All Myeloid-Derived Suppressor Cells Are Not Created Equal: How Gender Inequality Influences These Cells and Affects Cancer Therapy

Glioblastoma (GBM) is the most common primary malignant brain tumor, with very poor survival. It is characterized by immune-suppressive tumor microenvironment (TME). Myeloid-derived suppressor cells (MDSC) are one of the major contributors to immune-suppressive TME in GBM (1). These are pathologically activated neutrophils and monocytes with potent immune-suppressive activity (2). On the basis of their origin, two large populations of MDSCs are currently recognized: pathologically activated neutrophils referred as PMN-MDSC or, as in this report, granulocytic (gMDSC) and pathologically activated monocytes [monocytic (mMDSC)]. Although MDSCs and their classic counterpart neutrophils and monocytes share many phenotypic and morphologic characteristics, they have distinct transcriptomic and proteomic profiles, metabolism, and biochemical features, as well as functions. MDSC accumulation is strongly associated with negative clinical outcome in cancer and failure of cancer immunotherapy. Because of that, therapeutic targeting of MDSCs is actively pursued. In recent years, it has become evident that therapeutic approaches to targeting gMDSCs and mMDSCs should be different due to the differences in their biology. However, the specifics of therapeutic approaches need to be elucidated.

Epidemiologic evidence supports male-dominant sexual dimorphism in GBM. Male patients have a worse prognosis than females, underscoring the clinical relevance of biological sex in GBM (3). In general, sex differences in host immunity are well described. Females have a more active immune response, which is mediated by increased type I IFN signaling, proinflammatory cytokine production, and T-cell activation (4). In the context of tumor immunity, this could translate to more robust immunosurveillance. However, studies in patients treated with immune checkpoint inhibitors suggested that males might benefit more from this treatment compared with females. Thus, the hypothesis of a possible role of immune systems in sexual dimorphism in GBM seems appealing.

In this issue of Cancer Discovery, Bayik and colleagues (5) tried to determine sex-dependent immunologic changes in two orthotopic models of GBM: GL261 and SB28. They found that mMDSCs accumulated in male tumors, leading to an approximately 5.5- to 6.5-fold increase in the mMDSC/gMDSC ratio as compared with brain tissues in control animals. In contrast, no significant change was observed in females. Instead, in female mice, there was a 2-fold increase in the peripheral gMDSC frequency, while mMDSCs remained unchanged. Surprisingly, the presence of other myeloid cells (macrophages, dendritic cells) as well as natural killer cells and T cells did not differ between male and female tumor-bearing mice. The observed sexual dimorphism in MDSC was associated with survival differences. Female mice experienced longer survival compared with male mice. Female hosts reconstituted with male donor bone marrow had decreased survival compared to female-to-female transplant controls, demonstrating that the male immune system may have a more tumor-promoting role.

Bayik and colleagues then sought to determine the functional contribution of MDSC subsets to tumor progression by depleting MDSC using antibodies. Bulk MDSC depletion with anti-Gr1 antibody resulted in a survival benefit, but it was limited only to female mice. Depletion of gMDSC with anti-Ly6G antibody provided survival benefit only to female mice. Targeting mMDSC with Ly6C antibody did not affect mMDSC presence and survival of either male or female mice. The authors hypothesized that the lack of systemic mMDSC reduction was due to mMDSCs' proliferation and rapid replacement. Proliferative activity of mMDSCs was described previously (6). In this study, mMDSCs highly expressed the Ki-67 proliferation marker regardless of sex of the host.

To gain mechanistic insight into the differential roles of MDSC subsets, the authors generated mMDSCs and gMDSCs from the bone marrow. Both male and female MDSCs were functionally suppressive. To identify putative drug targets for these subsets, the authors used a network medicine approach that takes advantage of reported drug–target interactions using the differentially expressed gene profiles of mMDSCs and gMDSCs. This strategy identified fludarabine, a purine analogue, as a potential drug candidate to target mMDSCs. For gMDSC targeting, IL1β pathway inhibitors were enriched among the top targets. To test the therapeutic utility of these predicted drugs, they assessed their efficacy in preclinical models of GL261 and SB28 and observed that fludarabine significantly extended survival in male mice, with no significant benefit for female mice. In contrast, anti-IL1β antibody treatment significantly prolonged the survival of female mice. In SB28-bearing male mice, one cycle of fludarabine was sufficient to decrease mMDSC presence in tumors by 4-fold. Tumor proliferation rate was reduced, suggesting that inhibition of mMDSC infiltration into the tumors at an early stage can alter the growth dynamics. Interestingly, in female GL261-bearing animals the frequency of the proliferating tumor cells or tumor mMDSCs was unaffected by the drug. Treatment with anti-IL1β antibody reduced systemic gMDSCs specifically in GL261-bearing females. Anti-IL1β antibody also delayed tumor growth in female mice without direct impact on the proliferation of GL261 and SB28 cells in vitro (Fig. 1).

