Low-dose-melphalan-induced up-regulation of type-1 cytokine expression in the s.c. tumor nodule of MOPC-315 tumor bearers and the role of interferon γ in the therapeutic outcome

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Abstract
We have previously shown the importance of endogenous tumor necrosis factor (TNF) production for the curative effectiveness of low-dose melphalan (L-phenylalanine mustard) for mice bearing a large MOPC-315 tumor. In the current study we demonstrate that low-dose melphalan is actually associated with enhanced expression of mRNA for TNFα in the s.c. tumor nodule. Moreover, the expression of mRNA for interferon γ (IFNγ) and interleukin-12 (IL-12; p40) is also elevated at the tumor site. However, while elevation in the expression of mRNA for TNFα and IFNγ is evident within 24 h after the chemotherapy, elevation in the expression of mRNA for IL-12(p40) is first evident 72 h after the chemotherapy. Moreover, neutralizing anti-IFNγ mAb, like neutralizing anti-TNF mAb but not neutralizing anti-IL-12 mAb, reduced the curative effectiveness of low-dose melphalan for MOPC-315 tumor bearers. Studies into the mechanism through which IFNγ mediates its antitumor effect in low-dose-melphalan-treated MOPC-315 tumor-bearing mice revealed that MOPC-315 tumor cells, which are not sensitive to the direct antitumor effects of TNF, display some sensitivity to the antiproliferative activity of high concentrations of IFNγ. However, unlike TNFα, IFNγ is unable to promote the generation of anti-MOPC-315 cytotoxic T lymphocyte activity and, in fact, exerts an inhibitory activity on CTL generation. Taken together, our studies illustrate that low-dose melphalan therapy of MOPC-315 tumor bearers is associated with the rapid elevation in the expression of mRNA for IFNγ and TNF, two cytokines which are important for the curative effectiveness of low-dose melphalan, and which mediate their antitumor effect, in part, through distinct mechanisms.