Temporal pattern of expression and colocalization of microglia/macrophage phenotype markers following brain ischemic injury in mice

Background Emerging evidence indicates that, similarly to what happens for peripheral macrophages, microglia can express different phenotypes depending on microenvironmental signals. In spite of the large literature on inflammation after ischemia, information on M/M phenotype marker expression, their colocalization and temporal evolution in the injured brain is lacking. The present study investigates the presence of microglia/macrophage phenotype markers, their temporal expression, whether they are concomitantly expressed by the same subpopulation, or they are expressed at distinct phases or locations in relation to the ischemic lesion. Methods Volume of ischemic lesion, neuronal counts and TUNEL staining were assessed in C57Bl/6 mice at 6-12-24-48 h and 7d after permanent occlusion of the middle cerebral artery. At the same time points, the expression, distribution in the lesioned area, association with a definite morphology and coexpression of the microglia/macrophage markers CD11b, CD45, CD68, Ym1, CD206 were assessed by immunostaining and confocal microscopy. Results The results show that: 1) the ischemic lesion induces the expression of selected microglia/macrophage markers that develop over time, each with a specific pattern; 2) each marker has a given localization in the lesioned area with no apparent changes during time, with the exception of CD68 that is confined in the border zone of the lesion at early times but it greatly increases and invades the ischemic core at 7d; 3) while CD68 is expressed in both ramified and globular CD11b cells, Ym1 and CD206 are exclusively expressed by globular CD11b cells. Conclusions These data show that the ischemic lesion is accompanied by activation of specific microglia/macrophage phenotype that presents distinctive spatial and temporal features. These different states of microglia/macrophages reflect the complexity of these cells and their ability to differentiate towards a multitude of phenotypes depending on the surrounding micro-environmental signals that can change over time. The data presented in this study provide a basis for understanding this complex response and for developing strategies resulting in promotion of a protective inflammatory phenotype.


Background
Microglia, the major cellular contributors to post-injury inflammation, have the potential to act as markers of disease onset and progression and to contribute to neurological outcome of acute brain injury. They are normally present in the healthy brain where they actively survey their surrounding parenchyma by protracting and retracting their processes and they are endowed with the capacity to rapidly respond to injury or alterations in their microenvironment [1][2][3]. After acute brain injury, these resident cells are rapidly activated and undergo dramatic morphological and phenotypic changes. Typical morphological changes associated with microglia activation include thickening of ramifications and of cell bodies followed by acquisition of a rounded amoeboid shape. This intrinsic response is associated to recruitment of blood-born macrophages which migrate into the injured brain parenchyma [4,5]. This process is accompanied by expression of novel surface antigens and production of mediators that build up and maintain the inflammatory response of the brain tissue. Activated microglia and recruited macrophages (which are antigenically not distinguishable, henceforth referred to as M/ M), can affect neuronal function and promote neurotoxicity through the release of several harmful components such as IL-1β, TNF-α, proteases and reactive oxygen and nitrogen species [6,7]. On the other hand they also possess protective qualities and promote neurogenesis and lesion repair [8][9][10]. Indeed, microglia have been proposed to be beneficial by several mechanisms including glutamate uptake [11] removal of cell debris [12] and production of neurotrophic factors such as IGF-1 [13], GDNF [14] and BDNF [15,16].
Studies addressing phenotypic changes occurring in macrophages in peripheral inflammation and immunity have shown that these cells can undergo different forms of polarized activation. One is the classic or M1 activation, characterized by high capacity to present antigen, high production of NO and ROS and of proinflammatory cytokines. M1 cells act as potent effectors that kill micro-organisms and tumor cells, drive the inflammatory response and may mediate detrimental effects on neural cells. The second phenotype (M2) is an alternative apparently beneficial activation state, more related to a fine tuning of inflammation, scavaging of debris, promotion of angiogenesis, tissue remodeling and repair. Specific environmental signals are able to induce these different polarization states [17]. A similar possibility has been also recently raised for microglia, by showing that these cells, under certain conditions, can indeed be pushed to both extremes of the M1 and M2 differentiation spectrum [16,18]. More studies are needed to substantiate these observations.
In this frame the present study aims at getting insight on previously unexplored aspects of microglia phenotype changes induced by cerebral ischemia, namely, the presence of specific phenotype markers, their temporal expression, whether or not they are concomitantly expressed by the same subpopulation, whether they are expressed at distinct phases or locations in relation to the ischemic lesion. We focussed on a few molecules that are known to be expressed by macrophages in peripheral inflammation and that have been associated to different functions. They include: CD11b, a marker of M/M activation/recruitment, CD45 expressed on all nucleated hematopoietic cells [19], CD68 a marker of active phagocytosis, Ym1 a secretory protein that binds heparin and heparin sulphate and CD206 a C-type lectin carbohydrate binding protein, both of them expressed by alternatively activated macrophages and associated to recovery and function restoration [20,21].  1996) laws and policies. Before beginning any procedure, mice were housed for at least 1 week in their home cages at a constant temperature, with a 12 hour light-dark cycle, and ad libitum access to food and water in a selective pathogen-free (SPF) vivarium.

