Charge distribution of IgA-lambda in IgA nephropathy

Colocalization of IgG and IgA Heavy Chains with Kappa and Lambda Light Chains in Glomerular Deposits of IgA Nephropathy Patients Using High-Resolution Confocal Microscopy and Correlation with Oxford MEST-C Scores

Routine immunofluorescence microscopy of glomerular immunodeposits in IgA nephropathy shows IgA, C3, and lambda light chains, and sometimes IgG, IgM, and kappa light chains. However, a previous study using high-resolution confocal microscopy showed IgG in all IgA nephropathy cases, likely representing autoantibodies specific for galactose-deficient IgA1. Here, we used high-resolution confocal microscopy to examine the composition of glomerular immunodeposits and colocalization of kappa and lambda light chains with IgA or IgG heavy chains in kidney-biopsy samples from twenty patients with IgA nephropathy, seventeen without IgG, and nine with no or trace kappa light chains by routine immunofluorescence microscopy. IgG was detected in all biopsies by high-resolution confocal microscopy. Single-optical-plane images showed similar colocalization of IgG heavy chains with kappa and lambda light chains. Colocalization of IgA heavy chains was greater with lambda light chains than with kappa light chains. Colocalization of IgG heavy chain with kappa light chains was higher than with lambda light chains in biopsies with endocapillary hypercellularity and crescents, i.e., biopsies with active lesions. We confirmed the utility of high-resolution confocal microscopy to detect components of glomerular immunodeposits not apparent on routine immunofluorescence microscopy and for colocalization of different components, potentially clarifying the pathogenesis of IgA nephropathy.

Galactose-Deficient IgA1 B cells in the Circulation of IgA Nephropathy Patients Carry Preferentially Lambda Light Chains and Mucosal Homing Receptors

Background: Immunoglobulin A nephropathy (IgAN) primary glomerulonephritis is characterized by the deposition of circulating immune complexes (CIC) composed of polymeric (p)IgA1 molecules with altered O-glycans (Gd-IgA1) and anti-glycan antibodies in the kidney mesangium. The mesangial IgA deposits and serum IgA1 contain predominantly lambda (λ) light (L) chain, but the nature and origin of such IgA remains enigmatic Methods: We analyzed λ L chain expression in peripheral blood B cells of 30 IgAN patients, 30 healthy controls (HC), and 18 membranous nephropathy patients selected as disease controls (Non-IgAN). Results: In comparison to HC and Non-IgAN, in peripheral blood surface/membrane bound (mb)-Gd-IgA1⁺ cells from IgAN patients express predominantly λ L chain. In contrast, total mb-IgA⁺, mb-IgG⁺, and mb-IgM⁺ cells were preferentially positive for kappa (κ) L chain, in all analyzed groups. Although minor in comparison to κ L chain, λ L chain subsets of mb-IgG⁺, mb-IgM,⁺ and mb-IgA⁺ cells were significantly enriched in IgAN in comparison to Non-IgAN and/or HC. In contrast to HC, the peripheral blood of IgAN patients was enriched for λ⁺ mb-Gd-IgA1,⁺ CCR10,⁺ and CCR9⁺ cells, which preferentially home to the upper respiratory and digestive tract, respectively. Furthermore, we observed that mb-Gd-IgA1⁺ cell populations comprise more CD138⁺ cells and plasmablasts (CD38⁺) in comparison to total mb-IgA⁺ cells. Conclusions: Peripheral blood of IgAN patients is enriched for migratory λ⁺ mb-GdIgA1⁺ B cells, with the potential to home to mucosal sites where Gd-IgA1 could be produced during local respiratory or digestive tract infections.

Clinicopathological and prognostic study of primary IgA nephropathy with light chain λ restriction in the mesangial deposits

Introduction

Primary IgA nephropathy (IgAN) with light chain λ restriction in the mesangial deposits (IgAN-λ) has unique immunofluorescence (IF) features. Nevertheless, its clinicopathology and prognosis are still ambiguous.

