Depletion of cells of monocyte lineage prevents loss of renal microvasculature in murine kidney transplantation.

BACKGROUND: Acute rejection increases the risk of late renal allograft loss with tubular atrophy, interstitial fibrosis, and microvascular rarefaction. Evidence supports a role for macrophages in promoting allograft injury, but the pathogenic mechanisms are unclear. Using a model of acute rejection, we sought evidence of macrophage-mediated endothelial cell cytotoxicity leading to loss of the renal microvasculature. METHODS: We used a transgenic conditional ablation strategy to deplete circulating monocytes and infiltrating renal macrophages after kidney transplantation. CD11b-DTR mice (FVB/nj strain) are transgenic for the human diphtheria toxin receptor gene under the control of the CD11b promoter. Administration of diphtheria toxin results in rapid ablation of circulating monocytes and resident/infiltrating renal macrophages. Transplants were performed between fully mismatched strains (Balb/c donor into control nontransgenic FVB/nj recipient; allograft group), between FVB/nj littermates (isograft group), and from Balb/c donors into CD11b-DTR mice (DT-treated group). Diphtheria toxin was administered at days 3 and 5, and the effect of monocyte/macrophage depletion on changes in renal microvasculature was determined at day 7. RESULTS: Conditional monocyte and macrophage ablation effectively depleted infiltrating macrophages in murine renal allografts at day 7. Macrophage ablation reduced histologic features of rejection (arteritis, tubulitis) and the accompanying rarefaction of peritubular capillaries at 7 days. The identification of macrophages immunopositive for inducible nitric oxide synthase implicated nitric oxide generation as a possible mechanism of endothelial cell cytotoxicity. CONCLUSION: These data indicate a significant role for macrophages in causing acute rejection-related tissue injury that is, at least in part, targeted to the microcirculation.

Pro-resolution properties of macrophages in renal injury.

Macrophages are inflammatory cells with important roles in the propagation of renal injury. More recently they have also been shown to be important in the resolution of inflammation. Wang et al. show that macrophages can be modulated ex vivo by cytokine stimulation, localize to renal tissue, and slow progression of experimental glomerular inflammation. Thus strategies targeting macrophage function have considerable therapeutic potential.

Targeting genetically modified macrophages to the glomerulus.

Macrophages are key players in the development of the majority of renal diseases and are therefore ideal cellular vectors for site specifically targeting gene therapy to inflamed glomeruli. Macrophages can be genetically modified using viral vectors ex vivo then re-introduced into the body where they can home to the diseased site. This review summarises current experience in efficiently targeting modified macrophages to the inflamed glomerulus focussing on the factors controlling macrophage localisation, macrophage gene transfer methods, in vivo gene delivery and results of recent investigations using modified macrophage gene therapy for glomerular disease.

Macrophages in renal inflammation.

This review describes recent advances in macrophage biology in the context of renal inflammation. It highlights the importance of the activated macrophage for the progression and resolution of renal disease, and discusses recent and potential future approaches to modify macrophage function selectively within the kidney to activate them specifically to promote the healing of kidney disease.

Combinatorial model of chemokine involvement in glomerular monocyte recruitment: role of CXC chemokine receptor 2 in infiltration during nephrotoxic nephritis.

A sequential model involving chemokines has been proposed for leukocyte extravasation into areas of inflammation; however, site-specific aspects remain to be elucidated. Hence, we studied the role of chemokines produced by mesangial (MC) or glomerular endothelial cells (GEC) and their receptors in glomerular recruitment of monocytes. Stimulation of MC with TNF-alpha up-regulated mRNA and protein of CC and CXC chemokines but not constitutive expression of the CX(3)C chemokine fractalkine. While growth-related activity (GRO)-alpha was immobilized to MC proteoglycans, monocyte chemotactic protein (MCP)-1 was secreted into the soluble phase. Firm adhesion and sequestration of monocytes on activated MC was supported by the GRO-alpha receptor CXCR2 and to a lesser extent by CX(3)CR, whereas the MCP-1 receptor CCR2 contributed to their transendothelial chemotaxis toward activated MC. In contrast, fractalkine mRNA and protein was induced by TNF-alpha in transformed rat GEC, and both CXCR2 and CX(3)CR mediated monocyte arrest on GEC in shear flow. The relevance of these mechanisms was confirmed in a rat nephrotoxic nephritis model where acute glomerular macrophage recruitment was profoundly inhibited by blocking CXCR2 or CCR2. In conclusion, our results epitomize a combinatorial model in which chemokines play specialized roles in driving glomerular monocyte recruitment and emphasize an important role for CXCR2 in macrophage infiltration during early phases of nephrotoxic nephritis.

Macrophages transfected with adenovirus to express IL-4 reduce inflammation in experimental glomerulonephritis.

Nephrotoxic nephritis (NTN) is characterized by acute macrophage-dependent inflammation and serves as a model of human glomerulonephritis. In this study we have transfected rat macrophages with recombinant adenovirus expressing IL-4 (Ad-IL4) and demonstrated that these transfected macrophages develop fixed properties as a result of transfection, as shown by reduced NO production in response to IFN-gamma and TNF. Ad-IL4-transfected macrophages localized with enhanced efficiency to inflamed glomeruli after renal artery injection in rats with NTN compared with adenovirus expressing beta-galactosidase (Ad-beta gal)-transfected macrophages and produced elevated levels of the cytokine in glomeruli in vivo for up to 4 days. The delivery of IL-4-expressing macrophages produced a marked reduction in the severity of albuminuria (day 2 albuminuria, 61 +/- 15 mg/24 h) compared with unmodified NTN (day 2 albuminuria, 286 +/- 40 mg/24 h; p < 0.01), and this was matched by a reduction in the number of ED1-positive macrophages infiltrating the glomeruli. Interestingly, the injection of IL-4-expressing macrophages into single kidney produced a marked reduction in the numbers of ED1-positive macrophages in the contralateral noninjected kidney, an effect that could not be mimicked by systemic delivery of IL-4-expressing macrophages. This implies that the presence of IL-4-expressing macrophages in a single kidney can alter the systemic development of the inflammatory response. Macrophage transfection and delivery provide a valuable system to study and modulate inflammatory disease and highlight the feasibility of macrophage-based gene therapy.

