Inflammatory Foreign Body Response Induced by Neuro-Implants in Rat Cortices Depleted of Resident Microglia by a CSF1R Inhibitor and Its Implications

Abstract

Inflammatory encapsulation of implanted cortical-neuro-probes [the foreign body response (FBR)] severely limits their use in basic brain research and in clinical applications. A better understanding of the inflammatory FBR is needed to effectively mitigate these critical limitations. Combining the use of the brain permeant colony stimulating factor 1 receptor inhibitor PLX5622 and a perforated polyimide-based multielectrode array platform (PPMP) that can be sectioned along with the surrounding tissue, we examined the contribution of microglia to the formation of inflammatory FBR. To that end, we imaged the inflammatory processes induced by PPMP implantations after eliminating 89–94% of the cortical microglia by PLX5622 treatment. The observations showed that: (I) inflammatory encapsulation of implanted PPMPs proceeds by astrocytes in microglia-free cortices. The activated astrocytes adhered to the PPMP’s surfaces. This suggests that the roles of microglia in the FBR might be redundant. (II) PPMP implantation into control or continuously PLX5622-treated rats triggered a localized surge of microglia mitosis. The daughter cells that formed a “cloud” of short-lived (T1/2 ≤ 14 days) microglia around and in contact with the implant surfaces were PLX5622 insensitive. (III) Neuron degeneration by PPMP implantation and the ensuing recovery in time, space, and density progressed in a similar manner in the cortices following 89–94% depletion of microglia. This implies that microglia do not serve a protective role with respect to the neurons. (IV) Although the overall cell composition and dimensions of the encapsulating scar in PLX5622-treated rats differed from the controls, the recorded field potential (FP) qualities and yield were undistinguishable. This is accounted for by assuming that the FP amplitudes in the control and PLX5622-treated rats were related to the seal resistance formed at the interface between the adhering microglia and/or astrocytes and the PPMP platform rather than across the scar tissue. These observations suggest that the prevention of both astrocytes and microglia adhesion to the electrodes is required to improve FP recording quality and yield.

Ultrastructural analysis of neuroimplant-parenchyma interfaces uncover remarkable neuroregeneration along-with barriers that limit the implant electrophysiological functions

“…The electron-dense cytoplasm of these microglia was bordered by a clear plasma membrane and contained rough endoplasmic reticulum, mitochondria and dark inclusions which may plausibly be lysosomes and lipofuscin granules (Figures 3 and 4). In line with Figure 2 and our earlier immunohistological studies (Huang et al 2020;Sharon et al 2021), two weeks post-PPMP implantation, TEM images revealed the presence of astrocyte cell bodies and branches in close proximity to the platform surface and within the platform pores (Figures 3B and 4A). Astrocyte cell bodies could be identified by their pale nuclei that had a thin rim of heterochromatin and pale cytoplasm (Figures 3 and 4A).…”

“…PI was selected because it is a biocompatible polymer with a Young’s modulus of 2.5 GP. Importantly, based on studies demonstrating that PI implants can be thin-sectioned for histological examinations (Mercanzini et al, 2007;Mercanzini et al, 2008;Richter et al, 2013;Xie et al, 2014;Boehler et al, 2017;Huang et al, 2020) our laboratory has developed procedures to section implanted gMμE-PPMPs along with the surrounding brain parenchyma for light and transmission electron-microscope (TEM) studies (Huang et al, 2020;Sharon et al, 2021). In the present study, we fabricated 1.7 mm long, 280 μm wide and 16 μm thick nonfunctional gMμE-PPMPs.…”

“…Immunolabeling, imaging, image processing and analysis were conducted as detailed in previous studies from our laboratory (Huang et al, 2020;Sharon et al, 2021). Briefly, neurons were concomitantly labelled with two antibodies: one for neurite labelling (mouse anti-Neurofilament 160/200 monoclonal antibody (Sigma-Aldrich N2912, 1:10000-1:20000) and the other for neuronal nuclei (mouse anti-NeuN monoclonal antibody (Merck MAB377, 1:200)).…”

“…The observed TEM images and conclusions represent transmission electron microscope imaging of over >250 thin sections prepared from 25 different PPMPs implants. Immunolabeling, confocal imaging, image processing, analysis and statistics Immunolabeling, imaging, image processing and analysis were conducted as detailed in previous studies from our laboratory (Huang et al, 2020;Sharon et al, 2021). Briefly, neurons were concomitantly labelled with two antibodies: one for neurite labelling (mouse anti-Neurofilament 160/200 monoclonal antibody (Sigma-Aldrich N2912, 1:10000-1:20000) and the other for neuronal nuclei (mouse anti-NeuN monoclonal antibody (Merck MAB377, 1:200)).…”

“…To examine the interfaces formed between the brain parenchyma and implanted gMμE-PPMPs, cross-sections for transmission electron microscopy of gMμE-PPMPs along with the surrounding tissue were prepared. We selected to examine the ultrastructure of the inflammatory scar at two, four and eight weeks after electrode implantation, since our earlier immunohistological studies showed characteristic alterations in the distribution and densities of the microglia, astrocytes and neurons at these points in time (Figure 2 and Huang et al, 2020;Sharon et al, 2021). For the reader's convenience, the overall spatiotemporal relationships between microglia, astrocytes, neurons and PPMP implants is briefly presented in Figure 2 using conventional immunohistological imaging.…”