4 Identifying the immunoglobulin G transporter in equine tissues: A look at the neonatal Fc receptor

Abstract

The importance of immunoglobulin G (IgG) is well established regarding its contribution to maintaining health status. However, the mechanism for controlling IgG movement through tissues is less known. The neonatal Fc receptor (FcRn) has been identified as the receptor responsible for bidirectional transport of IgG across cells, maintenance of IgG levels in serum, and assisting with antigen presentation. Unfortunately, little is known about the FcRn in livestock. Therefore, the objective of this study was to provide critical fundamental information regarding the location of the FcRn in equine tissues, focusing on those with key health importance. It was our hypothesis that FcRn abundance would be highest in respiratory and gastrointestinal (GI) tissues. Tissues were collected from 6 horses of mixed breed, age (3–22yr), and sex (male n = 4; female n = 2) immediately following euthanasia for various chronic health conditions at North Dakota State University (n = 2) or Texas A&M University (n = 4). Sampling locations and procedures were standardized across universities to minimize variation. Tissue locations included the respiratory tract, GI tract, other visceral organs, the cornea, and the synovial membrane of the stifle and carpal joints. Tissues collected were fixed in 10% NBF before embedding in paraffin and cross sectioning at 5μm. Tissue cross sections were mounted on glass slides, stained for the FcRn, and counterstained with DAPI. Areas of interest were captured using the tiling module of Zeiss ZEN software and the Plan-Apochromat 20x/0.8 NA lens. Images were analyzed using Image Pro software with data represented as relative fluorescence (RF) within areas of interest to represent FcRn abundance. Data were analyzed using the general linear model procedure of SAS.Relative fluorescence differed between tissue types (P < 0.01). The highest RF was seen in joint synovium and the respiratory tract. Moderate RF was seen in GI tissues and the lowest RF was seen in other visceral organs. These data lay the foundation for all future studies regarding the FcRn and IgG and their role in disease prevention and treatment.