The presence of a tumour in F1 mice partially inhibits the GvH reaction following injection of parental spleen cells.

(A x CBA(T6)F1 mice bearing F1 mammary carcinomas for 9 days were injected i.v. with A strain spleen cells. The A spleen cells were from either non-immune donors or mice which had received an i.p. injection of F1 tumours cells 9 days previously, and were thus immune to the CBA component of the tumour. Fourteen days after receiving parental spleen cells, the F1-tumour-bearing mice were killed and their spleen ratios and indices were determined as an index of the severity of the graft-versus-host reaction (GvHR) induced. The spleen indices were compared with those in non-tumour-bearing F1 mice, receiving aliquots of the same parental cell suspensions. At the higher doses of A spleen cells, the presence of an F1 tumour reduced the GvHR. At the same time, in 3/5 experiments, the weight of the F1 tumour in mice injected with immune A spleen cells was less than that in F1 mice receiving the same number of tumour cells but no spleen cells. A reduction in GvHR and a decrease in tumour weight was not seen when F1 mice carrying an A strain tumour were injected with A strain spleen cells immune to an F1 tumour. Adding 23-day F1 tumour-bearing F1 spleen cells to A spleen cells did not reduce the GvHR induced in further non-tumour-bearing F1 recipients by the parental cells. This was evidence against the presence of suppressor cells in the tumour-bearing F1 spleen. When 51Cr-labelled A strain spleen cells were injected into F1 mice, some of which had a tumour and therefore an enlarged spleen, there was an inverse relation between the size of the spleen and the number of parental cells therein, per g spleen 4 h after injection. It is thus suggested that the reduction in GvHR in F1-tumour-bearing F1 mice, after injection of parental spleen cells, is due first to a reduction in the concentration of donor cells in the recipient spleen (i.e. the same number of donor cells in a larger spleen) and second to pre-occupation of the donor cells in reacting to the tumour.