DNA polymerase-α regulates the activation of type I interferons through cytosolic RNA:DNA synthesis.

Aberrant nucleic acids generated during viral replication are the main trigger for antiviral immunity, and mutations that disrupt nucleic acid metabolism can lead to autoinflammatory disorders. Here we investigated the etiology of X-linked reticulate pigmentary disorder (XLPDR), a primary immunodeficiency with autoinflammatory features. We discovered that XLPDR is caused by an intronic mutation that disrupts the expression of POLA1, which encodes the catalytic subunit of DNA polymerase-α. Unexpectedly, POLA1 deficiency resulted in increased production of type I interferons. This enzyme is necessary for the synthesis of RNA:DNA primers during DNA replication and, strikingly, we found that POLA1 is also required for the synthesis of cytosolic RNA:DNA, which directly modulates interferon activation. Together this work identifies POLA1 as a critical regulator of the type I interferon response.

Trex1 regulates lysosomal biogenesis and interferon-independent activation of antiviral genes.

The sensing of viral nucleic acids by the innate immune system triggers the production of type I interferons, which activates interferon-stimulated genes (ISGs) and directs a multifaceted antiviral response. ISGs can also be activated through interferon-independent pathways, although the precise mechanisms remain elusive. Here we found that the cytosolic exonuclease Trex1 regulated the activation of a subset of ISGs independently of interferon. Both Trex1(-/-) mouse cells and Trex1-mutant human cells had high expression of genes encoding antiviral molecules ('antiviral genes') and were refractory to viral infection. The interferon-independent activation of antiviral genes in Trex1(-/-) cells required the adaptor STING, the kinase TBK1 and the transcription factors IRF3 and IRF7. We also found that Trex1-deficient cells had an expanded lysosomal compartment, altered subcellular localization of the transcription factor TFEB and diminished activity of the regulator mTORC1. Together our data identify Trex1 as a regulator of lysosomal biogenesis and interferon-independent activation of antiviral genes and show that dysregulation of lysosomes can elicit innate immune responses.

Loss-of-function variants in SAT1 cause X-linked childhood-onset systemic lupus erythematosus.

OBJECTIVES: Families that contain multiple siblings affected with childhood onset of systemic lupus erythematosus (SLE) likely have strong genetic predispositions. We performed whole exome sequencing (WES) to identify familial rare risk variants and to assess their effects in lupus. METHODS: Sanger sequencing validated the two ultra-rare, predicted pathogenic risk variants discovered by WES and identified additional variants in 562 additional patients with SLE. Effects of a splice site variant and a frameshift variant were assessed using a Minigene assay and CRISPR/Cas9-mediated knock-in (KI) mice, respectively. RESULTS: The two familial ultra-rare, predicted loss-of-function (LOF) SAT1 variants exhibited X-linked recessive Mendelian inheritance in two unrelated African-American families. Each LOF variant was transmitted from the heterozygous unaffected mother to her two sons with childhood-onset SLE. The p.Asp40Tyr variant affected a splice donor site causing deleterious transcripts. The young hemizygous male and homozygous female Sat1 p.Glu92Leufs*6 KI mice spontaneously developed splenomegaly, enlarged glomeruli with leucocyte infiltration, proteinuria and elevated expression of type I interferon-inducible genes. SAT1 is highly expressed in neutrophils and encodes spermidine/spermine-N1-acetyltransferase 1 (SSAT1), a rate-limiting enzyme in polyamine catabolism. Young male KI mice exhibited neutrophil defects and decreased proportions of Foxp3 +CD4+ T-cell subsets. Circulating neutrophil counts and proportions of Foxp3 +CD4+ T cells correlated with decreased plasma levels of spermine in treatment-naive, incipient SLE patients. CONCLUSIONS: We identified two novel SAT1 LOF variants, showed the ability of the frameshift variant to confer murine lupus, highlighted the pathogenic role of dysregulated polyamine catabolism and identified SAT1 LOF variants as new monogenic causes for SLE.

