Phorbol ester-induced enhancement in lytic activity of CD8+ splenic T cells from low-dose melphalan-treated MOPC-315-tumor bearers

Importance of signaling via the IFN-alpha/beta receptor on host cells for the realization of the therapeutic benefits of cyclophosphamide for mice bearing a large MOPC-315 tumor

Here we show that low-dose cyclophosphamide (CY), that depends for its therapeutic effectiveness on the immunopotentiating activity of the drug for T cell-mediated tumor-eradicating immunity, is curative for approximately 80% of wild-type (WT) mice bearing a large s.c. MOPC-315 tumor, but only for approximately 10% of IFN-alpha/betaR-/- mice bearing a large s.c. MOPC-315 tumor. Histopathological examination of the s.c. tumors of such mice on day 4 after the chemotherapy revealed that the low dose of CY led to accumulation of T lymphocytes in both the WT and the IFN-alpha/betaR-/- mice. However, in the CY treated tumor bearing WT mice the T lymphocytes were present throughout the tumor mass and in direct contact with tumor cells, but in the CY treated tumor bearing IFN-alpha/betaR-/- mice most of the T lymphocytes remained in blood vessels. In addition to being important for CY-induced transendothelial migration of T lymphocytes into the tumor mass, we show here that signaling via the IFN-alpha/betaR is also important for CY-induced control of metastatic tumor progression in the spleen and liver of the tumor bearing mice. Finally, CY cured tumor bearing WT mice were resistant to a subsequent challenge with MOPC-315 tumor cells, but the few CY cured tumor bearing IFN-alpha/betaR-/- mice were not. Thus, signaling via the IFN-alpha/betaR on host cells in MOPC-315 tumor bearers is important for CY-induced: (a) transendothelial migration of T lymphocytes into the tumor mass and the eradication of the primary tumor, (b) control of metastatic tumor progression, and (c) resistance to a subsequent tumor challenge.

Importance of the B7-2 Molecule for Low Dose Melphalan-Induced Acquisition of Tumor-Eradicating Immunity by Mice Bearing a Large MOPC-315 Tumor

We have previously shown that low dose melphalan (l-phenylalanine mustard; l-PAM) therapy of hitherto immunosuppressed mice bearing a large (20-mm) s.c. MOPC-315 tumor leads to the acquisition of potent CD8+ T cell-mediated antitumor immunity which in turn eradicates the large tumor burden not eradicated by the direct antitumor effects of the drug. Here we show the preferential importance of the B7-2 costimulatory molecule for the curative effectiveness of low dose l-PAM for mice bearing a large MOPC-315 tumor by demonstrating that treatment with anti-B7-2 mAb, but not anti-B7-1 mAb, reduced the percentage of mice cured by the low dose l-PAM. In addition, we show the preferential importance of the B7-2 molecule for the low dose l-PAM-induced acquisition of the ability of tumor-infiltrating lymphocytes from MOPC-315 tumor bearers to secrete IL-2 and IFN-γ as well as to exert an anti-MOPC-315 CTL effect. The preferential importance of the B7-2 molecule may be due to the higher level of B7-2 than of B7-1 expression on B220+ cells and on tumor cells from the s.c. tumor nodule of low dose l-PAM-treated MOPC-315 tumor bearers and the selective up-regulation of the B7-2 molecule in the draining of these mice. Thus, the B7-2 molecule plays a dominant role in the acquisition of T cell-dependent tumor-eradicating immunity in low dose l-PAM-treated mice bearing a large MOPC-315 tumor, suggesting that one of the mechanisms by which chemotherapy may enhance antitumor immunity is through up-regulation of critical costimulatory molecules that enhance antitumor responses.

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 gamma in the therapeutic outcome

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 TNF alpha in the s.c. tumor nodule. Moreover, the expression of mRNA for interferon gamma (IFN gamma) and interleukin-12 (IL-12; p40) is also elevated at the tumor site. However, while elevation in the expression of mRNA for TNF alpha and IFN gamma 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-IFN gamma 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 IFN gamma 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 IFN gamma. However, unlike TNF alpha, IFN gamma 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 IFN gamma 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.

