Overexpression of Bovine FcRn in Mice Enhances T-Dependent Immune Responses by Amplifying T Helper Cell Frequency and Germinal Center Enlargement in the Spleen

Abstract

The neonatal Fc receptor (FcRn) plays key roles in IgG and albumin homeostasis, maternal IgG transport, and antigen presentation of IgG-opsonized antigens. Previously, we reported that transgenic (Tg) mice that overexpress the bovine FcRn (bFcRn) have augmented T-dependent humoral immune response with increased IgG protection, higher level of antigen-specific antibodies, greater number of antigen-specific B cells, and effective immune response even against weakly immunogenic epitopes. In the current study, we analyzed the localization of the bFcRn in secondary lymphoid organs, and focused to demonstrate the in vivo impact of its overexpression in the spleen on the course of antibody production. bFcRn was highly expressed by red pulp macrophages and marginal zone macrophages in the spleen and by subcapsular sinus macrophages and macrophage-like cells in the interfollicular areas in the lymph node cortex. We also demonstrated that splenic dendritic cells of Tg mice express bFcRn and intraperitoneal immunization of these mice with T-dependent antigens led to more than threefold increase in the number of antigen-specific activated T helper cells with increased size and numbers of germinal centers compared to wild-type controls. bFcRn expression in splenic B cells was also detected and that may also contribute to the enhanced B cell activation. Finally, we demonstrated that these Tg mice developed efficient immune response against very low dose of antigen, reflecting another important practical benefit of these Tg mice.

Accelerating antibody discovery using transgenic animals overexpressing the neonatal Fc receptor as a result of augmented humoral immunity

“…(We also used non‐immunized splenocytes as controls.) Supporting our earlier results, the analysis of the splenic cell population before and after immunization showed that the Tg spleen contained significantly more B and Ag‐specific CD4 + T‐helper cells and B cells, and twice to three times as many granulocytes, DCs, and plasma cells as their wt controls after immunization ( Fig. A ).…”

“…This immunohistochemical analysis showed that the bFcRn was highly expressed by red pulp and marginal zone MΦs in the spleen and by subcapsular sinus (SCS) MΦs and macrophage‐like cells in the interfollicular areas in the lymph node cortex. We also detected bFcRn expression in the splenic DCs and B cells using immunohistochemistry and flow cytometry, but we did not detect bFcRn expression in neutrophil granulocytes ( Fig. ).…”

“…On the other hand, we detected strong bFcRn expression in the outer and inner rings of the MZ, representing the MARCO + MZ MΦs and the CD169 + marginal metallophilic MΦs, in addition to the mFcRn‐positive components in the red pulp . The role of bFcRn in these cells remains to be analyzed as these cells may provide augmented stimulation for the humoral immune response.…”

“…Further analyses tracing the transport of detectable antigen in IC form may provide further understanding of this IC transport pathway. The role of MARCO + MZ MΦs in capturing ICs and stimulating B cells, including marginal zone B cells, is less characterized, but we cannot exclude the possibility that this process is also mediated by FcRn . Further studies are warranted to carefully analyze the expression of mFcRn and hFcRn in these cells and to validate this hypothesis.…”

“… Bovine FcRn (bFcRn) is expressed in antigen‐presenting cells in Tg spleen ( ). Splenocytes were stained with antibodies recognizing Gr‐1, CD 11b, CD 11c, F4/80, CD 19, and B220 markers.…”