Amino acid signatures of HLA Class-I and II molecules are strongly associated with SLE susceptibility and autoantibody production in Eastern Asians.

Human leukocyte antigen (HLA) is a key genetic factor conferring risk of systemic lupus erythematosus (SLE), but precise independent localization of HLA effects is extremely challenging. As a result, the contribution of specific HLA alleles and amino-acid residues to the overall risk of SLE and to risk of specific autoantibodies are far from completely understood. Here, we dissected (a) overall SLE association signals across HLA, (b) HLA-peptide interaction, and (c) residue-autoantibody association. Classical alleles, SNPs, and amino-acid residues of eight HLA genes were imputed across 4,915 SLE cases and 13,513 controls from Eastern Asia. We performed association followed by conditional analysis across HLA, assessing both overall SLE risk and risk of autoantibody production. DR15 alleles HLA-DRB1*15:01 (P = 1.4x10-27, odds ratio (OR) = 1.57) and HLA-DQB1*06:02 (P = 7.4x10-23, OR = 1.55) formed the most significant haplotype (OR = 2.33). Conditioned protein-residue signals were stronger than allele signals and mapped predominantly to HLA-DRB1 residue 13 (P = 2.2x10-75) and its proxy position 11 (P = 1.1x10-67), followed by HLA-DRB1-37 (P = 4.5x10-24). After conditioning on HLA-DRB1, novel associations at HLA-A-70 (P = 1.4x10-8), HLA-DPB1-35 (P = 9.0x10-16), HLA-DQB1-37 (P = 2.7x10-14), and HLA-B-9 (P = 6.5x10-15) emerged. Together, these seven residues increased the proportion of explained heritability due to HLA to 2.6%. Risk residues for both overall disease and hallmark autoantibodies (i.e., nRNP: DRB1-11, P = 2.0x10-14; DRB1-13, P = 2.9x10-13; DRB1-30, P = 3.9x10-14) localized to the peptide-binding groove of HLA-DRB1. Enrichment for specific amino-acid characteristics in the peptide-binding groove correlated with overall SLE risk and with autoantibody presence. Risk residues were in primarily negatively charged side-chains, in contrast with rheumatoid arthritis. We identified novel SLE signals in HLA Class I loci (HLA-A, HLA-B), and localized primary Class II signals to five residues in HLA-DRB1, HLA-DPB1, and HLA-DQB1. These findings provide insights about the mechanisms by which the risk residues interact with each other to produce autoantibodies and are involved in SLE pathophysiology.

Serum IgG Profiling of Toddlers Reveals a Subgroup with Elevated Seropositive Antibodies to Viruses Correlating with Increased Vaccine and Autoantigen Responses.

PURPOSE: The human antibody repertoire forms in response to infections, the microbiome, vaccinations, and environmental exposures. The specificity of such antibody responses was compared among a cohort of toddlers to identify differences between seropositive versus seronegative responses. METHODS: An assessment of the serum IgM and IgG antibody reactivities in 197 toddlers of 1- and 2-years of age was performed with a microfluidic array containing 110 distinct antigens. Longitudinal profiling was done from years 1 to 2. Seropositivity to RNA and DNA viruses; bacteria; live attenuated, inactive, and subunit vaccines; and autoantigens was compared. A stratification was developed based on quantitative variations in the IgG responses. Clinical presentations and previously known genetic risk alleles for various immune system conditions were investigated in relation to IgG responses. RESULTS: IgG reactivities stratified toddlers into low, moderate, and high responder groups. The high group (17%) had elevated IgG responses to multiple RNA and DNA viruses (e.g., respiratory syncytial virus, Epstein-Barr virus, adenovirus, Coxsackievirus) and this correlated with increased responses to live attenuated viral vaccines and certain autoantigens. This high group was more likely to be associated with gestational diabetes and an older age. Genetic analyses identified polymorphisms in the IL2RB, TNFSF4, and INS genes in two high responder individuals that were associated with their elevated cytokine levels and clinical history of eczema and asthma. CONCLUSION: Serum IgG profiling of toddlers reveals correlations between the magnitude of the antibody responses towards viruses, live attenuated vaccines, and certain autoantigens. A low responder group had much weaker responses overall, including against vaccines. The serum antibody screen also identifies individuals with IgG responses to less common infections (West Nile virus, parvovirus, tuberculosis). The characterization of the antibody responses in combination with the identification of genetic risk alleles provides an opportunity to identify children with increased risk of clinical disease.

