Analysis of association of the TIRAP (MAL) S180L variant and tuberculosis in three populations

Tirap controls Mycobacterium tuberculosis phagosomal acidification

Progression of tuberculosis is tightly linked to a disordered immune balance, resulting in inability of the host to restrict intracellular bacterial replication and its subsequent dissemination. The immune response is mainly characterized by an orchestrated recruitment of inflammatory cells secreting cytokines. This response results from the activation of innate immunity receptors that trigger downstream intracellular signaling pathways involving adaptor proteins such as the TIR-containing adaptor protein (Tirap). In humans, resistance to tuberculosis is associated with a loss-of-function in Tirap. Here, we explore how genetic deficiency in Tirap impacts resistance to Mycobacterium tuberculosis (Mtb) infection in a mouse model and ex vivo. Interestingly, compared to wild type littermates, Tirap heterozygous mice were more resistant to Mtb infection. Upon investigation at the cellular level, we observed that mycobacteria were not able to replicate in Tirap-deficient macrophages compared to wild type counterparts. We next showed that Mtb infection induced Tirap expression which prevented phagosomal acidification and rupture. We further demonstrate that the Tirap-mediated anti-tuberculosis effect occurs through a Cish-dependent signaling pathway. Our findings provide new molecular evidence about how Mtb manipulates innate immune signaling to enable intracellular replication and survival of the pathogen, thus paving the way for host-directed approaches to treat tuberculosis.

The Roles of Inflammasomes in Host Defense against Mycobacterium tuberculosis

Mycobacterium tuberculosis (MTB) infection is characterized by granulomatous lung lesions and systemic inflammatory responses during active disease. Inflammasome activation is involved in regulation of inflammation. Inflammasomes are multiprotein complexes serving a platform for activation of caspase-1, which cleaves the proinflammatory cytokines such as interleukin-1β (IL-1β) and IL-18 into their active forms. These cytokines play an essential role in MTB control. MTB infection triggers activation of the nucleotide-binding domain, leucine-rich-repeat containing family, pyrin domain-containing 3 (NLRP3) and absent in melanoma 2 (AIM2) inflammasomes in vitro, but only AIM2 and apoptosis-associated speck-like protein containing a caspase-activation recruitment domain (ASC), rather than NLRP3 or caspase-1, favor host survival and restriction of mycobacterial replication in vivo. Interferons (IFNs) inhibits MTB-induced inflammasome activation and IL-1 signaling. In this review, we focus on activation and regulation of the NLRP3 and AIM2 inflammasomes after exposure to MTB, as well as the effect of inflammasome activation on host defense against the infection.

Paradoxical Roles of the MAL/Tirap Adaptor in Pathologies

Toll-like receptors (TLRs) are at the forefront of pathogen recognition ensuring host fitness and eliciting protective cellular and humoral responses. Signaling pathways downstream of TLRs are tightly regulated for preventing collateral damage and loss of tolerance toward commensals. To trigger effective intracellular signaling, these receptors require the involvement of adaptor proteins. Among these, Toll/Interleukin-1 receptor domain containing adaptor protein (Tirap or MAL) plays an important role in establishing immune responses. Loss of function of MAL was associated with either disease susceptibility or resistance. These opposite effects reveal paradoxical functions of MAL and their importance in containing infectious or non-infectious diseases. In this review, we summarize the current knowledge on the signaling pathways involving MAL in different pathologies and their impact on inducing protective or non-protective responses.

The Single Nucleotide Polymorphism Mal-D96N Mice Provide New Insights into Functionality of Mal in TLR Immune Responses

MyD88 adaptor-like (Mal) protein is the most polymorphic of the four key adaptor proteins involved in TLR signaling. TLRs play a critical role in the recognition and immune response to pathogens through activation of the prototypic inflammatory transcription factor NF-κB. The study of single nucleotide polymorphisms in TLRs, adaptors, and signaling mediators has provided key insights into the function of the corresponding genes but also into the susceptibility to infectious diseases in humans. In this study, we have analyzed the immune response of mice carrying the human Mal-D96N genetic variation that has previously been proposed to confer protection against septic shock. We have found that Mal-D96N macrophages display reduced cytokine expression in response to TLR4 and TLR2 ligand challenge. Mal-D96N macrophages also display reduced MAPK activation, NF-κB transactivation, and delayed NF-κB nuclear translocation, presumably via delayed kinetics of Mal interaction with MyD88 following LPS stimulation. Importantly, Mal-D96N genetic variation confers a physiological protective phenotype to in vivo models of LPS-, Escherichia coli-, and influenza A virus-induced hyperinflammatory disease in a gene dosage-dependent manner. Together, these results highlight the critical role Mal plays in regulating optimal TLR-induced inflammatory signaling pathways and suggest the potential therapeutic advantages of targeting the Mal D96 signaling nexus.

Role of MyD88-adaptor-like gene polymorphism rs8177374 in modulation of malaria severity in the Pakistani population

Introduction

The present study was designed to investigate the association between rs8177374 polymorphism and malaria symptoms due to exposure of Plasmodium vivax and Plasmodium falciparum.

Materials and methods

A total of 454 samples were included in the study (228 malaria patients and 226 healthy individuals). Malaria patients, divided into P. vivax and P. falciparum groups on the basis of the causative species of Plasmodium, were categorized into mild and severe on the basis of clinical outcomes according to WHO criteria. Healthy individuals were used as controls. Allele specific PCR based strategy was used for the identification of rs8177374 SNP.

