Tatyana Dvorkin

CD11b+/Gr-1+ Immature Myeloid Cells Mediate Suppression of T Cells in Mice Bearing Tumors of IL-1β-Secreting Cells

Tumor cells secreting IL-1β are invasive and metastatic, more than the parental line or control mock-transfected cells, and concomitantly induce in mice general immune suppression of T cell responses. Suppression strongly correlates with accumulation in the peripheral blood and spleen of CD11b+/Gr-1+ immature myeloid cells and hematological alterations, such as splenomegaly, leukocytosis, and anemia. Resection of large tumors of IL-1β-secreting cells restored immune reactivity and hematological alterations within 7–10 days. Treatment of tumor-bearing mice with the physiological inhibitor of IL-1, the IL-1R antagonist, reduced tumor growth and attenuated the hematological alterations. Depletion of CD11b+/Gr-1+ immature myeloid cells from splenocytes of tumor-bearing mice abrogated suppression. Despite tumor-mediated suppression, resection of large tumors of IL-1β-secreting cells, followed by a challenge with the wild-type parental cells, induced resistance in mice; protection was not observed in mice bearing tumors of mock-transfected fibrosarcoma cells. Altogether, we show in this study that tumor-derived IL-1β, in addition to its proinflammatory effects on tumor invasiveness, induces in the host hematological alterations and tumor-mediated suppression. Furthermore, the antitumor effectiveness of the IL-1R antagonist was also shown to encompass restoration of hematological alterations, in addition to its favorable effects on tumor invasiveness and angiogenesis that have previously been described by us.

Differential effects of IL-1 alpha and IL-1 beta on tumorigenicity patterns and invasiveness

In this study, we show that distinct compartmentalization patterns of the IL-1 molecules (IL-1α and IL-1β), in the milieu of tumor cells that produce them, differentially affect the malignant process. Active forms of IL-1, namely precursor IL-1α (pIL-1α), mature IL-1β (mIL-1β), and mIL-1β fused to a signal sequence (ssIL-1β), were transfected into an established fibrosarcoma cell line, and tumorigenicity and antitumor immunity were assessed. Cell lines transfected with pIL-1α, which expresses IL-1α on the membrane, fail to develop local tumors and activate antitumor effector mechanisms, such as CTLs, NK cells, and high levels of IFN-γ production. Cells transfected with secretable IL-1β (mIL-1β and ssIL-1β) were more aggressive than wild-type and mock-transfected tumor cells; ssIL-1β transfectants even exhibited metastatic tumors in the lungs of mice after i.v. inoculation (experimental metastasis). In IL-1β tumors, increased vascularity patterns were observed. No detectable antitumor effector mechanisms were observed in spleens of mice injected with IL-1β transfectants, mock-transfected or wild-type fibrosarcoma cells. Moreover, in spleens of mice injected with IL-1β transfectants, suppression of polyclonal mitogenic responses (proliferation, IFN-γ and IL-2 production) to Con A was observed, suggesting the development of general anergy. Histologically, infiltrating mononuclear cells penetrating the tumor were seen at pIL-1α tumor sites, whereas in mIL-1β and ssIL-1β tumor sites such infiltrating cells do not penetrate inside the tumor. This is, to our knowledge, the first report on differential, nonredundant, in vivo effects of IL-1α and IL-1β in malignant processes; IL-1α reduces tumorigenicity by inducing antitumor immunity, whereas IL-1β promotes invasiveness, including tumor angiogenesis, and also induces immune suppression in the host.

Immune phenomena involved in the in vivo regression of fibrosarcoma cells expressing cell-associated IL-1α

Constitutive expression of cell‐associated, but not secreted, interleukin‐1α (IL‐1α) by oncogene‐transformed fibrosarcoma cells induced regressing tumors in mice, a phenomenon that was abrogated by the IL‐1 inhibitor, the IL‐1 receptor antagonist (IL‐1Ra). On the contrary, non‐IL‐1α‐expressing tumor cells induce progressive tumors in mice. In vivo and ex vivo experiments have shown that regression of IL‐1α‐positive fibrosarcoma cells depends on CD8+ T cells, which can also be activated in CD4+ T cell‐depleted mice, with some contribution of natural killer cells. In spleens of mice bearing the non‐IL‐1α‐expressing fibrosarcoma cells, some early and transient manifestations of antitumor‐specific immunity, such as activation of specific proliferating T cells, are evident; however, no development of cytolytic T lymphocytes or other antitumor protective cells could be detected. In spleens of mice bearing the non‐IL‐1α‐expressing fibrosarcoma cells, the development of early tumor‐mediated suppression was observed, and in spleens of mice injected with IL‐1α‐positive fibrosarcoma cells, protective immunity developed in parallel to tumor regression. Treatment of mice bearing violent fibrosarcoma tumors with syngeneic‐inactivated, IL‐1α‐positive fibrosarcoma cells, at a critical interval after injection of the malignant cells (Days 5–12), induced tumor regression, possibly by potentiating and amplifying transient antitumor cell immune responses or by ablation of tumor‐mediated suppression. Membrane‐associated IL‐1α may thus serve as an adhesion molecule, which allows efficient cell‐to‐cell interactions between the malignant and immune effector cells that bear IL‐1Rs and function as a focused cytokine with adjuvant activities at nontoxic, low levels of expression. Our results also point to the potential of using antitumor immunotherapeutic approaches using cell‐associated IL‐1α.