No abstract

In ruminants, protective immunoglobulins are transferred to the newborn via colostrum to mediate maternal immunity. There is a high selectivity in the transport of immunoglobulins from the maternal plasma across the mammary barrier into the colostrum, and only IgG1 is transferred in large amounts. We have recently analysed the expression of the neonatal Fc receptor (FcRn) in sheep mammary gland around parturition. Re-analysing this issue in bovine confirmed our previous data indicating that FcRn is homogeneously localized in the mammary gland acinar cells before parturition, however a remarkable difference was observed in the pattern after calving, where only the apical side of the cells was strongly stained. The presence of the FcRn in the acinar epithelial cells of the mammary gland and the obvious change in distribution before and after parturition indicate that FcRn plays an important role in the IgG transport during colostrum formation in ruminants.

Our previous studies have shown that overexpression of bovine FcRn (bFcRn) in transgenic (Tg) mice leads to an increase in the humoral immune response, characterized by larger numbers of Ag-specific B cells and other immune cells in secondary lymphoid organs and higher levels of circulating Ag-specific antibodies (Abs). To gain additional insights into the mechanisms underlying this increase in humoral immune response, we further characterized the bFcRn Tg mice. Our Western blot analysis showed strong expression of the bFcRn transgene in peritoneal macrophages and bone marrow derived dendritic cells; and a quantitative PCR analysis demonstrated that the expression ratios of the bFcRn to mFcRn were 2.6- and 10-fold in these cells, respectively. We also found that overexpression of bFcRn enhances the phagocytosis of Ag-IgG immune complexes (ICs) by both macrophages and dendritic cells and significantly improves Ag presentation by dendritic cells. Finally, we determined that immunized bFcRn mice produce a much greater diversity of Ag-specific IgM, whereas only the levels, but not the diversity, of IgG is increased by overexpression of bFcRn. We suggest that the increase in diversity of IgG in Tg mice is prevented by a selective bias towards immunodominant epitopes of ovalbumin, which was used in this study as a model antigen. These results are also in line with our previous reports describing a substantial increase in the levels of Ag-specific IgG in FcRn Tg mice immunized with Ags that are weakly immunogenic and, therefore, not affected by immunodominance.

We generated and characterized transgenic mice carrying a 102 kb bovine genomic fragment, encoding the neonatal Fc receptor alpha-chain (bFcRn). FcRn plays a crucial role in the maternal IgG transport and it also regulates the IgG and albumin homeostasis. Some of its functions and transcriptional regulation show species specific differences. The FcRn heterodimer is composed of the alpha-chain and beta-2-microglobulin (beta2 m). A bacterial artificial chromosome containing the bovine FcRn alpha-chain gene (bFCGRT) with its 44 kb 5' and 50 kb long 3' flanking sequences was microinjected into fertilized mouse oocytes. Two of the transgenic lines generated, showed copy number related and integration site independent bFcRn expression. The bFcRn alpha-chain forms a functional receptor with the mouse beta2-microglobulin and extends the half-life of the mouse IgG in transgenic mice. Our results underline the feasibility of creating BAC transgenic mouse models of economically important bovine genes.