Stem cell dysfunction and loss of renewal capacity are primary characteristics of tissue aging and decremental regeneration in response to injury. Alveolar type 2 cells (AT2) are key progenitor cells responsible for lung repair and are thought to be dysfunctional in diseases such as chronic obstructive pulmonary disease (COPD). AT2 cells are highly metabolic and rely on mitochondria, but how mitochondrial mechanisms influence their maintenance and cell fate is unclear. This gap is critical as no current therapies target lung repair or mitochondrial function in COPD. Here, we report that adenine nucleotide translocase 2 (ANT2), a key ATP/ADP transporter, is reduced in AT2 cells from COPD lungs, and that ANT2 loss impairs bioenergetics (ATP). We also identify, for the first time, ferroptotic susceptibility as a consequence of ANT2 loss in AT2 cells, leading to impaired self-renewal and progenitor capacity in alveolar organoids. Together, loss of ANT2 and the associated cellular dysfunction resulted in worsened lung damage or emphysema due to cigarette smoke in mice. Therapeutic restoration of ANT2 expression resulted in renewed AT2 stem cell function and prevention of emphysema by reducing oxidative stress and ferroptosis. These findings highlight the importance of ANT2 in metabolic regulation, plasticity, and cell resiliency of AT2 cells in the lung and that ANT2 is a potential target for lung repair.

CD4+Foxp3+ regulatory T cells (Treg) are essential for maintaining immune tolerance in both lymphoid and non-lymphoid tissues, including transplanted organs. Understanding the mechanisms that regulate Treg suppressor function and tissue migration remains a major focus in transplantation immunobiology. Here, we show that under minimal effector T cell (Teff) response settings, Treg are unexpectedly inefficient at migrating to inflamed allografts despite circulatory presence. In contrast, Teff infiltration into inflamed allografts induces a multifold increase in intragraft Treg accumulation. These observations were confirmed in two lympho-replete murine models in which Treg trafficking can be assessed with or without concurrent Teff activity. Intravital microscopy reveals that while Tregs transferred alone remain confined to perivascular spaces, the presence of Teff enables Treg migration deep into graft tissue. Mechanistically, Teff increase graft inflammation and Teff-driven Treg migration to allografts requires chemokine receptor signaling in Treg. In addition, we demonstrate that Teff-conditioned media directly promote chemokine receptor dependent Treg transmigration in vitro. Collectively, our findings demonstrate that Treg exhibit restricted migration and accumulation within inflamed allografts under controlled Teff responses, emphasizing the need to optimize Treg therapy designs to improve their trafficking to allografts under immunosuppression.

An early event after lung injury is extracellular matrix (ECM) remodeling and the formation of a provisional matrix. While megakaryocytes and platelets (Mgk/plt) play important roles in hemostasis, their impact on the matrix in lung injury is not fully understood. Using lung intravital microscopy, 3D large area scanning with multiphoton confocal hybrid imaging, and label-free quantitative proteomics, the matricellular protein thrombospondin-1 (TSP1) arising from Mgk/plt protects the lung from alveolar injury. Mgk/plt cell-specific Thbs1 knockout (cKO) mice show increased alveolar barrier disruption with exaggerated neutrophil-mediated injury. The cKO mice exhibit striking extracellular matrix re-organization with reduction in basement membrane matrix protein COL6A1 after injury. Moreover, cKO mice increase Mgk numbers to regions of fibrillar collagen deposition and alveolar leak. Our findings indicate Mgk/plt-derived TSP1 represents a key mechanism of matrix stabilization by protecting the basement membrane from neutrophil-mediated proteolytic damage, regulating injury severity and Mgk numbers at the alveolar interface.

Physiological adaptations to fasting enable humans to survive for prolonged periods without food and involve molecular pathways that may drive life-prolonging effects of dietary restriction in model organisms. Mobilization of fatty acids and glycerol from adipocyte lipid stores by canonical neutral lipases, including the rate limiting adipose triglyceride lipase (Pnpla2/ATGL), is critical to the adaptive fasting response. Here we discovered an alternative mechanism of lipolysis in adipocytes involving a lysosomal program. We functionally tested lysosomal lipolysis with pharmacological and genetic approaches in mice and in murine and human adipocyte and adipose tissue explant culture, establishing dependency on lysosomal acid lipase (LIPA/LAL) and the microphthalmia/transcription factor E (MiT/TFE) family. Our study establishes a model whereby the canonical pathway is critical for rapid lipolytic responses to adrenergic stimuli operative in the acute stage of fasting, while the alternative lysosomal pathway dominates with prolonged fasting. Survival during fasting requires release of adipose tissue lipid stores and is thought to be dependent on canonical lipases, including the rate limiting action of adipose triglyceride lipase. Here the authors show that lysosomes and lysosomal acid lipase play a critical role in adipocyte lipolysis with fasting in mice.

