The innate immune system is strongly implicated in the pathogenesis of Alzheimer's disease (AD). In contrast, the role of adaptive immunity in AD remains largely unknown. However, numerous clinical trials are testing vaccination strategies for AD, suggesting that T and B cells play a pivotal role in this disease. To test the hypothesis that adaptive immunity influences AD pathogenesis, we generated an immune-deficient AD mouse model that lacks T, B, and natural killer (NK) cells. The resulting "Rag-5xfAD" mice exhibit a greater than twofold increase in β-amyloid (Aβ) pathology. Gene expression analysis of the brain implicates altered innate and adaptive immune pathways, including changes in cytokine/chemokine signaling and decreased Ig-mediated processes. Neuroinflammation is also greatly exacerbated in Rag-5xfAD mice as indicated by a shift in microglial phenotype, increased cytokine production, and reduced phagocytic capacity. In contrast, immune-intact 5xfAD mice exhibit elevated levels of nonamyloid reactive IgGs in association with microglia, and treatment of Rag-5xfAD mice or microglial cells with preimmune IgG enhances Aβ clearance. Last, we performed bone marrow transplantation studies in Rag-5xfAD mice, revealing that replacement of these missing adaptive immune populations can dramatically reduce AD pathology. Taken together, these data strongly suggest that adaptive immune cell populations play an important role in restraining AD pathology. In contrast, depletion of B cells and their appropriate activation by T cells leads to a loss of adaptiveinnate immunity cross talk and accelerated disease progression.is the leading cause of age-related neurodegeneration, affecting over 5.2 million people in the United States alone (1). Pathologically, AD is characterized by two hallmark protein aggregates, amyloid-β (Aβ) plaques and neurofibrillary tangles, that are accompanied by neuroinflammation, including microgliosis, elevated cytokine production, and activation of complement pathways (2-5). Initially, microglia respond to and surround plaques, degrading Aβ by phagocytosis (for review, see refs. 6-8). However, chronic activation of these cells shift microglia to a more proinflammatory and less phagocytic state (9, 10). Although much of the data implicating microglia in AD has come from neuropathological investigation, recent genome-wide association studies have provided the first genetic evidence (to our knowledge) linking microglia dysfunction to AD, with the discovery of risk polymorphisms in several immune system genes: CR1, TREM2, CD33, HLA-DRB5, MS4A6A, and ABCA7 (8,(11)(12)(13)(14)(15).In contrast to the field's increasing understanding of the role of innate immunity in AD, comparatively little is known about whether the adaptive immune system might also influence AD. Those studies that have examined these peripheral populations have largely focused on questions about their potential as biomarkers or their role in active Aβ immunization (3, 16). However, the adaptive and innate immune systems rarely fu...
Multiple sclerosis is a disease that is characterized by inflammation, demyelination, and axonal damage; it ultimately forms gliotic scars and lesions that severely compromise the function of the central nervous system. Evidence has shown previously that altered growth factor receptor signaling contributes to lesion formation, impedes recovery, and plays a role in disease progression. Growth arrest-specific protein 6 (Gas6), the ligand for the TAM receptor tyrosine kinase family, consisting of Tyro3, Axl, and Mer, is important for cell growth, survival, and clearance of debris. In this study, we show that levels of membrane-bound Mer (205 kd), soluble Mer (ϳ150 kd), and soluble Axl (80 kd) were all significantly elevated in homogenates from established multiple sclerosis lesions comprised of both chronic active and chronic silent lesions. Whereas in normal tissue Gas6 positively correlated with soluble Axl and Mer, there was a negative correlation between Gas6 and soluble Axl and Mer in established multiple sclerosis lesions. In addition, increased levels of soluble Axl and Mer were associated with increased levels of mature ADAM17, mature ADAM10, and Furin, proteins that are associated with Axl and Mer solubilization. Soluble Axl and Mer are both known to act as decoy receptors and block Gas6 binding to membrane-bound receptors. These data suggest that in multiple sclerosis lesions, dysregulation of protective Gas6 receptor signaling may prolong lesion activity.
