companies:  Secretory IgA 

IgA synthesis exceeds the combined total of all the other immunoglobulin classes. 

Where IgA is found, Generally

IgA may be monomeric (i.e., a single molecule), dimeric (composed of two molecules) or trimeric (composed of three molecules). IgA monomers are joined together as dimers at the constant regions of their heavy chains by a J chain. IgA is secreted as one of two subclasses, IgA1 and IgA2. IgA1 predominates in the cirulcating blood wherein most of it occurs as a monomer. Most IgA on mucosal surfaces, such as the surfaces of the trachea, bronchi, and broncioles in the lungs, occurs as dimers or timers joined by J chains. IgA dimers and trimers have an increased ability to bind to and agglutinate target molecules which are more readily phagocytosed. IgA dimers and timers, because of the presence of their J chains, have the ability to attach to secretory component (see below). WO 02/076502. 

IgA Isotypes: 

There are two isotypes of IgA, IgA1 and IgA2.  The main difference between IgA1 and IgA2 subclasses is a 13 amino acid deletion in the IgA2 hinge region. This segment in IgA1 contains several serine and threonine amino acid residues that are O-glycosylated.

On mucosal surfaces (e.g., gut, respiratory tract, genital tract) both IgA1 and IgA2 are present, synthesized by local B cells. In the blood, IgA1 predominates, produced by B cells in the bone marrow, lymph nodes and spleen. IgA is the predominant class in external secretions such as breast milk, saliva, tears and mucus of the bronchial, enitourinary and digestive tracts.

IgA1: is heavily glycosylated and consists of 8% carbohydrate. The five O-linked glycosylation sites at the hinge region between the CH1 and CH2 domains of the alpha chain on IgA1 are distinctive and unusual features that are not found in most of the serum proteins, including IgA2. The O-glycosylation oat the hinge region of IgA1 has been reproted as significant with respect to hepatic clearance of IgA1 by asiloglycoprotein receptor (ASGPR). (Leung, Kindey International, 59, 2001, pp. 277-285) 

gA2: Three allotypes of IgA2(1) have been described; IgA2m and IgA2m(2).

IgA2 is only present in small amounts in human serum and makes up about 16% of the total IgA (Leibl, J. Chromatography B, 678 (1996) 173-180)

Secretory Component (SC):

In nature: 

Human secretory component (SC) is assocaited with secretory immunoglobulins (IgA and IgM) and serves to protect the immunoglobulin in the harsh mucosal environment. SC is derived form teh polymeric immunoglobulin receptor (pIgR) which transports polymeric immunoglobulins across epithelial cells into secretions. (Prinsloo, Protein Expr Purif 2006, 47(1) 179-85, 2005).

The term “secretory component” refers to a protein that specifically binds to J-chain-containing immunoglobulin, and is related to or derivable form identical to an extracellular portion of the polymeric immunoglobuliln receptor (pIgR). SC is the extracellular portion of the polymeric immunoglobulin receptor (pIgR), which usually gets assocaited during secretion with dimeric or polymeric IgA or pentameric IgM comprising a J chain. J chain containing IgA/IgM binds to the polymeric immunoglobulin receptor at the basolateral surface of epithelia cells and is taken up into the cell by transcytosis. This receptor complex then transits through the cellular compartments before being transported to the luminal surface of the epithelial cells. The transcytosed IgA/IgM-pIgR complex is then released through proteolysis, and the part of the polymeric immunoglobulin receptor (pIgR), referred to as the natural secretory component, stays associated with the J chain containing IgA/IgM, releasing secretoy IgA/IgM. (Corthesy, WO 2013/132052). 

Secretory component (Sc) exists in three molecuelar forms: as a membrane protein expressed on the outer surface of epithelial cells, it acts as a receptor for polymeric immunoglobulins. SC forms part of secretory IgA and IgM molecuesl in mucosal fluids where it is also found as a free glycoprotein. SC interacts with a specific binding site on the Fc region of SIgA and IgM, stabilises their quaternary structures and increases their resistance to proteolytic digestion (DOE, “The intestinal immune system” Gut, 1989, 30, 1679-1685).

The first line of defense against pathogens consists of mucosal secretions, with secretory IgA being one of the main effectors. Secretoy IgA is transported by the polymeric immunoglobulin receptor (pIgR) from usbmucosal sites into the lumen. The pIgR is expressed by lepithelial cells in a variety of mucosal tissues and in rodents is further implicated in transport into bile through the liver. The receptor can bind dimeric IgA, polymeric IgA and IgM but not monomeric IgA or IgG. The complex is internalized at the basolateral surface of the epithelium and transcytosed to the apical plasma membrane, where the extracellulur portion of the receptor called secretory component (Sc) is released by proteolytic cleavage and remains bound to IgA within a complex termed secretory IgA (sIgA). (J. Biological Chemistry, 274(44), 1999). 

