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

Challenges for the Newborn Following Influenza Virus Infection and Prospects for an Effective Vaccine

Newborns are at significantly increased risk of severe disease following infection with influenza virus. This is the collective result of their naïve status, altered immune responsiveness, and the lack of a vaccine that is effective in these individuals. Numerous studies have revealed impairments in both the innate and adaptive arms of the immune system of newborns. The consequence of these alterations is a quantitative and qualitative decrease in both antibody and T cell responses. This review summarizes the hurdles newborns experience in mounting an effective response that can clear influenza virus and limit disease following infection. In addition, the challenges, as well as the opportunities, for developing vaccines that can elicit protective responses in these at risk individuals are discussed.

Challenges for the Newborn Immune Response to Respiratory Virus Infection and Vaccination

The initial months of life reflect an extremely challenging time for newborns as a naïve immune system is bombarded with a large array of pathogens, commensals, and other foreign entities. In many instances, the immune response of young infants is dampened or altered, resulting in increased susceptibility and disease following infection. This is the result of both qualitative and quantitative changes in the response of multiple cell types across the immune system. Here we provide a review of the challenges associated with the newborn response to respiratory viral pathogens as well as the hurdles and advances for vaccine-mediated protection.

Neonatal and infant immunity for tuberculosis vaccine development: importance of age-matched animal models

Neonatal and infant immunity differs from that of adults in both the innate and adaptive arms, which are critical contributors to immune-mediated clearance of infection and memory responses elicited during vaccination. The tuberculosis (TB) research community has openly admitted to a vacuum of knowledge about neonatal and infant immune responses to Mycobacterium tuberculosis (Mtb) infection, especially in the functional and phenotypic attributes of memory T cell responses elicited by the only available vaccine for TB, the Bacillus Calmette-Guérin (BCG) vaccine. Although BCG vaccination has variable efficacy in preventing pulmonary TB during adolescence and adulthood, 80% of endemic TB countries still administer BCG at birth because it has a good safety profile and protects children from severe forms of TB. As such, new vaccines must work in conjunction with BCG at birth and, thus, it is essential to understand how BCG shapes the immune system during the first months of life. However, many aspects of the neonatal and infant immune response elicited by vaccination with BCG remain unknown, as only a handful of studies have followed BCG responses in infants. Furthermore, most animal models currently used to study TB vaccine candidates rely on adult-aged animals. This presents unique challenges when transitioning to human trials in neonates or infants. In this Review, we focus on vaccine development in the field of TB and compare the relative utility of animal models used thus far to study neonatal and infant immunity. We encourage the development of neonatal animal models for TB, especially the use of pigs.

Changes in blood monocyte Toll-like receptor and serum surfactant protein A reveal a pathophysiological mechanism for community-acquired pneumonia in patients with type 2 diabetes

BACKGROUND

The lung is one of the target organs of microangiopathy in diabetes mellitus (DM); patients with type 2 diabetes mellitus (T2DM) are vulnerable to pneumonia, and a variety of pathophysiological mechanisms has been described.

AIM

This study aimed to determine the pathophysiological mechanism of community-acquired pneumonia (CAP) in T2DM patients.

METHODS

A total of 90 individuals was included in this study comprised of three groups (n = 30): healthy control, T2DM and T2DM+ CAP groups. Toll-like receptor (TLR)2 and 4 protein and messenger RNA expression in peripheral blood monocytes(PBMC) was assessed by western blot and reverse transcription-polymerase chain reaction, respectively, and surfactant protein A (SP-A) levels were examined in serum samples by enzyme-linked immunosorbent assay.

RESULTS

In T2DM and T2DM+CAP groups, levels of both TLR2/4 protein and mRNA in PBMC were decreased compared with controls (P <0.05), with lower levels observed in the T2DM+CAP group in comparison with T2DM patients (P <0.05). The serum SP-A levels in T2DM+CAP individuals were significantly higher than the values obtained for T2DM patients (P <0.05). It also showed apparent increases when compared with that in controls although no statistical significance was detected.

CONCLUSION

In T2DM patients with pneumonia, TLR2/4 levels in PBMC and serum SP-A were altered, maybe playing an important role in the susceptibility to pneumonia in T2DM patients.

