Synthesis of complement protein C3 in the kidney is an important mediator of local tissue injury

Essential role of factor B of the alternative complement pathway in complement activation and opsonophagocytosis during acute pneumococcal otitis media in mice

We recently reported that the complement system plays a pivotal role in innate immune defense against Streptococcus pneumoniae during acute otitis media (OM) in mice. The current study was designed to determine which of the complement pathways are activated during acute pneumococcal OM and whether components of complement are expressed in the middle ear epithelium. Gene expression was determined by quantitative PCR, enzyme-linked immunosorbent assay (ELISA), and immunofluorescence staining. We found that S. pneumoniae induced increased gene expression of factor B of the alternative complement pathway and C3 in mouse middle ear epithelium. Activation of factor B and C3 in the middle ear lavage fluids was significantly greater than in simultaneously obtained serum samples as determined by Western blotting. Using mice deficient in complement C1qa, factor B, and factor B/C2, we found that complement C3 activation and opsonophagocytosis of S. pneumoniae were greatly attenuated in factor B- and factor B/C2-deficient mice. These findings support the concept that local complement activation is an important host innate immune response and that activation of the alternative complement pathway represents one of the innate immune defense mechanisms against pneumococcal infection during the early stage of acute OM.

Complement regulation of T-cell alloimmunity

PURPOSE OF REVIEW

The purpose of this review is to summarize recent findings implicating complement as an important regulator of T-cell immune responses. We then provide perspective for how these newly described mechanisms apply to allograft injury and how they could ultimately influence therapy.

RECENT FINDINGS

In addition to known effects of serum complement as an effector arm of antibody-initiated injury, T cells and antigen-presenting cells produce complement proteins and up-regulate complement receptors following cognate interactions. The locally released and activated, immune cell-derived complement signals predominantly through C3a and C5a binding to their receptors expressed on both partners to induce immune cell activation and differentiation. Complement deficiency or blockade limits T-cell-mediated autoimmunity and transplant rejection, whereas removal of the complement regulatory protein decay accelerating factor can enhance T-cell immunity and accelerate graft rejection.

SUMMARY

Emerging data indicate that immune cell-derived complement physiologically regulates immune cell survival and proliferation, modulating the strength and phenotype of adaptive T-cell immune responses involved in transplant rejection. The recognition of the diversity through which complement participates in allograft injury supports the need for continued design and testing of complement inhibitors in human transplant recipients.

Quantitative and qualitative urinary cellular patterns correlate with progression of murine glomerulonephritis

The kidney is a nonregenerative organ composed of numerous functional nephrons and collecting ducts (CDs). Glomerular and tubulointerstitial damages decrease the number of functional nephrons and cause anatomical and physiological alterations resulting in renal dysfunction. It has recently been reported that nephron constituent cells are dropped into the urine in several pathological conditions associated with renal functional deterioration. We investigated the quantitative and qualitative urinary cellular patterns in a murine glomerulonephritis model and elucidated the correlation between cellular patterns and renal pathology.

Urinary cytology and renal histopathology were analyzed in BXSB/MpJ (BXSB; glomerulonephritis model) and C57BL/6 (B6; control) mice. Urinary cytology revealed that the number of urinary cells in BXSB mice changed according to the histometric score of glomerulonephritis and urinary albumin; however, no correlation was detected for the levels of blood urea nitrogen and creatinine. The expression of specific markers for podocytes, distal tubules (DTs), and CDs was detected in BXSB urine. Cells immunopositive for Wilms tumor 1 (podocyte marker) and interleukin-1 family, member 6 (damaged DT and CD marker) in the kidney significantly decreased and increased in BXSB versus B6, respectively. In the PCR array analysis of inflammatory cytokines and chemokines, Il10, Cxcl2, C3, and Il1rn showed relatively higher expression in BXSB kidneys than in B6 kidneys. In particular, the highest expression of C3 mRNA was detected in the urine from BXSB mice. Furthermore, C3 protein and mRNA were localized in the epithelia of damaged nephrons.

These findings suggest that epithelial cells of the glomerulus, DT, and CD are dropped into the urine, and that these patterns are associated with renal pathology progression. We conclude that evaluation of urinary cellular patterns plays a key role in the early, noninvasive diagnosis of renal disease.

