Thomas Ernandez

Immunoregulatory role of TNFα in inflammatory kidney diseases

Tumor necrosis factor alpha (TNFalpha), a pleiotropic cytokine, plays important inflammatory roles in renal diseases such as lupus nephritis, anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis and renal allograft rejection. However, TNFalpha also plays critical immunoregulatory roles that are required to maintain immune homeostasis. These complex biological functions of TNFalpha are orchestrated by its two receptors, TNFR1 and TNFR2. For example, TNFR2 promotes leukocyte infiltration and tissue injury in an animal model of immune complex-mediated glomerulonephritis. On the other hand, TNFR1 plays an immunoregulatory function in a murine lupus model with a deficiency in this receptor that leads to more severe autoimmune symptoms. In humans, proinflammatory and immunoregulatory roles for TNFalpha are strikingly illustrated in patients on anti-TNFalpha medications: These treatments are greatly beneficial in certain inflammatory diseases such as rheumatoid arthritis but, on the other hand, are also associated with the induction of autoimmune lupus-like syndromes and enhanced autoimmunity in multiple sclerosis patients. The indication for anti-TNFalpha treatments in renal inflammatory diseases is still under discussion. Ongoing clinical trials may help to clarify the potential benefit of such treatments in lupus nephritis and ANCA-associated glomerulonephritis. Overall, the complex biology of TNFalpha is not fully understood. A greater understanding of the function of its receptors may provide a framework to understand its contrasting proinflammatory and immunoregulatory functions. This may lead the development of new, more specific anti-inflammatory drugs.

AKAP9 regulation of microtubule dynamics promotes Epac1-induced endothelial barrier properties

Adhesive forces at endothelial cell-cell borders maintain vascular integrity. cAMP enhances barrier properties and controls cellular processes through protein kinase A bound to A-kinase anchoring proteins (AKAPs). It also activates exchange protein directly activated by cAMP (Epac1), an exchange factor for Ras-related protein 1 (Rap1) GTPases that promotes cadherin- and integrin-mediated adhesion through effects on the actin cytoskeleton. We demonstrate that AKAP9 facilitates the microtubule polymerization rate in endothelial cells, interacts with Epac1, and is required for Epac1-stimulated microtubule growth. AKAP9 is not required for maintaining barrier properties under steady-state conditions. Rather, it is essential when the cell is challenged to make new adhesive contacts, as is the case when Epac activation enhances barrier function through a mechanism that, surprisingly, requires integrin adhesion at cell-cell contacts. In the present study, defects in Epac-induced responses in AKAP9-silenced cells were evident despite an intact Epac-induced increase in Rap activation, cortical actin, and vascular endothelial-cadherin adhesion. We describe a pathway that integrates Epac-mediated signals with AKAP9-dependent microtubule dynamics to coordinate integrins at lateral borders.

Regulation of human neutrophil Fcγ receptor IIa by C5a receptor promotes inflammatory arthritis in mice

OBJECTIVE Rheumatoid arthritis culminates in joint destruction that, in mouse models of disease, is supported by innate immune molecules, including Fcγ receptors (FcγR) and complement. However, these findings may not be predictive of the outcome in humans, given the structural differences between murine and human activating FcγR on neutrophils, a prominent component of joint exudates. The aim of this study was to examine the role of human neutrophil FcγRIIa in the development of arthritis and probe the underlying mechanism by which FcγRIIa initiates disease. METHODS K/BxN mouse serum transfer-induced arthritis was examined in mice expressing human FcγRIIa on neutrophils but lacking their own activating FcγR (γ-chain-deficient mice). The role of mast cells, complement (C3 and C5a), and CD18 integrins in FcγRIIa-initiated disease was examined using cell reconstitution approaches, inhibitors, and functional blocking antibodies, respectively. Crosstalk between the complement receptor C5aR and FcγRIIa on neutrophils was evaluated in vitro. RESULTS The expression of human FcγRIIa on neutrophils was sufficient to restore susceptibility to K/BxN serum-induced neutrophil recruitment, synovitis, and bone destruction in γ-chain-deficient mice. Joint inflammation was robust and proceeded even in the absence of mast cells and vascular permeability, features shown to contribute to disease in wild-type mice. Neutrophil recruitment was dependent on the presence of a CD18 integrin, lymphocyte function-associated antigen 1, and C5aR. In addition, C5aR significantly enhanced FcγRIIa-mediated phagocytosis and oxidative burst in vitro. CONCLUSION Human and murine activating FcγR on neutrophils are not functionally equivalent, and in humans, they may play a primary role in arthritis. Crosstalk between neutrophil C5aR and FcγRIIa is essential for disease progression, thus highlighting a new aspect of complement during the effector phase of inflammatory arthritis.

