Isolation and direct characterization of resident microglial cells from the normal and inflamed central nervous system

Reactive microgliosis

Damage to the central nervous system (CNS) elicits the activation of both astrocytes and microglia. This review is focused on the principal features that characterize the activation of microglia after CNS injury. It provides a critical discussion of concepts regarding microglial biology that include the relationship between microglia and macrophages, as well as the role of microglia as immunocompetent cells of the CNS. Mechanistic and functional aspects of microgliosis are discussed primarily in the context of microglial neuronal interactions. The controversial issue of whether reactive microgliosis is a beneficial or a harmful process is addressed, and a resolution of this dilemma is offered by suggesting different interpretations of the term 'activated microglia' depending on its usage during in vivo or in vitro experimentation.

Separate precursor cells for macrophages and microglia in mouse brain: immunophenotypic and immunoregulatory properties of the progeny

The brain contains two populations of macrophages: the microglia of brain parenchyma, and the central nervous system (CNS) macrophages located in the perivascular spaces, the leptomeninges and the choroid plexus. The microglia are characterized, in part, by their paucity of major histocompatibility complex (MHC) molecules and lack of constitutive antigen (Ag)-presenting activity for naïve CD4+ T-cells. Some CNS macrophages, on the other hand, constitutively express MHC molecules and present Ag to naïve CD4+ T-cells. We have reported that mouse brain contains precursor cells that, in the presence of colony-stimulating factor-1, the macrophage growth factor, give rise to clones of cells that differ in their ability to constitutively present Ag to naive CD4+ T cells. Here we report that this population of precursor cells can be separated into two discrete subpopulations based on differences in cell density and that the two cell populations give rise to progeny that differ in their content of cells constitutively expressing MHC class II and CD86 molecules, and the ability to present Ag to naïve CD4+ T-cells. A comparison of the level of CD45 staining of the progeny, an indication of a microglial or a CNS macrophage origin, suggests that one population of precursor cells yields immunologically immature microglia and the other CNS macrophages.

Microglia stimulate naive T-cell differentiation without stimulating T-cell proliferation

A major question relevant to the initiation and progression of inflammation and autoimmune processes within the central nervous system (CNS) is whether resident microglia or only infiltrating macrophage can productively interact with T-cells that enter the CNS either actively through extravasation or passively through defects in the blood brain barrier (BBB). We isolated microglia and macrophage from the brains of healthy adult mice and transgenic mice that displayed many features of multiple sclerosis and HIV leukoencephalopathy due to the astrocytic expression of interleukin (IL)-3 and compared their antigen-presenting cell (APC) functions. We found that unactivated microglia isolated from healthy nontransgenic mice and activated microglia isolated from transgenic siblings are relatively weak stimulators of naive T-cell proliferation compared to macrophage populations. The APC function of activated, but not unactivated, microglia could be increased by treatment acutely with lipopolysaccharide (LPS)/interferon gamma (IFN-gamma). However, this treatment also induced the apparent production of prostaglandins, which reduced T-cell proliferation when indomethacin was absent from the assay cultures. Strikingly, even in the absence of stimulated T-cell proliferation, both unactivated and activated microglia stimulated the differentiation of naive T-cells into Th1 effector cells, although neither microglial population was a more effective inducer than macrophages or splenic APCs. Thus, while microglia are clearly capable of productively interacting with naive T-cells, macrophages have a more robust APC function.

Antigen presenting capacity of brain microvasculature in altered peptide ligand modulation of experimental allergic encephalomyelitis

Co-immunization with an altered peptide ligand (LR) partially protects SJL mice from proteolipid protein peptide 139-151-induced experimental allergic encephalomyelitis [Kuchroo, V.K., Greer, J.M., Kaul, D., Ishioka, G.Y., Franco, A., Sette, A., Sobel, R.A., Lees, M.B., 1994. A single TCR antagonist peptide inhibits experimental allergic encephalomyelitis mediated by a diverse T cell repertoire. J. Immunol. 153, 3326-3336; Santambrogio, L., Lees, M.B., Sobel, R.A., 1998. Altered peptide ligand modulation of experimental allergic encephalomyelitis: immune responses within the CNS. J. Neuroimmunol. 81, 1-13]. Clinical protection was noted despite extensive central nervous system inflammation observed after co-immunization with native and altered peptides. To extend our previous reports on this model, we now compare MHC class II expression and antigen presenting cell activity of cells associated with the blood-brain barrier in diseased and protected mice. Immunohistochemical studies identified MHC class II products on both the endothelial and microglial/macrophage populations. Ex vivo experiments suggested a correlation between the reduced clinical disease observed in the co-immunized mice and the antigen presenting activity of cells at the blood-brain barrier. The results suggest that antigen presenting activity is primarily mediated by macrophage-lineage cells of the central nervous system.

Balancing function vs. self defense: the CNS as an active regulator of immune responses

Immunological privilege of the central nervous system (CNS) has often been viewed as the summation of mechanisms that are protective of, but extrinsic to, the CNS. Their primary role has then been seen as isolating the CNS from the organism as a whole. Experiments in recent years indicate that the CNS itself may have an innate immune system comprised of astrocytes and microglia capable of regulating the initiation and progression of immune responses. Thus, immunological privilege should be considered as an intrinsic property of the CNS that could involve direct CNS: immune cell interactions. Malfunctions of these intrinsic mechanisms could play significant roles augmenting or even initiating CNS-directed autoimmunity and inflammation.

Protective role of the virus-specific immune response for development of severe neurologic signs in simian immunodeficiency virus-infected macaques

The pathogenesis of human immunodeficiency virus-associated motor and cognitive disorders is poorly understood. In this context both a protective and a harmful role of the immune system has been discussed. This question was addressed in the present study by correlating the occurrence of neurologic disease in simian immunodeficiency virus (SIV)-infected macaques with disease progression and the humoral and cellular intrathecal antiviral immune response. Overt neurologic signs consisting of ataxia and apathy were observed at a much higher frequency in rapid progressor animals (6 of 12) than in slow progressors (1 of 7). Whereas slow progressors mounted a strong antiviral antibody (Ab) response as evidenced by enzyme-linked immunosorbent and immunospot assays, neither virus-specific Ab titers nor Ab-secreting cells could be found in the cerebrospinal fluid (CSF) or brain parenchyma of rapid progressors. Similarly, increased infiltration of CD8(+) T cells and cytotoxic T lymphocytes specific for viral antigens were detected only in the CSF of slow progressors. The finding that neurologic signs develop frequently in SIV-infected macaques in the absence of an antiviral immune response demonstrates that the immune system does not contribute to the development of motor disorders in these animals. Moreover, the lower incidence of neurologic symptoms in slow progressors with a strong intrathecal immune response suggests a protective role of the virus-specific immunity in immunodeficiency virus-induced central nervous system disease.

Interferon-gamma in progression to chronic demyelination and neurological deficit following acute EAE

The cytokine interferon-gamma (IFNgamma) is implicated in the induction of acute CNS inflammation, but it is less clear what role if any IFNgamma plays in progression to chronic demyelination and neurological deficit. To address this issue, we have expressed IFNgamma in myelinating oligodendrocytes of transgenic mice. MHC I immunostaining and iNOS mRNA were upregulated in their CNS, but such transgenic mice showed no spontaneous CNS inflammation or demyelination, and the incidence, severity, and histopathology of experimental autoimmune encephalomyelitis (EAE) were similar to nontransgenic controls. In contrast to control mice, which remit from EAE with resolution of glial reactivity and leukocytic infiltration, transgenics showed chronic neurological deficits. While activated microglia/macrophages persisted in demyelinating lesions for over 100 days, CD4(+) T lymphocytes were no longer present in CNS. IFNgamma therefore may play a role in chronic demyelination and long-term disability following the induction of demyelinating disease. Because IFNgamma may have neural as well as immune-infiltrating origins, these findings generate a new perspective on its role in the CNS.

