Low-dose 17α-ethinyl estradiol (EE) exposure exacerbates lupus renal disease and modulates immune responses to TLR7/9 agonists in genetically autoimmune-prone mice.

Estrogens have been shown to regulate the immune system and modulate multiple autoimmune diseases. 17α-ethinyl estradiol (EE), a synthetic analog of 17ß-estradiol, is prescribed commonly and found in oral contraceptives and hormone replacement therapies. Surprisingly, few studies have investigated the immunoregulatory effects of exposure to EE, especially in autoimmunity. In this study, we exposed autoimmune-prone female MRL/lpr mice to a human-relevant dose of EE through the oral route of exposure. Since lupus patients are prone to infections, groups of mice were injected with viral (Imiquimod, a TLR7 agonist) or bacterial (ODN 2395, a TLR9 agonist) surrogates. We then evaluated autoimmune disease parameters, kidney disease, and response to in vivo TLR7/9 pathogenic signals. EE-exposed mice had increased proteinuria as early as 7 weeks of age. Proteinuria, blood urea nitrogen, and glomerular immune complex deposition were also exacerbated when compared to controls. Production of cytokines by splenic leukocytes were altered in EE-exposed mice. Our study shows that oral exposure to EE, even at a very low dose, can exacerbate azotemia, increase clinical markers of renal disease, enhance glomerular immune complex deposition, and modulate TLR7/9 cytokine production in female MRL/lpr mice. This study may have implications for EE-exposure risk for genetically lupus-prone individuals.

Overactivity of Alternative Pathway Convertases in Patients With Complement-Mediated Renal Diseases.

Overactivation of the alternative pathway of the complement system is associated with the renal diseases atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy (C3G). C3 nephritic factors (C3NeF) play an important role in C3G pathogenesis by stabilizing the key enzymatic complex of complement, the C3 convertase. However, the reliability of assays detecting these autoantibodies is limited. Therefore, in this study, we validated and optimized a prototype hemolytic method for robust detection and characterization of factors causing convertase overactivity in large patient cohorts. The assay assesses convertase activity directly in the physiological milieu of serum and therefore is not restricted to detection of stabilizing autoantibodies such as C3NeF but may also reveal genetic variants resulting in prolonged convertase activity. We first defined clear cutoff values based on convertase activity in healthy controls. Next, we evaluated 27 C3G patient samples and found 16 positive for prolonged convertase activity, indicating the presence of factors influencing convertase stability. In three patients, the overactive convertase profile was persistent over disease course while in another patient the increased stability normalized in remission. In all these four patients, the convertase-stabilizing activity resided in the purified immunoglobulin (Ig) fraction, demonstrating the autoantibody nature. By contrast, the Igs of a familial aHUS patient carrying the complement factor B mutation p.Lys323Glu did not reveal convertase stabilization. However, in serum prolonged convertase activity was observed and segregated with the mutation in both affected and unaffected family members. In conclusion, we present a robust and reliable method for the detection, characterization, and evaluation over time of factors prolonging convertase activity (C3NeF or certain mutations) in patient cohorts. This assay may provide new insights in disease pathogenesis and may contribute to the development of more personalized treatment strategies.

Ubiquitin C-Terminal Hydrolase L1 is required for regulated protein degradation through the ubiquitin proteasome system in kidney.

Ubiquitin C-terminal hydrolase L1 (UCH-L1) is a major deubiquitinating enzyme of the nervous system and associated with the development of neurodegenerative diseases. We have previously shown that UCH-L1 is found in tubular and parietal cells of the kidney and is expressed de novo in injured podocytes. Since the role of UCH-L1 in the kidney is unknown we generated mice with a constitutive UCH-L1-deficiency to determine its role in renal health and disease. UCH-L1-deficient mice developed proteinuria, without gross changes in glomerular morphology. Tubular cells, endothelial cells, and podocytes showed signs of stress with an accumulation of oxidative-modified and polyubiquitinated proteins. Mechanistically, abnormal protein accumulation resulted from an altered proteasome abundance leading to decreased proteasomal activity, a finding exaggerated after induction of anti-podocyte nephritis. UCH-L1-deficient mice exhibited an exacerbated course of disease with increased tubulointerstitial and glomerular damage, acute renal failure, and death, the latter most likely a result of general neurologic impairment. Thus, UCH-L1 is required for regulated protein degradation in the kidney by controlling proteasome abundance. Altered proteasome abundance renders renal cells, particularly podocytes and endothelial cells, susceptible to injury.

