Frank L. Heppner

Dendritic cells permit immune invasion of the CNS in an animal model of multiple sclerosis

Immunization with myelin antigens leads to the development of experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis. The disease can also be induced by the transfer of encephalitogenic CD4+ T helper (TH) lymphocytes into naive mice. These T cells need to re-encounter their cognate antigen in the context of major histocompatibility complex (MHC) class II–bearing antigen-presenting cells (APCs) in order to recognize their target. The cell type and location of the APC mediating T-cell entry into the central nervous system (CNS) remain unknown. Here, we show that APCs of the lymphoreticular system and of the CNS parenchyma are dispensable for the immune invasion of the CNS. We also describe that a discrete population of vessel-associated dendritic cells (DCs) is present in human brain tissue. In mice, CD11c+ DCs alone are sufficient to present antigen in vivo to primed myelin-reactive T cells in order to mediate CNS inflammation and clinical disease development.

Experimental autoimmune encephalomyelitis repressed by microglial paralysis

Although microglial activation occurs in inflammatory, degenerative and neoplastic central nervous system (CNS) disorders, its role in pathogenesis is unclear. We studied this question by generating CD11b-HSVTK transgenic mice, which express herpes simplex thymidine kinase in macrophages and microglia. Ganciclovir treatment of organotypic brain slice cultures derived from CD11b-HSVTK mice abolished microglial release of nitrite, proinflammatory cytokines and chemokines. Systemic ganciclovir administration to CD11b-HSVTK mice elicited hematopoietic toxicity, which was prevented by transfer of wild-type bone marrow. In bone marrow chimeras, ganciclovir blocked microglial activation in the facial nucleus upon axotomy and repressed the development of experimental autoimmune encephalomyelitis. We conclude that microglial paralysis inhibits the development and maintenance of inflammatory CNS lesions. The microglial compartment thus provides a potential therapeutic target in inflammatory CNS disorders. These results validate CD11b-HSVTK mice as a tool to study the impact of microglial activation on CNS diseases in vivo.

A Cre-inducible diphtheria toxin receptor mediates cell lineage ablation after toxin administration.

A new system for lineage ablation is based on transgenic expression of a diphtheria toxin receptor (DTR) in mouse cells and application of diphtheria toxin (DT). To streamline this approach, we generated Cre-inducible DTR transgenic mice (iDTR) in which Cre-mediated excision of a STOP cassette renders cells sensitive to DT. We tested the iDTR strain by crossing to the T cell– and B cell–specific CD4-Cre and CD19-Cre strains, respectively, and observed efficient ablation of T and B cells after exposure to DT. In MOGi-Cre/iDTR double transgenic mice expressing Cre recombinase in oligodendrocytes, we observed myelin loss after intraperitoneal DT injections. Thus, DT crosses the blood-brain barrier and promotes cell ablation in the central nervous system. Notably, we show that the developing DT-specific antibody response is weak and not neutralizing, and thus does not impede the efficacy of DT. Our results validate the use of iDTR mice as a tool for cell ablation in vivo.

IL-17A and IL-17F do not contribute vitally to autoimmune neuro-inflammation in mice

The clear association of Th17 cells with autoimmune pathogenicity implicates Th17 cytokines as critical mediators of chronic autoimmune diseases such as EAE. To study the impact of IL-17A on CNS inflammation, we generated transgenic mice in which high levels of expression of IL-17A could be initiated after Cre-mediated recombination. Although ubiquitous overexpression of IL-17A led to skin inflammation and granulocytosis, T cell-specific IL-17A overexpression did not have a perceptible impact on the development and health of the mice. In the context of EAE, neither the T cell-driven overexpression of IL-17A nor its complete loss had a major impact on the development of clinical disease. Since IL-17F may be able to compensate for the loss of IL-17A, we also generated IL-17F-deficient mice. This strain was fully susceptible to EAE and displayed unaltered emergence and expansion of autoreactive T cells during disease. To eliminate potential compensatory effects of either cytokine, we treated IL-17F-deficient mice with antagonistic monoclonal antibodies specific for IL-17A and found again only a minimal beneficial impact on disease development. We conclude therefore that both IL-17A and IL-17F, while prominently expressed by an encephalitogenic T cell population, may only marginally contribute to the development of autoimmune CNS disease.

An unconventional role for miRNA: let-7 activates Toll-like receptor 7 and causes neurodegeneration

Activation of innate immune receptors by host-derived factors exacerbates CNS damage, but the identity of these factors remains elusive. We uncovered an unconventional role for the microRNA let-7, a highly abundant regulator of gene expression in the CNS, in which extracellular let-7 activates the RNA-sensing Toll-like receptor (TLR) 7 and induces neurodegeneration through neuronal TLR7. Cerebrospinal fluid (CSF) from individuals with Alzheimers disease contains increased amounts of let-7b, and extracellular introduction of let-7b into the CSF of wild-type mice by intrathecal injection resulted in neurodegeneration. Mice lacking TLR7 were resistant to this neurodegenerative effect, but this susceptibility to let-7 was restored in neurons transfected with TLR7 by intrauterine electroporation of Tlr7−/− fetuses. Our results suggest that microRNAs can function as signaling molecules and identify TLR7 as an essential element in a pathway that contributes to the spread of CNS damage.

