Background
Stroke triggers a profound immune activation and a massive infiltration of peripheral immune cells, including monocytes, that leads to neural inflammation and brain injury [
1‐
3]. Mouse and human monocytes are a heterogeneous population that includes pro-inflammatory (CCR2
+/Ly-6C
High) and reparative anti-inflammatory monocytes (CCR2
−/Ly-6C
Low) [
4]. Stroke severity is closely associated with CCR2
+ expression levels and the number of the monocytes in the post-ischemic brain [
2,
3,
5]. Despite the negative association of the pro-inflammatory monocytes in aggravating brain damage, the absence of early monocyte recruitment in the injured CNS impairs regenerative processes and delays stroke recovery [
6,
7]. This paradox suggests that monocyte-derived macrophages (MDMs) also have a beneficial function in resolving inflammation and promoting tissue repair, and that MDMs have a context-dependent role in stroke pathophysiology [
1,
8‐
14].
The dichotomy of MDM function is reflected by their phenotypic heterogeneity. Whether classically or alternatively activated, MDMs can change their behavior and phenotype depending on ischemic milieu [
15,
16]. In the post-ischemic brain, infiltrating CCR2
+ monocytes differentiate into tissue macrophages with alternative M2 phenotype features [
9]. Fate-mapping studies tracing monocytes based on CCR2 and/or CX3CR1 expression showed that monocytes in the injured brain can transition into microglia-like cells, revealing phenotype plasticity and functional convergence between MDMs and resident microglia [
7,
17‐
19]. In acute experimental autoimmune encephalomyelitis, MDMs also acquire microglia-specific markers during chronic inflammation and disease-associated microglia adopt an inflammatory phenotype [
15‐
17,
20]. These bidirectional changes in phenotype mediated by the tissue environment make it difficult to distinguish MDMs from resident microglia based on expression markers, which has obscured the origin and specific function of MDMs in the injured brain.
The clearance of cellular debris is a critical function of MDMs for tissue resolution and tissue remodeling. Phagocytic activity affects cellular metabolism [
16,
21,
22] and macrophage function can be influenced by hypoxia or nutrient alterations [
23‐
27]. In this current study, we sought to distinguish MDMs from microglia in the ischemic brain in order to define their function. We show that infiltrating MDMs in the post-ischemic brain convert to a microglia-like phenotype following phagocytic activity. We also show that MDMs, and not microglia, are the major phagocytic cell type in the injured brain the during acute/sub-acute phases. Together, the findings reveal an expanded role for peripheral immunity in CNS injury and repair processes.
Material and methods
Animals
Animals were maintained at a controlled temperature, humidity, and on a 12-h light/dark cycle. Each cage contained a maximum of 5 mice and was supplied with ventilation and irradiated bedding (1/8-in. Bed O’s Cobs, The Anderson, Maumee, OH). Sterilized food (PicoLab Rodent diet 5053, LabDiet, St. Louis, MO) and water were freely accessible in the cages. Male and female mice aged 3–4 months, C57BL/6 or GFP transgenic mice (C57BL/6-Tg (UBC-GFP), Jackson Laboratories, Bar Harbor, ME) were used.
Transient middle cerebral artery occlusion (MCAO)
Procedures for 30-min transient MCAO and post-stroke care have been previously described [
28]. Briefly, mice were anesthetized with an isoflurane/oxygen/nitrogen mixture. A 6-0 Teflon-coated black monofilament surgical suture (Doccol, Redland, CA) was inserted into the exposed external carotid artery and advanced into the internal carotid artery until it was wedged into the Circle of Willis, where it obstructs the origin of MCA. The filament was left in place for 30 min and then withdrawn. Cerebral blood flow (CBF) was measured prior to, during, and after stroke, and was monitored by Laser-Doppler flowmetry (Periflux System 5010; Perimed, Järfälla, Sweden). Only animals that had both > 80% reduction of pre-ischemic baseline CBF during MCAO and CBF > 80% of baseline after 10 min of reperfusion were included in the study. Body temperature was maintained at 37 ± 0.5 °C during, and for 30 min after, the MCA procedure via a rectal probe connected to a thermocouple-regulated heating water coil in the surgical board. Mice were placed in a recovery cage and their body temperatures were maintained at 37 ± 0.5 °C until they regained consciousness and resumed activity, after which they were returned to their home cages. Warm saline was administered subcutaneously to prevent dehydration during the acute phase. Softened food and hydrogel (Clear H
2O) were given during the first week of recovery. Mice typically started to regain body weight around 3–5 days post-stroke and continued to recover.