Focal cerebral ischemia
Permanent ischemia was obtained by permanent middle cerebral artery occlusion (pMCAO) [22,23]. Briefly, mice were anesthetized with Equitensin (pentobarbital 39 mM, chloral hydrate 256 mM, MgSO4 86 mM, ethanol 10% v/v, propyleneglycol 39.6% v/v) 100 μl/mouse administered by intraperitoneal (i.p.) injection. A vertical midline incision was made between the right orbit and tragus. The temporal muscle was excised, and the right MCA was exposed through a small burr hole in the left temporal bone. The dura mater was cut with a fine needle, and the MCA permanently occluded by electrocoagulation just proximal to the origin of the olfactory branch. Intraoperative rectal temperature was kept at 37.0 ± 0.5°C using a heating pad (LSI Letica). Mortality rate was 8.5%. Shamoperated mice received identical anesthesia and surgical procedure without artery occlusion.

Experimental design and blinding
Mice were assigned to surgery and experimental groups with surgery distributed equally across cages and days. To minimize the variability, all surgeries were performed by the same investigator, blinded to the experimental groups. All subsequent histological and immunohistological evaluations were also done by blinded investigators.

Brain transcardial perfusion
At selected time points mice were deeply anesthetized with Equitensin (120 μl/mouse i.p.) and transcardially perfused with 20 ml of PBS, 0.1 mol/liter, pH 7.4, followed by 50 ml of chilled paraformaldehyde (4%) in PBS. After carefully removing the brains from the skull, they were transferred to 30% sucrose in PBS at 4°C overnight for cryoprotection. The brains were then rapidly frozen by immersion in isopentane at -45°C for 3 min before being sealed into vials and stored at -70°C until use.

Quantification of infarct size
For lesion size determination, 20-μm coronal brain cryosections were cut serially at 320-μm intervals and stained with Cresyl Violet [8]. On each slice, the infarcted area was assessed blindly and delineated by the relative paleness of histological staining tracing the area on a video screen. The infarcted area and the percentage of brain swelling for edema correction were determined by subtracting the area of the healthy tissue in the ipsilateral hemisphere from the area of the contralateral hemisphere on each section [24,25]. Infarct volumes were calculated by the integration of infarcted areas on each brain slice as quantified with computer-assisted image analyzer and calculated by Analytical Image System (Imaging Research Inc., Brock University, St. Catharines, Ontario, Canada).

