Detection of nephritogenic antigen from the Lewis rat renal tubular basement membrane

Role of extracellular matrix renal tubulo-interstitial nephritis antigen (TINag) in cell survival utilizing integrin (alpha)vbeta3/focal adhesion kinase (FAK)/phosphatidylinositol 3-kinase (PI3K)/protein kinase B-serine/threonine kinase (AKT) signaling pathway

Tubulo-interstitial nephritis antigen (TINag) is an extracellular matrix protein expressed in tubular basement membranes. Combined mutations in TINag and nephrocystin-1 genes lead to nephronophthisis with reduced cell survival. Because certain extracellular matrix proteins are known to modulate cell survival, studies were initiated in Lewis rats lacking TINag to assess if they are more susceptible to cisplatin-induced injury. Cisplatin induced a higher degree of tubular cell damage and apoptosis in regions where TINag is expressed in a parental Wistar strain. This was accompanied by an accentuated increase in serum creatinine and Kim-1 RNA and renal expression of Bax, p53, and its nuclear accumulation, mtDNA fragmentation, and a decrease of Bcl-2. Cisplatin induced fulminant apoptosis of HK-2 cells with increased caspase3/7 activity, mtDNA fragmentation, and a reduced cell survival. These effects were partially reversed in cells maintained on TINag substratum. Far Western/solid phase assays established TINag binding with integrin αvβ3 comparable with vitronectin. Transfection of cells with αv-siRNA accentuated cisplatin-induced apoptosis, aberrant translocation of cytochrome c and Bax, and reduced cell survival. The αv-siRNA decreased expression of integrin-recruited focal adhesion kinase (FAK) and p-FAK, while increasing the expression of p53 and p-p53. Similarly, p-AKT was reduced although ILK was unaffected. Inhibition of PI3K had similar adverse cellular effects. These effects were ameliorated in cells on TINag substratum. In vivo, a higher degree of decrease in the expression of p-FAK and pAKT was observed in Lewis rats following cisplatin treatment. These in vivo and in vitro studies demonstrate an essential role of TINag in cellular survival to maintain proper tubular homeostasis utilizing integrin αvβ3 and downstream effectors.

Differential binding of cross-reactive anti-DNA antibodies to mesangial cells: the role of alpha-actinin

Target Ag display is a necessary requirement for the expression of certain immune-mediated kidney diseases. We previously had shown that anti-DNA Abs that cross-react with alpha-actinin may be important in the pathogenesis of murine and human lupus nephritis; in murine models, we had found that a significant proportion of pathogenic serum and kidney-deposited Igs are alpha-actinin reactive. Furthermore, a pathogenic anti-DNA/alpha-actinin Ab showed enhanced binding to immortalized mesangial cells (MCs) derived from a lupus prone MRL-lpr/lpr mouse as compared with MCs from BALB/c mice which are not susceptible to spontaneous lupus, suggesting that kidney alpha-actinin expression may be contributing to nephritis. In the current study, we established that two isoforms of alpha-actinin that are present in the kidney, alpha-actinin 1 and alpha-actinin 4, can both be targeted by anti-alpha-actinin Abs. We found novel sequence polymorphisms between MRL-lpr/lpr and BALB/c in the gene for alpha-actinin 4. Moreover, alpha-actinin 4 and a splice variant of alpha-actinin 1 were both expressed at significantly higher levels (mRNA and protein) in MCs from the lupus prone MRL-lpr/lpr strain. Significantly, we were able to confirm these differences in intact kidney by examining glomerular Ig deposition of anti-alpha-actinin Abs. We conclude that enhanced alpha-actinin expression may determine the extent of Ig deposition in the Ab-mediated kidney disease in lupus. Modulation of Ag expression may be a promising approach to down-regulate immune complex formation in the target organ in individuals with circulating pathogenic Abs.

Alpha-actinin is a cross-reactive renal target for pathogenic anti-DNA antibodies

