Thomas M. Walker

Taxol-mediated changes in fibrosarcoma-induced immune cell function: Modulation of antitumor activities

The anticancer drug taxol (paclitaxel) inhibits tumors through multiple cytotoxic and cytostatic mechanisms. Independently of these mechanisms, taxol induces distinct immunological efficacy when it acts as a second signal for activation of tumoricidal activity by interferon-γ(IFNγ)-primed murine normal host macrophages. We reported that tumor-distal macrophages, which mediate immunosuppression through dysregulated nitric oxide (NO) and tumor necrosis factor α (TNFα) production, are differentially regulated by taxol. Because taxol influences tumor cell growth dynamics and activates immune cell populations, we assessed the ex vivo immunosuppressive and antitumor activities of taxol-treated normal host and tumor-bearing host (TBH) macrophages. Pretreatment of such cells with taxol partly reconstituted T cell alloantigen reactivity, suggesting that taxol mediates a limited reversal of TBH macrophage immunosuppressive activity. Taxol-treated TBH macrophages significantly suppressed the growth of fibrosarcoma cells (Meth-KDE) through soluble effector molecules and promoted direct cell-mediated cytotoxicity, indicating that taxol enhanced tumor-induced macrophage antitumor activities. Tumor-induced helper T cells, however, showed a higher sensitivity to direct taxol-induced suppression. These data demonstrate that taxol exerts pleiotropic effects on antitumor immune responses with the capacity to abate the immunosuppressive activities of macrophages and promote macrophage-mediated anti-tumor activities simultaneously, but also directly modulating T cell reactivity. Collectively, these studies suggest that the antineoplastic drug taxol may impart antitumor activity through an immunotherapeutic capacity.

Ia- macrophages and cytokine networks contribute to tumor-induced suppression of CD4+ autoreactive T cells.

Tumor growth changes the functions and phenotypes of macrophages (M phi) and T cells. Suppression of CD4+ T cell autoresponses during tumor growth was contributed primarily by M phi. Tumor-induced alterations in the abilities of these cells to mediate autorecognition were assessed through syngeneic mixed lymphocyte reaction (SMLR) assays. Tumor-bearing host (TBH) M phi were significantly more suppressive (60-90%) than normal host (NH) M phi, and this suppression was caused partly by reduced Ia expression. TBH Ia- M phi were significantly more suppressive (50-80%) than their NH counterparts. The suppression mechanism was controlled partly by prostaglandin E2 (PGE2), because treating cultures with indomethacin and titrated NH and TBH Ia- M phi led to increased T-cell responsiveness, although responsiveness never reached levels of assays containing unseparated M phi. Blocking studies using anti-interferon-gamma (anti-IFN-gamma) monoclonal antibodies (mAb), anti-interleukin 4 (anti-IL-4) mAb, and indomethacin suggested that IFN-gamma, IL-4, and PGE2 contributed to tumor-induced M phi-mediated suppression. Our results suggested that a quantitative shift in M phi phenotype and a qualitative shift in M phi function in addition to differences in cytokine-directed accessory activities are partly responsible for tumor-induced suppression CD4+ T cell autoresponses.

Tumor growth changes the contribution of granulocyte-macrophage colony-stimulating factor during macrophage-mediated suppression of allorecognition

Tumor-bearing host (TBH) macrophages (M phi) suppress T cell alloresponses, and this study suggests granulocyte-macrophage colony-stimulating factor (GM-CSF), a molecule associated with suppressive M phi activity during tumor growth, signals more immunosuppression. In the absence of M phi, GM-CSF increased T cell proliferation in response to alloantigen. However, TBH M phi-mediated suppression of allorecogntion was further induced by GM-CSF. Allogeneic mixed lymphocyte reaction (MLR) cultures, containing normal host (NH) M phi, were either unaffected or enhanced. Prostaglandin E2 (PGE2), a highly suppressive monokine that decreases alloreactivity, did not seem to be involved in the suppression caused by the TBH M phi/GM-CSF interaction. M phi-CSF (M-CSF) addition to cultures did not reverse the suppression caused by TBH M phi and GM-CSF, and inhibition of PGE2 synthesis did not change the response to M-CSF. TBH Ia- M phi, a suppressor population that predominates among splenic M phi during tumor growth, demonstrated significantly lower reactivity in the presence of GM-CSF. In contrast, alloresponses suppressed by NH Ia- M phi demonstrated higher reactivity in the presence of GM-CSF. The data collectively suggest that TBH M phi respond differently to GM-CSF, and that tumor-induced changes in GM-CSF responsiveness affect M phi accessory ability.