Slice selection and quantitative analysis
Three brain coronal sections per mouse (+1.54, +0.50 and -0.94 mm from bregma, KBJ Franklin and G Paxinos, The Mouse Brain in Stereotaxic Coordinates, Academic Press), were used to quantify the stained area. On each slice, anatomically defined cortical regions of interest were demarcated, corresponding to the primary motor cortex, somatosensory cortex, insular cortex (granular, agranular) and secondary somatosensory cortex, representing the cortical regions involved in the largest lesion extension observed at 24 h after ischemia. Field selection was performed using a BX61 Olympus microscope equipped with a motorized stage acquiring the same focal plan throughout the samples [26]. For neuronal count, CD11b and CD68 quantification fields at 40× magnification were selected at 1.54 mm anterior to bregma (11 fields), at 0.50 mm anterior to bregma (11 fields) and at 0.94 mm posterior to bregma (11 fields). The first raw of fields was positioned at the lesion edge, spacing each field by 572.5 μm (distance between centres of the fields). Further raws of fields were positioned distanced by 389.3 μm each. For TUNEL, CD45, Ym1 and CD206 twenty-four quantification fields at 20× magnification were selected. Eight fields per slice were selected and fields were separated by 572.5 μm (distance between centres of fields), while distance between each raw was 389.3 μm. A schematic representation of the regions of interest and of the selected fields is depicted in Figure 1.

Neuronal Count
Cresyl Violet stained brain sections were used for neuronal count. Thirty-three fields at 40× were analyzed for each mouse. The amount of neuronal loss was calculated by pooling the number of stained neurons in the three ipsilateral sections and expressed as percentage of those in sham-operated animals. Fields were analyzed using ImageJ software http://rsbweb.nih.gov/ij/ and segmentation was used to discriminate neurons from glia on the basis of cell size.

TUNEL staining
To assess the presence of injured cells showing DNA damage, terminal deoxynucleotidyl transferaseYmediated dUTP nick end labeling (TUNEL) staining was performed on 20-μm sections by in situ cell death detection kit (Roche, Mannheim, Germany) according to the manufacturer instructions, as previously described [27]. DNase-treated sections were used as a positive control. After staining, the sections were visualized using fluorescent microscopy (Olympus IX70 Olympus Tokyo, Japan). Images of the area of interest were acquired using AnalySIS software (Olympus, Tokyo, Japan). For each mouse twenty-four fields at 20× were analyzed. TUNEL-positive cells were counted using ImageJ software http://rsbweb.nih.gov/ij/ and expressed as number per mm 2 for subsequent statistical analysis [28].

Immunofluorescence and confocal analysis
Immunofluorescence was performed on 20-μm coronal sections according to the previously described method [22]. Primary antibodies used were: anti-mouse CD45 Similarly to what reported for immunohistochemistry DAB staining, also in this case we considered only cell displaying CD45 rounded morphology (CD45 high , [22]). Appropriate negative controls without the primary antibodies were performed. None of the immunofluorescence reactions revealed unspecific fluorescent signal in the negative controls. Immunofluorescence was acquired using a scanning sequential mode to avoid bleed-through effects by an IX81 microscope equipped with a confocal scan unit FV500 with 3 laser lines: Ar-Kr (488 nm), He-Ne red (646 nm), and He-Ne green (532 nm) (Olympus, Tokyo, Japan) and a UV diode.
Two main areas of interest were considered, namely ischemic core and border zone [29] at both 24 h and 7d after pMCAo Three-dimensional images were acquired over a 10 μm z-axis with a 0.23 μm step size and processed using Imaris software (Bitplane, Zurich, Switzerland) and Photoshop cs2 (Adobe Systems Europe Ltd).

