Identification and characterization of B220⁺/B220^- subpopulations in murine Gr1⁺CD11b⁺ cells during tumorigenesis

Although all murine MDSCs are defined as Gr1⁺CD11b⁺, their true immunophenotype remains elusive. In this study, we found murine Gr1⁺CD11b⁺ cells can be divided into two subsets: Gr1⁺CD11b⁺B220^- and Gr1⁺CD11b⁺B220⁺, especially in the spleen tissues. Unlike the dominant B220^- subset, the B220⁺ subpopulation was not induced by tumor in vivo. Moreover, Gr1⁺CD11b⁺B220⁺ cells from tumor-bearing mice spleens were unable to induce arginase 1 and inducible nitric oxide synthase expression, inhibit T cell proliferation, or promote tumor growth in primary tumor site. Nevertheless, these cells suppressed tumor metastasis in vivo and reduced cancer cell motility in vitro, while Gr1⁺CD11b⁺B220^- cells from tumor-bearing mice spleens promoted tumor metastasis and enhanced cancer cell motility. Furthermore, both the polymorphonuclear (PMN-MDSCs) and monocytic MDSCs (Mo-MDSCs) could be further divided into B220^- and B220⁺ subsets; interestingly, tumor only induced the expansion of B220^- PMN-MDSCs and B220^- Mo-MDSCs, but not the B220⁺ counterparts. Compared with B220^- PMN-MDSCs and B220^- Mo-MDSCs, the Ly6G⁺Ly6C^-CD11b⁺B220⁺ and Ly6G^-Ly6C⁺CD11b⁺B220⁺ cells from tumor-bearing mice spleens exhibited a more mature phenotype without immunosuppressive activity. Additionally, IL-6 deficiency attenuated the tumor-induced accumulation of MDSCs, B220^- MDSCs and B220^- PMN-MDSCs but increased the percentages of Gr1⁺CD11b⁺B220⁺, Ly6G⁺Ly6C^-CD11b⁺B220⁺, and Ly6G^-Ly6C⁺CD11b⁺B220⁺ cells, indicating the opposing roles of the IL-6 signaling pathway in the expansion of B220^- MDSCs and their B220⁺ counterparts. Taken together, our findings indicate that the B220⁺ subset is a distinct subset of Gr1⁺CD11b⁺ cells functionally different from the B220^- subpopulation during tumorigenesis and induction of MDSCs to B220⁺ cells may be helpful for cancer therapy.