The effect of cyclosporin A on the primary immune response to allogeneic red cells in rabbits.

Cyclosporin A (CSA) has been used in an attempt to suppress the primary immune response of HgA(A)-negative rabbits to A-positive red cells. The immune response was assessed by measuring the survival of a small intravenous (i.v.) dose of 51Cr-labelled A-positive cells and by testing the serum of the immunized rabbits for anti-A. In one experiment, eight A-negative rabbits were given a first i.v. injection of A-positive red cells, and CSA (25 mg/kg/day) in olive oil was given by mouth for 17-34 days. There was no evidence of impaired alloimmunization compared with the responses in control animals treated with olive oil alone. In a second experiment, eight A-negative rabbits were given a first injection of A-positive muscularly (i.m.), and CSA (25 mg/kg/day) in miglyol was given by im.m. injection for 10 days. Six of these rabbits were rendered unresponsive, and the remaining two, who showed impaired survival of the monitoring red cells, produced only low anit-A titres. Seven out of eight controls given i.m. miglyol without CSA responded with good anti-A production. Rabbits that were unresponsive to A-positive red cells responded normally to sheep red blood cells 15 weeks after CSA treatment. Higher serum levels of CSA were found following i.m. administration of the drug but treatment by this route as associated with severe toxicity in some rabbits.