Selective immunomodulation by the antineoplastic agent mitoxantrone. II. Nonspecific adherent suppressor cells derived from mitoxantrone-treated mice.
Mitoxantrone exerts a potent suppressive influence upon humoral immune responses. The B cell is a likely target for this inhibitory effect, and we have reported evidence supporting this possibility. The impact of mitoxantrone upon T lymphocyte reactivity was assessed as a second mode of action of this novel antineoplastic drug. TH and TS lymphocyte induction were tested in the in vitro anti-sheep erythrocyte response, and a surprising differential effect of mitoxantrone was observed. Helper activity was abrogated and suppressor function was enhanced. In apparent disagreement with this result, mitoxantrone inhibited the in vivo induction of TS cells using trinitrophenylated spleen cells. Macrophages were investigated as potential mediators of these effects upon immunoregulatory function. Replacement of macrophages in mitoxantrone-treated spleen cell preparations by normal adherent cells allowed the induction and complete expression of TH lymphocyte function. Conversely, replacement of mitoxantrone-treated macrophages with normal adherent cells before induction of TS cells failed to generate TS cell function. Thus, TH cells were resistant and TS cells were completely susceptible to mitoxantrone. Furthermore, supplementation of normal TH cell cultures with splenic macrophages from mitoxantrone-treated mice inhibited the induction of helper function. Production of the lymphokines IL 2 and TRF in mitoxantrone-treated mice was normal. This is consistent with the retention of functional TH cells in drug-treated spleens. Macrophages in the spleens of mitoxantrone-treated mice were responsible for the abrogated helper function and the enhanced suppressor activity. Although TS cell induction was directly inhibited by the drug, the effect upon TH cell function was secondary to the action of mitoxantrone-induced suppressor macrophages. Mitogen-stimulated lymphokine production was normal. Thus, mitoxantrone is a selective immunomodulator. The macrophage-mediated suppression of TH cell induction and humoral immunity investigated in spleens from mitoxantrone-treated mice is an intriguing finding that may have significant implications for immunotherapy.