Cytosolic DNA-sensing pathways that signal via Stimulator of interferon genes (STING) mediate immunity to pathogens and also promote autoimmune pathology in DNaseII- and DNaseIII-deficient mice. In contrast, we report here that STING potently suppresses inflammation in a model of systemic lupus erythematosus (SLE). Lymphoid hypertrophy, autoantibody production, serum cytokine levels, and other indicators of immune activation were markedly increased in STING-deficient autoimmune-prone mice compared with STING-sufficient littermates. As a result, STING-deficient autoimmune-prone mice had significantly shorter lifespans than controls. Importantly, Toll-like receptor (TLR)-dependent systemic inflammation during 2,6,10,14-tetramethylpentadecane (TMPD)-mediated peritonitis was similarly aggravated in STING-deficient mice. Mechanistically, STING-deficient macrophages failed to express negative regulators of immune activation and thus were hyperresponsive to TLR ligands, producing abnormally high levels of proinflammatory cytokines. This hyperreactivity corresponds to dramatically elevated numbers of inflammatory macrophages and granulocytes in vivo. Collectively these findings reveal an unexpected negative regulatory role for STING, having important implications for STING-directed therapies.

Interferon regulatory factor 5-deficient (IRF5 (-/-) ) mice have been used for many studies of IRF5 biology. A recent report identifies a mutation in dedicator of cytokinesis 2 (DOCK2) as being responsible for the abnormal B-cell development phenotype observed in the IRF5 (-/-) line. Both dedicator of cytokinesis 2 (DOCK2) and IRF5 play important roles in immune cell function, raising the issue of whether immune effects previously associated with IRF5 are due to IRF5 or DOCK2. Here, we defined the insertion end-point of the DOCK2 mutation and designed a novel PCR to detect the mutation in genomic DNA. We confirmed the association of the DOCK2 mutation and the abnormal B-cell phenotype in our IRF5 (-/-) line and also established another IRF5 (-/-) line without the DOCK2 mutation. These two lines were used to compare the role of IRF5 in dendritic cells (DCs) and B cells in the presence or absence of the DOCK2 mutation. IRF5 deficiency reduces IFN-α, IFN-β and IL-6 production by Toll-like receptor 9 (TLR9)- and TLR7-stimulated DCs and reduces TLR7- and TLR9-induced IL-6 production by B cells to a similar extent in the two lines. Importantly however, IRF5 (-/-) mice with the DOCK2 mutation have higher serum levels of IgG1 and lower levels of IgG2b, IgG2a/c and IgG3 than IRF5 (-/-) mice without the DOCK2 mutation, suggesting that the DOCK2 mutation confers additional Th2-type effects. Overall, these studies help clarify the function of IRF5 in B cells and DCs in the absence of the DOCK2 mutation. In addition, the PCR described will be useful for other investigators using the IRF5 (-/-) mouse line.

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Endosomal Toll-like receptors (TLRs) play an important role in systemic autoimmune diseases such as SLE, where DNA- and RNA-associated autoantigens activate autoreactive B cells through TLR9- and TLR7-dependent pathways. Nevertheless, TLR9-deficient autoimmune prone mice develop more severe clinical disease, while TLR7-deficient and TLR7/9-double deficient autoimmune-prone mice develop less severe disease. To determine whether the regulatory activity of TLR9 is B cell intrinsic, we have now directly compared the functional properties of autoantigen activated WT, TLR9-deficient and TLR7-deficient B cells, in an experimental system where proliferation depends on BCR/TLR co-engagement. In vitro, TLR9-deficient cells are less dependent on survival factors for a sustained proliferative response than either WT or TLR7-deficient cells. The TLR9-deficient cells also preferentially differentiate toward the plasma cell lineage, as indicated by expression of CD138, sustained expression of IRF4, and other molecular markers of plasma cells. In vivo, autoantigen-activated TLR9-deficient cells give rise to greater numbers of autoantibody producing cells. Our results identify distinct roles for TLR7 and TLR9 in the differentiation of autoreactive B cells that explain the capacity of TLR9 to limit, and TLR7 to promote, the clinical features of SLE.

In mice that fail to express the phagolysosomal endonuclease, DNase II, and the type I IFN receptor, excessive accrual of undegraded DNA results in a STING-dependent, TLR-independent inflammatory arthritis. These double knockout (DKO) mice develop additional indications of systemic autoimmunity, including anti-nuclear autoantibodies and splenomegaly, not found in Unc93b1−/− DKO mice and therefore TLR-dependent. The DKO autoantibodies predominantly detect RNA-associated autoantigens, commonly targeted in TLR7-dominated SLE-prone mice. To determine whether an inability of TLR9 to detect endogenous DNA could explain the absence of dsDNA-reactive autoantibodies in DKO mice, we used a novel class of bifunctional autoantibodies, IgM/DNA DVD-Ig™ molecules, to activate B cells through a BCR/TLR9-dependent mechanism. DKO B cells could not respond to the IgM/DNA DVD-Ig™ molecule, despite a normal response to both anti-IgM and CpG ODN 1826. Thus DKO B cells only respond to RNA-associated ligands because DNase II-mediated degradation of self-DNA is required for TLR9 activation.