Transforming growth factor-beta-mediated down-regulation of antitumor cytotoxicity of spleen cells from MOPC-315 tumor-bearing mice engaged in tumor eradication following low-dose melphalan therapy

We have previously shown that treatment of mice bearing a large MOPC-315 plasmacytoma with a low dose of the anticancer drug melphalan (L-phenylalanine mustard; L-PAM) results in the acquisition of a potent CD8+ T-cell-mediated anti-MOPC-315 cytotoxic T lymphocyte (CTL) activity by the hitherto immunosuppressed tumor bearers, and this immunity contributes to complete tumor eradication. In the studies presented here, we sought to determine how the acquisition of this antitumor immunity following low-dose chemotherapy is possible, in light of the report that MOPC-315 tumor cells produce transforming growth factor-beta (TGF-beta), an immunosuppressive cytokine that can down-regulate the generation of CTL responses. We found that the acquisition of CTL activity following low-dose L-PAM therapy is not due to a chemotherapy-induced decrease in the sensitivity of MOPC-315 tumor bearer spleen cells to TGF-beta-mediated inhibition of CTL generation. Moreover, even spleen cells from MOPC-315 tumor-bearing mice, which had received L-PAM therapy 7 days earlier and had acquired CTL activity in vivo, were sensitive to the inhibitory activity of TGF-beta upon culture for as little as 1 day, with or without stimulator tumor cells. However, the production of TGF-beta by MOPC-315 tumors decreased drastically as a consequence of the low-dose chemotherapy. Thus, the curative effectiveness of low-dose L-PAM therapy for MOPC-315 tumor-bearing mice may be due, at least in part, to a reduction in TGF-beta production that enables the development of tumor-eradicating immunity.

Some approaches to improve the therapeutic effectiveness of adoptive chemoimmunotherapy with spleen cells from melphalan-treated BALB/c mice bearing a large MOPC-315 tumor

Spleen cells from BALB/c mice that are in the process of eradicating a large MOPC-315 tumor following low-dose L-PAM therapy (L-PAM TuB spleen cells) were previously shown to be capable of bringing about the complete regression of a large (15 to 20 mm) s.c. MOPC-315 tumor in a substantial percentage of T-cell-deficient (athymic nude) mice that had been treated with low-dose L-PAM (adoptive chemo-immunotherapy; ACIT). Here we show that aggressive depletion of CD4+T-cells by treatment both of spleen-cell donors and of recipients with anti-L3T4 monoclonal antibody (MAb) greatly improved the therapeutic effectiveness of L-PAM TuB spleen cells in ACIT. In the light of the apparent importance of cytotoxic T-lymphocytes (CTLs) for tumor eradication in low-dose L-PAM-treated MOPC-315-tumor bearers, it is interesting that treatment of L-PAM TuB spleen-cell donors with anti-L3T4 MAb was found to result in the generation of enhanced splenic anti-MOPC-315 cytotoxicity. Although most athymic nude mice in which the tumor had apparently completely regressed following ACIT remained tumor-free, approximately 1/3 of the mice relapsed. However, a substantial percentage of the relapsing mice were rescued by a low dose of L-PAM, which was not effective in causing tumor regression in athymic nude mice bearing a comparably large tumor if the mice had not been subjected previously to ACIT. Almost all athymic nude mice that had been "cured" of a large MOPC-315 tumor by ACIT but did not resist a subsequent MOPC-315 tumor challenge were rescued by low dose L-PAM. Thus, the therapeutic effectiveness of L-PAM TuB spleen cells in ACIT may be improved by aggressive depletion of CD4+ T-cells, suggesting that a low dose of L-PAM, which leads to the acquisition of potent splenic-tumor-eradicating immunity in BALB/c mice bearing a large MOPC-315 tumor, does not eliminate completely (or possibly not at all) the inhibitory activity of CD4+ T-cells. In addition, athymic nude mice that are not endowed with fully protective tumor-eradicating immunity following ACIT still have a substantial residual anti-tumor immune potential that can be exploited to bring about eradication of a large tumor burden following low-dose L-PAM therapy.