Phenotypes and alloantigen-presenting activity of individual clones of microglia derived from the mouse brain

Microglia exhibit clonal variability in eliciting cytotoxic T lymphocyte responses independent of class I expression

Microglia are important immunoregulatory cells within the central nervous system (CNS). Viral infection of primary microglia and splenic macrophage clones revealed that both exhibited a heterogeneous, but relatively low, sensitivity to cytolysis mediated by CD8(+) cytotoxic T lymphocytes (CTL). The majority of clones were poor in processing and presenting epitopes, despite triggering lysis when coated with peptide. These characteristics were retained by stable microglia lines. Reduced lysis did not correlate with class I expression and IFN-gamma treatment only partially enhanced recognition. In contrast, targeting the epitope into the endoplasmic reticulum restored cytolysis to levels achieved with exogenous peptide. An inherent resistance to cytolysis was revealed by efficient engagement of T cells in competition assays and the inability of saturating peptide to enhance cytolysis. These data suggest that microglia heterogeneity in antigen processing, in addition to low sensitivity to CTL lysis, contributes to limited CD8(+) T cell responses and viral CNS persistence.

Microglia derive from progenitors, originating from the yolk sac, and which proliferate in the brain

Microglia, the resident CNS macrophages, represent about 10% of the adult brain cell population. Although described a long time ago, their origin and developmental lineage is still debated. While del Rio-Hortega suggested that microglia originate from meningeal macrophages penetrating the brain during embryonic development, many authors claim that brain parenchymal microglia derive from circulating blood monocytes originating from bone marrow. We have previously reported that the late embryonic and adult mouse brain parenchyma contains potential microglial progenitors [F. Alliot, E. Lecain, B. Grima, B. Pessac, Microglial progenitors with a high proliferative capacity in the embryonic and the adult mouse brain, Proc. Natl. Acad. Sci. U.S.A. 88 (1991) 1541-1545]. We now report that they can be detected in the brain rudiment from embryonic day 8, after their appearance in the yolk sac and that their number increases until late gestation. We also show that microglia appear during embryonic development and that their number increases steadily during the first two postnatal weeks, when about 95% of microglia are born. Finally, the main finding of this study is that microglia is the result of in situ proliferation, as shown by the high proportion of parenchymal microglial cells that express PCNA, a marker of cell multiplication, in embryonic and postnatal brain. Taken together, our data support the hypothesis that terminally differentiated brain parenchymal microglia are derived from cells originating from the yolk sac whose progeny actively proliferates in situ during development.

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.

Correction of murine galactosialidosis by bone marrow-derived macrophages overexpressing human protective protein/cathepsin A under control of the colony-stimulating factor-1 receptor promoter

Galactosialidosis (GS) is a human neurodegenerative disease caused by a deficiency of lysosomal protective protein/cathepsin A (PPCA). The GS mouse model resembles the severe human condition, resulting in nephropathy, ataxia, and premature death. To rescue the disease phenotype, GS mice were transplanted with bone marrow from transgenic mice overexpressing human PPCA specifically in monocytes/macrophages under the control of the colony stimulating factor-1 receptor promoter. Transgenic macrophages infiltrated and resided in all organs and expressed PPCA at high levels. Correction occurred in hematopoietic tissues and nonhematopoietic organs, including the central nervous system. PPCA-expressing perivascular and leptomeningeal macrophages were detected throughout the brain of recipient mice, although some neuronal cells, such as Purkinje cells, continued to show storage and died. GS mice crossed into the transgenic background reflected the outcome of bone marrow-transplanted mice, but the course of neuronal degeneration was delayed in this model. These studies present definite evidence that macrophages alone can provide a source of corrective enzyme for visceral organs and may be beneficial for neuronal correction if expression levels are sufficient.

Reciprocal stimulation between TNF-alpha and nitric oxide may exacerbate CNS inflammation in experimental autoimmune encephalomyelitis

Nitric oxide (NO) and TNF-alpha are both highly active pleotypic modulators of cell function that are abundantly generated during inflammation. Experiments in animal systems have linked the generation of NO and TNF-alpha to autoimmune pathogenesis, and blockade of either NO or TNF-alpha has been shown to impede disease development. In this study, we show that NO and TNF-alpha can act mutually to stimulate each other's production. While IFN-gamma alone induces NO release from microglia, astrocytes are provoked into significant NO production only by a combination of IFN-gamma and TNF-alpha. Since both TNF-alpha and NO are abundantly generated during T-glial cell interaction, we asked whether and how NO affects TNF-alpha production. Using an in vitro system in which TNF-alpha secretion is induced in MBP-reactive T cells by co-culture with syngeneic astrocytes, we observed that the efficiency of TNF-alpha secretion was markedly increased, in a dose-dependent fashion, by addition of micromolar concentrations of a chemical generator of NO donor, sodium nitroprusside (SNP). Similarly, low concentrations of SNP significantly enhanced the IL-2 dependent growth of MBP-reactive T cells. These results suggest that autoimmune pathogenesis initiated by inflammatory responses within the CNS may result in part from a vicious cycle in which TNF-alpha and NO mutually provoke each other's production.

Alloantigen presentation to naive CD8+ T cells by mouse microglia: evidence for a distinct phenotype based on expression of surface-associated and soluble costimulatory molecules

We previously showed that approximately one-third of mouse primary microglial clones derived from individual precursor cells residing in normal brain constitutively present alloantigens (alloAgs) to naive CD8+ T cells (Moore et al.: J Neuroimmunol 41:203, 1992). To understand the basis for this alloAg presenting (alloAgP) activity, we developed a panel of microglial cell lines that were characterized by patterns of alloAgP activity similar to that of the primary clones. Flow cytometric analysis revealed that microglia with and without alloAgP activity expressed similar levels of major histocompatibility complex class I molecules; however, CD80 (B7-1) and CD86 (B7-2) expression was primarily restricted to the alloAgP- cell lines. Monoclonal antibody (Mab) to CD80 only partially blocked the proliferative response of allogeneic CD8+ T cells cocultured with the presenting cell lines, whereas Mab to CD86 completely inhibited the response, indicating a significant role for this molecule in T-cell activation. Using an immunoassay, recombinant mouse cytokines, cytokine-specific Mabs, and the reverse transcriptase-polymerase chain reaction to detect specific cytokine mRNAs, we found the synthesis of interleukin (IL)-1 alpha, IL-6, IL-12, and tumor necrosis factor-alpha (TNF-alpha) to be restricted to the alloAgP- cell lines. Costimulatory roles were then identified for these molecules. We conclude that the ability to present alloAg is a property of a subset of microglia that constitutively express CD86 and secrete costimulatory cytokines that promote the expansion of the alloAg-stimulated CD8+ T cells.

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.