Mir-17-92 regulates bone marrow homing of plasma cells and production of immunoglobulin G2c.

The polycistronic mir-17-92 cluster, also known as oncomir-1, was previously shown to be essential for early B lymphopoiesis. However, its role in late-stage B-cell differentiation and function remains unexplored. Here we ablate mir-17-92 in mature B cells and demonstrate that mir-17-92 is dispensable for conventional B-cell development in the periphery. Interestingly, mir-17-92-deficiency in B cells leads to enhanced homing of plasma cells to the bone marrow during T-cell-dependent immune response and selectively impairs IgG2c production. Mechanistically, mir-17-92 directly represses the expression of Sphingosine 1-phosphate receptor 1 and transcription factor IKAROS, which are, respectively, important for plasma cell homing and IgG2c production. We further show that deletion of mir-17-92 could reduce IgG2c anti-DNA autoantibody production and hence mitigate immune complex glomerulonephritis in Shp1-deficient mice prone to autoimmunity. Our results identify important roles for mir-17-92 in the regulation of peripheral B-cell function.

The NLRP3/ASC inflammasome promotes T-cell-dependent immune complex glomerulonephritis by canonical and noncanonical mechanisms.

Interleukin (IL)-1ß contributes to renal injury in immune complex glomerulonephritis. However, production of mature IL-1ß depends on activation of the inflammasome that cleaves pro-IL-1ß into its secretable form. A functional role of the NLRP3-containing inflammasome, which responds to various endogenous danger signals, was found in tubulointerstitial nephropathies, but its function in glomerular disease has not been established. To determine whether NLRP3 and its adapter molecule ASC contribute to glomerulonephritis, we induced T-cell-dependent autologous nephrotoxic serum nephritis in Nlrp3- and Asc-deficient mice. Renal expression of NLRP3/ASC inflammasome components and pro-IL-1ß increased during nephrotoxic serum nephritis and was abundant in renal dendritic cells. This was associated with renal production of mature IL-1ß, indicating inflammasome activation. Nlrp3 and Asc deficiency significantly attenuated glomerular injury, renal leukocyte infiltration, and T-cell activation. Production of mature IL-1ß was abrogated in Asc-deficient mice, consistent with a loss of inflammasome-dependent IL-1ß activation. Surprisingly, renal IL-1ß secretion remained intact in Nlrp3-deficient mice, indicating noncanonical pro-inflammatory effects of NLRP3 in autologous nephrotoxic serum nephritis. This may include NLRP3-mediated glomerular release of pro-inflammatory high-mobility group box 1 protein as a noncanonical function of NLRP3/ASC in glomerulonephritis. Thus, therapeutic blockade of the NLRP3/ASC/IL-1ß axis may be beneficial in glomerulonephritis.

Significance of glycosylphosphatidylinositol-anchored protein enrichment in lipid rafts for the control of autoimmunity.

Glycosylphosphatidylinositols (GPI) are complex glycolipids that are covalently linked to the C terminus of proteins as a post-translational modification and tether proteins to the plasma membrane. One of the most striking features of GPI-anchored proteins (APs) is their enrichment in lipid rafts. The biosynthesis of GPI and its attachment to proteins occur in the endoplasmic reticulum. In the Golgi, GPI-APs are subjected to fatty acid remodeling, which replaces an unsaturated fatty acid at the sn-2 position of the phosphatidylinositol moiety with a saturated fatty acid. We previously reported that fatty acid remodeling is critical for the enrichment of GPI-APs in lipid rafts. To investigate the biological significance of GPI-AP enrichment in lipid rafts, we generated a PGAP3 knock-out mouse (PGAP3(-/-)) in which fatty acid remodeling of GPI-APs does not occur. We report here that a significant number of aged PGAP3(-/-) mice developed autoimmune-like symptoms, such as increased anti-DNA antibodies, spontaneous germinal center formation, and enlarged renal glomeruli with deposition of immune complexes and matrix expansion. A possible cause for this was the impaired engulfment of apoptotic cells by resident peritoneal macrophages in PGAP3(-/-) mice. Mice with conditional targeting of PGAP3 in either B or T cells did not develop such autoimmune-like symptoms. In addition, PGAP3(-/-) mice exhibited the tendency of Th2 polarization. These data demonstrate that PGAP3-dependent fatty acid remodeling of GPI-APs has a significant role in the control of autoimmunity, possibly by the regulation of apoptotic cell clearance and Th1/Th2 balance.

