Quantification of bacterial phagocytosis by flow cytometry and spectrofluorimetry

Simultaneous cytofluorometric measurement of phagocytosis, burst production and killing of human phagocytes using Candida albicans and Staphylococcus aureus as target organisms

OBJECTIVE

Polymorphonuclear leukocytes (PMN) play a central role in the elimination of most extracellular pathogens, and an impairment of their functions predisposes an individual towards local and systemic bacterial and fungal infections. Here we describe a rapid and easy-to-perform cytofluorometric assay for investigation of PMN activity using Candida albicans and Staphylococcus aureus as target organisms.

METHODS

Phagocytes were stained with anti-CD13-RPE antibody, and microorganisms were stained with calcein-AM. Oxidative burst production was measured by oxidation of dihydroethidium. The percentage of killed target organisms after ingestion was determined by staining with ethidium-homodimer-1 after lysis of human cells. The dyes and procedures used in this method were chosen after comparison of different stains and cell preparation techniques described in previous assays.

RESULTS

Concerning phagocytosis, the percentages of active phagocytes and of ingested microorganisms were determined. Furthermore, the method allowed measurement of the resulting percentage of PMNs producing respiratory burst, and of the percentage of killed microorganisms. We minimized artifactual changes, which might have been the reason for the difficulties and conflicting results of other cytofluorometric methods.

CONCLUSIONS

The described method provides a new whole blood cytofluorometric assay, which combines rapid and simple handling with high reproducibility of results obtained by investigation of PMN activity using Candida albicans and Staphylococcus aureus as target organisms.

Flow cytometric and optical microscopic evaluation of poly(D, L-lactide-co-glycolide) microspheres phagocytosis by pig alveolar macrophages

The phagocytosis of fluorescent poly(D,L-lactide-co-glycolide) microspheres by fresh and frozen pig alveolar macrophages was investigated by optical microscopy on adherent cell culture and by flow cytometry with cell suspension. The kinetic of phagocytosis was studied on a 360 min period as a function of the ratio between microspheres and macrophages (MS:AM ratio from 1:1 to 10:1). No difference of phagocytosis between fresh and frozen macrophages was observed whatever the MS:AM ratio following flow cytometric evaluation while a significant phagocytosis pattern was noticed following optical microscopic evaluation for the highest ratio. The intensity of phagocytosis was dependent on the duration of incubation and dependent, but not proportionally, to the MS:AM ratio showing that the highest efficiency was obtained with the MS:AM ratio of 1:1. Flow cytometry analysis has shown a correlation between cell population and fluorescent events suggesting that phagocytosis of nonfluorescent antigen-loaded particles with different characteristics could be investigated.

Characterization of a murine monoclonal antibody to Cryptococcus neoformans polysaccharide that is a candidate for human therapeutic studies

The murine monoclonal antibody (MAb) 18B7 [immunoglobulin G1(kappa)] is in preclinical development for treatment of Cryptococcus neoformans infections. In anticipation of its use in humans, we defined the serological and biological properties of MAb 18B7 in detail. Structural comparison to the related protective MAb 2H1 revealed conservation of the antigen binding site despite several amino acid differences. MAb 18B7 was shown by immunofluorescence and agglutination studies to bind to all four serotypes of C. neoformans, opsonize C. neoformans serotypes A and D, enhance human and mouse effector cell antifungal activity, and activate the complement pathway leading to deposition of complement component 3 (C3) on the cryptococcal capsule. Administration of MAb 18B7 to mice led to rapid clearance of serum cryptococcal antigen and deposition in the liver and spleen. Immunohistochemical studies revealed that MAb 18B7 bound to capsular glucuronoxylomannan in infected mouse tissues. No reactivity of MAb 18B7 with normal human, rat, or mouse tissues was detected. The results show that both the variable and constant regions of MAb 18B7 are biologically functional and support the use of this MAb in human therapeutic trials.

