Residual immunity of athymic NCr/sed nude mice and the xenotransplantation of human tumors

Abstract

This study assessed the residual immunity possessed by NCr/Sed (nu/nu) athymic nude mice and examined strategies to reduce it in order to enhance the transplantability of human tumors for experimentation. Adult (8-week-old) female mice had fewer T cells (11%) and more B and NK (asialo-GM₁-positive [ASGM₁⁺]) cells in their spleen than euthymic (nu/+) controls. The number of phenotypically mature T cells increased with age, peaking at 16 weeks. ASGM₁⁺ cells also increased in number over time, although the NK-activity decreased after 12 weeks. B cells remained relatively constant in number. Athymic NCr/Sed nude mice displayed reactivity against a human squamous carcinoma xenograph (FaDu), in a Winn's test and TD₅₀ assay. Immunity against xenografts (TD₅₀ assay) was significantly lower (by a factor of 2) in 4-week-old than in 12-week-old nude mice. Similarly, a significant 2-fold reduction in TD₅₀ was obtained after a single intraperitoneal injection of cyclophosphamide into 8-week-old animals. Chronic (>8 weeks) exposure of the nude mice to subcutaneously administered β-estradiol markedly reduced the number of splenic NK cells and their cytolytic activity, but the TD₅₀ reduction was not statistically significant (p =0.1). Six Gray whole-body irradiations (WBI) had been shown to produce a highly significant, 3-fold reduction in the TD₅₀ for FaDu. Flow cytometric analysis of splenic lymphoid cells from whole-body-irradiated recipients revealed: (a) marked initial depletion in the absolute numbers of lymphoid cells; (b) marked and long-lasting depletion of T cells, with slow and minimal recovery only evident between 6 and 12 weeks; (c) rapid, almost complete, depletion of B cells with prompt and partial recovery after 2 weeks; (d) depletion of NK cells and NK activity, with recovery by 10 weeks. No change in the number or phagocytic capacity of resident peritoneal macrophages was seen. These data give further support to a postulated role for residual T cells in the xenoreactivity of NCr/Sed nude mice.