Presence of the di-leucine motif in the cytoplasmic tail of the pig FcRn α chain

The intracellular localization and association of porcine Ia-associated invariant chain with the MHC class I-related porcine neonatal Fc receptor for IgG

The neonatal Fc receptor (FcRn) transports IgG from mother to young and is involved in antigen presentation. FcRn is structurally similar to MHC class I, but its intracellular trafficking pathway is much more analogous to that of MHC class II. Ia-associated invariant chain (Ii) molecules play an additional role in directing MHC class II trafficking within the endocytic compartments by physical association with MHC class II. This study addresses the question of whether pig Ii chain plays this important role in FcRn trafficking to the endoplasmic reticulum. Red or green fluorescent protein-fused Ii or FcRn was constructed, and the intracellular localization of pig Ii with FcRn was detected using confocal microscopy. Immunoprecipitation and western blotting were used to test for their association. The results indicate that pig Ii chain specifically interacts with both FcRn H chain alone and FcRn-β2m complex, and the CLIP in Ii was required for FcRn-Ii association. A truncated FcRn deletion in the cytoplasmic tail changed the intracellular localization of FcRn. However, the truncated FcRn can still combine Ii. This indicated that the cytoplasmic tail of FcRn fails to affect FcRn association with Ii. These results suggest that association of FcRn with Ii chain is relevant, and appreciation of this process is important to the understanding of how IgG is transported.

Minipig as a potential translatable model for monoclonal antibody pharmacokinetics after intravenous and subcutaneous administration

Subcutaneous (SC) delivery is a common route of administration for therapeutic monoclonal antibodies (mAbs) with pharmacokinetic (PK)/pharmacodynamic (PD) properties requiring long-term or frequent drug administration. An ideal in vivo preclinical model for predicting human PK following SC administration may be one in which the skin and overall physiological characteristics are similar to that of humans. In this study, the PK properties of a series of therapeutic mAbs following intravenous (IV) and SC administration in Göttingen minipigs were compared with data obtained previously from humans. The present studies demonstrated: (1) minipig is predictive of human linear clearance; (2) the SC bioavailabilities in minipigs are weakly correlated with those in human; (3) minipig mAb SC absorption rates are generally higher than those in human and (4) the SC bioavailability appears to correlate with systemic clearance in minipigs. Given the important role of the neonatal Fc-receptor (FcRn) in the PK of mAbs, the in vitro binding affinities of these IgGs against porcine, human and cynomolgus monkey FcRn were tested. The result showed comparable FcRn binding affinities across species. Further, mAbs with higher isoelectric point tended to have faster systemic clearance and lower SC bioavailability in both minipig and human. Taken together, these data lend increased support for the use of the minipig as an alternative predictive model for human IV and SC PK of mAbs.

Identification and characterization of an alternatively spliced variant of the MHC class I-related porcine neonatal Fc receptor for IgG

The neonatal Fc receptor for immunoglobulin G (IgG) (FcRn) functions to transport maternal IgG to the fetal/neonatal animals and protects IgG from catabolism. The present study identified two pFcRn cDNAs (1.071 and 0.795kb) from intestinal epithelial cells. The corresponding mRNA transcripts were detected in porcine kidney cell line LLC-PK1, peripheral blood mononuclear cells, and porcine tissues by reverse transcription-PCR (RT-PCR) and Northern blot. Sequence analysis showed that the 1.071kb cDNA encodes the full-length pFcRn (pFcRn-L), whereas the 0.795kb cDNA codes for a truncated pFcRn (pFcRn-S) with deletion of 92 amino acids matching the alpha2 domain of pFcRn-L. pFcRn-L was constitutively expressed by epithelial cells; however, pFcRn-S was not detectable in porcine tissues and cell lines although its transcript was abundant. Despite the lack of native pFcRn-S, pFcRn-S was readily detected in transfected cells. Recombinant pFcRn-L was confirmed to bind IgG at pH 6.0, but not at pH 7.5; however, pFcRn-S failed to bind IgG at both pH 5.0-6.0 and 7.5. pFcRn-L was expressed on the cell surface and mainly localized in early endosomes. In contrast, pFcRn-S was absent from cell surface and primarily localized in the lysosome and pFcRn-S trafficking to lysosomes was independent of beta(2)-microglobulin (beta(2)m). The accumulation of pFcRn-S in the lysosome may explain the absence of native pFcRn-S protein expression. In addition, the trafficking of pFcRn-S to the lysosomal compartment suggests that in addition to sorting signals in its cytoplasmic tail, the FcRn structural integrity may be important for proper intracellular trafficking and function.

Ca2+-dependent calmodulin binding to FcRn affects immunoglobulin G transport in the transcytotic pathway

The Fcgamma receptor FcRn transports immunoglobulin G (IgG) so as to avoid lysosomal degradation and to carry it bidirectionally across epithelial barriers to affect mucosal immunity. Here, we identify a calmodulin-binding site within the FcRn cytoplasmic tail that affects FcRn trafficking. Calmodulin binding to the FcRn tail is direct, calcium-dependent, reversible, and specific to residues comprising a putative short amphipathic alpha-helix immediately adjacent to the membrane. FcRn mutants with single residue substitutions in this motif, or FcRn mutants lacking the cytoplasmic tail completely, exhibit a shorter half-life and attenuated transcytosis. Chemical inhibitors of calmodulin phenocopy the mutant FcRn defect in transcytosis. These results suggest a novel mechanism for regulation of IgG transport by calmodulin-dependent sorting of FcRn and its cargo away from a degradative pathway and into a bidirectional transcytotic route.

