Neutrophilic leukocyte granules: from structure to function

Light scattering by neutrophils: model, simulation, and experiment

We studied the elastic light-scattering properties of human blood neutrophils, both experimentally and theoretically. The experimental study was performed with a scanning flow cytometer measuring the light-scattering patterns (LSPs) of individual cells over an angular range of 5-60 deg. We determined the absolute differential light-scattering cross sections of neutrophils. We also proposed an optical model for a neutrophil as a sphere filled by small spheres and prolate spheroids that correspond to granules and segmented nucleus, respectively. This model was used in simulations of LSPs using the discrete dipole approximation and different compositions of internal organelles. A comparison of experimentally measured and simulated LSPs gives a good qualitative agreement in LSP shape and quantitative agreement in overall magnitude of the differential light-scattering cross section.

The bactericidal/permeability-increasing protein (BPI) in infection and inflammatory disease

Gram-negative bacteria (GNB) and their endotoxin present a constant environmental challenge. Endotoxins can potently signal mobilization of host defenses against invading GNB but also potentially induce severe pathophysiology, necessitating controlled initiation and resolution of endotoxin-induced inflammation to maintain host integrity. The bactericidal/permeability-increasing protein (BPI) is a pluripotent protein expressed, in humans, mainly neutrophils. BPI exhibits strong antimicrobial activity against GNB and potent endotoxin-neutralizing activity. BPI mobilized with neutrophils in response to invading GNB can promote intracellular and extracellular bacterial killing, endotoxin neutralization and clearance, and delivery of GNB outer membrane antigens to dendritic cells. Tissue expression by dermal fibroblasts and epithelia could further amplify local levels of BPI and local interaction with GNB and endotoxin, helping to constrain local tissue infection and inflammation and prevent systemic infection and systemic inflammation. This review article focuses on the structural and functional properties of BPI with respect to its contribution to host defense during GNB infections and endotoxin-induced inflammation and the genesis of autoantibodies against BPI that can blunt BPI activity and potentially contribute to chronic inflammatory disease.

How neutrophils kill microbes

Neutrophils provide the first line of defense of the innate immune system by phagocytosing, killing, and digesting bacteria and fungi. Killing was previously believed to be accomplished by oxygen free radicals and other reactive oxygen species generated by the NADPH oxidase, and by oxidized halides produced by myeloperoxidase. We now know this is incorrect. The oxidase pumps electrons into the phagocytic vacuole, thereby inducing a charge across the membrane that must be compensated. The movement of compensating ions produces conditions in the vacuole conducive to microbial killing and digestion by enzymes released into the vacuole from the cytoplasmic granules.

Wegener's granulomatosis

Clinical manifestations of Wegener's granulomatosis are nonspecific and indistinguishable from a variety of neoplastic, infectious, and inflammatory diseases. Ophthalmic disease is the presenting feature in nearly one sixth of patients with Wegener's granulomatosis and will ultimately develop in a majority. The discovery of antineutrophil cytoplasmic antibodies, particularly antiproteinase-3, has changed the clinical approach to evaluating patients suspected of having Wegener's granulomatosis. These antibodies are distinguished from other related autoantibodies because they produce a coarse granular pattern of cytoplasmic staining on indirect immunofluorescence with ethanol-fixed neutrophils. Treatment of Wegener's granulomatosis with oral cyclophosphamide and corticosteroids has decreased morbidity and improved survival, but side effects from long-term immunosuppressive therapy are common and sometimes serious. The effectiveness of trimethoprim-sulfamethoxazole in decreasing the number and severity of recurrences of Wegener's granulomatosis is being investigated. It remains to be determined if wide use of trimethoprim-sulfamethoxazole in limited Wegener's granulomatosis could further improve the quality of life for some patients.