Tumor progression in vitro: tumor-promoter-induced reversible decrease in natural immune susceptibility.

Growth of established murine tumor lines in media containing the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), was associated with reversible reductions in sensitivity to in vitro and in vivo parameters of natural resistance. L5178Y-F9 cells exposed to 100 ng TPA/ml for 2 days and returned to culture without TPA for 0-2 days, exhibited reductions in sensitivity to complement-mediated lysis by natural antibodies (Nab), activated macrophages and hypotonic lysis. The natural killer (NK) cell sensitive SL2-5 lymphoma was less sensitive to NK cells, complement-dependent NAb and hypotonic lysis after 2 days growth in 2 or 3 micrograms TPA/ml. Although TPA-treated L5178Y-F9 cells could acquire higher levels of serum NAb in vitro, this was complicated by the instability of the binding at 37 degrees C resulting in an effectively reduced capacity to bind NAb which was also demonstrated by TPA-treated SL2-5 cells. The tumor frequency of threshold s.c. inocula and the i.v. metastatic potential of the TPA-treated tumors was increased in syngeneic DBA/2 mice revealing possible correlations between reductions in the cellular characteristics assayed in vitro and decreased susceptibility to host-mediated defenses in vivo. Continued growth of the TPA-treated cells for a total of 2-8 days without TPA produced a reversal in the in vitro parameters, in the tumor frequency and in the metastatic potential, indicating the requirement for TPA to maintain the resistant phenotype. These data are consistent with the initial reversible nature of the promotion phase of multistage carcinogenesis. The reversible TPA-induced reductions in sensitivity to mediators of natural resistance may be an integral component of promotion, contributing to tumor survival in vivo and increasing the probability that the tumor will progress to a more malignant phenotype.