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Interleukin 12 (IL-12) is a heterodimeric cytokine with a number of biological effects that are consistent with its potential role as an antitumor agent. The antimetastatic and antitumor activities of IL-12 have been demonstrated in a number of murine tumor models. Both the inhibition of established experimental pulmonary or hepatic metastases and a reduction in spontaneous metastases have been achieved by treatment with murine IL-12. Systemic treatment of mice bearing subcutaneous tumors with IL-12 results in tumor growth inhibition, prolongation of survival, and, in some models, tumor regression. The antitumor effect of IL-12 in these models is dose-dependent and can be initiated against well-established tumors. Mice cured of their tumor by IL-12 treatment are specifically immune to rechallenge with the same tumor. A series of experiments have demonstrated that both T-cells and interferon-gamma (IFN-gamma) induction are necessary for the optimal antitumor effects of IL-12. However, the antitumor efficacy of IL-12 has not been observed after exogenous administration of murine IFN-gamma, suggesting that additional factors may be important for the antitumor effects of IL-12. In several tumor models, IL-12 is more active or has a larger therapeutic window than either IL-2 or IFN-alpha, two cytokines with demonstrated antitumor activity against human malignancies. Combining IL-12 with other cytokines or chemotherapeutic drugs can improve antitumor effects.

IL-12 is a heterodimeric cytokine that promotes cell-mediated immunity through its regulatory effects on T and NK cells. In some murine infectious disease models, IL-12 was shown to be produced endogenously in response to infection, and the exogenous administration of IL-12 to mice with either infectious diseases or tumors has resulted in significant therapeutic effects. IL-12 was protective early in the disease process, as well as against established disease. However, the biologic activities of IL-12 that are beneficial in the host response to these infectious diseases and malignancies can also be deleterious in certain disease states. Thus, IL-12 has considerable potential for the treatment of a variety of human disorders if used under the appropriate conditions. Likewise, antagonists of IL-12 may have a role in controlling diseases with pathologies that are mediated through immune mechanisms.

Macrophages (M phi s) undergo a physiological response known as the macrophage disappearance reaction (MDR) in response to certain stimuli in the peritoneal compartment. The types of stimuli that can cause the MDR, the relationship of the MDR to the host immunological response, and the possible role of the MDR in M phi activation are reviewed. The data indicate that the MDR occurs in response to both acute nonspecific inflammatory and specific immune delayed hypersensitivity processes and that the MDR may play an important role in M phi activation.

Although interleukin-12 (IL-12) has marked antitumor activity against the murine Renca renal cell carcinoma in vivo, no antiproliferative activity with IL-12 was observed against these tumor cells in vitro; in contrast, interferon-gamma (IFN-gamma) had growth inhibitory activity. Since one of the properties of IL-12 is its ability to stimulate production of IFN-gamma, the role of IFN-gamma in mediating the antitumor activity of IL-12 was evaluated. Substantially diminished antitumor activity was observed in mice injected with IL-12 and neutralizing antibody to murine IFN-gamma compared with mice receiving IL-12 alone, indicating that IFN-gamma was required for the optimal antitumor efficacy of IL-12. However, several lines of investigation suggest that the antitumor effect of IL-12 is not mediated solely through the induction of IFN-gamma. Exogenous administration of IFN-gamma to Renca tumor-bearing euthymic mice resulted in less antitumor efficacy than that which could be obtained with IL-12. In addition, the antitumor effect of IL-12 was reduced in nude mice compared with euthymic mice, but an approximately 10-fold higher level of serum IFN-gamma was induced in nude than in euthymic mice. Thus, these results indicate that induction of high serum levels of IFN-gamma is not sufficient to mediate the antitumor efficacy of IL-12.

Blocking conditions that are optimal for the detection of surface antigens on resident peritoneal macrophages (PM phi) by flow cytometry are not ideal for elicited or activated PM phi. A blocking step of 10% goat serum can be used routinely to detect the F4/80 and Mac-1 antigens on resident PM phi. In contrast, high concentrations (33-50% each) of combined goat and mouse sera were required to reduce nonspecific binding and to improve the detection of the F4/80 antigen on PM phi elicited by thioglycollate broth (TG) or activated by maleic anhydride divinyl ether copolymer (MVE-2). However, even low concentrations of goat serum masked the expression of the Mac-2 antigen on TG and MVE-2 PM phi. Thus, within a given elicited or activated PM phi population, different blocking conditions may be necessary to detect different surface antigens optimally. In addition to blocking, the use of isotypic controls that match the monoclonal antibody isotypes was found to be necessary for the optimal detection of antigen expression on TG and MVE-2 PM phi.

Activated macrophages (M phi) from mice given Salmonella typhimurium or Corynebacterium parvum were compared with resident peritoneal macrophages at the molecular level for permissiveness for herpes simplex virus type 1 (HSV-1) replication and for expression of interleukin-1 beta (IL-1 beta). Peritoneal macrophages were harvested from mice injected 7 days previously with live, avirulent S. typhimurium (Sal-PM phi) or heat-killed C. parvum (CP-PM phi) and infected with HSV-1 in vitro. Both Sal-PM phi and CP-PM phi were activated as evidenced by characteristic changes in an ectoenzyme, by increased permissiveness for infectious virus production and viral cytopathic effect, and by induction of IL-1 beta mRNA. Analysis at the molecular level revealed that both types of activated M phi demonstrated increased patterns of HSV-1 immediate-early gene expression and viral DNA replication as compared with resident cells. A novel finding was that viral infection reduced IL-1 beta mRNA in both types of activated M beta. This observation has implications for the efficacy of Salmonella vaccines given in proximity to HSV-1 infection and for potential deleterious effects of HSV-1 infection in immunosuppressed patients receiving immunotherapy.

In contrast with other macrophage (MO) populations, there is little information on the antiviral resistance in vitro of isolated liver MO (Kupffer cells, KC). We have demonstrated that the KC exhibits marked intrinsic resistance to infection in vitro with herpes simplex virus type 1 (HSV-1). Liver and peritoneal MO (PMO) were harvested from untreated mice (naive), from mice treated with drug vehicle, or from mice treated with either a synthetic nonspecific immunomodulator, maleic anhydride divinyl ether copolymer (MVE-2), or with MO colony-stimulating factor-1 (CSF-1). The studies revealed that resident KC, isolated by two different methods, are equally as nonpermissive for infection with HSV-1 as are resident PMO. When infected with HSV-1, resident KC showed no cytopathic effect, and no infectious virus was produced. Intravenous (i.v.) treatment of mice with MVE-2 (50 mg/kg 3 days before cell harvest) increased the number of KC recovered. However, neither i.v. treatment with MVE-2 nor CSF-1 (20,000 U daily for 4 days) had any pronounced effect on the permissiveness of KC or PMO to infection with HSV-1. These data support a role for KC in host antiviral resistance, and indicate that KC intrinsic antiviral resistance is maintained during immunotherapy with MVE-2 or CSF-1.