Regional topographical differences of canine microglial immunophenotype and function in the healthy spinal cord

Characterization of microglia/macrophage phenotypes in the spinal cord following intervertebral disc herniation

Dogs frequently suffer from traumatic spinal cord injury (SCI). Most cases of SCI have a favorable prognosis but 40-50% of dogs with paraplegia and absence of nociception do not regain ambulatory abilities, eventually leading to euthanasia. Microglia and infiltrating macrophages play a crucial role in inflammatory process after SCI. However, little is known about microglia/macrophage phenotypes representing a potential target for future therapeutic strategies. In the present study, the microglia/macrophage phenotype was characterized by immunohistochemistry in the morphologically unaltered canine spinal cord (10 control dogs) and during acute and subacute SCI (1-4 and 5-10 days post injury, 9 and 8 dogs, respectively) using antibodies directed against IBA1, MAC387, MHC-II, lysozyme, EGR2, myeloperoxidase, CD18, CD204 and lectin from Griffonia simplicifolia (BS-1). The expression of these markers was also analyzed in the spleen as reference for the phenotype of histiocytic cells. Histological lesions were absent in controls. In acute SCI, 4 dogs showed mild to moderate hemorrhages, 2 dogs bilateral gray matter necrosis and 6 dogs mild multifocal axonal swellings and myelin sheath dilation. One dog with acute SCI did not show histological alterations except for few dilated myelin sheaths. In subacute SCI, variable numbers of gitter cells, axonal changes and dilated myelin sheaths were present in all dogs and large areas of tissue necrosis in 2 dogs. Neuronal chromatolysis was found in 3 dogs with acute and subacute SCI, respectively. In control dogs, microglia/macrophage constitutively expressed IBA1 and rarely other markers. In acute SCI, a similar marker expression was found except for an increase in MAC387-positive cells in the spinal cord white matter due to an infiltration of few blood-borne macrophages. In subacute SCI, increased numbers of microglia/macrophages expressed CD18, CD204 and MHC-II in the gray matter SCI indicating enhanced antigen recognition, processing and presentation as well as cell migration and phagocytosis during this stage. Interestingly, only CD204-positive cells were upregulated in the white matter, which might be related to gray-white matter heterogeneity of microglia as previously described in humans. The present findings contribute to the understanding of the immunological processes during SCI in a large animal model for human SCI.

Isolation of microglia from retinas of chronic ocular hypertensive rats

Microglia are the principal glial cells involved in the processes of immune inflammation within both retina and optic nerve, especially under the context of glaucomatous neuropathy. Considering the distinguishing role of retinal microglia in glaucoma and the lack of established protocol for microglia isolation from animal glaucoma model, the present study aimed to develop and validate a method with characteristics of both simplicity and efficiency for retinal microglia isolation from chronic ocular hypertensive (COH) rats. A Percoll gradient of various concentrations was used to separate microglia from whole retinal cells of the COH rats and control group. The finally isolated microglia were identified by CD11b and Iba-1 immunofluorescence staining, and the cell viability was determined by trypan blue staining. Additionally, the proportion of microglia in the whole retina cells was identified by flow cytometry. Results showed that the survival rates of isolated retinal microglia with the Percoll gradient method were 67.2 ± 4% and 67.6 ± 3% in control and COH groups, respectively. The proportion of the microglia population in the whole retinal cells was about 0.4–0.93%. To conclude, the present study confirmed that the application of Percoll gradient could effectively separate microglia from retinas of COH rats, which will probably enrich the tool kit for basic researchers of glaucoma specialty and help with scientific investigations.

Current Insights Into the Pathology of Canine Intervertebral Disc Extrusion-Induced Spinal Cord Injury

