Immunological Attack by Adult Cells in the Developing Chick Embryo: Influence of the Vascularity of the Host Spleen and of Homograft Rejection by the Embryo on Splenomegaly

Abstract

Adult chicken spleen was implanted on to the chorio-allantoic membrane of chick embryos at 8,9,11,13 and 15 days of incubation. The initial manifestation of graft against the host attack by these adult cells was an increase of host spleen weight (splenomegaly). No splenomegaly appeared in the embryonic hosts before 15 days of incubation. The appearance of splenomegaly roughly coincided with the appearance of reticulum cell foci in the chimaeric spleens. Varying latent periods between the time of introduction of donor cells and the first appearance of increased spleen weights necessitated comparison of the host spleen weights 3 days after the onset of splenomegaly. This showed that implantation at 13 days produced the greatest splenomegaly. Only very slight splenomegaly was obtained after implantation of spleen at 15 days. A possible homograft rejection by the host and poor ‘take’ of the splenic implant on the chorio-allantoic membrane are both thought to be responsible. Intravenous injection of adult chicken spleen cells into embryos at 6, 8, 11 and 13 days of incubation again produced no splenomegaly before 15 days of incubation. Comparison of the host spleen weights 3 days after onset of splenomegaly showed that injection at 13–15 days gave the greatest splenomegaly. Injection before this time gave very slight splenomegaly and injections after 13 days produced a decreasing degree of splenomegaly with increasing age of embryo injected. Increasing the number of adult spleen cells at 15 days or giving large doses of blood at 8, 9, 11, 15 and 17 days of incubation and in the 1-and 2-day-old chick failed to increase the splenomegaly greatly or markedly to alter the above pattern. When 1-day-old chicks were injected with allogenic blood only slight splenomegaly occurred and when 2-day-old chicks were injected there was no splenomegaly 6 days later. The suitability of the chick embryo as an environment for the proliferation of immunologically competent cells is discussed. The reduction in splenomegaly when embryos older than 13-15 days are injected is discussed in relation to the appearance from this time of a weak homograft response by the embryo and to the greater difficulty of induction of both splenomegaly and tolerance to skin grafts soon after hatching. It is proposed that very small numbers of immunologically competent cells appear in the embryo soon after 15 days and progressively increase from this time until onset of competence of the host at 10 days post-hatching. Possible effects of such cell growth on the degree of induction of tolerance are discussed.