Pathophysiology of bacterial meningitis : Contributions by experimental animal models

M~decine et Maladies lnfectieuses - - 1989 - - 19 - 11 bis - 694 h 698

P A T H O P H Y S I O L O G Y OF B A C T E R I A L MENINGITIS : CONTRIBUTIONS BY EXPERIMENTAL A N I M A L M O D ELS* by M.G. Tii.UBER'" SUMMARY

Experimental animal models of meningitis have helped to elucidate major aspects of the pathophysiology of bacterial meningitis. Products derived from the meningeal pathogens (teichoic acid, lipopolysaccharides) trigger a cascade of events that includes the release of various cytokines, the induction of an inflammatory response in the subarachnoid space, the disruption of the blood brain barrier, alterations of cerebral blood flow, increased intracranial pressure, and the development of brain edema. Using antibodies and pharmacologic approaches, this chain of events can be modulated on various levels, thus providing the potential for adjunctive therapies to complement antibiotic treatment of bacterial meningitis.

Key-words : Meningitis- CSF inflammation - Brain edema - lntracranial pressure - Cerebral blood flow - Dexamethasone. In an attempt to better understand the pathophysiology of bacterial meningitis, we and others have used experimental animal models to disect out factors that may contribute to the development of brain damage during the disease (7). Over the last few years, this work has let to considerable insights into the disease that, in the near future, should allow to design rational new therapies to be used in adjunct with antibiotics. In this short review, several important aspects of the disease that have recently emerged will be summarized. Specifically, we will focus on three pathophysiologic hallmarks of the disease that have been studied in our laboratory: brain edema, intracranial pressure and cerebral blood flow. BACTERIAL PRODUCTS TRIGGER PATHOPHYSIOLOGY OF MENINGITIS Work from different laboratories has identified specific bacterial components that are biologically highly active and appear to be responsible for the initiation of the pathophysiologic events following bacterial seeding of the subarachnoid space (7). Recognition by the host of these bacterial products, as they are released by the * Communication prtsentte au XXXIX~me Congr~s de la Socitt6 de PathologicInfectieuse de Langue Fran~aise, tenu Paris le 8 dtcembre 1989 sous le titre: "Les infections exptrimentales". ** The Medical Service, San Francisco General Hospital, and the Department of Medicine, University of California, 1001 Potrero Ave.,San Francisco, CA 94110, USA.

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infecting organisms multiplying in the CSF, triggers an inflammatory cascade in the subarachnoid space. In the case of pneumococcal meningitis, cell wall fragments have been identified as the active components (17) and recent work (11,15) points to teichoic acid as the most active moiety in the pneumocoecal cell wall. In the case of Gram-negative organisms, endotoxins appear to be the major active bacterial product. Htemophilus influenzte, the most common meningeal pathogen, produces lipooligosaccharides (LOS) that are biologically highly active, when injected into the CSF of experimental animals (9). The physical state, in which LOS is presented to the host is probably in the form of outermembrane vesicles, which are released from H. influenzte and which show similar specific activity as other forms of LOS (2I). CYTOKINES ARE IMPORTANT MEDIATORS Although it is not excluded that bacterial products have some toxicity for the central nervous tissue by themselves, current evidence points to the large (and still growing) family of cytokines (interleukins, tumor necrosis factor, other inflammatory proteins) as important inflammatory mediators in meningitis. Several lines of evidence support this important role of cytokines. [1] Interleukin- 1 and tumor necrosis factor are readily detected in the CSF of experimental animals (and humans) with meningitis (2, 3). [2] Injection of active bacterial products (pneumococcal cell walls, LOS) triggers the release of

TABLE I CSF bacterial titers (Iogl0 cfu/mi) and brain edema (cortex; gram water/100 gram dry weight) in three groups of experimental pneumococcal meningitis of differing degrees of severity. Controls

N

Disease severity

Bacterial titers

Brain edema (gray matter)

Controls

12

none

sterile

437±14

Group I

9

mild

5.20±0.76@

456±11#

Group II

12

intermed.

8.10_+0.36@

440±16

Group III

12

severe

6.91_+0.89@

456±20#

@ The three groups are significantly different (p < 0.05) from each other. # Significantly different (p < 0.05) from controls. cytokines into the CSF (8). [3] If cytokines are injected intracisternally in experimental animals, this induces an inflammatory response that is similar to that caused by live organisms or biologically active bacterial products (4, 5). [4] Blockage of IL-1 or TNF, by antibodies or by administration of dexamethasone, which reduces the release of these cytokines into the CSF, downmodulates the subrachnoidal inflammation (2, 16). The relative contribution by the different cytokines have not yet been worked out in detail but it appears that several of them may play an important role and at least in one experimental study there is evidence for a synergistic effect between IL-1 and TNF. TNF was by itself inactive in inducing blood-brain barrier disruption in an experimental rat model, but when combined with subeffective concentratons of IL-1 caused marked blood-barrier changes (8).