The C5a receptor has a key role in immune complex glomerulonephritis in complement factor H-deficient mice.

Chronic serum sickness leads to the formation of glomerular immune complexes; however, C57BL/6 mice do not develop glomerulonephritis unless complement factor H (CFH) is absent from the plasma. Here we studied the role for C5a receptor (R) in this setting. The exaggerated humoral immune response in CFH(-/-) mice was normalized in CFH(-/-)C5aR(-/-) double knockout mice, highlighting the C5aR dependence. The CFH knockout mice developed proliferative glomerulonephritis with endocapillary F4/80+ macrophage infiltration, a process reduced in the double knockout mice. There was no interstitial inflammation by histologic criteria or flow cytometry for F4/80+ Ly6C(hi)CCR2(hi) inflammatory macrophages. There were, however, more interstitial CD3+ CD4+ T lymphocytes in CFH knockout mice with chronic serum sickness, while double knockout mice had greater than 5-fold more Ly6C(lo)CCR2(lo) anti-inflammatory macrophages compared to the CFH knockout mice. Mice lacking C5aR were significantly protected from functional renal disease as assessed by blood urea nitrogen levels. Thus, IgG- and iC3b-containing immune complexes are not inflammatory in C57BL/6 mice. Yet when these mice lack CFH, sufficient C3b persists in glomeruli to generate C5a and activate C5aR.

CD46 in innate and adaptive immunity: an update.

CD46 was discovered in 1986 during a search for novel C3b-binding proteins. CD46 is expressed ubiquitously and functions as a co-factor in the factor I-mediated proteolytic cleavage of C3b and C4b. Its vital role in preventing complement deposition on host tissue is underpinned by the fact that deficiency of CD46 is a predisposing factor for numerous disease conditions arising from complement-mediated 'self-attack'. However, in the last 10 years, it has become apparent that CD46 is also heavily involved in a new and somewhat surprising functional aspect of the complement system: the down-modulation of adaptive T helper type 1 (Th1) immune responses by regulating the production of interferon (IFN)-γ versus interleukin (IL)-10 within these cells. Specifically, this latter function of CD46 is a tantalizing discovery - it may not only have delivered the explanation as to why so many pathogens use and abuse CD46 as cell entry receptor but clearly has important clinical implications for the better understanding of Th1-mediated disease states and novel therapeutic approaches for their amelioration. Here, we summarize and discuss the current knowledge about CD46 and its expanding roles in the immune system.

A notable case report of May-Hegglin anomaly with immune complex-related nephropathy: a genetic and histological analysis.

May-Hegglin anomaly (MHA) is a rare autosomal dominant disease characterized by macrothrombocytopenia and leukocyte inclusions with microfilaments in the ribosomes. Mutations in the MYH9 gene, encoding non-muscle myosin heavy chain IIA (NMMHC-IIA) have been identified in patients with MHA and other MYH9-related diseases. Two young males (an older and younger brother) presented with macrothrombocytopenia and leukocyte inclusion bodies. Electron microscopy (EM) revealed parallel filaments in leukocyte inclusion bodies characteristic of MHA. Immunofluorescence microscopy (IF) showed NMMHC-IIA antibodies in 1 - 2 leukocyte inclusion bodies. These findings were consistent with MHA and they were identified to express the MYH9 mutation, D1424H. The older brother underwent a renal biopsy because of persistent proteinuria. Histology revealed mesangial proliferative glomerulonephritis with granular deposits of IgG and C1q. EM showed that the dense deposits were located in subendothelial cells, mesangial cells and Bowman's capsule. Immunocytochemistry revealed that NMMHC-IIA antibodies were localized in podocyte and endothelial cells in the glomerulus. Moreover, the expression of nephrin and podocin, slit diagram protein, was normal. An inflammatory mechanism may occur separately from MYH9-related disease. This report presents a case of MHA with immune complex-related nephropathy.

Abnormal immune complex processing and spontaneous glomerulonephritis in complement factor H-deficient mice with human complement receptor 1 on erythrocytes.