Methods

From January 2002 to December 2020, the clinical and pathologic data of 3872 patients who were diagnosed with having primary IgAN by renal biopsy in our hospital were reviewed. A total of 187 patients who met the selection criteria for IgAN-λ were enrolled to conduct a retrospective single-center study. The selection criteria were that IF features conform to light chain λ restriction in the mesangial deposits. According to age, sex, renal function (estimated glomerular filtration rate [eGFR]), and follow-up time, the control group was constructed with 1:3 matched cases of IgAN. The clinicopathologic and prognostic differences between the 2 groups were analyzed.

Results

Compared with that in the IgAN group, the serum fibrinogen level in the IgAN-λ group was significantly higher (P < 0.001). Furthermore, cluster analysis indicated the different clusters involved in fibrinogen between the IgAN-λ and IgAN groups and that fibrinogen is associated with factors reflecting renal function in IgAN-λ but proteinuria levels in IgAN. The light chain λ deposit in the mesangium is associated with the formation of crescents in those with IgAN-λ, but complement C3 deposition in those with IgAN. Our Kaplan-Meier analysis revealed that the prognosis of the IgAN-λ group was significantly worse than that of the IgAN group within >6 years of follow-up (P = 0.02). The multi-Cox analysis revealed that the light chain λ restriction in the mesangial deposits was an independent risk factor for poor outcomes (eGFR decreased from the baseline ≥ 30% continuously or reached end-stage renal disease [ESRD] or died).

Conclusion

The prognosis of those with IgAN-λ was worse than that of those with IgAN, which may be attributed to the light chain λ restriction in the mesangial deposits inducing a significant systemic inflammation manifested as severe clinical features and frequent crescent.

IgA Structure Variations Associate with Immune Stimulations and IgA Mesangial Deposition

IgA1 mesangial deposition is the hallmark of IgA nephropathy and Henoch-Schönlein purpura, the onset of which often follows infections. Deposited IgA has been reported as polymeric, J chain associated, and often, hypogalactosylated but with no information concerning the influence of the IgA repertoire or the link between immune stimuli and IgA structure. We explored these issues in the α1KI mouse model, which produces polyclonal human IgA1 prone to mesangial deposition. Compared with mice challenged by a conventional environment, mice in a specific pathogen-free environment had less IgA deposition. However, serum IgA of specific pathogen-free mice showed more galactosylation and much lower polymerization. Notably, wild-type, α1KI, and even J chain-deficient mice showed increased polymeric serum IgA on exposure to pathogens. Strict germfree conditions delayed but did not completely prevent deposition; mice housed in these conditions had very low serum IgA levels and produced essentially monomeric IgA. Finally, comparing monoclonal IgA1 that had different variable regions and mesangial deposition patterns indicated that, independently of glycosylation and polymerization, deposition might also depend on IgA carrying specific variable domains. Together with IgA quantities and constant region post-translational modifications, repertoire changes during immune responses might, thus, modulate IgA propensity to deposition. These IgA features are not associated with circulating immune complexes and C3 deposition and are more pertinent to an initial IgA deposition step preceding overt clinical symptoms in patients.

Enzymatic sialylation of IgA1 O-glycans: implications for studies of IgA nephropathy

Patients with IgA nephropathy (IgAN) have elevated circulating levels of IgA1 with some O-glycans consisting of galactose (Gal)-deficient N-acetylgalactosamine (GalNAc) with or without N-acetylneuraminic acid (NeuAc). We have analyzed O-glycosylation heterogeneity of naturally asialo-IgA1 (Ale) myeloma protein that mimics Gal-deficient IgA1 (Gd-IgA1) of patients with IgAN, except that IgA1 O-glycans of IgAN patients are frequently sialylated. Specifically, serum IgA1 of healthy controls has more α2,3-sialylated O-glycans (NeuAc attached to Gal) than α2,6-sialylated O-glycans (NeuAc attached to GalNAc). As IgA1-producing cells from IgAN patients have an increased activity of α2,6-sialyltransferase (ST6GalNAc), we hypothesize that such activity may promote premature sialylation of GalNAc and, thus, production of Gd-IgA1, as sialylation of GalNAc prevents subsequent Gal attachment. Distribution of NeuAc in IgA1 O-glycans may play an important role in the pathogenesis of IgAN. To better understand biological functions of NeuAc in IgA1, we established protocols for enzymatic sialylation leading to α2,3- or α2,6-sialylation of IgA1 O-glycans. Sialylation of Gal-deficient asialo-IgA1 (Ale) myeloma protein by an ST6GalNAc enzyme generated sialylated IgA1 that mimics the Gal-deficient IgA1 glycoforms in patients with IgAN, characterized by α2,6-sialylated Gal-deficient GalNAc. In contrast, sialylation of the same myeloma protein by an α2,3-sialyltransferase yielded IgA1 typical for healthy controls, characterized by α2,3-sialylated Gal. The GalNAc-specific lectin from Helix aspersa (HAA) is used to measure levels of Gd-IgA1. We assessed HAA binding to IgA1 sialylated at Gal or GalNAc. As expected, α2,6-sialylation of IgA1 markedly decreased reactivity with HAA. Notably, α2,3-sialylation also decreased reactivity with HAA. Neuraminidase treatment recovered the original HAA reactivity in both instances. These results suggest that binding of a GalNAc-specific lectin is modulated by sialylation of GalNAc as well as Gal in the clustered IgA1 O-glycans. Thus, enzymatic sialylation offers a useful model to test the role of NeuAc in reactivities of the clustered O-glycans with lectins.