Formation and maintenance of Alzheimer's disease beta-amyloid plaques in the absence of microglia

In Alzheimers disease, microglia cluster around β-amyloid deposits, suggesting that these cells are important for amyloid plaque formation, maintenance and/or clearance. We crossed two distinct APP transgenic mouse strains with CD11b-HSVTK mice, in which nearly complete ablation of microglia was achieved for up to 4 weeks after ganciclovir application. Neither amyloid plaque formation and maintenance nor amyloid-associated neuritic dystrophy depended on the presence of microglia.

Rorγt+ innate lymphocytes and γδ T cells initiate psoriasiform plaque formation in mice

Psoriasis is a common, relapsing inflammatory skin disease characterized by erythematous scaly plaques. Histological manifestations of psoriasis include keratinocyte dysregulation and hyperproliferation, elongated rete ridges, and inflammatory infiltrates consisting of T cells, macrophages, dendritic cells, and neutrophils. Despite the availability of new effective drugs to treat psoriasis, the underlying mechanisms of pathogenesis are still poorly understood. Recent studies have shown that Aldara cream, used to treat benign skin abnormalities, triggers psoriasis-like disease in humans and mice and have implicated Th17 cells in disease initiation. Using this as a model, we found a predominant role for the Th17 signature cytokines IL-17A, IL-17F, and IL-22 in psoriasiform plaque formation in mice. Using gene-targeted mice, we observed that loss of Il17a, Il17f, or Il22 strongly reduced disease the severity of psoriasis. However, we found that Th17 cells were not the primary source of these pathogenic cytokines. Rather, IL-17A, IL-17F, and IL-22 were produced by a skin-invading population of γδ T cells and RORγt(+) innate lymphocytes. Furthermore, our findings establish that RORγt(+) innate lymphocytes and γδ T cells are necessary and sufficient for psoriatic plaque formation in an experimental disease model that closely resembles human psoriatic plaque formation.

IL-22 is expressed by TH17 cells in an IL-23-dependent fashion, but not required for the development of autoimmune encephalomyelitis

Lately, IL-17-secreting Th cells have received an overwhelming amount of attention and are now widely held to be the major pathogenic population in autoimmune diseases. In particular, IL-22-secreting Th17 cells were shown to specifically mark the highly pathogenic population of self-reactive T cells in experimental autoimmune encephalomyelitis (EAE). As IL-17A itself was found to only play a minor role during the development of EAE, IL-22 is now postulated to contribute to the pathogenic function of Th17 cells. The goal of this study was to determine the role and function of IL-22 during the development of CNS autoimmunity in vivo. We found that CNS-invading encephalitogenic Th17 cells coexpress IL-22 and that IL-22 is specifically induced by IL-23 in autoimmune-pathogenic CD4+ T cells in a time- and dose-dependent manner. We next generated IL-22−/− mice, which—in contrast to the prediction that expression of inflammatory cytokines by CNS-invading T cells inevitably confers pathogenic function—turned out to be fully susceptible to EAE. Taken together, we show that self-reactive Th cells coexpress IL-17 and IL-22, but that the latter also does not appear to be directly involved in autoimmune pathogenesis of the CNS.

Inhibition of IL-12/IL-23 signaling reduces Alzheimer's disease-like pathology and cognitive decline

The pathology of Alzheimers disease has an inflammatory component that is characterized by upregulation of proinflammatory cytokines, particularly in response to amyloid-β (Aβ). Using the APPPS1 Alzheimers disease mouse model, we found increased production of the common interleukin-12 (IL-12) and IL-23 subunit p40 by microglia. Genetic ablation of the IL-12/IL-23 signaling molecules p40, p35 or p19, in which deficiency of p40 or its receptor complex had the strongest effect, resulted in decreased cerebral amyloid load. Although deletion of IL-12/IL-23 signaling from the radiation-resistant glial compartment of the brain was most efficient in mitigating cerebral amyloidosis, peripheral administration of a neutralizing p40-specific antibody likewise resulted in a reduction of cerebral amyloid load in APPPS1 mice. Furthermore, intracerebroventricular delivery of antibodies to p40 significantly reduced the concentration of soluble Aβ species and reversed cognitive deficits in aged APPPS1 mice. The concentration of p40 was also increased in the cerebrospinal fluid of subjects with Alzheimers disease, which suggests that inhibition of the IL-12/IL-23 pathway may attenuate Alzheimers disease pathology and cognitive deficits.

Sympathetic Innervation of Lymphoreticular Organs Is Rate Limiting for Prion Neuroinvasion

Transmissible spongiform encephalopathies are commonly propagated by extracerebral inoculation of the infectious agent. Indirect evidence suggests that entry into the central nervous system occurs via the peripheral nervous system. Here we have investigated the role of the sympathetic nervous system in prion neuroinvasion. Following intraperitoneal prion inoculation, chemical or immunological sympathectomy delayed or prevented scrapie. Prion titers in spinal cords were drastically reduced at early time points after inoculation. Instead, keratin 14-NGF transgenic mice, whose lymphoid organs are hyperinnervated by sympathetic nerves, showed reduction in scrapie incubation time and, unexpectedly, much higher titers of prion infectivity in spleens. We conclude that sympathetic innervation of lymphoid organs is rate limiting for prion neuroinvasion and that splenic sympathetic nerves may act as extracerebral prion reservoirs.