Experimental bacterial meningitis : studies on immunopathogenesis and immunoregulation

Bacterial meningitis is an inflammatory disease of the central nervous system (CNS) which occurs when bacteria gain entry to the subarachnoid space (SAS). Bacterial replication or Iysis leads to the exposure of cell components. These products elicit synthesis and release of IL-1ß, TNF-a, and/or other endogenous mediators into the cerebrospinal fluid (CSF) and intravascular space by endothelial cells. These mediators facilitate activation of polymorphonuclear leukocytes (PMNs ) as well as expression of and receptor affinity for adhesion and transendothelial passage of PMNs into the SAS. Within the SAS, PMNs are further activated by bacterial products or by cytokines to release molecules, including platelet activating factor, prostaglandins, nitric oxide and toxic oxygen metabolites that may contribute to the neurologic damage.

Intraperitoneal (i.p.) inoculation of Haemophilus influenzae type b (Hib) to 3 weeks old rats resulted in nonlethal meningitis. Using in situ hybridization (ISH), levels of cytokine mRNA expressing cells were determined in the brain, CSF and spleen from Hib-inoculated and uninfected control rats. IFN-y, IL-lß, IL-4, IL-6, IL-10, IL-12 and TNF-a mRNA levels were elevated at 12 h post inoculation (p.i.) in spleen and CSF. At this time point, strong expression of IL-6 and TGF-ß was detected in the brain, and also of L-10 at 48 h. The expression of IFN-y and IL-12 was very low throughout the observation time in the brain. Delayed cytokine induction occurred in CSF compared to spleen and brain. TGF-ß was high in CSF at 48 h, and some elevation of IL-lß, IL-6, IL-10, TNF-a, IFN-y and IL-12 was evident at 72 h p.i. This may suggest that measures promoting production of TGF-ß and/or IL-10 should be evaluated in treatment of bacterial meningitis.

Intracisternal inoculation of Hib or Streptococcus (S.) pneumoniae to 3 weeks old rats resulted in lethal meningitis. Levels of cytokine mRNA expression of IL-lß, IL4, IL-6, IL-10, IL-12, TNF-a, TNF-ß, IFN-y, and TGF-ß in the brain from Hib or S pneumoniae-inoculated and in uninfected control rats was evaluated. The production of IL-lß, IL-4, IL-6 and IFN-y was also evaluated by immunohistochemistry (IHC). In the brain of Hib inoculated rats, there was marked mRNA expression of IL-lß, IL-6, TNF-a, IL-12 and IFN-y. IL-lß, IL-6 and TNF-a were upregulated, with similar patterns of induction IFN-y and TNF-ß were up-regulated within 8 h p.i. IL-10 and TGF-ß were down-regulated at 18 h p.i., while IL-4 was not detected. In contrast, the brain of S. pneumoniae-inoculated rats showed lower levels of IL-lß, IL-6 and TNF-a, but higher levels of TNF-ß and detectable mRNA expression of IL- 4 when compared to the Hib-inoculated rats. IL-12, IFN-y, IL-10 and TGF-ß exhibited similar patterns of induction in the brains of Hib and S pneumoniae- inoculated rats. At 18 h p.i., IHC showed similar patterns of IL-lß, IL-4, IL-6 and IFN-y as mRNA expression The predominant cytokine messages in Hib-infected rats were associated with a pro-inflammatory response, while S. pneumoniae-infected rats responded with mixed pro- and anti-inflammatory responses.

Brains of infant rats challenged with Hib i.p. resulted in the time-dependent mRNA of expression of MIP-2, MlP-la, MCP-I and RANTES, which was maximal 24-48 h p.i. IHC showed significant increases in neutrophils and macrophages infiltrating the meninges, the ventricular system and the periventricular area. The kinetics of MIP-2, MlP-la, MCP-I and RANTES rnRNA expression paralleled those of the recruitment of inflammatory cells and disease severity. Blocking the function of MIP-2 and MlP-la resulted in significant reduction of neutrophils. Administration of anti-MCP-I Ab significantly decreased macrophage infiltration. Combined studies using ISH and IHC showed that MIP-2 and MlP-la positive cells were neutrophils and macrophages. MCP-I positive cells were neutrophils macrophages and astrocytes. Expression of RANTES was localized predominantly to resident astrocytes and microglia. Blocking of MIP-2 or MlP-la bioactivity in vivo results in decreased influx of neutrophils.

An immunoregulatory mechanism involving release of neutralizing autoantibodies (Aabs) to self-cytokines during bacterial infection is presented herein. High levels of cells expressing mRNA for IFN-y and TNF-a were detected 12 to 48 h p.i. in splenocytes, and large numbers of IFN-y-secreting cells were present in the spleen on day 3 p.i. These cytokines were undetectable at day 9 and 14 p.i. Increased titers of Aabs to both cytokines were observed, with a peak at day 7 p.i. and with very low levels at day 30. Similarly, the induction of five potentially important cytokines and their autoantibody responses in the CSF was examined. Protein levels of the cytokines IFN-y, TNF-a, TGF-ß, IL-4 and 10, were detected on day 3 p.i. with maximum induction at day 8. Thereafter, cytokine levels become undetectable. Increased Aab titers to these cytokines, except IL-4, were registered with peak levels between day 7 and 9. Upon reinoculation with Hib at day 30, regeneration of Aabs was recorded at day 37. These data suggest a role for autoimmunity in cytokine regulation and that a maintained balance of this mechanism may protect from sequelae.