Granules and secretory vesicles in human neonatal neutrophils

Neonates have an increased susceptibility to bacterial infections that may be due to defective adherence and migration of neonatal neutrophils (NN). Because receptors of relevance for these activities are located mainly in intracellular granules and secretory vesicles that have only recently been characterized in adult neutrophils (AN), we investigated whether the same structures are present in NN and to what extent they are mobilized in response to chemotactic and inflammatory mediators. Subcellular fractionation of NN on a three-layer Percoll density gradient revealed that secretory vesicles, identified by latent alkaline phosphatase and albumin, are present in NN. We also demonstrated the presence of gelatinase granules distinct from specific granules, although the content of their respective markers, gelatinase and lactoferrin, was markedly reduced. The low content of lactoferrin may explain an observed lower isopycnic density of specific granules in NN. Mobilization of granules by a variety of stimuli was slightly higher in NN compared with AN, whereas mobilization of secretory vesicles was normal. This shows that NN contain both secretory vesicles and all subsets of granules identified in AN, and that these are readily mobilized, although a marked structural difference exists between peroxidase-negative granules of NN and AN that may reflect differences during myelopoiesis.