Toll like receptor (TLR) 7 and 9, endosomal sensors for RNA and DNA, are key mediators of autoreactivity. Although generally considered homologous, they paradoxically have opposing effects on lupus: TLR7 exacerbates the disease while TLR9 protects from it How they mediate opposing effects in autoimmunity remains undetermined. We hypothesized that differences in signaling qualities of the Toll-Interleukin 1 Receptor (TIR) domains of TLR7 and TLR9 could be responsible for their opposing effects. To test this, we introduced the TIR domain of TLR9 into the endogenous Tlr7 locus and the TLR7 TIR domain into the endogenous Tlr9 locus of mice, creating chimeric molecules termed TLR779 and TLR997. Lupus-prone MRL/lpr mice carrying Tlr779 had greatly ameliorated disease, while MRL/lpr mice carrying Tlr997 had markedly exacerbated disease compared with respective TlrWT mice. These experiments establish that TLR7 and TLR9 TIR domains have divergent properties and control disease quality, thus explaining the longstanding "TLR paradox".

Patients afflicted with Stimulator of interferon gene (STING) gain-of-function mutations frequently present with debilitating interstitial lung disease (ILD) that is recapitulated in mice expressing the STINGV154M mutation (VM). Prior radiation chimera studies revealed an unexpected and critical role for non-hematopoietic cells in initiating ILD. To identify STING-expressing non-hematopoietic cell types required for the development of ILD, we use a conditional knockin (CKI) model and direct expression of the VM allele to hematopoietic cells, fibroblasts, epithelial cells, or endothelial cells. Only endothelial cell-targeted VM expression results in enhanced recruitment of immune cells to the lung associated with elevated chemokine expression and the formation of bronchus-associated lymphoid tissue, as seen in the parental VM strain. These findings reveal the importance of endothelial cells as instigators of STING-driven lung disease and suggest that therapeutic targeting of STING inhibitors to endothelial cells could potentially mitigate inflammation in the lungs of STING-associated vasculopathy with onset in infancy (SAVI) patients or patients afflicted with other ILD-related disorders.

In lupus, TLR7 and TLR9 mediate loss of tolerance to RNA and DNA, respectively. Yet, TLR7 promotes while TLR9 protects from disease, implying differences in signaling. To dissect this 'TLR paradox', we generated two TLR9 point mutants—lacking either ligand (TLR9K51E) or MyD88 (TLR9P915H) binding—in lupus-prone MRL/lpr mice. Ameliorated disease of Tlr9K51E mice compared to Tlr9-/- controls revealed a TLR9 'scaffold' protective function that is ligand- and MyD88-independent. Unexpectedly, Tlr9P915H mice were more protected than both Tlr9K51E and Tlr9WT mice, suggesting that TLR9 also possesses ligand-dependent, but MyD88-independent, regulatory signaling and MyD88-mediated proinflammatory signaling. Triple mixed bone marrow chimeras showed that TLR9-MyD88 independent regulatory roles were B cell-intrinsic and restrained differentiation into pathogenic age-associated B cells (ABC) and plasmablasts (PB). These studies reveal MyD88-independent regulatory roles of TLR9, shedding light on the biology of endosomal TLRs. In addition, we have created TLR7/TLR9 chimeric molecules in the germline of MRL/lpr lupus-prone mice, in which the TIR domains have been switched, and determined the effect on disease outcomes. In this way, we could map the differential regulatory and proinflammatory functions of these two distinct endosomal TLRs. This work was supported by NIH grant R37-AI118841.