Characterization of and functional antigen presentation by central nervous system mononuclear cells from mice infected with Theiler's murine encephalomyelitis virus

We examined the phenotype and function of cells infiltrating the central nervous system (CNS) of mice persistently infected with Theiler's murine encephalomyelitis virus (TMEV) for evidence that viral antigens are presented to T cells within the CNS. Expression of major histocompatibility complex (MHC) class II in the spinal cords of mice infected with TMEV was found predominantly on macrophages in demyelinating lesions. The distribution of I-As staining overlapped that of the macrophage marker sialoadhesin in frozen sections and coincided with that of another macrophage/microglial cell marker, F4/80, by flow cytometry. In contrast, astrocytes, identified by staining with glial fibrillary acidic protein, rarely expressed detectable MHC class II, although fibrillary gliosis associated with the CNS damage was clearly seen. The costimulatory molecules B7-1 and B7-2 were expressed on the surface of most MHC class II-positive cells in the CNS, at levels exceeding those found in the spleens of the infected mice. Immunohistochemistry revealed that B7-1 and B7-2 colocalized on large F4/80(+) macrophages/microglia in the spinal cord lesions. In contrast, CD4(+) T cells in the lesions expressed mainly B7-2, which was found primarily on blastoid CD4(+) T cells located toward the periphery of the lesions. Most interestingly, plastic-adherent cells freshly isolated from the spinal cords of TMEV-infected mice were able to process and present TMEV and horse myoglobin to antigen-specific T-cell lines. Furthermore, these cells were able to activate a TMEV epitope-specific T-cell line in the absence of added antigen, providing conclusive evidence for the endogenous processing and presentation of virus epitopes within the CNS of persistently infected SJL/J mice.

A revised view of the central nervous system microenvironment and major histocompatibility complex class II antigen presentation

There are numerous observations reporting that phagocytes expressing major histocompatibility complex (MHC) Class II molecules are associated with the central nervous system (CNS) in normal and pathological conditions. Although MHC Class II expression is necessary for antigen presentation to CD4 + T-cells, it is not sufficient and co-stimulatory molecules are also required. We review here recent in vivo studies demonstrating that the microglia and perivascular macrophages are unable to initiate a primary immune response in the CNS microenvironment, but may support secondary immune responses. Although in vitro studies show that microglia do not support a primary immune response leading to T-cell proliferation, they do show that microglia may protect the CNS from the unwanted attentions of autoreactive T-cells by inducing their apoptosis. The lack of cells in the CNS parenchyma with the ability to initiate a primary immune response has a cost, namely that pathogens may persist in the CNS undetected by the immune system.

Immune Invasion of the Central Nervous System Parenchyma and Experimental Allergic Encephalomyelitis, But Not Leukocyte Extravasation from Blood, Are Prevented in Macrophage-Depleted Mice

Organ-specific autoimmune diseases are characterized by infiltrates, including T lymphocytes and activated macrophages. Macrophages and secondarily activated tissue resident counterparts can both present Ag to and contribute to cytokine secretion by T lymphocytes. We have previously shown a crucial role of peripheral macrophages in experimental allergic encephalomyelitis (EAE), a Th1-mediated demyelinating disease that serves as a an animal model for multiple sclerosis (MS), by their depletion using mannosylated liposome-encapsulated dichloromethylene diphosphonate (Cl2MDP). Here we describe studies to investigate the mechanisms by which macrophages contribute to the lesion formation in EAE, by studying the effect of Cl2MDP-containing mannosylated liposomes (Cl2MDP-mnL) on adoptively transferred EAE in SJL/J mice. Adoptive transfer of EAE with myelin basic protein-reactive CD4+ T cells to SJL/J mice was abrogated by Cl2MDP-mnL treatment. CD4+ T cell and MHC II+ B220+ B cell extravasation from blood vessels and Th1 cytokine production were not inhibited. However, invasion of the central nervous system intraparenchymal tissues by lymphocytes, F4/80+, Mac-1+, and MOMA-1+ macrophages was almost completely blocked after treatment with Cl2MDP-mnL. Furthermore, in Cl2MDP-mnL-treated mice, the myelin sheaths appeared completely normal, whereas, in the control groups, marked demyelination occurred. Production of TNF-α and inducible nitric oxide synthase, both associated with macrophage/microglial activation, was inhibited. This intervention reveals a role for macrophages in regulating the invasion of autoreactive T cells and secondary glial recruitment that ordinarily lead to demyelinating pathology in EAE and multiple sclerosis.

Late-onset chronic inflammatory encephalopathy in immune-competent and severe combined immune-deficient (SCID) mice with astrocyte-targeted expression of tumor necrosis factor

To examine the role of tumor necrosis factor (TNF)-alpha in the pathogenesis of degenerative disorders of the central nervous system (CNS), transgenic mice were developed in which expression of murine TNF-alpha was targeted to astrocytes using a glial fibrillary acidic protein (GFAP)-TNF-alpha fusion gene. In two independent GFAP-TNFalpha transgenic lines (termed GT-8 or GT-2) adult (>4 months of age) animals developed a progressive ataxia (GT-8) or total paralysis affecting the lower body (GT-2). Symptomatic mice had prominent meningoencephalitis (GT-8) or encephalomyelitis (GT-2) in which large numbers of B cells and CD4+ and CD8+ T cells accumulated at predominantly perivascular sites. The majority of these lymphocytes displayed a memory cell phenotype (CD44high, CD62Llow, CD25-) and expressed an early activation marker (CD69). Parenchymal lesions contained mostly CD45+ high, MHC class II+, and Mac-1+ cells of the macrophage microglial lineage with lower numbers of neutrophils and few CD4+ and CD8+ T cells. Cerebral expression of the cellular adhesion molecules ICAM-1, VCAM-1, and MAdCAM as well as a number of alpha- and beta-chemokines was induced or upregulated and preceded the development of inflammation, suggesting an important signaling role for these molecules in the CNS leukocyte migration. Degenerative changes in the CNS of the GFAP-TNFalpha mice paralleled the development of the inflammatory lesions and included primary and secondary demyelination and neurodegeneration. Disease exacerbation with more extensive inflammatory lesions that contained activated cells of the macrophage/microglial lineage occurred in GFAP-TNFalpha mice with severe combined immune deficiency. Thus, persistent astrocyte expression of murine TNF-alpha in the CNS induces a late-onset chronic inflammatory encephalopathy in which macrophage/microglial cells but not lymphocytes play a central role in mediating injury.

