Immune therapeutic targeting of glioma cancer stem cells

Abstract

Glioblastoma multiforme (GBM) is a lethal cancer that responds poorly to radiotherapy and chemotherapy. Glioma cancer stem cells (gCSCs) have been shown to recapitulate the characteristic features of GBM and to mediate chemotherapy and radiation resistance. Immunotherapeutic targeting of this cell population holds therapeutic promise but must be considered in the context of the immunosuppressive properties mediated by the gCSC. Recent findings have indicated that this goal will be challenging because the gCSC can suppress both the innate and adaptive immune systems by a variety of gCSC-secreted products and cell-membrane interactions. In this review article, we will attempt to reconcile the disparate research findings regarding the potential of immune targeting of the gCSC and propose several novel solutions.

Fig. 1. The degree of gCSC mediated immune suppression associates with in vivo tumorigenicity but not with CD133 expression

The gCSCs (2 × 10⁶) were cultured in neural stem cell medium for 5 days, and then the supernatant medium was harvested for testing immunosuppressive properties. Additionally, the gCSCs (5 × 10⁴) were intracranially implanted in nude mice (6 per group). A. The percentage of inhibited T-cell proliferation does not correlate with gCSC CD133 expression levels. B. The number of induced Foxp3+ Tregs does not correlate with the gCSC CD133 expression levels. C. Increased T-cell apoptosis does not correlate with gCSC CD133 expression levels. D. The amount of gCSC-produced galectin-3 associates with CD133 expression. E. The percentage of inhibited T-cell proliferation inversely associates with the mean survival time of gCSC-implanted animals. F. The number of induced Foxp3+ Tregs inversely associates with the mean survival time of gCSC-implanted mice. G. Increased T-cell apoptosis inversely associates with the mean survival time of gCSC-implanted animals. H. The gCSC CD133 expression levels do not correlate with the mean survival of gCSC-implanted animals.

Fig. 2. Theoretical schema showing relationship between glioma cancer stem cells and the immune system

The gCSCs express p-STAT3, which in addition to inducing gCSC proliferation and self-renewal, is also capable of inducing p-STAT3 expression in a wide variety of immune cell populations via a feed forward mechanism. This up regulation of p-STAT-3 in the immune cells down modulates effector function such as T-cell activation, proliferation, and elaboration of effector cytokine production. The gCSC secretes a variety of factors: macrophage inhibitory cytokine, which inhibits macrophage phagocytosis; transforming growth factor (TGF-β1), which induces Tregs and polarizes the macrophage to an immunosuppressive M2 state; soluble colony stimulating factor (sCSF-1), which recruits circulating monocytes and also mediates differentiation and polarization of the monocyte into M2 tumor-associated macrophages; chemokine (C-C motif) ligand 2 (CCL-2), which enhances Treg tumor infiltration; and galectin-3, which triggers T-cell apoptosis. Additionally, the cell-surface expression of the costimulatory inhibitory molecule B7-H1, which mediates immune suppression by a variety of mechanisms, and the down modulation of the major histocompatibility complex (MHC) further enhance the immunosuppressive properties of the gCSCs.

Fig. 3. Decrease in immunosuppressive properties in gCSCs maintained in vitro over many passages

Comparison of early and late passage gCSCs (n = 4) showing decrease with later passages in (A) the production of TGF-β1 (p = 0.10), and sCSF-1 (p = 0.05), (B) the ability to induce macrophages (MΦ) to secrete the immunosuppressive cytokines IL-10 (p = 0.05) and IL-23 (n = 2, p = 0.05), and (C) the ability to inhibit T-cell proliferation (p = 0.008). Late passages were 15-20 passages after early passages. P-values calculated using student’s t-test.