The X-linked A- variant (rs1050828, Val68Met) in G6PDX accounts for glucose-6-phosphate (G6PD) deficiency in approximately 11% of African American males. This common, hypomorphic variant may impact pulmonary host defense and phagocyte function during pneumonia by altering levels of reactive oxygen species produced by host leukocytes. We used CRISPR-Cas9 technology to generate novel mouse strain with "humanized" G6PD A- variant containing non-synonymous Val68Met single nucleotide polymorphism. Male hemizygous or littermate wild-type (WT) controls were inoculated intratracheally with K. pneumoniae (KP2 serotype, ATCC 43816 strain,103 CFU inoculum). We examined leukocyte recruitment, organ bacterial burden, bone marrow neutrophil and macrophage (BMDM) phagocytic capacity, and hydrogen peroxide (H2O2) production. Unexpectedly, G6PD-deficient mice showed decreased lung bacterial burden (p=0.05) compared to controls 24-h post-infection. Extrapulmonary dissemination and bacteremia were significantly reduced in G6PD-deficient mice 48-h post-infection. Bronchoalveolar lavage fluid (BALF) IL-10 levels were elevated in G6PD-deficient mice (p=0.03) compared to controls at 24-h but were lower at 48-h (p=0.03). G6PD A- BMDMs show mildly decreased in vitro phagocytosis of pHrodo-labeled KP2 (p=0.03). Baseline, but not stimulated, H2O2 production by G6PD A- neutrophils was greater compared to WT neutrophils. G6PD A- variant demonstrate higher basal neutrophil H2O2 production and are protected against acute Klebsiella intrapulmonary infection.

Monocytes and macrophages are central to host defense but also contribute to inflammation-associated pathology. Efforts to manipulate monocyte and macrophage function are limited by our ability to effectively quantify the functional programs of these cells. We identified the gene Fth1, which encodes the ferritin H chain, as highly predictive of alveolar macrophage transcriptomic states during LPS-induced lung inflammation and developed an Fth1-mScarlet reporter mouse. In the steady-state lung, high Fth1-mScarlet expression is restricted to alveolar macrophages. In response to LPS-induced lung inflammation, Fth1 reporter activity is robustly increased in monocytes, with its expression reporting genes that are differentially expressed in monocytes versus macrophages. Consistent with this reporter-associated gene profile, within the Lyz2-GFP+CD11b+Ly6C+ gate, the highest Fth1 reporter expression was observed in CD11c+ cells, indicative of monocyte-to-macrophage differentiation. Although Fth1-mScarlet was induced in monocytes responding to either TLR4 ligation or M-CSF-induced macrophage differentiation in vitro, TLR4-dependent expression occurred with greater speed and magnitude. Considering this, we suggest that Fth1-mScarlet expression reports monocyte-to-macrophage differentiation, with increased expression in proinflammatory states. Dissecting macrophage differentiation from inflammatory programs will be enhanced when combining Fth1-mScarlet with other reporter systems. Thus, the Fth1-mScarlet model addresses an important lack of tools to report the diverse spectrum of monocyte and macrophage states in vivo.

LC3-associated phagocytosis (LAP) is a process wherein elements of autophagy conjugate LC3 to phagosomal membranes. We characterize the molecular requirements for LAP, and identify Rubicon as being required for LAP but not autophagy. Rubicon is recruited to LAPosomes and is required for the activity of a Class III PI(3)K complex containing UVRAG but lacking ATG14 and Ambra1. This allows for the sustained localization of PtdIns(3)P, which is critical for recruitment of downstream autophagic proteins and stabilization of the NOX2 complex to produce reactive oxygen species. Both PtdIns(3)P and reactive oxygen species are required for conjugation of LC3 to LAPosomes and subsequent association with LAMP1(+) lysosomes. LAP is induced by engulfment of Aspergillus fumigatus, a fungal pathogen that commonly afflicts immunocompromised hosts, and is required for its optimal clearance in vivo. Therefore, we have identified molecules that distinguish LAP from canonical autophagy, thereby elucidating the importance of LAP in response to A. fumigatus infection.

BACKGROUND:Donor genetic variation is associated with red blood cell (RBC) storage integrity and post-transfusion recovery. Our previous large-scale genome-wide association study demonstrated that the African G6PD deficient A- variant (rs1050828, Val68Met) is associated with higher oxidative hemolysis after cold storage. Despite a high prevalence of X-linked G6PD mutation in African American population (>10%), blood donors are not routinely screened for G6PD status and its importance in transfusion medicine is relatively understudied. STUDY DESIGN AND METHODS:To further evaluate the functional effects of the G6PD A- mutation, we created a novel mouse model carrying this genetic variant using CRISPR-Cas9. We hypothesize that this humanized G6PD A- variant is associated with reduced G6PD activity with a consequent effect on RBC hemolytic propensity and post-transfusion recovery. RESULTS:G6PD A- RBCs had reduced G6PD protein with ~5% residual enzymatic activity. Significantly increased in vitro hemolysis induced by oxidative stressors was observed in fresh and stored G6PD A- RBCs, along with a lower GSH:GSSG ratio. However, no differences were observed in storage hemolysis, osmotic fragility, mechanical fragility, reticulocytes, and post-transfusion recovery. Interestingly, a 14% reduction of 24-h survival following irradiation was observed in G6PD A- RBCs compared to WT RBCs. Metabolomic assessment of stored G6PD A- RBCs revealed an impaired pentose phosphate pathway (PPP) with increased glycolytic flux, decreasing cellular antioxidant capacity. DISCUSSION:This novel mouse model of the common G6PD A- variant has impaired antioxidant capacity like humans and low G6PD activity may reduce survival of transfused RBCs when irradiation is performed.