Statistical analysis
Statistical power (1-β) was assessed as post-hoc analysis by means of G*Power [30]. Statistical analysis was performed using standard software packages GraphPad Prism (GraphPad Software Inc., San Diego, CA, USA, version 4.0). All data are presented as mean and standard deviation (sd). The comparison between groups was performed using One-way ANOVA followed by appropriate post hoc test. p-values lower than 0.05 were considered statistically significant.
Cortex, the brain area involved in the ischemic lesion was considered for neuronal count ( Figure 2C). Six hours after ischemia, neuronal count performed in the ipsilateral cortex revealed a significant cell loss when compared to the corresponding area in the sham-operated group (84.9%). Neuronal counts progressively but slowly decreased reaching 64.9% at 7d. No significant difference was found between ispilateral and contralateral side in sham-operated animals (data not shown).
At 6 h after pMCAO rare TUNEL-positive cells were present in the injuried cortex indicating the presence of few dying cells (30.2 ± 14.2, expressed as cell density per mm 2 , Figure 2D The M/M markers expression was analyzed within the ischemic area based on the tissue sampling represented in Figure 1. At each time point, the sampled cortical area pertained to the ischemic territory, being the number of neurons in this region decreased compared to sham animals at every time points ( Figure 2C).
CD11b, a constitutive marker of microglia and macrophages was expressed at every time point considered as well as in sham-operated mice (5.6 ± 1.9, percent of stained area). Starting from 6 h the immunoreactivity increased and remained elevated at every subsequent time point considered, with no major differences throughout the experimental groups (9.5 ± 1.5, 11.7 ± 1.6, 10.1 ± 1.7, 13.1 ± 2.8, 13.0 ± 0.1, respectively at 6 h, 12 h, 24 h, 48 h and 7d, Figure 3B).
Outside the lesion, CD11b staining revealed thin ramifications and small soma ( Figure 3A No CD45-positive cells (CD45 high cells, see methods, Figure 3F) could be observed in sham-operated mice and in the contralateral hemisphere of ischemic mice. Six hours after ischemia these cells were clearly visible in the area considered (0.4 ± 0.2 percent of stained area). The immunoreactivity was further increased 12 and 24 h after ischemia (0.6 ± 0.2 and 1.1 ± 0.3, respectively). No further increase could be observed at 48 h (1.1 ± 0.4). CD45 staining was still present at 7d (0.9 ± 0.4, Figure 3F).

Localization of M/M markers with respect to the lesion
Twenty-four hours after pMCAO, the immunoreactivity for CD11b appeared to be evenly distributed in the ischemic area, being present both in the lesion border and in the ischemic core (Figure 4). At the same time CD45 staining showed a similar distribution being present throughout the entire ischemic area (Figure 4). CD45 cells visible at 10× magnification ( Figure 4) did not reveal the presence of CD45 low cells (corresponding to ramified microglia) appearing in the CD11b staining microphotograph. Conversely CD68 appeared to be mainly concentrated in the border zone, with rare cells present in the ischemic core. Notably, at longer time points (7d) along with the great increase of its expression ( Figure 3G), CD68 appears both in the border and in the core areas. Ym1 at 24 h after pMCAO appeared exclusively expressed in the ischemic core, similarly to CD206 (Figure 4). With the exception of CD68, all the markers considered showed a similar distribution at every time point (data not shown). Only immunoreactivity for CD11b could be observed outside the lesioned area in basal conditions (sham-operated mice). In the lesion, CD11b+ cells showing a ramified to globular morphology could be observed going from the border zone to the ischemic core (red). CD45 staining was present throughout the entire ischemic area (pink). Conversely CD68 appeared to be strongly concentrated in the border zone. Ym1 and CD206 were exclusively expressed in the ischemic core.

