Phagocytosis of heat-aggregated immunoglobulins by mesangial cells: an immunoperoxidase and acid phosphatase study.

It has been demonstrated previously that mesangial cells (MC) phagocytose particulate material and nonimmune proteins, but that cells are able to engulf aggregated immunoglobulins (Igs) has not been proven so far. To investigate this cell function, heat-aggregated antiperoxidase (HRP) Igs were injected intravenously into Lewis rats. Sequential immunoelectron microscopic studies revealed that the injected material accumulates progressively in the extracellular compartment from 10 minutes to 5 hours and disappear afterward. This disappearance was related in part to the incorporation of aggregated anti-HRP Igs within phagosomes and phagolysosomes by MC. Quantitation of the phagocytic process showed that it starts as early as 10 minutes after injection and reaches its peak at 2.5 hours. Endocytosis seems to be associated with the sequestration of injected Igs within large cytoplasmic invaginations and with micropinocytosis. To further support these observations, ultrastructural cytochemistry for acid phosphatase activity was performed. Quantitative studies showed that the number of acid phosphatase-labeled lysosomes in MC was up to 7-fold greater in rats receiving aggregated anti-HRP Igs than in noninjected ones. In conclusion, our combined immunoperoxidase and acid phosphatase studies show that MC possess a vacuolar (lysosomal) apparatus capable of handling heat-aggregated Igs. Whether these cells are also operational in the disposal of soluble immune complexes in spontaneous and experimental conditions remains to be proven. In addition, observations suggesting the drainage of macromolecules from mesangium to the juxtaglomerular apparatus and from mesangium to the urinary space are presented.