The epithelial Na+ channel (ENaC) γ subunit is essential for homeostasis of Na+, K+, and body fluid. Dual γ subunit cleavage before and after a short inhibitory tract allows dissociation of this tract, increasing channel open probability (PO), in vitro. Cleavage proximal to the tract occurs at a furin recognition sequence (143RKRR146, in the mouse γ subunit). Loss of furin-mediated cleavage prevents in vitro activation of the channel by proteolysis at distal sites. We hypothesized that 143RKRR146 mutation to 143QQQQ146 (γQ4) in 129/Sv mice would reduce ENaC PO, impair flow-stimulated flux of Na+ (JNa) and K+ (JK) in perfused collecting ducts, reduce colonic amiloride-sensitive short-circuit current (ISC), and impair Na+, K+, and body fluid homeostasis. Immunoblot of γQ4/Q4 mouse kidney lysates confirmed loss of a band consistent in size with the furin-cleaved proteolytic fragment. However, γQ4/Q4 male mice on a low Na+ diet did not exhibit altered ENaC PO or flow-induced JNa, though flow-induced JK modestly decreased. Colonic amiloride-sensitive ISC in γQ4/Q4 mice was not altered. γQ4/Q4 males, but not females, exhibited mildly impaired fluid volume conservation when challenged with a low Na+ diet. Blood Na+ and K+ were unchanged on a regular, low Na+, or high K+ diet. These findings suggest that biochemical evidence of γ subunit cleavage should not be used in isolation to evaluate ENaC activity. Furthermore, factors independent of γ subunit cleavage modulate channel PO and the influence of ENaC on Na+, K+, and fluid volume homeostasis in 129/Sv mice, in vivo.NEW & NOTEWORTHY The epithelial Na+ channel (ENaC) is activated in vitro by post-translational proteolysis. In vivo, low Na+ or high K+ diets enhance ENaC proteolysis, and proteolysis is hypothesized to contribute to channel activation in these settings. Using a mouse expressing ENaC with disruption of a key proteolytic cleavage site, this study demonstrates that impaired proteolytic activation of ENaC's γ subunit has little impact upon channel open probability or the ability of mice to adapt to low Na+ or high K+ diets.

Loss of NADPH oxidase (NOX2) exacerbates systemic lupus erythematosus (SLE) in mice and humans, but the mechanisms underlying this effect remain unclear. To identify the cell lineages in which NOX2 deficiency drives SLE, we employed conditional KO and chimeric approaches to delete Cybb in several hematopoietic cell lineages of MRL.Faslpr SLE-prone mice. Deletion of Cybb in macrophages/monocytes exacerbated SLE nephritis, though not to the degree observed in the Cybb global KOs. Unexpectedly, the absence of Cybb in B cells resulted in profound glomerulonephritis and interstitial nephritis, rivaling that seen with global deletion. Furthermore, we identified that NOX2 is a key regulator of TLR7, a driver of SLE pathology, both globally and specifically in B cells. This is mediated in part through suppression of TLR7-mediated NF-κB signaling in B cells. Thus, NOX2’s immunomodulatory effect in SLE is orchestrated not only by its function in the myeloid compartment, but through a pivotal role in B cells by selectively inhibiting TLR7 signaling.