Vaccines against respiratory viral pathogens for use in neonates: opportunities and challenges

The first six months of life reflect a time of high susceptibility to severe disease following respiratory virus infection. Although this could be improved significantly by immunization, current vaccines are not approved for use in these very young individuals. This is the result of the combined effects of poor immune responsiveness and safety concerns regarding the use of live attenuated vaccines or potent adjuvants in this population. Vaccines to effectively combat respiratory viral infection ideally would result in robust CD4(+) and CD8(+) T cell responses, as well as high-affinity Ab. Inclusion of TLR agonists or single-cycle viruses is an attractive approach for provision of signals that can act as potent stimulators of dendritic cell maturation, as well as direct activators of T and/or B cells. In this article, I discuss the challenges associated with generation of a robust immune response in neonates and the potential for adjuvants to overcome these obstacles.

Sinonasal epithelial cell expression of Toll-like receptor 9 is elevated in cystic fibrosis-associated chronic rhinosinusitis

BACKGROUND

Cystic fibrosis (CF) patients frequently suffer from chronic rhinosinusitis (CRS). The extent to which alterations in sinonasal innate immunity contribute to this disease process is unknown. Activation of sinonasal epithelial cell (SNEC) Toll-like receptors (TLRs), an important component of the innate immune system, may be associated with the hyperinflammatory state observed in sinonasal mucosa of CF patients with CRS. This study compares expression of Toll-like receptor 9 (TLR9), in SNRCs collected from CF subjects with CRS to that of normal control subjects.

METHODS

This was a prospective study measuring TLR9 on SNECs collected via endoscopic-guided middle meatal brushings from 8 adult controls and 14 adult subjects with CF-associated CRS.

RESULTS

TLR9 expression was significantly elevated in CF subjects at 91% ± 6% when compared with 76% ± 10% in normal controls (p = 0.001).

CONCLUSION

The significantly greater expression of sinonasal epithelial TLR9 in CF likely reflects increased antimicrobial innate immune activity in chronically colonized and frequently infected CF individuals. However, this finding contrasts with previously reported decreased epithelial TLR9 expression in eosinophilic CRS with nasal polyposis and may indicate differential modulation of innate immunity in Th1-predominent CF versus Th2-dominated CRS with nasal polyps, despite both being diseases of sinonasal mucosal inflammation.

Morphine modulation of toll-like receptors in microglial cells potentiates neuropathogenesis in a HIV-1 model of coinfection with pneumococcal pneumoniae

Chronic drug users account for a third of all cases of AIDS in the United States and the progression to AIDS dementia is accelerated in opiate drug abusers. Clinically, microglial activation better correlates with HIV-associated neurocognitive disorders (HAND) than productive HIV-1 infection in the CNS. Moreover, pneumococcal pneumonia is the most common opportunistic infection in individuals with HAND. We show that coinfection with Streptococcus pneumoniae may be a contributing factor in the increased prevalence of HAND in the opioid-dependent population. To date, there have been no studies published implicating the Toll-like receptors (TLR) in the neurocognitive disorders associated with NeuroAIDS in the context of opportunistic infection. Our studies show for the first time, in a morphine-dependent model, synergistic increase and activation of TLR expression in the presence of HIV-1 protein TAT and S. pneumoniae with a significant increase in proinflammatory cytokines (IL-6, TNF-α) levels. Furthermore, concurrent increases in reactive oxygen species and nitric oxide production leading to increased caspase 3 activation are also observed in both murine and human microglial cells. These effects are recapitulated with TLR 2, 4, and 9 cognate ligands (Pam3CSK4, LPS, and CpG) and significantly attenuated in TLR 2 and 4 knock-out mice and TLR2/4 double knock-out mice. Therefor, our findings clearly suggest for the first time that activation of TLRs on microglia cells by morphine and TAT in the context of S. pneumoniae infection may be a potential mechanism for the increased prevalence of HAND in HIV-infected opioid-dependent patients.

MAPK/AP-1 activation mediates TLR2 agonist-induced SPLUNC1 expression in human lung epithelial cells

BACKGROUND

Short Palate Lung and Nasal epithelium Clone 1 (SPLUNC1) is a newly described host defense protein, primarily expressed in large airway epithelial cells. Reduced SPLUNC1 has been reported in allergic and cigarette smoke-exposed airways. We found that Mycoplasma pneumoniae increases SPLUNC1 in airway epithelium in part via activating TLR2-NF-κB pathway. However, the contribution of additional signaling pathways to TLR2-mediated SPLUNC1 expression remains unclear. In the present study, we investigated if TLR2-induced mitogen-activated protein kinase (MAPK)/activator protein-1 (AP-1) signaling regulates SPLUNC1 expression in human lung epithelial cells.

