Behavior of in vitro cultured ameboid microglial cells migrating on Müller cell end-feet in the quail embryo retina

Generation of CSF1-Independent Ramified Microglia-Like Cells from Leptomeninges In Vitro

Although del Río-Hortega originally reported that leptomeningeal cells are the source of ramified microglia in the developing brain, recent views do not seem to pay much attention to this notion. In this study, in vitro experiments were conducted to determine whether leptomeninges generate ramified microglia. The leptomeninges of neonatal rats containing Iba1⁺ macrophages were peeled off the brain surface. Leptomeningeal macrophages strongly expressed CD68 and CD163, but microglia in the brain parenchyma did not. Leptomeningeal macrophages expressed epidermal growth factor receptor (EGFR) as revealed by RT-PCR and immunohistochemical staining. Cells obtained from the peeled-off leptomeninges were cultured in a serum-free medium containing EGF, resulting in the formation of large cell aggregates in which many proliferating macrophages were present. In contrast, colony-stimulating factor 1 (CSF1) did not enhance the generation of Iba1⁺ cells from the leptomeningeal culture. The cell aggregates generated ramified Iba1⁺ cells in the presence of serum, which express CD68 and CD163 at much lower levels than primary microglia isolated from a mixed glial culture. Therefore, the leptomeningeal-derived cells resembled parenchymal microglia better than primary microglia. This study suggests that microglial progenitors expressing EGFR reside in the leptomeninges and that there is a population of microglia-like cells that grow independently of CSF1.

Müller Cell Regulated Microglial Activation and Migration in Rats With N-Methyl-N-Nitrosourea-Induced Retinal Degeneration

During the pathogenesis of retinitis pigmentosa (RP), the roles of retinal microglial cells after activation have not been fully elucidated. Herein, experimental RP was induced in Sprague Dawley rats by intraperitoneal injection of N-methyl-N-nitrosourea (MNU) at 50 mg/kg, and the effects of MNU on the retinas were evaluated, respectively, by retinal histology and electroretinography recordings at serial time points. Time-dependent and gradual loss of photoreceptor cells, disrupted arrangement of the outer nuclear layer (ONL), and significant reductions in both a-wave and b-wave amplitudes were observed. Morphology changes were observed in retinal microglial cells; meanwhile, with time, the number of Iba1-positive microglia and their infiltration into the ONL gradually increased. Furthermore, physical interaction of microglial-Müller cell processes following microglial activation was observed after MNU injection. In addition, Müller cells increased CX3CL1 secretion, enhanced microglial cell migration, and upregulated the CX3CR1 expression of the latter. Our observations implied that, during the pathogenesis of RP by MNU, microglial cells exhibit a prominent morphology change and Müller cells can induce activated microglia infiltration by increasing secretion of the chemotaxis factor, CX3CL1, and promoting the migration of retinal microglial cells. This novel finding highlights a potential therapeutic target aimed at regulating the microglial response.

Cytokines and chemokines at the crossroads of neuroinflammation, neurodegeneration, and neuropathic pain

Cytokines and chemokines are proteins that coordinate the immune response throughout the body. The dysregulation of cytokines and chemokines is a central feature in the development of neuroinflammation, neurodegeneration, and demyelination both in the central and peripheral nervous systems and in conditions of neuropathic pain. Pathological states within the nervous system can lead to activation of microglia. The latter may mediate neuronal and glial cell injury and death through production of proinflammatory factors such as cytokines and chemokines. These then help to mobilize the adaptive immune response. Although inflammation may induce beneficial effects such as pathogen clearance and phagocytosis of apoptotic cells, uncontrolled inflammation can result in detrimental outcomes via the production of neurotoxic factors that exacerbate neurodegenerative pathology. In states of prolonged inflammation, continual activation and recruitment of effector cells can establish a feedback loop that perpetuates inflammation and ultimately results in neuronal injury. A critical balance between repair and proinflammatory factors determines the outcome of a neurodegenerative process. This review will focus on how cytokines and chemokines affect neuroinflammation and disease pathogenesis in bacterial meningitis and brain abscesses, Lyme neuroborreliosis, human immunodeficiency virus encephalitis, and neuropathic pain.

Microglia activation by SIV-infected macrophages: alterations in morphology and cytokine secretion

HIV infection in the brain and the resultant encephalitis affect approximately one third of individuals infected with HIV, regardless of treatment with antiretroviral drugs. Microglia are the resident phagocytic cell type in the brain, serving as a "first responder" to neuroinvasion by pathogens. The early events of the microglial response to productively infected monocyte/macrophages entering the brain can best be investigated using in vitro techniques. We hypothesized that activation of microglia would be specific to the presence of simian immunodeficiency virus (SIV)-infected macrophages as opposed to responses to macrophages in general. Purified microglia were grown and stimulated with control or SIV-infected macrophages. After 6 h, aliquots of the supernatant were analyzed for 23 cytokines using Millipore nonhuman primate-specific kit. In parallel experiments, morphologic changes and cytokine expression by individual microglia were examined by immunofluorescence. Surprisingly, the presence of macrophages was more important to the microglial response rather than whether the macrophages were infected with SIV. None of the cytokines examined were unique to co-incubation with SIV-infected macrophages compared with control macrophages, or their supernatants. Media from SIV-infected macrophages, however, did induce secretion of higher levels of IL-6 and IL-8 than the other treatments. As resident macrophages in the brain, microglia would be expected to have a strong response to infiltrate innate immune cells such as monocyte/macrophages. This response is triggered by incubation with macrophages, irrespective of whether or not they are infected with SIV, indicating a rapid, generalized immune response when infiltrating macrophages entering the brain.

