Protection from EAE in DOCK8 mutant mice occurs despite increased Th17 cell frequencies in the periphery

Impaired Treg-DC interactions contribute to autoimmunity in leukocyte adhesion deficiency type 1

Leukocyte adhesion deficiency type 1 (LAD-1) is a rare disease resulting from mutations in the gene encoding for the common β-chain of the β2-integrin family (CD18). The most prominent clinical symptoms are profound leukocytosis and high susceptibility to infections. Patients with LAD-1 are prone to develop autoimmune diseases, but the molecular and cellular mechanisms that result in coexisting immunodeficiency and autoimmunity are still unresolved. CD4+FOXP3+ Treg are known for their essential role in preventing autoimmunity. To understand the role of Treg in LAD-1 development and manifestation of autoimmunity, we generated mice specifically lacking CD18 on Treg (CD18Foxp3), resulting in defective LFA-1 expression. Here, we demonstrate a crucial role of LFA-1 on Treg to maintain immune homeostasis by modifying T cell-DC interactions and CD4+ T cell activation. Treg-specific CD18 deletion did not impair Treg migration into extralymphatic organs, but it resulted in shorter interactions of Treg with DC. In vivo, CD18Foxp3 mice developed spontaneous hyperplasia in lymphatic organs and diffuse inflammation of the skin and in multiple internal organs. Thus, LFA-1 on Treg is required for the maintenance of immune homeostasis.

Neutrophil extracellular traps and their histones promote Th17 cell differentiation directly via TLR2

Neutrophils perform critical functions in the innate response to infection, including through the production of neutrophil extracellular traps (NETs) - web-like DNA structures which are extruded from neutrophils upon activation. Elevated levels of NETs have been linked to autoimmunity but this association is poorly understood. By contrast, IL-17 producing Th17 cells are a key player in various autoimmune diseases but are also crucial for immunity against fungal and bacterial infections. Here we show that NETs, through their protein component histones, directly activate T cells and specifically enhance Th17 cell differentiation. This modulatory role of neutrophils, NETs and their histones is mediated downstream of TLR2 in T cells, resulting in phosphorylation of STAT3. The innate stimulation of a specific adaptive immune cell subset provides an additional mechanism demonstrating a direct link between neutrophils, NETs and T cell autoimmunity.

Cutting Edge: DOCK8 Regulates a Subset of Dendritic Cells That Is Critical for the Development of Experimental Autoimmune Encephalomyelitis

Dedicator of cytokinesis 8 (DOCK8) is a guanine nucleotide exchange factor with an essential role in cytoskeletal rearrangement, cell migration, and survival of various immune cells. Interestingly, DOCK8-deficient mice are resistant to the development of experimental autoimmune encephalomyelitis (EAE). To understand if EAE resistance in these mice results from an alteration in dendritic cell (DC) functions, we generated mice with conditional deletion of DOCK8 in DCs and observed attenuated EAE in these mice compared with control mice. Additionally, we demonstrated that DOCK8 is important for the existence of splenic conventional DC2 and lymph node migratory DCs and further established that migratory DC, rather than resident DC, are essential for the generation and proliferation of pathogenic T cell populations upon immunization with myelin Ag in adjuvant. Therefore, our data suggest that limiting migratory DCs through DOCK8 deletion and possibly other mechanisms could limit the development of CNS autoimmunity.