(A) Representative flow cytometry data of the expression of arginase I and PDL-1 plus the percentage of arginase I and PD-L1 in MDSCs derived from bone marrow in the presence of IL-6 and GM-CSF (WT), IL-6, GM-CSF and ISO (WT + ISO) or IL-6, GM-CSF, and ISO and Prop (WT + ISO + Prop) for 6 days. (B) T cells cocultured with WT or WT + ISO MDSCs in various ratios (n = 3). (C) Nanostring nCounter microarray analysis of WT or β2-AR^–/– MDSCs sorted by flow cytometry from 4T1 tumors of WT or β2-AR^–/– mice 25 days after tumor injection (WT or β2-AR^–/– MDSCs were pooled from 5 mice per group). (D) WT and β2-AR^–/– MDSCs were sorted from bone marrow of 4T1 tumor–bearing mice, cultured with LPS for 24 hours, and cytokines levels were analyzed in culture media using multiplex (n = 3). (E) Tumor growth kinetics in WT mice orthotopically injected with 4T1 cells (black square) or coinjected with 4T1 cells and WT MDSCs (blue circle) or 4T1 cells and β2-AR^–/– MDSCs (red square). MDSCs were sorted from the BM of tumor-bearing mice using an MDSC isolation kit. (F) Tumor growth kinetics in WT or β2-AR^–/– mice receiving i.v. transfer (3 × 10⁶ on days 3 and 6 after 4T1 injection) of MDSCs sorted the BM of tumor-bearing WT or β2-AR^–/– mice. Two-way ANOVA was used to analyze statistical significance among tumor growth in different groups. These data are presented as mean ± SEM. Other data are presented as median ± minimum to maximum. One-way ANOVA was used to analyze statistical significance among 3 groups, and the Student’s t test was used to analyze statistical significance between 2 groups. In all panels, *P < 0.05, **P < 0.01, ***P < 0.001, and ****P < 0.0001. A P value less than 0.05 was considered significant.