Results

MyD88-adaptor-like gene polymorphism was associated with susceptibility to malaria (p < 0.001). C allele frequency (0.74) was higher in the population compared to T allele frequency (0.26). CT genotype increased the susceptibility of malaria (OR: 2.661; 95% CI: 1.722–4.113) and was positively associated with mild malaria (OR: 5.609; 95% CI: 3.479–9.044, p = 0.00). On the other hand, CC genotype was associated with severe malaria (OR: 3.116; 95% CI: 1.560–6.224, p = 0.00). P. vivax infection rate was higher in CT genotype carriers compared to other genotypes (OR: 3.616; 95% CI: 2.219–5.894, p < 0.001).

Conclusion

MyD88-adaptor-like/TIR domain containing adaptor protein polymorphism for single nucleotide polymorphism rs8177374 is related with the susceptibility of malaria.

The Role of Host Genetics (and Genomics) in Tuberculosis

Familial risk of tuberculosis (TB) has been recognized for centuries. Largely through studies of mono- and dizygotic twin concordance rates, studies of families with Mendelian susceptibility to mycobacterial disease, and candidate gene studies performed in the 20th century, it was recognized that susceptibility to TB disease has a substantial host genetic component. Limitations in candidate gene studies and early linkage studies made the robust identification of specific loci associated with disease challenging, and few loci have been convincingly associated across multiple populations. Genome-wide and transcriptome-wide association studies, based on microarray (commonly known as genechip) technologies, conducted in the past decade have helped shed some light on pathogenesis but only a handful of new pathways have been identified. This apparent paradox, of high heritability but few replicable associations, has spurred a new wave of collaborative global studies. This review aims to comprehensively review the heritability of TB, critically review the host genetic and transcriptomic correlates of disease, and highlight current studies and future prospects in the study of host genomics in TB. An implicit goal of elucidating host genetic correlates of susceptibility to Mycobacterium tuberculosis infection or TB disease is to identify pathophysiological features amenable to translation to new preventive, diagnostic, or therapeutic interventions. The translation of genomic insights into new clinical tools is therefore also discussed.

A Common Variant in the Adaptor Mal Regulates Interferon Gamma Signaling

Humans that are heterozygous for the common S180L polymorphism in the Toll-like receptor (TLR) adaptor Mal (encoded by TIRAP) are protected from a number of infectious diseases, including tuberculosis (TB), whereas those homozygous for the allele are at increased risk. The reason for this difference in susceptibility is not clear. We report that Mal has a TLR-independent role in interferon-gamma (IFN-γ) receptor signaling. Mal-dependent IFN-γ receptor (IFNGR) signaling led to mitogen-activated protein kinase (MAPK) p38 phosphorylation and autophagy. IFN-γ signaling via Mal was required for phagosome maturation and killing of intracellular Mycobacterium tuberculosis (Mtb). The S180L polymorphism, and its murine equivalent S200L, reduced the affinity of Mal for the IFNGR, thereby compromising IFNGR signaling in macrophages and impairing responses to TB. Our findings highlight a role for Mal outside the TLR system and imply that genetic variation in TIRAP may be linked to other IFN-γ-related diseases including autoimmunity and cancer.

TIRAP rs8177374 gene polymorphism increased the risk of pulmonary tuberculosis in Zahedan, southeast Iran

OBJECTIVE

To evaluate the possible association between Toll-interleukin 1 receptor (TIR) domain containing adaptor protein (TIRAP; also known as MAL) rs1893352 and rs8177374 (S180L) gene polymorphisms and pulmonary tuberculosis (PTB) in a sample of Iranian population.

METHODS

This case-control study was performed on 174 PTB and 177 healthy subjects. Tetra amplification refractory mutation system-polymerase chain reaction (T-ARMS-PCR) was used to detect the polymorphisms.

RESULTS

Our finding showed that neither the overall Chi-square comparison of PTB and control subjects nor the logistic regression analysis indicated any association between rs1893352 polymorphism and PTB. Regarding rs8177374 polymorphism, the CT genotype as well as CT+TT increased the risk of PTB in comparison with CC genotype (OR=4.73, 95% CI=2.65-8.45, P<0.0001 and OR=6.47, 95% CI=3.68-11.38, P<0.0001, respectively). The rs8177374 T allele increased the risk of PTB in comparison with C allele (OR=4.21, 95% CI=2.43-7.26, P<0.0001).

CONCLUSIONS

Our finding indicates that TIRAP rs8177374 polymorphism is associated with PTB in a sample of Iranian population.

TIRAP C539T polymorphism contributes to tuberculosis susceptibility: evidence from a meta-analysis

BACKGROUND

Toll-interleukin 1 receptor domain containing adaptor protein (TIRAP), an important adaptor protein downstream of the Toll-like receptor (TLR) 2 and 4 pathways, is highly involved in the activation and coordination of the anti-mycobacterial immune response. We performed a meta-analysis to assess the association between TIRAP C539T polymorphism and tuberculosis (TB) risk.