Spinal cord injury (SCI) in dogs is commonly attributed to intervertebral disc extrusion (IVDE). Over the last years substantial progress was made in the elucidation of factors contributing to the pathogenesis of this common canine disease. A detailed understanding of the underlying histopathological and molecular alterations in the lesioned spinal cord represents a prerequisite to translate knowledge on the time course of secondary injury processes into the clinical setting. This review summarizes the current state of knowledge of the underlying pathology of canine IVDE-related SCI. Pathological alterations in the spinal cord of dogs affected by IVDE-related SCI include early and persisting axonal damage and glial responses, dominated by phagocytic microglia/macrophages. These processes are paralleled by a pro-inflammatory microenvironment with dysregulation of cytokines and matrix metalloproteinases within the spinal cord. These data mirror findings from a clinical and therapeutic perspective and can be used to identify biomarkers that are able to more precisely predict the clinical outcome. The pathogenesis of progressive myelomalacia, a devastating complication of SCI in dogs, is not understood in detail so far; however, a fulminant and exaggerating secondary injury response with massive reactive oxygen species formation seems to be involved in this unique neuropathological entity. There are substantial gaps in the knowledge of pathological changes in IVDE with respect to more advanced and chronic lesions and the potential involvement of demyelination. Moreover, the role of microglia/macrophage polarization in IVDE-related SCI still remains to be investigated. A close collaboration of clinical neurologists and veterinary pathologists will help to facilitate an integrative approach to a more detailed understanding of the molecular pathogenesis of canine IVDE and thus to identify therapeutic targets.

Lumbar spinal cord microglia exhibited increased activation in aging dogs compared with young adult dogs

Altered microglia function contributes to loss of CNS homeostasis during aging in the brain. Few studies have evaluated age-related alterations in spinal cord microglia. We previously demonstrated that lumbar spinal cord microglial expression of inducible nitric oxide synthase (iNOS) was equivalent between aging, neurologically normal dogs and dogs with canine degenerative myelopathy (Toedebusch et al. 2018, Mol Cell Neurosci. 88, 148-157). This unexpected finding suggested that microglia in aging spinal cord have a pro-inflammatory polarization. In this study, we reexamined our microglial results (Toedebusch et al. 2018, Mol Cell Neurosci. 88, 148-157) within the context of aging rather than disease by comparing microglia in aging versus young adult dogs. For both aging and young adult dogs, the density of microglia was significantly higher closest to the motor neuron cell body. However, there was no difference in densities between aging versus young adult dogs at all distances except for the furthest distance analyzed. The number of motor neurons with polarized microglia was higher in aging dogs; yet, the density per motor neuron of arginase-1-expressing microglia was reduced in aging dogs compared with young adult dogs. Finally, aging dogs had increased steady-state mRNA levels for genes consistent with activated microglia compared with young adult dogs. However, altered mRNA levels were limited to the lumbar spinal cord. These data suggested that aging dog spinal cord microglia exhibit regional immunophenotypic differences, which may render lumbar motor neurons more susceptible to age-related pathological insults.

Age- and location-related changes in microglial function

Inflammation in the central nervous system (CNS) is primarily regulated by microglia. No longer considered a homogenous population, microglia display a high degree of heterogeneity, immunological diversity and regional variability in function. Given their low rate of self-renewal, the microenvironment in which microglia reside may play an important role in microglial senescence. This study examines age-related changes in microglia in the brain and spinal cord. Using ex-vivo flow cytometry analyses, functional assays were performed to assess changes in microglial morphology, oxidative stress, cytokine production, and phagocytic activity with age in both the brain and spinal cord. The regional CNS environment had a significant effect on microglial activity with age. Blood-CNS barrier permeability was greater in the aging spinal cord compared with aging brain; this was associated with increased tissue cytokine levels. Aged microglia had deficits in phagocytosis at baseline and after stimulus-induced activation. The identification of age-specific, high scatter microglia together with the use of ex-vivo functional analyses provides the first functional characterization of senescent microglia. Age and regional-specificity of CNS disease should be taken into consideration when developing immune-modulatory treatments.

Dynamic Changes of Microglia/Macrophage M1 and M2 Polarization in Theiler's Murine Encephalomyelitis

