Partial tolerance of subcutaneously transplanted xenogeneic tumour cell graft by Fas‐mediated immunosuppression

SummaryCertain anti‐Fas antibodies, such as RMF2, induce apoptosis of Fas‐expressing cells. We applied the Fas/anti‐Fas system to induce killing of Fas‐expressing immunocytes with resultant immunosuppression. W7TM‐1 tumour cells, a rat T‐cell line, were inoculated subcutaneously in BALB/c mice and tumour growth was monitored in untreated mice and in mice treated with RMF2. Prior to treatment with RMF2, we examined the expression of Fas in isolated splenocytes and in tumour‐infiltrating lymphocytes by flow cytometry and immunohistochemistry, respectively. There was a remarkable increase in Fas‐positive lymphocytes, including natural killer (NK) cells, among splenocytes at day 5 after tumour cell inoculation. The number of Fas‐positive infiltrating lymphocytes also increased markedly, from day 5 to day 10. We then examined whether RMF2 could induce apoptosis of Fas‐positive activated lymphocytes isolated from the spleen at day 5 in vitro. Terminal deoxy (d) ‐UTP nick end labelling (TUNEL) and Annexin V staining methods showed apoptosis of isolated cells when incubated with RMF2, and typical apoptotic features were confirmed by 4′,6‐diamidino‐2‐phenylindole dihydrochloride (DAPI) staining. Furthermore, suppression of cellular and humoral immunity was noted in RMF2‐treated mice by mixed lymphocyte reaction and assay of serum levels of immunoglobulin G, respectively. Finally, treatment of animals with RMF2 daily from day 5 to day 9 could maintain the tumour size, while the tumour mass began to diminish in untreated mice immediately after reaching a maximum size. We confirmed the enhancing effects of long‐term treatment with RMF2, through the induction of immunosuppression, on the growth of unvascularized xenogeneic tumour cell grafts.