Statin Intolerance, Anti-HMGCR Antibodies, and Immune Checkpoint Inhibitor-Associated Myositis: A "Two-Hit" Autoimmune Toxicity or Clinical Predisposition?

Immune-related adverse events induced by immune checkpoint inhibitor (ICI) therapy may affect diverse organ systems, including skeletal and cardiac muscle. ICI-associated myositis may result in substantial morbidity and occasional mortality. We present a case of a patient with advanced non-small cell lung cancer who developed grade 4 myositis with concurrent myocarditis early after initiation of anti-programmed death ligand 1 therapy (durvalumab). Autoantibody analysis revealed marked increases in anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase antibody levels that preceded clinical toxicity, and further increased during toxicity. Notably, the patient had a history of intolerable statin myopathy, which had resolved clinically after statin discontinuation and prior to ICI initiation. This case demonstrates a potential association between statin exposure, autoantibodies, and ICI-associated myositis.

Late-Onset Immunotherapy Toxicity and Delayed Autoantibody Changes: Checkpoint Inhibitor-Induced Raynaud's-Like Phenomenon.

Immune checkpoint inhibitor (ICI)-induced immune-related adverse events (irAEs) may affect almost any organ system and occur at any point during therapy. Autoantibody analysis may provide insight into the mechanism, nature, and timing of these events. We report a case of ICI-induced late-onset Raynaud's-like phenomenon in a patient receiving combination immunotherapy. A 53-year-old woman with advanced non-small lung cancer received combination anti-cytotoxic T-lymphocyte antigen 4 and anti-programmed death 1 ICI therapy. She developed early (hypophysitis at 4 months) and late (Raynaud's at >20 months) irAEs. Longitudinal assessment of 124 autoantibodies was correlated with toxicity. Although autoantibody levels were generally stable for the first 18 months of therapy, shortly before the development of Raynaud's, a marked increase in multiple autoantibodies was observed. This case highlights the potential for delayed autoimmune toxicities and provides potential biologic insights into the dynamic nature of these events. KEY POINTS: A patient treated with dual anti-PD1 and anti-CTLA4 therapy developed Raynaud's-like signs and symptoms more than 18 months after starting therapy. In this case, autoantibody changes became apparent shortly before onset of clinical toxicity. This case highlights the potential for late-onset immune-related adverse events checkpoint inhibitors, requiring continuous clinical vigilance. The optimal duration of checkpoint inhibitor therapy in patients with profound and prolonged responses remains unclear.

Optimal germinal center responses require a multistage T cell:B cell adhesion process involving integrins, SLAM-associated protein, and CD84.

CD4(+) T cells deficient in signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) exhibit a selective impairment in adhesion to antigen-presenting B cells but not dendritic cells (DCs), resulting in defective germinal center formation. However, the nature of this selective adhesion defect remained unclear. We found that whereas T cell:DC interactions were primarily integrin dependent, T cell:B cell interactions had both an early integrin-dependent phase and a sustained phase that also required SAP. We further found that the SLAM family member CD84 was required for prolonged T cell:B cell contact, optimal T follicular helper function, and germinal center formation in vivo. Moreover, both CD84 and another SLAM member, Ly108, mediated T cell adhesion and participated in stable T cell:B cell interactions in vitro. Our results reveal insight into the dynamic regulation of T cell:B cell interactions and identify SLAM family members as critical components of sustained T cell:B cell adhesion required for productive humoral immunity.