METHODS

A systematic literature search for relevant studies up to February 27, 2014 was conducted in PUBMED, EMBASE, Web of science, CNKI, VIP, and Wanfang database. The association between gene and disease was assessed using odds ratios (ORs) with 95% confidence intervals (95%CIs) based on five genetic models.

RESULTS

A total of 16 qualified studies were enrolled in this meta-analysis. The results of pooling all studies detected statistically resistance of TIRAP C539T mutants to TB risk (T vs. C: OR 0.80, 95%CI 0.65-0.97; TC vs. CC: OR 0.71, 95%CI 0.55-0.92; TT+TC vs. CC: OR 0.74, 95% CI 0.58-0.94). Further subgroup analyses by ethnicity also demonstrated reduced risk of TB in European population (T vs. C: OR 0.71, 95%CI 0.52-0.95; TC vs. CC: OR 0.56, 95%CI 0.35-0.91; TT+TC vs. CC: OR 0.61, 95%CI 0.40-0.92), whereas no such effects were observed in other ethnicities.

CONCLUSION

This present meta-analysis suggests TIRAP C539T polymorphism is significantly correlated with reduced risk of TB infection, with stronger effect in European. Additional well-designed, larger-scale epidemiological studies among different ethnicities are needed.

Mal, more than a bridge to MyD88

The family of type 1 transmembrane proteins known as Toll-like receptors (TLRs) provide early immune system recognition and response to infection. In order to transmit their signal to the nucleus and initiate activation of pro-inflammatory and anti-microbial genes, TLRs must initiate a cytoplasmic signalling cascade, which is alternately controlled by 6 known signalling adaptors. These signaling adaptors are crucial for activating the correct immune response to any given TLR / pathogen interaction. This review will focus on one of those adaptors, MyD88 adaptor-like (Mal), also known as TIRAP. Mal is critical for signalling by the best studied of the TLRs, the Gram negative bacterial lipopolysaccharide (LPS) sensor, TLR4. Mal's role in TLR2 signalling in response to activation of the bacterial lipopeptide receptor, TLR2, is more contentious. Mal is a component of the so-called 'MyD88-dependent pathway' in TLR4 signalling. Recent advances in our understanding of the signalling pathways downstream of Mal highlight MyD88-indpendent roles, thus positioning Mal as multifunctional and integral for the molecular control of bacterial infections as well as inflammatory diseases. Here we describe the sequence of molecular events involved in the signalling pathways controlled by Mal, and the importance of Mal in driving host protection against a variety of bacteria, with specific attention to the evidence for Mal's role in TLR2 signalling, recent structural findings that have altered our understanding of Mal signalling, and evidence that single nucleotide polymorphisms (SNPs) of Mal are responsible for variations in population level resistance and susceptibility to bacterial infection.

The genetics of innate immunity sensors and human disease

Since their discovery, innate immunity microbial sensors have been increasingly studied and shown to play a critical role in innate responses to microbes in several experimental in vitro, ex vivo, and animal models. However, their role in the human response to infection in natural conditions has just started to be deciphered, by means of clinical studies of primary immunodeficiencies and epidemiological genetic studies. Here, we summarize the major findings concerning the genetic diversity of the various families of microbial sensors in humans, and of other molecules involved in the signaling pathways they trigger. Specifically, we review the genetic associations, revealed by both clinical and epidemiological genetics studies, of microbial sensors from five different families: Toll-like receptors, C-type lectin receptors, NOD-like receptors, RIG-I-like receptors, and cytosolic DNA sensors. In particular, we consider the relationships between variation at the genes encoding these molecules and susceptibility to and the severity of infectious diseases and other clinical conditions associated with immune dysfunction, including autoimmunity, inflammation, allergy, and cancer. Despite the fact that the genetic links between innate immunity sensors and human disorders remain still limited, human genetics studies are increasingly improving our understanding of the genuine functions of microbial sensors and downstream signaling molecules in the natural setting.

Innate immune gene polymorphisms in tuberculosis

Tuberculosis (TB) is a leading cause worldwide of human mortality attributable to a single infectious agent. Recent studies targeting candidate genes and "case-control" association have revealed numerous polymorphisms implicated in host susceptibility to TB. Here, we review current progress in the understanding of causative polymorphisms in host innate immune genes associated with TB pathogenesis. We discuss genes encoding several types of proteins: macrophage receptors, such as the mannose receptor (MR, CD206), dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN, CD209), Dectin-1, Toll-like receptors (TLRs), complement receptor 3 (CR3, CD11b/CD18), nucleotide oligomerization domain 1 (NOD1) and NOD2, CD14, P2X7, and the vitamin D nuclear receptor (VDR); soluble C-type lectins, such as surfactant protein-A (SP-A), SP-D, and mannose-binding lectin (MBL); phagocyte cytokines, such as tumor necrosis factor (TNF), interleukin-1β (IL-1β), IL-6, IL-10, IL-12, and IL-18; chemokines, such as IL-8, monocyte chemoattractant protein 1 (MCP-1), RANTES, and CXCL10; and other important innate immune molecules, such as inducible nitric oxide synthase (iNOS) and solute carrier protein 11A1 (SLC11A1). Polymorphisms in these genes have been variably associated with susceptibility to TB among different populations. This apparent variability is probably accounted for by evolutionary selection pressure as a result of long-term host-pathogen interactions in certain regions or populations and, in part, by lack of proper study design and limited knowledge of molecular and functional effects of the implicated genetic variants. Finally, we discuss genomic technologies that hold promise for resolving questions regarding the evolutionary paths of the human genome, functional effects of polymorphisms, and corollary impacts of adaptation on human health, ultimately leading to novel approaches to controlling TB.

