Cutting edge: mast cells critically augment myeloid-derived suppressor cell activity

Abstract

Myeloid-derived suppressor cells (MDSCs) are primarily recognized for their immunosuppressive properties in malignant disease. However, their interaction with other innate immune cells and their regulation of immune responses, such as in parasitic infection, necessitate further characterization. We used our previously published mouse model of MDSC accumulation to examine the immunoregulatory role of MDSCs in B16 melanoma metastasis and Nippostrongylus brasiliensis infection. In this study, we demonstrate that the activity of MDSCs is dependent on the immune stimuli and subset induced. Monocytic MDSCs predictably suppressed antitumor immune responses but granulocytic MDSCs surprisingly enhanced the clearance of N. brasiliensis infection. Intriguingly, both results were dependent on MDSC interaction with mast cells (MCs), as demonstrated by adoptive-transfer studies in MC-deficient (Kit(Wsh)(/)(Wsh)) mice. These findings were further supported by ex vivo cocultures of MCs and MDSCs, indicating a synergistic increase in cytokine production. Thus, MCs can enhance both immunosuppressive and immunosupportive functions of MDSCs.

Figure 1. MDSC depletion restores the anti-tumor response and prevents metastatic progression of B16 melanoma in A10Tg mice

(A) Number of lung metastases in WT and A10Tg animals challenged with B16 melanoma with (filled bar) or without (open bar) adoptive immunotherapy, indicated as (A), as described in Methods. Number of lung metastases of WT (B) or A10Tg (C) mice challenged with B16 and treated as in A with the addition of cyclophosphamide and gemcitabine, represented as (C) and (G), respectively. (D) Representative lungs of WT and A10Tg mice with AIT with and without CYP and GEM. (ND) denotes none detected and (NS), not significant. The data represents five mice per group. *p<0.05 in comparison to respective untreated controls.

Figure 2. A10Tg mice are resistant to Nb infection

(A) Eggs/gram feces were determined at indicated times in WT and A10Tg mice as well as A10Tgs treated with GEM or WT with adoptive transfer (AT) of MDSCs upon infection with 650 Nb L3 worms. (B) WT and A10Tg mice were examined on day seven for adult worm recovery as described in Methods. (C) Serum was collected at day seven and analyzed for the above cytokines using a Milliplex Mouse Cytokine Kit. *p<0.05 in comparison to infected WT controls. (D) Eggs/gram feces of WT mice with MDSC AT or WT controls, both depleted of T cells as described in methods. (E) Enumerated eggs/gram of feces of infected WT with AT of either CD11b⁺Ly6^highLy6C^low or CD11b⁺Ly6^lowLy6C^high population of MDSCs. (F) Enumerated eggs/gram feces of mast cell deficient Kit^Wsh/Wsh mice with and without MDSCs upon infection with 650 Nb L3 worms. The data represents five mice per group.

Figure 3. Mast cell/MDSC interaction augments cytokine production

BMMCs were co-cultured with A10Tg MDSCs as described in Methods. Supernatants were collected and analyzed for the production of (A) IL-6, (B) IL-13, (C) MIP-1μ, and (D) TNF-μ. The data represents pooled spleens of at least three mice per group. *p<0.05 in comparison to mast cells (MCs) alone.

Figure 4. MC/MDSC interaction is required for MDSC-mediated immune suppression

Photographic representation (A) and quantification (B) of B16 lung metastasis in control C57 and Kit^Wsh/Wsh mice with and without the AT of MDSCs. The data is representative of with at least five mice per group. *p<0.05 in comparison to WT alone or Kit^Wsh/Wsh with MDSCs.