RAD51 interconnects between DNA replication, DNA repair and immunity.

RAD51, a multifunctional protein, plays a central role in DNA replication and homologous recombination repair, and is known to be involved in cancer development. We identified a novel role for RAD51 in innate immune response signaling. Defects in RAD51 lead to the accumulation of self-DNA in the cytoplasm, triggering a STING-mediated innate immune response after replication stress and DNA damage. In the absence of RAD51, the unprotected newly replicated genome is degraded by the exonuclease activity of MRE11, and the fragmented nascent DNA accumulates in the cytosol, initiating an innate immune response. Our data suggest that in addition to playing roles in homologous recombination-mediated DNA double-strand break repair and replication fork processing, RAD51 is also implicated in the suppression of innate immunity. Thus, our study reveals a previously uncharacterized role of RAD51 in initiating immune signaling, placing it at the hub of new interconnections between DNA replication, DNA repair, and immunity.

Fatty Acid Amide Hydrolase Regulates Peripheral B Cell Receptor Revision, Polyreactivity, and B1 Cells in Lupus.

C57BL/6 mice bearing the Sle2(z) lupus-susceptibility congenic interval on chromosome 4 display high titers of polyclonal autoantibodies with generalized B cell hyperactivity, hallmarks of systemic lupus erythematosus. In B6.Sle2(z)HEL(Ig).sHEL BCR-transgenic mice, Sle2(z) did not breach central tolerance, but it led to heightened expression of endogenous Ig H and L chains in splenic B cells, upregulation of RAG, and serological polyreactivity, suggestive of excessive receptor revision. Fatty acid amide hydrolase (FAAH), a gene in the minimal subcongenic interval generated through recombinant mapping, was found to be upregulated in Sle2(z) B cells by microarray analysis, Western blot, and functional assays. Pharmacological inhibition of FAAH reversed the increase in receptor revision, RAG expression, and polyreactive autoantibodies in lupus-prone mice. These studies indicate that increased peripheral BCR revision, or selective peripheral expansion of BCR-revised B cells, may lead to systemic autoimmunity and that FAAH is a lupus-susceptibility gene that might regulate this process.

Differential outcome of TRIF-mediated signaling in TLR4 and TLR3 induced DC maturation.

Recognition of pathogen-associated molecular patterns by Toll-like receptors (TLRs) on dendritic cells (DCs) leads to DC maturation, a process involving up-regulation of MHC and costimulatory molecules and secretion of proinflammatory cytokines. All TLRs except TLR3 achieve these outcomes by using the signaling adaptor myeloid differentiation factor 88. TLR4 and TLR3 can both use the Toll-IL-1 receptor domain-containing adaptor inducing IFN-ß (TRIF)-dependent signaling pathway leading to IFN regulatory factor 3 (IRF3) activation and induction of IFN-ß and -α4. The TRIF signaling pathway, downstream of both of these TLRs, also leads to DC maturation, and it has been proposed that the type I IFNs act in cis to induce DC maturation and subsequent effects on adaptive immunity. The present study was designed to understand the molecular mechanisms of TRIF-mediated DC maturation. We have discovered that TLR4-TRIF-induced DC maturation was independent of both IRF3 and type I IFNs. In contrast, TLR3-mediated DC maturation was completely dependent on type I IFN feedback. We found that differential activation of mitogen-activated protein kinases by the TLR4- and TLR3-TRIF axes determined the type I IFN dependency for DC maturation. In addition, we found that the adjuvanticity of LPS to induce T-cell activation is completely independent of type I IFNs. The important distinction between the TRIF-mediated signaling pathways of TLR4 and TLR3 discovered here could have a major impact in the design of future adjuvants that target this pathway.

