Understanding the Effects of Both CD14-Mediated Innate Immunity and Device/Tissue Mechanical Mismatch in the Neuroinflammatory Response to Intracortical Microelectrodes

Abstract

Intracortical microelectrodes record neuronal activity of individual neurons within the brain, which can be used to bridge communication between the biological system and computer hardware for both research and rehabilitation purposes. However, long-term consistent neural recordings are difficult to achieve, in large part due to the neuroinflammatory tissue response to the microelectrodes. Prior studies have identified many factors that may contribute to the neuroinflammatory response to intracortical microelectrodes. Unfortunately, each proposed mechanism for the prolonged neuroinflammatory response has been investigated independently, while it is clear that mechanisms can overlap and be difficult to isolate. Therefore, we aimed to determine whether the dual targeting of the innate immune response by inhibiting innate immunity pathways associated with cluster of differentiation 14 (CD14), and the mechanical mismatch could improve the neuroinflammatory response to intracortical microelectrodes. A thiol-ene probe that softens on contact with the physiological environment was used to reduce mechanical mismatch. The thiol-ene probe was both softer and larger in size than the uncoated silicon control probe. Cd14^-/- mice were used to completely inhibit contribution of CD14 to the neuroinflammatory response. Contrary to the initial hypothesis, dual targeting worsened the neuroinflammatory response to intracortical probes. Therefore, probe material and CD14 deficiency were independently assessed for their effect on inflammation and neuronal density by implanting each microelectrode type in both wild-type control and Cd14^-/- mice. Histology results show that 2 weeks after implantation, targeting CD14 results in higher neuronal density and decreased glial scar around the probe, whereas the thiol-ene probe results in more microglia/macrophage activation and greater blood-brain barrier (BBB) disruption around the probe. Chronic histology demonstrate no differences in the inflammatory response at 16 weeks. Over acute time points, results also suggest immunomodulatory approaches such as targeting CD14 can be utilized to decrease inflammation to intracortical microelectrodes. The results obtained in the current study highlight the importance of not only probe material, but probe size, in regard to neuroinflammation.

Keywords: innate immunity; intracortical microelectrodes; neuroinflammation; shape memory polymer; softening electrode.

FIGURE 1

Immunohistochemical evaluation comparing the dual targeting of the innate immune response and mechanical mismatch to control at 2 weeks after implantation. All analyses were evaluated with respect to distance from the explanted microelectrode hole (μm). (A) Neuronal density evaluated as NeuN⁺ cells.(B) Astrocyte encapsulation evaluated as GFAP expression. (C) Blood-brain barrier permeability evaluated as IgG expression. (D) Microglial and macrophage activation evaluated as CD68 expression. ^∗ Denotes significance between silicon shank + WT and thiol-ene + Cd14^-/-; ^# denotes significant difference from background neuronal density.

FIGURE 2

Immunohistochemical evaluation of neuronal density. Neuronal density evaluated as NeuN⁺ cells with respect to distance from the explanted microelectrode hole (μm). (A) 2 weeks. (B) 16 weeks. (C) Representative images of 2 weeks neuronal density. (D) Representative images of 16 weeks neuronal density. Scale bar: 100 μm. ^@ Denotes significance between WT and Cd14^-/-; ^# denotes significant difference from background neuronal density.

FIGURE 3

Immunohistochemical evaluation of glial scarring assessed via astrocyte encapsulation. Astrocyte encapsulation evaluated as GFAP expression with respect to distance from the explanted microelectrode hole (μm). (A) 2 weeks. (B) 16 weeks. (C) Representative images of 2 weeks glial scar. (D) Representative images of 16 weeks glial scar. Scale bar: 100 μm. ^@ Denotes significance between WT and Cd14^-/-.

FIGURE 4

Immunohistochemical evaluation of activated microglia and macrophages. Microglial and macrophage activation evaluated as CD68 expression with respect to distance from the explanted probe hole (μm). (A) 2 weeks. (B) 16 weeks. (C) Representative images of 2 weeks activated microglia and macrophages. (D) Representative images of 16 weeks activated microglia and macrophages. Scale bar: 100 μm. ^∗ Denotes significance between silicon and thiol-ene probes.

FIGURE 5

Immunohistochemical evaluation of blood-brain barrier permeability. Blood–brain barrier permeability evaluated as IgG expression with respect to distance from the explanted microelectrode hole (μm). (A) 2 weeks. (B) 16 weeks. (C) Representative images of 2 weeks BBB permeability. (D) Representative images of 16 weeks BBB permeability. Scale bar: 100 μm. ^∗ Denotes significance between silicon and thiol-ene probes.

FIGURE 6

Bright field microscope image comparing dimensions of probes used in study. (A) uncoated silicon probe and (B) thiol-ene probe. Both images are taken at 5× magnification. Scale is in mm.