Human retinal microglia express phenotypic characteristics in common with dendritic antigen-presenting cells

Activation of retinal glial cells contributes to the degeneration of ganglion cells in experimental glaucoma

Elevation of intraocular pressure (IOP) is a major risk factor for neurodegeneration in glaucoma. Glial cells, which play an important role in normal functioning of retinal neurons, are well involved into retinal ganglion cell (RGC) degeneration in experimental glaucoma animal models generated by elevated IOP. In response to elevated IOP, mGluR I is first activated and Kir4.1 channels are subsequently inhibited, which leads to the activation of Müller cells. Müller cell activation is followed by a complex process, including proliferation, release of inflammatory and growth factors (gliosis). Gliosis is further regulated by several factors. Activated Müller cells contribute to RGC degeneration through generating glutamate receptor-mediated excitotoxicity, releasing cytotoxic factors and inducing microglia activation. Elevated IOP activates microglia, and following morphological and functional changes, these cells, as resident immune cells in the retina, show adaptive immune responses, including an enhanced release of pro-inflammatory factors (tumor neurosis factor-α, interleukins, etc.). These ATP and Toll-like receptor-mediated responses are further regulated by heat shock proteins, CD200R, chemokine receptors, and metabotropic purinergic receptors, may aggravate RGC loss. In the optic nerve head, astrogliosis is initiated and regulated by a complex reaction process, including purines, transmitters, chemokines, growth factors and cytokines, which contributes to RGC axon injury through releasing pro-inflammatory factors and changing extracellular matrix in glaucoma. The effects of activated glial cells on RGCs are further modified by the interplay among different types of glial cells. This review is concluded by presenting an in-depth discussion of possible research directions in this field in the future.

Shaping the Microglia in Retinal Degenerative Diseases Using Stem Cell Therapy: Practice and Prospects

Retinal degenerative disease (RDD) refers to a group of diseases with retinal degeneration that cause vision loss and affect people's daily lives. Various therapies have been proposed, among which stem cell therapy (SCT) holds great promise for the treatment of RDDs. Microglia are immune cells in the retina that have two activation phenotypes, namely, pro-inflammatory M1 and anti-inflammatory M2 phenotypes. These cells play an important role in the pathological progression of RDDs, especially in terms of retinal inflammation. Recent studies have extensively investigated the therapeutic potential of stem cell therapy in treating RDDs, including the immunomodulatory effects targeting microglia. In this review, we substantially summarized the characteristics of RDDs and microglia, discussed the microglial changes and phenotypic transformation of M1 microglia to M2 microglia after SCT, and proposed future directions for SCT in treating RDDs.

Microglia Contribution to the Regulation of the Retinal and Choroidal Vasculature in Age-Related Macular Degeneration

The retina is a highly metabolically active tissue with high-level consumption of nutrients and oxygen. This high metabolic demand requires a properly developed and maintained vascular system. The retina is nourished by two systems: the central retinal artery that supplies the inner retina and the choriocapillaris that supplies the outer retina and retinal pigment epithelium (RPE). Pathological neovascularization, characterized by endothelial cell proliferation and new vessel formation, is a common hallmark in several retinal degenerative diseases, including age-related macular degeneration (AMD). A limited number of studies have suggested that microglia, the resident immune cells of the retina, have an important role not only in the pathology but also in the formation and physiology of the retinal vascular system. Here, we review the current knowledge on microglial interaction with the retinal vascular system under physiological and pathological conditions. To do so, we first highlight the role of microglial cells in the formation and maintenance of the retinal vasculature system. Thereafter, we discuss the molecular signaling mechanisms through which microglial cells contribute to the alterations in retinal and choroidal vasculatures and to the neovascularization in AMD.

Immune cells in the retina and choroid: Two different tissue environments that require different defenses and surveillance

In the eye immune defenses must take place in a plethora of differing microenvironments ranging from the corneal and conjunctival epithelia facing the external environment to the pigmented connective tissue of the uveal tract containing smooth muscle, blood vessels and peripheral nerves to the innermost and highly protected neural retina. The extravascular environment of the neural retina, like the brain parenchyma, is stringently controlled to maintain conditions required for neural transmission. The unique physiological nature of the neural retina can be attributed to the blood retinal barriers (BRB) of the retinal vasculature and the retinal pigment epithelium, which both tightly regulate the transport of small molecules and restrict passage of cells and macromolecules from the circulation into the retina in a similar fashion to the blood brain barrier (BBB). The extracellular environment of the neural retina differs markedly from that of the highly vascular, loose connective tissue of the choroid, which lies outside the BRB. The choroid hosts a variety of immune cell types, including macrophages, dendritic cells (DCs) and mast cells. This is in marked contrast to the neural parenchyma of the retina, which is populated almost solely by microglia. This review will describe the current understanding of the distribution, phenotype and physiological role of ocular immune cells behind or inside the blood-retinal barriers and those in closely juxtaposed tissues outside the barrier. The nature and function of these immune cells can profoundly influence retinal homeostasis and lead to disordered immune function that can lead to vision loss.

Concurrent cell type-specific isolation and profiling of mouse brains in inflammation and Alzheimer's disease

Nonneuronal cell types in the CNS are increasingly implicated as critical players in brain health and disease. While gene expression profiling of bulk brain tissue is routinely used to examine alterations in the brain under various conditions, it does not capture changes that occur within single cell types or allow interrogation of crosstalk among cell types. To this end, we have developed a concurrent brain cell type acquisition (CoBrA) methodology, enabling the isolation and profiling of microglia, astrocytes, endothelia, and oligodendrocytes from a single adult mouse forebrain. By identifying and validating anti-ACSA-2 and anti-CD49a antibodies as cell surface markers for astrocytes and vascular endothelial cells, respectively, and using established antibodies to isolate microglia and oligodendrocytes, we document that these 4 major cell types are isolated with high purity and RNA quality. We validated our procedure by performing acute peripheral LPS challenge, while highlighting the underappreciated changes occurring in astrocytes and vascular endothelia in addition to microglia. Furthermore, we assessed cell type-specific gene expression changes in response to amyloid pathology in a mouse model of Alzheimer's disease. Our CoBrA methodology can be readily implemented to interrogate multiple CNS cell types in any mouse model at any age.