The ENaC gamma subunit is essential for homeostasis of Na + , K + , and body fluid. Dual subunit cleavage before and after a short inhibitory tract allows dissociation of this tract, increasing channel open probability (P O ), in vitro . Cleavage proximal to the tract occurs at a furin recognition sequence ( 143 RKRR 146 in mouse). Loss of furin-mediated cleavage prevents in vitro activation of the channel by proteolysis at distal sites. We hypothesized that 143 RKRR 146 mutation to 143 QQQQ 146 ( Q4 ) in 129/Sv mice would reduce ENaC P O , impair flow-stimulated flux of Na + (J Na ) and K + (J K ) in perfused collecting ducts, reduce colonic amiloride-sensitive short circuit current (I SC ), and impair Na + , K + , and body fluid homeostasis. Immunoblot of Q4/Q4 mouse kidney lysates confirmed loss of a band consistent in size with the furin-cleaved proteolytic fragment. However, Q4/Q4 male mice on a low Na + diet did not exhibit altered ENaC P O or flow-induced J Na , though flow-induced J K modestly decreased. Colonic amiloride-sensitive I SC in Q4/Q4 mice was not altered. Q4/Q4 males, but not females, exhibited mildly impaired fluid volume conservation when challenged with a low Na + diet. Blood Na + and K + were unchanged on a regular, low Na + , or high K + diet. These findings suggest that biochemical evidence of gamma subunit cleavage should not be used in isolation to evaluate ENaC activity. Further, factors independent of gamma subunit cleavage modulate channel P O and the influence of ENaC on Na + , K + , and fluid volume homeostasis in 129/Sv mice, in vivo .

Epithelial Na+ channels (ENaCs) are activated by proteolysis of the α and γ subunits at specific sites flanking embedded inhibitory tracts. To examine the role of α subunit proteolysis in channel activation in vivo, we generated mice lacking the distal furin cleavage site in the α subunit (αF2M mice). On a normal Na+ control diet, no differences in ENaC protein abundance in kidney or distal colon were noted between wild-type (WT) and αF2M mice. Patch-clamp analyses revealed similar levels of ENaC activity in kidney tubules, while no physiologically relevant differences in blood chemistry or aldosterone levels were detected. Male αF2M mice did exhibit diminished ENaC activity in the distal colon, as measured by amiloride-sensitive short-circuit current (ISC). Following dietary Na+ restriction, WT and αF2M mice had similar natriuretic and colonic ISC responses to amiloride. However, single-channel activity was significantly lower in kidney tubules from Na+-restricted αF2M mice compared with WT littermates. ENaC α and γ subunit expression in kidney and distal colon were also enhanced in Na+-restricted αF2M vs. WT mice, in association with higher aldosterone levels. These data provide evidence that disrupting α subunit proteolysis impairs ENaC activity in vivo, requiring compensation in response to Na+ restriction. KEY POINTS: The epithelial Na+ channel (ENaC) is activated by proteolytic cleavage in vitro, but key questions regarding the role of ENaC proteolysis in terms of whole-animal physiology remain to be addressed. We studied the in vivo importance of this mechanism by generating a mouse model with a genetic disruption to a key cleavage site in the ENaC's α subunit (αF2M mice). We found that αF2M mice did not exhibit a physiologically relevant phenotype under normal dietary conditions, but have impaired ENaC activation (channel open probability) in the kidney during salt restriction. ENaC function at the organ level was preserved in salt-restricted αF2M mice, but this was associated with higher aldosterone levels and increased expression of ENaC subunits, suggesting compensation was required to maintain homeostasis. These results provide the first evidence that ENaC α subunit proteolysis is a key regulator of channel activity in vivo.

Epithelial Na+ channels (ENaCs) control extracellular fluid volume by facilitating Na+ absorption across transporting epithelia. In vitro studies showed that Cys-palmitoylation of the γENaC subunit is a major regulator of channel activity. We tested whether γ subunit palmitoylation sites are necessary for channel function in vivo by generating mice lacking the palmitoylated cysteines (γC33A,C41A) using CRISPR/Cas9 technology. ENaCs in dissected kidney tubules from γC33A,C41A mice had reduced open probability compared with wild-type (WT) littermates maintained on either standard or Na+-deficient diets. Male mutant mice also had higher aldosterone levels than WT littermates following Na+ restriction. However, γC33A,C41A mice did not have reduced amiloride-sensitive Na+ currents in the distal colon or benzamil-induced natriuresis compared to WT mice. We identified a second, larger conductance cation channel in the distal nephron with biophysical properties distinct from ENaC. The activity of this channel was higher in Na+-restricted γC33A,C41A versus WT mice and was blocked by benzamil, providing a possible compensatory mechanism for reduced prototypic ENaC function. We conclude that γ subunit palmitoylation sites are required for prototypic ENaC activity in vivo but are not necessary for amiloride/benzamil-sensitive Na+ transport in the distal nephron or colon.

