Failed Induction of the T_H1 System in T_H2 Dominant Patients: The Cancer-Permissive Immune Macroenvironment

Tumor microenvironment infiltration by cells of the T helper cell type 1 (T_H1) system, including T_H1 cells, M1 macrophages, natural killer cells, and CD8⁺ T cells, is associated with better cancer prognosis. In contrast, tumor microenvironment infiltration by cells of the T_H2 system, including T_H2 cells, M2 macrophages, and innate lymphoid cells type 2, as well as immune suppressive myeloid-derived suppressor cells and regulatory T cells, is associated with poorer cancer prognosis. Beyond the tumor itself and a myriad of other modifying factors, such as genetic and epigenetic influences on tumorigenesis, the overall immune state of the patient, termed the macroenvironment, has also been shown to significantly influence cancer outcomes. Alterations in the tricarboxylic acid (TCA) cycle (TCA cycle breaks) involving loss of function of succinate dehydrogenase, isocitrate dehydrogenase, and fumarate hydratase have been shown to be associated with an intracellular metabolic shift away from oxidative phosphorylation and into glycolysis in cells that are transforming into cancer cells. The same loss of function of succinate dehydrogenase and isocitrate dehydrogenase has also been identified as inducing a shift in macrophages toward glycolysis that is associated with M1 macrophage polarization. M1 macrophages make interleukin 12, which stimulates T_H1 cells and natural killer cells to produce interferon gamma (IFN-γ), which in turn stimulates M1 macrophage activity, forming an activation loop. IFN-γ also drives activation of CD8⁺ T cells. Thus, M1 macrophage activation initiates and sustains activation of the T_H1 system of cells. In this fashion, TCA cycle breaks at succinate dehydrogenase and isocitrate dehydrogenase that promote cellular transformation into cancer cells are also associated with upregulation of the T_H1 system that provides anti-cancer immune surveillance. The T_H1 and T_H2 systems are known to inhibit each other's activation. It is this author's hypothesis that, in patients whose macroenvironment is sufficiently T_H2-dominant, the metabolic shift toward glycolysis induced by TCA cycle breaks that gives rise to mutagenic changes in tissue parenchymal cells is not counterbalanced by adequate activation of M1 macrophages, thus giving rise to cancer cell development. For instance, the atopic T_H2-high asthma phenotype, a T_H2 dominance-based comorbidity, is associated with a more than doubled incidence of colon, breast, lung, and prostate cancer, compared with non-asthmatics. Failure of TCA cycle breaks to induce M1 polarization of tissue-resident macrophages yields a tissue environment in which the tissue-resident macrophages fail to routinely perform M1-associated functions such as phagocytizing newly developing cancer cells. Failure of M1 phenotypic expression in both tissue-resident macrophages and monocyte-derived macrophages recruited to the tumor microenvironment yields both a loss of direct antitumor M1 macrophage actions and failure of T_H1 system activation in general, including failure of CD8⁺ T cell activation, yielding a cancer-permissive tumor microenvironment and a poorer prognosis in patients with existing cancers. This paper proposes a conceptual framework that connects established elements in the existing research and points to the utility of a patient profiling process, aimed at personalization of treatment through identification and targeting of elements in each patient's tumor microenvironment and macroenvironment that contribute to unfavorable prognosis.