The IgA system: a comparison of structure and function in different species

The predominant immunoglobulin isotype on most mucosal surfaces is secretory immunoglobulin A (SIgA), a polypeptide complex comprising two IgA monomers, the connecting J chain, and the secretory component. The molecular stability and strong anti-inflammatory properties make SIgA particularly well suited to provide protective immunity to the vulnerable mucosal surfaces by preventing invasion of inhaled and ingested pathogens. In contrast to SIgA, IgA in serum functions as an inflammatory antibody through interaction with FcalphaR on immune effector cells. Although IgA appears to share common features and protective functions in different species, significant variations exist within the IgA systems of different species. This review will give an overview of the basic concepts underlying mucosal IgA defence which will focus on the variations present among species in structure, antibody repertoire development, pIgR-mediated transport, colostral IgA content, hepatobiliary transport, and function with particular emphasis on the IgA system of the pig and dog. These interspecies variations emphasise the importance of elucidating and analysing the IgA system within the immune system of the species of interest rather than inferring roles from conclusions made in human and mouse studies.

Characterization of basolateral-targeting signals in the neonatal Fc receptor

The neonatal Fc receptor, FcRn, transports proteins through cells, avoiding degradative compartments. FcRn is used in many physiological processes where proteins must remain intact while they move through cells. These contexts include the transport of IgG antibodies from mother to offspring, and the protection of IgG and albumin from catabolism. In polarized cell models, FcRn in the plasma membrane is predominantly at the basolateral surface. This distribution depends on two signals that overlap endocytosis signals. One of these signals resembles a YXXPhi motif, but with a tryptophan in place of the critical tyrosine residue; the other is a DDXXXLL signal. We examined the effects of mutations in and around these signals on the basolateral targeting of rat FcRn in rat inner medullary collecting duct cells. We also studied a second acidic cluster, Glu331/Glu333, some distance from either endocytosis signal. Some amino acid substitutions in the W-2 and W+3 positions disrupted the tryptophan-based basolateral-targeting signal without impairing its function in endocytosis. The tryptophan-based basolateral targeting and endocytosis signals are thus distinct but overlapping, as has been seen for collinear tyrosine-based signals. Surprisingly, the tryptophan-based basolateral-targeting signal required the aspartate pair of the dileucine-based signal. This acidic cluster, separated by two amino acids from the Phi residue of the tryptophan signal, is therefore a component of both of the basolateral-targeting signals. The acidic cluster Glu-331/Glu333 was not required for basolateral targeting, but its replacement reduced endocytosis.

Characterization of the porcine neonatal Fc receptor--potential use for trans-epithelial protein delivery

The neonatal Fc receptor transports maternal immunoglobulin across the gut wall and has the potential to deliver genetically engineered proteins bearing immunoglobulin Fc domains across the gut to the mucosal immune system. Here we have characterized the porcine neonatal Fc receptor and tested its utility as a model system to study this kind of protein delivery. The complete DNA sequence obtained from an EST revealed 70-80% homology to mouse and human receptors, respectively, and tyrptophan and di-leucine endocytosis motifs were identified in the cytoplasmic tail. Reverse transcription-polymerase chain reaction analysis showed expression of the receptor mRNA in gut, liver, kidney and spleen tissue, aortic endothelial cells and monocytes. Pig kidney cell lines showed saturable pH-dependent binding and uptake of porcine immunoglobulin G (IgG) and also bovine, mouse and human IgG. Polyclonal antibodies raised against the receptor immunoprecipitated a protein of 40,000 MW when the cDNA was expressed in cells and the receptor required assembly with porcine beta2-microglobulin for transport from the endoplasmic reticulum to recycling and early endosomes. Immunohistochemical analysis showed the receptor expressed in epithelial cells of the gut of young and adult animals. The ability of the receptor to deliver immunoglobulin across the gut was demonstrated by feeding piglets bovine colostrum as a source of bovine IgG. Bovine IgG was delivered into the pig circulation. Pigs express the neonatal Fc receptor and the receptor has the potential to deliver protein antigens to the pig immune system.

Fc receptors in livestock species

Many of the receptors for the Fc domain of immunoglobulins in cattle, sheep, pigs and horses have been cloned and characterized recently. This review summarises recent developments and relates them to the current understanding of the primary structure, cellular specificity and binding properties of Fc receptors (FcRs). Although there is an obvious overall similarity to their human and mouse counterparts, some Fc receptors in domestic animals are unusual, perhaps most notably the bovine Fcgamma2R, which although related to other mammalian FcgammaRs, belongs to a novel gene family and the porcine FcgammaRIIIA, which associates with a molecule that contains significant homology to the cathelin family of antimicrobial proteins. Accumulating data suggest the possibility of a different role for the FcRn in ruminants, which may secrete IgG onto the mucosal surfaces, rather than absorbing it, as was suggested by mouse studies. These differences may be linked to the diversity of immunoglobulin classes in different mammalian species, and may contribute to different immune functions. The observations made so far emphasize the importance of elucidating and analyzing the roles of these molecules within the immune system of each host animal, rather than inferring roles from conclusions made in human and mouse studies. A better understanding of Fc receptor expression on immune effector cells should help in developing new immunization protocols, while knowledge of the Fc receptors involved in immunoglobulin transport, especially in the mammary gland, may help to develop new products which could be used not only for veterinary purposes but perhaps also for human therapy.