The study of Chinese medicinal herbal formula Shen San Fang in the treatment of experimental IgA nephropathy

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide, yet there is no effective or specific therapy. Shen San Fang (S3F) is a traditional Chinese herbal medicinal formula that has been used in China for many years to treat patients with hematuria. The aim of this study is to test the therapeutic value of S3F in an experimental model of IgAN. IgAN was induced in Lewis rats by continuous oral immunization with bovine gamma-globulin (BGG) in the drinking water for 8 weeks, followed by intravenous injection of 1 mg BGG daily for 3 successive days. The rats were randomly divided into four groups (five rats/group): control, control receiving S3F, induction of IgAN, and IgAN receiving S3E S3F decoction was fed to rats beginning week 4 from the first day of oral sensitization with BGG. The S3F treatment was continued until the rats were sacrificed or for a 4-week period. Hematuria, renal immunohistochemistry for IgA and transforming growth factor-beta 1 (TGF-beta1), renal histopathology, and renal content of TGF-beta1 were measured. Rats developing IgAN had marked hematuria, profound mesangial proliferation and mesangial expansion, intense and diffuse glomerular IgA deposition, increased glomerular TGF-beta1 expression, and raised renal TGF-beta1 levels. S3F treatment resulted in a significant reduction of hematuria, decreased mesangial IgA deposition, weaker immunostaining of TGF-beta1 in glomerulus, and a lower renal TGF-beta1 concentration. Our animal data suggests a therapeutic value for the Chinese medicinal formula S3F in experimental IgAN. This beneficial effect was due to reduced glomerular IgA deposition and TGF-beta1 expression. Our preliminary findings hold promise for future human therapy.

Polymeric IgA1 from patients with IgA nephropathy upregulates transforming growth factor-beta synthesis and signal transduction in human mesangial cells via the renin-angiotensin system

The effects of polymeric IgA1 (pIgA1) and monomeric IgA1 (mIgA1) from patients with IgA nephropathy (IgAN) on the renin-angiotensin system (RAS) and TGF-beta synthesis were examined in cultured human mesangial cells (HMC). Both pIgA1 and mIgA1 induced renin gene expression in HMC, in a dose-dependent manner. Similar findings were observed for TGF-beta gene and protein expression. The values measured in HMC incubated with pIgA1 were significantly higher than those in HMC incubated with equivalent amounts of mIgA1. When similar experiments were performed with the addition of either captopril or losartan, there was a significant increase in the renin gene expression by HMC, whereas the synthesis of TGF-beta was markedly reduced. The TGF-beta signal transduction pathways in HMC were studied by measuring the receptor-regulated Smad proteins (Smad 2 and 3) and common-partner Smad proteins (Smad 4). pIgA1 from patients with IgAN upregulated Smad activity in HMC, and the activity observed in HMC that had been preincubated with pIgA1 was readily suppressed with optimal concentrations of captopril or losartan. The effects of pIgA1 on the RAS were further examined in HMC incubated with IgA isolated from 30 patients with IgAN, 30 healthy subjects, and disease control subjects with other diseases. pIgA1 induction of angiotensin II or TGF-beta synthesis in HMC was significantly greater with preparations from patients with IgAN, compared with healthy or disease control subjects. The findings support a pathogenetic role of pIgA1 in IgAN through upregulation of the RAS and TGF-beta, leading to chronic renal failure with renal fibrosis.

