Gr‐1 antibody induces STAT signaling, macrophage marker expression and abrogation of myeloid‐derived suppressor cell activity in BM cells

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Abstract
The Gr‐1 (RB6‐8C5) Ab binds with high affinity to mouse Ly‐6G molecules and to a lower extent to Ly‐6C and has been widely used for cell depletion in infected or tumor‐bearing mice. Here we found that Gr‐1 treatment of BM cells in vitro and in vivo showed no depleting effects. The epitope recognized by the Gr‐1 Ab overlapped with Ly‐6G (1A8 Ab) but not Ly‐6C (ER‐MP20 Ab). In vitro the Gr‐1 Ab transmitted signals via STAT‐1, STAT‐3 and STAT‐5 into BM cells, similar to GM‐CSF. In healthy mice injected with the Gr‐1 Ab, the Ab remained attached to the surface of myeloid cells for at least four days. Gr‐1 Ab induced myeloid cell expansion, upregulation of macrophage markers, but not the DC marker CD11c. Suppressor activity of two distinct Gr‐1high and Gr‐1low expressing BM‐myeloid‐derived suppressor cell subsets was transiently ablated by Gr‐1 Ab injection. Depleting effects of Gr‐1 Ab could only be observed on inflammatory Ly‐6CintLy‐6Ghigh neutrophils from the peritoneal cavity, which occurred via apoptosis and was associated with the absence of Mcl‐1 expression. Together, Gr‐1 Ab induces signals leading to myelopoiesis and affects myeloid‐derived suppressor cell activity, suggesting functional roles for Ly‐6C/G molecules in macrophage differentiation and neutrophil apoptosis.