Polymorphisms in the promoter of the CD14 gene and their associations with susceptibility to pulmonary tuberculosis

Although the role of CD14 in recognizing Mycobacterium tuberculosis is well-understood, the possible role of polymorphisms in susceptibility to develop tuberculosis remains unclear. This study evaluates whether there is an association of polymorphisms within the promoter of the CD14 gene with susceptibility to pulmonary tuberculosis. In a case-control study, we genotyped the eight known single nucleotide polymorphisms SNPs within the promoter of the CD14 gene of 698 Han Chinese subjects. Statistically significant differences between tuberculosis patients and healthy controls were found for G-1619A, T-1359G, A-1145G, and C-159T. The haplotype-GGGT, composed of these four SNPs, exhibited a significant association with the disease. Furthermore, expression levels of soluble CD14 were significantly higher in tuberculosis patients with the GGGT haplotype than with other haplotypes, while IgE expression levels were significantly reduced. Our results suggest that these four SNPs within the promoter of the CD14 gene are associated with susceptibility to pulmonary tuberculosis.

Structural insights into TIR domain specificity of the bridging adaptor Mal in TLR4 signaling

MyD88 adaptor-like protein (Mal) is a crucial adaptor that acts as a bridge to recruit the MyD88 molecule to activated TLR4 receptors in response to invading pathogens. The specific assembly of the Toll/interleukin-1 receptor (TIR) domains of TLR4, Mal and MyD88 is responsible for proper signal transduction in the TLR4 signaling pathway. However, the molecular mechanism for the specificity of these TIR domains remains unclear. Here, we present the crystal structure of the TIR domain of the human Mal molecule (Mal-TIR) at a resolution of 2.4 Å. Unexpectedly, Mal-TIR exhibits an extraordinarily long AB loop, but no αB helix or BB loop, distinguishing it from other TIR domains. More importantly, the Mal-TIR AB loop is capable of mediating direct binding to the TIR domains of TLR4 and MyD88 simultaneously. We also found that Mal-TIR can form a back-to-back dimer that may resemble the dimeric assembly of the entire Mal molecule. Our data demonstrate the bridge role of the Mal-TIR domain and provide important information about TIR domain specificity.

Evaluation of Toll-like receptor and adaptor molecule polymorphisms for susceptibility to tuberculosis in a Colombian population

Immunological studies have supported the idea that innate immunity is critical for the control of Mycobacterium tuberculosis (Mtb) infection in humans. Despite the overwhelming evidence showing the critical role of Toll-like receptors (TLRs) in the in vitro recognition of Mtb, the in vivo significance of individual TLRs has been more difficult to demonstrate consistently. We were interested in examining the role of genes of TLRs and molecules involved in their signalling cascades, and a case-control study was designed to test the association of polymorphisms of these innate immune genes with pulmonary tuberculosis (TB) in a Colombian population. In this study, we did not find an association with TLR2, TLR4, TLR9, MyD88 or MAL/TIRAP polymorphic variants. These findings suggest that those genes are not involved as risk factors for pulmonary TB in our population.

Human TLRs and IL-1Rs in host defense: natural insights from evolutionary, epidemiological, and clinical genetics

Toll-like receptors (TLRs) and interleukin-1 receptors (IL-1Rs) have TIR intracellular domains that engage two main signaling pathways, via the TIR-containing adaptors MyD88 (which is not used by TLR3) and TRIF (which is used only by TLR3 and TLR4). Extensive studies in inbred mice in various experimental settings have attributed key roles in immunity to TLR- and IL-1R-mediated responses, but what contribution do human TLRs and IL-1Rs actually make to host defense in the natural setting? Evolutionary genetic studies have shown that human intracellular TLRs have evolved under stronger purifying selection than surface-expressed TLRs, for which the frequency of missense and nonsense alleles is high in the general population. Epidemiological genetic studies have yet to provide convincing evidence of a major contribution of common variants of human TLRs, IL-1Rs, or their adaptors to host defense. Clinical genetic studies have revealed that rare mutations affecting the TLR3-TRIF pathway underlie herpes simplex virus encephalitis, whereas mutations in the TIR-MyD88 pathway underlie pyogenic bacterial diseases in childhood. A careful reconsideration of the contributions of TLRs and IL-1Rs to host defense in natura is required.

Innate immune recognition of Mycobacterium tuberculosis

Tuberculosis (TB), caused by Mycobacterium tuberculosis (MTB), is a major health problem, with 10 million new cases diagnosed each year. Innate immunity plays an important role in the host defense against M. tuberculosis, and the first step in this process is recognition of MTB by cells of the innate immune system. Several classes of pattern recognition receptors (PPRs) are involved in the recognition of M. tuberculosis, including Toll-like receptors (TLRs), C-type lectin receptors (CLRs), and Nod-like receptors (NLRs). Among the TLR family, TLR2, TLR4, and TLR9 and their adaptor molecule MyD88 play the most prominent roles in the initiation of the immune response against tuberculosis. In addition to TLRs, other PRRs such as NOD2, Dectin-1, Mannose receptor, and DC-SIGN are also involved in the recognition of M. tuberculosis. Human epidemiological studies revealed that genetic variation in genes encoding for PRRs and downstream signaling products influence disease susceptibility, severity, and outcome. More insight into PRRs and the recognition of mycobacteria, combined with immunogenetic studies in TB patients, does not only lead to a better understanding of the pathogenesis of tuberculosis but also may contribute to the design of novel immunotherapeutic strategies.

