Strain-specific innate immune signaling pathways determine malaria parasitemia dynamics and host mortality.

Malaria infection triggers vigorous host immune responses; however, the parasite ligands, host receptors, and the signaling pathways responsible for these reactions remain unknown or controversial. Malaria parasites primarily reside within RBCs, thereby hiding themselves from direct contact and recognition by host immune cells. Host responses to malaria infection are very different from those elicited by bacterial and viral infections and the host receptors recognizing parasite ligands have been elusive. Here we investigated mouse genome-wide transcriptional responses to infections with two strains of Plasmodium yoelii (N67 and N67C) and discovered differences in innate response pathways corresponding to strain-specific disease phenotypes. Using in vitro RNAi-based gene knockdown and KO mice, we demonstrated that a strong type I IFN (IFN-I) response triggered by RNA polymerase III and melanoma differentiation-associated protein 5, not Toll-like receptors (TLRs), binding of parasite DNA/RNA contributed to a decline of parasitemia in N67-infected mice. We showed that conventional dendritic cells were the major sources of early IFN-I, and that surface expression of phosphatidylserine on infected RBCs might promote their phagocytic uptake, leading to the release of parasite ligands and the IFN-I response in N67 infection. In contrast, an elevated inflammatory response mediated by CD14/TLR and p38 signaling played a role in disease severity and early host death in N67C-infected mice. In addition to identifying cytosolic DNA/RNA sensors and signaling pathways previously unrecognized in malaria infection, our study demonstrates the importance of parasite genetic backgrounds in malaria pathology and provides important information for studying human malaria pathogenesis.

Ifih1 gene dose effect reveals MDA5-mediated chronic type I IFN gene signature, viral resistance, and accelerated autoimmunity.

Type I IFNs (IFN-I) are normally produced during antiviral responses, yet high levels of chronic IFN-I expression correlate with autoimmune disease. A variety of viral sensors generate IFN-I in their response, but other than TLRs, it is not fully known which pathways are directly involved in the development of spontaneous immune pathologies. To further explore the link between IFN-I induced by viral pathways and autoimmunity, we generated a new transgenic mouse line containing multiple copies of Ifih1, a gene encoding the cytoplasmic dsRNA sensor MDA5 with proven linkage to diabetes and lupus. We show that MDA5 overexpression led to a chronic IFN-I state characterized by resistance to a lethal viral infection through rapid clearance of virus in the absence of a CD8(+) or Ab response. Spontaneous MDA5 activation was not sufficient to initiate autoimmune or inflammatory pathology by itself, even though every immune cell population had signs of IFN activation. When combined with the lupus-susceptible background of the FcγR2B deficiency, MDA5 overexpression did accelerate the production of switched autoantibodies, the incidence of glomerulonephritis, and early lethality. Thus, MDA5 transgenic mice provide evidence that chronic elevated levels of IFN-I are not sufficient to initiate autoimmunity or inflammation although they might exacerbate an ongoing autoimmune pathology.

Linking susceptibility genes and pathogenesis mechanisms using mouse models of systemic lupus erythematosus.

Systemic lupus erythematosus (SLE) represents a challenging autoimmune disease from a clinical perspective because of its varied forms of presentation. Although broad-spectrum steroids remain the standard treatment for SLE, they have many side effects and only provide temporary relief from the symptoms of the disease. Thus, gaining a deeper understanding of the genetic traits and biological pathways that confer susceptibility to SLE will help in the design of more targeted and effective therapeutics. Both human genome-wide association studies (GWAS) and investigations using a variety of mouse models of SLE have been valuable for the identification of the genes and pathways involved in pathogenesis. In this Review, we link human susceptibility genes for SLE with biological pathways characterized in mouse models of lupus, and discuss how the mechanistic insights gained could advance drug discovery for the disease.

Immunoglobulin E plays an immunoregulatory role in lupus.

The (patho)physiological role of IgE in nonallergic inflammatory diseases is not well understood. Here, we explored the effect of IgE deficiency on the inflammatory response in FcγRIIB-deficient mice as well as in mice carrying both a deletion of FcγRIIB and the chromosomal translocation of Y-linked autoimmune acceleration (Yaa) that hastens and results in a more aggressive lupuslike disease in these mice. The findings show that deficiency of IgE delays disease development and severity as demonstrated by reduced autoantibody production and amelioration of organ pathologies. This was associated with decreased numbers of plasma cells and reduced levels of IgG2b and IgG3. Unexpectedly, the loss of IgE also caused a striking decrease of immune cell infiltration in secondary lymphoid organs with a marked effect on the presence of dendritic cells, monocytes, neutrophils, and eosinophils in these organs and decreased activation of basophils. The presence of autoreactive IgE in human systemic lupus erythematosus subjects was also associated with increased basophil activation and enhanced disease activity. These findings argue that IgE facilitates the amplification of autoimmune inflammation.

