Regulation of Streptococcus pneumoniae distribution by Toll-like receptor 2 in vivo

Cryptococcus neoformans-astrocyte interactions: effect on fungal blood brain barrier disruption, brain invasion, and meningitis progression

Cryptococcus neoformans is an opportunistic, neurotropic, and encapsulated fungus that causes life-threatening cryptococcal meningitis (CM), especially in regions of the world where AIDS is endemic. The polysaccharide capsule of C. neoformans is the fungus major virulent factor, being copiously released during infection and causing immunosuppressive defects in the host. Although the capsular material is commonly associated with reactive astrocytes in fatal CM, little is known about the molecular and cellular interactions among astroglia and C. neoformans. As astrocytes also make up the neurovascular unit at the blood-brain barrier (BBB), which C. neoformans must transverse to colonize the central nervous system and cause CM; these cells may play a significant regulatory role in the prevention and progression of infection. For example, astrocytes are implicated in neurological disease including the regulation of cerebral intracranial pressure, immune function, and water homeostasis. Hence, in this review, we provide a general overview of astroglia biology and discuss the current knowledge on C. neoformans-astrocyte interactions including their involvement in the development of CM. This "gliocentric view" of cerebral cryptococcosis suggests that therapeutic interventions particularly targeting at preserving the neuroprotective function of astrocytes may be used in preventing and managing C. neoformans BBB transmigration, brain invasion, colonization, and meningitis.

Choroid plexus and the blood-cerebrospinal fluid barrier in disease

The choroid plexus (CP) forming the blood-cerebrospinal fluid (B-CSF) barrier is among the least studied structures of the central nervous system (CNS) despite its clinical importance. The CP is an epithelio-endothelial convolute comprising a highly vascularized stroma with fenestrated capillaries and a continuous lining of epithelial cells joined by apical tight junctions (TJs) that are crucial in forming the B-CSF barrier. Integrity of the CP is critical for maintaining brain homeostasis and B-CSF barrier permeability. Recent experimental and clinical research has uncovered the significance of the CP in the pathophysiology of various diseases affecting the CNS. The CP is involved in penetration of various pathogens into the CNS, as well as the development of neurodegenerative (e.g., Alzheimer´s disease) and autoimmune diseases (e.g., multiple sclerosis). Moreover, the CP was shown to be important for restoring brain homeostasis following stroke and trauma. In addition, new diagnostic methods and treatment of CP papilloma and carcinoma have recently been developed. This review describes and summarizes the current state of knowledge with regard to the roles of the CP and B-CSF barrier in the pathophysiology of various types of CNS diseases and sets up the foundation for further avenues of research.

Identifying host genetic risk factors in the context of public health surveillance for invasive pneumococcal disease

Host genetic factors that modify risk of pneumococcal disease may help target future public health interventions to individuals at highest risk of disease. We linked data from population-based surveillance for invasive pneumococcal disease (IPD) with state-based newborn dried bloodspot repositories to identify biological samples from individuals who developed invasive pneumococcal disease. Genomic DNA was extracted from 366 case and 732 anonymous control samples. TagSNPs were selected in 34 candidate genes thought to be associated with host response to invasive pneumococcal disease, and a total of 326 variants were successfully genotyped. Among 543 European Americans (EA) (182 cases and 361 controls), and 166 African Americans (AA) (53 cases and 113 controls), common variants in surfactant protein D (SFTPD) are consistently underrepresented in IPD. SFTPD variants with the strongest association for IPD are intronic rs17886286 (allelic OR 0.45, 95% confidence interval (CI) [0.25, 0.82], with p = 0.007) in EA and 5' flanking rs12219080 (allelic OR 0.32, 95%CI [0.13, 0.78], with p = 0.009) in AA. Variants in CD46 and IL1R1 are also associated with IPD in both EA and AA, but with effects in different directions; FAS, IL1B, IL4, IL10, IL12B, SFTPA1, SFTPB, and PTAFR variants are associated (p≤0.05) with IPD in EA or AA. We conclude that variants in SFTPD may protect against IPD in EA and AA and genetic variation in other host response pathways may also contribute to risk of IPD. While our associations are not corrected for multiple comparisons and therefore must be replicated in additional cohorts, this pilot study underscores the feasibility of integrating public health surveillance with existing, prospectively collected, newborn dried blood spot repositories to identify host genetic factors associated with infectious diseases.

Pattern recognition by Toll-like receptors

The mammalian immune system senses pathogens through pattern recognition receptors and responds with activation. The Toll-like receptors (TLRs) that are expressed on antigen presenting cells such as macrophages and dendritic cells play a critical role in this process. Their signaling activates these cells and leads to an innate immune response with subsequent initiation of an adaptive immune response. Each TLR recognizes specific structures and induces common inflammatory cytokines. However, some TLRs have specific functions, such as induction of Type I interferons. The TLR dependent cytokine response is reflected in the induction of common and specific signaling pathways leading to adequate immune responses for different pathogens. Some TLRs are also activated by endogenous structures that are released during infection, but these structures may promote or sustain autoimmune diseases under certain circumstances. In addition, TLRs directly shape adaptive immune responses of T and B cells and play an important role in homeostasis of gut epithelium and lung repair after injury.