Evolving paradigms that determine the fate of an allograft

Despite the many advances in both immunological knowledge and the practical application of clinical immunosuppression, the holy grail of indefinite graft survival with immune tolerance in clinical solid organ transplantation remains a distant dream. The tremendous progress made in understanding the molecular and cellular basis of allograft rejection has not been translated into durable modalities that have advanced clinical care and outcomes. Indeed, currently used drugs and treatment protocols, largely directed at inhibiting alloreactive T cells, have not optimally improved allograft survival or function. A shift in emphasis, focusing on under appreciated immune pathways must now be considered to make further improvement. We highlight three areas of recent interest, complement, NK cells and lymphatics, which reinforce the concept that the transplant community must direct attention on how the immune system as a whole responds to a transplant. The current challenge is to integrate molecular, cellular and anatomic concepts to achieve the equivalent of a unified field theory of the immune response to organ transplants.

Novel aspects of complement in kidney injury

Complement activation is integral to the development and progression of multiple forms of kidney disease. The liver is the principal source of serum complement, but various kidney cell types and bone marrow-derived immune cells can produce a full array of complement proteins. Locally produced and activated complement yields cleavage products that function as vital intermediaries, amplifying inflammation in ischemia-reperfusion injury and transplant rejection, among other pathological states. Additional new studies indicate that during cognate T-cell-antigen presenting cell interactions, both cell types produce alternative pathway complement components. The resultant activation products have an essential role in T-cell activation, expansion, and differentiation, which in turn has a profound impact on the development of immune-mediated kidney disease. The recognition of an expanded role for kidney cell-derived and immune cell-produced complement as pathogenic to the kidney supports the need for future studies to test the efficacy of complement inhibitors in the prevention and/or treatment of selected kidney diseases.

A novel, complement-mediated way to enhance the interplay between macrophages, dendritic cells and T lymphocytes

Recently it has been reported that human C3-deficiency is associated with impairments in dendritic cell differentiation. Here we investigated how complement C3 influences the phenotype and functional activity of human dendritic cells. We show that human monocyte-derived dendritic cells (MDCs) when incubated with native, hemolytically active C3, bind the activation fragments of C3 covalently. This reaction directs MDCs to increase expression of MHCII, CD83 and CD86, moreover it results in a significantly enhanced secretion of TNF-alpha, IL-6 and IL-8. A further functional consequence of C3b-fixation is the elevated capacity of the dendritic cells to stimulate allogeneic T cells. The distinct role of covalently fixed C3-fragments is strongly supported by our results obtained with MDCs where CD11b expression was downregulated by siRNA. To reveal the possible in vivo significance of the present findings we modelled a phenomenon occurring during inflammation, where C3 is produced locally by activated macrophages. In these cocultures MDCs were found to fix substantial amounts of macrophage derived C3-fragments on their cell membrane. Our data provide compelling evidence that antigen presenting cells arising in complement-sufficient environment mature to competent stimulators of T cells.

Enhanced complement activation is part of the unfavourable cardiovascular risk profile in South Asians

South Asian immigrants in western societies exhibit a high burden of diabetes and subsequent vascular complications. Diabetic vascular complications are associated with vascular inflammation. We hypothesize that enhanced complement activation is involved. Therefore, levels of complement C3 and SC5b-9 - the soluble end product of complement activation - in a group of 200 South Asians were compared with an age- and sex-matched control group of native Caucasians. In addition, the association between complement levels and albuminuria, an indicator of renal damage and a cardiovascular risk marker, was assessed in the diabetic South Asian group. Compared with native Caucasians, South Asians had significantly higher levels of both serum C3 and plasma SC5b-9, even when only non-diabetic South Asians were considered. Diabetic South Asians had significantly higher C3 levels compared with non-diabetic South Asians. In diabetic South Asians, higher levels of SC5b-9 were associated with an increased prevalence of albuminuria (odds ratio 5.4, 95% confidence interval 1.8-15.8). These results suggest that enhanced complement activation is part of the unfavourable cardiovascular risk profile in South Asians.

