Inhibition of murine prostate tumor growth and activation of immunoregulatory cells with recombinant canarypox viruses

Background: Immunization with modified tumor cells carrying recombinant imm unomodulatory genes is being explored as-cancer immunotherapy. In this stud y, we examine whether canarypox ALVAC viruses carrying immunostimulatory cy tokine genes (granulocyte-macrophage colony-stimulating factor, interleukin 2, interleukin 12, and tumor necrosis factor-alpha) can induce antitumor i mmunity (to rechallenge):in the RM-1 model of a highly aggressive, weakly i mmunogenic murine prostate cancer. Methods: For antitumor activity studies, RM-1 murine prostate cancer cells were infected with the parental ALVAC vi rus or one or two recombinant ALVAC-cytokine viruses and then injected into male C57BL/6 mice. For rechallenge studies, other mice were first given an injection subcutaneously with irradiated (nonproliferating) recombinant AL VAC-infected RM-1 cells and then (10 days later) with untreated RM-1 cells. For the determination of which immune cells were required for antitumor ac tivity, mice were immunodepleted of CD4, CD8, or natural killer (NK) NK1.1 cells with the corresponding monoclonal antibodies and were then given an i njection of ALVAC-cytokine-infected RM-1 cells. For all experiments, tumor outgrowth and animal survival were monitored. Results After subcutaneous in jection into mice, RM-1 cells infected with one (except ALVAC-interleukin 2 ) or two ALVAC-cytokine recombinants had statistically significantly greate r antitumor activity than RM-1 cells infected with parental ALVAC (P < .001 for all; two-sided test), The antitumor,activity of RM-1 cells infected wi th any two ALVAC-cytokine recombinants,vas greater than, but not statistica lly significantly different from, that of RM-1 cells infected with any one ALVAC-cytokine recombinant, NK1.1 cells were necessary for antitumor activi ty, but tumor-specific CD4(+) regulatory T cells were also induced that inh ibited CDS' RM-1-specific cytotoxic T cells, resulting in the lack of immun ity to a rechallenge by RM-1 cells. Discussion: Canarypox: viruses can tran sfer immunostimulatory cytokine genes into RM-1 prostate cancer cells. When such cells were injected into mice, the cytokines induced an antitumor res ponse against this highly aggressive, weakly immunogenic tumor. This respon se, however, did not protect the mouse against a rechallenge with RM-1 cell s because suppressor CD4(+) T cells were induced that inhibited tumor-speci fic CD8(+) cytotoxic T cells.