Splenectomy and adoptive transfer of splenocytes
Splenectomies and adoptive transfers of splenocytes in mice were performed as previously described [
28]. Briefly, mice were anesthetized with isoflurane and a ~1-cm incision was made on the left side of the abdominal cavity under the rib cage. The spleen was removed by cutting the mesentery and connective tissue, and the splenic vessels were cauterized. Meloxicam (5 mg/kg, P.O.) was administered as an analgesic prior to surgery, in addition to buprenorphine (0.5 mg/kg, S.C., every 12 h for the first 48 h after surgery) and bupivacaine (0.1 ml of 0.25–0.5%, S.C., on incision site, before incision). For trafficking studies, splenocytes were harvested from GFP (green fluorescent protein) transgenic mice (C57BL/6-Tg(UBC-GFP)30Scha/J, Jackson laboratory) or C57BL/6 splenocytes labeled with green fluorescence by PKH67 (Green Fluorescent Cell Linker Kit; Sigma). The spleen was excised and minced using scissors, then pipetted in ice-cold Hanks' balanced salt solution (Life Technologies) without Ca
2+ and Mg
2+. The mixture passed through a 70 µm strainer and then centrifuged at 3000 rpm for 10 min at 4 °C. Erythrocytes were removed with Red Blood Cell Lysis buffer (Sigma). Isolated splenocytes (1–2 × 10
7) were transferred into asplenic MCAO mice via the retro-orbital venous sinus and killed 24 h after for analyses.
Isolation of brain immune cells
Mice were anesthetized with isoflurane and pentobarbital before being perfused with ice-cold phosphate-buffered saline (PBS) containing heparin. Brains were removed and the hemispheres were separated before being placed into ice-cold Hanks’ balanced salt solution without Ca2+ and Mg2+ (HBSS, Life Technologies, Grand Island, NY). Tissue was enzymatically and mechanically dissociated using a MACS Neural Tissue Dissociation Kit with Papain (Miltenyi Biotec, Bergisch Gladbach, Germany) and then treated with a myelin debris removal solution (Miltenyi Biotec). Isolated brain immune cells were used for either flow cytometer analyses or cultured for ex vivo phagocytic activity assays.
Flow cytometry analysis
From single cell preparations, the total number of GFP
+ cells from each hemisphere was determined by multiplying dilution factors to GFP
+ event reads. For cell staining, primary antibodies used were: phycoerythrin CD11b (PE-Vio770 anti-mouse CD11b REA, 1:50); CD45 (Vio-blue anti-mouse CD45 REA, 1:50); Ki-67 (PE anti-mouse Ki67 REA, 1:50); CD11c (PE anti-mouse CD11c REA, 1:50); and a mixture of lineage markers (Lin) conjugated with allophycocyanin (APC) (APC-Lin) against T cells (APC anti-mouse CD90.2 REA), B cells (APC anti-mouse CD45R/B220 REA), natural killer cells (APC anti-mouse NK-1.1 REA, and APC anti-mouse CD49b REA), and granulocytes (APC anti-mouse Ly-6G REA) [
29]. Following incubation at room temperature for 20 min in dark the cells were washed with PBS, and then analyzed with a MACS Quant VYB flow cytometer (Miltenyi Biotec, San Diego, CA). Antibodies used for flow cytometry were purchased from Miltenyi Biotec. Antibody specificity was determined by analyzing cell only, as well as single and double antibody controls for validation.
