Addressing diagnostic challenges in primary immunodeficiencies: Laboratory evaluation of Toll-like receptor- and NF-κB-mediated immune responses

CD62-L down-regulation after L18-MDP stimulation as a complementary flow cytometry functional assay for the diagnosis of XIAP deficiency

X-linked inhibitor of apoptosis (XIAP) deficiency is an infrequent inborn error of immunity caused by mutations in XIAP gene. Most cases present with absence of XIAP protein which can be detected by flow cytometry (FC), representing a rapid diagnostic method. However, since some genetic defects may not preclude protein expression, it is important to include a complementary functional test in the laboratory workup of these patients. L-selectin (CD62-L) is a molecule that is cleaved from the surface membrane of leukocytes upon stimulation of different receptors such as toll like receptors (TLRs) and nucleotide-binding oligomerization domain-like receptors (NLRs), including NOD2. Considering that XIAP deficiency impairs NOD2 signaling, we decided to assess CD62-L down-regulation by FC post-stimulation of neutrophils and monocytes with L18-muramyl Di-Peptide (L18-MDP), a NOD2 specific agonist, in order to develop a novel assay for the functional evaluation of patients with suspicion of XIAP defects. Whole blood samples from 20 healthy controls (HC) and four patients with confirmed molecular diagnosis of XIAP deficiency were stimulated with 200 ng/mL of L18-MDP for 2 h. Stimulation with 100 ng/mL of lipopolysaccharide (LPS) was carried out in parallel as a positive control of CD62-L shedding. CD62-L expression was evaluated by FC using an anti CD62-L- antibody and down-regulation was assessed by calculating the difference in CD62-L expression before and after stimulation, both in terms of percentage of CD62-L expressing cells (Δ%CD62-L) and median fluorescence intensity (ΔMFI%). Neutrophils and monocytes from XIAP deficient patients displayed a significantly diminished response to L18-MDP stimulation compared with HC (p < 0.0001), indicating a severely altered mechanism of CD62-L down-regulation following activation of NOD2-XIAP axis. On the other hand, the response to LPS stimulation was comparable between patients and heathy controls, suggesting preserved CD62-L shedding with a different stimulus. FC detection of CD62-L down-regulation in monocytes and neutrophils after whole blood stimulation with L18-MDP results in an effective and rapid functional test for the identification of XIAP deficient patients.

Identification and characterization of Toll-like receptor 14d in Northeast Chinese lamprey (Lethenteron morii)

Toll-like receptors (TLRs) play an important role in innate immunity of defense against bacterial or viral pathogens. To study the biological characteristics and functions of the TLR genes, TLR14d was identified from Northeast Chinese lamprey (Lethenteron morii) and named LmTLR14d. LmTLR14d coding sequence (cds) is 3285 bp in length and encodes 1094 amino acids (aa). The results showed that LmTLR14d has the typical structure of TLR molecule, which contains the extracellular domain of leucine-rich repeats (LRR), transmembrane domain, and intracellular domain of Toll/interleukin-1 receptor (TIR). The phylogenetic tree showed that LmTLR14d is a homologous gene of TLR14/18 in bony fish. Quantitative real-time PCR (qPCR) revealed that LmTLR14d was expressed in various healthy tissues, including immune and non-immune tissues. Pseudomonas aeruginosa infection up-regulated LmTLR14d in the supraneural body (SB), gill, and kidney tissues of infected Northeast Chinese lamprey. Immunofluorescence results showed that LmTLR14d was located in the cytoplasm of HEK 293T cells in clusters, and its subcellular localization was determined by the TIR domain. The immunoprecipitation results showed that LmTLR14d could recruit L.morii MyD88 (LmMyD88) but not L.morii TRIF (LmTRIF). Dual luciferase reporter results showed that LmTLR14d significantly enhanced the activity of L.morii NF-κβ (LmNF-κβ) promoter. Furthermore, co-transfection of LmTLR14d with MyD88 significantly enhanced the L.morii NF-κβ (LmNF-κβ) promoter activity. LmTLR14d can induce the expression of inflammatory cytokine genes il-6 and tnf-α downstream of NF-κB signal. This study suggested that LmTLR14d might play an important role in the innate immune signal transduction process of lamprey and revealed the origin and function of teleost-specific TLR14.

