Does deleting dendritic cells delete autoimmunity?

The Challenge of the Pathogenesis of Parkinson's Disease: Is Autoimmunity the Culprit?

The role of autoimmunity in Parkinson's disease (PD), as one of the most popular research subjects, has been intensively investigated in recent years. Although the ultimate cause of PD is unknown, one major area of interest remains identifying new therapeutic targets and options for patients suffering from PD. Herein, we present a comprehensive review of the impacts of autoimmunity in neurodegenerative diseases, especially PD, and we have composed a logical argument to substantiate that autoimmunity is actively involved in the pathogenesis of PD through several proteins, including α-synuclein, DJ-1, PINK1, and Parkin, as well as immune cells, such as dendritic cells, microglia, T cells, and B cells. Furthermore, a detailed analysis of the relevance of autoimmunity to the clinical symptoms of PD provides strong evidence for the close correlation of autoimmunity with PD. In addition, the previously identified relationships between other autoimmune diseases and PD help us to better understand the disease pattern, laying the foundation for new therapeutic solutions to PD. In summary, this review aims to integrate and present currently available data to clarify the pathogenesis of PD and discuss some controversial but innovative research perspectives on the involvement of autoimmunity in PD, as well as possible novel diagnostic methods and treatments based on autoimmunity targets.

LYN- and AIRE-mediated tolerance checkpoint defects synergize to trigger organ-specific autoimmunity

Studies of the genetic factors associated with human autoimmune disease suggest a multigenic origin of susceptibility; however, how these factors interact and through which tolerance pathways they operate generally remain to be defined. One key checkpoint occurs through the activity of the autoimmune regulator AIRE, which promotes central T cell tolerance. Recent reports have described a variety of dominant-negative AIRE mutations that likely contribute to human autoimmunity to a greater extent than previously thought. In families with these mutations, the penetrance of autoimmunity is incomplete, suggesting that other checkpoints play a role in preventing autoimmunity. Here, we tested whether a defect in LYN, an inhibitory protein tyrosine kinase that is implicated in systemic autoimmunity, could combine with an Aire mutation to provoke organ-specific autoimmunity. Indeed, mice with a dominant-negative allele of Aire and deficiency in LYN spontaneously developed organ-specific autoimmunity in the eye. We further determined that a small pool of retinal protein-specific T cells escaped thymic deletion as a result of the hypomorphic Aire function and that these cells also escaped peripheral tolerance in the presence of LYN-deficient dendritic cells, leading to highly destructive autoimmune attack. These findings demonstrate how 2 distinct tolerance pathways can synergize to unleash autoimmunity and have implications for the genetic susceptibility of autoimmune disease.

RNA sensing by conventional dendritic cells is central to the development of lupus nephritis

Glomerulonephritis is a common and debilitating feature of systemic lupus erythematosus (SLE). The precise immune mechanisms that drive the progression from benign autoimmunity to glomerulonephritis are largely unknown. Previous investigations have shown that a moderate increase of the innate Toll-like receptor 7 (TLR7) is sufficient for the development of nephritis. In these systems normalization of B-cell TLR7 expression or temporal depletion of plasmacytoid dendritic cells (pDCs) slow progression; however, the critical cell that is responsible for driving full immunopathology remains unidentified. In this investigation we have shown that conventional DC expression of TLR7 is essential for severe autoimmunity in the Sle1Tg7 model of SLE. We show that a novel expanding CD11b(+) conventional DC subpopulation dominates the infiltrating renal inflammatory milieu, localizing to the glomeruli. Moreover, exposure of human myeloid DCs to IFN-α or Flu increases TLR7 expression, suggesting they may have a role in self-RNA recognition pathways in clinical disease. To our knowledge, this study is the first to highlight the importance of conventional DC-TLR7 expression for kidney pathogenesis in a murine model of SLE.

Hyperactivated MyD88 signaling in dendritic cells, through specific deletion of Lyn kinase, causes severe autoimmunity and inflammation

Deletion of lyn, a Src-family tyrosine kinase expressed by B, myeloid, and dendritic cells (DCs), triggers lupus-like disease in mice, characterized by autoantibody production and renal immune complex deposition leading to chronic glomerulonephritis. B cells from these mice are hyperactive to antigen-receptor stimulation owing to a loss of inhibitory signaling mediated by Lyn kinase. The hyperactive B-cell responses are thought to underlie the development of autoimmunity in this model. Lyn-deficient mice also manifest significant myeloexpansion. To test the contribution of different immune cell types to the lupus-like disease in this model, we generated a lyn(flox/flox) transgenic mouse strain. To our surprise, when we crossed these mice to Cd11c-cre animals, generating DC-specific deletion of Lyn, the animals developed spontaneous B- and T-cell activation and subsequent production of autoantibodies and severe nephritis. Remarkably, the DC-specific Lyn-deficient mice also developed severe tissue inflammatory disease, which was not present in the global lyn(-/-) strain. Lyn-deficient DCs were hyperactivated and hyperresponsive to Toll-like receptor agonists and IL-1β. To test whether dysregulation of these signaling pathways in DCs contributed to the inflammatory/autoimmune phenotype, we crossed the lyn(f/f) Cd11c-cre(+) mice to myd88(f/f) animals, generating double-mutant mice lacking both Lyn and the adaptor protein myeloid differentiation factor 88 (MyD88) in DCs, specifically. Deletion of MyD88 in DCs alone completely reversed the inflammatory autoimmunity in the DC-specific Lyn-mutant mice. Thus, we demonstrate that hyperactivation of MyD88-dependent signaling in DCs is sufficient to drive pathogenesis of lupus-like disease, illuminating the fact that dysregulation in innate immune cells alone can lead to autoimmunity.

Autoimmunity, dendritic cells and relevance for Parkinson's disease

Innate and adaptive immune responses in neurodegenerative diseases have become recently a focus of research and discussions. Parkinson’s disease (PD) is a neurodegenerative disorder without known etiopathogenesis. The past decade has generated evidence for an involvement of the immune system in PD pathogenesis. Both inflammatory and autoimmune mechanisms have been recognized and studies have emphasized the role of activated microglia and T-cell infiltration. In this short review, we focus on dendritic cells, on their role in initiation of autoimmune responses, we discuss aspects of neuroinflammation and autoimmunity in PD, and we report new evidence for the involvement of neuromelanin in these processes.

Mechanisms of necroptosis in T cells

In caspase 8-deficient mouse T cells, necroptosis occurs via a Ripk3- and Ripk1-dependent pathway independent of autophagy and programmed necrosis.

Cell populations are regulated in size by at least two forms of apoptosis. More recently, necroptosis, a parallel, nonapoptotic pathway of cell death, has been described, and this pathway is invoked in the absence of caspase 8. In caspase 8–deficient T cells, necroptosis occurs as the result of antigen receptor–mediated activation. Here, through a genetic analysis, we show that necroptosis in caspase 8–deficient T cells is related neither to the programmed necrosis as defined by the requirement for mitochondrial cyclophilin D nor to autophagy as defined by the requirement for autophagy-related protein 7. Rather, survival of caspase 8–defective T cells can be completely rescued by loss of receptor-interacting serine-threonine kinase (Ripk) 3. Additionally, complementation of a T cell–specific caspase 8 deficiency with a loss of Ripk3 gives rise to lymphoproliferative disease reminiscent of lpr or gld mice. In conjunction with previous work, we conclude that necroptosis in antigen-stimulated caspase 8–deficient T cells is the result of a novel Ripk1- and Ripk3-mediated pathway of cell death.