Size-dependent binding of IgA to HepG2, U937, and human mesangial cells

IgA nephropathy (IgAN) is characterized by increased circulating IgA and mesangial IgA deposition. The mechanism of mesangial IgA deposition remains poorly understood in IgAN. In this report, we studied the binding characteristics of serum IgA from patients with IgAN and healthy controls to different cell types, including a liver-cell line (HepG2), a monocytic cell line (U937), and human mesangial cells (HMCs). Jacalin-bound proteins (JBPs) were purified from serum IgA by means of jacalin affinity chromatography. Total IgA concentrations were significantly higher in patients with IgAN than in controls (P <.001). JBPs were further separated by means of size exclusion chromatography, and six pooled fractions with molecular weight ranging from 50 to 1,000 kD were obtained. The concentration of low-molecular-weight (LMW) IgA complexes (150-300 kD) and high-molecular-weight (HMW) IgA complexes (300-1,000 kD) were significantly higher in patients than in healthy controls (P <.001 and.05, respectively). Cultured human mesangial cells bound more IgA of 300 to 610 kD in IgA isolated from patients with IgAN (P <.01). The binding of IgA (LMW and HMW) from patients with IgAN to HepG2 was significantly higher than that of IgA preparations from controls. U937 significantly bound more IgA of 150 to 825 kD in IgA isolated from patients with IgAN (P <.01). Different and distinct binding patterns were observed in the three cell types for IgA with different molecular weights. HMCs bound more HMW than LMW IgA. We noted preferential binding of LMW (150 to 300 kDa) and intermediate (350-710 kDa) IgA to HepG2 than of IgA complexes of more than 710 kDa. U937 mainly bound LMW and intermediate size IgA (150 to 710 kDa) with no binding of IgA with size greater than 710 kD. Our findings suggest that monocytes, hepatocytes, and mesangial cells have unique properties with regard to their binding to different forms of IgA. These characteristic properties may alter the catabolism of circulating IgA and, hence, predispose their deposition in the kidney mesangium in IgAN.

Increased sialylation of polymeric lambda-IgA1 in patients with IgA nephropathy

The mechanism of mesangial IgA deposition is poorly understood in IgA nephropathy (IgAN). Abnormal glycosylation of carbohydrate moieties in the hinge region of the IgA molecule has recently attracted much attention. In this report, we studied galactosylation and sialylation profiles in kappa- and lambda-IgA1 from patients with IgAN. Total serum IgA1 was isolated from patients with IgAN or healthy controls by jacalin-affinity chromatography. Six fractions of molecular weight (MW) 50-1,000 kDa were separated by fast protein liquid chromatography (FPLC). Four lectin-binding assays were used to study the sialylation and the presence of terminal galactose or N-acetylgalactosamine (GalNAc) in the O-linked carbohydrate moieties of kappa- or lambda-IgA1. Maackia amurensis agglutinin (MAA) and Sambucus nigra agglutinin (SNA) lectin recognize alpha(2,3)- and alpha(2,6)-linked sialic acid, respectively. Peanut agglutinin (PNA) and Helix aspersa (HA) lectin recognize terminal galactose and GalNAc, respectively. Reduced HA was demonstrated in macromolecular kappa or lambda-IgA1 (300-825 kDa) isolated from patients with IgAN (P < 0.05 compared with healthy controls). Lambda- but not kappa-IgA1 from patients with IgAN bound less to PNA (P < 0.05). The alpha(2,3)-linked sialic acid content in lambda- but not kappa-IgA1 of MW 150-610 kDa from patients was higher than that of controls (P < 0.005). The alpha(2,6)-linked sialic acid content in lambda-IgA1 (300-825 kDa) and kappa-IgA1 (150-610 kDa) from patients was also higher than that of controls. This unusual glycosylation and sialylation pattern of the lambda-IgA1 may have important implications for the pathogenesis of IgAN, as both the masking effect of sialic acid on galactose and the reduced galactosylation will hinder the clearance of macromolecular lambda-IgA1 by asialoglycoprotein receptor of hepatocytes. The negative charge from sialic acid may also favor mesangial deposition of macromolecular lambda-IgA1 in IgAN.