Alternative pre-mRNA splicing as a mechanism for terminating Toll-like Receptor signaling

While inflammation induced by Toll-like receptor (TLR) signaling is required to combat infection, persistent inflammation can damage host tissues and contribute to a myriad of acute and chronic inflammatory disorders. Thus, it is essential not only that TLR signaling be activated in the presence of pathogens but that TLR signaling is ultimately terminated. One mechanism that limits persistent TLR signaling is alternative pre-mRNA splicing. In addition to encoding the canonical mRNAs that produce proteins that promote inflammation, many genes in the TLR signaling pathway also encode alternative mRNAs that produce proteins that are dominant negative inhibitors of signaling. Many of these negative regulators are induced by immune challenge, so production of these alternative isoforms represents a negative feedback loop that limits persistent inflammation. While these alternative splicing events have been investigated on a gene by gene basis, there has been limited systemic analysis of this mechanism that terminates TLR signaling. Here we review what is known about the production of negatively acting alternative isoforms in the TLR signaling pathway including how these inhibitors function, how they are produced, and what role they may play in inflammatory disease.

NF-κB mediates lipopolysaccharide-induced alternative pre-mRNA splicing of MyD88 in mouse macrophages

Although a robust inflammatory response is needed to combat infection, this response must ultimately be terminated to prevent chronic inflammation. One mechanism that terminates inflammatory signaling is the production of alternative mRNA splice forms in the Toll-like receptor (TLR) signaling pathway. Whereas most genes in the TLR pathway encode positive mediators of inflammatory signaling, several, including that encoding the MyD88 signaling adaptor, also produce alternative spliced mRNA isoforms that encode dominant-negative inhibitors of the response. Production of these negatively acting alternatively spliced isoforms is induced by stimulation with the TLR4 agonist lipopolysaccharide (LPS); thus, this alternative pre-mRNA splicing represents a negative feedback loop that terminates TLR signaling and prevents chronic inflammation. In the current study, we investigated the mechanisms regulating the LPS-induced alternative pre-mRNA splicing of the MyD88 transcript in murine macrophages. We found that 1) the induction of the alternatively spliced MyD88 form is due to alternative pre-mRNA splicing and not caused by another RNA regulatory mechanism, 2) MyD88 splicing is regulated by both the MyD88- and TRIF-dependent arms of the TLR signaling pathway, 3) MyD88 splicing is regulated by the NF-κB transcription factor, and 4) NF-κB likely regulates MyD88 alternative pre-mRNA splicing per se rather than regulating splicing indirectly by altering MyD88 transcription. We conclude that alternative splicing of MyD88 may provide a sensitive mechanism that ensures robust termination of inflammation for tissue repair and restoration of normal tissue homeostasis once an infection is controlled.

Malt1 deficient mice develop osteoporosis independent of osteoclast-intrinsic effects of Malt1 deficiency

This study tested the hypothesis that mucosa associated lymphoid tissue 1 (Malt1) deficiency causes osteoporosis in mice by increasing osteoclastogenesis and osteoclast activity. A patient with combined immunodeficiency (CID) caused by MALT1 deficiency had low bone mineral density resulting in multiple low impact fractures that was corrected by hematopoietic stem cell transplant (HSCT). We have reported that Malt1 deficient Mϕs, another myeloid cell type, are hyper-responsive to inflammatory stimuli. Our objectives were to determine whether Malt1 deficient mice develop an osteoporosis-like phenotype and whether it was caused by Malt1 deficiency in osteoclasts. We found that Malt1 deficient mice had low bone volume by 12 weeks of age, which was primarily associated with reduced trabecular bone. Malt1 protein is expressed and active in osteoclasts and is induced by receptor activator of NF-κB ligand (RANKL) in preosteoclasts. Malt1 deficiency did not impact osteoclast differentiation or activity in vitro. However, Malt1 deficient (Malt1^-/- ) mice had more osteoclasts in vivo and had lower levels of serum osteoprotegerin (OPG), an endogenous inhibitor of osteoclastogenesis. Inhibition of Malt1 activity in Mϕs induced MCSF production, required for osteoclastogenesis, and decreased OPG production in response to inflammatory stimuli. In vitro, MCSF increased and OPG inhibited osteoclastogenesis, but effects were not enhanced in Malt1 deficient osteoclasts. These data support the hypothesis that Malt1 deficient mice develop an osteoporotic phenotype with increased osteoclastogenesis in vivo, but suggest that this is caused by inflammation rather than an effect of Malt1 deficiency in osteoclasts.

