Phagocytosis of bacteria by proximal tubular epithelium in experimental pyelonephritis

Pathogenesis of urinary tract infection: an update

PURPOSE OF REVIEW

Urinary tract infection is the second most common bacterial infection in children. It may cause renal scarring leading to secondary hypertension and chronic kidney disease. Recent information has greatly improved our understanding of the pathogenesis of urinary tract infection and renal scarring.

RECENT FINDINGS

Urothelium, an anatomical barrier for innate immune responses, expresses toll-like receptors with the capacity to recognize pathogen-associated molecular patterns. Engagement of toll-like receptors can lead to uroepithelial cell activation and production of inflammatory mediators. These include complement proteins, other bactericidal peptides, cytokines, chemokines, defensins and adhesion molecules. The resulting inflammatory infiltrate serves to aid bacterial clearance but can also lead to renal damage. Furthermore, interactions between urinary proteins, such as Tamm-Horsfall protein, and TLR-4 add to the complexity of this defense system. Interindividual variability in cellular response may in part be responsible for variable clinical outcomes. Polymorphisms in a number of candidate genes in this host defense mechanism may be involved in determining those patients who are susceptible to recurrent infections and renal scarring following urinary tract infection.

SUMMARY

Further understanding of the basic molecular mechanisms of urinary tract infection and translating these bench data to the bedside holds the promise of improving diagnosis and therapeutic strategies of treating urinary tract infection and preventing recurrence and renal scarring.

Blocking of monocyte chemoattractant protein-1 during tubulointerstitial nephritis resulted in delayed neutrophil clearance

The chemokine monocyte chemoattractant protein (MCP)-1 has been implicated in the monocyte/macrophage infiltration that occurs during tubulointerstitial nephritis (TIN). We investigated the role of MCP-1 in rats with TIN by administering a neutralizing anti-MCP-1 antibody (Ab). We observed significantly reduced macrophage infiltration and delayed neutrophil clearance in the kidneys of TIN model rats treated with the anti-MCP-1 Ab. To exclude the possibility that an observed immune complex could affect the resolution of apoptotic neutrophils via the Fc receptor, TIN model rats were treated with a peptide-based MCP-1 receptor antagonist (RA). The MCP-1 RA had effects similar to those of the anti-MCP-1 Ab. In addition, MCP-1 did not affect macrophage-mediated phagocytosis of neutrophils in vitro. Deposition of the anti-MCP-1 Ab in rat kidneys resulted from its binding to heparan sulfate-immobilized MCP-1, as demonstrated by the detection of MCP-1 in both pull-down and immunoprecipitation assays. We conclude that induction of chemokines, specifically MCP-1, in TIN corresponds with leukocyte infiltration and that the anti-MCP-1 Ab formed an immune complex with heparan sulfate-immobilized MCP-1 in the kidney. Antagonism of MCP-1 in TIN by Ab or RA may alter the pathological process, most likely through delayed removal of apoptotic neutrophils in the inflammatory loci.

Epithelial secretion of C3 promotes colonization of the upper urinary tract by Escherichia coli

To assess the role of complement in renal infection, we studied a model of Escherichia coli-induced pyelonephritis in mice deficient in complement components C3 and C4. Renal infection occurred less frequently in C3- and C4-deficient mice compared with wild-type mice. In vitro, renal epithelial cells internalized fewer bacteria in the absence of C3 or in the presence of blockade of C3 bound to the bacteria. Moreover, upregulation of epithelial C3 production by stimulation with lipopolysaccharide enhanced bacterial internalization. Here we provide evidence that uropathogenic E. coli might use host C3 to invade the renal epithelium and that local complement production is sufficient for the bacteria to achieve this effect.

Demonstration of bacterial antigen in macrophages in experimental pyelonephritis

The immunomorphological characteristics of interstitial macrophages with PAS-positive granules were studied in experimental Escherichia coli pyelonephritis in rats, using an anti-E. coli antibody. Immunohistochemical, immunoelectron microscopical, as well as light- and electron microscopical findings were compared at twelve time points between 2 days and 13 weeks after infection. Macrophages with PAS-positive granules were present in the inflammatory infiltrates from the 7th day. The granules were phagolysosomes, filled predominantly with myelin figures. The myelin figures originated mainly from the constituents of the bacterial wall and reacted with the anti-E. coli antibody even 13 weeks after infection. The storage of bacterial residues with preserved antigenic structures for several weeks after infection indicates disturbed phagolysosomal elimination of the bacterial substances in the PAS-positive macrophages. In the formation of macrophages with PAS-positive granules, lysosomal overloading with large amounts of bacteria and cell debris is assumed, leading to consumption of the lysosomal enzymes, consequent incomplete breakdown and retention of the bacterial residues.

Role of superoxide dismutase in the pathogenesis of pyelonephritis: immunological localization of superoxide dismutase in human renal tissues

The enzyme superoxide dismutase affords a protective effect from renal scarring secondary to acute pyelonephritis in primates. To investigate the relationship between renal superoxide dismutase content and age we selected formalin-fixed normal human renal tissue from subjects of varying age, ranging from premature infant to adult, for immunostaining with human anti-superoxide dismutase antibody using the peroxidase-antiperoxidase technique. Sections that demonstrated acute pyelonephritis were immunostained for comparison. Immunostaining for superoxide dismutase was detected consistently in the proximal tubular cell cytoplasm in all specimens regardless of subject age. Superoxide dismutase was not detected in other segments of the nephron. In kidneys that demonstrated acute pyelonephritis we detected enhanced immunostaining in the proximal tubules, as well as increased background staining related to the inflammatory cells present. These results in conjunction with recent demonstrations of proximal tubular cell endocytosis of bacteria suggest that superoxide dismutase has an important role in mediating the initial events of pyelonephritis within the proximal tubular cell.

Acute human pyelonephritis: leukocytic infiltration of tubules and localization of bacteria

The fine structural details of how leukocytes appear in the lumen of tubules and the localization of bacteria in the tubulo-interstitial space were studied by light and electronmicroscopy in renal cortical biopsy specimens from three patients with acute pyelonephritis. The cells of interstitial infiltrates infiltrated and sometimes disrupted the cortical collecting tubules preferentially, while inflammatory infiltration of the proximal and distal convoluted tubules occurred more rarely. Since the emigration of tubular wall-localized individual leukocytes into the lumen was not observed even in long series of thin sections, focal inflammatory disruption of the uriniferous ducts was considered to be the morphological basis of the intratubular accumulation of leukocytes. The structural simplicity of the collecting tubular cells is suggested to be the reason for their preferential involvement in the drainage of the interstitial suppuration, although a role for specific carbohydrate receptors cannot be excluded. The bacteria were usually found within the neutrophilic granulocytes and macrophages of the interstitial infiltrates, and within and among the cells of leukocyte casts. Additionally, pure bacterial colonies were noticed in the lumen of a few collecting tubules. The problem of the adherence of the bacteria to the surface of the tubular cells is discussed.