Cytokine expression by inflammatory cells obtained from the spinal cords of Lewis rats with experimental autoimmune encephalomyelitis induced by inoculation with myelin basic protein and adjuvants

Inflammatory cells were obtained from the spinal cords of rats with acute experimental autoimmune encephalomyelitis (EAE) induced by inoculation with myelin basic protein (MBP) and adjuvants. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to investigate the expression of mRNA for interleukin-2 (IL-2), IL-4, IL-10 and interferon-gamma (IFN-gamma) by cells from groups of rats studied 10-21 days after inoculation. On all days of study, the inflammatory cells, which were predominantly lymphocytes, expressed mRNA for IL-2, IL-4, IL-10 and IFN-gamma. In the mRNA from normal rat spinal cord tissue, there was little expression of cytokine mRNA. Cells from a short-term MBP-reactive T cell line expressed all the cytokines. Densitometry was used to measure the products of PCR, to assess the expression of each cytokine relative to that of beta-actin. IL-2 mRNA was expressed throughout the course of disease and reached a peak on day 18, during late clinical recovery. IFN-gamma was expressed throughout the course of the disease and was also high during late recovery. IL-4 mRNA was present in the spinal cord throughout the course of the disease, with a slight rise during late recovery. Relative expression of IL-10 rose to a peak on days 17-19, during late recovery from clinical disease. This study indicates that IL-2, IL-4, IL-10 and IFN-gamma are expressed by inflammatory cells in the spinal cord in EAE, with the relative expression of all cytokines being high during late clinical recovery.

Central Nervous System Microglial Cell Activation and Proliferation Follows Direct Interaction with Tissue-Infiltrating T Cell Blasts

Central nervous system (CNS)-resident macrophages (microglia) normally express negligible or low level MHC class II, but this is up-regulated in graft-vs-host disease (GvHD), in which a sparse CNS T cell infiltrate is observed. Relative to microglia from the normal CNS, those from the GvHD-affected CNS exhibited a 5-fold up-regulation of characteristically low CD45, MHC class II expression was increased 10- to 20-fold, and microglial cell recoveries were enhanced substantially. Immunohistologic analysis revealed CD4+αβTCR+CD2+ T cells scattered infrequently throughout the CNS parenchyme, 90% of which were blast cells of donor origin. An unusual clustering of activated microglia expressing strongly enhanced levels of CD11b/c and MHC class II was a feature of the GvHD-affected CNS, and despite the paucity of T lymphocytes present, activated microglial cell clusters were invariably intimately associated with these T cells. Moreover, 70% of T cells in the CNS were associated with single or clustered MHC class II+ microglia, and interacting cells were predominantly deep within the tissue parenchyme. Approximately 3.7% of the microglia that were freshly isolated from the GvHD-affected CNS were cycling, and proliferating cell nuclear Ag-positive microglia were detected in situ. Microglia from GvHD-affected animals sorted to purity by flow cytometry and cultured, extended long complex processes, exhibited spineous processes, and were phagocytic and highly motile. These outcomes are consistent with direct tissue macrophage-T cell interactions in situ that lead to activation, proliferation, and expansion of the responding tissue-resident cell.

Neutralizing TNF-alpha activity modulates T-cell phenotype and function in experimental autoimmune uveoretinitis

Inhibiting TNF-alpha activity prevents tissue destruction without inhibiting retinal T cell infiltration in experimental autoimmune uveoretinitis (EAU) in Lewis rats. To further determine the role of TNF-alpha in autoimmune uveitis we characterized T cells isolated from retinae after treatment with a TNF-alpha antagonist. TNF-alpha activity was neutralized in vivo with a p55 TNF-alpha receptor-Ig fusion protein (sTNFr-Ig), administered 8 and 10 days after induction of EAU with heterologous retinal antigens. Retinal T-cell phenotype expression was examined by flow cytometry with respect to OX22 status (CD45RBlow or CD45RBhigh), activation (OX40 and CD25 expression) and rate of T-cell apoptosis (Annexin V+PI- expression). Lymphocyte reactivity was assessed by proliferation responses and cytokine production to retinal antigens. Despite greater than 40% of CD4+ T cells being activated at the height of disease, the proportion of OX22low expression was reduced and T cells exhibited reduced IFN-gamma and elevated IL-4 production. Retinal T cells maintained antigen-specific proliferation and demonstrated a low apoptotic rate. Although in both animal groups, comparable numbers of T cells were isolated, neutralizing TNF activity suppressed Th1 effector mechanisms protecting against target organ damage.

Marrow-derived activated macrophages are required during the effector phase of experimental autoimmune uveoretinitis in rats

PURPOSE

Experimental autoimmune uveoretinitis (EAU), an established model for human endogenous (autoimmune) posterior uveitis, is a CD4+ T cell-mediated disease inducible in Lewis rats by intradermal inoculation with retinal antigens. Immunohistochemical studies have previously documented the lymphocyte profiles during various stages of the disease process. The purpose of the present study was to investigate the role of macrophages in EAU.

METHODS

EAU was induced in Lewis rats, and the effect of macrophage depletion, using the drug dichlorodimethylene diphosphonate (Cl2MDP) encapsulated in liposomes and administered intravenously, was assessed based on the clinical and histological profile of the disease.

RESULTS

The results have shown that in control animals macrophages occur early, feature prominently throughout the course of the disease and display considerable heterogeneity: marrow-derived ED1+ cells and ED3+ cells are the major infiltrating cells, with many cells also expressing ED7 and ED8. In contrast, few cells expressed the ED2 antigen during EAU, even though ED2+ "resident" macrophages occur in the normal choroid. Macrophage depletion, using intravenously injected dichloromethylene diphosphonate (Cl2MDP) enclosed in liposomes, caused a delay in the onset and a reduction in the severity of EAU when administered during the "effector" stage of the disease, i.e. 9-11 days after inoculation with retinal antigen. The delay in disease onset was greater when liposomes were mannosylated and was accompanied by a reduction in the overall inflammatory cell infiltrate into the eye and reduced tissue damage. In addition, there was a reduction in the level of expression of MHC Class II antigen and CR3 (ED7) antigen, a marker of macrophage activation, in Cl2MDP-treated animals compared to controls.

CONCLUSION

These results suggest that blood-borne, activated macrophages are major effectors of tissue damage during EAU.

Interferon-gamma antagonizes transforming growth factor-beta2-mediated immunosuppression in murine Toxoplasma encephalitis

The in vivo modulating activity of recombinant transforming growth factor (TGF)-beta2 on acute toxoplasmosis was evaluated in both Toxoplasma gondii susceptible C57BL/6 and resistant BALB/c mice. TGF-beta2 lethally exacerbated Toxoplasma encephalitis in C57BL/6, but not in BALB/c mice. In C57BL/6 mice, TGF-beta2 induced a profound dose-dependent increase of the intracerebral parasitic load as well as a reduction of IFN-gamma levels in serum and cerebrospinal fluid with a coincident decrease of MHC class II antigen expression of macrophages, microglial cells, and B cells. Furthermore, TGF-beta2-treated C57BL/6 mice showed a reduced activation of CD4+ and CD8+ T cells and a diminished recruitment of immune cells to the brain. The TGF-beta2-mediated development of lethal toxoplasmosis in C57BL/6 mice was abolished by treatment with recombinant interferon (IFN)-gamma.

