Pathogenic significance of IgA receptor interactions in IgA nephropathy

IgA nephropathy (IgAN), the most common primary glomerulonephritis worldwide, frequently progresses to renal failure. The pathogenesis of this disease involves the deposition of undergalactosylated IgA1 complexes in the glomerular mesangium. How the IgA1 complexes are generated and why they are deposited in the mesangium remains unclear. We propose a model wherein two types of IgA receptors participate in sequential steps to promote the development of IgAN, with FcalphaRI (CD89) being initially involved in the formation of circulating IgA-containing complexes and, subsequently, transferrin receptor (CD71) in mediating mesangial deposition of IgA1 complexes.     

Difference in IgA1 O-glycosylation between IgA deposition donors and IgA nephropathy recipients

IgA nephropathy (IgAN) is the most common form of primary glomerulonephritis, and disease recurrence often occurs after transplantation. On the other hands, Asymptomatic IgA deposition (IgAD) is occasionally observed in donated kidney. It is recognized that IgAD does not progress to IgAN, but the mechanism has not demonstrated yet. In IgAN, aberrant IgA1 O-glycan structure in the hinge region (HR) of serum IgA is suggested as one of the most convincing key mediators. However, little is known about IgA1 O-glycan structure in IgAD patients. Herein, we investigated the prevalence of IgAD in living renal transplant donors in our cohort. IgAD was observed in 21(13.0%) among 161 renal transplant donors and have statistically significant blood relationship with IgAN recipients (28.6% in relatives vs. 9.8% in non-relatives, respectively; p?=?0.0073). Next, we evaluated the IgA1 O-glycan structure of serum IgA from IgAN recipients (n?=?26), IgAD donors (n?=?17), and non-IgAD helthy donors (n?=?27) using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI–TOF MS). The numbers of GalNAc and Gal and the Gal/GalNAc ratio in the HR of the IgAN recipients had significantly lower comparing to the IgAD and non-IgAD healthy donors. The decreased Gal/GalNAc ratio in IgAN recipients means the increased ratio of galactose-deficient IgA1. To the best of our knowledge, this is the first report to compare the O-glycan structures in IgAN recipients and IgAD donors using MALDI–TOF MS. We concluded that IgAD was more common in IgAN related donors. Overall, decreased GalNAc and Gal contents in HR could play a material pathogenic role in IgAN.     

Aberrant glycosylation of IgA from patients with IgA nephropathy

Despite the prominent role of IgA, particularly IgA₁, in the pathogenesis of IgA nephropathy (IgAN), the precise role of this molecule in the process remains unclear. Four biotin-conjugated lectins in sandwich-type enzyme-linked immunosorbent assays were devised to determine the glycosylation profiles of total IgA and its subclasses. We took advantage of differential binding properties of these lectins to sugar residues to dissect the oligosaccharide chainsO-linked to the hinge and thoseN-linked to the Fc region of total IgA and IgA subclasses in 47 patients with IgAN and an equal number of controls. The proportion of sialylated IgA₁ was higher in patients compared with controls (p<0.02), whereas IgA₂ in patients appeared less well sialylated. A reduction of galactose in pathological IgA as detected by RCA-I became significant after treatment of the molecule with neuraminidase (p<0.01). Defective galactosylation was also observed for patient IgA₁ when it was probed with ECL, a lectin that has a specificity for Gal 1,4N-acetylglucosamine groupings onN-linked oligosaccharides. The RCA and ECL results, therefore, suggest that increased sialylation on the IgA₁ is onO-linked oligosaccharides in the hinge region. This was partly confirmed by a small increase in the binding of PNA to IgA₁ from the patient group. This lectin binds preferentially to Gal 1,3N-acetylgalactosamine groups that are found onO-linked oligosaccharides.     

IgA:IgM and IgA:IgA hybrid hybridomas secrete heteropolymeric immunoglobulins that are polyvalent and bispecific

