Activated microglial cells acquire an immature dendritic cell phenotype and may terminate the immune response in an acute model of EAE Beatriz Almolda ⁎, Berta González, Bernardo Castellano Department of Cellular Biology, Physiology and Immunology, Institute of Neurosciences, Autonomous University of Barcelona, Bellaterra 08193, Spain abstract article info Article history: Received 4 February 2010 Received in revised form 17 March 2010 Accepted 31 March 2010 Keywords: Fascin CD1 B7 MHC MS model ED2 Antigen presentation, a key mechanism in immune responses, involves two main signals: the first is provided by the engagement of a major histocompatibility complex (MHC), class I or class II, with their TCR receptor in lymphocytes, whereas the second demands the participation of different co-stimulatory molecules, such as CD28, CTLA-4 and their receptors B7.1 and B7.2. Specific T-cell activation and deactivation are achieved through this signalling. The aim of our study is to characterise, in the acute experimental autoimmune encephalomyelitis (EAE) model in Lewis rat, the temporal expression pattern of these molecules as well as the cells responsible for their expression. To accomplish that, MBP-immunised female Lewis rats were daily examined for the presence of clinical symptoms and sacrificed, according to their clinical score, at different phases during EAE. Spinal cords were cut with a cryostat and processed for immunohistochemistry: MHC-class I and MHC-class II, co-stimulatory molecules (B7.1, B7.2, CD28, CTLA-4) and markers of dendritic cells (CD1 for immature cells and fascin for mature cells). Our results show that microglial cells are activated in the inductive phase and, during this phase and peak, they are able to express MHC-class I, MHC-class II and CD1, but not B7.1 and B7.2. This microglial phenotype may induce the apoptosis or anergy of infiltrated CD28+ lymphocytes observed around blood vessels and in the parenchyma. During the recovery phase, microglial cells express high MHC-class I and class II and, those located in the surroundings of blood vessels, displayed the B7.2 co-stimulatory molecule. These cells are competent to interact with CTLA-4+ cells, which indicate an active role of microglial cells in modulating the ending of the immune response by inducing lymphocyte activity inhibition and Treg activation. Once clinical symptomatology disappeared, some foci of activated microglial cells (MHC-class II+/B7.2+) were still present in concomitance with CTLA-4+ cells, suggesting a prolonged involvement of microglia in lymphocyte inhibition and tolerance promotion. In addition to microglia, during the inductive and recovery phases, we also found perivascular ED2+ cells and fascin+ cells which are able to migrate to the parenchyma and may play a role in lymphocytic regulation. Further studies to understand the specific function played by these cells are warranted. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Antigen presentation is a crucial process in T-cell activation and modulation of immune responses. Two main signals are involved in this process. The first, provided by the engagement of either the MHC- class I or MHC-class II on antigen-presenting cells (APCs) with the T- cell receptor (TCR) on T lymphocytes, controls the specificity of the immune response, as MHC-class I is recognised by CD8+ T cells whereas MHC-class II interacts with CD4+ T cells (Janeway, 1992). The second signal, the co-stimulatory signal, is antigen non-specific, involves the interaction of different T-cell surface receptors with their respective ligands on APCs (Lanzavecchia, 1997; Lenschow et al., 1996) and is essential for the full T-cell activation, as TCR-MHC binding in the absence of co-stimulation can lead to T-cell apoptosis or anergy (Kishimoto and Sprent, 1999). Different combinations of co- stimulatory molecules and receptors providing stimulatory or inhibitory signals have been described (Nurieva et al., 2009), however the signal provided by the B7 molecules, B7.1 and B7.2 on APCs with their receptors CD28 and CTLA-4 in lymphocytes appear to be the predominant molecular interactions for T-cell activation (Salomon and Bluestone, 2001; Sharpe and Freeman, 2002). Binding of B7.1 or B7.2 with CD28 provides a potent stimulatory signal in T cells, whereas binding of the related but higher-affinity CTLA-4 receptor, delivers an inhibitory signal (Karandikar et al., 1996; Sansom, 2000). Journal of Neuroimmunology 223 (2010) 39–54 ⁎ Corresponding author. Unitat d'Histologia, Torre M5, Facultat de Medicina, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain. Tel.: +34 935811826; fax: 34 935812392. E-mail address: beatriz.almolda@uab.cat (B. Almolda). 0165-5728/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jneuroim.2010.03.021 Contents lists available at ScienceDirect Journal of Neuroimmunology journal homepage: www.elsevier.com/locate/jneuroim