Mast cell histamine promotes the immunoregulatory activity of myeloid-derived suppressor cells

Abstract

It has been shown recently that MCs are required for differential regulation of the immune response by granulocytic versus monocytic MDSCs. Granulocytic MDSCs promoted parasite clearance, whereas monocytic MDSCs enhanced tumor progression; both activities were abrogated in MC-deficient mice. Herein, we demonstrate that the lack of MCs also influences MDSC trafficking. Preferential trafficking to the liver was not seen in MC-deficient mice. In addition, evidence that the MC mediator histamine was important in MDSC trafficking and activation is also shown. MDSCs express HR1-3. Blockade of these receptors by HR1 or HR2 antagonists reversed the histamine enhancement of MDSC survival and proliferation observed in cell culture. In addition, histamine differentially influenced Arg1 and iNOS gene expression in MDSCs and greatly enhanced IL-4 and IL-13 message, especially in granulocytic MDSCs. Evidence that histamine influenced activity seen in vitro translated to in vivo when HR1 and HR2 antagonists blocked the effect of MDSCs on parasite expulsion and tumor metastasis. All of these data support the MDSC-mediated promotion of Th2 immunity, leading to the suggestion that allergic-prone individuals would have elevated MDSC levels. This was directly demonstrated by looking at the relative MDSC levels in allergic versus control patients. Monocytic MDSCs trended higher, whereas granulocytic MDSCs were increased significantly in allergic patients. Taken together, our studies indicate that MCs and MC-released histamine are critical for MDSC-mediated immune regulation, and this interaction should be taken into consideration for therapeutic interventions that target MDSCs.

Keywords: Nippostrongylus brasiliensis; Th2; allergic disease; cetirizine; cimetidine; parasite.

Figure 1.. MDSCs preferentially migrate to the liver in a MC-dependent manner.

(A) WT mice given labeled MDSCs, analyzed 18 h later. MDSC in liver, peripheral blood (PB), spleen, and bone marrow (BM). Data compiled in B. (C) Wsh or WT was given labeled MDSCs, infected with Nb, and examined for MDSC staining on Day 7. Cell percentage of PKH26GL⁺ cells out of total Gr1⁺CD11b⁺cells (D) and number (E) from C. (F) Eggs/gm feces (EPG) determined on Day 7. MDSCs given Days −1, 2, and 5. (G) Four-hour MDSC migration in response to B16 melanoma, MCs, or media alone. *P < 0.05; ***P < 0.0005. Mean ± sd; n ≥ 5/group.

Figure 2.. Histamine increases MDSC survival and proliferation.

(A) Survival assessed by trypan blue exclusion over time course of MDSCs alone (circle) or with histamine (triangle). (B and C) Proliferation assays of MDSCs cultured alone, with histamine (H), or in combination with indicated agents, as measured by counts per minute (CPM). (D) MDSCs were cultured with histamine or media. *P < 0.05; **P < 0.005; ***P < 0.0005. Mean ± sd; n ≥ 10/group and at least two independent experiments.

Figure 3.. Histamine influences MDSC gene expression.

Ly6C⁺ or Ly6G⁺ MDSCs were cultured ± histamine and analyzed (48 h) for Arg1 (A) or iNOS (B) message. Fold change of histamine-treated/media alone (A and B). Fold change of IL-13 and IL-4 message from histamine-treated Ly6G⁺/Ly6C⁺ cells (C). After normalization to GAPDH, data were analyzed using the ΔΔCt method.***P < 0.0005. Mean ± sd; n ≥ 5 from two experiments.

Figure 4.. HR1 and HR2 antagonists block MDSC-mediated Nb clearance.

(A and B) Eggs/gm feces in Nb-infected ± MDSCs (M) on indicated days (D) and ± daily CT treatment (A) or every other day CIM treatment (B). (C) Day 14 analysis of total MDSCs. (D) Flow cytometric determination of liver monocytic or granulocytic populations. *P < 0.05; **P < 0.005; ***P < 0.0005. Mean ± sd; n ≥ 5 with two independent experiments.

Figure 5.. Allergic patients have increased circulating MDSCs.

Symptomatic allergic patients were compared with nonallergic controls. Cells were isolated from peripheral blood gating strategy, presented in Supplemental Fig. 5. Percent total MDSCs (B) and subpopulations, CD14⁺ (C) or CD15⁺ (D), were determined by flow cytometry. (A) Representative allergic patient versus control and (B–D) compiled.*P < 0.05. Mean ± sd. Allergic patients, n = 6; controls, n = 6.

Figure 6.. Model of MDSC/MC interaction.

MCs are required for MDSC activity. MCs release mediators, such as histamine, that induce MDSC activation, proliferation, and Th2 cytokine production. This enhanced cytokine production culminates in Th2-skewed immune responses that promote allergy and parasitic clearance and diminish anti-tumor responses.