Role for Toll-like receptor 2 in the immune response to Streptococcus pneumoniae infection in mouse otitis media

Streptococcus pneumoniae is the most common pathogen associated with otitis media. To examine the role of Toll-like receptor 2 (TLR2) in host defense against Streptococcus pneumoniae infection in the middle ear, wild-type (WT; C57BL/6) and TLR2-deficient (TLR2(-/-)) mice were inoculated with Streptococcus pneumoniae (1 x 10(6) CFU) through the tympanic membrane. Nineteen of 37 TLR2(-/-) mice showed bacteremia and died within 3 days after the challenge, compared to only 4 of 32 WT mice that died. Of those that survived, more severe hearing loss in the TLR2(-/-) mice than in the WT mice was indicated by an elevation in auditory-evoked brain stem response thresholds at 3 or 7 days postinoculation. The histological pathology was characterized by effusion and tissue damage in the middle ear, and in the TLR2(-/-) mice, the outcome of infection became more severe at 7 days. At both 3 and 7 days postchallenge, the TLR2(-/-) mice had higher blood bacterial titers than the WT mice (P < 0.05), and typical bacteria were identified in the effusion from both ears of both mouse groups by acridine orange staining. Moreover, by 3 days postchallenge, the mRNA accumulation levels of NF-kappaB, tumor necrosis factor alpha, interleukin 1beta, MIP1alpha, Muc5ac, and Muc5b were significantly lower in the ears of TLR2(-/-) mice than in WT mice. In summary, TLR2(-/-) mice may produce relatively low levels of proinflammatory cytokines following pneumococcal challenge, thus hindering the clearance of bacteria from the middle ear and leading to sepsis and a high mortality rate. This study provides evidence that TLR2 is important in the molecular pathogenesis and host response to otitis media.

Effects of Toll-like receptor 2 agonist Pam(3)CysSK(4) on inflammation and brain damage in experimental pneumococcal meningitis

TLR2 signaling participates in the pathogenesis of pneumococcal meningitis. In infant rats, the TLR2 agonist Pam(3)CysSK(4) was applied intracisternally (0.5 microg in 10 microl saline) alone or after induction of pneumococcal meningitis to investigate the effect of TLR2 activation on cerebrospinal fluid (CSF) inflammation and hippocampal apoptosis. A dose effect of Pam(3)CysSK(4) on apoptosis was investigated by intracisternal application of 0.5 microg in 10 microl saline and 40 microg in 20 microl saline. Pam(3)CysSK(4) neither induced apoptosis in sham-operated mice nor aggravated apoptosis in acute infection. However, Pam(3)CysSK(4) induced pleocytosis, TNF-alpha and MMP-9 in CSF in sham-infection but not during acute meningitis. We conclude that TLR2 signaling triggered by Pam(3)CysSK(4) at a dosage capable to induce a neuroinflammatory response does not induce hippocampal apoptosis in the infant rat model of experimental pneumococcal meningitis.

Innate immunity to pneumococcal infection of the central nervous system depends on toll-like receptor (TLR) 2 and TLR4

BACKGROUND

Recent studies have suggested that, in addition to Toll-like receptor (TLR) 2, other pattern recognition receptors mediate activation of the immune response after infection of the central nervous system (CNS) with Streptococcus pneumoniae (SP).

METHODS

Using a mouse meningitis model, we investigated the influence of TLR4 single deficiency (TLR4(-/-)), TLR2/TLR4 double deficiency (TLR2/4(-/-)), and TLR2/TLR4/TLR9 triple deficiency (TLR2/4/9(-/-)) on the immune response of the CNS to SP infection. To identify the cell populations that mediate the responses to SP, we generated TLR2/4(-/-)-wild-type (wt) bone marrow (BM) chimeras.

RESULTS

Compared with infected wt mice, infected TLR2/4(-/-) and TLR2/4/9(-/-) mice had similar reductions in brain cytokine levels, pleocytosis, and cerebral pathologic findings, whereas no such effect was noted in infected TLR4(-/-) mice. The attenuated immune response was paralleled by an impaired host defense that resulted in worsening of disease. Analysis of the chimeric mice after infection showed that mere TLR2/4 deficiency, either of radioresistant cells or of transplanted BM-derived cells, was sufficient to mount a substantial cerebral immune response, such as that noted in wt mice.

CONCLUSION

In murine SP meningitis, TLR2 and TLR4 expressed on radioresistant and transplanted BM-derived cells were major cellular sensors of invading SP inducing inflammatory responses.