Mature microglia resemble immature antigen-presenting cells

Owing to the difficulties of isolating adequate numbers of microglia from adult tissue, much of our understanding of their function is based on characterizations of microglia that develop in mixed glial cultures. To learn more about the nature of these cells in vivo, we have compared the phenotypes of murine microglia isolated from adults, neonates, and from mixed glial cultures with spleen cells from fetuses, neonates, and adults. In the adult CNS, the only resident population of cells that express CD45, a protein tyrosine phosphatase, are the F4/80+ and FcR+ cells: the microglia. In contrast to all other differentiated cells of hemopoietic origin, microglial CD45 levels fail to increase from the neonatal period through adulthood. Rather, their levels are indistinguishable from the low levels found on a small population of embryonic day 16 liver cells. Conversely, we find that the F4/80 values of microglia are elevated as compared to splenic macrophages. Strikingly, microglia that develop in mixed glial cultures display a more activated phenotype, with low F4/80 values, weak MHC class II expression, and the appearance of a subset of cells positive for the dendritic cell marker, NLDC145. Additionally, CD45 values are elevated to a level intermediate between that of adult microglia and adult spleen, a level similar to that found on microglia activated in vivo. Consistent with this activated phenotype, indomethacin revealed the ability of mixed glial culture microglia to present a peptide antigen to naive T-cells expressing a defined T-cell receptor. Although adult microglia did express costimulatory molecules, B7.2, ICAM-1, and CD40, and could be induced to express MHC class II, they failed to present antigen in the same assay. Interestingly, these same cells could stimulate T-cell proliferation in a mixed lymphocyte reaction but not in an allogeneic specific manner. Taken together these data suggest that adult microglia remain in a relatively immature and unactivated state of differentiation as compared to other tissue macrophages.

Direct ex vivo flow cytometric analysis of human microglial cell CD4 expression: examination of central nervous system biopsy specimens from HIV-seropositive patients and patients with other neurological disease

OBJECTIVE

To define a clear ex vivo flow cytometric phenotype for adult human microglia that would distinguish it from all other macrophage lineage cells in the central nervous system (CNS) or blood, and to utilize this phenotype to examine the activation state and CD4 expression of microglia freshly derived from CNS tissue of HIV-positive patients and those with other neurological diseases.

DESIGN

Fresh human CNS tissue from both HIV-uninfected and HIV-infected individuals was obtained by biopsy or resection, and cells isolated immediately, labelled for flow cytometry and analysed.

METHODS

A Percoll density gradient isolation technique and phenotypic characteristics used for rodent microglia were applied and modified.

RESULTS

Resident microglia could clearly be defined by the flow cytometric phenotype CD45low CD4- CD11b+ CD11chigh major histocompatibility complex (MHC) class II+ CD26- CD14-. Assuming normally low-level MHC class II expression in the healthy CNS, it was likely that MHC class II positivity reflected underlying pathology necessitating biopsy or resection and appeared to be a 'leaky' activation marker. Microglia activation was observed in specimens from only six (35%) out of 17 HIV-uninfected but all four (100%) HIV-infected patients, defined strictly as any level of upregulation of CD4 expression, to produce the phenotype CD45low/medium CD4low CD11b+ CD1.1Chigh MHC class II+/+2 CD26- CD14-. Where examined by immunohistology, CD68 was also upregulated in these cases.

CONCLUSIONS

When activated in situ, microglia express low levels of CD4 and this is always seen in tissue from HIV-infected patients. Using the flow cytometric phenotype established here, microglia from HIV-infected tissue can now be isolated in pure form and studied directly ex vivo.

The central nervous system environment controls effector CD4+ T cell cytokine profile in experimental allergic encephalomyelitis

In experimental allergic encephalomyelitis (EAE), CD4+ T cells infiltrate the central nervous system (CNS). We derived CD4+ T cell lines from SJL/J mice that were specific for encephalitogenic myelin basic protein (MBP) peptides and produced both Th1 and Th2 cytokines. These lines transferred EAE to naive mice. Peptide-specific cells re-isolated from the CNS only produced Th1 cytokines, whereas T cells in the lymph nodes produced both Th1 and Th2 cytokines. Mononuclear cells isolated from the CNS, the majority of which were microglia, presented antigen to and stimulated MBP-specific T cell lines in vitro. Although CNS antigen-presenting cells (APC) supported increased production of interferon (IFN)-gamma mRNA by these T cells, there was no increase in the interleukin (IL)-4 signal, whereas splenic APC induced increases in both IFN-gamma and IL-4. mRNA for IL-12 (p40 subunit) was up-regulated in both infiltrating macrophages and resident microglia from mice with EAE. We have thus shown that a Th1 cytokine bias within the CNS can be induced by CNS APC, and that IL-12 is up-regulated in microglial cells within the CNS of mice with EAE. Microglia may therefore control Th1 cytokine responses within the CNS.

A population of interstitial cells in the anterior pituitary with a hematopoietic origin and a rapid turnover: a relationship with folliculo-stellate cells?

The non-hormone secreting folliculo-stellate (FS) cells in the anterior pituitary (AP) appear heterogeneous. Some of these cells have been described as having a neuroectodermal origin and being glial, while some others have been suggested to be monocytic or dendritic cells (DC). We have analyzed here the hematopoietic origin of interstitial cell populations in the AP. In the rat AP, the relative densities of S100+ FS cells and major histocompatibility complex (MHC) class II-expressing DC-like cells show a parallel increase in the postnatal period between the age of 3 weeks to 2 months. We first looked for the presence of donor derived cells in the AP of lethally irradiated bone marrow (BM)-transplanted rats. Donor derived myeloid cells carrying the n haplotype of the MHC class I antigen (RT1.An) reacting with the OX27 moAb, could not be detected in the AP three months after transplantation. It appeared, however, that OX27+ DC-like cells a-priori were virtually absent from the rat AP. Therefore this transplantation model was not suitable for our studies. We then turned to a model of transgenic mice expressing a suicide gene in subpopulations of dendritic cells. Mice were lethally irradiated and received a BM transplant from the transgenic animals, with or without a treatment with ganciclovir (GCV) that specifically kills the dividing cells expressing the suicide gene. This model has already been used to identify and delete mainly dendritic cell populations, viz N418+ and ER-BMDM1+ dendritic cells in the marginal zones of the spleen and in the thymic medulla. We observed in the AP a 30% reduction of the ER-BMDM1+ FS-like cells and a 50-100% reduction of interstitial cells expressing the F4/80, Mac-1 and MOMA-1 markers in the mice receiving the transgenic BM and treated with GCV, compared to control mice that were not treated with GCV or that received non-transgenic BM. When a treatment with granulocyte-macrophage colony-stimulating factor (GM-CSF) was initiated during the GCV treatment, we observed an even stronger reduction of the above-mentioned interstitial cell populations. These data indicate that in the mouse AP a population of stellate cells exists with a hematopoietic origin, that expresses markers of myeloid cells, and that has a rapid turnover.

Obtention and characterization of primary astrocyte and microglial cultures from adult monkey brains

Simple methods for obtention of primary cultures of isolated astrocytes and microglia from adult simian brain have been developed. Characterization of these two glial cell populations were performed by morphological observations and by immunocytochemistry. The astroglial cultures were obtained by an indirect method. After L-leucine methyl-ester treatment and trypsinizations, more than 99% of cells expressed glial fibrillary acidic protein (GFAP), whereas no macrophages or microglia could be detected. Likely, the 1% remaining cells were immature astrocytes or cells that lost their GFAP expression. Cultured simian astrocytes expressed vimentin, laminin, and fibronectin. We also found a constitutively low expression of major histocompatibility complex (MHC) class II by cultured astrocytes which was significantly enhanced by lipopolysaccharide (LPS), interferon gamma (IFN-gamma), or tumor necrosis factor alpha (TNF-alpha) treatments. Microglial cultures were obtained by a direct method of isolation using Percoll gradient separations and compared to simian monocyte-derived macrophages or alveolar macrophages. Microglial cells differed from macrophages by their proliferation upon granulocyte-macrophage colony stimulating factor (GM-CSF) treatment and by their typical morphology when observed by scanning electron microscopy. As macrophages, they expressed in vitro CD68, CD64, CD14, CD11b, MHC class II, and fibronectin. However, contrary to macrophages, simian cultured microglia expressed laminin. This observation suggests that microglia represent a new potential source of this extracellular matrix protein in the brain.

