Prolonged administration of antithymocyte serum in mice. I. Observations on cellular and humoral immunity.

A colony of 400 CBA/He mice was divided into three groups: (a) mice injected continually throughout life with rabbit antimouse thymocyte serum (ATS), (b) mice treated in the same manner with normal rabbit serum (NRS), and (c) mice left untreated. The cellular immune competence of all groups was tested by the transplantation of allografts or rat xenografts, by testing the ability of their lymphoid cells to mount a graft-versus-host reaction, by measuring the response of their lymphocytes to oxazolone skin-painting and the response to phytohaemagglutinin, and finally by measuring the organ distribution of the θ-positive, thymus-dependent lymphocyte population. Determinations were made of the antibody responses to sheep erythrocytes, allografts and rat xenografts, bovine serum albumin, keyhole limpet haemocyanin, and polyoma virus. The ATS-treated mice grew and reproduced normally and did not succumb to bacterial infections. Polyoma virus, probably introduced in the rabbit serum, induced tumours in only some of the ATS-treated mice. Only two lymphomas (0·8%) occurred in this same group; tumours were not seen in the other groups. These results do not support any hypothesis which attributes to lymphocytes a non-immunological function, nor do they support a simple form of the immunological surveillance theory in relation to cancer. Several possible explanations for the increased incidence of tumours in transplant patients are proposed, which take into account the difficulty experienced in this study in abrogating a `cellular immune response' with antilymphocytic serum alone.