Assessment of phagocytosis
Using a previously published protocol [
1], immune cells (1 × 10
5 cells/ well) from either brains or splenocytes were plated on 24-well plates (2 × 10
5 cells/well) and incubated at 37 °C with 5% CO
2 for 1 h. After washing away non-adherent cells, 1 μm (diameter) red fluorescent microsphere beads (beads
580/605; F-13083 from ThermoFisher) were added to cell suspensions and incubated at 37 °C with 5% CO
2 for 4 h. Non-phagocytosed beads were removed by multiple PBS washes and cells were resuspended in PBS. Phagocytic activity was determined by measuring the number of beads
580/605+ cells, using cells incubated at 4 °C with beads as negative controls. For in vivo studies, a mixture of GFP
+ splenocytes (1–2 × 10
7 cells in 100 ul PBS) and beads
580/605 (4 × 10
7 beads in 100 ul PBS) was retro-orbitally injected into asplenic MCAO mice 1 day before killing. MDMs [GFP
+] and microglia [GFP-] in the blood and brain were analyzed 24 h after infusion for incorporation of beads
580/605.
Immunohistochemistry
Mice were transcardially perfused with 4% paraformaldehyde in 0.1 mol/L phosphate buffer. Brains were collected and post-fixed overnight, placed into a 30% sucrose solution, and cryosectioned at a 30 µm thickness. Sections were washed in PBS, incubated with 1% bovine serum albumin and 5% normal goat serum for 1 h at room temperature, and incubated with anti-rabbit Iba-1 (1:1000, Wako, Richmond, VA, 019–19741) and anti-chicken GFP (1:1,000, Aves Lab, Davis, CA, AB16901) overnight at 4 °C. This was followed by incubation with either Alexa Fluor 488 goat anti-chicken IgG (1:2000, Life Technologies, A11039) or Alexa Fluor 594 goat anti-rabbit IgG (1:2000, Life Technologies, A11012) secondary antibodies for 1 h. After washing with PBS, the sections were mounted using Fluoroshield reagent (Sigma-Aldrich, F6057). Confocal image stacks were taken with an inverted A1R-HD25 confocal microscope (Nikon Instruments Inc., Melville, NY) using a 40x (NA 1.3) oil objective in 0.3 µm z-steps and at 0.43 µm/pixel. Displayed images represent maximum projections of the obtained z-stack for each imaged site and examined under a laser scanning confocal microscope (Carl Zeiss, Thornwood, NY, USA).
Statistics
Comparisons between CD45 subsets were evaluated using Student’s t test. Multiple comparisons between groups were made using ANOVA followed by a post hoc Bonferroni comparison. Two-way ANOVA were performed for in vivo studies measuring the (1) effect of stroke and (2) effect of times as well as for ex vivo studies measuring the (1) effect of stroke and (2) effect of phagocytosis. Statistical analyses were performed using Prism software (GraphPad Software Inc., La Jolla, CA), and differences were considered significant if p < 0.05.
Discussion
Defining cell type-specific roles for microglia and MDMs in the post-stroke brain has been impeded by the overlapping expression of molecular markers, phenotypic heterogeneity within each cell type, and an ability of each cell type to modify their phenotype and function in response to CNS environments [
15,
17,
36‐
39]. The adoptive transfer of GFP-tagged splenocytes used in this study allowed MDMs to be distinguished from resident microglia in the post-ischemic brain. This strategy also enabled us to define the origin and function of MDMs during acute phase of stroke. Stroke induces a massive infiltration of monocytes into the brain, and we showed that there is an equal distribution of CD45
High and CD45
Low subsets in the infiltrating MDMs. The unexpected and high proportion of CD45
Low MDMs indicates that CD45
High MDMs change to a CD45
Low microglia-like phenotype in the post-ischemic brain. Unlike microglia, MDMs in the post-stroke brain were proliferative, expressed CD11c, and were competent phagocytes. Additionally, CD45
High MDMs have higher phagocytic activity when compared to the CD45
Low subset. Moreover, the CD45
High to CD45
Low phenotype conversion in MDMs requires phagocytosis. Together, this study shows that MDMs are the major phagocytic cell type in the ischemic brain and have plastic phenotypes as part of tissue repair following cerebral ischemia.