BACKGROUND:Microvasculature dysfunction is a common finding in pathologic remodeling of the heart and is thought to play an important role in the pathogenesis of hypertrophic cardiomyopathy (HCM), a disease caused by sarcomere gene mutations. We hypothesized that microvascular dysfunction in HCM was secondary to abnormal microvascular growth and could occur independent of ventricular hypertrophy.METHODS:We used multimodality imaging methods to track the temporality of microvascular dysfunction in HCM mouse models harboring mutations in the sarcomere genes Mybpc3 (cardiac myosin binding protein C3) or Myh6 (myosin heavy chain 6). We performed complementary molecular methods to assess protein quantity, interactions, and post-translational modifications to identify mechanisms regulating this response. We manipulated select molecular pathways in vivo using both genetic and pharmacological methods to validate these mechanisms.RESULTS:We found that microvascular dysfunction in our HCM models occurred secondary to reduced myocardial capillary growth during the early postnatal time period and could occur before the onset of myocardial hypertrophy. We discovered that the E3 ubiquitin protein ligase MDM2 (murine double minute 2) dynamically regulates the protein stability of both HIF1α (hypoxia-inducible factor 1 alpha) and HIF2α (hypoxia-inducible factor 2 alpha)/EPAS1 (endothelial PAS domain protein 1) through canonical and noncanonical mechanisms. The resulting HIF imbalance leads to reduced proangiogenic gene expression during a key period of myocardial capillary growth. Reducing MDM2 protein levels by genetic or pharmacological methods normalized HIF protein levels and prevented the development of microvascular dysfunction in both HCM models.CONCLUSIONS:Our results show that sarcomere mutations induce cardiomyocyte MDM2 signaling during the earliest stages of disease, and this leads to long-term changes in the myocardial microenvironment.

Neurodevelopmental disorders (NDDs) are frequently associated with dendritic abnormalities in pyramidal neurons that affect arbor complexity, spine density, and synaptic communication 1,2. The underlying genetic causes are often complex, obscuring the molecular pathways that drive these disorders 3. Next-generation sequencing has identified recurrent de novo missense mutations in a handful of genes associated with NDDs, offering a unique opportunity to decipher the molecular pathways 4. One such gene is PACS1, which encodes the multi-functional trafficking protein PACS1 (or PACS-1); a single recurrent de novo missense mutation, c607C>T (PACS1R203W), causes developmental delay and intellectual disability (ID) 5,6. The processes by which PACS1R203W causes PACS1 syndrome are unknown, and there is no curative treatment. We show that PACS1R203W increases the interaction between PACS1 and the α-tubulin deacetylase HDAC6, elevating enzyme activity and appropriating control of its posttranscriptional regulation. Consequently, PACS1R203W reduces acetylation of α-tubulin and cortactin, causing the Golgi to fragment and enter developing neurites, leading to increased dendrite arborization. The dendrites, however, are beset with diminished spine density and fewer functional synapses, characteristic of ID pathology. Treatment of PACS1 syndrome mice with PACS1- or HDAC6-targeting antisense oligonucleotides restores neuronal structure and synaptic transmission, suggesting PACS1R203W/HDAC6 may be targeted for treating PACS1 syndrome neuropathology.

The endosomal Toll-like receptor 7 (TLR7) is a major driver of murine and human systemic lupus erythematosus (SLE). The role of TLR7 in lupus pathogenesis is enhanced when the regulatory role of TLR9 is absent. TLR7 signaling in plasmacytoid DCs (pDC) is generally thought to be a major driver of the IFN response and disease pathology; however, the cell types in which TLR7 acts to mediate disease have not been distinguished. To address this, we selectively deleted TLR7 in either CD11c+ cells or CD19+ cells; using a TLR7-floxed allele, we created on the lupus-prone MRL/lpr background, along with a BM chimera strategy. Unexpectedly, TLR7 deficiency in CD11c+ cells had no impact on disease, while TLR7 deficiency in CD19+ B cells yielded mild suppression of proteinuria and a trend toward reduced glomerular disease. However, in TLR9-deficient MRL/lpr mice with accelerated SLE, B cell-specific TLR7 deficiency greatly improved disease. These results support revision of the mechanism by which TLR7 drives lupus and highlight a cis regulatory interaction between the protective TLR9 and the pathogenic TLR7 within the B cell compartment. They suggest B cell-directed, dual TLR7 antagonism/TLR9 agonism or dual TLR7/9 antagonism as a potential future therapeutic strategy to treat SLE.