Oncogenic transformation and cancer progression have been intimately linked with inflammatory conditions (reviewed in [1
]). Accordingly, we observed that 4T1 tumors developed faster in BALB/c mice when they over-expressed IL-1β, although both tumor lines exhibited similar growth kinetics in vitro (
M.E. and R.N.A., unpublished observations and [11
]). MDSC are known to accumulate in tumor bearing individuals, particularly under inflammatory conditions but they are also observed under various other pathological conditions, including infectious diseases [31
]. The fact that multiple pathological conditions result in similar biological outcomes might explain the heterogeneity of MDSC but at the same time represents a challenge when studying these cells [31
]. Understanding the pathways behind this heterogeneity under various conditions might allow unravelling the origin of their complexity. Here, we found that the enhanced accumulation of MDSC in mice bearing IL-1β-secreting 4T1 tumors was almost exclusively attributable to the expansion of a novel subset of MDSC. MDSC populations are mainly defined by their expression of Ly6C/G and CD11b, and this newly identified subset of PMN-MDSC was distinct from known MDSC subsets by its lack of Ly6C expression.
Our data provide strong evidence that IL-1β is involved in the regulation of the Ly6Cneg
MDSC subset and suggest that the predominance of Ly6Cneg
MDSC may enhance tumor progression in mice with 4T1/IL-1β tumors. Although the mode of action of this pleitotropic cytokine in this setting remains to be elucidated its ability to enhance the survival of PMN [32
] might in part explain the strong accumulation of MDSC in these mice. A regulatory role for Ly6Cneg
MDSC in mice with 4T1/IL-1β tumors is further supported by the delayed tumor growth after depletion of Gr-1+
cells. IL-10-dependent regulatory capacities of PMN in the settings of bacterial infections have recently been demonstrated [33
]. Interestingly, LPS and INFγ activated MDSC from 4T1/IL-1β-tumor bearing mice were shown to produce significantly more IL-10 than those from 4T1-tumor bearing mice [16
], yet, it remains to be shown whether this IL-10-dependent immune regulation occurs in the tumor environment [34
]. Moreover, a novel subpopulation of human MDSC has recently been described possessing strong T cell suppressive potential. This subset was induced from normal peripheral blood mononuclear cells using cytokine mixtures containing IL-1β [35
-MDSC and Ly6Clow
-MDSC might represent separate lineages of MDSC characterized by a different susceptibility to factors in the tumor/host environment and equipped with a differential capacity to interfere with adaptive and innate immune responses. Alternatively, variations in the level of expression by PMN-MDSC of Ly6C might mark distinct states of differentiation within one MDSC lineage. Conceivably, such a differentiation within the tumor-microenvironment would likely be susceptible to tumor-derived signals, including TDFs. In support of this, it has recently been shown that different tumor microenvironments harbor distinct subsets of tumor associated macrophages (TAMs) that could be classified according to the “M1” (antitumor) versus “M2” (protumor) macrophage activation paradigm [36
] and all of which could be derived from a common monocyte precursor population [36
]. A similar plasticity has been reported to exist within tumor-associated neutrophils (TANs) that could polarize under the influence of TGF-β present in the tumor-microeenvironment towards antitumorigenic “N1” (when blocking TGF-β) versus protumorigenic “N2” (presence of TGF-β) subsets [37
]. Whether or not Ly6Cneg
-MDSC can be classified according to this paradigm requires further experimental investigation.
NK cells are generally described as prototypic innate anti-tumor cells [27
] and an impaired NK cell compartment is associated with enhanced susceptibility to tumor development [39
]. Consequently, a coherent ‘survival’ strategy of tumors might involve impairing the activity of NK cells, which is indeed frequently observed in tumor bearing individuals [18
]. The block in the development of NK cells from 4T1/IL-1β-tumor bearing mice is similar to that observed in mice bearing EL4 tumors [44
] and reminiscent of NK cells from transgenic mice expressing the CD27-ligand CD70 ectopically on all B cells [45
]. The reduced level of CD27 expression by NK cells might thus be an indication of engagement of CD27 by its ligand CD70 suggesting that constitutive CD27-CD70 interactions might cause the observed block in NK cell development in 4T1/IL-1β-tumor bearing mice. As CD70 expression is restricted to activated T and B cells its expression might be induced upon exposure to IL-1β. However, NK cells in CD70-tg mice were not functionally impaired and expressed high levels of NKG2D, suggesting that the functional inhibition of NK cells in 4T1/IL-1β-tumor bearing mice is independent of the developmental defect.
Suppression of NK cell function in tumor bearing mice has been shown to involve MDSC-derived cytokines including TGF-β1 [18
]. Yet, gene expression profiles of Ly6Clow
MDSC and Ly6Cneg
MDSC showed no significant difference in TGF-β mRNA expression (M.E. and R.N.A., unpublished observations), indicating that in our mouse model the observed reduction of NKG2D expression by NK cells was independent of TGF-β. Interestingly, NK cells from mice that constitutively express the NKG2D-ligand Rae-1ε exhibited a reduced cell surface expression of NKG2D and were functionally impaired, while their development was not affected [46
]. Our finding that cell-to-cell contact was necessary for MDSC-induced down-modulation of NKG2D supports the concept that NKG2D-NKG2D ligand interactions contribute to functional inhibition of NK cells.
Nausch et al
. demonstrated that some Mono-MDSC but not PMN-MDSC from RMA-S tumor-bearing mice activated NK cells via expression of the NKG2D-ligand Rae-1 [47
]. Although we did not measure Rae-1 expression by Mono-MDSC this subset was by far outnumbered by the considerably expanding Ly6Cneg
MDSC so that a potential activating activity of Mono-MDSC was likely overwhelmed by the suppressive activity of Ly6Cneg
MDSC. Importantly, the RMA-S tumor model used by Nausch et al.
differed from ours in that the granulocytic (PMN) MDSC in their studies expressed intermediate levels of Ly6C [47
], suggesting that RMA-S tumor cells may not create a heightened inflammatory tumor environment.
In this work we identified a novel MDSC subpopulation characterized by its lack of Ly6C expression and its inhibition of NK cell function. Our findings extend the complexity of this immunosuppressive myeloid cell population and demonstrate how inflammation, via the production of IL-1β, regulates MDSC phenotype and function.