Tumor necrosis factor blockade in actively induced experimental autoimmune encephalomyelitis prevents clinical disease despite activated T cell infiltration to the central nervous system

Recently, we demonstrated that experimental autoimmune encephalomyelitis (EAE) in the rat, passively transferred using myelin basic protein (MBP)-reactive encephalitogenic CD4+ T cells, was preventable by administration of a p55-tumor necrosis factor-IgG fusion protein (TNFR-IgG). This was despite quantitatively and qualitatively normal movement of these MBP-specific T cells to the central nervous system (CNS). To extend these findings, the effect of TNFR-IgG on EAE actively induced by injection of MBP in complete Freund's adjuvant was examined. This form of EAE in the rat typically involves an acute, self-limiting neurological deficit, substantial CNS inflammation, but minimal demyelination. Here we show that administration of TNFR-IgG prior to onset of disease signs completely prevented the neurological deficit or markedly reduced its severity. This blockade of clinical disease was dissociated from weight loss which occurred at the same tempo and magnitude as in control rats exhibiting neurological signs of disease such as paralysis. The timing of TNF blockade was critical as established clinical disease was relatively refractory to TNFR-IgG treatment. Activated CD4+ T cells expressing normal or elevated levels of VLA4, major histocompatibility complex class II, MRC OX40 and CD25 were isolated from or immunohistochemically localized in the CNS of clinically healthy rats treated before disease onset. There was a reduction of the amount of other inflammatory leukocytes in the CNS of these treated animals but, more importantly, the activation state of inflammatory leukocytes, as well as that of microglia isolated from treated animals, was reduced. Thus, TNFR-IgG, when administered before disease onset, appears to act by inhibiting an effector function of activated T cells and possibly other inflammatory leukocytes necessary to bring about the neurological deficit. However, while TNF is a critically important cytokine for the early events leading to initiation of EAE, it is not a necessary factor in the acute neurological deficit characteristic of this form of EAE, once disease onset has occurred.

Increased severity of experimental autoimmune encephalomyelitis, chronic macrophage/microglial reactivity, and demyelination in transgenic mice producing tumor necrosis factor-alpha in the central nervous system

Tumor necrosis factor-alpha (TNF-alpha) is an inflammatory cytokine implicated in a number of autoimmune diseases. Apoptotic cell death is induced by TNF-alpha in vitro, and has been suggested as one cause of autoimmune pathology, including autoimmune demyelinating diseases where oligodendrocytes are a target of immune attack. TNF-alpha also regulates macrophage activity which could contribute to autoimmune inflammation. We have expressed TNF-alpha at disease-equivalent levels in the central nervous system of transgenic mice, using a myelin basic protein (MBP) promoter. These mice were normal and showed no spontaneous pathology, but they developed experimental autoimmune encephalomyelitis (EAE) with greater severity than nontransgenic controls when immunized with MBP in adjuvant. Unlike nontransgenic controls, EAE then progressed to a nonabating demyelinating disease. Macrophage/microglial reactivity was evident in demyelinating lesions in spinal cord, but T cells were not detected during chronic disease. The participation of TNF-alpha in the demyelinating process is thus more probably due to the perpetuation of macrophage/microglial activation than to direct cytotoxicity of myelin or oligodendroglia.

Expression pattern and cellular origin of cytokines in the normal and Toxoplasma gondii-infected murine brain

In the normal brain, low levels of cytokines are observed, whereas inflammatory disorders of the central nervous system are characterized by an up-regulation of cytokine production. The cellular sources for cytokines in the central nervous system are largely undefined. In the present study, we have analyzed intracerebral cytokine production in normal and Toxoplasma gondii-infected mice using immunohistochemistry, in situ hybridization, flow cytometry of brain-derived leukocytes, and reverse transcriptase polymerase chain reaction detection in various subpopulations of inflammatory cells. In the normal brain, neurons and choroid plexus epithelia expressed interleukin (IL)-1 beta and IL-10. Microglia/macrophages produced IL-1 beta, IL-10, and tumor necrosis factor-alpha In Toxoplasma encephalitis, these cell types exhibited increased levels of the respective cytokines. In addition, microglia/macrophages showed a de novo expression of inducible nitric oxide synthase. CD4+ and CD8+ T cells, which were recruited to the brain, produced IL-2, IL-10, tumor necrosis factor-alpha, and interferon-gamma. IL-4 was exclusively detectable in CD4+ T cells, whereas CD8+ T cells showed expression of IL-1 beta. As chronic Toxoplasma encephalitis was not associated with neuronal degeneration and an up-regulation of neurotrophic factors, some cytokines may also exert neurotrophic and/or neuroprotective properties.

Tolerance to rat liver allografts: IV. Acceptance depends on the quantity of donor tissue and on donor leukocytes

Liver allografts in some rat strains are often spontaneously accepted across a complete major histocompatibility barrier without the requirement for immunosuppression while other nonliver allografts are rejected. In previous studies, we have shown that spontaneous acceptance is dependent on liver passenger leukocytes. Depletion of passenger leukocytes by donor irradiation allows rejection, with DA recipients of irradiated PVG livers having a median survival time (MST) of 16 days. Here we show that, in this model, spontaneous acceptance is reconstituted by intravenous injection of donor leukocytes. Intravenous injection of 3-5x10(7) PVG liver leukocytes significantly prolonged DA survival time (MST=96 days, P=0.026), as did 5x10(7) spleen leukocytes (MST>100 days, P=0.002). Deletion of T cells from the reconstituting inoculum reduced survival time (MST=78 days, P=0.039), whereas deletion of B cells or monocytes/macrophages had no effect on survival time. In contrast, PVG hearts are regularly rejected by DA recipients, and PVG liver or spleen leukocytes, even at doses of greater than 3x10(8) cells/recipient, were unable to induce heart acceptance. To investigate the possibility that acceptance of the irradiated liver but not the heart might be due to the large mass of the liver, two kidneys and two hearts of PVG origin were transplanted to each DA recipient together with 1.5x10(8) PVG leukocytes. These organs survived for greater than 200 days, thereby showing that a large mass of donor tissue, in association with donor leukocytes, leads to acceptance of organs that are rejected if transplanted singly. It appears likely that spontaneous liver transplant tolerance is a high-dose or activation-associated immune phenomenon.

Microglia induce CD4 T lymphocyte final effector function and death

Microglia, a type of tissue macrophage, are the only cells in the central nervous system (CNS) parenchyma to express some major histocompatibility complex (MHC) class II constitutively or to upregulate expression readily. They are thought to play a role in CD4 T cell activation in autoimmune diseases such as multiple sclerosis, as well as in neurodegenerative conditions, Alzheimer's disease in particular. We show here that highly purified MHC class II+ microglia when tested directly ex vivo do indeed support an effector response by an encephalitogenic myelin basic protein-reactive CD4 T cell line from which production of the proinflammatory cytokines, interferon gamma and tumor necrosis factor, is elicited, but not interleukin (IL)-2 secretion or proliferation. After this interaction, the T cells die by apoptosis. Other nonmicroglial but CNS-associated macrophages isolated in parallel stimulate full T cell activation, including IL-2 production, proliferation, and support T cell survival. Neither CNS-derived population expresses B7.1/B7.2. Resident macrophages that terminate effector T cells in tissues constitute a novel and broadly applicable regulatory measure of particular relevance to processes of self-tolerance against sequestered antigens.