BRAIN EDEMA Brain water content increased early in the course of experimental meningitis caused by pneumococci (13). However, when different pneumococcal strains were compared, only two of three strains caused brain edema, while a third strain failed to cause edema despite high CSF bacterial titers (Table I ; M.G. T~iuber, M. Niemtller, H. Kuster, U. Borschberg, unpublished observations). The reason why one strain failed to cause brain edema is not clear but may be related to the amount or type of cell wall fragments released by the infecting organisms. Purified pneumococcal cell wall fragments, when injected intracisternally, cause brain edema to similar degrees than live organisms (11). Interestingly, the strain which failed to cause edema has an unusual pattern of autolytic control (E. Tuomanen, unpublished observation). Conceivably, the pattern of cell wall release could influence the pathophysiologic pattern induced by various bacterial strains (18). These possibilities need to be further explored. Brain edema is also caused by meningitis due to Escherichia coli (14) and H. influenzGe (10). In the case

695

TABLE II Influence of granulocytes on brain edema (hemispheres; gram water/100 gram dry weight), intracranial pressure and CSF lactate and protein concentrations in endotoxininduced experimental meningitis. Control Neutropenic N Brain edema

8

7

374.0±9.0 373.5+5.4

Normal 12 389.5+8.2 a

lntracranial pressure (mmHg) 0.5±1.0

-0.5+1,4

5.9+2.4a

Lactate (mmol/1)

1.9±0.2

5.27_+0.85b

12.0+3.5

Protein (mg/1)

25±14

81+43 b

316+176

a p < 0.01 vs. controls and neutropenics b significantly different (p < 0.01) from control and normal of Gram-negative meningitis, endotoxin released from the infecting organisms seems to be instrumental in causing brain edema (14). Most important, we found that antibiotic therapy of E. coil meningitis with a third generation cephalosporin (cefotaxime) led to a massive increase in endotoxin concentrations in the CSF and that this increase in turn was associated with a marked increase in brain edema (14). Monoclonal antibodies or polymyxin B, both of which blocked the activity of endotoxin, also prevented the increase in brain edema associated with institution of antibiotic therapy (14). Many other studies have in the mean time confirmed that initiation of bactericidal antibiotic therapy in meningitis causes an inflammatory burst triggered by the accelerated release of bacterial products (1). Thus, with some possible exceptions, brain edema is a relatively uniform response to meningitis due to different organisms, and antibiotic asso-

TABLE III Cerebral blood flow (mi/100 Gram/rain) in three groups of rabbits with experimental pneumococcal meningitis of differing degrees of disease severity.

Group

Disease severity

Cortex

Brain stem

White matter

Controls (N = 12)

none

52.8+5.9

51.3+5.8

27.6+7.3

Group I (N = 9)

mild

68.9+6.2#

54.7+7.7

25.9+2.4

Group II (N = 12)

intermed.

60.8+8.5#

55.9+6.2

21.7+4.0#

Group III (N = 12),

severe

49.4+11.6

44.3+7.6

19.0+4.0#

# Significantly different (p < 0.05) from control values. ciated lysis of the infecting organism may lead to a sharp rise in brain edema. While bacterial products (cell walls, endotoxins) appear important stimuli for the development of brain edema in meningitis, other mediators (cytokines, granulocytes and their products, unsaturated fatty acids, excitatory amino acids) may also be important. The role of granulocytes in this context appears to be complex. When normal and nentropenic animals with pneumococcal meningitis were compared after 24 hours of infection, w e found no difference in the degree of brain edema between the two groups (12), suggesting that granulocytes were not important. However, stimulation of granulocytes in the CSF of rabbits with pneumococcal meningitis by intracisternal injection of fMLP (formyl-Meth-Leu-Phen, a chemotactic tripeptid) caused an additional increase in brain edema (12). Moreover, when we examined the development of brain edema following the intracistemal injection of rough endotoxin (from Salmonella minnesota Re 595), we found that normal rabbits developed brain edema, while neutropenic rabbits did not (Table II: M.G. T~iuber, S. Kennedy, E. Sande, and D. Gunderson, unpublished observation). This last observation is consistent with results in rats, where injection of LOS or IL-I produces profound disruption of the blood-brain barrier only in normal but not in neutropenic rats (6, 19). Furthermore, blockage of WBC ingress into CSF by antibodies against cell adhesion glycoproteins or by pentoxiphyUin attenuates several aspects of the pathophysiology of meningitis (6, 16). Thus, the contribution of granulocytes to brain edema varies to some extend according to the experimental conditions examined. It appears, however, that granulocytes, at least in certain stages of the disease, play a role in mediating damage to the CNS and further studies will have to examine whether therapies aimed at effects caused by granulocytes can be of clinical benefit.

