Defects in immune regulation have been implicated in the pathogenesis of diabetes in mouse and in man. In vitro assays using autologous regulatory (Treg) and responder effector (Teff) T cells have shown that suppression is impaired in diabetic subjects. In this study, we addressed whether the source of this defect is intrinsic to the Treg or Teff compartment of diabetic subjects. We first established that in type 1 diabetes (T1D) individuals, similar levels of impaired suppression were seen, irrespective of whether natural (nTreg) or adaptive Treg (aTreg) were present. Then using aTreg, we examined the ability of T1D aTreg to suppress Teff of healthy controls, as compared with the ability of control aTreg to suppress Teff of diabetic subjects. Taking this approach, we found that the aTregs from T1D subjects function normally in the presence of control Teff, and that the T1D Teff were resistant to suppression in the presence of control aTreg. This escape from regulation was seen with nTreg as well and was not transferred to control Teff coincubated with T1D Teff. Thus, the "defective regulation" in T1D is predominantly due to the resistance of responding T cells to Treg and is a characteristic intrinsic to the T1D Teff. This has implications with respect to pathogenic mechanisms, which underlie the development of disease and the target of therapies for T1D.

Patients with multiple sclerosis (MS) manifest demyelination and neurodegeneration mediated in part by CD4(+) T cells that have escaped regulation. Resistance of pathogenic effector T cells (T(effs)) to suppression by regulatory T cells (T(regs)) has been demonstrated in several autoimmune diseases. Although impairment in T(reg) number and function has been observed in relapsing-remitting MS (RRMS), T(eff) resistance has not been well studied in this disease. To determine whether T(eff) resistance contributes to failed tolerance in RRMS, we performed T(reg) suppression assays with T(effs) from either RRMS patients not on immunomodulatory therapy or healthy individuals. T(eff) resistance was present in the T(effs) of RRMS patients with active disease but not from patients with inactive disease. Interleukin-6 (IL-6) and phosphorylation of signal transducer and activator of transcription 3 (pSTAT3) promote T(eff) resistance to T(regs), and we found an increase in IL-6 receptor α (IL-6Rα) expression and elevated IL-6 signaling as measured by pSTAT3 in our RRMS subjects. Further, the impaired suppression in RRMS subjects correlated with an increase in IL-6Rα surface expression on CD4(+) T cells and an increase in pSTAT3 in response to IL-6. To address whether the enhanced pSTAT3 contributed to T(eff) resistance in active RRMS patients, we blocked STAT3 phosphorylation and found that impaired suppression was reversed. Therefore, enhanced IL-6R signaling through pSTAT3, in some cases through increased IL-6Rα expression, contributed to T(eff) resistance in active RRMS. These markers may aid in determining disease activity and responsiveness to immunomodulatory therapies in RRMS.

IL-2 receptor (IL-2R) signaling is essential for optimal stability and function of CD4+CD25hiFOXP3+ regulatory T cells (Treg); a cell type that plays an integral role in maintaining tolerance. Thus, we hypothesized that decreased response to IL-2 may be a common phenotype of subjects who have autoimmune diseases associated with variants in the IL2RA locus, including T1D and MS, particularly in cells expressing the high affinity IL-2R alpha chain (IL-2RA or CD25). To examine this question we used phosphorylation of STAT5 (pSTAT5) as a downstream measure of IL-2R signaling, and found a decreased response to IL-2 in CD4+CD25hi T cells of T1D and MS, but not SLE patients. Since the IL2RArs2104286 haplotype is associated with T1D and MS, we measured pSTAT5 in controls carrying the rs2104286 risk haplotype to test whether this variant contributed to reduced IL-2 responsiveness. Consistent with this, we found decreased pSTAT5 in subjects carrying the rs2104286 risk haplotype. Reduced IL-2R signaling did not result from lower CD25 expression on CD25hi cells; instead we detected increased CD25 expression on naive Treg from controls carrying the rs2104286 risk haplotype, and subjects with T1D and MS. However the rs2104286 risk haplotype correlated with increased soluble IL-2RA levels, suggesting that shedding of the IL-2R may account in part for the reduced IL-2R signaling associated with the rs2104286 risk haplotype. In addition to risk variants in IL2RA, we found that the T1D-associated risk variant of PTPN2rs1893217 independently contributed to diminished IL-2R signaling. However, even when holding genotype constant at IL2RA and PTPN2, we still observed a significant signaling defect in T1D and MS patients. Together, these data suggest that multiple mechanisms converge in disease leading to decreased response to IL-2, a phenotype that may eventually lead to loss of tolerance and autoimmunity.

Interleukin-6 (IL-6) is a key pathogenic cytokine in multiple autoimmune diseases including rheumatoid arthritis and multiple sclerosis, suggesting that dysregulation of the IL-6 pathway may be a common feature of autoimmunity. The role of IL-6 in type 1 diabetes (T1D) is not well understood. We show that signal transducer and activator of transcription 3 (STAT3) and STAT1 responses to IL-6 are significantly enhanced in CD4 and CD8 T cells from individuals with T1D compared to healthy controls. The effect is IL-6-specific because it is not seen with IL-10 or IL-27 stimulation, two cytokines that signal via STAT3. An important determinant of enhanced IL-6 responsiveness in T1D is IL-6 receptor surface expression, which correlated with phospho-STAT3 levels. Further, reduced expression of the IL-6R sheddase ADAM17 in T cells from patients indicated a mechanistic link to enhanced IL-6 responses in T1D. IL-6-induced STAT3 phosphorylation was inversely correlated with time from diagnosis, suggesting that dysregulation of IL-6 signaling may be a marker of early disease. Finally, whole-transcriptome analysis of IL-6-stimulated CD4+ T cells from patients revealed previously unreported IL-6 targets involved in T cell migration and inflammation, including lymph node homing markers CCR7 and L-selectin. In summary, our study demonstrates enhanced T cell responses to IL-6 in T1D due, in part to, an increase in IL-6R surface expression. Dysregulated IL-6 responsiveness may contribute to diabetes through multiple mechanisms including altered T cell trafficking and indicates that individuals with T1D may benefit from IL-6-targeted therapeutic intervention such as the one that is being currently tested (NCT02293837).

Regulatory T cells are involved in the maintenance of tolerance. Alterations in their functional capacity are implicated in the development of autoimmunity. In the case of common autoimmune disorders the defects in suppression may be partial, and may be due to a loss of Treg function, or a resistance to suppression by responder T cells. Thus in order to assess Treg function, an in vitro assay that is sensitive enough to demonstrate modest alterations in suppression, and which can differentiate between impaired suppression due to Treg dysfunction, and responder cell resistance is ideal. In this chapter we describe a CFSE based proliferation assay that utilizes a bead based activation system, which is reproducible, consistent and able to distinguish between defects in Treg function and the resistance of responder T cells.