RNA sensing by conventional dendritic cells is central to the development of lupus nephritis.

Glomerulonephritis is a common and debilitating feature of systemic lupus erythematosus (SLE). The precise immune mechanisms that drive the progression from benign autoimmunity to glomerulonephritis are largely unknown. Previous investigations have shown that a moderate increase of the innate Toll-like receptor 7 (TLR7) is sufficient for the development of nephritis. In these systems normalization of B-cell TLR7 expression or temporal depletion of plasmacytoid dendritic cells (pDCs) slow progression; however, the critical cell that is responsible for driving full immunopathology remains unidentified. In this investigation we have shown that conventional DC expression of TLR7 is essential for severe autoimmunity in the Sle1Tg7 model of SLE. We show that a novel expanding CD11b(+) conventional DC subpopulation dominates the infiltrating renal inflammatory milieu, localizing to the glomeruli. Moreover, exposure of human myeloid DCs to IFN-α or Flu increases TLR7 expression, suggesting they may have a role in self-RNA recognition pathways in clinical disease. To our knowledge, this study is the first to highlight the importance of conventional DC-TLR7 expression for kidney pathogenesis in a murine model of SLE.

Two functional lupus-associated BLK promoter variants control cell-type- and developmental-stage-specific transcription.

Efforts to identify lupus-associated causal variants in the FAM167A/BLK locus on 8p21 are hampered by highly associated noncausal variants. In this report, we used a trans-population mapping and sequencing strategy to identify a common variant (rs922483) in the proximal BLK promoter and a tri-allelic variant (rs1382568) in the upstream alternative BLK promoter as putative causal variants for association with systemic lupus erythematosus. The risk allele (T) at rs922483 reduced proximal promoter activity and modulated alternative promoter usage. Allelic differences at rs1382568 resulted in altered promoter activity in B progenitor cell lines. Thus, our results demonstrated that both lupus-associated functional variants contribute to the autoimmune disease association by modulating transcription of BLK in B cells and thus potentially altering immune responses.

Cutting Edge: Inhibiting TBK1 by Compound II Ameliorates Autoimmune Disease in Mice.

TANK-binding kinase 1 (TBK1) is a serine/threonine protein kinase that plays a crucial role in innate immunity. Enhanced TBK1 function is associated with autoimmune diseases and cancer, implicating the potential benefit of therapeutically targeting TBK1. In this article, we examined a recently identified TBK1 inhibitor Compound II on treating autoimmune diseases. We found that Compound II is a potent and specific inhibitor of TBK1-mediated IFN response. Compound II inhibited polyinosinic-polycytidylic acid-induced immune activation in vitro and in vivo. Compound II treatment also ameliorated autoimmune disease phenotypes of Trex1(-/-) mice, increased mouse survival, and dampened the IFN gene signature in TREX1 mutant patient lymphoblasts. In addition, we found that TBK1 gene expression is elevated in systemic lupus erythematosus patient cells, and systemic lupus erythematosus cells with high IFN signature responded well to Compound II treatment. Together, our findings provided critical experimental evidence for inhibiting TBK1 with Compound II as an effective treatment for TREX1-associated autoimmune diseases and potentially other interferonopathies.

B cell-intrinsic CD84 and Ly108 maintain germinal center B cell tolerance.

