Localization of neonatal Fc receptor for IgG in aggregated lymphoid nodules area in abomasum of Bactrian camels (Camelus bactrianus) of different ages

Organized MALT is a critical part of the mucosal immune system [44]. Wang et al. reported that there were developed ALNA in abomasum of Bactrian camel [38–40], which belonged to the organized MALT. Our result showed that FcRn was expressed in non-FAE of ALNA at different levels in all groups, which was compatible with the expression of FcRn in human enterocytes (FcRn was expressed in adult human enterocyte) [15]. However, this was very different from the FcRn expression in the intestinal mucosal epithelium of rat [8], which was only expressed in newborns enterocyte and rapidly declined after weaning in mouse. Researches have demonstrated that FcRn was pH-dependent binding to IgG, with relatively strong binding at acidic pH (pH???6.5) and negligible binding at physiological pH (7.3–7.4) [43]. Some studies reported that the pH was 5.55 in the camel abomasum [45], which was just within the range of optimal pH value for FcRn binding to IgG. Thus, FcRn expression in the mucosal epithelial cells of ALNA in abomasum of Bactrian camels of different ages could provide a powerful evidence for FcRn participating in the transmembrane transport of IgG and associated antigens (especially the transportation of HCAb).

Compared with non-FAE, the FAE is a kind of specialized epithelium, on which the unique microfolds cell (M cell) could efficiently uptake and transport macromolecules and microorganisms in gut lumen to the underlying lymphoid tissue [46]. The MOD measuring results of FcRn expression in epithelial cell of this area found that the FcRn expression level in FAE and non-FAE had no difference in the fetus and young groups, respectively (P??0.05), while that in FAE was significantly lower than that in non-FAE in pubertal and middle-aged groups, respectively (P??0.05). The expression characteristics were similar to those of pIgR, transport receptor of SIgA, in this area [46]. In ALNA of the fetus to middle-aged Bactrian camels, although the FcRn was expressed in mucosal immune inductive sites FAE, in ALNA of the fetus to old Bactrian camels, mainly in effector sites non-FAE. As for whether FcRn participated in M cells uptaking and transporting associated antigens in FAE of ALNA in abomasum of Bactrian camels remains a further study.

In addition, our results showed that FcRn was expressed in the vessel endothelium and smooth muscle in this area, which was similar to the FcRn expression characteristics in the vessel and smooth of mice and humans [47]. It suggested that the FcRn expression in these sites was mainly related to regulating the half-life of IgG and albumin and homeostasis in this local region.

In the present study, both macrophages and DCs in sLFs of ALNA in abomasum of Bactrian camels highly expressed FcRn, respectively. Researches have shown that there was mononuclear phagocyte system composed of monocytes, DCs and macrophages in sLFs. And in this system, different types of cells had different subtypes, respectively, and were distributed in special regions [48–53]. They played an important role in antigen capture, processing and presentation, secreting cytokines and regulating immune tolerance [54, 55]. These results provided an evidence that FcRn could participate in regulating the immune response and tolerance in the sLFs of ALNA in abomasum of Bactrian camels.