Gene transfer into inflamed glomeruli using macrophages transfected with adenovirus.

In vivo gene transfer to sites of inflammatory disease provides a novel method both for studying the effects of cytokines and growth factors, and for therapeutic intervention. Macrophages play a pivotal role in the development and control of inflammation and are therefore logical cells to use for genetic modification and in vivo gene delivery. In this study we show that macrophages (both cell lines and primary cultures) can be transfected by recombinant adenoviruses expressing beta-galactosidase, that the macrophages become activated by the transfection process as determined by generation of nitric oxide and can be easily manipulated to localise to inflamed glomeruli after direct injection into the renal artery of rats with an experimentally induced glomerular inflammation caused by nephrotoxic nephritis. The injection of transfected macrophages reduces the severity of injury in this model of glomerulonephritis as shown by a reduction in the degree of albuminuria. This approach provides a favourable system for gene delivery in inflammatory disease and shows that both the functional properties of the transfected macrophage as well the transgene it is engineered to produce are relevant for in vivo gene transfer. Gene Therapy (2000) 7, 263-270.

New approaches to modify glomerular inflammation.

Glomerulonephritis remains the leading cause of end-stage renal failure and treatments for these conditions remain non-specific and with significant side effects. The cellular and molecular basis of acute and chronic inflammation is increasingly understood and the work in a number of animal models of nephritis demonstrates the potential of specific molecular interventions. These include preventing the migration of inflammatory cells by inhibiting the effects of chemokines or blocking endothelial/leucocyte adhesion interactions. Within damaged tissue it is possible to decrease the activity of pro-inflammatory cytokines, such as interleukin-1 (IL-1) and tumour necrosis factor (TNF) by using their natural antagonists, namely interleukin-1 receptor antagonist (IL-1ra) and soluble TNF receptors. In addition the behaviour of macrophages can be altered by the effects of anti-inflammatory cytokines including interleukin-4 (IL-4), interleukin-13 (IL-13), interleukin-10 (IL-10), interleukin-6 (IL-6) and transforming growth factor-beta (TGF-beta). By deactivating the inflammatory response of macrophages these cytokines can favour resolution of disease. The ability to use these approaches in clinical practice remains elusive, however the prospect of using gene transfer technology to deliver anti-inflammatory factors directly to the site of inflammation and our increasing understanding of the complexity of the control of inflammation bring such therapies closer.

Initial cytokine exposure determines function of macrophages and renders them unresponsive to other cytokines.

The functional properties of infiltrating macrophages (Mphi) must be tightly regulated to facilitate appropriate responses to complex conditions in an inflammatory focus. This study was designed to ascertain whether uncommitted Mphi that have been exposed to combinations of cytokines with opposing functions develop properties dictated by one cytokine or by cytokine mixtures. Uncommitted rat bone marrow-derived Mphi (BMDMs) were incubated with IFN-gamma, TNF-alpha, TGF-beta, IL-4, IL-6, and IL-10 alone or sequentially in combinations. After 48 h, function was assessed by nitric oxide (NO) generation, uptake of apoptotic neutrophils, and beta-glucuronidase expression. IFN-gamma followed 4 h later by TNF-induced NO generation. The pretreatment of BMDMs before IFN-gamma priming with TNF, TGF-beta, and IL-4 suppressed NO generation by 87%, 92%, and 85%, respectively; IL-10 had no effect. The same cytokines administered at 4 h after IFN priming had no effect on NO generation. The uptake of apoptotic polymorphonuclear leukocytes was augmented by TNF (40% vs 29% controls; p < 0.05) and decreased by IFN-gamma, IL-10, and IL-4. The TNF response was unaffected by subsequent treatment with IFN-gamma, IL-4, or IL-10. Similarly, the decreased polymorphonuclear leukocyte uptake induced by IFN-gamma, IL-4, or IL-10 was unaffected by the subsequent addition of TNF. Beta-glucuronidase expression was increased by TGF-beta and decreased by IFN-gamma. These responses were not modified by cytokines with the opposing function. Thus, the functional response of BMDMs to complex mixtures of cytokines was determined by the first cytokine to which they were exposed. Once activated, BMDMs become unresponsive to alternative activating signals, a finding which has obvious implications for Mphi function in vivo.

Inhibiting inflammatory cytokines.

Acute glomerulonephritis is a common cause of renal dysfunction and ultimately renal failure. The inflammation involved is a tightly regulated response with pro- and anti-inflammatory cytokines playing key roles. Interleukin-1 (IL-1) and tumor necrosis factor (TNF) are the principal pro-inflammatory cytokines produced by intrinsic cells and infiltrating leukocytes. IL-1 and TNF can be directly antagonized using IL-1 receptor antagonist (IL-1ra) or binding proteins such as soluble receptors or antibodies. Alternatively, cytokines with anti-inflammatory properties can be used to decrease IL-1 and TNF synthesis, increase the production of their natural antagonists and deactivate inflammatory cells such as macrophages. This review will focus on these anti-inflammatory cytokines, principally IL-4, IL-6, IL-10 and IL-13, and highlight recent research of their activities in existing models of renal disease. The results of these experiments offer a promising new avenue of treatment.