Role of leukocytes in cerebral autoregulation and hyperemia in bacterial meningitis in rabbits

The effect of leukocytes on regional cerebral blood flow (rCBF) and cerebrovascular autoregulation in experimental meningitis was determined in rabbits. Four groups of animals were studied. Cerebrospinal fluid (CSF) leukocyte migration was prevented in two groups by pretreatment with 1.5 mg/kg of IB4, a monoclonal antibody directed against CD11/18 leukocyte adhesion receptors. Intracisternal inoculation was performed with saline (control and control-IB4 groups) or Haemophilus influenzae type b (Hib and Hib-IB4 groups). Eighteen hours later, rCBF was determined with radiolabeled microspheres. Autoregulation was assessed by graded hemorrhagic hypotension. Compared with untreated meningitis (Hib group), IB4-pretreated meningitis (Hib-IB4 group) was associated with a reduced CSF leukocyte count (1,980 ± 880 vs. 200 ± 110 cells/μl; P < 0.05) and an elevated CSF colony count (2.87 ± 0.08 vs. 5.63 ± 0.72 log₁₀ colony-forming units/ml; P < 0.05). Compared with control, baseline CBF was elevated in both untreated and IB4- pretreated meningitis (51 ± 2, 54 ± 2, 66 ± 5, and 102 ± 17 ml.100 g^-1 · min^-1 in control, control-IB4, Hib, and Hib-IB4 groups, respectively). The degree of hyperemia in meningitis was related to the CSF colony count, with a high CBF occurring in animals with high colony counts. During hypotension, CBF remained at or above baseline in the Hib group and both control groups, indicating preservation of cerebrovascular autoregulation in untreated Hib meningitis. In the Hib-IB4 group, the elevated baseline CBF was not maintained during hypotension, falling to 51% of baseline at a cerebral perfusion pressure of 30 mmHg and indicating impairment of cerebrovascular autoregulation. These results suggest that CSF leukocytes are not primarily responsible for the hyperemic response in Hib meningitis. Cerebral hyperemia may be induced either directly by bacterial components or indirectly by components of the inflammatory cascade that precede CSF leukocyte migration.