BackgroundAxl, together with Tyro3 and Mer, constitute the TAM family of receptor tyrosine kinases. In the nervous system, Axl and its ligand Growth-arrest-specific protein 6 (Gas6) are expressed on multiple cell types. Axl functions in dampening the immune response, regulating cytokine secretion, clearing apoptotic cells and debris, and maintaining cell survival. Axl is upregulated in various disease states, such as in the cuprizone toxicity-induced model of demyelination and in multiple sclerosis (MS) lesions, suggesting that it plays a role in disease pathogenesis. To test for this, we studied the susceptibility of Axl-/- mice to experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis.MethodsWT and Axl-/- mice were immunized with myelin oligodendrocyte glycoprotein (MOG)35-55 peptide emulsified in complete Freund's adjuvant and injected with pertussis toxin on day 0 and day 2. Mice were monitored daily for clinical signs of disease and analyzed for pathology during the acute phase of disease. Immunological responses were monitored by flow cytometry, cytokine analysis and proliferation assays.ResultsAxl-/- mice had a significantly more severe acute phase of EAE than WT mice. Axl-/- mice had more spinal cord lesions with larger inflammatory cuffs, more demyelination, and more axonal damage than WT mice during EAE. Strikingly, lesions in Axl-/- mice had more intense Oil-Red-O staining indicative of inefficient clearance of myelin debris. Fewer activated microglia/macrophages (Iba1+) were found in and/or surrounding lesions in Axl-/- mice relative to WT mice. In contrast, no significant differences were noted in immune cell responses between naïve and sensitized animals.ConclusionsThese data show that Axl alleviates EAE disease progression and suggests that in EAE Axl functions in the recruitment of microglia/macrophages and in the clearance of debris following demyelination. In addition, these data provide further support that administration of the Axl ligand Gas6 could be therapeutic for immune-mediated demyelinating diseases.
A signaling role for T cell leukemia-1 (TCL1) during T cell development or in premalignant T cell expansions and mature T cell tumors is unknown. In this study, TCL1 is shown to regulate the growth and survival of peripheral T cells but not precursor thymocytes. Proliferation is increased by TCL1-induced lowering of the TCR threshold for CD4+ and CD8+ T cell activation through both PI3K-Akt and protein kinase C-MAPK-ERK signaling pathways. This effect is submaximal as CD28 costimulation coupled to TCL1 expression additively accelerates dose-dependent T cell growth. In addition to its role in T cell proliferation, TCL1 also increases IFN-γ levels from Th1-differentiated T cells, an effect that may provide a survival advantage during premalignant T cell expansions and in clonal T cell tumors. Combined, these data indicate a role for TCL1 control of growth and effector T cell functions, paralleling features provided by TCR-CD28 costimulation. These results also provide a more detailed mechanism for TCL1-augmented signaling and help explain the delayed occurrence of mature T cell expansions and leukemias despite tumorigenic TCL1 dysregulation that begins in early thymocytes.
AKT3, a member of the serine/threonine kinase AKT family, is involved in a variety of biological processes. AKT3 is expressed in immune cells, and is the major AKT isoform in the CNS representing 30% of the total AKT expressed in spinal cord, and 50% in the brain. Myelin-oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE) is a mouse model in which lymphocytes and monocytes enter the CNS, resulting in inflammation, demyelination, and axonal injury. We hypothesized that during EAE, deletion of AKT3 would negatively impact the CNS of AKT3-/- mice making them more susceptible to CNS damage. During acute EAE, AKT3-/-mice were more severely affected than wildtype (WT) mice. Evaluation of spinal cords showed that during acute and chronic disease, AKT3-/- spinal cords had more demyelination compared to WT spinal cords. Quantitative RT-PCR determined higher levels of IL-2, IL-17, and IFNγ mRNA in spinal cords from AKT3-/- mice than WT. Experiments using bone marrow chimeras demonstrated that AKT3-/- mice receiving AKT3-deficient bone marrow cells had elevated clinical scores relative to control WT mice reconstituted with WT cells, indicating that altered function of both CNS cells and bone marrow-derived immune cells contributed to the phenotype. Immunohistochemical analysis revealed decreased numbers of FoxP3+ Tregs in the spinal cord of AKT3-/- mice compared to WT mice, whereas in vitro suppression assays showed that AKT3-deficient T-helper cells were less susceptible to Treg-mediated suppression than their WT counterparts. These results indicate that AKT3 signaling contributes to the protection of mice against EAE.
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