Malt1 blocks IL-1β production by macrophages in vitro and limits dextran sodium sulfate-induced intestinal inflammation in vivo

This study tested the hypothesis that Malt1 deficiency in macrophages contributes to dextran sodium sulfate (DSS)-induced intestinal inflammation in Malt1-deficient mice. In people, combined immunodeficiency caused by a homozygous mutation in the MALT1 gene is associated with increased susceptibility to bacterial infections and chronic inflammation, including severe inflammation along the gastrointestinal tract. The consequences of Malt1 deficiency have largely been attributed to its role in lymphocytes, but Malt1 is also expressed in macrophages, where it is activated downstream of TLR4 and dectin-1. The effect of Malt1 deficiency in murine macrophages and its contribution to DSS-induced colitis have not been investigated. Our objectives were to compare the susceptibility of Malt1^+/+ and Malt1^-/- mice to DSS-induced colitis, to determine the contribution of macrophages to DSS-induced colitis in Malt1^-/- mice, and to assess the effect of innate immune stimuli on Malt1^-/- macrophage inflammatory responses. We found that Malt1 deficiency exacerbates DSS-induced colitis in mice, accompanied by higher levels of IL-1β, and that macrophages and IL-1 signaling contribute to pathology in Malt1^-/- mice. Malt1^-/- macrophages produce more IL-1β in response to either TLR4 or dectin-1 ligation, whereas inhibition of Malt1 proteolytic (paracaspase) activity blocked IL-1β production. TLR4 or dectin-1 stimulation induced Malt1 protein levels but decreased its paracaspase activity. Taken together, these data support the hypothesis that Malt1^-/- macrophages contribute to increased susceptibility of Malt1^-/- mice to DSS-induced colitis, which is dependent on IL-1 signaling. Increased IL-1β production by MALT1-deficient macrophages may also contribute to chronic inflammation in people deficient in MALT1.

Functional Evaluation of an IKBKG Variant Suspected to Cause Immunodeficiency Without Ectodermal Dysplasia

Hypomorphic IKBKG mutations in males are typically associated with anhidrotic ectodermal dysplasia with immunodeficiency (EDA-ID). Some mutations cause immunodeficiency without EDA (NEMO-ID). The immunological profile associated with these NEMO-ID variants is not fully documented. We present a 2-year-old patient with suspected immunodeficiency in which a hemizygous p.Glu57Lys IKBKG variant was identified. At the age of 1 year, he had an episode of otitis media that evolved into a bilateral mastoiditis (Pseudomonas spp). Hypogammaglobulinemia, specific (polysaccharide) antibody deficiency, and low switched memory B cell subsets were noticed. The mother was heterozygous for the variant but had no signs of incontinentia pigmenti. Patient peripheral blood mononuclear cells produced low amounts of IL-6 after stimulation with IL-1β, Pam₃CSK_4, and FSL-1. In patient fibroblasts, IκB-α was degraded normally upon stimulation with IL-1β or TNF-α. Transduction of wild-type and variant NEMO in NEMO^-/- deficient SV40 fibroblasts revealed a slight but significant reduction of IL-6 production upon stimulation with IL-1β and TNF-α. In conclusion, we demonstrated that p.Glu57Lys leads to specific immunological defects in vitro. No other pathogenic PID variants were identified through whole exome sequencing. As rare polymorphisms have been described in IKBKG and polygenic inheritance remains an option in the presented case, this study emphasizes the need for thorough functional and genetic evaluation when encountering and interpreting suspected disease-causing NEMO-ID variants.

Toll-Like Receptor 8 Is a Major Sensor of Group B Streptococcus But Not Escherichia coli in Human Primary Monocytes and Macrophages