Microglia in the primate macula: specializations in microglial distribution and morphology with retinal position and with aging

Microglia, the principal resident immune cell in the retina, play constitutive roles in immune surveillance and synapse maintenance, and are also associated with retinal disease, including those occurring in the macula. Perspectives on retinal microglia function have derived largely from rodent models and how these relate to the macula-bearing primate retina is unclear. In this study, we examined microglial distribution and cellular morphology in the adult rhesus macaque retina, and performed comparative characterizations in three retinal locations along the center-to-periphery axis (parafoveal, macular, and the peripheral retina). We found that microglia density peaked in the parafoveal retina and decreased in the peripheral retina. Individual microglial morphology reflected macular specialization, with macular microglia demonstrating the largest and most complex dendritic arbors relative to other retinal locations. Comparing retinal microglia between young and middle-aged animals, microglial density increased in the macular, but not in the peripheral retina with age, while microglial morphology across all locations remained relatively unchanged. Our findings indicate that microglial distribution and morphology demonstrate regional specialization in the retina, correlating with gradients of other retinal cell types. As microglia are innate immune cells implicated in age-related macular diseases, age-related microglial changes may be related to the increased vulnerability of the aged macula to immune-related neurodegeneration.

HLA class II genotypes are not associated with age related macular degeneration in a case-control, population-based study

Multiple lines of evidence support an immunologic basis and genetic disposition for the development of age-related macular degeneration (AMD). Comprehensive human leukocyte antigens (HLA) class II typing at four loci (DRB1, DQA1, DQB1, and DPB1) was assessed using next generation sequencing methods and tested for association with age-related macular degeneration (AMD) in a case-control study of 456 AMD cases and 499 controls from the population-based Study of Osteoporotic Fractures (SOF) cohort. No statistically significant associations were identified for any of the class II loci and a previously identified association between DRB1*13:01 was not replicated in this dataset. These results reported here suggest that common HLA class II genetic variation does not contribute to AMD disease risk.

The role of microglia in diabetic retinopathy

There is growing evidence that chronic inflammation plays a role in both the development and progression of diabetic retinopathy. There is also evidence that molecules produced as a result of hyperglycemia can activate microglia. However the exact contribution of microglia, the resident immune cells of the central nervous system, to retinal tissue damage during diabetes remains unclear. Current data suggest that dysregulated microglial responses are linked to their deleterious effects in several neurological diseases associated with chronic inflammation. As inflammatory cytokines and hyperglycemia disseminate through the diabetic retina, microglia can change to an activated state, increase in number, translocate through the retina, and themselves become the producers of inflammatory and apoptotic molecules or alternatively exert anti-inflammatory effects. In addition, microglial genetic variations may account for some of the individual differences commonly seen in patient's susceptibility to diabetic retinopathy.

Age-related changes in the optic nerve of Sprague-Dawley rats: an ultrastructural and immunohistochemical study

The optic nerve is a unique part of the central nervous system. It lacks neuronal cell bodies and consists of axons of the retinal ganglion cells together with the supporting neuroglial cells. In the present study, aging of the optic nerve was studied in female Sprague-Dawley rats aged 3, 12, 24 and 30 months old, ultrastructurally, immunohistochemically and morphometrically trying to answer the question why aging is a common risk factor for many ocular diseases especially glaucoma. Additionally, studying the optic nerve aging offered a good opportunity to gain further insight into the effects of aging on white matter. Both nerve fibers and neuroglial cells demonstrated several age related changes which were more profound in 30 months old rats. Optic nerve axons displayed watery degeneration and dark degeneration. Myelin disturbances including widening, whorls, splitting and vacuolations of the myelin lamellae were also observed. Neuroglial cells appeared to be more frequent than in younger rats especially microglia cells and developed dense cytoplasmic inclusions. GFAP-positive astrocytes delineated age-related progressive increase in number, size as well as length and thickness of their processes. CD68 immunohistochemical staining revealed age-related changes in the morphology, location and number of CD68 positive microglia cells.

Autoimmune responses against photoreceptor antigens during retinal degeneration and their role in macrophage recruitment into retinas of RCS rats

Autoimmunity may contribute to retinal degeneration. The studies examined the evolution of autoimmune responses against retina in naïve dystrophic RCS rats over the course of retinal degeneration. We showed that anti-retinal autoantibodies and T cells are generated in response to the availability of antigenic material released from dying photoreceptor cells during retinal degeneration but with distinctive activation trends. Passive transfer of anti-retinal antibodies enhanced disease progression by disrupting the BRB, upregulating MCP-1, attracting blood macrophages into retina, and augmenting apoptotic photoreceptor cell death. Our findings directly link anti-retinal autoantibodies to activated macrophage entry and their possible role in neurodegeneration.

Small interfering RNA-mediated suppression of Ccl2 in Müller cells attenuates microglial recruitment and photoreceptor death following retinal degeneration

BACKGROUND

The recruitment and activation of inflammatory cells is thought to exacerbate photoreceptor death in retinal degenerative conditions such as age-related macular degeneration (AMD). We investigated the role of Müller cell-derived chemokine (C-C motif) ligand (Ccl)2 expression on monocyte/microglia infiltration and photoreceptor death in light-mediated retinal degeneration, using targeted small interfering (si)RNA.

METHODS

Adult Sprague-Dawley rats were injected intravitreally with 1 μg of either Ccl2 siRNA or scrambled siRNA, and were then exposed to 1000 lux of light for a period of 24 hours. The mice were given an overdose of barbiturate, and the retinas harvested and evaluated for the effects of bright-light exposure. Ccl2 expression was assessed by quantitative PCR, immunohistochemistry, and in situ hybridization. Monocytes/microglia were counted on retinal cryostat sections immunolabeled with the markers ED1 and ionized calcium binding adaptor (IBA)1, and photoreceptor apoptosis was assessed using terminal dUTP nick end labeling.

RESULTS

Intravitreal injection of Ccl2 siRNA significantly reduced the expression of Ccl2 following light damage to 29% compared with controls. In retinas injected with Ccl2 siRNA, in situ hybridization and immunohistochemistry on retinal cryostat sections showed a substantial decrease in Ccl2 within Müller cells. Cell counts showed significantly fewer ED1-positive and IBA1-positive cells in the retinal vasculature and outer nuclear layer of Ccl2 siRNA-injected retinas, compared with controls. Moreover, there was significantly less photoreceptor apoptosis in Ccl2 siRNA-injected retinas compared with controls.

CONCLUSIONS

Our data indicate that Ccl2 expression by Müller cells promotes the infiltration of monocytes/microglia, thereby contributing to the neuroinflammatory response and photoreceptor death following retinal injury. Modulation of exaggerated chemokine responses using siRNA may have value in reducing inflammation-mediated cell death in retinal degenerative disease such as AMD.