Progress in molecular and genetic studies of IgA nephropathy

Several new findings emerged recently from biochemical, genetic, and molecular studies of patients with IgA nephropathy. It appears that immunoglobulin A1-secreting cells of IgA nephropathy patients produce increased amounts of aberrantly glycosylated IgA1 in which the O-linked glycans in the hinge region are deficient in the content of galactose. The galactose-deficient IgA1 in the circulation is recognized by naturally occurring antibodies with anti-glycan specificity, and immune complexes are formed. These circulating immune complexes escape hepatic degradation and eventually are deposited in the kidney mesangium. Resident mesangial cells bind the IgA-containing immune complexes with the involvement of a novel IgA receptor and become activated. A familial form of IgA nephropathy has been linked to chromosome 6q22-23. Recent progress in molecular analyses of IgA nephropathy thus defines this disease as an autoimmune process with a novel IgA mesangial receptor and certain genetically determined traits.

Charge-dependent binding of polymeric IgA1 to human mesangial cells in IgA nephropathy

BACKGROUND

IgA nephropathy (IgAN) is characterized by raised serum IgA1 and predominant mesangial IgA1 deposits of polymeric nature. The mechanism of polymeric IgA1 (pIgA1) deposition in the kidney mesangium is poorly understood in IgAN. It has been suggested that increased sialic acid content and anionic charge of the pIgA1 molecules may be operational in the IgA1 deposition in human mesangial cells (HMCs). The present study examined the binding of pIgA1 with different surface charges to HMCs. The binding characteristics of IgA1 to HMCs in the presence of polycation (poly-L-lysine) or polyanion (heparin) were also investigated.

METHODS

IgA1 was purified in sera from patients with IgAN and from healthy controls by jacalin affinity chromatography. IgA1 was further separated into pIgA1 and monomeric IgA1 (mIgA1) by fast protein liquid chromatography (FPLC). pIgA1 or mIgA1 with different net charges on their surface were resolved by ion exchange chromatography (IEC) with a Mono Q column. The binding characteristics of pIgA1 and mIgA1 to HMCs in the presence or absence of polycation or polyanion were examined by flow cytometry.

RESULTS

In patients with IgAN, the absolute amount of mIgA1 and pIgA1 is significantly higher than that of healthy controls (P < 0. 001). There was significant increase in binding of pIgA1 from patients with IgAN to HMC and cell lysate. pIgA1 that interacted strongly with the ion exchanger also bound more to HMCs when compared with IgA1 interacted weakly with the ion exchanger (P < 0. 001). The anionic charged pIgA1 from patients was significantly higher than that of healthy controls (P < 0.001). Preincubation with poly-L-lysine increased the binding of pIgA1 to HMCs. The binding of pIgA1 to HMCs was decreased by preincubation with heparin.

CONCLUSIONS

The binding of IgA to HMCs is charge dependent. Polymeric IgA with the highest net negative charge binds more to HMCs. Preincubation with polyanion decreased the binding of polymeric IgA to HMCs. These results suggest an important role for anionic charge in IgA1 deposition onto the kidney mesangial cells.

Absence of CD89, polymeric immunoglobulin receptor, and asialoglycoprotein receptor on human mesangial cells