Coexpression of M/M markers at 24 h and 7d after pMCAO
Twenty-four hours after ischemia CD68 was expressed in hypertrophic ameboid CD11b cells present in the ischemic core and in ramified microglia in the border zone where CD68 positive cells were mostly located (Figure 4 and 5A-B). A similar pattern of coexpression could be observed at 7d. At this time point the expression of CD68 was greatly increased both in globular CD11b+ cells in the ischemic core and in ramified CD11b cells laying in the border zone ( Figure 4 and Figure 5C-D).
At 24 h after pMCAO Ym1 positive cells co-labeled with CD11b globular cells within the ischemic core, where they were exclusively located (Figure 4 and Figure  6A-B-E-F). Seven days after ischemia Ym1 and CD11b At 24 h after pMCAO, the few CD68+ cells found in the ischemic core did not colocalize with Ym1+ cells that were present exclusively in this area (Figure 4 and Figure 8A-B). In the magnification of Figure 8E-F it is possible to observe that even when these markers appear closely related, they actually belong to distinct cells. At longer times (7d) Ym1 cells not colocalizing with CD68 are still present, however coexpression with CD68 can also be seen (Figure 8C-D-G-H).
A small fraction of CD206 positive cells show coexpression with CD68 at 24 h after ischemia in the ischemic core ( Figure 9A-E-F). Similar situation is observed at 7d after pMCAO when a dramatic increase in CD68 positive cells is apparent in the ischemic core ( Figure 9C-G-H). CD206 marker is not present in the border zone at both time points (Figure 9B-D).
Ym1 and CD206 appeared to be coexpressed in the ischemic core both at 24 h ( Figure 10A-C-D) and 7d ( Figure 10B-E-F) after pMCAO. None of the two markers was present in the border zone at both time points considered ( Figure 4).
As expected all CD11b globular, CD68 globular, Ym1 and CD206 positive cells were all positive for CD45 high in both ischemic core and border zone (data not shown), being CD45 a common marker for immune cell populations [31,32].
A summary of M/M markers coexpression 24 h and 7d after the ischemic lesion is reported in Figure 11.
Lastly, to provide additional details on the functional status of M/M, we assessed their relationship with neurons (NeuN+). We analyzed CD11b/CD68 and CD11b/ Ym1 double positive cells as these populations showed to increase at different time points, thus suggesting distinct functional states. CD11b stain of M/M membranes was chosen for documenting the morphology of M/M when contacting neurons. We found that neurons were often enwrapped by CD11b positive cells in both ischemic core and border zone at both 24 h and 7d ( Figure 12). In most cases CD11b cells surrounding neurons were positive for CD68 at both zones ( Figure  12A-B-C-D), suggesting an active phagocytosis. None of the CD11b/Ym1 double positive cells at 24 h appeared to be engaging a phagocytic interaction with neurons, being these cells never in contact with NeuN positive cells ( Figure 12E). At 7d, a few CD11b/Ym1 double Figure 11 Summary of M/M markers coexpression 24 h and 7d after the ischemic lesion. At 24 h after injury (A) immunoreactivity for CD11, which is readly increased after ischemia, is expressed in ramified and globular cells. CD68 is present in a percentage of both globular and ramified CD11b+ cells. Ym1 and CD206 that are present mostly in the core of the lesion, are expressed by a fraction of globular CD11b+ cells and can be present on the same cells. A few cells coexpressing CD206 and CD68 can be found in the area between the core and the border zone where the two markers are mainly located respectively. At 7d after injury (B) Ym1 decreases while CD68 expression greatly increases and from the border zone where it was at earlier times it invades the ischemic core (see also data in Figure 3). A few CD68+ cells appear now to express Ym1. positive cells showed a phagocytic appearance enveloping neurons ( Figure 12G), coherently with their partially CD68 positive phenotype at this time point (Figure 8). In the border zone, at both time points, Ym1 was not detectable and only single CD11b positive cells did envelop neurons ( Figure 12F-H).

