Human CD4⁺ T cells present within the microenvironment of human lung tumors are mobilized by the local and sustained release of IL-12 to kill tumors in situ by indirect effects of IFN-γ

By implanting nondisrupted pieces of human lung tumor biopsy tissues into SCID mice, it has been possible to establish viable grafts of the tumor, as well as the tumor-associated microenvironment, including inflammatory cells, fibroblasts, tumor vasculature, and the extracellular matrix. Using this xenograft model, we have evaluated and characterized the effects of a local and sustained release of human rIL-12 (rhIL-12) from biodegradable microspheres. In response to rhIL-12, the human CD45⁺ inflammatory cells present within the xenograft mediate the suppression or the complete arrest of tumor growth in SCID mice. Analysis of the cellular events reveals that human CD4⁺ and CD8⁺ T cells are induced by rhIL-12 to produce and secrete IFN-γ. Serum levels of human IFN-γ in mice bearing rhIL-12-treated tumor xenografts correlate directly with the degree of tumor suppression, while neutralizing Abs to human IFN-γ abrogate the IL-12-mediated tumor suppression. Gene expression profiling of tumors responding to intratumoral rhIL-12 demonstrates an up-regulation of IFN-γ and IFN-γ-dependent genes not observed in control-treated tumors. Genes encoding a number of proinflammatory cytokines, chemokines (and their receptors), adhesion molecules, activation markers, and the inducible NO synthase are up-regulated following the introduction of rhIL-12, while genes associated with tumor growth, angiogenesis, and metastasis are decreased in expression. NO contributes to the tumor killing because an inhibitor of inducible NO synthase prevents IL-12-induced tumor suppression. Cell depletion studies reveal that the IL-12-induced tumor suppression, IFN-γ production, and the associated changes in gene expression are all dependent upon CD4⁺ T cells.