Early focal expression of the chemokine Ccl2 by Müller cells during exposure to damage-inducing bright continuous light

PURPOSE

To investigate the time course and localization of Ccl2 expression and recruitment of inflammatory cells associated with light-induced photoreceptor degeneration.

METHODS

Sprague-Dawley (SD) rats were exposed to 1000 lux light for up to 24 hours, after which some animals were allowed to recover in dim light (5 lux) for 3 or 7 days. During and after exposure to light, the animals were euthanatized and the retinas processed. Ccl2 expression was assessed by qPCR, immunohistochemistry, and in situ hybridization at each time point. Counts were made of perivascular monocytes/microglia immunolabeled with ED1, and photoreceptor apoptosis was assessed with TUNEL.

RESULTS

Upregulation of Ccl2 expression was evident in the retina by 12 hours of exposure and correlated with increased photoreceptor death. Ccl2 expression reached its maximum at 24 hours, coinciding with peak cell death. Immunohistochemistry and in situ hybridization showed that Ccl2 is expressed by Müller cells from 12 hours of exposure, most intensely in the superior retina, in the region of the incipient light-induced lesion. After the Müller cell-driven expression of Ccl2, there was a substantial recruitment of monocytes to the local retina and choroidal vasculature. This coincided spatially with the expression of Ccl2 in the superior retina. Peak monocyte infiltration followed maximum Ccl2 expression by up to 3 days. Furthermore, Ccl2 immunoreactivity was observed in many infiltrating monocytes after a 24-hour exposure.

CONCLUSIONS

The data indicate that photoreceptor death promotes region-specific expression of Ccl2 by Müller cells, which facilitates targeting of monocytes to sites of injury. The data suggest that recruitment of monocytes to developing lesions is secondary to signaling events in the retina.

MHC class II expression by beta2 integrin (CD18)-positive microglia, macrophages and macrophage-like cells in rabbit retina

The aim of this study was to investigate the developmental expression of major histocompatibility complex class II (MHCII) by microglia and macrophages and their relationship to blood vessels in the retina, a representative tissue of the central nervous system. Such information is crucial to understanding the role of these cells in immune surveillance. Wholemount preparations of retinas from late embryonic, postnatal and adult rabbits were subjected to three-colour fluorescence microscopy using beta2 integrin (CD18) and MHCII antibodies and biotinylated Griffonia simplicifolia B4 isolectin labelling of blood vessels. CD18+ cells consistently exhibited characteristics of macrophages or microglia in the vascularized and non-vascularized regions of the retina, respectively. At all ages, MHCII was expressed by a high proportion of cells in the vascularized region, which contained macrophage-like 'parenchymal cells' as well as typical perivascular macrophages. MHCII expression by ramified microglia, first detected on postnatal day 30, was lower in the peripheral retina and intermediate in the avascular region of the myelinated streak. The observed localization of MHCII+ cells in relation to blood vessels and location-dependent differences in MHCII expression point to the possibility that these cells may be distributed strategically within the retina to provide multiple lines of defence against immune challenge arriving via the retinal vasculature.

Reactivation of uveitogenic T cells by retinal astrocytes derived from experimental autoimmune uveitis-prone B10RIII mice

PURPOSE

To determine the involvement of retinal astrocytes (RACs) in T cell-mediated experimental autoimmune uveitis (EAU).

METHODS

Frozen sections of eyes from naive mice or mice with EAU were stained for glial fibrillary acidic protein (GFAP) or major histocompatibility complex (MHC) class II molecules and were examined by confocal microscopy. RACs were isolated and cocultured with interphotoreceptor retinoid-binding protein (IRBP) peptide-specific T cells. The proliferation and cytokine production of responder T cells were determined by [(3)H]-thymidine incorporation and ELISA, respectively.

RESULTS

The development of intraocular inflammation was associated with increased GFAP-positive cells in the retina. RACs from EAU-prone mice (B10RIII) activated uveitogenic T cells in vitro, enhanced T-cell proliferation and the production of proinflammatory cytokines, and increased the numbers of IL-17(+) IRBP T cells in the inflamed eye. The interaction between local RACs and IRBP-specific T cells was regulated by a distinct pattern of costimulatory molecules. In addition, the ability of IRBP-specific T cells to interact with RACs was dependent on whether the latter were derived from EAU-prone (B10RIII) or EAU-low susceptible (C57Bl/6) strains of mice.

CONCLUSIONS

This study suggests that the RACs in EAU-prone mice contribute to the reactivation of pathogenic T cells in the eye, leading to intraocular inflammation and tissue damage.

Identification of novel dendritic cell populations in normal mouse retina

PURPOSE

Whether tissue resident or infiltrating antigen-presenting cells (APCs) are involved in modulating immune responses in the retina and initiating inflammation is controversial. In this histologic study, the authors examine the retinas of mice strains with different susceptibility to experimental autoimmune uveoretinitis (EAU) for tissue resident APC.

METHODS

Retinal wholemounts from normal and inflamed eyes of B10R III, C57BL/6, BALB/c, and ABH Biozii mice were immunostained for APC markers (33D1, CD11c, CD11b, major histocompatibility complex [MHC] class II, F4/80, CD80, CD86, CD205, mPDCA, B220, and GR1) and analyzed by confocal fluorescence microscopy using emission fingerprinting and three-dimensional reconstruction techniques. Hematoxylin and eosin-stained histologic sections were used to evaluate EAU disease scores and to assess outer blood retina barrier (retinal pigment epithelium [RPE]) structures.

RESULTS

A population of 33D1(+) cells was identified exclusively in the peripheral margins and juxtapapillary areas of the retina in normal, nonimmunized C57BL/6 adult mice. These cells were also MHC class II(high), and their location corresponded to sites of earliest inflammation in EAU. Numbers in the papillary area were very low (less than 10), but this region marked the predominant anatomic site for initiation of inflammation in this moderately susceptible strain. The distribution and phenotype of these cells within the retinas differed between mouse strains exhibiting different disease susceptibility. In EAU-resistant BALB/c mice, many more 33D1(+) dendritic cells were present in the normal retina but were MHC class II(low/-). Conversely, no 33D1(+) or MHC class II (+) dendriform cells could be found in the normal retinas of highly EAU-susceptible B10.RIII mice.

CONCLUSIONS

A novel population of 33D1(+) DCs was identified in normal mouse retina. The function of these cells remains to be defined, but increased numbers correlate positively with structural abnormalities in the RPE and increased resistance of the strain to EAU.

