THYMIC ANTITUMOR EFFECTORS IN MICE CURED OF LYMPHOMA BY CYCLOPHOSPHAMIDE PLUS TNF-ALPHA THERAPY - PHENOTYPIC AND FUNCTIONAL-CHARACTERIZATION UP TO 20 MONTHS AFTER INITIAL TUMOR INOCULATION

As reported previously, cyclophosphamide plus tumor necrosis factor-cu treatment of C57BL/6 mice bearing advanced EL4 lymphoma induced appro x. 60% long-term (i.e., >60 days) survivors. These mice developed prot ective immunity, as evidenced by 1) rejection (100% survival) of EL4 t umor re-implanted on day 60 (day 0 = initial tumor implantation); and 2) development of significant levels of specific EL4 tumor cell killin g activity by both splenocytes and thymocytes. Using this model, age-r elated changes in functionally and phenotypically definable thymocyte subsets were assessed. In thymocytes from 90 to 308 day survivors, spe cific immune memory was long term; both CD4(+) and CD8(+) cells were r equired for the ex vivo stimulation of lytic activity, but the specifi c anti-EL4 cytotoxic effector was CD4(-)CD8(+). On day 520, the surviv ing mice were randomized into 2 groups. One group received a second re challenge with EL4 tumor cells and all survived, The other group was sacrificed on day 520, Their thymocytes, exposed to X-irradiated EL4, developed anti-EL4 lytic activity and, in comparison with thymocytes o f young and age-matched control mice, were markedly enriched in CD4(-) CD8(+)CD44(+) cells. On day 625, thymocytes from the survivors of the day 520 re-challenge were evaluated and were found to have developed s pecific anti-EL4 lytic activity. Phenotypically, they had returned tow ard the pattern seen in age-matched control mice although CD4(-)CD8(+) CD44(+) cells remained increased. These mice were greater than or equa l to 2 years old, the median life span of C57BL/6 mice. Thus, mice cur ed of tumor by an immune-modulating regimen rejected re-implanted prim ary tumor and maintained specific thymic anti-tumor immune memory for life. (C) 1998 Wiley-Liss, Inc.