Role of the innate immune system in host defence against bacterial infections: focus on the Toll-like receptors

The innate immunity plays a critical role in host protection against pathogens and it relies amongst others on pattern recognition receptors such as the Toll-like receptors (TLRs) and the nucleotide-binding oligomerization domains proteins (NOD-like receptors, NLRs) to alert the immune system of the presence of invading bacteria. Since their recent discovery less than a decade ago, both TLRs and NLRs have been shown to be crucial in host protection against microbial infections but also in homeostasis of the colonizing microflora. They recognize specific microbial ligands and with the use of distinct adaptor molecules, they activate different signalling pathways that in turns trigger subsequent inflammatory and immune responses that allows a immediate response towards bacterial infections and the initiation of the long-lasting adaptive immunity. In this review, we will focus on the role of the TLRs against bacterial infections in humans in contrast to mice that have been used extensively in experimental models of infections and discuss their role in controlling normal flora or nonpathogenic bacteria. We also highlight how bacteria can evade recognition by TLRs.

Adjuvant TACE inhibitor treatment improves the outcome of TLR2-/- mice with experimental pneumococcal meningitis

Background

Streptococcus (S.) pneumoniae meningitis has a high lethality despite antibiotic treatment. Inflammation is a major pathogenetic factor, which is unresponsive to antibiotics. Therefore adjunctive therapies with antiinflammatory compounds have been developed. TNF484 is a TNF-alpha converting enzyme (TACE) inhibitor and has been found efficacious in experimental meningitis. Toll-like receptor 2 (TLR2) contributes to host response in pneumococcal meningitis by enhancing bacterial clearing and downmodulating inflammation. In this study, TNF484 was applied in mice, which lacked TLR2 and exhibited a strong meningeal inflammation.

Methods

10³ CFU S. pneumoniae serotype 3 was inoculated subarachnoidally into C57BL/6 wild type (wt) mice or TLR2^-/-, CD14^-/- and CD14^-/-/TLR2^-/- mice. Severity of disease and survival was followed over 9 days. Response to antibiotics (80 mg/kg ceftriaxone i.p. for 5 days) and/or TACE inhibitor treatment (1 mg/kg s.c. twice daily for 4 days) was evaluated. Animals were sacrificed after 12, 24, and 48 h for analysis of bacterial load in cerebrospinal fluid (CSF) and brain and for TNF and leukocyte measurements in CSF.

Results

TLR2^-/- mice were significantly sicker than the other mouse strains 24 h after infection. All knockout mice showed higher disease severity after 48 h and died earlier than wt mice. TNF release into CSF was significantly more elevated in TLR2^-/- than in the other strains after 24 h. Brain bacterial numbers were significantly higher in all knockout than wt mice after 24 h. Modulation of outcome by antibiotic and TACE inhibitor treatment was evaluated. With antibiotic therapy all wt, CD14^-/- and TLR2^-/-/CD14^-/- mice, but only 79% of TLR2^-/- mice, were rescued. TACE inhibitor treatment alone did not rescue, but prolonged survival in wt mice, and in TLR2^-/- and CD14^-/- mice to the values observed in untreated wt mice. By combined antibiotic and TACE inhibitor treatment 95% of TLR2^-/- mice were rescued.

Conclusion

During pneumococcal meningitis strong inflammation in TLR2-deficiency was associated with incomplete responsiveness to antibiotics and complete response to combined antibiotic and TACE inhibitor treatment. TACE inhibitor treatment offers a promising adjuvant therapeutic strategy in pneumococcal meningitis.

Strategies to prevent neuronal damage in paediatric bacterial meningitis

PURPOSE OF REVIEW

The mortality of bacterial meningitis can reach 30%, and up to 50% of survivors suffer from persisting neurological deficits as a consequence of the disease. The incidence of neurological sequelae of bacterial meningitis has not improved over the last decade. Adjunctive therapeutic options are limited, and ongoing research into the pathophysiology of brain damage in bacterial meningitis aims at providing the scientific basis for future development of more efficient adjunctive options.

RECENT FINDINGS

In a population with good access to health care, dexamethasone given before or at the time of initiation of antibiotic therapy acts beneficially in paediatric pneumococcal meningitis, but not in meningococcal meningitis. In experimental animal models, brain-derived neurotrophic factor protected against brain injury and improved hearing while melatonin, which has antioxidant properties among other effects, reduced neuronal death. Transgene technology can be used to provide new insights into the pathophysiology of the disease and to identify potential therapeutic targets.

SUMMARY

Although dexamethasone improves outcome of bacterial meningitis under defined circumstances, the morbidity of bacterial meningitis still remains unacceptably high. Experimental models may help to identify new therapeutic strategies to further improve the neurological outcome in young children suffering from bacterial meningitis.

Toll-like receptor 2 deficiency delays pneumococcal phagocytosis and impairs oxidative killing by granulocytes

Phagocytosis and killing of Streptococcus pneumoniae was compared in blood-derived wild-type (WT) and Toll-like receptor 2 (TLR2)-deficient (TLR2-/-) polymorphonuclear leukocytes (PMN). Phagocytosis of green fluorescent protein-transformed pneumococci was delayed in TLR2-/- PMN. These cells exhibited also a lower oxidative bactericidal activity against S. pneumoniae than WT PMN, suggesting that TLR2 modulates bacterial clearance in PMN.