Signaling lymphocyte activation molecules (SLAMs) play an integral role in immune regulation. Polymorphisms in the SLAM family receptors are implicated in human and mouse model of lupus disease. The lupus-associated, somatically mutated, and class-switched pathogenic autoantibodies are generated in spontaneously developed germinal centers (GCs) in secondary lymphoid organs. The role and mechanism of B cell-intrinsic expression of polymorphic SLAM receptors that affect B cell tolerance at the GC checkpoint are not clear. In this study, we generated several bacterial artificial chromosome-transgenic mice that overexpress C57BL/6 (B6) alleles of different SLAM family genes on an autoimmune-prone B6.Sle1b background. B6.Sle1b mice overexpressing B6-derived Ly108 and CD84 exhibit a significant reduction in the spontaneously developed GC response and autoantibody production compared with B6.Sle1b mice. These data suggest a prominent role for Sle1b-derived Ly108 and CD84 in altering the GC checkpoint. We further confirm that expression of lupus-associated CD84 and Ly108 specifically on GC B cells in B6.Sle1b mice is sufficient to break B cell tolerance, leading to an increase in autoantibody production. In addition, we observe that B6.Sle1b B cells have reduced BCR signaling and a lower frequency of B cell-T cell conjugates; the reverse is seen in B6.Sle1b mice overexpressing B6 alleles of CD84 and Ly108. Finally, we find a significant decrease in apoptotic GC B cells in B6.Sle1b mice compared with B6 controls. Our study establishes a central role for GC B cell-specific CD84 and Ly108 expression in maintaining B cell tolerance in GCs and in preventing autoimmunity.

Allelic heterogeneity in NCF2 associated with systemic lupus erythematosus (SLE) susceptibility across four ethnic populations.

Recent reports have associated NCF2, encoding a core component of the multi-protein NADPH oxidase (NADPHO), with systemic lupus erythematosus (SLE) susceptibility in individuals of European ancestry. To identify ethnicity-specific and -robust variants within NCF2, we assessed 145 SNPs in and around the NCF2 gene in 5325 cases and 21 866 controls of European-American (EA), African-American (AA), Hispanic (HS) and Korean (KR) ancestry. Subsequent imputation, conditional, haplotype and bioinformatic analyses identified seven potentially functional SLE-predisposing variants. Association with non-synonymous rs17849502, previously reported in EA, was detected in EA, HS and AA (P(EA) = 1.01 × 10(-54), PHS = 3.68 × 10(-10), P(AA) = 0.03); synonymous rs17849501 was similarly significant. These SNPs were monomorphic in KR. Novel associations were detected with coding variants at rs35937854 in AA (PAA = 1.49 × 10(-9)), and rs13306575 in HS and KR (P(HS) = 7.04 × 10(-7), P(KR) = 3.30 × 10(-3)). In KR, a 3-SNP haplotype was significantly associated (P = 4.20 × 10(-7)), implying that SLE predisposing variants were tagged. Significant SNP-SNP interaction (P = 0.02) was detected between rs13306575 and rs17849502 in HS, and a dramatically increased risk (OR = 6.55) with a risk allele at each locus. Molecular modeling predicts that these non-synonymous mutations could disrupt NADPHO complex assembly. The risk allele of rs17849501, located in a conserved transcriptional regulatory region, increased reporter gene activity, suggesting in vivo enhancer function. Our results not only establish allelic heterogeneity within NCF2 associated with SLE, but also emphasize the utility of multi-ethnic cohorts to identify predisposing variants explaining additional phenotypic variance ('missing heritability') of complex diseases like SLE.

T/B-cell interactions are more transient in response to weak stimuli in SLE-prone mice.

Changes in immune function during the course of systemic lupus erythematosus (SLE) are well characterized. Class-switched antinuclear antibodies are the hallmark of SLE, and T/B-cell interactions are thus critical. However, changes in immune function contributing to disease susceptibility are unknown. Here, we have analyzed primary T and B cells from a mouse model of SLE prior to the onset of disease. To allow cognate T-cell activation with low affinity, we have developed a lower potency peptide ligand for the OTII TCR. T- and B-cell couples formed less frequently and retained their polarity less efficiently preferentially in response to low-affinity stimulation in SLE-prone mice. This matched decreased recruitment of actin and Vav1 and an enhanced PKCΘ recruitment to the cellular interface in T cells. The induction of the GC B-cell marker GL7 was increased in T/B cell couples from SLE-prone mice when the T-cell numbers were limited. However, the overall gene expression changes were marginal. Taken together, the enhanced cell-couple transience may allow a more efficient sampling of a large number of T/B cell couples, preferentially in response to limiting stimuli, therefore enhancing the immune reactivity in the development of SLE.