Spontaneous activation of RNA-sensing pathways in autoimmune disease.

Multiple intracellular RNA sensing innate immune pathways have been linked to autoimmune disease. RNA-related ligands taken up by the endocytic pathway activate TLRs, and affect primarily immune cells. This type of activation is enhanced by nucleic acid-specific antibodies and induces an inflammatory program. In contrast, spontaneous activation of cytoplasmic RNA sensing pathways targets mostly non-hematopoietic tissues and their effect on autoimmune disease is secondary to the release of interferon in the circulation. The fact that pathologies result from spontaneous activation of innate pathways implies that endogenous RNA ligands that might be sensed as pathogenic are commonly found in both immune and non-immune cells.

Innate pathways to B-cell activation and tolerance.

B cells represent an important link between the adaptive and innate immune systems as they express both antigen-specific B-cell receptors (BCRs) as well as various Toll-like receptors (TLRs). Several checkpoints in B-cell development ensure that self-specific cells are eliminated from the mature B-cell repertoire to avoid harmful autoreactive responses. These checkpoints are controlled by BCR-mediated events but are also influenced by TLR-dependent signals from the innate immune system. Additionally, B-cell-intrinsic and extrinsic TLR signaling are critical for inflammatory events required for the clearance of microbial infections. Factors secreted by TLR-activated macrophages or dendritic cells directly influence the fate of protective and autoreactive B cells. Additionally, naive and memory B cells respond differentially to TLR ligands, as do different B-cell subsets. We review here recent literature describing intrinsic and extrinsic effects of TLR stimulation on the fate of B cells, with particular attention to autoimmune diseases.

Integration of the beta-catenin-dependent Wnt pathway with integrin signaling through the adaptor molecule Grb2.

BACKGROUND: THE COMPLEXITY OF WNT SIGNALING LIKELY STEMS FROM TWO SOURCES: multiple pathways emanating from frizzled receptors in response to wnt binding, and modulation of those pathways and target gene responsiveness by context-dependent signals downstream of growth factor and matrix receptors. Both rac1 and c-jun have recently been implicated in wnt signaling, however their upstream activators have not been identified. METHODOLOGY/PRINCIPAL FINDINGS: Here we identify the adapter protein Grb2, which is itself an integrator of multiple signaling pathways, as a modifier of beta-catenin-dependent wnt signaling. Grb2 synergizes with wnt3A, constitutively active (CA) LRP6, Dvl2 or CA-beta-catenin to drive a LEF/TCF-responsive reporter, and dominant negative (DN) Grb2 or siRNA to Grb2 block wnt3A-mediated reporter activity. MMP9 is a target of beta-catenin-dependent wnt signaling, and an MMP9 promoter reporter is also responsive to signals downstream of Grb2. Both a jnk inhibitor and DN-c-jun block transcriptional activation downstream of Dvl2 and Grb2, as does DN-rac1. Integrin ligation by collagen also synergizes with wnt signaling as does overexpression of Focal Adhesion Kinase (FAK), and this is blocked by DN-Grb2. CONCLUSIONS/SIGNIFICANCE: These data suggest that integrin ligation and FAK activation synergize with wnt signaling through a Grb2-rac-jnk-c-jun pathway, providing a context-dependent mechanism for modulation of wnt signaling.

Wnt signaling induces matrix metalloproteinase expression and regulates T cell transmigration.

Wnts are a family of secreted glycoproteins with diverse developmental roles, including regulation of cell migration; however, little is known about wnt signaling in mature T cells. We find that endothelial-cell-derived wnts, acting through Frizzled receptors, induce matrix metalloproteinase (MMP) 2 and MMP9 expression in effector T cells. Blocking wnt signaling, or MMP activity, reduces T cell migration through the basement membrane in vitro and into inflamed skin in vivo. Wnt signaling stabilizes beta-catenin protein in T cells and directly targets the MMP promoters through tandem TCF sites. Thus, our data support a necessary and previously unexpected role for wnt signaling in T cell extravasation.

