Monica J. Carson

CNS immune privilege: hiding in plain sight.

Summary:  Central nervous system (CNS) immune privilege is an experimentally defined phenomenon. Tissues that are rapidly rejected by the immune system when grafted in sites, such as the skin, show prolonged survival when grafted into the CNS. Initially, CNS immune privilege was construed as CNS isolation from the immune system by the blood–brain barrier (BBB), the lack of draining lymphatics, and the apparent immunoincompetence of microglia, the resident CNS macrophage. CNS autoimmunity and neurodegeneration were presumed automatic consequences of immune cell encounter with CNS antigens. Recent data have dramatically altered this viewpoint by revealing that the CNS is neither isolated nor passive in its interactions with the immune system. Peripheral immune cells can cross the intact BBB, CNS neurons and glia actively regulate macrophage and lymphocyte responses, and microglia are immunocompetent but differ from other macrophage/dendritic cells in their ability to direct neuroprotective lymphocyte responses. This newer view of CNS immune privilege is opening the door for therapies designed to harness autoreactive lymphocyte responses and also implies (i) that CNS autoimmune diseases (i.e. multiple sclerosis) may result as much from neuronal and/or glial dysfunction as from immune system dysfunctions and (ii) that the severe neuronal and glial dysfunction associated with neurodegenerative disorders (i.e. Alzheimer’s disease) likely alters CNS‐specific regulation of lymphocyte responses affecting the utility of immune‐based therapies (i.e. vaccines).

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. GLIA 22:72–85, 1998.

Microglia as Liaisons Between the Immune and Central Nervous Systems: Functional Implications for Multiple Sclerosis

Multiple sclerosis is a chronic demyelinating inflammatory disease of the central nervous system (CNS). As the tissue macrophage of the CNS, microglia have the potential to regulate and be regulated by cells of the CNS and by CNS‐infiltrating immune cells. The exquisite sensitivity of microglia to these signals, coupled with their ability to develop a broad range of effector functions, allows the CNS to tailor microglial function for specific physiological needs. However, the great plasticity of microglial responses can also predispose these cells to amplify disproportionately the irrelevant or dysfunctional signals provided by either the CNS or immune systems. The consequences of such an event could be the conversion of self‐limiting inflammatory responses into chronic neurodegeneration and may explain in part the heterogeneous nature of multiple sclerosis. GLIA 40:218–231, 2002.

Heterogeneous expression of the triggering receptor expressed on myeloid cells-2 on adult murine microglia

Microglial activation is an early and common feature of almost all neuropathologies, including multiple sclerosis, Alzheimers disease and mechanical injury. To better understand the relative contributions microglia make toward neurodegeneration and neuroprotection, we used TOGA® to identify molecules expressed by microglia and regulated by inflammatory signals. Triggering receptor expressed on myeloid cells‐2 (TREM‐2) was among the mRNAs identified as being expressed by unactivated microglia, but down‐regulated by lipopolysaccharide/interferon γ. In the healthy CNS, not all microglia expressed TREM‐2. Microglial expression of TREM‐2 varied not only between brain regions but also within each brain region. Brain regions with an incomplete blood–brain barrier had the lowest percentages of TREM‐2‐ expressing microglia, whereas the lateral entorhinal and cingulate cortex had the highest percentages. A novel form of TREM‐2b that lacked a transmembrane domain was detected, perhaps indicating a soluble form of the protein. Taken together, these data suggest that (1) subsets of microglia are specialized to respond to defined extracellular signals; and (2) regional variations in TREM‐2 expression may contribute to the varying sensitivities of different brain regions to similar pathological signals.

No hypothermic response to serotonin in 5-HT7 receptor knockout mice

With data from recently available selective antagonists for the 5-HT7 receptor, it has been hypothesized that 5-hydroxytryptamine (5-HT)-induced hypothermia is mediated by the 5-HT7 receptor, an effect previously attributed to other receptor subtypes. It has been established that the biologically active lipid oleamide allosterically interacts with the 5-HT7 receptor to regulate its transmission. The most well characterized effects of oleamide administration are induction of sleep and hypothermia. Here, we demonstrate, by using mice lacking the 5-HT7 receptor, that 5-HT-induced hypothermia is mediated by the 5-HT7 receptor. Both 5-HT and 5-carboxamidotryptamine, a 5-HT1 and 5-HT7 receptor agonist, in physiological doses fail to induce hypothermia in 5-HT7 knockout mice. In contrast, oleamide was equally effective in inducing hypothermia in mice lacking the 5-HT7 receptors as in wild-type mice. When administered together, 5-HT and oleamide showed additive or greater than additive effects in reducing body temperature. Taken together, the results show that 5-HT-induced hypothermia is mediated by the 5-HT7 receptor, and that oleamide may act through an independent mechanism as well as at an allosteric 5-HT7 receptor site to regulate body temperature.