METHODS

Human lung epithelial NCI-H292 cells were stimulated with a TLR2 agonist Palmitoyl (3)-Cys-Ser-Lys (4)-OH (Pam(3)CSK(4)). MAPK/AP-1 activation and its role in SPLUNC1 regulation were investigated by Western blot, c-Jun activation assay, chromatin immunoprecipitation (ChIP) and real-time PCR. SPLUNC1 promoter activity was assessed by a luciferase reporter assay.

RESULTS

Pam(3)CSK(4) increased SPLUNC1 expression in NCI-H292 cells in a dose- and time-dependent manner, and enhanced SPLUNC1 promoter activity. Pam(3)CSK(4)-treated cells demonstrated activated MAPK and c-Jun compared to untreated cells. ChIP assay indicated increased c-Jun binding to the SPLUNC1 promoter following Pam(3)CSK(4) stimulation. Inhibition of ERK1/2 significantly reduced Pam(3)CSK(4)-mediated c-Jun activation and SPLUNC1 expression.

CONCLUSIONS

Our results for the first time demonstrate that TLR2-mediated MAPK/AP-1 activation up-regulates lung epithelial SPLUNC1 expression at the transcriptional level. Understanding SPLUNC1 gene regulation should provide more specific therapeutic targets to restore deficient SPLUNC1 production in diseased airways.

Association of human TLR1 and TLR6 deficiency with altered immune responses to BCG vaccination in South African infants

The development of effective immunoprophylaxis against tuberculosis (TB) remains a global priority, but is hampered by a partially protective Bacillus Calmette-Guérin (BCG) vaccine and an incomplete understanding of the mechanisms of immunity to Mycobacterium tuberculosis. Although host genetic factors may be a primary reason for BCG's variable and inadequate efficacy, this possibility has not been intensively examined. We hypothesized that Toll-like receptor (TLR) variation is associated with altered in vivo immune responses to BCG. We examined whether functionally defined TLR pathway polymorphisms were associated with T cell cytokine responses in whole blood stimulated ex vivo with BCG 10 weeks after newborn BCG vaccination of South African infants. In the primary analysis, polymorphism TLR6_C745T (P249S) was associated with increased BCG-induced IFN-γ in both discovery (n = 240) and validation (n = 240) cohorts. In secondary analyses of the combined cohort, TLR1_T1805G (I602S) and TLR6_G1083C (synonymous) were associated with increased IFN-γ, TLR6_G1083C and TLR6_C745T were associated with increased IL-2, and TLR1_A1188T was associated with increased IFN-γ and IL-2. For each of these polymorphisms, the hypo-responsive allele, as defined by innate immunity signaling assays, was associated with increased production of TH1-type T cell cytokines (IFN-γ or IL-2). After stimulation with TLR1/6 lipopeptide ligands, PBMCs from TLR1/6-deficient individuals (stratified by TLR1_T1805G and TLR6_C745T hyporesponsive genotypes) secreted lower amounts of IL-6 and IL-10 compared to those with responsive TLR1/6 genotypes. In contrast, no IL-12p70 was secreted by PBMCs or monocytes. These data support a mechanism where TLR1/6 polymorphisms modulate TH1 T-cell polarization through genetic regulation of monocyte IL-10 secretion in the absence of IL-12. These studies provide evidence that functionally defined innate immune gene variants are associated with the development of adaptive immune responses after in vivo vaccination against a bacterial pathogen in humans. These findings could potentially guide novel adjuvant vaccine strategies as well as have implications for IFN-γ-based diagnostic testing for TB.

Tuberculosis (TB) is one of the leading infectious causes of death worldwide. The current vaccine for TB, BCG, is widely used but it is not highly effective in preventing disease. We investigated the role of host genetics in the immune response to BCG vaccination. We found that variants of innate immunity genes (TLR1 and TLR6) were associated with BCG-induced immune responses after vaccination. These findings may guide new strategies for vaccine development as well as diagnosis of TB.

Flagellin-induced corneal antimicrobial peptide production and wound repair involve a novel NF-kappaB-independent and EGFR-dependent pathway

Background

The bacterial protein flagellin plays a major role in stimulating mucosal surface innate immune response to bacterial infection and uniquely induces profound cytoprotection against pathogens, chemicals, and radiation. This study sought to determine signaling pathways responsible for the flagellin-induced inflammatory and cytoprotective effects on human corneal epithelial cells (HCECs).

