Role of purinergic signaling in experimental pneumococcal meningitis.

Excessive neutrophilic inflammation contributes to brain pathology and adverse outcome in pneumococcal meningitis (PM). Recently, we identified the NLRP3 inflammasome/interleukin (IL)-1ß pathway as a key driver of inflammation in PM. A critical membrane receptor for NLRP3 inflammasome activation is the ATP-activated P2 purinoceptor (P2R) P2X7. Thus, we hypothesized involvement of ATP and P2Rs in PM. The functional role of ATP was investigated in a mouse meningitis model using P2R antagonists. Brain expression of P2Rs was assessed by RT-PCR. ATP levels were determined in murine CSF and cell culture experiments. Treatment with the P2R antagonists suramin or brilliant blue G did not have any impact on disease course. This lack of effect might be attributed to meningitis-associated down-regulation of brain P2R expression and/or a drop of cerebrospinal fluid (CSF) ATP, as demonstrated by RT-PCR and ATP analyses. Supplemental cell culture experiments suggest that the reduction in CSF ATP is, at least partly, due to ATP hydrolysis by ectonucleotidases of neutrophils and macrophages. In conclusion, this study suggests that ATP-P2R signaling is only of minor or even no significance in PM. This may be explained by down-regulation of P2R expression and decreased CSF ATP levels.

Impaired mast cell-driven immune responses in mice lacking the transcription factor NFATc2.

The three calcium-dependent factors NFATc1, c2, and c3 are expressed in cells of the immune system and play pivotal roles in modulating cellular activation. With regard to NFATc2, it was reported that NFATc2-deficient mice display increased immune responses in several models for infection and allergy in vivo. This led to the assumption that NFATc2 is involved in the maintenance of immune homeostasis. Using the synthetic TLR7 agonist imiquimod as an adjuvant in epicutaneous peptide immunization, we observed that both the inflammatory reaction and the peptide-specific CTL response are severely impaired in NFATc2-deficient mice. Detailed analyses revealed that early production of proinflammatory cytokines, lymph node hypertrophy, and migration of Langerhans cells are strongly reduced in NFATc2-deficient animals. With the aid of mast cell-deficient mice and reconstitution experiments using mast cells derived from either NFATc2-deficient mice or wild-type controls, we were able to show that NFATc2 expressed in mast cells is critical for the initiation of inflammation, migration of Langerhans cells, and the development of full-blown CTL responses following epicutaneous immunization. Thus, NFATc2 is an important factor controlling mast cell accessory function at the interface of innate and adaptive immunity.

Meningitis-associated hearing loss: protection by adjunctive antioxidant therapy.

Hearing loss is the most frequent long-term complication of pneumococcal meningitis, affecting up to 40% of survivors. Unfortunately, adjuvant therapy with dexamethasone has failed to satisfactorily reduce its incidence. Therefore, we evaluated the use of antioxidants for the adjunctive therapy of meningitis-associated deafness. Eighteen hours after intracisternal injection of 7.5 x 10(5) colony-forming units of Streptococcus pneumoniae, rats were treated systemically either with ceftriaxone and the antioxidants and peroxynitrite scavengers Mn(III)tetrakis(4-benzoic acid)-porphyrin (MnTBAP) or N-acetyl-L-cysteine (NAC) or placebo (1 ml phosphate-buffered saline) for 4 days. Hearing was assessed by auditory brainstem response audiometry. Adjunctive antioxidant therapy significantly reduced the long-term hearing loss (14 days after infection) for square wave impulses (mean hearing loss +/- SD: ceftriaxone and placebo, 45+/-26 dB; ceftriaxone and MnTBAP, 9+/-23 dB; ceftriaxone and NAC, 19+/-30 dB) as well as 1 kHz (ceftriaxone and placebo, 28+/-19 dB; ceftriaxone and MnTBAP, 10+/-16 dB; ceftriaxone and NAC, 10+/-17 dB), and 10 kHz tone bursts (ceftriaxone and placebo, 62+/-27 dB; ceftriaxone and MnTBAP, 16+/-13 dB; ceftriaxone and NAC, 25+/-26 dB). Furthermore, both antioxidants attenuated the morphological correlates of meningogenic hearing loss, namely, long-term blood-labyrinth barrier disruption, spiral ganglion neuronal loss, and fibrous obliteration of the perilymphatic spaces. Adjuvant antioxidant therapy is highly otoprotective in meningitis and therefore is a promising future treatment option.