Meta-analysis on the association of TIRAP S180L variant and tuberculosis susceptibility

The missense variant S180L in TIRAP (Toll-interleukin-1 receptor domain-containing adaptor protein) gene is implicated in attenuating TLRs signal transaction and may affect individual response to Mycobacterium tuberculosis infection. Several studies investigated the association between TIRAP S180L and risk of tuberculosis (TB), but the results were controversial. In this study, we quantitatively synthesized nine studies relevant to the association between TIRAP S180L polymorphism and TB risk with total 6584 TB cases and 7294 controls using meta-analysis. We found that the variant allele Leu180 and heterozygous genotype Ser/Leu were not significantly associated with risk of TB (allelic OR = 0.99, 95%CI: 0.88-1.11; Ser/Leu vs Ser/Ser: OR = 0.99, 95%CI: 0.87-1.13) with heterogeneity P values > 0.05. In subgroup analysis, none of the significant associations were observed for S180L and TB risk in Africans (allelic OR = 0.58, 95%CI: 0.29-1.61; heterozygous OR = 0.65, 95%CI: 0.32-1.32) or Asians (allelic OR = 1.30, 95%CI: 0.97-1.74; heterozygous OR = 1.17, 95%CI: 0.84-1.65) or risk of pulmonary tuberculosis (PTB) (allelic OR = 0.92, 95%CI: 0.69-1.22; heterozygous OR = 0.98, 95%CI: 0.86-1.12). This meta-analysis indicates that TIRAP S180L polymorphism is unlikely to substantially contribute to TB susceptibility.

Association analysis of the LTA4H gene polymorphisms and pulmonary tuberculosis in 9115 subjects

Immunoregulatory eicosanoids have been implicated in protection from mycobacterial infection in cell and animal models. Recently, a study of the zebrafish embryo demonstrated that mutants of the lta4h gene, which encodes the leukotriene A4 hydrolase (LTA4H) enzyme of the eicosanoid pathway, have hypersusceptibility to Mycobacterium marinum infection. It also reported that heterozygosity at the two single nucleotide polymorphisms rs1978331 and rs2660898 located in introns of the LTA4H gene, a human homologue of lta4h, is associated with protection from pulmonary tuberculosis. To replicate this association we genotyped six LTA4H gene polymorphisms in samples from 3703 pulmonary tuberculosis patients and 5412 healthy controls collected in Russia. We found no evidence of the protective effect of heterozygosity at the polymorphisms rs1978331 and rs2660898 (P = 0.29 and 0.49) and no association of the alleles of any of the six polymorphisms (P = 0.13-0.81). These results suggest that common polymorphisms in the LTA4H gene do not play any major role in susceptibility to clinical pulmonary tuberculosis.

Microsatellite polymorphisms in intron 2 of the toll-like receptor 2 gene and their association with susceptibility to pulmonary tuberculosis in Han Chinese

BACKGROUND

Toll-like receptor 2 (TLR2) is essential for the immune response to tuberculosis (TB). The goal of the present study was to investigate whether the guanine-thymine (GT) repeat microsatellite polymorphism in intron 2 of the TLR2 gene might be correlated with susceptibility to TB in Han Chinese.

METHODS

The number of (GT)n repeats was determined by gene scanning from 244 patients with TB and 233 control subjects. The expression of TLR2 on CD14+ peripheral blood mononuclear cells was determined using flow cytometry.

RESULTS

No association in allelic polymorphism between control subjects and patients with TB was found. However, the S/M genotype of the microsatellite polymorphism was more frequent in TB patients than in healthy controls (p=0.01). The S/L genotype was more popular in controls than in patients with TB (p=0.007). TLR2 expression was higher in subjects with the S/L genotype than in those with the S/M genotype (p<0.05).

CONCLUSIONS

Our data suggest that the S/M genotype of the microsatellite (GT)n polymorphisms in intron 2 of the TLR2 gene may increase susceptibility to TB in Chinese, and the S/L genotype may act as a negative risk factor.

Shared pathways to infectious disease susceptibility?

The recent advent of genomic approaches for association testing is starting to enable a more comprehensive understanding of the role of human immune response in determining infectious disease outcomes. Progressing from traditional linkage approaches using microsatellite markers to high-resolution genome-wide association scans, these new approaches are leading to the robust discovery of a large number of disease susceptibility genes and the beginnings of an appreciation of their connections. In this commentary, we discuss how this technology development has led to increasingly complex and common infectious diseases being unraveled, and how this is starting to dissect pathogen-specific human responses. Intriguingly, these still preliminary findings suggest that pathogen innate detection mechanisms may not be as shared among diseases as immune response mechanisms.

Targeting Toll-like receptors: emerging therapeutics?

