The Induction of Tolerance of Skin Homografts in Rats with Pooled Cells from Multiple Donors
Authors: Billingham R. E. , Silvers W. K. 
Published: December 2022

Summary
An investigation has been made of the possibility of conferring tolerance in respect of skin homografts from any donor in a heterogeneous rat population by inoculation of newborn animals with pooled cell suspensions prepared from multiple, randomly selected donors. Subjects for these experiments were Wistar rats, from a large, closed, but noninbred colony, whose heterogeneity was such that the median survival time of skin homografts exchanged between them was about 12 days, although a small proportion of the animals (10%) were genetically tolerant, accepting their grafts for longer than 50 days.
Subcutaneous injection of newborn animals with 15,000,000 to 20,000,000 pooled splenic cells from standard panels of 10 donors was almost ineffective in prolonging the lives of test skin homografts, but following intravenous inoculation, the grafts on about 30% of the subjects lived for longer than 50 days—an arbitrary criterion of a high degree of tolerance. Similar dosages of pooled newborn or infant donor splenic cells conferred a high degree of tolerance upon about 60% of the subjects and pooled adult bone marrow cells were even more effective since more than 80% of the animals were highly tolerant. Evidence is presented that if newborn Wistar rats are injected with cells from newborn or adult spleens or from adult bone marrow prepared from the derived inbred Lewis strain, instead of from pooled Wistar donors, then the proportion that became tolerant of Lewis grafts (60–70%) does not differ significantly according to the type of cell inoculated, so that within the dosage limits employed, cells from infant and adult spleen, and from adult bone marrow, must be considered as potentially equally effective in conferring tolerance. It is argued that the different results obtained with the different tissues when pooled cells from heterogeneous donors were employed are expressions of the varying extents to which immunologic interactions occurred on the part of immunologically competent, lymphoid-type cells present in the inoculums. These reactions may have led to the destruction of all cells of some donor genotypes before they had had an opportunity to induce tolerance in respect of some important transplantation antigens of the population. The high degree of efficacy of pooled marrow is attributable to its very low content of immunologically competent cells, whereas that of infant spleen probably depends upon the fact that here the majority of the abundant lymphoid cells are too immature to interact, or may even become tolerant of many of the “foreign” antigens present.
Evidence is presented that the “polyvalent” tolerance induced by pooled donor cells is highly colony-specific; although it extends to the majority of members of the colony, and also to members of the derived Lewis inbred strain, it is completely ineffective in respect of skin homografts derived from unrelated chocolate B.N. inbred strain donors. It is suggested on the basis of these findings that the probability of being able to confer polyvalent tolerance in a truly wild mammalian population is extremely remote because of the large number of different histocompatibility genes present that would have to be represented in the pooled, tolerance-conferring inoculums.
During the course of this study evidence has been obtained that, contrary to the accepted view, the duration of the tolerance-responsive period in the rat closely resembles that characteristic of the mouse. A preliminary account of “runt disease”—the outcome of the reaction of inoculated adult lymphoid tissue cells against a homologous infant host—as it expresses itself in rats is given.