Bovine FcgammaRIII with a single extracellular domain

Identification of the Linear Fc-Binding Site on the Bovine IgG1 Fc Receptor (boFcγRIII) Using Synthetic Peptides

The bovine IgG1 Fc receptor (boFcγRIII) is a homologue to human FcγRIII (CD16) that binds bovine IgGI with medium-low affinity. In order to identify the Fc-binding site on the bovine IgG1 Fc receptor (boFcγRIII), peptides derived from the second extracellular domain (EC2) of boFcγRIII were synthesized and conjugated with the carrier protein. With a Dot-blot assay, the ability of the peptides to bind bovine IgG1 was determined, and the IgG1-binding peptide was also identified via truncation and mutation. The minimal peptide AQRVVN corresponding to the sequence 98-103 of boFcγRIII bound bovine IgG1 in Dot-blot, suggesting that it represents a linear ligand-binding site located in the putative A-B loop of the boFcγRIII EC2 domain. Mutation analysis of the peptide showed that the residues of Ala98, Gln99, Val101, Val102 and Asn103 within the Fc-binding site are critical for IgG1 binding on boFcγRIII. The functional peptide identified in this paper is of great value to the IgG-Fc interaction study and FcR-targeting drug development.

Cloning and characterization of ovine immunoglobulin G Fc receptor III (FcγRIII)

Receptors for the Fc regions of immunoglobin G (IgG) play a critical role in immunoregulation and immune defenses against pathogens. In this study, we describe the cloning, eukaryotic expression and IgG subclass specificity of ovine Fc gamma receptor III (FcγRIII). The newly cloned ovine FcγRIII cDNA contains a 940 bp open-reading frame (ORF), and is predicted to encode a 250 amino acid transmembrane glycoprotein composed of two immunoglobulin-like extracellular domains, a transmembrane region and a short cytoplasmic tail. The overall identity of the ovine FcγRIII amino acid sequence to its cattle, pig and human counterparts was 83.2%, 62.0%, 60.7%, respectively. Overlapping PCR was performed with the extracellular domain of ovine FcγRIII and the transmembrane and intracellular region of ovine Fc gamma chain to construct a chimeric receptor. Rosetting analysis showed that transfected COS-7 cells required Fc receptor gamma chain for the expression of FcγRIII on the surface. COS-7 cells expressing FcγRIII were able to bind chicken erythrocytes sensitized with ovine IgG1, but not IgG2. Identification of ovine FcγRIII will further our understanding of the ovine immune system.

Genetic characterization and ligand specificity of the ovine Fc gamma receptor I (ovFc gamma RI)

Receptors for the Fc region of IgG (Fc gamma Rs) play a critical role in the immune system and host protection against infection. In this study, we describe the cloning, mRNA expression and IgG subclass specificity of ovine Fc gamma receptor I (ovFc gamma RI). The ovFc gamma RI cDNA contains a 1047bp open-reading frame, and is predicted to encode a 349 amino acid trans-membrane glycoprotein composed of three immunoglobulin-like domains, a trans-membrane region and a short cytoplasmic tail. The overall identity of the ovine Fc gamma RI to its cattle, human and mouse counterparts at the level of the amino acid sequence was 92%, 61% and 54%, respectively. Rosetting analysis shows that COS-7 cells were transfected with an expressional vector carrying the cDNA open-reading frame of ovFc gamma RI and expressed this receptor on the surface. Identification of ovine Fc gamma RI will aid in the understanding of molecular basis of the ovine immune system and further studies of the receptor function.

Cloning and characterization of ovine immunoglobulin G Fc receptor II (FcgammaRII)

Immunoglobulin G (IgG) Fc receptors (FcgammaRs) bind to immune complexes through interactions with the Fc region of IgG to initiate or inhibit the defense mechanism of the leukocytes on which they are expressed. In this study, we describe the cloning, sequencing and characterization of ovine FcgammaRII. By screening a translated expression sequence tag (EST) database with the protein sequence of bovine IgG Fc receptor II, we identified a putative ovine homologue. Using rapid amplification of cDNA ends (RACE), we isolated the cDNA encoding ovine FcgammaRII from peripheral blood leucocyte RNA. The ovine FcgammaRII cDNA contains an 894bp open-reading frame, encoding a 297 amino acid transmembrane glycoprotein composed of two immunoglobulin-like extracellular domains, a transmembrane region and a cytoplasmic tail with an immunoreceptor tyrosine-based inhibitory motif (ITIM). The glycoprotein encoded by the cloned cDNA was then expressed on the surface of COS-7 cells and immunoglobulin-binding assays show that it binds ovine IgG1, but not IgG2. Identification of the ovine FcgammaRII will aid in the understanding of the molecular basis of IgG-FcgammaR interaction.