Microglia and macrophages play a central role for demyelination in Theiler's murine encephalomyelitis (TME) virus infection, a commonly used infectious model for chronic-progressive multiple sclerosis. In order to determine the dynamic changes of microglia/macrophage polarization in TME, the spinal cord of Swiss Jim Lambert (SJL) mice was investigated by gene expression profiling and immunofluorescence. Virus persistence and demyelinating leukomyelitis were confirmed by immunohistochemistry and histology. Electron microscopy revealed continuous myelin loss together with abortive myelin repair during the late chronic infection phase indicative of incomplete remyelination. A total of 59 genes out of 151 M1- and M2-related genes were differentially expressed in TME virus-infected mice over the study period. The onset of virus-induced demyelination was associated with a dominating M1 polarization, while mounting M2 polarization of macrophages/microglia together with sustained prominent M1-related gene expression was present during the chronic-progressive phase. Molecular results were confirmed by immunofluorescence, showing an increased spinal cord accumulation of CD16/32(+) M1-, arginase-1(+) M2- and Ym1(+) M2-type cells associated with progressive demyelination. The present study provides a comprehensive database of M1-/M2-related gene expression involved in the initiation and progression of demyelination supporting the hypothesis that perpetuating interaction between virus and macrophages/microglia induces a vicious circle with persistent inflammation and impaired myelin repair in TME.

Passage-dependent morphological and phenotypical changes of a canine histiocytic sarcoma cell line (DH82 cells)

DH82 cells represent a permanent macrophage cell line isolated from a dog with histiocytic sarcoma (HS) and are commonly used in various fields of research upon infection and cancer, respectively. Despite its frequent use, data on cell surface antigen expression of this cell line are fragmentary and in part inconsistent. We therefore aimed at a detailed morphological and antigenic characterization of DH82 cells with respect to passage-dependent differences. Cellular morphology of early (≤ 13) and late (≥ 66) passages of DH82 cells was evaluated via scanning electron microscopy. Moreover, cells were labelled with 10 monoclonal antibodies directed against CD11c, CD14, CD18, CD44, CD45, CD80, CD86, MHC-I, MHC-II, and ICAM-1 for flow cytometric analysis. Early passage cells were characterized by round cell bodies with abundant small cytoplasmic projections whereas later passages exhibited a spindle-shaped morphology with large processes. The percentage of CD11c-, CD14-, CD18-, CD45-, and CD80 positive cells significantly decreased in late passages whereas the expression of CD44, CD86, MHC-I, MHC-II and ICAM-1 remained unchanged. DH82 cells represent a remarkably heterogeneous cell line with divergent antigenic and morphologic properties. The present findings have important implications for future studies, which should consider distinct characteristics with regard to the used passage.

Isolation and ex vivo characterization of the immunophenotype and function of microglia/macrophage populations in normal dog retina

Microglia are the primary resident immune cells of the retina and are involved in the pathogenesis of various retinal diseases. In this study, we optimized experimental conditions to isolate microglia from canine retinas and characterized ex vivo their immunophenotype and function using flow cytometry (FACS). The most suitable protocol included a mechanical dissociation of the retina and an enzymatic digestion using DNAse and collagenase. Extraction was carried out by density gradient centrifugation, and retinal microglia accumulated on distinct interfaces of 1.072 and 1.088 g/mL of a Percoll gradient. Immunophenotypical characterization was performed with monoclonal antibodies CD11b, CD11c, CD18, CD45, CD44, B7-1 (CD80), B7-2 (CD86), CD1c, ICAM-1 (CD54), CD14, MHCI, MHCII, CD68, CD3, CD4, CD8α, and CD21. The most prevalent microglia population in the normal canine retina is CD11b(high)CD45(low). Functionally, retinal microglia exhibited phagocytosis and reactive oxygen species (ROS) generation activities. To conclude, ex vivo examinations of retinal microglia are feasible and possibly reflect the in vivo conditions, avoiding artifacts observed in tissue culture. The established method will be relevant to examine microglia from diseased canine retinas in order to elucidate their roles in degenerative processes.

Functional diversity of microglia - how heterogeneous are they to begin with?

Microglia serve in the surveillance and maintenance, protection and restoration of the central nervous system (CNS) homeostasis. By their parenchymal location they differ from other CNS-associated myeloid cells, and by origin as well as functional characteristics they are also–at least in part–distinct from extraneural tissue macrophages. Nevertheless, microglia themselves may not comprise a uniform cell type. CNS regions vary by cellular and chemical composition, including white matter (myelin) content, blood–brain barrier properties or prevailing neurotransmitters. Such a micromilieu could instruct as well as require local adaptions of microglial features. Yet even cells within circumscribed populations may reveal some specialization by subtypes, regarding house-keeping duties and functional capacities upon challenges. While diversity of reactive phenotypes has been established still little is known as to whether all activated cells would respond with the same program of induced genes and functions or whether responder subsets have individual contributions. Preferential synthesis of a key cytokine could asign a master control to certain cells among a pool of activated microglia. Critical functions could be sequestered to discrete microglial subtypes in order to avoid interference, such as clearance of endogenous material and presentation of antigens. Indeed, several and especially a number of recent studies provide evidence for the constitutive and reactive heterogeneity of microglia by and within CNS regions. While such a principle of “division of labor” would influence the basic notion of “the” microglia, it could come with the practival value of addressing separate microglia types in experimental and therapeutic manipulations.