Discussion
Our study shows that the ischemic lesion is accompanied by activation of specific M/M phenotype that presents distinctive spatial and temporal features. We have demonstrated that: 1) the ischemic lesion induces the expression of the selected M/M markers that develop over time, each with a specific pattern; 2) the selected markers are associated with globular or ramified CD11b morphology, 3) each marker has a given localization in the lesioned area with no apparent major changes during time, with the exception of CD68.
We have firstly determined the histopathological features of the lesion induced by pMCAO. From the analysis of the temporal evolution of the lesion it appears that the percentage of neuronal loss is somehow stable from 24 h up to 7d although the persistence of TUNEL-positive cells at this late point indicates that some cells may still be in degeneration at that late time. It should be noted that assessing the lesion volume by the paleness of the cresyl violet staining may lead to misleading conclusions since, as detailed below, invading inflammatory cells may contribute to the apparent reduction of the lesioned area at 7d. Actually the quantification of the CD11b and CD45 high immunoreactivity indicates that inflammatory cells rapidly increase in number and/or size early after the injury and at every time point considered.
M/M play a pivotal role in surveillance and response to altered CNS conditions [1,2,4]. An emerging concept is that, similarly to what happens for peripheral macrophages, these cells can exert different antithetic functions depending on environmental signals, acting as major players in the pro-inflammatory cytotoxic response, but also participating in the immunosuppressive and self-repair processes [6,33,34]. The phenotype markers considered in this study include classical markers of M/M activation (CD11b and CD45) and markers expressed by alternatively activated macrophages (CD68, Ym1 and CD206). Although evidence of M2 activation state in the brain has been reported in M/M in AD models [35,36], following global ischemia [37], in models of experimental autoimmune encephalomyelitis [38] or spinal cord injury [16,39], information on M2 marker expression, coexpression and temporal evolution in the injuried brain is lacking. We could observe that these phenotype markers are exclusively expressed by CD11b cells and that each of them shows distinct features in terms of time course of activation and localization in relation to the ischemic lesion.
CD11b is expressed on the surface of many leukocytes and is a widely used maker of M/M. It belongs to a family of cell surface receptors known as integrins. It is covalently bound to a beta 2 subunit to form integrin α M β 2 (Mac-1, CD11b/CD18) which is implicated in diverse responses including cell-mediated killing, phagocytosis, chemotaxis and cellular activation. CD11b has the ability to recognize a wide series of ligands such as fibrinogen, iC3b fragment of the third complement component, ICAM-1, denaturated products, blood coagulation factor X [40]. Our data show that CD11b staining increases at early time points after ischemia, rapidly reaching a plateau of activation. Notably, CD11b positive cells display a different morphology in relation to the lesion, namely they are ramified in the border zone and ameboid in the ischemic core. Similarly to CD11b, also CD45 high cells increase rapidly after ischemia. These cells, display a rounded morphology and most probably correspond to recruited macrophages, neutrophils and lymphocytes [22,32,41]. This study does not specifically address the question of differentiating between invading macrophages and resident microglia. An important future direction will be to identify specific molecular/ phenotypical markers for these two cell populations, since there is evidence that they may play a different role in the progression of brain injury [9,34,42].
CD68 or macrosialin is a member of the lysosomal/ endosomal-associated membrane glycoprotein (LAMP) family and a member of the scavenger receptor family which recognizes a wide range of anionic macromolecules such as oxidatively modified lipoprotein, apoptotic cells and cell surface antigens of microorganisms. Its localization and predominance in phagocytic macrophages implicates CD68 in phagocytosis [43,44]. We observed that the early increase in CD68 immunoreactivity is concentrated in the border zone and expressed in ramified CD11b positive cells. At later time points a dramatic increase in CD68 expression appears both in the border zone and the ischemic core and is apparent in globular CD11b cells. At both time points and in both zones, CD11b/CD68 double positive cells appear to physically interact with neurons and show a phagocytic-like morphology characterized by neuron engulfment. The phagocytic activity of alternatively activated M/M is associated to clearance of cells debris, of damaged or dying cells and of infiltrating neutrophils thus resulting in the elimination of several potentially cytotoxic substances [4,18,21,45,46]. However the overall functional meaning of phagocytosis in acute brain injury is still an open question. Actually the protective effect of manipulations such as stem cell infusion may be associated with a decrease in CD68 expression [47].
Ym1 belongs to the lectin family and is constitutively expressed by liver, lung and bone marrow, consistently with the fact that these are the original sources of myeloid cells [48]. It is synthesized and secreted by activated macrophages during inflammation and exhibits a pHdependent, specific activity towards GlcN oligomers and heparin. Ym1 may control leukocyte trafficking by competing with them for binding sites on local extracellular matrix, an action resulting in down-regulation of inflammation. Our findings show that, similarly to what reported in peripheral macrophages, Ym1 is activated transiently suggesting that it may be involved in the establishment of an inflammatory management control of the injured region [48]. Its expression is restricted to the ischemic core and it colocalizes with CD11b globular cells and with some CD68 cells at later times only. None of the CD11b/ Ym1 double positive cells is associated with phagocytosis of neurons at 24 h, whilst at 7d some of them show a phagocytic appearance and envelop neurons, coherently with their partially CD68 positive phenotype at this time point. An increase in Ym1 expression has been associated to the beneficial effect of stem cell infusion in mice subjected to global ischemia [37], in line with a protective role in acute brain injury.
Another marker of alternatively activated macrophages is CD206 or mannose receptor [45,49]. This is an endocytic receptor that binds both microbial glycans and self glycoproteins carrying terminal mannose, fucose and N-acetylglucosamine by interaction with its carbohydrate recognition domains (CRDs). Its known function is related to recognition and endocytosis of the carbohydrate portion of antigens for processing and presentation [50]. Our results show that CD206 expression significantly increases over time and colocalizes with Ym1 positive cells and with a fraction of CD68 positive cells that increase at later time points.
Lastly, our data have been obtained in a model of permanent ischemia and may not be extended to an ischemia with reperfusion paradigm. Notably the present data and our previous results [22] indicate that the ratio of CD45 high /CD45 low is dramatically different in these two conditions, being much higher after pMCAO. This may be due to either a higher number of infiltrating cells and/or a lower survival of resident cells, thus indicating that in transient ischemia the composition of the specific M/M populations in the lesioned area is different.