Internal limiting membrane peeling as prophylaxis of macular pucker formation in eyes undergoing retinectomy for severe proliferative vitreoretinopathy

BACKGROUND

The purpose of this was to analyze the effect of internal limiting membrane (ILM) peeling on the anatomical and functional outcomes in patients undergoing retinectomy for proliferative vitreoretinopathy-related retinal detachment, especially regarding the postoperative development of macular pucker.

METHODS

In all, a consecutive and prospective series of 84 eyes of 84 patients were included in the study. All eyes underwent retinectomy with silicone oil tamponade for retinal detachment because of proliferative vitreoretinopathy. In Group A (33 eyes), the ILM was also peeled; in Group B (51 eyes), the ILM was left intact. Each patient gave consent to be included in the study, and no patient was lost to follow-up. Postoperatively, careful slit-lamp examination with a contact lens was used to determine whether primary ILM peeling was effective in preventing macular pucker formation. Various statistical methods were used to analyze the significance of the results with a P value of ≤ 0.05 interpreted as significant.

RESULTS

In Group A, the mean age of the patients was 57.2 ± 12.8 years and in Group B 54.6 ± 14.5 years. Median follow-up in Group A was 28.2 ± 7.2 months and in Group B 27.4 ± 6.5 months. The mean time interval between the last retinectomy and silicone oil removal was 9.2 ± 6.1 months in Group A and 8.8 ± 3.0 months in Group B. The mean follow-up after silicone oil removal was 17.4 ± 10.3 months in Group A and 15.1 ± 9.3 months in Group B. The mean logarithm of the minimum angle of resolution visual acuity at the final follow-up visit was 1.89 ± 0.87 in Group A and 1.85 ± 0.83 in Group B (P = 0.6, t-test). Extramacular epiretinal cellular proliferation occurred in 3 eyes (9%) in Group A in the first month after retinectomy and in 3 eyes (5.8%) in Group B (P = 0.27, Fisher exact test). These epiretinal membranes, extending to the edge of the retinotomy, were stable during the follow-up period. No case of macular pucker was observed in Group A, but macular pucker was observed in 9 eyes (17.6%) in Group B at the final examination (P = 0.008, Fisher exact test).

CONCLUSION

Primary peeling of the ILM allowed complete removal of all the epiretinal membranes and successfully prevented the development of macular pucker. Retinectomy and silicone oil tamponade proved an effective treatment modality for eyes with retinal detachment due to proliferative vitreoretinopathy.

Adaptive Müller cell responses to microglial activation mediate neuroprotection and coordinate inflammation in the retina

Purpose

Microglia and Müller cells are prominent participants in retinal responses to injury and disease that shape eventual tissue adaptation or damage. This investigation examined how microglia and Müller cells interact with each other following initial microglial activation.

Methods

Mouse Müller cells were cultured alone, or co-cultured with activated or unactivated retinal microglia, and their morphological, molecular, and functional responses were evaluated. Müller cell-feedback signaling to microglia was studied using Müller cell-conditioned media. Corroborative in vivo analyses of retinal microglia-Müller cell interactions in the mouse retina were also performed.

Results

Our results demonstrate that Müller cells exposed to activated microglia, relative to those cultured alone or with unactivated microglia, exhibit marked alterations in cell morphology and gene expression that differed from those seen in chronic gliosis. These Müller cells demonstrated in vitro (1) an upregulation of growth factors such as GDNF and LIF, and provide neuroprotection to photoreceptor cells, (2) increased pro-inflammatory factor production, which in turn increased microglial activation in a positive feedback loop, and (3) upregulated chemokine and adhesion protein expression, which allowed Müller cells to attract and adhere to microglia. In vivo activation of microglia by intravitreal injection of lipopolysaccharide (LPS) also induced increased Müller cell-microglia adhesion, indicating that activated microglia may translocate intraretinally in a radial direction using Müller cell processes as an adhesive scaffold.

Conclusion

Our findings demonstrate that activated microglia are able to influence Müller cells directly, and initiate a program of bidirectional microglia-Müller cell signaling that can mediate adaptive responses within the retina following injury. In the acute aftermath following initial microglia activation, Müller cell responses may serve to augment initial inflammatory responses across retinal lamina and to guide the intraretinal mobilization of migratory microglia using chemotactic cues and adhesive cell contacts. Understanding adaptive microglia-Müller cell interactions in injury responses can help discover therapeutic cellular targets for intervention in retinal disease.