A role for Ly108 in the induction of promyelocytic zinc finger transcription factor in developing thymocytes.

The promyelocytic zinc finger transcription factor (PLZF) is required for the development of activated phenotypes in NKT and other innate T lymphocytes. Although strong TCR stimulation has been implicated in the induction of PLZF, the factors regulating PLZF expression are incompletely understood. We show in this study that costimulation of preselection double-positive thymocytes through the signaling lymphocyte activation molecule family receptor Ly108 markedly enhanced PLZF expression compared with that induced by TCR stimulation alone. Costimulation with Ly108 increased expression of early growth response protein (Egr)-2 and binding of Egr-2 to the promoter of Zbtb16, which encodes PLZF, and resulted in PLZF levels similar to those seen in NKT cells. In contrast, costimulation with anti-CD28 failed to enhance Egr-2 binding and Zbtb16 expression. Moreover, mice lacking Ly108 showed decreased numbers of PLZF-expressing CD4(+) T cells. Together, these results support a potential role for Ly108 in the induction of PLZF.

Genetic predisposition to autoimmunity--what have we learned?

Rapid advances in genetic technologies have led to the identification of more than 85 loci that contribute to susceptibility to autoimmune diseases. These susceptibility genes are distributed throughout the innate and adaptive immune systems, indicating that dysregulations in both immune systems participate in the development of autoimmunity. A significant subset of these susceptibility genes are shared between multiple autoimmune diseases. However, the dysregulation of specific pathways, such as the pathogen recognition receptors of the innate immune system and the TNF supergene family, are significantly involved in some autoimmune diseases. Although these findings dramatically increase the details available concerning the nature of genetic predisposition to autoimmunity, a mechanistic understanding of the processes involved has not been achieved. Future studies must focus on correlating phenotypes with specific genotypes to improve our understanding of the immune processes that are dysregulated during the development of autoimmunity.

Inducible expression of kallikrein in renal tubular cells protects mice against spontaneous lupus nephritis.

OBJECTIVE: To ascertain whether engineered expression of kallikreins within the kidneys, using an inducible Cre/loxP system, can ameliorate murine lupus nephritis. METHODS: In mice with a lupus-prone genetic background, we engineered the expression of tamoxifen-inducible Cre recombinase under the control of a kidney-specific promoter whose activation initiates murine kallikrein-1 expression within the kidneys. These transgenic mice were injected with either tamoxifen or vehicle at age 2 months and then were monitored for 8 months for kallikrein expression and disease. RESULTS: Elevated expression of kallikrein was detected in the kidney and urine of tamoxifen-injected mice but not in controls. At age 10 months, all vehicle-injected mice developed severe lupus nephritis, as evidenced by increased proteinuria (mean ± SD 13.43 ± 5.65 mg/24 hours), increased blood urea nitrogen (BUN) and serum creatinine levels (39.86 ± 13.45 mg/dl and 15.23 ± 6.89 mg/dl, respectively), and severe renal pathology. In contrast, the tamoxifen-injected mice showed significantly reduced proteinuria (6.6 ± 4.12 mg/24 hours), decreased BUN and serum creatinine levels (15.71 ± 8.17 mg/dl and 6.64 ± 3.39 mg/dl, respectively), and milder renal pathology. Tamoxifen-induced up-regulation of renal kallikrein expression increased nitric oxide production and dampened renal superoxide production and inflammatory cell infiltration, alluding to some of the pathways through which kallikreins may be operating within the kidneys. CONCLUSION: Local expression of kallikreins within the kidney has the capacity to dampen lupus nephritis, possibly by modulating inflammation and oxidative stress.