Ultrastructural histopathology of experimental Haemophilus influenzae type B meningitis in the infant rat. II. Phagocytosis and lysis of microorganisms by leptomeningeal leukocytes.

We examined the events associated with phagocytosis, lysis and digestion of Haemophilus influenzae type b (Hib) by subarachnoid leukocytes in infant rats with experimental Hib meningitis. In early stages of infection, large numbers of bacteria were attached to the surfaces of neutrophils and macrophages invaded the subarachnoid space and actively ingested Hib. The bacteria, coccobacillary in shape with an approximate length of 1.0 micrometers and 0.3 micrometer in width, were interiorized after fusion of leukocyte microplicae which had arisen around them. Ingested Hib were sequestered within large, membrane-bound vacuoles containing five or more microbes. Following the fusion of primary lysosomes with the membrane of the phagocytic vacuole, bacteria were lysed and degraded. In later stages of infection, macrophages possessed large numbers of inclusions containing extensively digested Hib and myelin figures. Histochemical analysis of subarachnoid leukocytes revealed that macrophages actively synthesized acid phosphatase and that this enzyme aided in the digestion of phagocytosed bacteria. Peroxidase was also demonstrated within phagocytic vacuoles of neutrophils. Our results suggest that subarachnoid macrophages and neutrophils actively lyse and digest ingested Hib through the direct action of their hydrolytic enzymes.