Identification of the linear epitope for Fc-binding on the bovine IgG2 Fc receptor (boFcgamma2R) using synthetic peptides

To identify the linear epitope for Fc-binding on the bovine IgG2 Fc receptor (boFcgamma2R), peptides derived from the membrane-distal extracellular domain (EC1) of boFcgamma2R corresponding to the homologous region of human FcalphaRI were synthesized. Binding of bovine IgG2 to the different peptides was tested by Dot-blot assay, and the peptide showing maximal binding was further modified by truncation and mutation. The minimum effective peptide 82FIGV85 located in the putative F-G loop of the EC1 domain was found to bind bovine IgG2 specifically and inhibit the binding of bovine IgG2 to the receptor. The Phe82, Ile83 and Val85 residues within the linear epitope were shown to be critical for IgG2-binding. Such functional epitope peptide should be very useful for understanding the IgG-Fcgamma interaction and development of FcR-targeting drugs.

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.

Glycosylation of FcgammaRIII in N163 as mechanism of regulating receptor affinity

Human FcgammaRIII (CD16) is a low-affinity receptor for immunoglobulin G (IgG). There are two different isoforms of this protein: CD16a (transmembranous, expressed on natural killer cells and on macrophages) and CD16b (glycosylphosphatidylinositol-linked, expressed on neutrophilic granulocytes in two allelic forms NA1 and NA2). Both forms of the protein have a variable glycosylation pattern. The NA1 allele of CD16B has four asparagine (N)-linked glycosylation sites. One of them (N163) is localized in the ligand-binding site of domain II. This site is shared by the NA2 allele and CD16A. To examine the functional role of the glycosylation we mutated the four glycosylation sites of the NA1 allele (N39, N75, N163, N170) into glutamine (Q). HEK293 cells were stably transfected with the single mutants and wild-type CD16 as control. We determined binding of human IgG to transfected cells using immunofluorescence studies with anti-human IgG antibody. Monomeric IgG bound to N163Q transfectants with higher affinity than to other transfectants, showing that glycosylation in N163 influences the affinity of CD16 to its ligand. In addition, preincubation of WT-CD16-transfected cells with Tunicamycin (an inhibitor of N-glycosylation) resulted in an increased binding of monomeric IgG whereas N163Q-CD16-transfected cells remained unaffected. Therefore, glycosylation in N163 is a mechanism of regulating affinity of FcgammaRIII to its ligand IgG.

Immunological characterization of a gammadelta T-cell stimulatory ligand on autologous monocytes

Bovine gammadelta T cells are stimulated to proliferate by autologous monocytes. This is referred to as the autologous mixed leucocyte reaction (AMLR). It has been shown previously that the stimulatory component is constitutively expressed on the monocyte plasma membrane and is a protein or has a protein moiety. Here we showed that gammadelta T-cell responses to the monocytes requires interaction with the T-cell receptor because Fab1 fragments of a monoclonal antibody (mAb) that reacts with the delta chain of the T-cell receptor blocked proliferation in the AMLR. Monocyte molecules involved in stimulation were also characterized further by biochemical and immunological methods. A mAb, named M5, was generated by immunizing mice with bovine monocytes and shown to block the ability of monocytes to stimulate in the AMLR. Treatment of monocytes or monocyte membranes with high salt, chelating agents or phospholipase C did not affect their ability to stimulate gammadelta T-cell proliferation or reactivity with mAb M5 indicating the ability of monocytes to stimulate does not involve peripheral membrane components or a glycosyl-phosphatidylinsositol (GPI)-anchored components. Hence it was concluded that the stimulation occurred as a result of intergral membrane proteins including that recognized by mAb M5. The ligand for mAb M5 was on all bovine monocytes and to a lower level on granulocytes but not on lymphocytes. MAb M5 also reacted with sheep monocytes but not with human monocytes or murine macrophages, in agreement with a previous reports that sheep monocytes but not human or mouse mononuclear phagocytes have the capacity to stimulate bovine gammadelta T cells in in vitro cultures. The level of expression of the M5 ligand was not altered by gamma-irradiation or culture of monocytes with lipopolysaccharide but it was decreased following culture with interferon-gamma-containing cell culture supernatants.