Polyvalent bispecific antibodies were secreted by hybrid hybridoma cells when both parental clones expressed a naturally polymerizing immunoglobulin. Hybrid hybridomas made from IgA lambda 2 anti-trinitrophenyl (TNP) and IgA kappa anti-phosphocholine (PC) parental cells secreted polymeric IgA antibodies that bound both TNP and PC. Some of the TNP binding was dissociated from the PC binding under conditions of mild reduction and alkylation suggesting that the bispecific polymeric IgA contained disulfide-linked parental monomers as well as bispecific hybrid monomers. Hybrid hybridomas constructed from IgA lambda 2 anti-TNP and IgM kappa anti-ox erythrocyte parental cells secreted bispecific, polymeric immunoglobulin that contained mu-, alpha-, kappa-, and lambda 2-chains. The mu and kappa-chains dissociated from the alpha- and lambda 2-chains under conditions of mild reduction and alkylation, indicating that both parental monomers had been incorporated into the same polymeric immunoglobulin to form a heteropolymeric antibody molecule. Heterologous pairing of alpha and mu heavy chains in monomers was not detected. Hybrid hybridomas constructed from IgA lambda 2 and IgG3 lambda 2 or IgA lambda 2 and IgG1 kappa parents co-secreted both parental immunoglobulins, but the antibodies secreted by these clones did not form heteropolymers or exhibit heterologous heavy chain pairing. These findings establish that polyvalent, bispecific, polymeric immunoglobulin molecules can be produced by hybrid hybridomas when both parents express a naturally polymerizing class of heavy chain but not when only one parent does. Hybrid hybridomas that produce heteropolymeric immunoglobulins are sources of high avidity bispecific antibodies that may find a number of basic and practical applications. The hybridoma cells that produce these antibodies may provide useful tools for investigating the in situ determinants of immunoglobulin chain association and the regulation of antibody assembly and secretion.     

IgA1 protease

IgA1 proteases are proteolytic enzymes that cleave specific peptide bonds in the human immunoglobulin A1 (IgA1) hinge region sequence. Several species of pathogenic bacteria secrete IgA1 proteases at mucosal sites of infection to destroy the structure and function of human IgA1 thereby eliminating an important aspect of host defence. IgA1 proteases are known as autotransporter proteins as their gene structure encodes the information to direct their own secretion out of the bacterial cell. The iga gene structure is also thought to contribute to the antigenic heterogeneity demonstrated by the IgA1 proteases during infections and the cleavage specificity of the IgA1 proteases for human IgA1. The IgA1 proteases have therefore been implicated as important virulence factors that contribute to bacterial infection and colonisation. The development of strategies to inactivate these IgA1 proteases has become the subject of recent research, as this has the potential to reduce bacterial colonisation at mucosal surfaces.     

IgA and IgA diphtheria antitoxin responses from human tonsil lymphocytes.

Human tonsil lymphocytes were stimulated with diphtheria toxoid and then cultured in a Marbrook culture system so that antibodies could be measured in the culture supernatant. Specific antibodies were measured with excess radiolabeled antigen and antisera specific for each immunoglobulin class. Good IgG and IgA diphtheria antitoxin responses have been obtained and responding culture supernatants were shown to neutralize toxin. The relationship between antitoxin response in vitro and immunization of donors with toxoid was investigated. It was found that at least two immunizations after the age of 6 months were necessary to prime the tonsils for an in vitro antibody response. The IgG and IgA in culture supernatants were demonstrated by immunodiffusion and were measured by radioimmunoassay. By sucrose density gradient ultracentrifugation, it was shown that 40% of the IgA produced in the cultures was greater than 7S. Evidence was obtained that neither the IgA nor the specific IgA antitoxin bears secretory piece. It appears that human lymphocytes from tonsils produce polymer IgA in vitro without secretory piece.     

孢素联合糖皮质激素对IgA肾病患者IgA、C3及IgA/C3影响研究

目的:探讨孢素联合糖皮质激素对IgA肾病患者IgA、C3及IgA/C3的影响。方法:我院收治的IgA肾病住院患者90例,按用药不同分为对照组与实验组。对照组予以醋酸泼尼松片口服,实验组在对照组基础上予以予环孢素软胶囊口服,治疗结束后对患者的血肌酐、血尿酸、24 h尿蛋白定量及IgA、C3、IgA/C3进行检测。结果:与对照组相比,实验组24 h尿蛋白定量水平较低,P0.05;IgA水平及IgA/C3水平较低,P0.05,差异具有统计学意义;两组患者的C3、血肌酐、血尿酸水平无显著差异,无统计学意义(P0.05)。结论:孢素联合糖皮质激素能够显著降低Ig A肾病患者Ig A水平及IgA/C3,对C3水平无影响,对临床有指导意义。     

Molecular basis of IgA nephropathy

IgA nephropathy (IgAN) is the most common glomerulonephritis worldwide and remains an important cause of end-stage renal failure. However, the basic molecular mechanism(s) underlying abnormal IgA synthesis, selective mesangial deposition with ensuing mesangial cell proliferation and extracellular matrix expansion remains poorly understood. Notably, the severity of tubulointerstitial lesions better predicts the renal progression than the degree of glomerular lesions. The task of elucidating the molecular basis of IgAN is made especially challenging by the fact that both environmental and genetic components likely contribute to the development and progression of IgAN. This review will summarize the earlier works on the structure of the IgA molecule, mechanisms of mesangial IgA deposition and pathophysiologic effects of IgA on mesangial cells following mesangial deposition. Recently, a series of important advances in the area of communication between the glomerular mesangium and renal tubular cells have emerged. These novel findings regarding the molecular pathogenesis of IgAN will be helpful in designing future directions for therapy.     

