Alaina L. Garland

Molecular basis for pH-dependent mucosal dehydration in cystic fibrosis airways.

Significance Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which codes for a chloride/bicarbonate channel whose absence leads to dehydration and acidification of CF airways. A contributing factor to CF lung disease is dysregulation of the epithelial Na+ channel (ENaC), which exacerbates mucus dehydration. Here, we show that ENaC hyperactivity in CF airways is direct consequence of acidic airway surface liquid (ASL) and that ASL hydration is restored by raising ASL pH. Additionally, we show that short palate lung and nasal epithelial clone 1, the most abundant gene in airway epithelia, is the extracellular pH-sensitive factor that inhibits ENaC in normal but not CF airways. We suggest that future CF therapy be directed toward raising the pH of CF airways. The ability to maintain proper airway surface liquid (ASL) volume homeostasis is vital for mucus hydration and clearance, which are essential aspects of the mammalian lung’s innate defense system. In cystic fibrosis (CF), one of the most common life-threatening genetic disorders, ASL dehydration leads to mucus accumulation and chronic infection. In normal airways, the secreted protein short palate lung and nasal epithelial clone 1 (SPLUNC1) effectively inhibits epithelial Na+ channel (ENaC)-dependent Na+ absorption and preserves ASL volume. In CF airways, it has been hypothesized that increased ENaC-dependent Na+ absorption contributes to ASL depletion, and hence increased disease. However, this theory is controversial, and the mechanism for abnormal ENaC regulation in CF airways has remained elusive. Here, we show that SPLUNC1 is a pH-sensitive regulator of ENaC and is unable to inhibit ENaC in the acidic CF airway environment. Alkalinization of CF airway cultures prevented CF ASL hyperabsorption, and this effect was abolished when SPLUNC1 was stably knocked down. Accordingly, we resolved the crystal structure of SPLUNC1 to 2.8 Å. Notably, this structure revealed two pH-sensitive salt bridges that, when removed, rendered SPLUNC1 pH-insensitive and able to regulate ASL volume in acidic ASL. Thus, we conclude that ENaC hyperactivity is secondary to reduced CF ASL pH. Together, these data provide molecular insights into the mucosal dehydration associated with a range of pulmonary diseases, including CF, and suggest that future therapy be directed toward alkalinizing the pH of CF airways.

β Cell-Specific CD4+ T Cell Clonotypes in Peripheral Blood and the Pancreatic Islets Are Distinct

Type 1 diabetes is an autoimmune disease mediated by β cell-specific CD4+ and CD8+ T cells. Tracking β cell-specific T cells is one approach to monitor the diabetogenic response in at risk or diabetic individuals. Such analyses, however, are limited to PBL because T cells infiltrating the pancreatic islets are normally inaccessible. A key issue is whether peripheral β cell-specific T cells accurately reflect those cells infiltrating the target tissue. We investigated the properties of CD4+ T cells specific for a mimetic epitope recognized by the BDC2.5 clonotypic TCR in NOD mice. Soluble IAg7-Ig (sIAg7-Ig) multimer complexes covalently linked to a mimetic BDC peptide (sIAg7-mBDC) were used to identify or isolate CD4+ T cells from PBL and the islets of NOD mice. A temporal increase in sIAg7-mBDC binding (g7-mBDC+) T cells corresponding with the progression of β cell autoimmunity was detected in both PBL and islets in NOD female mice. In contrast to T cells in PBL, however, the majority of islet g7-mBDC+ T cells exhibited a type 1 phenotype, and mediated diabetes upon transfer into NOD.scid recipients. TCR-β and CDR-β gene usage of single islet-infiltrating g7-mBDC+ CD4+ T cells from individual NOD mice showed a restricted repertoire dominated by one or two clones typically expressing TCR β-chain variable TRBV-15. In contrast, a distinct and diverse TCR repertoire was detected for PBL-derived g7-mBDC+ T cells. These results demonstrate that PBL and islet CD4+ T cells specific for a given β cell epitope can differ regarding pathogenicity and TCR repertoire.

