The in vivo behaviour of complement-coated red cells: studies in C6-deficient, C3-depleted and normal rabbits.

Two series of experiments were performed on rabbits to investigate the role of fixed C3 in the non-lytic destruction of red cells. In the first series, intravenous injection of a potentially lytic IgM cold antibody (anti-I) into C6-deficient rabbits caused a severe thrombocytopenia and neutropenia and a fall in haemoglobin concentration and PCV with only minimal haemoglobinaemia; all were of short duration. A sharp fall in plasma C3 concentration, the demonstration of C3 on the red cells and the occurrence of immune-adherence suggested that red cell-complement intermediates in the form EA C1423 were present in the circulation at this time. Subsequently, when the fixed-C3 activity of the circulating red cells was markedly diminishing, the haemoglobin and PCV and platelet and neutrophil counts recovered towards the pre-injection values, indicating the sequestration rather than the acute destruction of these cells. In contrast to the findings in the C6-deficient rabbits, injection of the same IgM cold antibody into C3-depleted rabbits caused no significant haematological changes. In a second series of experiments, red cells in the form EC43(5) and EC4 were injected intravenously into C6-deficient, C3-depleted and normal rabbits. EC4 survived normally during the period of observation, whereas EC43(5) were removed exponentially (:1½–4 min) from the circulation at sites in the reticuloendothelial system. In the liver, the main site of sequestration, EC43(5) attached to Kupffer cells, where some were immediately ingested. With time, unphagocytosed EC43(5) returned to the circulation at a slow exponential rate (:25–100 min), apparently as damaged, spherocytic cells. It is suggested that the return of EC43(5) to the circulation from sites of attachment on fixed macrophages is due to the progressive inactivation of fixed-C3. It would therefore appear that the presence of fixed C3 in an active form is essential for the non-lytic sequestration and damage of red cells which have been exposed to IgM cold antibody.