Expression of complement components differs between kidney allografts from living and deceased donors

A disparity remains between graft survival of renal allografts from deceased donors and from living donors. A better understanding of the molecular mechanisms that underlie this disparity may allow the development of targeted therapies to enhance graft survival. Here, we used microarrays to examine whole genome expression profiles using tissue from 53 human renal allograft protocol biopsies obtained both at implantation and after transplantation. The gene expression profiles of living-donor kidneys and pristine deceased-donor kidneys (normal histology, young age) were significantly different before reperfusion at implantation. Deceased-donor kidneys exhibited a significant increase in renal expression of complement genes; posttransplantation biopsies from well-functioning, nonrejecting kidneys, regardless of donor source, also demonstrated a significant increase in complement expression. Peritransplantation phenomena, such as donor death and possibly cold ischemia time, contributed to differences in complement pathway gene expression. In addition, complement gene expression at the time of implantation was associated with both early and late graft function. These data suggest that complement-modulating therapy may improve graft outcomes in renal transplantation.

C3a mediates epithelial-to-mesenchymal transition in proteinuric nephropathy

Tubulointerstitial inflammation and progressive fibrosis are common pathways that lead to kidney failure in proteinuric nephropathies. Activation of the complement system has been implicated in the development of tubulointerstitial injury in clinical and animal studies, but the mechanism by which complement induces kidney injury is not fully understood. Here, we studied the effect of complement on the phenotype of tubular epithelial cells. Tubular epithelial cells exposed to serum proteins adopted phenotypic and functional characteristics of mesenchymal cells. Expression of E-cadherin protein decreased and expression of both alpha-smooth muscle actin protein and collagen I mRNA increased. Exposure of the cells to the complement anaphylotoxin C3a induced similar features. Treating with a C3a receptor (C3aR) antagonist prevented both C3a- and serum-induced epithelial-to-mesenchymal transition. In the adriamycin-induced proteinuria model, C3aR-deficient mice demonstrated less injury, preserved renal function, and improved survival compared with wild-type mice. Furthermore, the kidneys of C3aR-deficient mice had significantly less interstitial collagen I and alpha-smooth muscle actin. In summary, the complement anaphylotoxin C3a is an important mediator of glomerular and tubulointerstitial injury and can induce tubular epithelial-to-mesenchymal transition.

Gene expression analysis reveals new possible mechanisms of vancomycin-induced nephrotoxicity and identifies gene markers candidates

Vancomycin, one of few effective treatments against methicillin-resistant Staphylococcus aureus, is nephrotoxic. The goals of this study were to (1) gain insights into molecular mechanisms of nephrotoxicity at the genomic level, (2) evaluate gene markers of vancomycin-induced kidney injury, and (3) compare gene expression responses after iv and ip administration. Groups of six female BALB/c mice were treated with seven daily iv or ip doses of vancomycin (50, 200, and 400 mg/kg) or saline, and sacrificed on day 8. Clinical chemistry and histopathology demonstrated kidney injury at 400 mg/kg only. Hierarchical clustering analysis revealed that kidney gene expression profiles of all mice treated at 400 mg/kg clustered with those of mice administered 200 mg/kg iv. Transcriptional profiling might thus be more sensitive than current clinical markers for detecting kidney damage, though the profiles can differ with the route of administration. Analysis of transcripts whose expression was changed by at least twofold compared with vehicle saline after high iv and ip doses of vancomycin suggested the possibility of oxidative stress and mitochondrial damage in vancomycin-induced toxicity. In addition, our data showed changes in expression of several transcripts from the complement and inflammatory pathways. Such expression changes were confirmed by relative real-time reverse transcription–polymerase chain reaction. Finally, our results further substantiate the use of gene markers of kidney toxicity such as KIM-1/Havcr1, as indicators of renal injury.

New boundaries for complement in renal disease

Complement is an important component of the innate immune system whose function is integrated with the adaptive immune response. Since complement proteins are produced in virtually any cell in the body, it is important to question which pools of complement are responsible for what actions. This is particularly so in the case of complement-mediated renal disease, where distinct sites may require individualized approaches for therapy. From experimental and clinical evidence to date, it seems that the circulating pool of complement underlies much of the pathology traditionally associated with glomerular disease, including capillary wall injury. In contrast, the renal tubulointerstitium is the domain of local synthesis of complement, notably the axial component C3, principally expressed by the tubular epithelium. This means that therapeutic targeting will have to ensure penetration of the interstitial space in certain disorders. Likewise, monitoring of disease activity may benefit from evaluating this extravascular pool. Therapeutic and diagnostic applications in human disease are already taking this into account, with transplantation leading the way.