Inducible adeno-associated virus-mediated IL-2 gene therapy prevents autoimmune diabetes

IL-2 and TGF-β1 play key roles in the immunobiology of Foxp3-expressing CD25+CD4+ T cells (Foxp3+Treg). Administration of these cytokines offers an appealing approach to manipulate the Foxp3+Treg pool and treat T cell-mediated autoimmunity such as type 1 diabetes. However, efficacy of cytokine treatment is dependent on the mode of application, and the potent pleiotropic effects of cytokines like IL-2 may lead to severe side effects. In the current study, we used a gene therapy-based approach to assess the efficacy of recombinant adeno-associated virus vectors expressing inducible IL-2 or TGF-β1 transgenes to suppress ongoing β cell autoimmunity in NOD mice. Intramuscular vaccination of recombinant adeno-associated virus to 10-wk-old NOD female mice and a subsequent 3 wk induction of IL-2 was sufficient to prevent diabetes and block the progression of insulitis. Protection correlated with an increased frequency of Foxp3+Treg in the periphery as well as in the draining pancreatic lymph nodes and islets. IL-2 induced a shift in the ratio favoring Foxp3+Treg versus IFN-γ–expressing T cells infiltrating the islets. Induction of IL-2 had no systemic effect on the frequency or activational status of T cells and NK cells. Induction of TGF-β1 had no effect on the Foxp3+Treg pool or the progression of β cell autoimmunity despite induced systemic levels of activated TGF-β1 that were comparable to IL-2. These results demonstrate that inducible IL-2 gene therapy is an effective and safe approach to manipulate Foxp3+Treg and suppress T cell-mediated autoimmunity and that under the conditions employed, IL-2 is more potent than TGF-β1.

Adiponectin Lowers Glucose Production by Increasing SOGA

Adiponectin is a hormone that lowers glucose production by increasing liver insulin sensitivity. Insulin blocks the generation of biochemical intermediates for glucose production by inhibiting autophagy. However, autophagy is stimulated by an essential mediator of adiponectin action, AMPK. This deadlock led to our hypothesis that adiponectin inhibits autophagy through a novel mediator. Mass spectrometry revealed a novel protein that we call suppressor of glucose by autophagy (SOGA) in adiponectin-treated hepatoma cells. Adiponectin increased SOGA in hepatocytes, and siRNA knockdown of SOGA blocked adiponectin inhibition of glucose production. Furthermore, knockdown of SOGA increased late autophagosome and lysosome staining and the secretion of valine, an amino acid that cannot be synthesized or metabolized by liver cells, suggesting that SOGA inhibits autophagy. SOGA decreased in response to AICAR, an activator of AMPK, and LY294002, an inhibitor of the insulin signaling intermediate, PI3K. AICAR reduction of SOGA was blocked by adiponectin; however, adiponectin did not increase SOGA during PI3K inhibition, suggesting that adiponectin increases SOGA through the insulin signaling pathway. SOGA contains an internal signal peptide that enables the secretion of a circulating fragment of SOGA, providing a surrogate marker for intracellular SOGA levels. Circulating SOGA increased in parallel with adiponectin and insulin activity in both humans and mice. These results suggest that adiponectin-mediated increases in SOGA contribute to the inhibition of glucose production.

Reduced IL-2 expression in NOD mice leads to a temporal increase in CD62LLOFoxP3+CD4+ T cells with limited suppressor activity

IL‐2 plays a critical role in the induction and maintenance of FoxP3‐expressing regulatory T cells (FoxP3+Tregs). Reduced expression of IL‐2 is linked to T‐cell‐mediated autoimmune diseases such as type 1 diabetes (T1D), in which an imbalance between FoxP3+Tregs and pathogenic T effectors exists. We investigated the contribution of IL‐2 to dysregulation of FoxP3+Tregs by comparing wildtype NOD mice with animals congenic for a C57BL/6‐derived disease‐resistant Il2 allele and in which T‐cell secretion of IL‐2 is increased (NOD.B6Idd3). Although NOD mice exhibited a progressive decline in the frequency of CD62LhiFoxP3+Tregs due to an increase in CD62LloFoxP3+Tregs, CD62LhiFoxP3+Tregs were maintained in the pancreatic lymph nodes and islets of NOD.B6Idd3 mice. Notably, the frequency of proliferating CD62LhiFoxP3+Tregs was elevated in the islets of NOD.B6Idd3 versus NOD mice. Increasing levels of IL‐2 in vivo also resulted in larger numbers of CD62LhiFoxP3+Tregs in NOD mice. These results demonstrate that IL‐2 influences the suppressor activity of the FoxP3+Tregs pool by regulating the balance between CD62Llo and CD62Lhi FoxP3+Tregs. In NOD mice, reduced IL‐2 expression leads to an increase in nonsuppressive CD62LloFoxP3+Tregs, which in turn correlates with a pool of CD62LhiFoxP3+Tregs with limited proliferation.