Emerging evidence indicates that microglia and MDMs undergo phenotype changes and functional convergence in the CNS [
15,
17,
39]. The conversion of CD45
High MDMs to a CD45
Low phenotype we observed in the post-stroke brain is consistent with these previous reports. Our studies, however, did not find compelling evidence for the converse conversion of microglia to an inflammatory MDM phenotype. The vast majority of GFP
− cells in the R3’ subset was CD45
Low, but there was a small CD45
High sub-population (Fig.
2b). This small subset was likely endogenous MDMs that entered the injured hemisphere, but some cells could have possibly been endogenous microglia that acquired an MDM phenotype.
In an animal model of Alzheimer’s disease, CD45
High cells had a higher phagocytic capacity and expressed TREM2 and CD11c, both of which are markers that resemble disease-associated microglia [
34,
40]. The origin of CD11c
+ mononuclear phagocytes was not defined in this study, but a peripheral origin has been reported for a portion of CD11c
+ dendritic cells in stroke [
41]. In our analyses, we found that CD11c
+ cells were predominantly CD45
High MDMs (R4’; Fig.
2f−h). Thus, our findings indicate that microglia do not acquire a disease-modifying MDM phenotype in the post-stroke brain. Since CD11c
+ dendritic cells play a role in inducing Th1 cell-mediated immunity and exert protective effects [
41,
42], we speculate that MDM-derived CD11c
+ cells modulate adaptive immune function following stroke. Previous studies have also reported that microglia proliferate in ischemic brains [
43,
44], but we found that Ki-67
+ cells were abundant in the CD45
High MDMs of R4’ and only minimal in the R1’–R3’ subsets (Fig.
2c-e). This observation indicates that proliferation is restricted to infiltrating MDMs.
Clearing cellular debris by phagocytes in the injured CNS is critical for tissue repair and remodeling [
1,
45], but establishing the relative phagocytic activity of MDMs and microglia has been challenging. In our study, we found that MDMs (P4’) were the major phagocytes in the ischemic brain (Fig.
4d), with a higher phagocytic capacity in CD45
High MDMs compared to CD45
Low MDMs (Fig.
6c). This finding is consistent with a report that microglia-induced phagocytosis can be suppressed by peripherally derived macrophages [
20]. Differences in phagocytic capability between CD45
High and CD45
Low subsets within the injured brain, in conjunction with phenotype changes, posed an intriguing question as to whether phagocytosis is necessary for the phenotype conversion in MDMs. Our findings support the hypothesis that phagocytosis has a causal role in the CD45
High and CD45
Low conversion within MDMs.
A limitation of this study is that the adoptive transfer procedure to infuse GFP-tagged splenocyte to asplenic mice does not truly reflect post-stroke in normal status. The rationale for removing the spleen 2 days prior to cell transfusion was to allow time for the reduction of the endogenous monocyte pool in the blood, which facilitates trafficking of the exogenously infused GFP-tagged splenocytes. Parabiosis can provide an in vivo tool to study MM trafficking. Sharing hematopoietic cells between parabionts (e.g., WT and GFP mice), however, will greatly diminish the number of fluorescently tagged MMs in circulation. This reduction will likely result in high endogenous non-tagged MM trafficking into the brain, when compared to our model system with GFP + splenocyte transfer into asplenic mice. An additional caveat in our approaches is the underestimation of the number of MDMs that infiltrate and/or accumulate during the post-stroke survival periods due to limiting the post-infusion time to only 24 h. While bone marrow transplantation between Wt and GFP-tg mice provides the number of MDMs infiltrated over time, the approach would not provide in situ trafficking at a given post-stroke time point. Thus, the advantage of in situ trafficking used in this study is the ability to clearly define the origin and function of temporally trafficking MDMs. In summary, the ability to distinguish MDMs from microglia, the study revealed that MDMs are the major phagocytes, not microglia, in the ischemic milieu and that they undergo phenotype changes to microglia-like cells upon phagocytosis. The study provides a defined expansive role of MDMs in phagocytosis and tissue repair in cerebral ischemia.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.