FcRn Overexpression in Transgenic Mice Results in Augmented APC Activity and Robust Immune Response with Increased Diversity of Induced Antibodies

Abstract

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.

NFκB induces overexpression of bovine FcRn

“…[16][17][18][19] We previously reported the effects of FcRn overexpression accomplished by Tg modification in mice and rabbits. [20][21][22][23][24][25] These effects include quantitative as well as qualitative augmentation in the humoral immune response of these animals. More specifically, FcRn overexpression increases the number of Ag-specific B cells in the spleen generated in response to immunization, and increases antibody diversity.…”

“…Among the mechanisms that contribute to the boosted immune response, the augmented Ag-IgG IC presentation via professional APCs such as dendritic cells that express bFcRn seems to be especially important. 21,26 It is known that FcRn prevents degradation of circulating IgG (both Ag-specific and Ag-non-specific) and extends its half-life, but, because of the many more Ag-specific IgG producing B cells, the Ag-specific IgG fraction is disproportionally high relative to total IgG in these Tg animals. 20 Therefore, these FcRn Tg animals mount an enhanced immune response against epitopes that are otherwise weakly immunogenic or evade recognition by the immune system.…”

“…29 In this paper, we report on studies in which we investigated the NFκB responsiveness of the bFcRn that is expressed, among many other cells, in macrophages and dendritic cells of bFcRn Tg mice and significantly enhance their Ag-presenting capability. 21 Using several complementary strategies, we could show that bFcRn harbors three NFκB binding sites and that the bFcRn expression can be triggered by exposure to lipopolysaccharide (LPS), which causes toll-like receptor (TLR)-4-dependent, NFκB-mediated stimulation 30 in vitro in primary bovine aorta endothelial cells (BAECs) or peritoneal macrophages derived from Tg mice that carry and express bFcRn. We could also show bFcRn upregulation in the spleen of bFcRn Tg mice that were treated by intraperitoneally (i.p.)…”

“…To investigate whether an elevated bFcRn mRNA level that occurs after i.p. LPS treatment is followed by increased bFcRn expression at protein level, we generated an FcRn-specific mAb (Z15_6D5/8 mAb) that binds specifically to bFcRn and does not cross-react with the mouse FcRn, based on flow cytometry using peritoneal macrophages (Gr1 -, CD11b + ) from bFcRn Tg (that expresses bFcRn based on our previous observation 21 ) and wild type (wt) mice (Fig. 6A).…”

Simulation of Monoclonal Antibody Pharmacokinetics in HumansUsing a Minimal Physiologically Based Model

“…However, it is not ideal to have to impose a correlation between the baseline IgG and FcRn concentration without some experimental evidence to support it. Experiments in transgenic animals with overexpression of FcRn have shown that they have higher levels of circulating IgG and improved immune function (46), suggesting higher levels of FcRn can result in higher plasma IgG levels. However, information on FcRn abundance and the variability within the human population and the correlation of the FcRn abundance with the steadystate IgG level is needed to further refine the minimal PBPK model.…”