IgA nephropathy (IgAN) is characterized by raised serum IgA and predominant mesangial IgA deposits of polymeric nature. The expression of IgA receptor molecules in white blood cells and glomerular mesangial cells has recently attracted much attention in relation to the uptake of IgA by these cells. This study investigates the expression of IgA Fc receptor (Fc alphaR1 or CD89), asialoglycoprotein receptor (ASGPR), and polymeric Ig receptor (pIgR) in cultured glomerular mesangial cells. Using a sensitive nested reverse transcription-PCR, mRNA encoding for Fc alphaR1, pIgR, or the H2 chain of ASGPR was not demonstrated on human mesangial cells. U937, HepG2, and HT29 cell lines, used as positive controls, strongly expressed the Fc alphaR1, ASGPR, and pIgR mRNA, respectively, under similar experimental conditions. Flow cytometry also demonstrated the presence of surface proteins for Fc alphaR1, ASGPR, and pIgR on the respective control cell lines but not on human mesangial cells. Expression of Fc alphaR1 mRNA on cultured U937 cells was upregulated by tumor necrosis factor-alpha. However, tumor necrosis factor-alpha, interleukin-1beta, or transforming growth factor-beta failed to induce the expression of Fc alphaR1 on human mesangial cells. Human serum IgA or secretory IgA bound to human mesangial cells, HepG2, or the U937 cell line in a dose-dependent manner. The binding of purified IgA to human mesangial cells was not blocked by preincubation with human IgG, IgM, orosomucoid, asialo-orosomucoid, anti-CD89 antibody (My43), or anti-secretory component antibody. The present study concluded that there was an absence of Fc alphaR1, ASGPR, or pIgR on human mesangial cells. These findings suggest that the predominant binding of human IgA to human mesangial cells is mediated by other mechanisms.

Increased sialylation of polymeric immunoglobulin A1: mechanism of selective glomerular deposition in immunoglobulin A nephropathy?

Immunoglobulin A nephropathy (IgAN) is characterized by raised serum IgA and predominant mesangial IgA deposits of polymeric nature. The abnormal glycosylation of the carbohydrate moieties in the hinge region of the IgA molecule has recently attracted much attention. In this study we investigated the galactosylation and sialylation of monomeric and polymeric IgA1 isolated from patients with IgAN. Total IgA1 in serum samples from patients with IgAN or from healthy controls was isolated with a jacalin-agarose column as jacalin-bound protein (JBP). Monomeric and polymeric IgA1 were distinctly separated by fast protein liquid chromatography. Lectin binding assays were designed to examine the sialylation and the expression of terminal galactose and N-acetyl galactosamine of the O-linked carbohydrate in the hinge region of the IgA molecule. Reduced terminal galactosylation was demonstrated in serum IgA and monomeric IgA1 isolated from patients with IgAN as compared with results in healthy control subjects. However, a reduction in terminal galactosylation was not found in polymeric IgA1 isolated from patients with IgAN. Instead, increased sialylation of IgA1 (alpha2-3 linked to galactose) was demonstrated in polymeric IgA1. This abnormality of IgA1 could bear considerable implication on the pathogenesis of IgAN, because the masking effect of sialic acid may hinder the clearance of polymeric IgA1 by the asialoglycoprotein receptor (ASGP-R) of the liver cells. An increase in the sialylated content would also render the polymeric IgA from patients with IgAN more anionic. These immunochemical properties may contribute to the selective glomerular deposition of polymeric IgA1 in IgAN.

Analysis of charge distribution of lambda- and kappa-IgA in IgA nephropathy by focused antigen capture immunoassay

The finding that eluted mesangial IgA and serum IgA from patients with IgA nephropathy (IgAN) had a restricted anionic charge contrasting with normal serum IgA prompted us to examine the charge of kappa- and lambda-subclasses of IgA. Previous studies are not totally satisfactory because either total IgA without further dividing into subclass or only kappa- and lambda-IgA1 was examined. A new approach is described to study the electrostatic property of total IgA and its light chain subclasses. The new focused antigen capture immunoassay (FACIA) allows us to separate the immunoglobulins by isoelectric point, then to capture by the heavy chain class and finally to visualize according to the light chain class. This method works well with whole blood or serum without the need of prior purification by affinity chromatography. The serum total IgA and lambda-IgA levels in patients with IgAN were significantly higher than that of healthy controls. Elevated lambda-IgA levels in patients with IgAN resulted in reduced kappa/lambda ratio of total IgA. Similar to our previous findings in IgA1 with O-linked oligosaccharide side chain, lambda-IgA from patients with IgAN or health controls is highly anionic whereas kappa-IgA is relatively cationic. The higher anionic/cationic ratio observed in total IgA from patients compared with controls was due to the higher concentration of lambda-IgA from the former group. Raised anionic lambda-IgA in IgAN may be contributory to the immunopathogenesis through its selective mesangial binding.