Conclusions
In the ischemic lesion M/M express markers that show distinct temporal expression, distribution and association with a definite cell morphology suggesting that different M/M populations are acting at the site of injury according to well defined phenotype, time of activation and pattern of localization. Conceivably, at 24 h after insult, ramified and phagocytic M/M surround the lesion, possibly acting as a barrier against further expansion of the lesion, whilst globular M/M committed to a protective phenotype (i.e. expressing Ym1 and CD206) populate the ischemic core with the primary function of resolving inflammation and promoting wound healing. At later time points (7d) the phagocytic behavior of M/M becomes prevalent in the whole lesioned area with numerous globular phagocytic M/M invading the core territory. The observed switch towards the phagocytic phenotype is accompanied by the progressive reduction of the expression of the protective Ym1.
At 24 h protective Ym1 positive cells do not appear to be involved in neuron phagocytosis, differently from CD68 positive cells that show a close physical interaction with neurons. At 7d, when neuronal damage becomes irreversible in the core area, Ym1 positive cells decrease and start to show phagocytic behavior being partially co-localized with CD68. These cells now show the ability to envelop neurons in a phagocytic-like manner. Overall this effect suggests that endogenous protective mechanisms take place soon after injury (24 h-48 h) and last at least up to 7d when phagocytosis of neurons and debris removal are prevalent. Whether this effect is beneficial or detrimental cannot be clearly established. Phagocytosis (CD68 positive cells) can result in a protective function if properly balanced. In normal brain, phagocytic function of microglia have been suggested to support neurogenesis [51]. After an acute injury microglia are supposed to remove cellular parts, as well as whole cells [52], an action that might be necessary to remove irreversibly damaged cells to make space for new neuronal projections and fresh connections or newly generated neurons.
The different states of M/M in the ischemic lesion reflect the complexity of these cells and their ability to differentiate towards a multitude of phenotypes depending on the surrounding microenvironmental signals that can change over time. The inflammatory response that follows cerebral ischemia is regarded as a promising target for stroke therapy. The data presented in this study provide a basis for understanding this complex response and for developing strategies resulting in promotion of a protective inflammatory phenotype.