Ultrastructural histopathology of experimental Haemophilus influenzae type B. Meningitis in the infant rat. I. Leukocytes of the spinal leptomeninges.

We examined the origin, nature and role of cellular defense mechanisms in experimental Haemophilus influenzae type b (Hib) meningitis in the infant rat using ultrastructural techniques. An extensive inflammatory exudate composed of granulocytes, lymphocytes and macrophages was observed on the leptomeninges of all animals with meningitis. At 1-3 days following Hib-inoculation, neutrophils were the major leukocytic element within the exudate while at 7-9 days the leukocyte population was dominated by macrophages. Irrespective of day of inoculation, leukocytes were observed undergoing diapedesis through the thin-walled blood vessels of the subarachnoid space and pial connective tissue, a phenomenon which suggested that the cellular elements composing the inflammatory exudate originated directly from the systemic circulation. Only neutrophils and macrophages demonstrated phagocytosis of free bacteria in the cerebrospinal fluid. The presence of plasma cells suggests that antibody to Hib may be synthesized locally within the subarachnoid space. Furthermore, cell-to-cell contacts among leukocytes may also contribute to the initiation and maintenance of the humoral immune response to Hib.