Detection of hepatitis B virus DNA and RNA in kidneys of HBV related glomerulonephritis

Glomerular deposition of hepatitis B virus (HBV) antigens are observed in chronic HBsAg carriers with different glomerulonephritides yet the etiologic role of HBV remains uncertain. We examined the paraffin section of kidney biopsies from 40 chronic HBsAg carriers with membranous nephropathy (MGN), mesangiocapillary glomerulonephritis (MCGN) or IgA nephropathy (IgAN) for HBV DNA and HBV RNA using in situ hybridization (ISH). Glomerular HBV antigens were present in all biopsies by immunofluorescence. HBsAg or HBcAg mRNA was also studied in RNA extracted from frozen renal tissue using a two-step polymerase chain reaction (PCR) following reverse transcription (RT). HBcAg DNA was not easily detected with ISH alone, but was readily found in 31 biopsies (78%) following PCR. HBV DNA was detected mainly in the cytoplasm of proximal tubular epithelia but not in glomerular cells. HBsAg and/or HBcAg mRNA were detected by RT-PCR in extracted RNA from 13 biopsies (33%). The PCR findings were further confirmed by (a) Southern blot hybridization using a cloned HBV probe and (b) absence of PCR product following treating RNA with RNase or omitting the RT. It is plausible that HBV DNA in renal tubules represents endocytosis of HBV DNA in the urinary filtrate and the HBV RNA extracted from kidney biopsies could derive from infiltrating cells bearing HBV RNA. Hence, ISH with specific HBV core gene RNA probe was performed subsequently. HBcAg RNA, localized in the nuclei and cytoplasm of glomerular and tubular cells, was detected in 56%, 20%, and 36% of renal biopsies in chronic HBsAg carriers with MGN, MCGN, and IgAN, respectively. Our findings indicate the presence of viral transcription in glomerular cells and renal tubular epithelia, supporting an etiological role of HBV in some chronic HBsAg carriers who develop coexisting glomerulonephritides.

Increased binding of polymeric lambda-IgA to cultured human mesangial cells in IgA nephropathy

IgA nephropathy (IgAN) is characterized by raised plasma lambda-IgA1 and mesangial polymeric lambda-IgA1 deposits. It remains uncertain whether the predominant glomerular lambda-IgA1 deposits represent a selective uptake of polymeric IgA or a non-specific uptake due to elevated circulating lambda-IgA1 levels in response to an unidentified antigen. In this study, we explored whether there is an increased binding of monomeric IgA1 (mIgA1) or polymeric IgA1 (pIgA1) from patients with IgAN to cultured human mesangial cells (HMC). Total IgA1 in plasma from patients or healthy controls was isolated by jacalin-agarose column as jacalin-bound proteins (JBP). Monomeric IgA1 and pIgA1 were distinctly separated by FPLC. HMC were incubated with IgA preparations and IgA bound to HMC was determined by flow cytometry analysis using standard curves constructed by known concentrations of kappa-IgA1 or lambda-IgA1. In order to avoid any increased binding of IgA to HMC due to elevated kappa- or lambda-IgA concentrations in JBP samples from patients, JBP samples from patients or controls were appropriately diluted to achieve comparable levels of total IgA1. No differences in the total mIgA1 or pIgA1 concentration, percentage of mIgA1 or pIgA1, or the kappa/lambda ratio of mIgA1 or pIgA1 were found between adjusted JBP samples from patients or healthy controls. We found a sharp rise in percentage of pIgA1 among IgA1 bound to HMC (70%), despite the fact that only 3% of the IgA1 in the adjusted JBP samples were polymeric, suggesting that pIgA1 had a higher affinity to HMC than mIgA1. Furthermore, the kappa/lambda ratios of pIgA1 bound to HMC were significantly lower than the kappa/lambda ratios of pIgA1 in adjusted JBP only with IgAN patients but not healthy controls (P = 0.0026). Our data suggest a preferential mesangial binding of polymeric lambda-IgA1 from patients with IgAN. These polymeric lambda-IgA immune complexes are likely to be "pathogenic" and are important in the pathogenesis of IgAN.