Mechanisms overseeing myeloid-derived suppressor cell production in neoplastic disease

Abstract

Perturbations in myeloid cell differentiation are common in neoplasia, culminating in immature populations known as myeloid-derived suppressor cells (MDSCs). MDSCs favor tumor progression due to their ability to suppress host immunity or promote invasion and metastasis. They are thought to originate from the bone marrow as a result of exposure to stromal- or circulating tumor-derived factors (TDFs). Although great interest has been placed on understanding how MDSCs function, less is known regarding how MDSCs develop at a transcriptional level. Our work explores the premise that MDSCs arise because cancer cells, through the production of certain TDFs, inhibit the expression of interferon regulatory factor-8 (IRF8) that is ordinarily essential for controlling fundamental properties of myeloid cell differentiation. Our interest in IRF8 has been based on the following rationale. First, it is well-recognized that IRF8 is a 'master regulator' of normal myelopoiesis, critical not only for producing monocytes, dendritic cells (DCs), and neutrophils, but also for controlling the balance of all three major myeloid cell types. This became quite evident in IRF8^-/- mice, whereby the loss of IRF8 leads to a disproportionate accumulation of neutrophils at the expense of monocytes and DCs. Second, we showed that such myeloid populations from IRF8^-/- mice exhibit similar characteristics to MDSCs from tumor-bearing mice. Third, in a reciprocal fashion, we showed that enforced expression of IRF8 in the myeloid system significantly mitigates tumor-induced MDSC accumulation and improves immunotherapy efficacy. Altogether, these observations support the hypothesis that IRF8 is an integral negative regulator of MDSC biology.

Keywords: Cancer; IRF8; MDSCs; Myelopoiesis; Regulatory myeloid cells.

Fig. 1

Use of an IRF8-EGFP reporter mouse model to track changes in IRF8 levels during pathologic insults. a EGFP levels (right y-axis) of blood CD11b⁺Gr-1⁺ cells (left y-axis) in non-tumor-bearing (NTB; n = 7) mice or 4T1 tumor-bearing mice (>250 mm³; n = 4). *P < 0.05 as determined by an unpaired t test. b EGFP levels of splenic CD11b⁺Ly6C^loLy6G⁺ cells from PBS (grey-shaded area) or G-CSF (black line)-treated mice (10 µg/day for 5 days) of one of five representative mice per group

Fig. 2

Expansion of GMPs influenced by various pathologic insults. Percentage of GMPs (defined as Lineage (Lin)⁻Sca-1⁻ckit⁺CD16/32⁺CD150⁻) within the Lin⁻Sca-1⁻ckit⁺ fraction of the bone marrow of the following groups of mice: non-tumor-bearing (NTB) wild-type (n = 19), IRF8^−/− (n = 3), 4T1 (n = 14) or AT-3 (n = 8) tumor-bearing (>250 mm³), or NTB wild-type mice following treatment with recombinant G-CSF protein (10 µg/day for 5 days; n = 5). Data compiled from multiple experiments. *P < 0.01 as determined by an unpaired t test

Fig. 3

Model for the regulation of IRF8 in GMPs as an important molecular event underlying PMN-MDSC development. STAT3-activating TDFs (several of which are shown and are indicated by blue dots) circulate in the blood, then enter and permeate the bone marrow microenvironment to induce PMN-MDSCs by inhibiting IRF8 expression at the GMP stage. Exposure of GMPs to such TDFs not only expands the GMP pool, but skews myeloid differentiation toward a robust granulocytic response by ensuring that IRF8 levels remain low or downregulated (shown by the relative IRF8 gradient). This does not rule out the possibility that PMN-MDSCs may also emerge at earlier (e.g., HSC hematopoietic progenitor cells, CMP common myeloid progenitor cells) or later myeloid progenitor stages. In addition to IRF8, a number of other transcription factors (examples are shown) have been implicated in PMN-MDSC development/expansion. Although their precise relationship with IRF8 remains unresolved, it is known that C/EBPβ is a target of STAT3, while RORC1 may be a putative target of STAT3 signaling based on its role in emergency granulopoiesis (denoted by the dashed red line). Our model further posits that such molecular interactions converge at the GMP stage, although such interactions may occur at other progenitor stages as well. Once PMN-MDSCs accumulate in the periphery, they traffic to the tumor microenvironment, where they aid neoplastic growth through immune system-dependent (i.e., immune suppression) or independent mechanisms (i.e., angiogenesis). Pink cells broadly indicate other stromal cells of the tumor microenvironment