Complement receptor 1 (CR1) on human erythrocytes (Es) and complement factor H (CFH) on rodent platelets perform immune adherence, which is a function that allows the processing of immune complexes (ICs) bearing C3 by the mononuclear phagocyte system. Similar immune adherence occurs in the glomerular podocyte by CR1 in humans and CFH in rodents. As a model for human IC processing, we studied transgenic mice lacking CFH systemically but with human CR1 on Es. These CR1(hu)Tg/CFH(-/-) mice spontaneously developed proliferative glomerulonephritis, which was accelerated in a chronic serum sickness model by active immunization with heterologous apoferritin. ICs containing Ag, IgG and C3 bound to Es in CR1(hu)Tg/CFH(-/-) mice. In this setting, there was increased IC deposition in glomeruli, attributable to the presence of CR1 on Es, together with the absence of CFH on platelets and podocytes. In the absence of plasma CFH, the accumulated ICs activated complement, which led to spontaneous and chronic serum sickness-induced proliferative glomerulonephritis. These findings illustrate the complexities of complement-dependent IC processing by blood cells and in the glomerulus, and the importance of CFH as a plasma complement regulator.

Autoimmune diseases in small animals.

There are many autoimmune diseases recognized in humans; many of these have counterparts in companion animals. The diseases discussed in this article do not constitute the entire spectrum of autoimmune disease in these species. They are the common and better-described diseases of dogs and cats that have a well-documented autoimmune etiology. There are myriad autoimmune diseases that affect humans; similar diseases yet unrecognized in companion animals likely will be characterized in the future. The role of genetics in predisposition to autoimmunity is a common characteristic of these diseases in humans and animals. Likewise, the suggested role of environmental or infectious agents is another commonality between humans and their pets.

Urokinase-type plasminogen activator and arthritis progression: role in systemic disease with immune complex involvement.

INTRODUCTION: Urokinase-type plasminogen activator (u-PA) has been implicated in fibrinolysis, cell migration, latent cytokine activation, cell activation, T-cell activation, and tissue remodeling, all of which are involved in the development of rheumatoid arthritis. Previously, u-PA has been reported to play a protective role in monoarticular arthritis models involving mBSA as the antigen, but a deleterious role in the systemic polyarticular collagen-induced arthritis (CIA) model. The aim of the current study is to determine how u-PA might be acting in systemic arthritis models. METHODS: The CIA model and bone marrow chimeras were used to determine the cellular source of u-PA required for the arthritis development. Gene expression of inflammatory and destructive mediators was measured in joint tissue by quantitiative PCR and protein levels by ELISA. The requirement for u-PA in the type II collagen mAb-induced arthritis (CAIA) and K/BxN serum transfer arthritis models was determined using u-PA(-/-) mice. Neutrophilia was induced in the peritoneal cavity using either ovalbumin/anti-ovalbumin or the complement component C5a. RESULTS: u-PA from a bone marrow-derived cell was required for the full development of CIA. The disease in u-PA(-/-) mice reconstituted with bone marrow from C57BL/6 mice was indistinguishable from that in C57BL/6 mice, in terms of clinical score, histologic features, and protein and gene expression of key mediators. u-PA(-/-) mice were resistant to both CAIA and K/BxN serum transfer arthritis development. u-PA(-/-) mice developed a reduced neutrophilia and chemokine production in the peritoneal cavity following ovalbumin/anti-ovalbumin injection; in contrast, the peritoneal neutrophilia in response to C5a was u-PA independent. CONCLUSIONS: u-PA is required for the full development of systemic arthritis models involving immune complex formation and deposition. The cellular source of u-PA required for CIA is bone marrow derived and likely to be of myeloid origin. For immune complex-mediated peritonitis, and perhaps some other inflammatory responses, it is suggested that the u-PA involvement may be upstream of C5a signaling.

The complement system in systemic autoimmune disease.

