In vivo depletion of CD4⁺FOXP3⁺ Treg cells by the PC61 anti‐CD25 monoclonal antibody is mediated by FcγRIII⁺ phagocytes

Abstract

Depletion of CD4 1 CD25 1 FoxP3 1 Treg using PC61 mAb (anti-murine CD25 rat IgG1) is widely used to characterize Treg function in vivo. However, the mechanism of Treg depletion remains largely unknown. Herein, we report the PC61 mAb's mechanism of action. In peripheral blood, a single injection of PC61 mAb eliminated $70% of CD4 1 FoxP3 1 cells with the remaining Treg expressing low or no CD25. Functional blockade of Fcc receptors with 2.4G2 mAb significantly inhibited PC61 mAb activity. Furthermore, Fcc receptor (FccR)III À/À mice were resistant to Treg depletion. FccRIII is expressed on immune cells including NK cells and macrophages that are the major effector cells for Ab-dependentcellular-cytotoxicity and Ab-dependent-cellular-phagocytosis, respectively. Depletion of NK cells had no effect, whereas depletion of phagocytes, including macrophages, by clodronate liposome significantly inhibited Treg depletion. Furthermore, in vitro, PC61 mAb can mediate Ab-dependent-cellular-phagocytosis of CD25 1 cells by WT or FccRIIB À/À , but not FccRIII À/À , macrophages. Altogether these data demonstrate the critical role of FccRIII 1 phagocytes in mediating Treg depletion by PC61 mAb. This finding may be useful in guiding the development of human Treg targeting therapy.Key words: CD25 . Fcc receptor . PC61 Ab . Phagocytes . Treg Supporting Information available online IntroductionNaturally occurring Treg play a critical role in the maintenance of peripheral tolerance. They can suppress not only CD4 1 and CD8 1 T cells, but also NK cells, macrophages and dendritic cells [1]. Treg are marked by expression of FoxP3 1 transcription factor (Foxp3), which confers the Treg's regulatory activity. Induction of Foxp3 expression in naïve T cells can convert them into Treg [2,3]. Before the discovery of Foxp3, Treg were identified by expression of CD25, an IL-2 receptor a subunit. Initially, CD25 was simply considered a Treg marker that was not associated with Treg function because of its expression on activated T cells without suppressive activity. More recently, however, it became clear that CD25, thus also IL-2, is essential for the generation, peripheral expansion and maintenance of Treg. CD25-deficient mice and mice lacking IL-2 or Foxp3 have a reduced Treg number and spontaneously develop a fatal lymphoproliferative autoimmune syndrome that can be rescued by adoptive transfer of Treg from WT mice [1]. Temporary neutralization of IL-2 using anti-IL-2 mAb also reduces Treg number, induces autoimmune gastritis in BALB/c mice and accelerates autoimmune diabetes in NOD mice [4]. Additionally, ectopic expression of IL-2 receptor b in the thymus increases Treg production [5]. A recent study using transgenic mice expressing GFP-linked Foxp3 shows that the peripheral expansion of CD25 À Treg is significantly reduced compared with that of CD25 Results and discussion Treg depletion by PC61 mAbWe first tested the extent of Treg depletion after CD25 mAb injection in C57BL/6 mice. In naïve mice (Fig. 1A left panel), around 70% of CD4 1 Fo...

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