Regulator of G-protein signaling-21 (RGS21) is an inhibitor of bitter gustatory signaling found in lingual and airway epithelia.

Background: RGS21 is expressed in tastant-responsive lingual epithelium, but with unknown function. Results: RGS21 accelerated intrinsic GTPase activity of multiple Gα subunits; RGS21 over- and underexpression in epithelial cells modulated bitterant responsiveness. Conclusion: RGS21 is a negative regulator of bitterant signal transduction. Significance: RGS21 represents a nonreceptor regulatory component of gustatory signaling that alters sensitivity of bitterant responsiveness in an endogenous, cellular context. The gustatory system detects tastants and transmits signals to the brain regarding ingested substances and nutrients. Although tastant receptors and taste signaling pathways have been identified, little is known about their regulation. Because bitter, sweet, and umami taste receptors are G protein-coupled receptors (GPCRs), we hypothesized that regulators of G protein signaling (RGS) proteins may be involved. The recent cloning of RGS21 from taste bud cells has implicated this protein in the regulation of taste signaling; however, the exact role of RGS21 has not been precisely defined. Here, we sought to determine the role of RGS21 in tastant responsiveness. Biochemical analyses confirmed in silico predictions that RGS21 acts as a GTPase-accelerating protein (GAP) for multiple G protein α subunits, including adenylyl cyclase-inhibitory (Gαi) subunits and those thought to be involved in tastant signal transduction. Using a combination of in situ hybridization, RT-PCR, immunohistochemistry, and immunofluorescence, we demonstrate that RGS21 is not only endogenously expressed in mouse taste buds but also in lung airway epithelial cells, which have previously been shown to express components of the taste signaling cascade. Furthermore, as shown by reverse transcription-PCR, the immortalized human airway cell line 16HBE was found to express transcripts for tastant receptors, RGS21, and downstream taste signaling components. Over- and underexpression of RGS21 in 16HBE cells confirmed that RGS21 acts to oppose bitter tastant signaling to cAMP and calcium second messenger changes. Our data collectively suggests that RGS21 modulates bitter taste signal transduction.

β cell-specific IL-2 therapy increases islet Foxp3+Treg and suppresses type 1 diabetes in NOD mice

Interleukin-2 (IL-2) is a critical cytokine for the homeostasis and function of forkhead box p3–expressing regulatory T cells (Foxp3+Tregs). Dysregulation of the IL-2–IL-2 receptor axis is associated with aberrant Foxp3+Tregs and T cell–mediated autoimmune diseases such as type 1 diabetes. Treatment with recombinant IL-2 has been reported to enhance Foxp3+Tregs and suppress different models of autoimmunity. However, efficacy of IL-2 therapy is dependent on achieving sufficient levels of IL-2 to boost tissue-resident Foxp3+Tregs while avoiding the potential toxic effects of systemic IL-2. With this in mind, adeno-associated virus (AAV) vector gene delivery was used to localize IL-2 expression to the islets of NOD mice. Injection of a double-stranded AAV vector encoding IL-2 driven by a mouse insulin promoter (dsAAVmIP-IL2) increased Foxp3+Tregs in the islets but not the draining pancreatic lymph nodes. Islet Foxp3+Tregs in dsAAVmIP-IL2–treated NOD mice exhibited enhanced fitness marked by increased expression of Bcl-2, proliferation, and suppressor function. In contrast, ectopic IL-2 had no significant effect on conventional islet-infiltrating effector T cells. Notably, β-cell–specific IL-2 expression suppressed late preclinical type 1 diabetes in NOD mice. Collectively, these findings demonstrate that β-cell–specific IL-2 expands an islet-resident Foxp3+Tregs pool that effectively suppresses ongoing type 1 diabetes long term.

Autoreactive Effector/Memory CD4+ and CD8+ T Cells Infiltrating Grafted and Endogenous Islets in Diabetic NOD Mice Exhibit Similar T Cell Receptor Usage