HLA and eye disease: a synopsis

Human leukocyte antigen (HLA) gene products have been implicated in the pathogenesis of an increasing number of eye diseases, mainly inflammatory in nature. This perspective reviews the current hypotheses for why HLA polymorphisms are associated with specific eye diseases. Statistical problems in studies involving HLA associations are discussed, and possible solutions outlined. The relevance of HLA testing in routine ophthalmic practice, its practical and cost implications is also assessed.

Human retinal microglia express candidate receptors for HIV-1 infection

BACKGROUND/AIMS

Microglia are the primary antigen presenting cells in the central nervous system and the retina, and can harbour viral antigens that may damage neural tissue via the release of neurotoxins. All cells bearing CD4 molecules and co-receptors (members of the chemokine receptor and Fcgamma receptor families) are potential targets for the human immunodeficiency virus (HIV). In this study, retinal microglia (in vitro and in situ) were investigated for the expression of candidate HIV-1 binding receptors.

METHODS

Cultured human retinal microglia and frozen sections of human retinas were used. Immunohistochemistry was used to investigate expression of cell surface receptors necessary for HIV-1 infection: CD4, CC chemokine receptor 5 (CCR5), and Fcgamma receptors.

RESULTS

Human retinal microglia expressed detectable levels of CD4, CD16, CD64, and CCR5 in vitro and Fcgamma receptor I (CD64) in situ.

CONCLUSIONS

Human retinal microglia express several candidate receptors required for viral binding and as such may be a potential reservoir for HIV-1 infection.

Transient expression of MIDC-8 in the normal mouse brain

In this study, we have immunohistochemically characterized the expression of mononuclear phagocyte markers CD14, CD36, CD68, CD204 and MARCO by parenchymal microglia in the developing and adult mouse brain. We further investigated whether these cells express two well-characterized phenotypic markers of dendritic cells: CD205 (DEC-205/NLDC-145) and MIDC-8 antigen. Our results confirm the lack of expression of dendritic cell markers by microglia. We noted that these cells do not appear to express markers associated with monocytes and macrophages during the course of development, but do express CD68 and CD204 antigens in the adult. Unexpectedly, we also noted the transient expression of MIDC-8 antigen on cells within the medial ganglionic eminence and by neuroepithelial cells lining the lateral ventricles and in the medial lemniscus between E15 and E19. We discuss this finding in the context of neural and haematopoietic differentiation.

Activated microglia in human retinitis pigmentosa, late-onset retinal degeneration, and age-related macular degeneration

Many gaps exist in our knowledge of human retinal microglia in health and disease. We address the hypothesis that primary death of rod photoreceptors leads to activation of resident microglia in human retinas with retinitis pigmentosa (RP), late-onset retinal degeneration (L-ORD), or age-related macular degeneration (AMD). Regions of ongoing photoreceptor cell death were studied by immunocytochemistry with microglia- and other retinal cell-specific markers. In normal human retinas, quiescent microglia were small, stellate cells associated with inner retinal blood vessels. In retinas with RP, L-ORD, or AMD, numerous activated microglia were present in the outer nuclear layer in regions of ongoing rod cell death. These microglia were enlarged, amoeboid cells that contained rhodopsin-positive cytoplasmic inclusions. We conclude that activated microglia migrate to the outer nuclear layer and remove rod cell debris. In other central nervous system diseases such as stroke, activated microglia phagocytose debris from the primary injury and also secrete molecules that kill nearby normal neurons. By analogy with these diseases, we suggest that microglia activated by primary rod cell death may kill adjacent photoreceptors. Activated microglia may be a missing link in understanding why initial rod cell death in the human diseases RP, L-ORD, and AMD leads to death of the cones that are critical for high acuity daytime vision.

TGF-beta 1 synergizes with GM-CSF to promote the generation of glial cell-derived dendriform cells in vitro

Microglia are often considered a type of tissue macrophages analogous Langerhans' cells, while dendritic cells (DC) can be generated in vitro from cultured microglia in the presence of GM-CSF. In this study, we show that TGF-beta 1, in the presence of GM-CSF, promoted the growth and differentiation of glial cell-derived dendritic cells (GC-DC). TGF-beta 1-driven GC-DC exhibited an immature state reflected by low CD11c expression, augmented endocytosis, and reduced antigen presentation. Expression of Fas was inhibited in GM-CSF+TGF-beta 1-supplemented cell cultures and may relate to a long life span of GC-DC treated with GM-CSF+TGF-beta 1. IL-10 and IL-12 mRNA on GC-DC was not affected upon exposure to GM-CSF alone or to GM-CSF+IFN-gamma, GM-CSF+IL-10 or GM-CSF+TGF-beta 1. In sharp contrast, TGF-beta 1, in the presence of GM-CSF, dramatically up-regulated the expression of TNF-alpha and TGF-beta 1 mRNA. These results demonstrate that TGF-beta 1 seems to play a crucial role in the differentiation, functional skewing, and cytokine profile of GC-DC. TGF-beta 1-driven GC-DC awaits further investigation to facilitate a better understanding of the glia-T cell dialog as well as the pathogenesis and immunotherapy of central nervous system inflammatory and degenerative diseases.

Effects of post-mortem delay and storage duration on the expression of GFAP in normal human adult retinae

Glial fibrillary acidic protein (GFAP) is an established marker of retinal glia and has been shown to be modulated by several cytokines and retinal pathology. The influence of a number of factors, including post-mortem delay, storage duration and retinal pathology, on the distribution and morphology of macroglia and GFAP antigenicity was examined in human retina. The effects of these parameters on GFAP expression were estimated using immunohistochemistry, confocal microscopy and image analysis. Changes in expression of antigenicity were analysed in human retinal cryosections at three levels: constitutive,aberrant and total. The results indicated that short-term and long-term storage duration had no significant effect on GFAP immunoreactivity at all three levels of expression (P > 0.2).However, a significant increase in GFAP immunoreactivity and distribution at all three levels of expression was associated with prolonged post-mortem delay (> 30 h) (P < 0.05). This study highlights the importance of rigorous matching of post-mortem delay between control specimens in histological studies of human retinae. The study further demonstrates the utility of Eye Bank retinae fixed and stored in 2% paraformaldehyde, provided that appropriate controls are applied.

TGF-beta1-conditioned glial cell-derived dendritic cells inhibit expansion of MBP-reactive T cells in vitro

Resident microglial cells contribute to activation and expansion of T cells under inflammatory conditions within the CNS. However, there is no evidence how interactions between microglia and T cells affect CNS inflammation. We evaluated the effect of glial cell-derived dendritic cells (GC-DC) in expanding and eliminating myelin basic protein (MBP)-reactive T cells. GC-DC untreated with TGF-beta1 (GC-DC0) primed antigen specific T cell proliferation, whereas GC-DC treated with TGF-1 (GC-DCbeta) effectively inhibited expansion of T cells via inducing IFN-y-expressing CD8+ T cells. Augmented IFN-gamma and/orTNF-alpha might also affect the elimination of MBP-reactive T cells. These results indicate that TGF-beta1-mediated functional skewing of GC-DC plays a critical role for the elimination of MBP-reactive T cells.