Anti-DNA Abs commonly found in patients with systemic lupus erythematosus are thought to play an important pathogenic role in lupus nephritis. Anti-DNA Abs may contribute to renal disease by cross-reactivity with renal Ags, the identity of which remain elusive. To identify a target Ag for pathogenic anti-DNA Abs, we performed Western blotting and immunoprecipitations of mesangial cell lysates from the lupus-prone MRL-lpr/lpr mouse and a nonautoimmune BALB/c mouse with the pathogenic anti-DNA Ab R4A. We found that R4A (but not a nonpathogenic Ab mutant of R4A) binds to and immunoprecipitates a 100-kDa protein expressed on the cell surface and in lysates of MRL-lpr/lpr mesangial cells. DNase treatment of the lysate and of the R4A Ab did not effect binding, indicating that the binding of R4A to the 100-kDa protein was direct and not mediated by an antigenic bridge containing DNA. Binding was greatly diminished in BALB/c lysates, suggesting that Ag expression or availability at the level of the target organ may be a factor in determining susceptibility to lupus nephritis. Following identification of this 100-kDa protein as nonmuscle alpha-actinin, binding of R4A to alpha-actinin was confirmed by Western blot, ELISA, inhibition studies, and immunofluorescence. High titers of anti-alpha-actinin Abs were present in sera and kidney eluates of lupus mice with active nephritis. These results indicate that the nephritogenicity of some anti-DNA Abs may be mediated via cross-reactivity with alpha-actinin. Furthermore, variations in target Ag display between individuals may underlie differential susceptibility to anti-DNA Ab-induced renal disease.

Autoimmunity in heart disease: mechanisms and genetic susceptibility

Recent studies of heart disease suggest that immunologically mediated processes often accompany cardiac injury and can contribute to pathogenesis. Murine models of myocarditis have provided insight into the mechanisms by which autoimmune responses to cardiac antigens arise and cause tissue pathology. It is now evident that T cells, cytokines and antibodies can all contribute to cardiac injury. Furthermore, murine models have demonstrated that both the propensity to develop autoreactivity following cardiac injury and the vulnerability of the heart to these responses are under genetic control. Continued studies will help to identify susceptibility genes and might aid in the development of strategies to protect individuals at risk from immunologically mediated damage following cardiac injury.

Antibody-mediated autoimmune myocarditis depends on genetically determined target organ sensitivity

Injury to cardiac myocytes often leads to the production of anti-myosin antibodies. While these antibodies are a marker of myocardial injury, their contribution to pathogenesis in diseases such as autoimmune myocarditis or rheumatic fever is much less clear. We demonstrate in this report that monoclonal anti-myosin antibodies can mediate myocarditis in a susceptible mouse strain. Additionally, we show disease susceptibility depends on the presence of myosin or a myosin-like molecule in cardiac extracellular matrix. This study demonstrates that susceptibility to autoimmune heart disease depends not only on the activation of self-reactive lymphocytes but also on genetically determined target organ sensitivity to autoantibodies.

Tubulointerstitial changes as a major determinant in the progression of renal damage

Tubulointerstitial injury is an invariant finding in the chronically diseased kidney, irrespective of the type of disease or the compartment in which the disease originates. Such histologic changes are functionally significant in that scores for such damage, rather than glomerular injury, correlate with decline of renal function. This review summarizes (1) clinical evidence attesting to tubulointerstitial changes as an index of functional impairment, (2) mechanisms by which tubulointerstitial injury impairs renal function, and (3) interactions of pathologic processes in the vascular, glomerular, tubular, and interstitial compartments that culminate in tubulointerstitial injury. This report concludes with a review of interstitial fibrosis, a pathologic process regarded as an irreversible outcome from tubulointerstitial injury.

Distribution of tubulointerstitial nephritis antigen and evidence for multiple forms

A monoclonal antibody (A8) to a basement membrane component (TIN antigen), which is associated with autoimmune tubulointerstitial nephritis, was developed and utilized to characterize tissue distribution and properties of TIN antigen by immunofluorescence microscopy and immunoblotting. Results were confirmed with polyclonal goat anti-rabbit and human autoantibodies. TIN antigen was found in basement membranes of kidney cortex, small intestines, skin, and cornea, but was not detected in the renal medulla. Within the kidney cortex proximal tubular basement membrane (TBM) showed the strongest staining. TIN antigen was also detected in Bowman's capsule, distal TBM, peritubular capillaries, and focally in the interstitium, but not in glomerular basement membrane or mesangial matrix. Immunoblotting of SDS-extracted human, rabbit, mouse, and Brown Norway rat TBM with A8 revealed predominantly a 58 kD TIN antigen; however, other reactive components were detected in minor quantities. Bovine TBM contained components of 52 kD, 45 kD and 35 kD in varying concentrations. Immunoblotting of isolated rabbit TIN antigen revealed the major 58 kD component that was characterized previously, and minor components of 300 kD, 175 kD, 160 kD and 50 kD. TIN antigen was not detected in Lewis rat TBM by immunofluorescence or immunoblotting. These studies suggest the following: 1) TIN antigen may be synthesized as a high molecular weight glycoprotein that is processed to smaller forms; 2) it may be covalently associated with other basement membrane components; 3) the antibody reactive epitope may be present on multiple TBM components; and 4) high molecular weight forms may represent aggregates of TIN antigen.(ABSTRACT TRUNCATED AT 250 WORDS)