Islet transplantation provides a “cure” for type 1 diabetes but is limited in part by recurrent autoimmunity mediated by β cell-specific CD4+ and CD8+ T cells. Insight into the T cell receptor (TCR) repertoire of effector T cells driving recurrent autoimmunity would aid the development of immunotherapies to prevent islet graft rejection. Accordingly, we used a multi-parameter flow cytometry strategy to assess the TCR variable β (Vβ) chain repertoires of T cell subsets involved in autoimmune-mediated rejection of islet grafts in diabetic NOD mouse recipients. Naïve CD4+ and CD8+ T cells exhibited a diverse TCR repertoire, which was similar in all tissues examined in NOD recipients including the pancreas and islet grafts. On the other hand, the effector/memory CD8+ T cell repertoire in the islet graft was dominated by one to four TCR Vβ chains, and specific TCR Vβ chain usage varied from recipient to recipient. Similarly, islet graft- infiltrating effector/memory CD4+ T cells expressed a limited number of prevalent TCR Vβ chains, although generally TCR repertoire diversity was increased compared to effector/memory CD8+ T cells. Strikingly, the majority of NOD recipients showed an increase in TCR Vβ12-bearing effector/memory CD4+ T cells in the islet graft, most of which were proliferating, indicating clonal expansion. Importantly, TCR Vβ usage by effector/memory CD4+ and CD8+ T cells infiltrating the islet graft exhibited greater similarity to the repertoire found in the pancreas as opposed to the draining renal lymph node, pancreatic lymph node, or spleen. Together these results demonstrate that effector/memory CD4+ and CD8+ T cells mediating autoimmune rejection of islet grafts are characterized by restricted TCR Vβ chain usage, and are similar to T cells that drive destruction of the endogenous islets.

IFN-γ receptor deficiency prevents diabetes induction by diabetogenic CD4+, but not CD8+, T cells.

IFN‐γ is generally believed to be important in the autoimmune pathogenesis of type 1 diabetes (T1D). However, the development of spontaneous β‐cell autoimmunity is unaffected in NOD mice lacking expression of IFN‐γ or the IFN‐γ receptor (IFNγR), bringing into question the role IFN‐γ has in T1D. In the current study, an adoptive transfer model was employed to define the contribution of IFN‐γ in CD4+ versus CD8+ T cell‐mediated β‐cell autoimmunity. NOD.scid mice lacking expression of the IFNγR β chain (NOD.scid.IFNγRBnull) developed diabetes following transfer of β cell‐specific CD8+ T cells alone. In contrast, β cell‐specific CD4+ T cells alone failed to induce diabetes despite significant infiltration of the islets in NOD.scid.IFNγRBnull recipients. The lack of pathogenicity of CD4+ T‐cell effectors was due to the resistance of IFNγR‐deficient β cells to inflammatory cytokine‐induced cell death. On the other hand, CD4+ T cells indirectly promoted β‐cell destruction by providing help to CD8+ T cells in NOD.scid.IFNγRBnull recipients. These results demonstrate that IFN‐γR may play a key role in CD4+ T cell‐mediated β‐cell destruction.

Evaluation of a SPLUNC1-derived peptide for the treatment of cystic fibrosis lung disease

In cystic fibrosis (CF) lungs, epithelial Na+ channel (ENaC) hyperactivity causes a reduction in airway surface liquid volume, leading to decreased mucocilliary clearance, chronic bacterial infection, and lung damage. Inhibition of ENaC is an attractive therapeutic option. However, ENaC antagonists have failed clinically because of off-target effects in the kidney. The S18 peptide is a naturally occurring short palate lung and nasal epithelial clone 1 (SPLUNC1)-derived ENaC antagonist that restores airway surface liquid height for up to 24 h in CF human bronchial epithelial cultures. However, its efficacy and safety in vivo are unknown. To interrogate the potential clinical efficacy of S18, we assessed its safety and efficacy using human airway cultures and animal models. S18-mucus interactions were tested using superresolution microscopy, quartz crystal microbalance with dissipation, and confocal microscopy. Human and murine airway cultures were used to measure airway surface liquid height. Off-target effects were assessed in conscious mice and anesthetized rats. Morbidity and mortality were assessed in the β-ENaC-transgenic (Tg) mouse model. Restoration of normal mucus clearance was measured in cystic fibrosis transmembrane conductance regulator inhibitor 172 [CFTR(inh)-172]-challenged sheep. We found that S18 does not interact with mucus and rapidly penetrated dehydrated CF mucus. Compared with amiloride, an early generation ENaC antagonist, S18 displayed a superior ability to slow airway surface liquid absorption, reverse CFTR(inh)-172-induced reduction of mucus transport, and reduce morbidity and mortality in the β-ENaC-Tg mouse, all without inducing any detectable signs of renal toxicity. These data suggest that S18 is the first naturally occurring ENaC antagonist to show improved preclinical efficacy in animal models of CF with no signs of renal toxicity.