Polymeric IgA binds to the polymeric immunoglobuilin receptor on the basolateral surface of epithelial cells and is taken up by the cells by transcytosis. The receptor-IgA complex then passes through the cell before being secreted on the luminal side, still attached to the receptor. Proteolysis of the receptor occurs and the dimeric IgA along with its secretory components which is wrapped around the the IgA are free to difuse throughout the lumen. Proteolytic components protect IgA from digestion by gastric acids and enzymes of the digestive system. 

recobminant production:

Human secretory component can be produced by recombinant techinques. 

Secretory IgA

Where sIgA is found in vivo

Mucosal surfaces comprising the GI, respiratory and urogenital mucosae represent a large port of entry for most of the pathogens and thus have to be efficiently protected. The goal is achieved by a combination of constitutive, non specific substances (e.g., mucus, lyzozyme, lactoferrin and defensins and specific immune mechanisms mediated by cellular and antibody resposnes. In vivo, the chief antibody at mucosal surfaes is secretory IgA (SIgA), a complex structure of IgA produced by activated B cells in the mucosal epithelium and of the secretory component (SC), a polypeptide of 85 kDa corresponding to the extracellular part of the poly immunoglobulin receptor (pIgR) expressed by epithelial cells (RFavre, J. Chromatography B, 786, 2003, 143-151).

Structure of sIgA and How Produced:

Unlike IgA in the serum and crebrospinal fluid which largely comprisees 7S monomers, secretory IgA is an 11S dimer made up of two IgA monomers joined by a covalently linked peptide named J chain. Both IgA and IgM plasma cells synthesise J chain which greatly enhances binding of an additional polypeptide, secretory component (SC), thereby completing the asembly of secretory IgA and IgM molecuels. One molecuel of J chain joins two IgA monoemrs whereas three or more J chain molecuels are involved in forming the IgM pentamers within the plasma cell before secretion. (DOE, “The intestinal immune system” Gut, 1989, 30, 1679-1685). 

In vivo, sIgA is produced by two different cell types, the plasma cell and the epithelial cell. Plasma cells synthesize and assemble alpha H and L chains with J chains into polymeric IgA. The pIgA secreted by the plasma cell binds to a polymeric Ig receptor (pIgR) expressed on the basolateral surface of the mucosal epithelium. The IgA-pIgR complex is transcytosed to the apical surface. During transit, a disulfide bond is formed betweent he IgA and the pIgR. At the apical surface, the IgA molecuel is released by proteolytic cleavage of the receptor. This cleavage results in a fragment, aobut 70k MW, bieng retained on the IgA moelcule. This fragment is the SC fragment, which is attached by disulfide bonds to the IgA moelcule. The IgA-SC complex is thereby rleased into external secretions. (Morrison (US 6,300,104).

Production of sIgA in vitro

Kelly (US 2015/0017181) discloses that plasma dervied immunoglobulin mixed with secretory component leads to the formation fo secretory-like IgA and/or secretory like IgM, i.e., J chain containing IgA dimers, tetramers or other polymer forms or J chain containing IgM pentamers complexed with SC even without purificaiton from plasma prior to mixing witht he SC. However, it is preferred to enrich the composition for such J chain containingIgA and/or L chain-containing IgM prior to mixing with the SC. 

Morrison (US 6,300,104; see also WO98/57993) discloses a method of producing secretory Ig molecules, such as secretory IgA, for treating infection, by transfecting a cell producing an IgG with a polynucleotide encoding an SC.

Regulation of IgA Production 

Polymeric Ig receptor (pIgR) and the asialoglycoprotein binding receptor (ASGR) expressed by the liver are important in the regulation of serum IgA levels. IgA2 has been showed to be cleared more rapidly than IgA1 from the circulation (Rifai, J. Exp. Med. 191(12), 2000, 2171-2181).

Where and How IgA is found:

IgA comprises dimers of IgA monomers, lined by the so-called J-chain and having a secretory component, is important with defense against certain bacteria like gonorrhea and viruses like polio. Secretory IgA can survive in harsh environments such as the disgestive and respiratory tracts which make it a preferred immunoglobulin for application in products for treating and/or preventing infection and/or inflammation of mucosal surfaces, such as the gastro intestinal mucosa, mucosa of the respiratory tract and also of the skin. In contrast to erum, in which IgG predominates, the predominant type of antibody in milk is IgA. IgA in milk is also structurally different from that in the serum. In serum, IgA is structurally similar to IgG although a small proportion consists of a dimeric immunoglobulin (i.e., 2 IgA monomers coupled together, by a polypeptide called the J-chain). Milk IgA is predominatly secretory (IgA, a dimeric molecule with two additional peptide chains termed J-chain and secretory component). 

IgA only represent about 3-4% of teh total protein of normal human serum. In plasma, IgA monomers exist in an equilibrium with non-covalently associated IgA dimers; however, J chain containing IgA dimers are also present. Total dimeric IgA constitues aobut 10-25% of total IgA. In the secretions of the mucous membranes and glands, J-chain containing IgA dimers with an additional secretory component are predominant. (Menyawi US 14/377,535)