CD4-Positive T Cell-Mediated Neuroprotection Requires Dual Compartment Antigen Presentation

Our laboratory discovered that CD4-positive (CD4+) T cells of the immune system convey transitory neuroprotection to injured mouse facial motoneurons (FMNs) (Serpe et al., 1999, 2000, 2003). A fundamental question in the mechanisms responsible for neuroprotection concerns the identity of the cell(s) that serves as the antigen-presenting cell (APC) to activate the CD4+ T cells. Here, we first establish that CD4+ T cells reactive to non-CNS antigen fail to support FMN survival and, second, demonstrate a two-compartment model of CD4+ T cell activation. Mouse bone marrow (BM) chimeras were developed that discriminate between resident antigen-presenting host cell and BM-derived antigen-presenting donor cell expression of major histocompatibility complex II within central and peripheral compartments, respectively. After facial nerve transection, neither compartment alone is sufficient to result in activated CD4+ T cell-mediated FMN survival. Rather, CD4+ T cell-mediated neuroprotection appears to depend on both resident microglial cells in the central compartment and a BM-derived APC in the peripheral compartment. This is the first in vivo report demonstrating a neuroprotective mechanism requiring APC functions by resident (i.e., parenchymal) microglial cells.

The cellular response in neuroinflammation: The role of leukocytes, microglia and astrocytes in neuronal death and survival

Neuroinflammation is a complex integration of the responses of all cells present within the CNS, including the neurons, macroglia, microglia and the infiltrating leukocytes. The initiating insult, environmental factors, genetic background and age/past experiences all combine to modulate the integrated response of this complex neuroinflammatory circuit. Here, we explore how these factors interact to lead to either neuroprotective versus neurotoxic inflammatory responses. We specifically focus on microglia and astrocytic regulation of autoreactive T cell responses.

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. J. Neurosci. Res. 55:1–8, 1999.

A Rose by Any Other Name? The Potential Consequences of Microglial Heterogeneity During CNS Health and Disease

SummaryMicroglial activation and macrophage infiltration into the CNS are common features of CNS autoimmune disease and of chronic neurodegenerative diseases. Because these cells largely express an overlapping set of common macrophage markers, it has been difficult to separate their respective contributions to disease onset and progression. This problem is further confounded by the many types of macrophages that have been termed microglia. Several approaches, ranging from molecular profiling of isolated cells to the generation of irradiation chimeric rodent models, are now beginning to generate rudimentary definitions distinguishing the various types of microglia and macrophages found within the CNS and the potential roles that these cells may play in health and disease.

Dual induction of TREM2 and tolerance-related transcript, Tmem176b, in amyloid transgenic mice: implications for vaccine-based therapies for Alzheimer's disease.

Vaccine-based autoimmune (anti-amyloid) treatments are currently being examined for their therapeutic potential in Alzheimers disease. In the present study we examined, in a transgenic model of amyloid pathology, the expression of two molecules previously implicated in decreasing the severity of autoimmune responses: TREM2 (triggering receptor expressed on myeloid cells 2) and the intracellular tolerance-associated transcript, Tmem176b (transmembrane domain protein 176b). In situ hybridization analysis revealed that both molecules were highly expressed in plaque-associated microglia, but their expression defined two different zones of plaque-associated activation. Tmem176b expression was highest in the inner zone of amyloid plaques, whereas TREM2 expression was highest in the outer zone. Induced expression of TREM2 occurred co-incident with detection of thioflavine-S-positive amyloid deposits. Transfection studies revealed that expression of TREM2 correlated negatively with motility, but correlated positively with the ability of microglia to stimulate CD4+ T-cell proliferation, TNF (tumour necrosis factor) and CCL2 (chemokine ligand 2) production, but not IFNγ (interferon γ) production. TREM2 expression also showed a positive correlation with amyloid phagocytosis in unactivated cells. However, activating cells with LPS (lipopolysaccharide), but not IFNγ, reduced the correlation between TREM2 expression and phagocytosis. Transfection of Tmem176b into both microglial and macrophage cell lines increased apoptosis. Taken together, these data suggest that, in vivo, Tmem176b+ cells in closest apposition to amyloid may be the least able to clear amyloid. Conversely, the phagocytic TREM2+ microglia on the plaque outer zones are positioned to capture and present self-antigens to CNS (central nervous system)-infiltrating lymphocytes without promoting pro-inflammatory lymphocyte responses. Instead, plaque-associated TREM2+ microglia have the potential to evoke neuroprotective immune responses that may serve to support CNS function during pro-inflammatory anti-amyloid immune therapies.