Immunological and aetiological aspects of macular degeneration

Aetiological and immunological aspects of AMD, a leading cause of blindness in Western countries, have been reviewed. Developmental studies suggest that anatomical features unique to the fovea result in a critical relationship between metabolic demand and blood supply at the macula, which is maintained throughout life. Recent studies show a sufficient degree of consistency in the link between smoking and both dry and wet AMD to regard it as causative. Dry AMD is considered to be the natural endstage of the disease; epidemiological and morphological studies point to choroidal vascular atrophy as the causative event and it is suggested that signals associated with acute vascular compromise lead to the development of subretinal neovascularisation. The relationship between sub-pigment epithelial deposits, including basal laminar deposit, and the pathogenesis of AMD is examined. Much of the literature is consistent with a choroidal origin for the constituents of drusen. The blood-retinal barrier preserves the physiological environment of the neural retina and limits inflammatory responses. The factors, including cytokines, adhesion molecules and the presence of resident immunocompetent cells (microglia), which determine the immune status of the retina are considered. Historical descriptions of the involvement of inflammatory cells are provided, evidence implicating inflammation in the pathogenesis of AMD involving macrophages, giant cells and microglia has been derived from observations of human and animal subretinal neovascular lesions. The role of humoral factors such as anti-retinal autoantibodies and acute phase proteins together with clinical observations has been surveyed. Taken together these data demonstrate the involvement of both cellular and humoral immunity in the pathogenesis of AMD. It remains to be determined to what degree the influence of immunity is causative or contributory in both wet and dry AMD, however, the use of anti-inflammatory agents to ameliorate the condition further indicates the existence of an inflammatory component.

Ocular immune privilege: a review

The definition of the term 'immune privilege' has evolved over the last century. Current usage refers to a state within a particular organ or tissue in which elements of normal immunity are absent. The fact that this deficiency is thought to be generally beneficial has compelled others to go a step further and venture that immune privilege acts to minimize expression of immunopathology. The purpose of this article is to review which parts of the eye hold immune privileged status, what mechanisms contribute to it, and what clinical benefits may have driven the development of these unique immune environments. The article ends with an examination of recent studies which have sought to use components of ocular immune privilege to prevent systemic autoimmune disease.

Circumferential migration of ameboid microglia in the margin of the developing quail retina

Central-to-peripheral migration of QH1-positive microglial precursors occurs in the vitrealmost part of the developing quail retina. This study shows that some QH1-positive ameboid cells with morphological features of migrating cells are already present in the margin of the retina before microglial precursors migrating centrally to peripherally arrive in this zone. Because the earlier cells are oriented parallel to the ora serrata, we deduce that some microglial cells migrate circumferentially in the margin of the retina, whereas other microglial precursors migrate from central to peripheral zones. Microglial cells that migrate circumferentially are first seen on embryonic day 6 (E6) and advance in a temporal-to-dorsal-to-nasal direction from the temporoventral quadrant of the retina. When cells migrating centrally to peripherally reach the retinal margin, they meet those migrating circumferentially. From E6 on, some QH1-positive dendritic cells in the ciliary body bear processes that penetrate the retina, where they are oriented circumferentially. These observations suggest that microglial cells that migrate circumferentially in the retinal margin share a common origin with dendritic cells of the ciliary body. Therefore, microglial cells of the quail retina appear to make up a heterogeneous population, with some cells originating from the pecten/optic nerve head area and others from the ciliary body.

The human hyaloid system: cellular phenotypes and inter-relationships

We have investigated the expression of leucocyte markers, phenotypic characteristics and cellular relationships of the normal human fetal hyaloid vasculature using immunohistochemistry, light and electron microscopy. Antibodies against von Willebrand Factor, alpha-smooth muscle actin, glial fibrillary acidic protein, vimentin, major histocompatibility complex classes-I and -II, CD45 (leucocyte-common antigen) and calcitonin gene-related peptide were used to identify the cellular constituents of the hyaloid vasculature in whole mounts. Additional morphological features were described at the ultrastructural level. Endothelial cells throughout the hyaloid system were immunoreactive to von Willebrand Factor and major histocompatibility complex class-I antibodies. Pericytes were immunoreactive to alpha-smooth muscle actin antibody; labeled cells were distributed along large branches of the hyaloid artery, vasa hyaloidea propria, tunica vasculosa lentis and pupillary membrane but no immunoreactivity was detected on small connecting capillaries. Vessel and non-vessel-associated hyalocytes on the hyaloid artery, vasa hyaloidea propria, tunica vasculosa lentis, pupillary membrane and vitreous were immunoreactive to major histocompatibility complex classes-I and -II, CD45 and calcitonin gene-related peptide antibodies. Anti-glial fibrillary acidic protein reactivity was detected on Bergmeister's papilla but not on the hyaloid artery. Cells immunoreactive for vimentin were present throughout the hyaloid vasculature including small connecting capillaries. Ultrastructural observations of the hyaloid vasculature revealed junctional complexes, including zonulae adherens, macula adherens and possible zonulae occludens, between adjacent endothelial cells. Fenestrae were not observed in the gestational ages included in the present study. The use of whole mounts in conjunction with specific antisera has provided novel immunohistochemical definitions of the structure and cellular constituents of the human hyaloid. The results indicate that hyalocytes are a heterogeneous population of leucocyte-lineage cells.

Antigen-presenting cells in experimental autoimmune uveoretinitis

The present study attempts to identify the antigen-presenting cells in the retina, utilizing bone marrow-transplanted chimeric rats. Two types of chimeras were used: one produced by transplanting bone marrow cells from F1 hybrids of Lewis and Brown Norway (BN) into sublethally irradiated Brown Norway rats (LBN/F1-->BN), followed by adoptive transfer of S-antigen-specific T cells obtained from Lewis rats; the second produced by transplanting bone marrow cells from BN rats into sublethally irradiated F1 hybrids (BN-->LBN/F1), followed by adoptive transfer of S-antigen-specific T cells obtained from F1 hybrids. As controls, Lewis, F1 hybrids and BN rats also received adoptive transfer of syngeneic uveitogenic T cell lines. All animals were killed on the seventh day after adoptive transfer and their eyes and pineal glands were analysed immunohistochemically, utilizing antibody directed against Lewis specific MHC class II molecules(OX-3). The analyses revealed the development of uveoretinitis and pinealitis in both types of chimeras and in the Lewis and F1 hybrid rats. BN rats did not develop uveoretinitis. OX-3-positive cells were found in the retina and the pineal glands of both types of chimeras, and in the Lewis and F1 hybrid rats but not in the BN rats. These cells in the retina expressed dendritic morphology and perivascular distribution. Retinal pigment epithelia, Müller cells and the vascular endothelia of both chimeras, the two strains, and the F1 hybrid rats did not demonstrate OX-3-positive staining. These results suggest that the bone marrow-derived cells in the retina and pineal gland may present S-antigen to T cells, initiating the cascade of uveoretinitis and pinealitis.