There is a growing interest in the targeting of Toll-like receptors (TLRs) for the prevention and treatment of cancer, rheumatoid arthritis, inflammatory bowel disease and systemic lupus erythematosus (SLE). Several new compounds are now undergoing preclinical and clinical evaluation, with a particular focus on TLR7 and TLR9 activators as adjuvants in infection and cancer, and inhibitors of TLR2, TLR4, TLR7 and TLR9 for the treatment of sepsis and inflammatory diseases. Here, we focus on TLRs that hold the most promise for drug discovery research, highlighting agents that are in the discovery phase and in clinical trials,and on the emerging new aspects of TLR-mediated signalling - such as control by ubiquitination and regulation by microRNAs - that might offer further possibilities of therapeutic manipulation.

Current findings, challenges and novel approaches in human genetic susceptibility to tuberculosis

The evidence for a human genetic component in susceptibility to tuberculosis (TB) is incontrovertible. Quite apart from studies of rare disease events illustrating the importance of key genes in humans and animals, TB at the population level is also influenced by the genetics of the host. Heritability of disease concordance and immune responses to mycobacterial antigens has been clearly shown, and ranges up to 71%. Linkage studies, designed to identify major susceptibility genes in a disease, have produced a number of candidate loci but few, except for regions on chromosome 5p15, 20p and 20q, have been replicated. The region on 5p15 regulates the intensity of the response to the tuberculin skin test, and another locus on 11p14 appears to control resistance to the bacterium. In addition, numerous genes and pathways have been implicated in candidate gene association studies, with validation of polymorphisms in IFNG, NRAMP1, and NOS2A and equivocal results for IL10, CCL2, DC-SIGN, P2RX7, VDR, TLR2, TLR9 and SP110. Other more recently researched candidate genes such as TNFRSF1B remain to be validated, preferably in meta-analyses. New approaches have provided early evidence for the importance of gene-gene interactions in regulating resistance to disease, and also the prospect that applying host genetics in the field of vaccinomics could lead to a more targeted approach in designing interventions to aid the human immune system in combating mycobacteria. Genome-wide association studies and admixture mapping are approaches that remain to be applied to TB, and it is not clear, as is the case with other complex diseases, how much of the heritability of the TB susceptibility phenotype will be determined by multiple genes of small effect versus rare variants with disproportionately large effects.

Genetic susceptibility to different clinical forms of tuberculosis in the Peruvian population

Racial variation, twin studies, segregation analyses, linkage and association studies all suggest that genetic factors play an important role in predisposition to tuberculosis. Many previous studies have been performed with pulmonary TB patients, as the most prevalent form of clinical TB (nearly 95%), and very few of them have considered extrapulmonary TB. The present study evaluates the effects of variation in eight candidate genes (LTA, TNF, IL1B, IL1RN, IL10, TGFB1, TIRAP and P2X7) with pulmonary, pleural, miliary and other extrapulmonary forms of TB in a Peruvian population from the North of Lima. 626 TB cases and 513 healthy controls were enrolled in this study. LTA(+368) and IL10(-592) were associated with different clinical forms of TB (P<0.05). LTA(+368) genotype A/A was protective for pleural TB, LTA(+368) G/A was correlated with susceptibility to miliary TB. Genotypes A/A and G/A were associated with protection and susceptibility respectively when considering all extrapulmonary TB forms versus either healthy controls or pulmonary TB patients. Carriers of IL10(-592)*C were under-represented among those with pulmonary TB and all TB forms (P<0.001). IL10(-1082)-IL10(-592) haplotypes showed different distributions among patients with pulmonary TB and all TB forms (P<0.01) when compared to healthy controls. In addition, IL10(-1082)-IL10(-592) haplotypes showed differences between pleural, miliary and all forms of extrapulmonary TB when compared with pulmonary TB (P<0.05). All findings are consistent with an under-representation of the IL10(-1082)*A-IL10(-592)*A haplotype in pulmonary TB patients. These results suggest that the polymorphisms LTA(+368) and IL10(-592), or variants in strong linkage disequilibrium, variably affect susceptibility to the differing clinical forms of TB in Peruvians.

MyD88 adaptor-like D96N is a naturally occurring loss-of-function variant of TIRAP

Signals elicited by TLRs following the detection of microbes are integrated and diversified by a group of four cytoplasmic adaptor molecules featuring an evolutionarily conserved Toll/IL-1R signaling domain. Single nucleotide polymorphisms (SNPs) in TLRs and their adaptor molecules have been shown to influence susceptibility to a range of infectious and other diseases. The adaptor MyD88 adaptor-like (Mal)/Toll/IL-1R-containing adaptor protein is involved in TLR2 and 4 signal transduction by recruiting another adaptor molecule, MyD88, to the plasma membrane. In this study, we used naturally occurring variants of Mal as tools to study the molecular biology of Mal in more detail in cellular model systems and to thereby identify functionally interesting variants whose corresponding nonsynonymous SNPs might be of further epidemiological interest. Of seven reported variants for Mal, we found Mal D96N associated with reduced NF-kappaB signaling and cytokine production after overexpression in HEK293 and Huh-7 cells. The D96N mutation prevented Mal from recruiting its signaling partner MyD88 to the plasma membrane and altered posttranslational modification of Mal. These findings led us to investigate the frequency of heterozygosity for the corresponding SNP rs8177400 in a Caucasian case-control study on the etiology of lymphoma, a disease in which TLRs have been implicated. Although rs8177400 did not modify lymphoma risk in general, its frequency of heterozygosity was accurately determined to 0.97%. Our data add rs8177400 (D96N) to the list of functionally important variants of Mal and warrant further research into its immunological, epidemiological, and diagnostic relevance.