Complement is part of the innate immune system. Its major function is recognition and elimination of pathogens via direct killing and/or stimulation of phagocytosis. Activation of the complement system is, however, also involved in the pathogenesis of the systemic autoimmune diseases. Activation via the classical pathway has long been recognized in immune complex-mediated diseases such as cryoglobulinemic vasculitis and systemic lupus erythematosus (SLE). In SLE, the role of complement is somewhat paradoxical. It is involved in autoantibody-initiated tissue damage on the one hand, but, on the other hand, it appears to have protective features as hereditary deficiencies of classical pathway components are associated with an increased risk for SLE. There is increasing evidence that the alternative pathway of complement, even more than the classical pathway, is involved in many systemic autoimmune diseases. This is true for IgA-dominant Henoch Schönlein Purpura, in which additional activation of the lectin pathway contributes to more severe disease. In anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis the complement system was considered not to be involved since immunoglobulin deposition is generally absent in the lesions. However, recent studies, both in human and animal models, demonstrated complement activation via the alternative pathway as a major pathogenic mechanism. Insight into the role of the various pathways of complement in the systemic autoimmune diseases including the vasculitides opens up new ways of treatment by blocking effector pathways of complement. This has been demonstrated for monoclonal antibodies to C5 or C5a in experimental anti-phospholipid antibody syndrome and ANCA-associated vasculitis.

Rheumatoid arthritis and systemic lupus erythematosus as immune complex diseases.

Rheumatoid factors are 9s IgM autoantibodies directed against the hinge regions of 7s IgG's that have been changed consequent to their encounter with a foreign antigen, such as those produced by oral bacteria. Occasionally self-aggregating 7s IgG's serve this function. When these complexes are taken up by phagocytes in the joint, they form the "RA cell," a cell analogous to the LE cell of Hargraves. The circulating complexes, which activate complement cascades in the joint, are not specific for RA, being found in other rheumatic and autoimmune diseases as well as having a low prevalence in the normal population. Recently, other antigens resulting in autoimmune complex formation with greater specificity for RA have been described. These antibodies, known as anti-cyclic citrullinated peptide (anti-CCP) antibodies recognize citrullinated protein residues, which are present as antigenic determinants in patients with RA. This is in contrast to systemic lupus erythematosus (SLE), another autoimmune disease characterized by immune complexes in the systemic circulation. In the case of SLE, 7s IgG's directed against several nuclear antigens localize mainly in the kidneys and blood vessels. They also produce cerebral and pulmonary disease by activating complement systemically. Genetic defects in the complement cascade associated with SLE result in inadequate clearance of immune complexes as well as apoptotic blebs containing autoantigens.

Mesangial immune complex glomerulonephritis due to complement factor D deficiency.

Complement factor D is a serine protease essential for the activation of the alternative pathway and is expressed in the kidney, adipocytes, and macrophages. Factor D is found at relatively high levels in glomeruli suggesting that this component of the complement cascade could influence renal pathophysiology. In this study, we utilize mice with a targeted deletion of the activating complement factor D gene and compare these results to mice with targeted deletion of the inhibitory complement factor H gene. Eight-month-old mice with a deleted factor D gene spontaneously develop albuminuria and have reduced creatinine clearance due to mesangial immune complex glomerulonephritis. These mesangial deposits contain C3 and IgM. In contrast to the mesangial location of the immune deposits in the factor D-deficient mice, age-matched factor H-deficient mice develop immune deposits along the glomerular capillary wall. Our observations suggest that complement factor D or alternative pathway activation is needed to prevent spontaneous accumulation of C3 and IgM deposits within the mesangium. Our studies show that the complement factor D gene knockout mice are a novel model of spontaneous mesangial immune complex glomerulonephritis.

Unexpected development of autoimmunity in BAFF-R-mutant MRL-lpr mice.

BAFF-R is the predominant receptor that mediates B-cell activating factor (BAFF)-dependent B-cell signalling and plays a critical role in late-stage B-cell maturation and survival. BAFF has been implicated in the development of autoimmunity and systemic lupus erythematosus (SLE). To define the role of BAFF-R in autoimmunity and SLE, we crossed A/WySnJ mice with MRL-lpr mice and generated BAFF-R-mutant MRL-lpr mice. The BAFF-R mutation markedly impaired the development of immature, mature and marginal zone B cells in the spleens of MRL-lpr mice. Unexpectedly, the BAFF-R mutation in MRL-lpr mice did not result in decreased autoantibody production, hypergammaglobulinaemia or immune complex-mediated glomerulonephritis. Rather, the ability of BAFF-R-mutant lpr splenic B cells to produce immunoglobulins in vitro was not decreased, although germinal centre formation, antibody response and B-cell proliferation were impaired. Further studies found increased numbers of B cells in the bone marrow of BAFF-R-mutant MRL-lpr mice compared to the BAFF-R-intact lupus mice. ELISPOT analysis revealed that BAFF-R-mutant MRL-lpr mice had more antibody-secreting cells in their bone marrow than the control mice. Thus, these findings could explain the development of autoimmunity and hypergammaglobulinaemia observed in BAFF-R-mutant MRL-lpr mice.

