Image_2_Plasma Cell Alloantigen 1 and IL-10 Secretion Define Two Distinct Peritoneal B1a B Cell Subsets With Opposite Functions, PC1high Cells Being Protective and PC1low Cells Harmful for the Growing Fetus.jpg


<p>B cells possess various immuno regulatory functions. However, research about their participation in tolerance induction toward the fetus is just emerging. Accumulating evidence supports the idea that B cells can play seemingly conflicting roles during pregnancy, either protecting or harming the fetus. Previous findings indicated the presence of two different peritoneal B cell subsets, defined by the expression of the plasma cell alloantigen 1 (PC1) and with distinct immune modulatory functions. Here, we aimed to study the participation of these two B cell subsets, on pregnancy outcome in a murine model of disturbed fetal tolerance. The frequencies and cell numbers of peritoneal and splenic CD19<sup>+</sup>IL-10<sup>+</sup> and CD19<sup>+</sup>CD5<sup>+</sup>IL-10<sup>+</sup>PC1<sup>+</sup> cells were assessed in virgin as well as normal pregnant (NP) and abortion-prone (AP) females during the course of gestation. Peritoneal PC1<sup>low</sup> or PC1<sup>high</sup> B1a B cells were sorted, analyzed for their ability to secrete IL-10 and adoptively transferred into NP or AP females. On gestation day (gd) 12, the abortion rate as well as the frequencies and cell numbers of regulatory T cells, TH1 and TH17 cells were determined in spleens and decidua. In addition, mRNA expression of IL-10, TGF-β, IFN-γ, and TNF-α was analyzed in decidual tissue. Peritoneal CD19<sup>+</sup>IL-10<sup>+</sup> and CD19<sup>+</sup>CD5<sup>+</sup>IL-10<sup>+</sup>PC1<sup>+</sup> frequencies fluctuated during the progression of normal pregnancies while no significant changes were observed in spleen. AP females showed significantly reduced frequencies of both B cell populations and exhibited an altered peritoneal PC1<sup>high</sup>/PC1<sup>low</sup> ratio at gd10. Adoptive transfers of PC1<sup>low</sup> B1a B cells into NP females increased the abortion rate in association with a reduced splenic regulatory T/TH17 ratio. By contrast, the transfer of PC1<sup>high</sup> B1a B cells into AP females significantly diminished the fetal rejection rate and significantly reduced the numbers of splenic TH17 cells. Our results suggest that the peritoneum harbors two distinct B1a B cell subsets that can be distinguished by their PC1 expression. Whereas PC1<sup>high</sup> B1a B cells seem to support fetal survival, PC1<sup>low</sup> cells B1a B cells may compromise fetal well-being.</p

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