Tumor growth alters macrophage responsiveness to macrophage colony-stimulating factor during reactivity against allogeneic and syngeneic MHC class II molecules

Tumor-induced changes in macrophage (M phi)2 accessory activities significantly suppress T-cell recognition of allogeneic and syngeneic major histocompatibility complex (MHC) class II molecules. Because these changes are often associated with altered responses to stimulatory and inhibitory cytokines, we investigated the possibility that tumor growth alters the contribution of a macrophage regulatory cytokine, macrophage colony-stimulating factor (M-CSF), during reactivity against allogeneic and syngeneic MHC class II molecules. T-cell reactivity against allogeneic MHC class II molecules was significantly suppressed by tumor-bearing host (TBH) M phi in the presence of M-CSF. M-CSF-induced suppression was independent of TBH M phi prostaglandin E2 (PGE2) synthesis. T-cell reactivity against syngeneic MHC class II molecules increased in the presence of M-CSF when normal host (NH) M phi served as the source of syngeneic molecules. However, T-cell reactivity against syngeneic MHC class II molecules in the presence of M-CSF did not change when TBH M phi served as stimulator/accessory cells. Although T-cell reactivity against NH syngeneic MHC class II molecules was additively increased by M-CSF and indomethacin (a PGE2 synthesis inhibitor) treatment, reactivity against TBH syngeneic MHC class II molecules increased solely through PGE2 synthesis inhibition. Admixtures of both NH and TBH M phi in the absence or presence of M-CSF suggest that tumor-induced suppression was not strictly due to decreased expression of MHC class II molecules. Collectively, these data suggest that TBH M phi are partly suppressive through altered responsiveness to M-CSF.

Macrophages stimulated by receptor-ligand interactions exhibit differences in cell-cycle kinetics during tumor growth: stimulation at Mac-1 and Mac-3 receptors alters DNA synthesis.

Phenotypic and functional changes associated with tumor-bearing host (TBH) macrophages (M phi) are partly responsible for immunosuppression during tumor growth. Flow cytofluorometric analyses revealed differences in cell-cycle kinetics between normal host (NH) and TBH M phi that were stimulated at specific receptors. Receptor-ligand interactions were induced by antibodies against Mac-1, -2, -3, and Ia receptors and changes in DNA synthesis were measured over a 12-h time course by incorporation of propidium iodide. TBH M phi showed higher DNA synthesis than NH M phi over this time course irrespective of the receptor induced. NH M phi stimulated at the Mac-1 receptor demonstrated higher DNA synthesis than control NH M phi although TBH M phi stimulated at this receptor and control TBH M phi failed to show any differences. Both NH and TBH M phi exhibited small, short-term decreases in DNA synthesis when stimulated at the Mac-2 receptor. TBH M phi that were stimulated at the Mac-3 receptor demonstrated higher DNA synthesis than their control counterparts while NH M phi stimulated at this receptor and control NH M phi showed identical levels of DNA synthesis. No differences in DNA synthesis were found among normal or TBH M phi that were stimulated through Ia. Differences in DNA synthesis did not appear to be attributable to differences in receptor expression. Further analysis of Mac-1 and Mac-3 stimulated cells revealed that DNA synthesis in NH M phi stimulated at the Mac-1 receptor returned to control levels at 48 h.(ABSTRACT TRUNCATED AT 250 WORDS)