Peroxisome proliferator-activated receptor alpha protects against glomerulonephritis induced by long-term exposure to the plasticizer di-(2-ethylhexyl)phthalate.

Safety concerns about di-(2-ethylhexyl)phthalate (DEHP), a plasticizer and a probable endocrine disruptor, have attracted considerable public attention, but there are few studies about long-term exposure to DEHP. DEHP toxicity is thought to involve peroxisome proliferator-activated receptor alpha (PPARalpha), but this contention remains controversial. For investigation of the long-term toxicity of DEHP and determination of whether PPARalpha mediates toxicity, wild-type and PPARalpha-null mice were fed a diet that contained 0.05 or 0.01% DEHP for 22 mo. PPARalpha-null mice that were exposed to DEHP exhibited prominent immune complex glomerulonephritis, most likely related to elevated glomerular oxidative stress. Elevated NADPH oxidase, low antioxidant enzymes, and absence of the PPARalpha-dependent anti-inflammatory effects that normally antagonize the NFkappaB signaling pathway accompanied the glomerulonephritis in PPARalpha-null mice. The results reported here indicate that PPARalpha protects against the nephrotoxic effects of long-term exposure to DEHP.

NADPH-oxidase-driven oxygen radical production determines chondrocyte death and partly regulates metalloproteinase-mediated cartilage matrix degradation during interferon-gamma-stimulated immune complex arthritis.

In previous studies we have found that FcgammaRI determines chondrocyte death and matrix metalloproteinase (MMP)-mediated cartilage destruction during IFN-gamma-regulated immune complex arthritis (ICA). Binding of immune complexes (ICs) to FcgammaRI leads to the prominent production of oxygen radicals. In the present study we investigated the contribution of NADPH-oxidase-driven oxygen radicals to cartilage destruction by using p47phox-/- mice lacking a functional NADPH oxidase complex. Induction of a passive ICA in the knee joints of p47phox-/- mice resulted in a significant elevation of joint inflammation at day 3 when compared with wild-type (WT) controls as studied by histology. However, when IFN-gamma was overexpressed by injection of adenoviral IFN-gamma in the knee joint before ICA induction, a similar influx of inflammatory cells was found at days 3 and 7, comprising mainly macrophages in both mouse strains. Proteoglycan depletion from the cartilage layers of the knee joints in both groups was similar at days 3 and 7. Aggrecan breakdown in cartilage caused by MMPs was further studied by immunolocalisation of MMP-mediated neoepitopes (VDIPEN). VDIPEN expression in the cartilage layers of arthritic knee joints was markedly lower (between 30 and 60%) in IFN-gamma-stimulated arthritic p47phox-/- mice at day 7 than in WT controls, despite significant upregulation of mRNA levels of various MMPs such as MMP-3, MMP-9, MMP-12 and MMP-13 in synovia and MMP-13 in cartilage layers as measured with quantitative RT-PCR. The latter observation suggests that oxygen radicals are involved in the activation of latent MMPs. Chondrocyte death, determined as the percentage of empty lacunae in articular cartilage, ranged between 20 and 60% at day 3 and between 30 and 80% at day 7 in WT mice, and was completely blocked in p47phox-/- mice at both time points. FcgammaRI mRNA expression was significantly lower, and FcgammaRII and FcgammaRIII were higher, in p47phox-/- mice than in controls. NADPH-oxidase-driven oxygen radical production determines chondrocyte death and aggravates MMP-mediated cartilage destruction during IFN-gamma-stimulated IC-mediated arthritis. Upregulation of FcgammaRI by oxygen radicals may contribute to cartilage destruction.

The contested role of uracil DNA glycosylase in immunoglobulin gene diversification.

Class switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin (Ig) genes are initiated by the activation-induced cytosine deaminase AID. The resulting uracils in Ig genes were believed to be removed by the uracil glycosylase (UNG) and the resulting abasic sites treated in an error-prone fashion, creating breaks in the Ig switch regions and mutations in the variable regions. A recent report suggests that UNG does not act as a glycosylase in CSR and SHM but rather has unknown activity subsequent to DNA breaks that were created by other mechanisms.