Macrophage handling of soluble immune complexes ingestion and digestion of surface‐bound complexes at 4, 20 and 37 °C

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Abstract
Radioassays have been developed to measure, as separate events, the ingestion and degradation of macrophage-bound guinea pig IgG anti-DNP (2,4-dinitrophenyl)- DNP-BSA (bovine serum albumin) complexes of defined size and subclass. Complex ingestion was observed to be temperature-dependent and was effectively blocked only by a combination of inhibitors of respiration and glycolysis indicating that the process is under the same metabolic control as fluid phase pinocytosis. On the other hand, the half-life of membrane-bound complexes at 37 °C (t1/2 = 5.6 ± 1.7 min) was considerably shorter than the value expected from membrane turnover studies (t1/2 = 22.4 min) suggesting that complexes are selectively cleared from the cell surface. The rate of ingestion at 20°C was independent of complex size and of the IgG subclass used in complex formation, but was affected by in vivo stimulation of the macrophages before assay. Complex digestion was shown to be highly temperature-dependent and, at 37 °C, proceeded at a rate (t1/2 = 1.5-5 h) which was 20-60-fold slower than the rate of ingestion indicating that the latter is unlikely to influence digestion kinetics. On the other hand, the selective action of 2-deoxyglucose in blocking digestion, but not ingestion, suggests that pinosome-lysosome fusion may play a part in determining the overall catabolic rate. A 2 to 3-fold difference in digestion rates was observed between the proteins employed as antibody (guinea pig IgG1 or IgG2) and as antigen (DNP-BSA). This finding suggests that the intrinsic susceptibility of ingested proteins to enzymatic hydrolysis may be the prime determinant of digestion rate. As with ingestion, no discrimination was observed in the degradation of complexes of different size or IgG antibody subclass. The observations in this and the preceding study (Eur. J. Immunol. 1980. 10: 317) indicate that complex size is important in determining the level of uptake by phagocytes but not subsequent events associated with catabolism.