Human Lupus Serum Induces Neutrophil-Mediated Organ Damage in Mice That Is Enabled by Mac-1 Deficiency

Systemic lupus erythematosus (SLE) is a chronic, multiorgan inflammatory autoimmune disorder associated with high levels of circulating autoantibodies and immune complexes. We report that passive transfer of human SLE sera into mice expressing the uniquely human FcγRIIA and FcγRIIIB on neutrophils induces lupus nephritis and in some cases arthritis only when the mice additionally lack the CD18 integrin, Mac-1. The prevailing view is that Mac-1 on macrophages is responsible for immune complex clearance. However, disease permitted by the absence of Mac-1 is not related to enhanced renal immune complex deposition or in situ C1q/C3 complement activation and proceeds even in the absence of macrophages. Instead, disease is associated with increased FcγRIIA-induced neutrophil accumulation that is enabled by Mac-1 deficiency. Intravital microscopy in the cremasteric vasculature reveals that Mac-1 mitigates FcγRIIA-dependent neutrophil recruitment in response to deposited immune complexes. Our results provide direct evidence that human SLE immune complexes are pathogenic, demonstrate that neutrophils are primary mediators of end organ damage in a novel humanized lupus mouse model, and identify Mac-1 regulation of FcγRIIA-mediated neutrophil recruitment as a key step in development of target organ damage.

Neutrophils: game changers in glomerulonephritis?

Glomerulonephritides represent a diverse array of diseases that have in common immune cell-mediated effector mechanisms that cause organ damage. The contribution of neutrophils to the pathogenesis of proliferative glomerulonephritis (GN) is not well recognized. Most equate neutrophils with killing pathogens and causing collateral tissue damage during acute inflammation. However, these phagocytes are endowed with additional characteristics that have been traditionally reserved for cells of the adaptive immune system. They communicate with other cells, exhibit plasticity in their responses and have the potential to coordinate and inform the subsequent immune response, thus countering the notion that they arrive, destroy and then disappear. Therefore, neutrophils, which are the first to arrive at a site of inflammation, are potential game changers in GN.

The Changing Landscape of Renal Inflammation

Kidney inflammation is a major contributor to progressive renal injury, leading to glomerulonephritis (GN) and chronic kidney disease. We review recent advances in our understanding of leukocyte accumulation in the kidney, emphasizing key chemokines involved in GN. We discuss features of renal inflammation such as the evolving concept of immune cell plasticity. We also describe certain aspects of organ-specific tissue microenvironments in shaping immune cell responses, as well as the current knowledge of how regulatory T lymphocytes impact on other immune effector cell populations to control inflammation. It is clear that present and future research in these areas may contribute to the development of novel targeted therapeutics, with the hope of alleviating the burden of end-stage renal disease (ESRD).

Uninephrectomy and apical fluid shear stress decrease ENaC abundance in collecting duct principal cells

Acute nephron reduction such as after living kidney donation may increase the risk of hypertension. Uninephrectomy induces major hemodynamic changes in the remaining kidney, resulting in rapid increase of single-nephron glomerular filtration rate (GFR) and fluid delivery in the distal nephron. Decreased sodium (Na) fractional reabsorption after the distal tubule has been reported after uninephrectomy in animals preserving volume homeostasis. In the present study, we thought to specifically explore the effect of unilateral nephrectomy on epithelial Na channel (ENaC) subunit expression in mice. We show that γ-ENaC subunit surface expression was specifically downregulated after uninephrectomy, whereas the expression of the aldosterone-sensitive α-ENaC and α1-Na-K-ATPase subunits as well as of kidney-specific Na-K-Cl cotransporter isoform and Na-Cl cotransporter were not significantly altered. Because acute nephron reduction induces a rapid increase of single-nephron GFR, resulting in a higher tubular fluid flow, we speculated that local mechanical factors such as fluid shear stress (FSS) were involved in Na reabsorption regulation after uninephrectomy. We further explore such hypothesis in an in vitro model of FSS applied on highly differentiated collecting duct principal cells. We found that FSS specifically downregulates β-ENaC and γ-ENaC subunits at the transcriptional level through an unidentified heat-insensitive paracrine basolateral factor. The primary cilium as a potential mechanosensor was not required. In contrast, protein kinase A and calcium-sensitive cytosolic phospholipase A2 were involved, but we could not demonstrate a role for cyclooxygenase or epoxygenase metabolites.