The comparative effectiveness of rabbit anti-mouse thymocyte sera in potentiating the survival of mouse skin or tumour allografts made across the H-2 barrier.

A group of fifteen rabbits were immunized five times, at weekly intervals, by intravenous injection of mouse thymocytes. After an interval of 3 months the rabbits were then re-immunized three times at intervals of 7 days. The rabbits were bled prior to the commencement of immunization to obtain normal rabbit serum (NRS), and 7 days after the first, third, fifth, sixth and eigth immunizations. Thus pulse (P), P1, P3, P5, P6 and P8 rabbit anti-mouse thymocyte serum (RAMTS) was obtained. Further pairs of rabbits were immunized three times, at intervals of 7 days, by injection of thymocytes pre-incubated with NRS, or P3, P6 or P8 RAMTS. Thus RAMT(NRS)S, RAMT(P3)S, RAMT(P6)S and RAMT(P8)S were obtained. The immunosuppressive potency of the antisera were compared by their ability to promote the survival of A-strain skin grafts made to CBA mice and A-strain mammary carcinoma transplants in CBA mice. Administration of P3, P5, P6, P8, RAMT(P3)S and RAMT(P6)S led to progressive tumour growth, but only P3 and P5 prolonged skin graft survival. Pre-incubation of tumour cells in the several RAMTS samples did not enhance their growth in sub-lethally irradiated (350 rad) CBA hosts. It is thus suggested that the preferential survival of tumour allografts was due to their ability to outgrow a degree of host response able to destroy a skin graft.