To validate preclinical observations in patients, Bayik and colleagues analyzed fresh tumor tissues with flow cytometry from a cohort of male patients with isocitrate dehydrogenase (IDH)–wild-type GBM. They found that mMDSCs were more abundant than gMDSCs. Moreover, these tumor-infiltrating mMDSCs were positive for Ki-67, confirming that they were proliferating cells. To evaluate the prognostic value of gMDSCs, they analyzed The Cancer Genome Atlas (TCGA) GBM database for mRNA levels of OLR1 (LOX1), a gMDSC-specific marker (7), and IL1B and found that there were no differences in the expression of these genes between male and female patients. Further investigation of IL1β protein levels by IHC of tumor tissue from patients with IDH–wild-type GBM established that this cytokine was abundant in the tumor microenvironment regardless of patient sex. Notably, the expression levels of OLR1 and IL1B were associated with patient survival only when stratified on the basis of sex. In both TCGA and the Chinese Glioma Genome Atlas, high OLR1 expression inversely correlated with the survival of females but had no effect in male patients with GBM. A similar trend was observed with IL1B levels. OLR1 expression positively correlated with IL1B expression, suggesting that these genes are part of the same signaling network.

Collectively, these studies indicated that in GBM male hosts have enhanced accumulation of mMDSCs, whereas a gMDSC–IL1β axis associates with females and represents a therapeutic target for female patients. This interesting report describes, for the first time, sexual dimorphism of MDSCs and suggested specific targets for different populations of these cells. However, as with any novel study, it raises number of questions. First, is observed sexual dimorphism associated with other types of cancer or specific only to GBM? If the latter, what can then explain such tumor specificity?

Investigators observed sexual dimorphism only in MDSCs, not in macrophages and dendritic cells. Considering that monocytes/mMDSCs are the main source of tumor-associated macrophages (TAM), it is peculiar that similar dimorphism was not observed in accumulation of those cells. The reason for mMDSC accumulation in tumors of male mice is not clear. Because the authors did not observe systemic expansion of these cells, mMDSC accumulation would be the result of increased migration or possibly blocked differentiation to macrophages. In the latter case, it could explain lack of sexual dimorphism in TAMs. The other possible explanation is loss of gMDSC in tumor tissues. Granulocytic cells are more sensitive than monocytic cells to tumor microenvironment. It is possible that if mMDSCs and gMDSCs are equally recruited to tumors, mMDSCs survive better and thus would be a predominant population. Indirectly, it can explain the lack of shift in mMDSCs/gMDSCs in tumors of female mice. Because these mice have much higher expansion of gMDSCs in the periphery, more gMDSCs would come to tumors and despite their loss the shift of balance toward mMDSCs would not be detectable.

Authors presented strong correlation between expression of OLR1, gene encoding gMDSC-specific LOX1, and clinical outcome in female patients. Although the data are suggestive, there is a caveat that requires further clarification. OLR1 is expressed on different cells, including macrophages and endothelial cells, present in TME. Therefore, in whole tumor tissues this may affect interpretation of the results.

There is now clear evidence that because of their biological differences gMDSCs and mMDSCs are sensitive to different therapeutic targeting. Examples include targeting of their migration (8), epigenetics (9), or metabolism (10). Moreover, it has become clear that successful therapy would require a combination of targeting of mMDSCs and gMDSCs (11). This study provides evidence that different approaches may be beneficial for different MDSC populations. Previous studies demonstrated the effect of chemotherapeutics on MDSCs, including nucleoside analogues, 5-fluorouracil, gemcitabine, and others. In this study, the authors demonstrate the potent effect of fludarabine, but only on mMDSCs, whereas IL1β neutralization worked against gMDSCs. Because these populations differentially accumulated in male and female mice, it opens an opportunity to select therapy based on patients' gender. However, such a potentially impactful conclusion would require strong confirmation in future studies.

D.I. Gabrilovich is a consultant at T-Rx Pharmaceutical, Quentis, Elstar, Third Rock Ventures, EMD Serono, Shattuck, Merck, Verseau Theraputic, Takeda, Riley FBR, and Compass Theraputic, and reports receiving commercial research grants from Merck, M1NA Therapeutics, Coures Therapeutics, Syndax, Syntrix, and GI Innovation. No other potential conflicts of interest were disclosed.