Isolation of human umbilical vein endothelial cells (HUVEC).

Angiogenesis is a complex multi-step process, where in response to angiogenic stimuli, new vessels are created from the existing vasculature. These steps include: degradation of the basement membrane, proliferation and migration (sprouting) of endothelial cells (EC) into the extracellular matrix, alignment of EC into cords, lumen formation, anastomosis, and formation of a new basement membrane. Many in vitro assays have been developed to study this process, but most only mimic certain stages of angiogenesis, and morphologically the vessels often do not resemble vessels in vivo. Here we demonstrate an optimized in vitro angiogenesis assay that utilizes human umbilical vein EC and fibroblasts. This model recapitulates all of the key early stages of angiogenesis, and importantly the vessels display patent intercellular lumens surrounded by polarized EC. Vessels can be easily observed by phase-contrast and time-lapse microscopy, and recovered in pure form for downstream applications.

Matrix metalloproteinase-7 activation of mouse paneth cell pro-alpha-defensins: SER43 down arrow ILE44 proteolysis enables membrane-disruptive activity.

Small intestinal Paneth cells secrete alpha-defensin microbicidal peptides as mediators of innate enteric immunity. In mice, production of mature Paneth cell alpha-defensins, termed cryptdins (Crps), requires proteolytic activation of inactive precursors (pro-Crps) by the convertase matrix metalloproteinase-7. Proteolysis of mouse (pro-Crp4)(20-92) produces the specific cleavage intermediates pro-Crp4(44-92), pro-Crp4(54-92), and pro-Crp4(59-92). To identify which cleavage event enables bactericidal activity, recombinant pro-Crp4-processing intermediates were purified to homogeneity and assayed for bactericidal peptide activity. The in vitro bactericidal activities of pro-Crp4-processing intermediates were very similar to fully processed Crp4, contrasting the lack of bactericidal and membrane-disruptive activity shown by pro-Crp4(20-92). Thus, cleavage of pro-Crp4(20-92) at Ser(43) downward arrowIle(44) is sufficient to activate bactericidal activity, and amino acids in the pro-Crp4(20-43) of the proregion maintain the precursor in an inactive state. Because cationic Arg residues are determinants of Crp4 bactericidal peptide activity, we hypothesized that Asp and Glu residues in pro-Crp4(20-43) neutralize Crp4 Arg side chains in pro-Crp4(20-92). Therefore, a pro-Crp4(20-92) variant with Gly substitutions at all pro-Crp4(20-43) Asp and Glu positions ((DE/G)-pro-Crp4) was prepared, and it was bactericidal and lysed phospholipid vesicles under conditions where native pro-Crp4(20-92) lacks activity. These findings show that MMP-7 proteolysis of pro-Crp4(20-92) at Ser(43) downward arrowIle(44) converts inactive precursors to bactericidal forms by removal of covalently associated, inhibitory acidic amino acids from proximity with the Crp4 component of the molecule.

Endothelial cell co-stimulation through OX40 augments and prolongs T cell cytokine synthesis by stabilization of cytokine mRNA.

Human endothelial cells (ECs) constitutively express OX40L and co-stimulate memory CD4(+) T cell proliferation that is dependent upon OX40-OX40L interaction. In vivo, OX40 prolongs T cell survival; however, an unanswered question is whether it can also prolong synthesis of proliferation-sustaining cytokines such as IL-2. Here we show that EC co-stimulation results in the secretion of T cell IL-2, IL-3 and IFN-gamma and that in the absence of OX40 signals synthesis largely ceases by 12-18 h, but is prolonged up to 60 h in the presence of OX40 signaling. Blocking OX40-mediated cytokine expression at later times suppresses T cell proliferation and this can be overcome by addition of exogenous IL-2. We find that OX40 signaling has discrete effects on T cell activation as it does not affect expression of IL-10, CD25, CD69 or soluble IL-2R. Also, OX40 does not appear to alter IL-2 transcription, but rather acts to stabilize a subset of cytokine mRNAs, increasing their half-lives by 3-6-fold. We further show that OX40L induces activation of p38 mitogen-activated protein kinase (MAPK) and phosphotidyl-inositol-3-kinase (PI3K) in T cells, and using specific inhibitors, we find that increased mRNA half-life is dependent upon both these pathways but is independent of c-jun-N-terminal kinase (JNK). Thus, EC co-stimulation through OX40 leads to prolonged T cell cytokine synthesis and enhanced proliferation.