Pharmacokinetics of antituberculosis drugs in pulmonary tuberculosis patients with type 2 diabetes

Altered pharmacokinetics of antituberculosis drugs may contribute to an increased risk of tuberculosis treatment failure for diabetic patients. We previously found that rifampin exposure was 2-fold lower in diabetic than in nondiabetic tuberculosis patients during the continuation phase of treatment. We now examined the influence of diabetes on the pharmacokinetics of antituberculosis drugs in the intensive phase of tuberculosis treatment, and we evaluated the effect of glycemic control. For this purpose, 18 diabetic and 18 gender- and body weight-matched nondiabetic tuberculosis patients were included in an Indonesian setting. Intensive pharmacokinetic sampling was performed for rifampin, pyrazinamide, and ethambutol at steady state. The bioavailability of rifampin was determined by comparing rifampin exposure after oral versus intravenous administration. Pharmacokinetic assessments were repeated for 10 diabetic tuberculosis patients after glycemic control. No differences in the areas under the concentration-time curves of the drugs in plasma from 0 to 24 h postdose (AUC(0-24)), the maximum concentrations of the drugs in plasma (C(max)), the times to C(max) (T(max)), and the half-lives of rifampin, pyrazinamide, and ethambutol were found between diabetic and nondiabetic tuberculosis patients in the intensive phase of tuberculosis treatment. For rifampin, oral bioavailability and metabolism were similar in diabetic and nondiabetic patients. The pharmacokinetic parameters of antituberculosis drugs were not correlated with blood glucose levels or glucose control. We conclude that diabetes does not alter the pharmacokinetics of antituberculosis drugs during the intensive phase of tuberculosis treatment. The reduced exposure to rifampin of diabetic patients in the continuation phase may be due to increased body weight and possible differences in hepatic induction. Further research is needed to determine the cause of increased tuberculosis treatment failure among diabetic patients.

Innate immunity

Recent years have witnessed an explosion of interest in the innate immune system. Questions about how the innate immune system senses infection and empowers a protective immune response are being answered at the molecular level. These basic science discoveries are being translated into a more complete understanding of the central role innate immunity plays in the pathogenesis of many human infectious and inflammatory diseases. It is particularly exciting that we are already seeing a return on these scientific investments with the emergence of novel therapies to harness the power of the innate immune system. In this review we explore the defining characteristics of the innate immune system, and through more detailed examples, we highlight recent breakthroughs that have advanced our understanding of the role of innate immunity in human health and disease.

Polymorphic variation in TIRAP is not associated with susceptibility to childhood TB but may determine susceptibility to TBM in some ethnic groups

Host recognition of mycobacterial surface molecules occurs through toll like receptors (TLR) 2 and 6. The adaptor protein TIRAP mediates down stream signalling of TLR2 and 4, and polymorphisms in the TIRAP gene (TIRAP) have been associated with susceptibility and resistance to tuberculosis (TB) in adults. In order to investigate the role of polymorphic variation in TIRAP in childhood TB in South Africa, which has one of the highest TB incidence rates in the world, we screened the entire open reading frame of TIRAP for sequence variation in two cohorts of childhood TB from different ethnic groups (Xhosa and mixed ancestry). We identified 13 SNPs, including seven previously unreported, in the two cohorts, and found significant differences in frequency of the variants between the two ethnic groups. No differences in frequency between individual SNPs or combinations were found between TB cases and controls in either cohort. However the 558C→T SNP previously associated with TB meningitis (TBM) in a Vietnamese population was found to be associated with TBM in the mixed ancestry group. Polymorphisms in TIRAP do not appear to be involved in childhood TB susceptibility in South Africa, but may play a role in determining occurrence of TBM.

Low frequency of the TIRAP S180L polymorphism in Africa, and its potential role in malaria, sepsis, and leprosy

Background

The Toll-like receptors (TLRs) mediate innate immunity to various pathogens. A mutation (S180L) in the TLR downstream signal transducer TIRAP has recently been reported to be common in Europeans and Africans and to roughly half the risks of heterogeneous infectious diseases including malaria, tuberculosis, bacteremia, and invasive pneumococal disease in heterozygous mutation carriers.

Methods

We assessed the TIRAP S180L variant by melting curve and RFLP analysis in 1095 delivering women from malaria-endemic Ghana, as well as in a further 1114 individuals participating in case control studies on sepsis and leprosy in Germany, Turkey and Bangladesh.

Results

In Ghana, the TIRAP S180L polymorphism was virtually absent. In contrast, the mutation was observed among 26.6%, 32.9% and 12% of German, Bangladesh and Turkish controls, respectively. No significant association of the heterozygous genotype with sepsis or leprosy was observed. Remarkably, homozygous TIRAP 180L tend to increase the risk of sepsis in the German study (P = 0.04).

Conclusion

A broad protective effect of TIRAP S180L against infectious diseases per se is not discernible.

Toll-like receptor polymorphisms in malaria-endemic populations

Background

Toll-like receptors (TLR) and related downstream signaling pathways of innate immunity have been implicated in the pathogenesis of Plasmodium falciparum malaria. Because of their potential role in malaria pathogenesis, polymorphisms in these genes may be under selective pressure in populations where this infectious disease is endemic.

