Distribution of FcRn Across Species and Tissues

Abstract

The neonatal Fc receptor (FcRn) is a major histocompatibility complex class I type molecule that binds to, transports, and recycles immunoglobulin G (IgG) and albumin, thereby protecting them from lysosomal degradation. Therefore, besides the knowledge of FcRn affinity, FcRn protein expression is critical in understanding the pharmacokinetic behavior of Fc-containing biotherapeutics such as monoclonal antibodies. The goal of this investigation was to achieve for the first time a comparative assessment of FcRn distribution across a variety of tissues and species. FcRn was mapped in about 20 tissues including placenta from human and the most frequently used species in non-clinical safety testing of monoclonal antibodies (mouse, rat, cynomolgus monkey). In addition, the FcRn expression pattern was characterized in two humanized transgenic mouse lines (Tg32 and Tg276) expressing human FcRn under different promoters, and in the severe combined immunodeficient (SCID) mouse. Consecutive sections were stained with specific markers, namely, anti-CD68 for macrophages and anti-von Willebrand Factor for endothelial cells. Overall, the FcRn expression pattern was comparable across species and tissues with consistent expression of FcRn in endothelial cells and interstitial macrophages, Kupffer cells, alveolar macrophages, enterocytes, and choroid plexus epithelium. The human FcRn transgenic mouse Tg276 showed a different and much more widespread staining pattern of FcRn. In addition, immunodeficiency and lack of IgG in SCID mice had no negative effect on FcRn expression compared with wild-type mice.

Keywords: FcRn; SCID mouse; cynomolgus monkey; human; humanized transgenic mice Tg32 and Tg276; immunohistochemistry; mouse; neonatal Fc receptor; rat; species comparison.

Figure 1.

Immunohistochemical localization of FcRn in the placenta of different species. In human (A) and cynomolgus monkey (B), FcRn was detected in syncytiotrophoblasts (dashed arrow) and in endothelial cells of fetal vessels (arrow). In rat (C), FcRn was detected in epithelial cells of the yolk sac endoderm (asterisk) and in endothelial cells of fetal vessels in the LZ (arrow). In wild-type mice (D), FcRn was detected in epithelial cells of the yolk sac endoderm (asterisk). In humanized mouse line Tg32 (E), FcRn was detected in trophoblastic giant cells of the BZ (dashed arrow) and in endothelial cells of fetal vessels in the LZ (arrow). In humanized mouse line Tg276 (F), FcRn was detected in all cells of the BZ, and in endothelial cells of fetal vessels (arrow) and in syncytiotrophoblasts (dashed arrow) in the LZ. Scale bar = 50 µm. Abbreviations: FcRn, neonatal Fc receptor; LZ, labyrinth zone; BZ, basal zone.

Figure 2.

Immunohistochemical localization of neonatal Fc receptor (FcRn) across tissues of cynomolgus monkey. Tissues included in this study were liver (A), small intestine (jejunum; B), large intestine (colon; C), skeletal muscle (D), heart muscle (E), sciatic nerve (F), brain (G), eye (H), skin (I), lung (J), thyroid (K), spleen (L), lymph node (M), bone marrow (N), adrenal gland (O), and testis (P). Scale bar = 50 µm.

Figure 3.

Identification of neonatal Fc receptor (FcRn)–positive cell types. Consecutive sections from human small intestine were stained for FcRn (A), macrophage marker CD68 (B), and endothelial marker von Willebrand Factor (vWF; C). FcRn staining colocalized with a subset of macrophages (arrow) and with a subset of vWF-positive blood vessels (asterisk). Scale bar = 50 µm.

Figure 4.

Comparison of neonatal Fc receptor (FcRn) localization between human (A, E, I), humanized transgenic mouse lines Tg32 (B, F, J) and Tg276 (C, G, K), and wild-type mouse (D, H, L). Stained sections of brain (A–D), skin (E–H), and lung (I–L) are presented. In brain sections, endothelial staining of vessels is marked with an asterisk. Tg276 specific staining in each tissue is marked with arrows (subset of neuronal cells (C), keratinocytes (G, dashed arrow), cells of the sebaceous gland (G, arrow), alveolar epithelial cells (K, arrow), and less intense bronchiolar epithelial cells (K dashed arrow)). Additional pictures of staining in choroid plexus are marked with an “i” in the bottom right corner of the brain sections. Scale bar = 50 µm.

Figure 5.

FcRn expression in liver samples from human (A), cynomolgus monkey (B), humanized transgenic mouse lines Tg32 (C) and Tg276 (D), rat (E), wild-type mouse (F), severe combined immunodeficient mouse (G), and neonatal Fc receptor (FcRn) knock-out mouse (H). All of the species exhibited strong staining of FcRn (excluding the knock-out mouse) in Kupffer cells (dashed arrow), and weak (A–C, E, and F) to strong (D and G) staining in sinusoidal endothelial cells (arrow). FcRn expression was also detected in the bile ducts (asterisk) of both humanized transgenic mouse lines and rat. The FcRn knock-out mouse did not show any staining (internal negative control). Scale bar = 50 µm.