Exudative macular degeneration and intravitreal triamcinolone: 18 month follow up

PURPOSE

To evaluate the safety and efficacy of intravitreal triamcinolone after 18 months of follow up in patients with age-related macular degeneration and subfoveal or juxtafoveal choroidal neovascularization considered unsuitable for laser photocoagulation.

METHODS

Thirty eyes of 28 patients, referred from general eye clinics as well as the private clinic of one of the authors to a hospital-based retinal out-patient clinic, were treated with an intravitreal injection of triamcinolone (4 mg). The primary outcome measure was the proportion of eyes with loss of six or more lines on a Bailey-Lovie Chart. The incidence of adverse events associated with treatment was also observed.

RESULTS

Of the 20 eyes with initial visual acuity (VA) of 6/60 or better, the vision was maintained (+/-1 Bailey-Lovie lines) in 11 eyes (55%), while six eyes (30%) suffered severe visual loss (six or more lines). The VA improved by five to six lines in three of 10 eyes with initial vision of 3/60 or worse. Three of four eyes receiving a second injection suffered either progressive cataract or elevated intra-ocular pressure (IOP) requiring cataract surgery and/or filtering surgery. One of 26 eyes (3%) receiving a single injection showed progression of cataract and elevation of IOP within 6 weeks of treatment and required anti-glaucoma medication for 6 weeks. Progression of nuclear sclerosis 8-12 months after treatment was observed in six of 26 eyes (23%) receiving a single injection.

CONCLUSIONS

The results of the present study suggest that a single intravitreal injection of 4 mg triamcinolone is reasonably well tolerated by the human eye. The rate of development of severe visual loss was less than reported for historical controls. Because the results are preliminary and uncontrolled, the treatment should not be used routinely until its benefit to patients is established by a prospective, randomized controlled study.

Dual implication of 2',3'-cyclic nucleotide 3' phosphodiesterase as major autoantigen and C3 complement-binding protein in the pathogenesis of multiple sclerosis

Multiple sclerosis (MS) is characterized by intra-blood-brain barrier immunoglobulin synthesis that persists lifelong. Subcellular fractionation and two-dimensional electrophoresis were used in conjunction with immune precipitation and immunoblotting to identify antigenic determinants for this immunoglobulin. We report that 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNP), a protein associated with oligodendrocyte/myelin membranes, also present in lymphocytes and retina, is one major target for the humoral response. Antibodies to CNP are detected in sera of 74% of MS patients. The antibodies are IgM and are present in serum in high titer as well as in cerebrospinal fluid. The antibody response is temporally persistent, consistent with systemic immune activation and persistent antigenic stimulation. Moreover, CNP is isolated as an immune complex from MS brain. CNP is expressed as two isoforms, with CNPII identical to CNPI but with a 20-amino acid extension at the amino terminus of CNPII; however, the antibody response is exclusively restricted to CNPI. In contrast, both isoforms bind the C3 complement, providing a plausible mechanism in MS central nervous system (CNS) for opsonization of myelin membrane CNP, mediated via the C3 receptor, and phagocytosis of CNP-Ig immune complexes, mediated by membrane Ig Fc receptors of macrophages and CNS microglia.

Cathepsin E immunoreactivity in human ocular tissues: influence of aging and pathological states

We studied the antigenic expression of the aspartic proteinase cathepsin E in normal and pathologic human ocular tissues obtained from donors of different age. In the retina the enzyme was immunolocalized in neurons of outer and inner plexiform layers and in few ganglionic neurons. Muller cells were also sometimes immunoreactive for cathepsin E. An increase of neuronal enzyme immunoreactivity with age was evident. Immunocompetent blood cells invading the vitreous body were strongly immunostained for the enzyme. The enzyme is possibly involved in the retinal protein metabolism and might play immunological roles in certain pathologic events.

Early activation of microglia in the pathogenesis of fatal murine cerebral malaria

Microglia are pluripotent members of the macrophage/monocyte lineage that can respond in several ways to pathological changes in the central nervous system. To determine their role in the pathogenesis of fatal murine cerebral malaria (FMCM) we have conducted a detailed study of the changes in morphology and distribution of retinal microglia during the progression of the disease. Adult CBA/T6 mice were inoculated with Plasmodium berghei ANKA. These mice died 7 days post inoculation (p.i.) with the parasite while exhibiting cerebral symptoms, increased permeability of the blood-brain barrier, and monocyte adherence to the vascular endothelium. Mice were injected i.v. with Monastral blue 2 h prior to sacrifice to identify "activated" monocytes, and their isolated retinae were incubated with the Griffonia simplicifolia (GS) lectin or reacted for the nucleoside diphosphatase enzyme to visualize microglia and the vasculature. Changes in microglial morphology were seen within 2-3 days p.i., that is, at least 3 days prior to the onset of cerebral symptoms and 4 days before death. Morphological changes included retraction of ramified processes, soma enlargement, an increasingly amoeboid appearance, and vacuolation. There was also increased staining intensity and redistribution of "activated" microglia toward retinal vessels, but no increase in density of NDPase-positive cells. The GS lectin only labeled a small population of microglia in the uninfected adult mouse retina. However, there was a striking increase in the focal density of GS-positive microglia during the progression of the disease. Extravasation of monocytes also was observed prior to the onset of cerebral symptoms. These results provide the first evidence that microglial activation is a critical component of the pathological process during FMCM.