Comparison of phenotypic and functional properties of immediately ex vivo and cultured human adult microglia

Microglial cells are resident cells of the CNS and are implicated as regulators and effectors of immune responses which occur within this compartment. The precise role of parenchymal microglia remains speculative because distinctions between these cells, perivascular "microglia," and blood-derived monocytes/macrophages are not well defined. The current study describes the phenotype and function of microglia immediately upon isolation from the non-inflamed adult human CNS and the phenotypic changes which occur in these cells when maintained in tissue culture. We find that the characteristic phenotype of immediately ex vivo parenchymal microglia (CD11c+/CD45low/CD14) corresponds to that found in situ in the "normal" human brain. The phenotype differs from that of perivascular "microglia" in situ and PBDM (both CD45hi/CD14++). The immediately ex vivo microglia express B7-2 and HLA class II molecules and can support alloantigen-induced proliferation by CD4+ T cells freshly isolated from peripheral blood. Following in vitro culture, the cells are characterized by a bipolar morphology, continued lower levels of CD45 expression compared to PBDM, and slight upregulation of B7-1 and HLA-DR antigen expression. CD14 becomes expressed at high levels on the cells, suggesting that CD14 can serve as an apparent marker of microglia activation which is not based on changes in morphology or APC capacity. Further, treatment of the cells with IFN-gamma and LPS causes further upregulation of HLA-DR and clear expression of B7-1 molecules on the surface. The capacity to characterize phenotypic and functional properties of microglia before and after activation provides an opportunity to determine means to manipulate the immune regulatory and effector properties of this cell type.

Inhibition of tumor necrosis factor activity minimizes target organ damage in experimental autoimmune uveoretinitis despite quantitatively normal activated T cell traffic to the retina

Recent studies demonstrated that administration of a p55-tumor necrosis factor (TNF) receptor IgG-fusion protein (TNFR-IgG) prevented the clinical onset of experimental autoimmune encephalomyelitis but did not alter the number or tissue distribution of autoantigen-specific CD4+ effector T cells which trafficked into the central nervous system. To determine whether specific target tissues of autoimmune damage remain intact after TNFR-IgG treatment despite the presence of inflammatory cells within the tissues, we examined rats with experimental autoimmune uveoretinitis (EAU), as in this model, the main target of autoreactive CD4+ T cells, the retinal rod outer segments (ROS), can be examined readily by light microscopy. As judged by direct ophthalmoscopy, the onset of inflammation in the anterior chamber of the eye in EAU following administration of TNFR-IgG was delayed by 6 days compared to untreated controls, but the magnitude of the response was only slightly less than controls. Histological examination of the retinae and direct assessment of retinal inflammation revealed a disproportionate sparing of ROS in the TNFR-IgG-treated animals despite a level of retinal inflammation not substantially less than controls in which ROS damage was marked. Analysis of retinal leukocytes by immunofluorescence microscopy and flow cytometry indicated that approximately equal numbers of CD4+ alpha beta TCR+ lymphocytes were present in treated and control retinae, more than 30% of CD4+ cells in both experimental groups expressed the CD25 or MRC OX40 activation markers and most cells, which would include the CD4+ T lymphocytes, were activated as evidenced by MHC class II expression. Fewer activated macrophages and granulocytes were present in the treated retinae, possibly reflecting the lower level of tissue damage and subsequent accumulation of these inflammatory cells. The results demonstrate directly that a tissue specifically targeted for autoimmune destruction can be protected despite the influx of fully activated CD4+ T cells.

Progressive activation of adult microglial cells in vitro

Bulk-isolated microglial cells from the adult rat brain grown in N2 medium supplemented with 10% fetal calf serum survived for at least 2 weeks, and their purity was >99% at day 1 and >93% at day 7. The phenotype of freshly plated cells was comparable to that of "resting," ramified microglia in vivo. With time in culture and with different schedules, depending on the parameter considered, microglia acquired antigenic (e.g., positivity for vimentin, ED1, major histocompatibility complex class I antigens, leukocyte common antigen, and to a lesser extent CD4) and functional (e.g., proliferation, phagocytosis) features characteristic of "activated" microglia as described in situ. Production of nitrite and prostaglandin E2 in response to lipopolysaccharide increased greatly with time in culture. Phagocytosis was also accompanied by increased release of nitrite and prostaglandin E2, the latter being more affected than the first by the age of the cultures. The culture system described may be suitable to study the factors that can modulate "activation" of adult microglia.

Immune surveillance and autoantigen recognition in the central nervous system

The central nervous system (CNS) is considered to be a severely disadvantaged site for the elicitation of immune responses. First, there is no specialised lymphatic drainage. Second, both glia and neuronal cells normally do not express appreciable levels of major histocompatibility complex molecules. Third, the vasculature of the CNS is, at least in most places, lined by endothelial cells that have tight junctions and form a barrier (the blood-brain barrier) against most molecules and cells present in the circulation. Fourth, the cells most important in the initiation of immune response, the leucocyte dendritic cell, are not present. Nevertheless, the existence of inflammatory diseases of this tissue, occurring naturally as in multiple sclerosis or in animals after peripheral immunisation with CNS autoantigens, indicates that the immune system can access and recognise antigens in this site. How this is achieved has become clearer in recent years and primarily seems to involve extravasation of activated but not resting T cells across the blood-brain barrier, and recognition of antigen on macrophage-like perivascular cells, rather than cells within the CNS parenchyme such as astrocytes or microglia. The processes involved in immunological patrolling of the CNS and development of autoimmune inflammatory disease are reviewed.

Unimpaired autoreactive T-cell traffic within the central nervous system during tumor necrosis factor receptor-mediated inhibition of experimental autoimmune encephalomyelitis

The critical role of tumor necrosis factor (TNF) as a mediator in autoimmune inflammatory processes is evident from in vivo studies with TNF-blocking agents. However, the mechanisms by which TNF, and possibly also its homologue lymphotoxin alpha, contributes to development of pathology in rheumatoid arthritis and Crohn disease and in animal models like experimental autoimmune encephalomyelitis is unclear. Possibilities include regulation of vascular adhesion molecules enabling leukocyte movement into tissues or direct cytokine-mediated effector functions such as mediation of tissue damage. Here we show that administration of a TNF receptor (55 kDa)-IgG fusion protein prevented clinical signs of actively induced experimental autoimmune encephalomyelitis. Significantly, the total number of CD4+ T lymphocytes isolated from the central nervous system of clinically healthy treated versus diseased control animals was comparable. By using a CD45 congenic model of passively transferred experimental autoimmune encephalomyelitis to enable tracking of myelin basic protein-specific effector T lymphocytes, prevention of clinical signs of disease was again demonstrated in treated animals but without quantitative or qualitative impediment to the movement of autoreactive T lymphocytes to and within the central nervous system. Thus, despite the uninterrupted movement of specific T lymphocytes into the target tissue, subsequent disease development was blocked. This provides compelling evidence for a direct effector role of TNF/lymphotoxin alpha in autoimmune tissue damage.