Methods

A post-PCR Ligation Detection Reaction-Fluorescent Microsphere Assay (LDR-FMA) was developed to determine the frequencies of TLR2, TLR4, TLR9, MyD88-Adaptor Like Protein (MAL) single nucleotide polymorphisms (SNPs), and TLR2 length polymorphisms in 170 residents of two regions of Kenya where malaria transmission is stable and high (holoendemic) or episodic and low, 346 residents of a malaria holoendemic region of Papua New Guinea, and 261 residents of North America of self-identified ethnicity.

Results

The difference in historical malaria exposure between the two Kenyan sites has significantly increased the frequency of malaria protective alleles glucose-6-phoshpate dehydrogenase (G6PD) and Hemoglobin S (HbS) in the holoendemic site compared to the episodic transmission site. However, this study detected no such difference in the TLR2, TLR4, TLR9, and MAL allele frequencies between the two study sites. All polymorphisms were in Hardy Weinberg Equilibrium in the Kenyan and Papua New Guinean populations. TLR9 SNPs and length polymorphisms within the TLR2 5' untranslated region were the only mutant alleles present at a frequency greater than 10% in all populations.

Conclusion

Similar frequencies of TLR2, TLR4, TLR9, and MAL genetic polymorphisms in populations with different histories of malaria exposure suggest that these innate immune pathways have not been under strong selective pressure by malaria. Genotype frequencies are consistent with Hardy-Weinberg Equilibrium and the Neutral Theory, suggesting that genetic drift has influenced allele frequencies to a greater extent than selective pressure from malaria or any other infectious agents in these populations.

TIR-containing adaptors in Toll-like receptor signalling

While the Toll-like receptors (TLRs) are responsible for the recognition and response to pathogen ligands, increasing evidence suggests that the family of five cytosolic Toll/IL-1 receptor (TIR) adaptor proteins also play a crucial role in the specificity of the response. Genetic studies in mice, and increasingly in human polymorphic populations, have given us a greater understanding the role these adaptors play in orchestrating and coordinating the multifaceted immune response to multiple exogenous threats. Importantly, with growing evidence of the critical role TLRs play in responses to host danger signals and autoimmune disease, a more comprehensive understanding and appreciation of the role these adaptors play in disease progression may provide future targets for therapeutic intervention in human disease. Importantly, growing evidence supports the concept of pathway specific and inflammatory control by a better understanding of how these adaptors interact with other signalling mediators, where they localise within the cell and the inflammatory programs they initiate as a way of manipulating immune responses. This review deals with our current understanding of these TIR-containing adaptor proteins and how mutagenesis of specific residues and domains has increased our knowledge of their function in TLR immune responses.

Toll-like receptors: their roles in bacterial recognition and respiratory infections

Although respiratory infections cause significant morbidity and mortality throughout the world, the immunologic factors that mediate host susceptibility to these infections remain poorly understood. The lung contains a vast surface at the host-environment interface and acts as a crucial barrier to invading pathogens. The lung is equipped with specialized epithelial and hematopoietic cells, which express pattern recognition receptors that act as both sentinels and mediators of pulmonary innate immunity. Toll-like receptors (TLRs) mediate a particularly critical role in pathogen recognition and subsequent initiation of the host immune response. In this review, we will summarize current knowledge of TLRs and their bacterial ligands and explore their role in respiratory infections. Moreover, we will highlight recent advances in the role of TLRs in pulmonary infections from a human immunogenetics perspective.

MCP-1 promoter variant -362C associated with protection from pulmonary tuberculosis in Ghana, West Africa

Current endeavour focuses on human genetic factors that contribute to susceptibility to or protection from tuberculosis (TB). Monocytes are crucial in containing Mycobacterium tuberculosis infection, and the monocyte chemoattractant protein-1 (MCP-1) cytokine plays a role in their recruitment to the site of infection. The G allele of the MCP-1 promoter polymorphism at position -2581 relative to the ATG transcription start codon has been described to be associated in Mexican and Korean TB patients with increased susceptibility to TB. We genotyped this and additional MCP-1 variants in sample collections comprising more than 2000 cases with pulmonary TB and more than 2300 healthy controls and 332 affected nuclear families from Ghana, West Africa, and more than 1400 TB patients and more than 1500 controls from Russia. In striking contrast to previous reports, MCP-1 -2581G was significantly associated with resistance to TB in cases versus controls [odds ratio (OR) 0.81, corrected P-value (P(corr)) = 0.0012] and nuclear families (OR 0.72, P(corr) = 0.04) and not with disease susceptibility, whereas in the Russian sample no evidence of association was found (P = 0.86). Our and other results do not support an association of MCP-1 -2581 with TB. In the Ghanaian population, eight additional MCP-1 polymorphisms were genotyped. MCP-1 -362C was associated with resistance to TB in the case-control collection (OR 0.83, P(corr) = 0.00017) and in the affected families (OR 0.7, P(corr) = 0.004). Linkage disequilibrium (LD) and logistic regression analyses indicate that, in Ghanaians, the effect results exclusively from the MCP-1 -362 variant, whereas the effect of -2581 may in part be explained by its LD with -362.