The ordered array of perivascular macrophages is disrupted by IL-1-induced inflammation in the rabbit retina

In this study we have shown that an antibody to CD18 identified a population of cells in the rabbit retina that resembled the perivascular macrophage found in other regions of the central nervous system. In the normal retina these cells possessed a ramified morphology and presented in an ordered array on the vitreal surface in association with the epiretinal vessels. Approximately 50% of the perivascular macrophages constitutively expressed MHC class II. In response to interleukin-1 beta (IL-1 beta)-induced inflammation, these cells became activated, as evidenced by a change from a ramified to an ameboid morphology and increased expression of MHC class II, and migrated away from the vessels. These changes were first detected around 3 h post-intraocular challenge coincident with the onset of inflammation. At the peak of the inflammatory response (approximately 24 h post-challenge), many activated perivascular macrophages were no longer associated with the vessels and formed long "cord" of MHC class II+ cells associated with underlying deposits of fibrin. In eyes challenged with heat-inactivated IL-1, no change in the morphology or distribution of the perivascular macrophage was noted. At 3 weeks post-challenge with IL-1, the number and distribution of the perivascular macrophages were restored to baseline values, although with a reduced cell size. Since these changes closely resemble those that occur in non-lymphoid dendritic cells in the skin, heart, and/or kidney following activation with cytokines or bacterial products, the results suggest that the perivascular macrophage represents the dendritic cell of the retina and may thus play an important role in immune surveillance in the eye and maintenance of the blood-retina barrier.

Microglia in the nerve fiber layer of the cat retina: detection of postnatal changes by a new monoclonal antibody

This paper describes changes in the appearance and distribution of microglia in postnatal cat retina as demonstrated by a new antibody, H386F. This fractionated IgM antibody was created via an intrasplenic immunization of a single BALB/C mouse with about 2-3 x 10(5) large, whole cells isolated from 46 minced cat retinae. To confirm that the labeled cells are microglia, the staining properties of H386F were compared with those of four commercially available antibodies, OX-33, OX-41, OX-42, and ED-1, that have been used by others to distinguish between microglia and other cells in rat brain. These experiments show that H386F is the only antibody of the five to label only microglia in both the cat retina and hippocampus.

The influence of penetrating keratoplasty and cyclosporin A therapy on MHC class II (Ia)-positive cells in the rat iris and choroid

BACKGROUND

The presence of Ia-positive cells (MHC class II equivalent) has been previously reported in the iris and choroid of various species. They have been reported to have both round and dendritic morphologies; the latter may represent classic dendritic cells, potent antigen-presenting cells (APCs). It is possible that the dendritic-like cells play a important role in (auto)immune processes of uveal and other ocular tissues. Using the flat or whole mount technique, the distribution of Ia-positive cells in the rat iris and choroid was investigated following penetrating keratoplasty (PKP) and following treatment with cyclosporin A (CsA).

METHODS

Lewis (LW) rats received corneal buttons from Lewis-Brown Norway (LW-BN) donors and were randomly assigned to the following groups: (i) operated, untreated (n = 24); (ii) operated, CsA-treated (10 mg/kg i.m.; n = 22). Controls were groups (iii) normal LW rats (n = 13); (iv) unoperated, CsA-treated (16 days' treatment; n = 8); (v) anterior perforation of the anterior chamber (n = 3); (vi) eight corneal sutures only (n = 4); (vii) syngeneic operated (LW to LW; n = 4). Animals of groups (i) and (ii) were killed on the 5th, 9th and 13th postoperative days and on appearance of the corneal rejection (group i, day 13; group ii, day 16). Both eyes were enucleated, immediately fixed, and iris-choroid flat mounts were examined for Ia-positive cells using APAAP immunohistochemistry.

RESULTS

In the normal Lewis rat iris, scattered Ia-positive cells of both nondendritic and dendritic morphology were observed. CsA treatment in the unoperated rat did not result in a significant decrease in the percentage of dendritic cells in the iris or choroid. Anterior chamber perforation, the placement of sutures in the cornea and syngeneic PKP resulted in a moderate increase in iris Ia-positive cells. Allogeneic transplantation resulted in a large increase in both types of Ia-positive cells, particularly on day 13 with corneal rejection. In group ii, an initial decrease in Ia-positive cells until day 13 was observed; upon rejection (day 16), the histological picture was similar to that of untreated animals. Alterations in the operated choroid were also apparent following CsA treatment.

CONCLUSION

Corneal transplantation in the Lewis rat results in an increase in Ia-positive cells in the iris; CsA therapy can delay but not prevent this reaction. Changes in choroidal Ia-positive cells following PKP were not apparent, their numbers being affected only by CsA treatment following grafting.

Mouse microglial cell lines differing in constitutive and interferon-gamma-inducible antigen-presenting activities for naive and memory CD4+ and CD8+ T cells

We developed a panel of non-virus transformed cell lines derived from individual microglial precursors residing in the brains of normal mice. These colony stimulating factor-1-dependent cell lines are B7-1+ (CD80), Mac-1+, Mac-2+, Mac-3+, CD45+, MHC class I+, colony stimulating factor-1 receptor+, and they ingest antibody-coated particles. However, the cell lines differ in their expression of B7-2 (CD86), F4/80, Ly-6C and MHC class II molecules. They also differ in their ability to constitutively process and present antigens to naive CD4+ and CD8+ T cells, memory CD4+ and CD8+, and in the manner by which interferon gamma modulates their antigen-presenting activities. These cell lines should be valuable as models for studies on the immunobiology of the microglia.

Development of microglial topography in human retina

The development of microglial topography in wholemounts of human retina has been examined in the age range 10-25 weeks gestation (WG) using histochemistry and immunohistochemistry for CD45 and major histocompatibility complex class II antigens. Microglia were present in three planes corresponding to the developing nerve fibre layer/ganglion cell layer, the inner plexiform layer and the outer plexiform layer. Distribution patterns of cells through the retinal thickness and across the retinal surface area varied with gestational age. Microglia were elongated in superficial retina, large and ramified in the middle plane, and small, rounded and less ramified in deep retina. Intensely labeled, rounded profiles seen at the pars caeca of the ciliary processes, the retinal margin and at the optic disc may represent precursors of some retinal microglia. At 10 WG, the highest densities of microglia were present in middle and deep retina in the far periphery and at the retinal margin, with few superficial microglia evident centrally at the optic disc. At 14 WG, high densities of microglia were apparent superficially at the optic disc; microglia of middle and deep retina were distributed at more central locations although continuing to concentrate in the retinal periphery. Microglia appear to migrate into the developing human retina from two mains sources, the retinal margin and the optic disc, most likely originating from the blood vessels of the ciliary body and iris, and the retinal vasculature, respectively. The data suggest that the development of microglial topography occurs in two phases, an early phase occurring prior to vascularization, and a late phase associated with the development of the retinal vasculature.

