Differing complement-mediated opsonic activity of rabbit interstitial fluids from autologous serum

Lack of appropriate methods for withdrawing extravascular or interstitial fluid from an animal host has limited in vitro study on the role of complement in the local defence of the extravascular space. In the present study, we obtained fluids from membrane diffusion chambers (porosity 0.22 micron) implanted into the kidneys, peritoneal cavity and soft tissues in rabbits. The complement-mediated opsonic activity (CMOA) of these fluids for Staphylococcus aureus ATCC 502A and Escherichia coli 01 was then compared to that of autologous sera. Soft tissue and renal interstitial fluids were as opsonic for E. coli as autologous sera but were however, poor opsonins for S. aureus. The peritoneal fluid was marginally effective in opsonization of both bacterial strains. While chelation of the fluids with MgEGTA (to block the classical pathway) did not diminish CMOA for E. coli, it reduced the CMOA for S. aureus by half. Conversely, heat-inactivation of the fluids and serum eliminated the opsonic activity for E. coli but only decreased the opsonic activity for S. aureus by half. Following a 24 h in vivo growth of E. coli in the implanted chambers, the CMOA was drastically reduced. Concomitant to the reduction in functional complement in the fluids, E. coli recovered from the chambers were found coated, though not maximally, with C3b as evidenced by studies with fluorescent antibody. The differences in opsonic content of extravascular fluids observed here might explain why certain sites of the body may be more vulnerable to attack by some bacterial species which are not effectively opsonized and therefore phagocytized.