696

The observation that indomethacin reduces brain edema in pneumococcal meningitis suggests that prostaglandins are another mediator of edema (20). Corticosteroids (dexamethasone and methyl-prednisolone) prevent the development of brain edema '(13), but their broad antiinflammatory action precludes firm conclusions about specific pathways involved in the development of edema. Nevertheless, dexamethasone clearly reduces the release of cytokines during meningitis (2) and since TNF appears to be important for the development of brain edema (16), it is conceivable that the effects of steroids on brain edema are at least in part a consequence of their effects on cytokine production and release.

INTRACRANIAL

PRESSURE

Ina'acranial pressure is the result of the sum of the total brain volume, intracranial blood volume and CSF volume. Intracranial pressure starts to rise early in the course of experimental meningitis (13). However, in very sick animals, intracranial pressure decreased progressively (r = -0.45, p < 0.03 for a score of disease severity vs. intracranial pressure) (M.G. T~iuber, M. NiemOller, H. Kuster, U. Borschberg, unpublished observations). As is the case for brain edema, bacterial products (pneumococcal cell wall fragments, endotoxin) cause increased intracranial pressure in the absence of live organisms. However, when endotoxin was injected intracisternally in neutropenic rabbits, intracranial pressure failed to rise (Table II, on Kennedy, E. Sande, and D. Gunderson, unpublished observations). On the other hand, WBC in the CSF were not necessary for the development of intracranial hypertension when examined 24 hours after pneumococcal infection (12). However, fMLP stimulation of granulocytes in the CSF of rabbits with pneumococcal meningitis decreased the intracranial pressure (while brain edema increased), suggesting that

these animals got more sick as a result of WBC stimulation (12). Interestingly, not all corticosteroids are effective in reducing increased intracranial pressure: while dexamethasone was effective in this regard, methyl prednisolone was not, despite similar effects on brain edema (13). These findings document a dissociation between brain edema and intracranial pressure : pressure may be increased in the absence of brain edema and may decrease, as edema becomes more pronounced. CEREBRAL BLOOD FLOW We measured cerebral blood flow in animals infected with three different pneumococcal strains that caused different degrees of clinical illness (and corresponding differences in CSF chemistry) after 24 hours of infection (M.G. Tauber, M. Niem611er, H. Kuster, U. Borschberg, unpublished observations). Differences in disease severity in the three groups were not a result of different bacterial titers in the CSF. Cortical blood flow increased initially as a result of infection and, on the average, was not markedly reduced even in the group of very sick animals (Table III). However, we found a decrease of blood flow with increasing severity of the disease (r = -0.57; p < 0.01) such that the sickest animals had cortical blood flow reductions by about 30 % of normal. Blood flow in the brain stem (pons) did not changes substantially in the three experimental groups, while flow in the subcortical white matter was progressively reduced with increasing severity of the disease (Table III). Changes in blood flow in infected animals correlated with

CSF lactate concentrations and with arterial blood pressure. This last finding suggested that autoregulation of cerebral blood flow may be lost in meningitis. This has recently been examined in detail (19). The authors found that infected animals had a complete loss of autoregulations such that cerebral blood flow became dependent of systemic arterial pressure. In addition, blood flow and intracranial pressure showed a positive correlation, suggesting that cerebral blood flow is an important determinant of intracranial pressure, at least during acute changes. Since cerebral perfusion is highly vulnerable to any changes of systemic arterial blood pressure, it is important in the management of patients with meningitis to stabilize blood pressure without major peaks or hypotensive phases. CONCLUSIONS Many aspects of the pathophysiology of bacterial meningitis have been clarified in recent years. Based on this improved understanding, rational therapeutic approaches that supplement antibiotic treatment become conceivable. Interventions can be directed against harmful bacterial products (antibodies, polymyxin B), against cytokines (antibodies, steroids), against WBC's (antibodies, pentoxyphyllin) or against some of the pathophysiologic consequences of the disease, such as brain edema, intracranial pressure or alterations of cerebral blood flow. All these therapeutic modalities will have to be tested carefully in animal models and then be evaluated in clinical trial.

R E S U M E : PHYSIOPATHOLOGIE DES MENINGITES BACTERIENNES : CONTRIBUTION DES MODELES EXPERIMENTAUX Les moddles experimentaux de m~ningite ont contribud d dlucider des aspects essentiels de la physiopathologie des m~ningites bactdriennes. Des substances provenant du germe infectant (acides teichoiques, lipopolysaccharides) activent une cascade d'dvdnements : relargage de diverses cytokines, induction d"une inflammation mdningde, ouverture de la barridre Mmato-encdphalique, altdration de la perfusion cdrdbrale, hypertention intra-cr~nienne, oeddme cdrdbral. A l'aide d'anticorps et d'agents pharmacologiques, cette cascade d'dv~nements peut ~tre manipulde d diffdrents niveaux, rdalisant une possibilitd de traitement adjuvant de l'antibiothdrapie des mdningites bactdriennes. Mots-cl~s : M#ningite - Inflammation rn~ning#e - Oeddme c#r#bral - Hypertension intracrfinienne - D#bit vasculaire c~r~bral - Dexamdthasone. BIBLIOGRAPHIE 1.

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