Biological functions of extravasated serum IgG in rat brain

During blood-brain barrier opening serum IgG could be extravasated. The function of intraparenchymal IgG, however, is unknown. Its biological effects in the acute phase were currently investigated. From rat autoserum IgG was purified and injected into the cortex. Similarly, IgG-Fab fragment was prepared and administered likewise. As for the control group, only vehicle was injected. Animals were sacrificed on days 1, 2 and 4 after the infusion and were histologically evaluated. On days 1 and 2, the infusion of IgG caused significant intraparenchymal infiltration of neutrophils which expressed LFA-1-alpha. It also induced CR3 up-regulation in microglia and endothelial ICAM-1 expression. On day 4, these findings had disappeared. HE stained brain sections and the TUNEL method did not reveal significant nerve cell death in IgG injected animals during the experiment as compared to the controls. IgG-Fab did not cause significant changes either. Extravasated IgG has been viewed to have biochemical functions. Its Fc fragment seemed to cause microglial and endothelial activation, followed by leukocytic infiltration. This sequence itself was not neurotoxic. Therefore, it is suggested that extravasated IgG is one of the inducers that modulate cellular responses in the acute phase of brain damage.