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.

Different subsets of T cells in conjunction with natural killer cells, macrophages, and activated microglia participate in the intracerebral immune response to Toxoplasma gondii in athymic nude and immunocompetent rats

Oral infection of athymic nude and immunocompetent Lewis rats with Toxoplasma gondii induced a chronic nonlethal encephalitis. The histopathological pattern of Toxoplasma encephalitis was significantly different in both groups of animals and there were substantially larger numbers of Toxoplasma cysts in the brains of athymic rats. Combined immunohistochemical and flow cytometric analyses of intracerebral leukocytes identified alpha beta TCR+ CD4+ and CD8+ T cells; macrophages, and natural killer cells as inflammatory cell populations in immunocompetent rats, whereas in athymic rats natural killer cells, macrophages, and gamma delta TCR+ CD8+ CD3+ T cells contributed to the intracerebral inflammatory infiltrates. These findings not only point to a major participation of alpha beta TCR+ T cells to the intracerebral immune response, but also indicate that they are not essential to prevent the development of a lethal Toxoplasma encephalitis. In addition, microglia were strongly activated in both strains with simultaneous up-regulation of major histocompatibility complex class I and II antigens and CD4. Activation of microglia was most prominent in athymic rats, demonstrating that immunodeficiency does not preclude an up-regulation of these molecules including the human immunodeficiency virus receptor CD4 on microglial cells.

Immunological aspects of experimental allergic encephalomyelitis and multiple sclerosis

Multiple sclerosis (MS) is the most frequent, demyelinating disease of the central nervous system (CNS) in Northern Europeans and North Americans. Despite intensive research its etiology is still unknown, but a T cell-mediated autoimmune pathogenesis is likely to be responsible for the demyelination. This hypothesis is based both on findings in MS patients and studies of an experimental animal model for demyelinating diseases, experimental allergic encephalomyelitis (EAE). Experiments in EAE have not only demonstrated which myelin antigens are able to induce the demyelinating process but also have determined the characteristics of encephalitogenic T cells, that is, their fine specificity, major histocompatibility complex (MHC) restriction, lymphokine secretion, activation requirements, and T cell receptor (TCR) usage. Based on these findings, highly specific and efficient immune interventions have been designed in EAE and have raised hopes that similar approaches could modulate the disease process in MS. Although the examination of the myelin-specific T cell response in MS patients has shown parallels to EAE, this remains an area of intensive research because a number of questions remain. This review summarizes the important lessons from EAE, examines recent findings in MS, and discusses current concepts about how the disease process develops and which steps might be taken to modulate it.

Coronavirus induction of class I major histocompatibility complex expression in murine astrocytes is virus strain specific

Neurotropic strains of mouse hepatitis viruses (MHV) such as MHV-A59 (A59) and MHV-4 (JHMV) cause acute and chronic encephalomyelitis and demyelination in susceptible strains of mice and rats. They are widely used as models of human demyelinating diseases such as multiple sclerosis (MS), in which immune mechanisms are thought to participate in the development of lesions in the central nervous system (CNS). The effects of MHV infection on target cell functions in the CNS are not well understood, but A59 has been shown to induce the expression of MHC class I molecules in glial cells after in vivo and in vitro infection. Changes in class I expression in infected cells may contribute to the immunopathogenesis of MHV infection in the CNS. In this communication, a large panel of MHV strains was tested for their ability to stimulate class I expression in primary astrocytes in vitro. The data show that the more hepatotropic strains, such as MHV-A59, MHV-1, MHV-2, MHV-3, MHV-D, MHV-K, and MHV-NuU, were potent inducers of class I expression in astrocytes during acute infection, measured by radioimmunoassay. The Kb molecule was preferentially expressed over Db. By contrast, JHMV and several viral strains derived from it did not stimulate the expression of class I molecules. Assays of virus infectivity indicated that the class I-inducing activity did not correlate with the ability of the individual viral strain to replicate in astrocytes. However, exposure of the viruses or the supernatants from infected astrocytes to ultraviolet light abolished the class I-inducing activity, indicating that infectious virus is required for class I expression. These data also suggest that class I expression was induced directly by virus infection, and not by the secretion of a soluble substance into the medium by infected astrocytes. Finally, analyses of A59/JHMV recombinant viral strains suggest that class I-inducing activity resides in one of the A59 structural genes.

Cervical lymphoid tissue but not the central nervous system supports proliferation of virus-specific T lymphocytes during coronavirus-induced encephalitis in rats

The CD4+ T lymphocyte response in the central nervous system (CNS) and cervical lymph nodes (CLNs) of rats with different susceptibility to coronavirus-induced encephalitis was investigated. The majority of CD4+ T lymphocytes entering the virus-infected CNS in the course of the infection are primed cells that neither proliferate ex vivo nor can be stimulated to proliferation by viral antigens or mitogen in vitro. In contrast, T lymphocytes taken from CLNs of the same animals revealed a strong proliferative response. Restimulation of CLN lymphocytes by viral antigens disclosed a striking difference between the disease-resistant rat strain Brown Norway (BN) and the susceptible Lewis (LEW) strain. Whereas BN lymphocytes responded as early as 5 days post infection, it took more than 11 days until a comparable proliferation was detectable in LEW lymphocytes. From these data we postulate that the majority of T lymphocytes entering the virus-infected brain after sensitisation and expansion in cervical lymph nodes is unresponsive to further proliferation signals and that the kinetics and magnitude of T lymphocyte stimulation in CLNs play an important role in the clinical course of the infection.

Microglial response to degeneration of serotonergic axon terminals

The neurotoxic drug p-chloramphetamine (PCA) causes widespread degeneration of fine, unmyelinated serotonergic (5-HT) axons in the forebrain. PCA toxicity is selective for 5-HT axon terminals; preterminal axons and cell bodies are spared. Degeneration is followed by slowly progressive axonal sprouting and partial reinnervation. PCA is injected subcutaneously; this route of administration avoids mechanical disruption of the blood brain barrier. The present study analyzed the response of microglia and astrocytes in rat brain to selective ablation of 5-HT axons by PCA. Several microglial markers were analyzed with immunocytochemical methods. An increase in the number of microglial processes and in immunoreactive staining was observed with antibodies directed against CR-3, MHC-I, CD4, and rat LCA. The microglial response was maximal 3 weeks after PCA treatment, became less evident 6 weeks after treatment, and by 9 weeks no difference was observed between treated and control rats. No change was detected in MHC-II or the macrophage marker ED1, nor in expression of GFAP by astrocytes. Thus, degeneration of 5-HT axon terminals affects only a subset of the microglial markers examined; in comparison, retrograde reaction to facial nerve transection causes a robust increase in all of these markers and in GFAP. The microglial response to PCA-induced axon loss is slow in onset and small in magnitude. These findings indicate that CNS microglia are activated by degeneration of fine, unmyelinated 5-HT axon terminals; furthermore, sensitive microglial markers can detect a subtle axonal lesion that provokes no detectable increase in GFAP expression by astrocytes.