Ontogeny and cellular expression of MHC and leucocyte antigens in human retina

We have investigated the ontogeny of MHC class I, class II, CD45, and macrophage antigens in whole mounts of normal human fetal retina at 10-25 weeks gestation (WG) using monoclonal antibodies and immunogold histochemistry. MHC class I antigens were expressed on retinal vascular endothelial cells and provided a useful marker of vessel organization from 14-25 WG. Microglial cells expressed immunoreactivity to MHC class I, class II, and CD45 antigens from 10 WG (pre-vascularization) and macrophage S22 (Mac S22) antigen from 14 WG (post-vascularization), although none of the antigens tested were detected on neuronal or macroglial elements. Microglia expressing MHC, CD45, and macrophage antigens occurred in both ramified and rounded forms with no close correlation being observed between morphology and antigenicity. The numbers of immunoreactive cells labeled with each of the four markers increased steadily throughout gestation in all specimens studied. Equivalent numbers of microglia expressed MHC class I, class II, and CD45 antigens in retinae at similar gestational ages; however, our data indicate that microglia expressing Mac S22 antigen comprise approximately 40% or less of the population of MHC and CD45-immunoreactive cells during development. Topographical analyses suggest that MHC class I, class II, and CD45-positive microglia enter the retina from both the peripheral retinal margin and the optic disc from at least 10 WG; Mac S22-positive cells appear in association with the development of the retinal vasculature and enter the retina via the optic disc after 14 WG.

Exudative macular degeneration and intravitreal triamcinolone. A pilot study

PURPOSE

To study the efficacy of the anti-inflammatory agent triamcinolone (Kenacort A-40) in patients with exudative age-related macular degeneration and subfoveal and juxtafoveal choroidal new vessels, considered unsuitable for laser photocoagulation.

METHOD

Thirty eyes of 28 patients were treated with intravitreal injection of triamcinolone. The subsequent visual acuity (VA) of treated eyes was compared with published VA outcomes of untreated eyes. Patients were classified into three types according to their responses to treatment.

RESULTS

Within two weeks of receiving treatment, exudation decreased and vision improved in the majority of Types I and II patients (87%), the trend continuing in longer term follow-up. The overall VA outcome for treated eyes was significantly better than published VA data for untreated exudative macular lesions.

CONCLUSIONS

The preliminary results are encouraging and no serious side effects of a single injection of triamcinolone have been detected in patients followed for up to 18 months. The treatment should, however, continue to be regarded as unproven and only administered in the context of a prospective, case-controlled clinical trial.

Flow cytometric identification of a minority population of MHC class II positive cells in the normal rat retina distinct from CD45lowCD11b/c+CD4low parenchymal microglia

AIMS

This study aimed to isolate and classify by flow cytometry, the cell surface phenotype of microglia in the normal rat retina with a view to identifying putative antigen presenting cells (APC) within the retina, which has to date not been possible by immunohistochemistry.

METHODS

Normal rat retinal microglia were isolated and classified using a modification of an isolation technique employing graduated Percoll density gradient cell separation and flow cytometric phenotypic criteria used for CNS microglia.

RESULTS

Retinal microglia can be defined by flow cytometry on the basis of their CD45lowCD11b/c+CD4low cell surface expression. Constitutive MHC class II expression in the normal rat retina was confined almost exclusively to a very minor population of cells expressing neither low (microglia) nor high levels of CD45. Three colour flow cytometric analysis confirmed that these MHC class II positive cells were ED2+.

CONCLUSIONS

Using this sensitive isolation technique we have identified the cell surface characteristics of ramified, resident microglia, and found that they do not constitutively express MHC class II. There is, however, constitutive MHC class II expression on a phenotypically distinct population of cells (CD45low/highED2+). We propose these cells are the counterpart of the perivascular macrophages found in the CNS which present antigen to extravasating T cells, although their exact retinal location can only be confirmed by immunohistochemical analysis. The role of parenchymal microglia as APC remains undefined. Future isolation of microglia and putative perivascular cells using this technique will help identify the role these cells play in the initiation and perpetuation of immune responses within the retina.

Human retinal microglia: expression of immune markers and relationship to the glia limitans

The immunoreactivity, morphology and relationship to the glia limitans of microglia were investigated in flatmounts and sections of normal human retina, using immunogold histochemistry, electron microscopy (EM), and antibodies directed against CD45, major histocompatability complex class I (MHC-I), MHC-II, and human macrophage antigens. Immunoreactivity was evident for all antibodies tested, including MHC-I, which labeled both microglia and retinal vascular endothelium. Most consistent labeling was obtained using antibodies to CD45, MHC-II, and anti-human macrophage (S22) antigen. Immunoreactive cells were seen in the perivascular space (perivascular cells), where they were closely adherent to the vessel profile, and in the retinal parenchyma (microglia). Some parenchymal microglia were also vessel associated and by EM were seen to be closely related to the glia limitans (paravascular microglia). Paravascular microglia were shown by optical densitometry, to express higher levels of MHC antigens than neighboring, non-vessel associated, parenchymal microglia. In addition, paravascular microglia were macrophage (S22) antigen positive, while other parenchymal microglia did not express macrophage antigens. Quantitative data indicate that similar populations of microglia are immunoreactive to CD45, MHC-I, and MHC-II, while relatively few microglia (approximately 10%) are immunoreactive for human macrophage (S22) antigens, supporting previous suggestions that microglia are a heterogeneous population.

Microglia in the avian retina: immunocytochemical demonstration in the adult quail

Immunocytochemical techniques were used in conjunction with the QH1 antibody to study the morphological characteristics and distribution of microglia in the avascular retina of an avian species (the quail). The majority of microglial cells appeared in the outer and inner plexiform layers throughout the entire retina, whereas a few microglial cells in the nerve fiber layer were seen only in the central zone of the retina, near the optic nerve head. In the outer plexiform layer, microglial cells were star-shaped, with processes that ramified profusely in the horizontal plane. Fine process tips extended outward radially, insinuating themselves among the photoreceptors. A regular mosaic-like arrangement of microglial cells was evident in the outer plexiform layer, with no overlapping between adjacent cell territories. Microglial cells in the inner plexiform layer ramified through the entire width of this layer, showing radial and horizontal processes. Microglia in the inner plexiform layer also tended to be regularly distributed in a mosaic-like fashion, although there was slight overlapping between adjacent cell territories. Microglia density in this layer was approximately twice that in the outer plexiform layer. This pattern of microglial distribution was similar to that described in vascular retinae of several species of mammals, a finding that suggest that blood vessels are not responsible for the final locations of microglia in the adult retina, and that microglial precursors must migrate through long distances before they reach their precise destination.