HLA and IgA deficiency

The distribution of 29 HLA-A and B antigens was compared in 50 Caucaso?ds with an IgA deficit and in 300 healthy controls. The patients were divided in 3 groups: 1) Partial selective IgA deficit (40); 2) Total selective Iga deficit (7); 3) IgA deficit associated with hypogammaglobulinemia (3). The patients viewed as a whole, we observed an increased frequency for the antigens HLA-Aw19, HLA-B5 and HLA-BW17. Yet, the modifications are not cleanly significant, with p less than 0.05, but p corrected not significant. We also considered the 3 groups separated and we did not remark any particular association with HLA. The data concerning HLA and congenital immune insufficiencies are reviewed. The most authors at once studied several immune defects. Only one Hungarian work was performed on IgA deficit. We do not confirm HLA-A1 and HLA-B8 increased frequencies, as it was reported, in Hungary, by Bajtai and al. There is no evident association between one HLA-A or B gene and the IgA deficit. The possible relation of IgA insufficiency with autoimmunity and allergy would justify complementary investigations, especially about HLA-D and Ia genes repartition in this disease.     

Family studies of IgA deficiency

Seventeen immunoglobulin A (IgA)-deficient subjects and other members from 13 families were examined at HLA-A, B and DR, C4A, C4B, and Bf loci. Of the 29 independent haplotypes in the IgA-deficient: subjects, 22 included deletions, duplications, or defects at the C4 or 21-hydroxylase loci. It is suggested that there may be a gene regulating serum IgA concentrations in this same region of chromosome 6. Three main supratypes explain most of the previously reported tHLA associations with IgA deficiency. These are A1, Cw7, B8, C4AQ0, C4B1, BfS, DR3, Bw65(14), C4A2, C4B1/2, BfS, and Bw57(17), C4A6, C4B1, BfS. All three are proposed to carry a gene for IgA deficiency, while other supratypes carrying the same B allele generally do not. Other supratypes possibly associated with IgA deficiency were also identified. A survey of about 150 individuals with at least 1 of the 3 main supratypes revealed only 2 IgA-deficient subjects, and these were among the 20 that had 2 of these supratypes. This suggests the possibility of a recessive mode of inheritance, with penetrance determined by another factor which is not major histocompatibility complex-linked. All the supratypes found in this group of IgA-deficient subjects would then carry the putative recessive allele for IgA deficiency.     

Resistance of normal serum IgA and secretory IgA to bacterial IgA proteases: evidence for the presence of enzyme-neutralizing antibodies in both serum and secretory IgA, and also in serum IgG

Normal serum IgA and secretory IgA (sIgA) of subclass IgA1 were isolated from pooled human serum and milk, respectively. They were tested for their susceptibility to bacterial IgA proteases from Haemophilus influenzae, Streptococcus pneumoniae, Neisseria gonorrhoeae, and Neisseria meningitidis that cleave IgA of only the IgA1 subclass. They were also tested for susceptibility to a novel IgA-protease from Clostridium ramosum that cleaves IgA of the IgA1 as well as the IgA2 subclass of the A2m(1) allotype. Both normal serum IgA1 and sIgA1 exhibited resistance to most IgA proteases. The one exception was the IgA protease from C. ramosum which readily cleaved both the serum IgA1 and sIgA1 into Fab and Fc fragments. Secretory component (SC) had nothing to do with the resistance of these IgAs. The resistance of these IgAs to most of the IgA proteases was found to be due to their enzyme-neutralizing antibody activity, since the Fab but not the Fc fragment of sIgA1 showed enzyme-inhibitory activity against these IgA proteases. Similar enzyme-neutralizing antibody activity was found in the pepsin-digested normal serum IgG-(Fab')2 fragment. These results indicate that the induction of the enzyme-neutralizing antibodies against the bacterial IgA proteases took place not only in mucosal sIgA but also in serum IgA and IgG. No enzyme-neutralizing antibody activity against the novel IgA-protease of C. ramosum was detected in any immunoglobulin preparations used in the present study or in the serum of a patient who carries the IgA protease-producing strain of C. ramosum in his feces.