Studies on intravascular phagocytosis in the lung

Anti-D initially stimulates an Fc-dependent leukocyte oxidative burst and subsequently suppresses erythrophagocytosis via interleukin-1 receptor antagonist

Previous results have demonstrated that anti-D therapy in children with chronic auto-immune thrombocytopenic purpura (AITP) induced a significant increase in several pro- and anti-inflammatory plasma cytokines within 2 hours of administration. To investigate the biologic basis of these early in vivo responses, we developed a flow cytometric assay to measure Fc-dependent responses of human peripheral leukocytes with fluorescently labeled and anti-D-opsonized red blood cells (RBCs). When anti-D-opsonized RBCs were incubated with peripheral blood leukocytes, the earliest detectible event observed was a significant oxidative burst in both monocytes (P <.05) and granulocytes (P <.0001), characterized by the production of hydrogen peroxide (H2O2), peroxynitrite (ONOO-), superoxide (O -2), and hydroxyl (OH) by 10 minutes which declined by 1 hour. By 2 hours, the opsonized RBCs were phagocytosed, particularly by granulocytes (P <.001), but the phagocytosis subsequently declined by 6 hours of incubation. The decline in phagocytosis was correlated with a significant production of interleukin-1 receptor antagonist (IL1ra) by both monocytes (P =.036) and granulocytes (P =.0002) within 4 hours. None of these events occurred if the RBCs were coated with anti-D F(ab)'2 fragments. When recombinant IL1ra was titrated into the assay, phagocytosis of the opsonized RBCs was significantly inhibited (P =.002). Taken together, these results suggest that at least one mechanism of action of anti-D is via the production of the anti-inflammatory cytokine IL1ra which can negatively regulate the ability of leukocytes to phagocytose opsonized cells.

Quantifying rat pulmonary intravascular mononuclear phagocytes

Cells of the mononuclear phagocyte system (MPS) protect the host by clearing effete and foreign particulates from the circulation. The current study was designed to identify, quantify, harvest, and provide a partial functional characterization of the systemic host-defense cell located in the pulmonary microvasculature of the rat. Critical colloid doses of test particulates (monastral blue B [MBB] or polystyrene beads) were infused intra-arterially into anesthetized rats so that phagocytically active pulmonary intravascular phagocytes could be identified. Morphologic characterization of in situ phagocytes was performed using electron microscopy. The number of active phagocytes was then determined using tissue samples processed for light microscopy. Finally, sequential perfusion of the pulmonary vasculature with buffer, chelating agent, and collagenase allowed elution and preliminary functional characterization of the pulmonary intravascular mononuclear phagocyte (PIMP). Electron microscopy demonstrated that both mononuclear phagocytes and neutrophils contributed to pulmonary sequestration of circulating particulates. Light microscopy showed that the microvasculature of each alveolus contained 0.50+/-0.19 active mononuclear phagocytes and 0.14+/-0.12 active neutrophils. A chelation/collagenase elution technique was then used to harvest the PIMP. Histologic evaluation of the postperfusion lungs indicated that 80% of the active phagocytes were removed by the technique. In total, the elution fluids contained 2.63+/-1.04 x 10(7) cells, with 1.60+/-0.78 x 10(7), 0.49+/-0.17 x 10(7), and 0.54+/-0.26 x 10(7) of those cells being mononuclear phagocytes, neutrophils, and lymphocytes, respectively. Functionally, the mononuclear phagocyte population exhibited a spectrum of phagocytic activities, with 51.5+/-19.5% of the cells being inactive, 33.9+/-13.4% exhibiting moderate phagocytic activity, and 14.6+/-9.8% demonstrating intensive phagocytic capacity. The current study provides the first quantified demonstration that mononuclear phagocytes are primarily responsible for sequestering blood-borne foreign particulates in the pulmonary circulation of the rat. Approximately 2 x 10(7) PIMP existed in the lungs of 300 gram rats. The functionally heterogeneous mononuclear phagocytes exhibited phagocytic capacities ranging from avidly phagocytic (14.6+/-9.8%) through moderately active (33.9+/-13.4%) to inactive. The lung microvasculature's large pool of inactive mononuclear phagocytes may provide a recruitable mechanism to allow significant increases in clearance of circulating particulates. A resident pool of activatable mononuclear phagocytes might explain previous clinical observations of increased particulate localization in the lung microvasculature of septic patients.

Ultrastructural and cytochemical evaluation of sepsis-induced changes in the rat pulmonary intravascular mononuclear phagocytes

Sepsis stimulates an increase in the number and activity of mononuclear phagocytes in systemic host-defence organs. The present study was conducted to define the ultrastructural and cytochemical characteristics of the mononuclear phagocytes that sequester in the lung microvasculature of septic rats. Fourteen rats were challenged with a single intraperitoneal injection of saline (0.5 ml/100 g), E. coli (2 x 10(7)/100 g) or glucan (4 mg/100 g), and euthanased 2, 4, or 7 d later. The lungs were inflation fixed and processed for transmission electron microscopy. Cellular morphology was used to identify the intravascular mononuclear phagocytes and acid phosphatase (AcPase) expression was monitored as an index of cellular differentiation and activation. Control rats contained a limited number of monocytes in the pulmonary vasculature. In contrast, large numbers of activated mononuclear phagocytes were seen in the microvasculature within 48 h of treatment with either microbial product. The recruited pulmonary intravascular mononuclear phagocytes (PIMP) exhibited AcPase-reactive Golgi complexes, accumulation of secretory vesicles and other features of cell activation consistent with enhanced biosynthetic activity. Subsequent electron microscopy, conducted 4 and 7 d posttreatment, suggested that a progressive decline in the number and activity of PIMPs then occurred. In order to quantify the sepsis-induced accumulation of AcPase-positive PIMP, the experimental challenges were repeated in 11 rats and, 48 h later, tissue samples were evaluated by light microscopy for tartrate-insensitive acid phosphatase. Control rats exhibited 0.148 +/- 0.107 AcPase-positive PIMP/alveoli. E. coli and glucan challenged animals exhibited significant (P < 0.01) increases in AcPase-positive mononuclear phagocytes, with 0.782 +/- 0.073 and 0.636 +/- 0.170 PIMP/alveoli respectively. The results demonstrate that focal sepsis stimulates a significant, but transient, recruitment of activated mononuclear phagocytes into the rat pulmonary microvasculature.