Dysregulated IFN-γ signals promote autoimmunity in STAT1 gain-of-function syndrome.

Heterozygous signal transducer and activator of transcription 1 (STAT1) gain-of-function (GOF) mutations promote a clinical syndrome of immune dysregulation characterized by recurrent infections and predisposition to humoral autoimmunity. To gain insights into immune characteristics of STAT1-driven inflammation, we performed deep immunophenotyping of pediatric patients with STAT1 GOF syndrome and age-matched controls. Affected individuals exhibited dysregulated CD4+ T cell and B cell activation, including expansion of TH1-skewed CXCR3+ populations that correlated with serum autoantibody titers. To dissect underlying immune mechanisms, we generated Stat1 GOF transgenic mice (Stat1GOF mice) and confirmed the development of spontaneous humoral autoimmunity that recapitulated the human phenotype. Despite clinical resemblance to human regulatory T cell (Treg) deficiency, Stat1GOF mice and humans with STAT1 GOF syndrome exhibited normal Treg development and function. In contrast, STAT1 GOF autoimmunity was characterized by adaptive immune activation driven by dysregulated STAT1-dependent signals downstream of the type 1 and type 2 interferon (IFN) receptors. However, in contrast to the prevailing type 1 IFN-centric model for STAT1 GOF autoimmunity, Stat1GOF mice lacking the type 1 IFN receptor were only partially protected from STAT1-driven systemic inflammation, whereas loss of type 2 IFN (IFN-γ) signals abrogated autoimmunity. Last, germline STAT1 GOF alleles are thought to enhance transcriptional activity by increasing total STAT1 protein, but the underlying biochemical mechanisms have not been defined. We showed that IFN-γ receptor deletion normalized total STAT1 expression across immune lineages, highlighting IFN-γ as the critical driver of feedforward STAT1 elevation in STAT1 GOF syndrome.

Fate-Mapping of GM-CSF Expression Identifies a Discrete Subset of Inflammation-Driving T Helper Cells Regulated by Cytokines IL-23 and IL-1ß.

Pathogenic lymphocytes initiate the development of chronic inflammatory diseases. The cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) (encoded by Csf2) is a key communicator between pathogenic lymphocytes and tissue-invading inflammatory phagocytes. However, the molecular properties of GM-CSF-producing cells and the mode of Csf2 regulation in vivo remain unclear. To systematically study and manipulate GM-CSF+ cells and their progeny in vivo, we generated a fate-map and reporter of GM-CSF expression mouse strain (FROG). We mapped the phenotypic and functional profile of auto-aggressive T helper (Th) cells during neuroinflammation and identified the signature and pathogenic memory of a discrete encephalitogenic Th subset. These cells required interleukin-23 receptor (IL-23R) and IL-1R but not IL-6R signaling for their maintenance and pathogenicity. Specific ablation of this subset interrupted the inflammatory cascade, despite the unperturbed tissue accumulation of other Th subsets (e.g., Th1 and Th17), highlighting that GM-CSF expression not only marks pathogenic Th cells, but that this subset mediates immunopathology and tissue destruction.

The transcription factor musculin promotes the unidirectional development of peripheral Treg cells by suppressing the TH2 transcriptional program.

Although master transcription factors (TFs) are key to the development of specific T cell subsets, whether additional transcriptional regulators are induced by the same stimuli that dominantly repress the development of other, non-specific T cell lineages has not been fully elucidated. Through the use of regulatory T cells (Treg cells) induced by transforming growth factor-ß (TGF-ß), we identified the TF musculin (MSC) as being critical for the development of induced Treg cells (iTreg cells) by repression of the T helper type 2 (TH2) transcriptional program. Loss of MSC reduced expression of the Treg cell master TF Foxp3 and induced TH2 differentiation even under iTreg-cell-differentiation conditions. MSC interrupted binding of the TF GATA-3 to the locus encoding TH2-cell-related cytokines and diminished intrachromosomal interactions within that locus. MSC-deficient (Msc-/-) iTreg cells were unable to suppress TH2 responses, and Msc-/- mice spontaneously developed gut and lung inflammation with age. MSC therefore enforced Foxp3 expression and promoted the unidirectional induction of iTreg cells by repressing the TH2 developmental program.

Immunostimulatory Tim-1-specific antibody deprograms Tregs and prevents transplant tolerance in mice.

T cell Ig mucin (Tim) molecules modulate CD4(+) T cell responses. In keeping with the view that Tim-1 generates a stimulatory signal for CD4(+) T cell activation, we hypothesized that an agonist Tim-1-specific mAb would intensify the CD4(+) T cell-dependant allograft response. Unexpectedly, we determined that a particular Tim-1-specific mAb exerted reciprocal effects upon the commitment of alloactivated T cells to regulatory and effector phenotypes. Commitment to the Th1 and Th17 phenotypes was fostered, whereas commitment to the Treg phenotype was hindered. Moreover, ligation of Tim-1 in vitro effectively deprogrammed Tregs and thus produced Tregs unable to control T cell responses. Overall, the effects of the agonist Tim-1-specific mAb on the allograft response stemmed from enhanced expansion and survival of T effector cells; a capacity to deprogram natural Tregs; and inhibition of the conversion of naive CD4(+) T cells into Tregs. The reciprocal effects of agonist Tim-1-specific mAbs upon effector T cells and Tregs serve to prevent allogeneic transplant tolerance.

CD4(+)CD25(-)Foxp3(-) Th1 cells are the source of IL-10-mediated immune suppression in chronic cutaneous leishmaniasis.

Nonhealing forms of leishmaniasis in humans are commonly associated with elevated levels of the deactivating cytokine IL-10, and in the mouse, normally chronic infections can be cleared in the absence of IL-10. Using a Leishmania major strain that produces nonhealing dermal lesions in a T helper type 1 (Th1) cell-polarized setting, we have analyzed the cellular sources of IL-10 and their relative contribution to immune suppression. IL-10 was produced by innate cells, as well as CD4(+)CD25(+)Foxp3(+) and CD4(+)CD25(-)Foxp3(-) T cells in the chronic lesion. Nonetheless, only IL-10 production by antigen-specific CD4(+)CD25(-)Foxp3(-) T cells, the majority of which also produced IFN-gamma, was necessary for suppression of acquired immunity in Rag(-/-) reconstituted mice. Surprisingly, Rag(-/-) mice reconstituted with naive CD4(+) T cells depleted of natural T regulatory cells developed more severe infections, associated with elevated levels of IL-10 and, especially, Th2 cytokines in the site. The data demonstrate that IL-10-producing Th1 cells, activated early in a strong inflammatory setting as a mechanism of feedback control, are the principal mediators of T cell-derived IL-10-dependent immune suppression in a chronic intracellular infection.