It has been hypothesized that physiological shear forces acting on medical devices implanted in the brain significantly accelerate the rate to device failure in patients with chronically indwelling neuroprosthetics. In hydrocephalus shunt devices, shear forces arise from cerebrospinal fluid flow. The shunt’s unacceptably high failure rate is mostly due to obstruction with adherent inflammatory cells. Astrocytes are the dominant cell type bound directly to obstructing shunts, rapidly manipulating their activation via shear stress-dependent cytokine secretion. Here we developed a total internal reflection fluorescence microscopy combined with a microfluidic shear device chip (MSDC) for quantitative analysis and direct spatial-temporal mapping of secreted cytokines at the single-cell level under physiological shear stress to identify the root cause for shunt failure. Real-time secretion imaging at 1-min time intervals enabled successful detection of a significant increase of astrocyte IL-6 cytokine secretion under shear stress greater than 0.5 dyne/cm2, validating our hypothesis and highlighting the importance of reducing shear stress activation of cells.

Background The composition of tissue obstructing neuroprosthetic devices is largely composed of inflammatory cells with a significant astrocyte component. In a first-of-its-kind study, we profile the astrocyte phenotypes present on hydrocephalus shunts. Methods qPCR and RNA in-situ hybridization were used to quantify pro-inflammatory (A1) and anti-inflammatory (A2) reactive astrocyte phenotypes by analyzing C3 and EMP1 genes, respectively. Additionally, CSF cytokine levels were quantified using ELISA. In an in vitro model of astrocyte growth on shunts, different cytokines were used to prevent the activation of resting astrocytes into the A1 and A2 phenotypes. Obstructed and non-obstructed shunts were characterized based on the degree of actual tissue blockage on the shunt surface instead of clinical diagnosis. Results The results showed a heterogeneous population of A1 and A2 reactive astrocytes on the shunts with obstructed shunts having a significantly higher proportion of A2 astrocytes compared to non-obstructed shunts. In addition, the pro-A2 cytokine IL-6 inducing proliferation of astrocytes was found at higher concentrations among CSF from obstructed samples. Consequently, in the in vitro model of astrocyte growth on shunts, cytokine neutralizing antibodies were used to prevent activation of resting astrocytes into the A1 and A2 phenotypes which resulted in a significant reduction in both A1 and A2 growth. Conclusions Therefore, targeting cytokines involved with astrocyte A1 and A2 activation is a promising intervention aimed to prevent shunt obstruction.

Understanding the composition characteristics of the glial scar contributing to the high failure rate of neuroprosthetic devices implanted in the brain has been limited, to date, with the evaluation of cells, tissue, and biomarkers obstructing the implant. However, there remains a critical knowledge gap in gene expression profiles of the obstructing cells. This first-time study investigates the phenotypic expression specific to astrocyte scarring from those cells on hydrocephalus shunt surfaces at the time of failure, aimed at the development of therapeutic approaches to target reactive astrocytes for improved functional outcome. Recent evidence has indicated that the tissue obstructing shunts is over 80% inflammatory, with a more exaggerated astrocytic response. To understand how to mitigate the astrocyte immune response to shunts, we performed gene expression profiling of the C3 and EMP1 genes to quantify if astrocytes were classically activated and pro-inflammatory (A1) or alternatively activated and anti-inflammatory (A2), respectively. Shunt catheters were removed from patients at the time of failure and categorized by obstructed vs non-obstructed shunts. RNAscope fluorescent in situ hybridization and quantitative PCR analysis of the C3 and EMP1 expressed genes revealed that a heterogeneous mixed population of both the A1 and A2 reactive phenotype exist on the shunt surface. However, the number of A2 reactive astrocytes are significantly higher on obstructed shunts compared to A1 reactive astrocytes. ELISA data also confirmed higher levels of IL-6 for obstructed shunts involved in A2 reactive astrocyte proliferation and glial scar formation on the shunt surface. Since TNF-α and IL-1β propel resting astrocytes into an A2 reactive state, by simply blocking the secretion or action of these cytokines, astrocyte activation and attachment on obstructing shunts could be inhibited.