytes and macrophages surrounding implants can be greatly affected by material properties, both those inherent to the selected material and those engineered to confer additional functionality. Properties with effects on macrophages include topographical cues, bulk mechanical properties such as elasticity and stiffness, porosity, functionalization with ligands, and release of biomolecules. (A color version of this figure is available in the online journal.)