Systemic lupus erythematosus (SLE) is a chronic, life threatening autoimmune disorder, leading to multiple organ pathologies and kidney destruction. Analyses of numerous murine models of spontaneous SLE have revealed a critical role for endosomal Toll-like receptors (TLRs) in the production of autoantibodies and development of other clinical disease manifestations. Nevertheless, the corresponding TLR9-deficient autoimmune-prone strains consistently develop more severe disease pathology. Injection of BALB/c mice with 2,6,10,14-tetramethylpentadecane (TMPD), commonly known as pristane, also results in the development of SLE-like disease. We now show that Tlr9−/− BALB/c mice injected intraperitoneally with TMPD develop more severe autoimmunity than their TLR-sufficient cohorts. Early indications include an increased accumulation of TLR7-expressing Ly6Chi inflammatory monocytes at the site of injection, upregulation of interferon regulated gene expression in the peritoneal cavity, and an increased production of myeloid lineage precursors (CMP and GMP) in the bone marrow. TMPD-injected Tlr9−/− BALB/c mice develop higher autoantibody titers against RNA, neutrophil cytoplasmic antigens (ANCA), and myeloperoxidase (MPO) than TMPD-injected WT BALB/c mice. The TMP-injected Tlr9−/− mice, and not the WT mice, also develop a marked increase in glomerular IgG deposition and infiltrating granulocytes, much more severe glomerulonephritis, and a reduced lifespan. Together the data point to a major role for TLR7 in the response to self-antigens in this model of experimental autoimmunity. Therefore the BALB/c pristane model recapitulates other TLR7-driven spontaneous models of SLE and is negatively regulated by TLR9.

The protein kinase Btk has been implicated in the development, differentiation, and activation of B cells through its role in the BCR and TLR signaling cascades. These receptors and in particular, the BCR and either TLR7 or TLR9 also play a critical role in the activation of autoreactive B cells by RNA- or DNA-associated autoantigens. To explore the role of Btk in the development of autoreactive B cells, as well as their responses to nucleic acid-associated autoantigens, we have now compared Btk-sufficient and Btk-deficient mice that express a prototypic RF BCR encoded by H- and L-chain sdTgs. These B cells bind autologous IgG2a with low affinity and only proliferate in response to IgG2a ICs that incorporate DNA or RNA. We found that Btk-sufficient RF(+) B cells mature into naïve FO B cells, all of which express the Tg BCR, despite circulating levels of IgG2a. By contrast, a significant proportion of Btk-deficient RF(+) B cells acquires a MZ or MZ precursor phenotype. Remarkably, despite the complete inability of RF(+) Xid/y B cells to respond to F(ab')2 anti-IgM, RF(+) Xid/y B cells could respond well to autoantigen-associated ICs. These data reveal unique features of the signaling cascades responsible for the activation of autoreactive B cells.

Influenza A virus (IAV) and SARS-CoV-2 are pandemic viruses causing millions of deaths, yet their clinical manifestations are distinctly different. With the hypothesis that upper airway immune and epithelial cell responses are also distinct, we performed single-cell RNAsequencing (scRNA-Seq) on nasal wash cells freshly collected from adults with either acute COVID-19 or influenza or from healthy controls. We focused on major cell types and subtypes in a subset of donor samples. Nasal wash cells are enriched for macrophages and neutrophils for both influenza and COVID-19 compared to healthy controls. Hillock-like epithelial cells, M2-like macrophages, and age-dependent B cells are enriched in COVID-19 samples. A global decrease in interferon (IFN)-associated transcripts in neutrophils, macrophages, and epithelial cells is apparent in COVID-19 compared to influenza. The innate immune response to SARS-CoV-2 appears to be maintained in macrophages, despite evidence for limited epithelial immune sensing. Cell-to-cell interaction analyses reveal a decrease in epithelial interactions in COVID-19 and highlight differences in macrophage-macrophage interactions for COVID-19 and influenza. Our study demonstrates that scRNA-Seq can define host and viral transcriptional activity at the site of infection and reveal distinct local epithelial and immune cell responses for COVID-19 and influenza that may contribute to their divergent disease courses.