Macrophage phenotype as a predictor of constructive remodeling following the implantation of biologically derived surgical mesh materials
Introduction
Biologic materials composed of extracellular matrix (ECM) have been harvested from a wide variety of tissues and organs and have been used in a similarly wide variety of preclinical and clinical applications [1], [2]. It has been shown that ECM based materials, if prepared and utilized appropriately, are capable of acting as inductive templates for the formation of site-specific functional host tissues following implantation [3], [4], [5]. Alternatively, if processing methods do not effectively decellularize the source tissue, involve chemicals that create non-degradable molecular cross-links, or leave residual reagents in the ECM, then the in vivo remodeling response is less desirable and characterized by chronic inflammation, fibrotic encapsulation, and scar tissue formation [6], [7], [8]. The mechanisms by which biologic mesh materials elicit either “constructive remodeling” or chronic inflammation, however, are only partially understood.
The process of tissue remodeling following implantation has been shown to be invariably associated with a robust macrophage response beginning as early as two days post-implantation and continuing for several months depending on the mesh material and the clinical application in which it is used [8]. The prolonged presence of macrophages at a site in which the remodeling outcome can range from scarring to healthy functional tissue formation suggests a central, and perhaps determinant, role for macrophages in tissue remodeling following surgical mesh implantation.
Activated macrophages possess diverse, plastic phenotypes that play an important role in the host inflammatory response and the process of tissue repair and remodeling following injury [9], [10], [11], [12], [13], [14]. Macrophage phenotype is dependent upon interactions with microbial and non-microbial components as well as the cytokines and chemokines secreted by other cells within the microenvironment [10], [15], [16]. Macrophage phenotype has been broadly characterized as M1, or “classically” activated, and M2, or “alternatively” activated, mimicking the Th1/Th2 nomenclature [15]; however, it is well recognized that macrophages are a heterogeneous cell population and that M1 and M2 represent extremes on a spectrum of macrophage phenotypes [9], [10], [16]. M1 refers to macrophages activated by bacterial lipopolysaccharide (LPS) and interferon-γ (IFN-γ) and possessing characteristics which include production of large amounts of pro-inflammatory signaling and effector molecules, efficient antigen presentation, killing of intracellular pathogens, tumor destruction, and promotion of polarized Th1 responses. M2 refers to macrophages which are activated by interleukin-4 (IL-4), IL-10, IL-13, or a combination thereof, and possessing immunoregulatory or tissue remodeling characteristics which include minimal production of pro-inflammatory molecules, expression of scavenger, mannose, and galactose receptors, increased phagocytic activity, and participation in polarized Th2 reactions. M2 macrophages have been further shown to consist of subdivisions including M2a, M2b, and M2c depending on the activating signals and functional characteristics [10]. The exact role of each M2 subtype in tissue remodeling is not well defined. However, heterogeneity and plasticity of macrophage phenotype are increasingly recognized as playing an important role in the response to pathogens and tissue injury as well as in the development and progression of a variety of diseases including obesity, atherosclerosis, and cancer [17], [18], [19].
The macrophage population present immediately following tissue injury possesses predominantly M1 characteristics [20], [21]. Transition to an M2 phenotype occurs concurrently with resolution of the inflammatory process and the initiation of the remodeling phase of wound healing [20]. This process represents the default mammalian host response to tissue injury, and generally results in the formation of localized scar tissue. Certain biologic mesh materials have been shown to modulate this default host response and facilitate the formation of site-appropriate functional host tissue instead of scar tissue [6], [7]. Recent work shows that the presence of distinct phenotypic populations of macrophages at early time points following implantation may be predictive of downstream outcomes consistent with constructive tissue remodeling [6], [7].
The present study examined both the in vivo host remodeling response to 14 FDA approved, biologically derived surgical mesh materials and the in vitro chemotactic response of a muscle progenitor-like cell line toward the secreted products of phenotypically distinct macrophages. The mesh materials were all composed of naturally occurring biomaterials but varied in their tissue and species source and method of production. The in situ polarization of macrophages following device implantation was determined and correlated to the observed real time and downstream tissue remodeling outcomes. The in vitro chemotactic studies were conducted to examine a potential mechanism by which lineage directed cells participate in constructive, site-specific remodeling.
Section snippets
Test articles
The biologic mesh materials evaluated, their manufacturer, and composition are listed in Table 1. All materials were received sterile and trimmed to size (1 cm × 1 cm) prior to implantation. It should be noted that all of the devices examined in the present study were commercially available materials in their original packaging. Therefore, while not quantitatively measured in the present study, all materials were assumed to meet FDA mandated standards for endotoxin content (<20 EU).
Animal model
Fifty-six
Histomorphologic analysis
Each material elicited a distinct host tissue remodeling response that was characterized by varying degrees of cellular infiltration, vascularization, connective tissue deposition and tissue remodeling. Despite differences in the host tissue response to each material, the responses could be characterized as falling into one of three general qualitative and quantitative groups (Fig. 1, Fig. 2). The groups were characterized by chronic inflammation and foreign body response (quantitative score < 5;
Discussion
The present study investigated the host response to 14 commercially available biologic surgical mesh materials and showed that macrophage phenotype at 14 days post-implantation is correlated with both the early histomorphologic response and the later tissue remodeling outcome. Each material was distinct in its source tissue and methods of preparation and elicited a response that was characterized by a distinct histomorphologic progression of tissue remodeling at 14 and 35 days, which is similar
Conclusion
The histomorphologic progression of remodeling associated with each of the implanted materials examined in the present study is similar to that reported clinically for each of the devices examined. The present study clearly shows that there is a strong correlation between the early macrophage response to implanted ECM scaffold materials and the outcome of tissue remodeling. Further, the results of the present study suggest that downstream effects may be due to the recruitment and survival of
Acknowledgements
Funding for this study was provided through a grant from the National Institutes of Health (R01 AR054940-01). Additionally, Bryan Brown was supported by NIH fellowship F31 EB007914.
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