Antifungal activity of a human antiglucuronoxylomannan antibody

The human immunoglobulin M (IgM) monoclonal antibody (MAb) 2E9 binds the glucuronoxylomannan (GXM) of Cryptococcus neoformans serotypes A, B, and D. This study was undertaken to determine the opsonic efficacy of 2E9 and its ability to promote the antifungal activity of human polymorphonuclear neutrophils (PMNs) against C. neoformans. We incubated purified PMNs with fluorescein isothiocyanate-labeled C. neoformans cells that were treated with the GXM IgM 2E9, IgM antibodies that do not bind GXM, and rabbit and human factor-B-deficient serum as complement sources. PMN-associated C. neoformans cells fluoresced and were detected with a fluorescence-activated cell sorter. The amount of phagocytosis was defined as the percent fluorescing PMNs, which was 37% for yeast cells opsonized with 2E9 plus rabbit serum and 57% for yeast cells opsonized with 2E9 plus factor-B-deficient serum. Phagocytosis was significantly greater for yeast cells that were treated with 2E9 plus a complement source than for yeast cells treated with the complement sources alone or treated with the control IgMs alone or with the complement sources. Fluorescence quenching and light and electron microscopy of the phagocytosis mixtures revealed that 2E9-opsonized yeast cells were internalized by PMNs. Maximal inhibition of C. neoformans growth occurred when PMNs were cocultured with yeast cells that were opsonized with 2E9 plus a complement source. Our data demonstrate that the human GXM IgM 2E9 can mediate PMN phagocytosis and C. neoformans growth inhibition in vitro. These findings strongly suggest that antibody-mediated deposition of complement components on the cryptococcal capsule can augment PMN complement receptor-mediated antifungal activity. Antibody activation of complement-mediated effector cell antifungal mechanisms may play a role in host defense against cryptococcosis and represents a goal for the use of MAbs to treat or prevent human C. neoformans infections.

A simple flow cytometric technique to quantify rod outer segment phagocytosis in cultured retinal pigment epithelial cells

PURPOSE

The primary aim of this study was to develop and characterize a simple flow cytometric method of quantifying rod outer segment (ROS) phagocytosis in cultured retinal pigment epithelial (RPE) cells. A secondary aim was to compare the kinetics of ROS phagocytosis in an immortal human RPE cell line with untransformed human RPE cells.

METHODS

Flow cytometry was performed on RPE cells that had been challenged with fluorescein isothiocyanate-labeled ROS (FITC-ROS) and phagocytosis was calculated by subtracting background cellular autofluorescence.

RESULTS

Non-specific uptake of fluorescent label was negligible and RPE cells phagocytosed FITC-ROS and unlabeled ROS with equal efficacy. The kinetics of FITC-ROS phagocytosis in the D407 RPE cell line differed from early passage untransformed human RPE cultures. FITC-ROS phagocytosis proceeded at a fairly linear rate for the first 12 h in the 3 human cell cultures studied, but was rapid for the first 3 h before slowing in the D407 cells. Within all cell populations, there was a heterogeneity of phagocytic activity which varied with time.

CONCLUSIONS

This automated technique for measuring phagocytosis is rapid, simple, highly accurate, avoids radiation hazards, and permits study of heterogeneity within cell populations. The biochemistry, physiology and pathophysiology of the interactions between retinal pigment epithelial (RPE) cells and photoreceptors continue to be areas of considerable research interest (1, 2, 3). Vital to such work is the ability to accurately quantify rod outer segment (ROS) phagocytosis by RPE cells. Current in vitro techniques of measuring ROS phagocytosis use either automated or manual methods to count phagosomes. While manual counting techniques offer the advantage of visual quality control, they are highly labor intensive, there is a practical limitation to the number of phagosomes that can be counted, and measurements suffer from relatively large standard errors (3). Automated methods include scintillation counting and flow cytometry. Problems with radiolabels include radiation hazards, nonspecific radiolabel uptake, and limited visual control (3). Flow cytometry, on the other hand, circumvents nearly all of these problems and may prove to be the optimal phagocytosis assay.

Neutrophil function in elderly persons assessed by flow cytometry

It is well known that the immune response declines with senescence and it is suggested that these changes render an individual susceptible to infection, autoimmune phenomena and cancer. Bacterial and viral infections are a major cause of illness and death amongst aged subjects, and once infection is established, the elderly also have a diminished capacity to prevent its spread (1). The cellular and molecular basis for this age-related decline in immunocompetence are still unknown and, possibly, are related to an alteration in cell transduction mechanisms (2).