Impact of Theiler's virus infection on hippocampal neuronal progenitor cells: differential effects in two mouse strains

AIMS

Disease-associated alterations in hippocampal neurogenesis are discussed as an important factor contributing to long-term consequences of central nervous system diseases. Therefore, the study aimed to determine the impact of Theiler's murine encephalomyelitis virus infection on hippocampal cell proliferation, neuronal progenitor cells and neurogenesis as well as the influence of microglia on respective disease-associated alterations.

METHODS

The impact of the infection was evaluated in two mouse strains which differ in the disease course, with an acute polioencephalitis followed by virus elimination in C57BL/6 mice and a chronic demyelinating disease in SJL/J mice.

RESULTS

Infection with the low neurovirulent BeAn strain did not exert significant acute effects regardless of the mouse strain. In the chronic phase, the number of neuronal progenitor cells and early postmitotic neurones was significantly reduced in infected SJL/J mice, whereas no long-term alterations were observed in C57BL/6 mice. A contrasting course of microglia activation was observed in the two mouse strains, with an early increase in the number of activated microglia cells in SJL/J mice and a delayed increase in C57BL/6 mice. Quantitative analysis did not confirm a correlation between the number of activated microglia and the number of neuronal progenitor cells and early postmitotic neurones. However, flow cytometric analyses revealed alterations in the functional state of microglial cells which might have affected the generation of neuronal progenitor cells.

CONCLUSIONS

Theiler's murine encephalomyelitis virus infection can exert delayed effects on the hippocampal neuronal progenitor population with long-term alterations evident 3 months following infection. These alterations proved to depend on strain susceptibility and might contribute to detrimental consequences of virus encephalitis such as cognitive impairment.

Alteration in mononuclear cell subpopulations in dogs immunized with gentamicin-attenuated Leishmania infantum

The impact of immunization with gentamicin-attenuated Leishmania infantum (H-line) on the immunophenotypic profile of popliteal lymph node (PLN) and peripheral blood mononuclear cells (PBMCs) of dogs was assessed by flow cytometry and immunohistochemistry. Compared with the dogs infected with L. infantum wild-type (Group WT), there was a significantly higher percentage of CD4+, CD44+ T cells and CD14+, MHC-II+ cells and a lower percentage of CD4+ CD25+ regulatory T cells in PLN of the immunized dogs with L. infantum H-line (Group H). The percentage of CD4+ and CD8+ T cells in PBMCs of immunized dogs was higher than that in dogs of Group WT. The CD4:CD8 ratio in PLN of dogs of Group H was significantly higher than that in dogs of Group WT. A significantly higher percentage of CD21+ B cells and a lower percentage of CD79b+ cells were found in PLN of the immunized dogs compared with dogs of Group WT. Immunohistochemical investigation showed no parasites in the PLN of immunized dogs whereas there were parasites in the PLN of 60% of dogs infected with L. infantum WT. In this study, the immunophenotypic profile of mononuclear cells of the immunized dogs correlates with cellular immunity.

