Macrophage phenotype and remodeling outcomes in response to biologic scaffolds with and without a cellular component
Introduction
The extracellular matrix (ECM) represents the secreted products, both structural and functional, of the resident cells of each tissue and organ. The composition and ultrastructure of the ECM is determined by factors that influence the phenotype of its resident cells including mechanical forces, biochemical milieu, oxygen requirements, pH and inherent gene expression patterns, among others. In turn, the ECM influences cell attachment, migration, proliferation and three-dimensional organization, serving as an “information highway” between cells [1], [2], [3]. For these reasons, multiple forms of allogeneic and xenogeneic ECM from sources such as small intestine, urinary bladder, and skin have been investigated as biologic scaffolds for tissue reconstruction in both preclinical studies and human clinical applications [4], [5]. Some studies have shown improved tissue remodeling outcomes when site appropriate autologous cells are either seeded onto the ECM scaffold prior to implantation or placed in contact with the scaffold in-situ [6], [7], [8]. However, the survival and fate of such a cellular component during the remodeling process following in vivo implantation is largely unknown, and the effect of the presence of these cells upon the host macrophage response has not been investigated.
Macrophages are a heterogeneous subset of the mononuclear cell population [9], [10], [11] involved in the host response to implanted materials. Macrophages are activated in response to tissue damage or infection, causing an increase in the production of cytokines, chemokines, and other inflammatory molecules to which they are exposed [9], [12], [13], [14]. Recently, macrophage phenotype has been characterized based on distinct functional properties, surface markers, and the cytokine profile of the microenvironment [9], [14], [15]. Polarized macrophages are referred to as either M1 or M2 cells, mimicking the Th1/Th2 nomenclature [9]. However, M1 and M2 represent extremes along a continuum that includes multiple macrophage phenotypes (M1, M2a, M2b and M2c) [14]. M1, classically activated proinflammatory, macrophages are known to be induced by IFN-γ alone or in combination with LPS, TNF and GM-CSF. In general, M1 activated macrophages express IL-12high, IL-23high, IL-10low; metabolize arginine; produce high levels of inducible nitric oxide synthetase (iNOS); secrete toxic reactive oxygen and nitric oxygen intermediates and inflammatory cytokines such as IL-1β, IL-6, and TNF; and are inducer and effector cells in Th1 type inflammatory responses [15]. In contrast, M2, alternatively activated, macrophages are induced by exposure to a variety of signals including the cytokines IL-4, IL-13, and IL-10, immune complexes, and glucocorticoid or secosteroid (vitamin D3) hormones. M2 activated macrophages express IL-12low, IL-23low, and IL-10high; have high levels of scavenger, mannose, and galactose receptors; produce arginase in the place of arginine, subsequently producing ornithine and polyamines; are involved in polarized Th2 reactions; and possess the ability to facilitate tissue repair and regeneration [10], [12], [13], [15].
Macrophages are a plastic cell population capable of sequentially changing their polarization in response to local stimuli during the process of wound healing [16], [17], [18]. The macrophages participating in the host response to an implanted material are exposed to multiple stimuli including cytokines and effector molecules secreted by cells including other macrophages that are participating in the host response, microbial agents, epitopes associated with the implanted biomaterial, and the degradation products of the biomaterial, among others. Therefore, it is logical to assume that the host macrophage response after implantation of a biomaterial is modulated via “cross-talk” between macrophages and the other cells involved in the host response as well as factors within the local microenvironment. The effects of macrophage phenotype upon the tissue remodeling outcome following the implantation of a biomaterial are largely unknown, but recognition of the predominant phenotypic profile may provide a tool by which a constructive and functional tissue remodeling outcome can be predicted and/or promoted.
The objectives of the present study were twofold: (1) to determine the effects of the presence of cells, either autologous or xenogeneic, within an implanted ECM scaffold material upon the phenotype of the macrophages participating in the host response, and (2) to determine the relationship between the M1/M2 profile of the macrophages participating in the host response and the downstream tissue remodeling outcome.
Section snippets
Overview
Sixty-four Sprague–Dawley rats were randomly divided into four separate groups of sixteen each. 1 cm × 1 cm defects were created in the ventrolateral abdominal wall musculature, and repaired using one of the following materials: (1) cellular autograft (autologous body wall tissue), (2) acellular allograft (allogeneic rat body wall ECM), (3) cellular xenograft (xenogeneic pig urinary bladder tissue), or (4) acellular xenograft (xenogeneic pig urinary bladder ECM). See Table 1. The treatment
Results
All of the animals in this study survived the surgical procedure and post-operative period without complications.
Discussion
The present study examined the effects of the presence of a cellular component within a scaffold derived from extracellular matrix upon the polarization of the macrophages participating in the host response following implantation. The study also examined the relationship between macrophage polarization and host tissue remodeling events until 28 days post implantation. The results of the study indicate that the presence of a cellular component within an ECM derived scaffold shifts the macrophage
Conclusion
The present study showed that the presence of a cellular component within an extracellular matrix scaffold modulates the phenotype of the macrophages participating in the host response following implantation. It was observed that those test articles that contained a cellular component, even an autologous cellular component, elicited a predominantly M1 type macrophage response and resulted in the deposition of dense connective tissue and/or scarring. Those test articles that did not contain a
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