Lymphocytes and macrophages outnumber oligodendroglia in normal fish spinal cord

As shown by staining with a monoclonal antibody against fish CD45, leukocytes are present in very large numbers in the fish central nervous system. Their subtypes were distinguished by electron microscopy and found to include all major hematogenous forms except thrombocytes, the most numerous being tissue macrophages and lymphocytes. As a population, they differ fundamentally from ramified microglia, the restricted form of myeloid cells present in the central nervous system in mammals. They are rare in most grey matter regions but are concentrated in myelinated fiber tracts as well as in certain strata of the radial glial network. The macrophages engulf discarded myelin and outnumber the oligodendrocytes in normal spinal cord white matter, where the density of lymphocytes is > 5000-fold greater than reported in rat.

Resident macrophages (ramified microglia) of the adult brown Norway rat central nervous system are constitutively major histocompatibility complex class II positive

A flow cytometric phenotype for isolated adult central nervous system (CNS) ramified microglia was previously defined (CD45low CD11b/c+) in the Lewis strain rat, that clearly distinguished these cells from all blood-derived leucocytes, the latter being CD45high. Consistent with the reported lack of major histocompatibility complex (MHC) expression in the CNS, isolated microglia were mostly MHC class II-. Employing these phenotypic criteria, we now show that a proportion of microglia in Brown Norway (BN) strain rats are constitutively MHC class II+. In spinal cord, up to 25% of microglia are distinctly positive and most have some level of expression. In situ staining of MHC class II+ microglial cells in BN rats indicates that positive cells are typical of ramified microglia on the grounds of both morphological appearance and anatomical location. In Lewis (LEW) rats, the few MHC class II-expressing cells isolated from the normal CNS are CD45high blood-derived cells and not resident microglia. After infection of both LEW and BN rats with a neurotropic murine hepatitis virus (MHV-JHM), MHC class II was rapidly upregulated on microglia in the BN but not in the LEW strain. In the latter, inflammatory cells were the predominant MHC class II-expressing population. Nevertheless, most microglia in the LEW strain could, after some delay, be induced to express MHC class II after transfer of an experimental autoimmune encephalomyelitis (EAE)-inducing encephalitogenic T cell line. Paradoxically, strains resistant to EAE (exemplified by the BN) contained more constitutive MHC class II-expressing microglia than susceptible ones, when a variety of strains were examined. The results clearly establish that the normal CNS may contain MHC class II-expressing cells that are a resident rather than a transient blood-derived population. It is significant that this expression is strain related, but there is no evidence that microglial cell constitutive MHC class II expression predisposes to EAE susceptibility.

Microglia in degenerative neurological disease

Microglia express many leukocyte surface antigens which are upregulated in such chronic degenerative neurological diseases as Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS). These surface antigens include leukocyte common antigen, immunoglobulin Fc receptors, MHC class I and class II glycoproteins, beta 2-integrins, and the vitronectin receptor. Ligands for these receptors are also found. They include immunoglobulins, complement proteins of the classical pathway, T lymphocytes of the cytotoxic/suppressor and helper/inducer classes, and vitronectin. T lymphocytes marginate along capillary venules, with some penetrating into the tissue matrix. Immunoglobulins and complement proteins are synthesized locally in brain, although they may also come from the bloodstream if the blood-brain barrier is compromised. The membrane attack complex, which is formed from C5b-9, the terminal components of complement, has been identified in AD and multiple sclerosis brain tissue. In addition, proteins designed to defend against bystander lysis caused by the membrane attack complex, including protectin, C8 binding protein, clusterin, and vitronectin, are associated with damaged neuronal processes in AD. Autodestruction may play a prominent part in these 2 diseases.

Microglia progenitor cells: a subpopulation in cultures of mouse neopallial astroglia

We have shown that astroglia cultures initiated from newborn mouse neopallium contain microglia progenitor cells. Astroglia secrete growth factor(s) that transform the progenitor cells into microglia. One of the trophic factors is bone marrow macrophage growth factor CSF-1. Mouse embryonic fibroblast cells STO also secrete trophic factor(s) that acts synergistically or additively with CSF-1 on microglia progenitor cells. Using limiting dilution analysis we estimated that in the presence of CSF-1 and STO cells, 1 in every 8 cells in astroglia cultures is a potential microglia progenitor cell.

Pathogenesis of virus-induced demyelination

Demyelination is a component of several viral diseases of humans. The best known of these are subacute sclerosing panencephalitis (SSPE) and progressive multifocal leukoencephalopathy (PML). There are a number of naturally occurring virus infections of animals that involve demyelination and many of these serve as instructive models for human demyelinating diseases. In addition to the naturally occurring diseases, many viruses have been shown to be capable of producing demyelination in experimental situations. In discussing virus-associated demyelinating disease, the chapter reviews the architecture and functional organization of the CNS and considers what is known of the interaction of viruses with CNS cells. It also discusses the immunology of the CNS that differs in several important aspects from that of the rest of the body. Experimental models of viral-induced demyelination have also been considered. Viruses capable of producing demyelinating disease have no common taxonomic features; they include both DNA and RNA viruses, enveloped and nonenveloped viruses. The chapter attempts to summarize the important factors influencing viral demyelination, their common features, and possible mechanisms.

Experimental autoimmune uveoretinitis: a model system for immunointervention: a review

Experimental autoimmune uveoretinitis (EAU) is a useful model of human posterior uveitis and as such, permits the analysis of strategies for immuno-intervention. Modulation of the autoimmune response may be attempted at the stages of induction of EAU, during homing of autoreactive lymphocytes to the target organ, the retina, or during the effector stage of the disease. This paper presents a brief overview of current immuno-therapeutic modalities and assesses the usefulness for extrapolation to human disease.

Class II major histocompatibility complex (Ia) antigen-bearing dendritic cells within the iris and ciliary body of the rat eye: distribution, phenotype and relation to retinal microglia

The density, distribution and surface phenotype of dendritic cells (DC) and macrophage populations within the ciliary body and iris of Wistar Furth rats were studied by a combination of flat mounting, tangential sectioning, pre-embedding fixation, with a single and double immunohistochemical techniques. Monoclonal antibodies included anti-Ia (OX6) and other dendritic cell/macrophage (ED1 and ED8) or mature tissue macrophage markers (ED2). Single and double staining revealed a network (approximately 400 cells/mm2) of Ia+ cells within the epithelium of the ciliary processes with the morphological and surface phenotypic characteristics of DC populations in other tissues. A minor proportion of DC co-expressed ED1 and ED8, but not ED2. In contrast the immunopositive cells in the lamina propria displayed a more generalized phenotype, including ED2 expression, and pleiomorphic morphology suggesting a preponderance of cells of macrophage lineage. Flat mounts of iris revealed a remarkably regular network of Ia+ DC at a density of 450 cells/mm2. The network of DC in the ciliary epithelium terminated at the cilioretinal junction where they formed a continuous syncytium with retinal microglia which did not display Ia staining. The demonstration of networks of cells with relevant morphological and phenotypical properties of professional antigen-presenting cells at strategic locations within the eye has important implications in relation to ocular immune regulation and on the theories of the mechanism of anterior chamber-associated immune deviation (ACAID). Namely, until now it has been assumed that 'immune privilege' in the anterior chamber of the eye is partly a consequence of there being a paucity of class II+ cells in the surrounding tissues. Dendritic cells in the eye may function as antigen-presenting cells, sampling endogenous and exogenous intraocular antigens and possibly migrating from the eye to draining lymphoid organs (the spleen) where they may generate systemic immune responses. Equally dendritic cells could potentially regulate local immune responses for example in various forms of autoimmune uveoretinal inflammatory disease.