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The role of astrocytes and microglia as antigen‐presenting cells in the brain is still controversial. In this study we have analyzed and compared aspects of the molecular machinery that underlies MHC class II trafficking in immunocompetent astrocytes and microglia. We show that IFN‐γ‐stimulated microglia possess active cathepsin L and cathepsin S, and efficiently degrade the invariant chain, unlike IFN‐γ‐stimulated astrocytes that express cathepsin L but not cathepsin S. The lack of cathepsin S proves to be dramatic for the antigen‐presentation capacity of astrocytes, whichis nearly abolished when these cells are stimulated by a combination of IFN‐γ and TNF‐α. TNF‐α indeed decreases cathepsin L activity as we show here, leading to alterations in invariantchain processing, and hence in MHC class II trafficking in astrocytes. Cystatin C inhibits cathepsin L activity in astrocytes, but does not regulate cathepsin L and cathepsin S activity in microglia. We therefore identify cathepsin L and cathepsin S as key components in the regulation of the immune potential of astrocytes and microglia, and provide evidence for a cell‐specific regulation exerted byIFN‐γ and TNF‐α on the expression and activity of cathepsins.

The mannose receptor is a pattern-recognition receptor involved in innate and adaptive immunity. The receptor is mainly expressed by macrophages and, within the brain, by astrocytes and microglia. This study reports for the first time the effects of two classical proinflammatory (interferon-gamma, IFNgamma) and anti-inflammatory (interleukin-4, IL-4) cytokines on the levels of expression and activity of the mannose receptor expressed by mouse microglia, the brain resident macrophages. As observed for macrophages, IFNgamma treatment led to a decrease and IL-4 to an increase of mannose receptor expression. Consequently, the rates of pinocytosis were strongly upregulated by IL-4 and inhibited by IFNgamma. This latter, however, resumed with time and reached again the constitutive rate of pinocytosis. This recovery resulted from an increased pinocytic activity of the few mannose receptor molecules still expressed by IFNgamma-treated microglia. This may suggest a brain-specific regulation of the effects of IFNgamma since such a phenomenon has not been observed in macrophages. Together, these observations demonstrate that cytokine-stimulated immunocompetent microglia express a functional mannose receptor.

Astrocytes and microglia, two glial cell populations of the CNS, have been described to be involved in many immune processes. We used defined combinations of cytokines, interferon gamma (IFN-gamma)/interleukin-1 alpha (IL-1 alpha) and IFN-gamma/tumor necrosis factor alpha (TNF alpha), to simulate different in vitro immune environments observed in disease or inflammation. In these conditions, we analyzed and compared the regulating effects of these cytokines on cell surface and total expression of MHC II and on the capacity of murine astrocytes and microglia to present peptide and native antigens to specific primed T cells. Neither IL-1 alpha nor TNF alpha affected the IFN-gamma-induced antigen presentation capacity of microglia. Astrocytes, however, were severely impaired in their capacity to present native antigens and, to a minor extent, a peptide antigen. Total expression of MHC II was not affected by these cytokines in microglia, whereas in astrocytes it was reduced by IL-1 alpha and increased by TNF alpha. Both cytokines downregulated MHC II expression at the surface of astrocytes, but not of microglia. This shows that TNF alpha affects the of IFN-gamma-immunocompetent astrocytes to process and present antigen, probably either by altering membrane traffic of MHC II and of antigen and/or enzymatic activities associated with these mechanisms, while IL-1 alpha does so by downregulating MHC II expression. Altogether, our results illustrate how differently astrocytes and microglia react toward a defined, similar immune environment. One type of cell, the astrocytes, downregulate their T-cell stimulation and MHC II trafficking, and probably also their antigen processing, functions while the other, the microglia, maintain their antigen presentation potential.

Recently, numerous innovative approaches have attempted to overcome the shortcomings of standard tissue culturing by providing custom-tailored substrates with superior features. In particular, tunable surface chemistry and topographical micro-and nanostructuring have been highlighted as potent effectors to control cell behavior. Apart from tissue engineering and the development of biosensors and diagnostic assays, the need for custom-tailored platform systems is accentuated by a variety of complex and poorly characterized biological processes. One of these processes is cell-to-cell communication mediated by tunneling nanotubes ͑TNTs͒, the reliable statistical analysis of which is consistently hampered by critical dependencies on various experimental factors, such as cell singularization, spacing, and alignment. Here, the authors developed a microstructured platform based on a combination of controlled surface chemistry along with topographic parameters, which permits the controllable attachment of different cell types to complementary patterns of cell attracting/nonattracting surface domains and-as a consequencerepresents a standardized analysis tool to approach a wide range of biological questions. Apart from the technical complementation of mainstream applications, the developed surfaces could successfully be used to statistically determine TNT-based intercellular connection processes as they are occurring in standard as well as primary cell cultures.

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