Methodology/Principal Findings

Flagellin purified from Pseudomonas aeruginosa (strain PAK) or live bacteria were used to challenge cultured HCECs. The activation of signaling pathways was assessed with Western blot, and the secretion of cytokine/chemokine and production of antimicrobial peptides (AMPs) were measured with ELISA and dot blot, respectively. Effects of flagellin on wound healing were assessed in cultured porcine corneas. L94A (a site mutation in TLR5 binding region) flagellin and PAK expressing L94A flagellin were unable to stimulate NF-κB activation, but were potent in eliciting EGFR signaling in a TGF-α–related pathway in HCECs. Concomitant with the lack of NF-κB activation, L94A flagellin was ineffective in inducing IL-6 and IL-8 production in HCECs. Surprisingly, the secretion of two inducible AMPs, LL-37 and hBD2, was not affected by L94A mutation. Similar to wild-type flagellin, L94A induced epithelial wound closure in cultured porcine cornea through maintaining EGFR-mediated signaling.

Conclusions/Significance

Our data suggest that inflammatory response mediated by NF-κB can be uncoupled from epithelial innate defense machinery (i.e., AMP expression) and major epithelial proliferation/repair pathways mediated by EGFR, and that flagellin and its derivatives may have broad therapeutic applications in cytoprotection and in controlling infection in the cornea and other mucosal tissues.

Effect of salvia miltiorrhizae on the expressions of TLR4 protein in the liver of rats with SAP or OJ

To investigate the effect of salvia miltiorrhizae on the expressions of TLR4 protein in the liver of rats with severe acute pancreatitis (SAP) and obstructive jaundice (OJ), and explore the protective mechanism of salvia miltiorrhizae on the liver of rats. A total of 288 mice was used in SAP- (n = 108) and OJ-associated experiments (n = 180). The rats were randomly divided into sham-operated, model control and treated group. Based on the different time points after operation, these groups were subdivided into 3, 6 and 12 h subgroups (SAP rats, n = 12) or 7, 14, 21 and 28 days subgroups (OJ rats, n = 15). At the corresponding time points after operation, blood and liver specimens were collected to determine the contents of endotoxin and TNF-alpha in the blood as well as the expression levels of TLR4 protein in the liver. Compared with the corresponding model control group, though the number of dead SAP or OJ rats in the treated group declined, no statistical difference was noted; The levels of plasma endotoxin in SAP (at 6 and 12 h) or OJ rats in the treated group decreased significantly (P < 0.001 and P < 0.01, respectively); The levels of serum TNF-alpha in SAP (at 12 h) or OJ rats (on 14 days) declined (P < 0.001 and P < 0.01, respectively); The staining intensity as well as the product of staining intensity and positive rate of TRL4 protein only significantly declined on 7 and 28 days in OJ rats (P < 0.01). On 7 days, treated group in positive rate of TLR4 protein were significantly lower than that in model control group (P < 0.01). The pathological changes in different treated groups of SAP and OJ rats were improved. Salvia miltiorrhizae is able to reduce the levels of plasma endotoxin and inhibit effectively the expressions of TLR4 protein in the liver of SAP or OJ rats, thereby decreasing inflammatory reaction and exerting protective effect on liver function.

Respiratory infections in immunocompromised patients

PURPOSE OF REVIEW

Immunosuppressive states and therapies are becoming common in clinical practice. Recent advances and trends in bacterial, fungal, viral and parasitic pulmonary infections in immunosuppressed patients are described.

RECENT FINDINGS

Pulmonary infections can jeopardize the prognosis of immunosuppressed patients. The number of patients infected with multidrug-resistant bacteria or opportunistic pathogens like rapid-growing environmental mycobacteria, Strongyloides stercoralis or Rhodococcus equi is increasing with the increased numbers of immunosuppressed patients due to HIV/AIDS and the potent immunosuppressive therapies used in solid-organ and haematopoietic transplantations, cancer and systemic illnesses. The slow development of more effective antibiotics underlines the necessity of preventive measures, development of rapid detection tests for pathogens and appropriate treatment regimens to avoid development of further resistance.

SUMMARY

Adequate prophylaxis, clinical suspicion, microbiological and molecular investigations, drug susceptibility-based antibiotic treatment and new drug development are strategies required to face up to the challenge of pulmonary infections in immunodepressed patients.