Spatio-temporal development of axonopathy in canine intervertebral disc disease as a translational large animal model for nonexperimental spinal cord injury

Spinal cord injury (SCI) represents a devastating central nervous system disease that still lacks sufficient therapies. Here, dogs are increasingly recognized as a preclinical animal model for the development of future therapies. The aim of this study was a detailed characterization of axonopathy in canine intervertebral disc disease, which produces a mixed contusive and compressive injury and functions as a spontaneous translational animal model for human SCI. The results revealed an early occurrence of ultrastructurally distinct axonal swelling. Immunohistochemically, enhanced axonal expression of β-amyloid precursor protein, non-phosphorylated neurofilament (n-NF) and growth-associated protein-43 was detected in the epicenter during acute canine SCI. Indicative of a progressive axonopathy, these changes showed a cranial and caudally accentuated spatial progression in the subacute disease phase. In canine spinal cord slice cultures, immunoreactivity of axons was confined to n-NF. Real-time quantitative polymerase chain reaction of naturally traumatized tissue and slice cultures revealed a temporally distinct dysregulation of the matrix metalloproteinases (MMP)-2 and MMP-9 with a dominating expression of the latter. Contrasting to early axonopathy, diminished myelin basic protein immunoreactivity and phagocytosis were delayed. The results present a basis for assessing new therapies in the canine animal model for translational research that might allow partial extrapolation to human SCI.

The role of pro- and anti-inflammatory cytokines in the pathogenesis of spontaneous canine CNS diseases

Dogs are comparatively frequently affected by various spontaneously occurring inflammatory and degenerative central nervous system (CNS) conditions, and immunopathological processes are a hallmark of the associated neuropathology. Due to the low regenerative capacity of the CNS a sophisticated understanding of the underlying molecular basis for disease initiation, progression and remission in canine CNS diseases represents a prerequisite for the development of novel therapeutical approaches. In addition, as many spontaneous canine CNS diseases share striking similarities with their human counterpart, knowledge about the immune pathogenesis may in part be translated for a better understanding of certain human diseases. In addition to cytokine-driven differentiation of peripheral leukocytes including different subsets of T cells recent research suggests a pivotal role of these mediators also in phenotype polarization of resident glial cells. Cytokines thus represent the key mediators of the local and systemic immune response in CNS diseases and their orchestration significantly decides on either lesion progression or remission. The aim of the present review is to summarize the growing number of data focusing on the molecular basis of the immune response during spontaneous canine CNS diseases and to detail the effect of cytokines on the immune pathogenesis of selected idiopathic, infectious, and traumatic canine CNS diseases. Steroid-responsive meningitis arteritis (SRMA) represents a unique idiopathic disease of leptomeningeal blood vessels characterized by excessive IgA secretion into the cerebrospinal fluid. Recent reports have given sophisticated insights into the cytokine-driven, immune-mediated pathogenesis of SRMA that is characterized by a biased T helper 2 cell response. Canine distemper associated leukoencephalitis represents an important spontaneously occurring disease that allows investigations on the basic pathogenesis of immune-mediated myelin loss. It is characterized by an early virus-induced up-regulation of pro-inflammatory cytokines with chronic bystander immune-mediated demyelinating processes. Lastly, canine spinal cord injury (SCI) shares many similarities with the human counterpart and most commonly results from intervertebral disk disease. The knowledge of its pathogenesis is largely restricted to experimental studies in rodents, and the impact of immune processes that accompany secondary injury is discussed controversially. Recent investigations on canine SCI highlight the pivotal role of pro-inflammatory cytokine expression that is paralleled by a dominating reaction of microglia/macrophages potentially indicating a polarization of these immune cells into a neurotoxic and harmful phenotype. This report will review the role of cytokines in the immune processes of the mentioned representative canine CNS diseases and highlight the importance of cytokine/cytokine interaction as a useful therapeutic target in canine CNS diseases.

Seizure activity in dogs is associated with enhanced TIMP-2 expression of microglia

In the pathogenesis of epilepsy aberrant synaptic plasticity plays an important role. Matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) are responsible for nervous tissue remodelling resulting in synaptic plasticity in the central nervous system (CNS) and might therefore be crucially involved in epileptogenesis. To assess the potential pathogenetic role of microglial MMPs and TIMPs in seizure induction, twenty-four dogs suffering from different intracranial diseases with and without seizure activity were comparatively examined. Microglial cells were isolated by density gradient centrifugation and their expression profiles of MMP-2, MMP-9, MMP-12, MMP-13, MMP-14, TIMP-1, TIMP-2, and RECK (reversion-inducing cysteine-rich protein with Kazal motifs) were examined via quantitative real-time PCR (qPCR). Interestingly, a significant up-regulation of TIMP-2 expression was found for the first time in dogs suffering from seizures. In conclusion, microglial TIMP expression might be involved in seizure generation.