TLR8 is the major endosomal sensor of degraded RNA in human monocytes and macrophages. It has been implicated in the sensing of viruses and more recently also bacteria. We previously identified a TLR8-IFN regulatory factor 5 (IRF5) signaling pathway that mediates IFNβ and interleukin-12 (IL-12) induction by Staphylococcus aureus and is antagonized by TLR2. The relative importance of TLR8 for the sensing of various bacterial species is however still unclear. We here compared the role of TLR8 and IRF5 for the sensing of Group B Streptococcus (GBS), S. aureus, and Escherichia coli in human primary monocytes and monocyte-derived macrophages (MDM). GBS induced stronger IFNβ and TNF production as well as IRF5 nuclear translocation compared to S. aureus grown to the stationary phase, while S. aureus in exponential growth appeared similarly potent to GBS. Cytokine induction in primary human monocytes by GBS was not dependent on hemolysins, and induction of IFNβ and IL-12 as well as IRF5 activation were reduced with TLR2 ligand costimulation. Heat inactivation of GBS reduced IRF5 and NF-kB translocation, while only the viable E. coli activated IRF5. The attenuated stimulation correlated with loss of bacterial RNA integrity. The E. coli-induced IRF5 translocation was not inhibited by TLR2 costimulation, suggesting that IRF5 was activated via a TLR8-independent mechanism. Gene silencing of MDM using siRNA revealed that GBS-induced IFNβ, IL-12-p35, and TNF production was dependent on TLR8 and IRF5. In contrast, cytokine induction by E. coli was TLR8 independent but still partly dependent on IRF5. We conclude that TLR8-IRF5 signaling is more important for the sensing of GBS than for stationary grown S. aureus in human primary monocytes and MDM, likely due to reduced resistance of GBS to phagosomal degradation and to a lower production of TLR2 activating lipoproteins. TLR8 does not sense viable E. coli, while IRF5 still contributes to E. coli-induced cytokine production, possibly via a cytosolic nucleic acid sensing mechanism.

Paeonol protects endotoxin-induced acute kidney injury: potential mechanism of inhibiting TLR4-NF-κB signal pathway

STUDY DESIGN AND METHODS

In order to determine the therapeutic effect and mechanism of paeonol on acute kidney injury induced by endotoxin, an acute kidney injury model was established by intraperitoneal administration of lipopolysaccharide in mice in vivo and on LPS-induced dendritic cells in vitro. Renal tissues were used for histologic examination. Concentrations of blood urea nitrogen and serum creatinine were detected, inflammatory cytokines were determined by ELISA. The relative proteins' expression of TLR4-NF-κB signal pathway was assessed by Western blot, the localization and expression of phospho-NF-κB p65 in kidney was monitored by immunohistochemistry.

RESULTS

Treatment of paeonol successfully cuts histopathological scores and dilutes the concentrations of blood urea nitrogen and serum creatinine as index of renal injury severity. In addition, paeonol reduces pro-inflammatory cytokines and increases anti-inflammatory cytokines stimulated by LPS in a dose-dependent manner. Paeonol also inhibits the expression of phosphorylated NF-κB p65, IκBα and IKKβ, and restrains NF-κB p65 DNA-binding activity. Paeonol treatment also attenuates the effects of LPS on dendritic cells, with significant inhibition of pro-inflammatory cytokines release, then TLR4 expression and NF-κB signal pathway have been suppressed.

CONCLUSIONS

These results indicated that paeonol has protective effects on endotoxin-induced kidney injury. The mechanisms underlying such effects are associated with its successfully attenuate inflammatory and suppresses TLR4 and NF-κB signal pathway. Therefore, paeonol has great potential to be a novel and natural product agent for treating AKI or septic-AKI.

Toll-like receptor signaling in primary immune deficiencies

Toll-like receptors (TLRs) recognize common microbial or host-derived macromolecules and have important roles in early activation of the immune system. Patients with primary immune deficiencies (PIDs) affecting TLR signaling can elucidate the importance of these proteins to the human immune system. Defects in interleukin-1 receptor-associated kinase-4 and myeloid differentiation factor 88 (MyD88) lead to susceptibility to infections with bacteria, while mutations in nuclear factor-κB essential modulator (NEMO) and other downstream mediators generally induce broader susceptibility to bacteria, viruses, and fungi. In contrast, TLR3 signaling defects are specific for susceptibility to herpes simplex virus type 1 encephalitis. Other PIDs induce functional alterations of TLR signaling pathways, such as common variable immunodeficiency in which plasmacytoid dendritic cell defects enhance defective responses of B cells to shared TLR agonists. Dampening of TLR responses is seen for TLRs 2 and 4 in chronic granulomatous disease (CGD) and X-linked agammaglobulinemia (XLA). Enhanced TLR responses, meanwhile, are seen for TLRs 5 and 9 in CGD, TLRs 4, 7/8, and 9 in XLA, TLRs 2 and 4 in hyper IgE syndrome, and for most TLRs in adenosine deaminase deficiency.