Rebecca Martin/Sandbox1: Difference between revisions

At least two isotypes exist, termed IgA1 and IgA2. IgA2 can further be categorized into 2 allotypes: IgA2 m(1) and IgA2 m(2). While IgA2 is found in most mammalian species, IgA1 is found only in higher apes. An approximately equal ratio of secretory IgA1 (sIgA1) to secretory IgA2 (sIgA2) reside at the mucosal surface, with the exception of the colon, where the majority is sIgA2 <ref name=" nineten">PMID:19109255</ref>. In the serum, about 90% of the IgA is monomeric IgA1 <ref name ="ten" />. While both isoforms are able to bind polysaccharide, IgA1 preferentially binds protein antigen, while IgA2 preferentially binds lipopolysaccharide lipid A.  

The receptors for IgA include the Fcα Receptor (FcαRI; CD89) and the polyimmunologlobulin receptor (pIgR). When binding to FcαRI results in the dimerization, the consequent signaling results in effector functions, including respiratory burst, mucosal surface, phaocytosis, and eosinophil degranulation. Binding to the pIgR results in transoocytosis and IgA secretion <ref name="five" />. Unlike other antibody isotypes, IgA exists in mutiple oligomeric states <ref name="nineseven" />. The most common of which are the monomeric, dimeric, and secretory forms <ref name="ten" />, adding to the complexity of structural functions for IgA. Exploring IgA's structure and protein interactions illuminates the unique and critical function IgA plays in humoral immunity.

::Proteins containing the classic immunoglobulin-like domain are found predominantly in the immune system <ref name="att" />. In fact, the antibody's closest related structires are those that recognize antigen: MHC and TCRs.

::The V-type domain is found in a wider variety of proteins, including the Ig-binding molecules, such as the pIgR and the FcalphaR <ref name="att" />. Viral hemagluttinin is yet another example.

== IgA1 and IgA2: a Structural Comparison ==

<scene name='Rebecca_Martin/Sandbox1/Igd/1'>IgD</scene> Hinge region is 64 amino acids in length. Note similarity to IgA1.

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== The J Chain allows IgA to form Dimers==

:The secretory compenent is the first 585 residues of the pIgR <ref name="seven"/>. The C terminal end of the secretory component is linked to the pIgR, but maintains no specific fold. The ability for the secretory to move freely facilitates its proteolytic cleavage and the secretion of sIgA. Structurally, the secretory component is comprised of 5 V-type immunoglobulin-like domains (D1-5) with 5-7 glycan chains, which increase the chains resistance to proteases. These glycosylation sites are located on one side of the protein and do not interfere with IgA binding. A long (10 amino acids) linker region exists between D3 and D4, so the D4 and D5 regions fold in on D2 and D3 in a compact J-shape. D1-3 are 12nm in length, while D4-5 are 10 nm long. Thus, D1 remains accessable. The one-sided glycans allow free access of D1's CDR regions and the Cys 502 at D5 to interact with IgA. It is thought that when D1 interacts with IgA's Fc region and the J chain,  allowing the secretory component to unfold and disulfide formation between D5 C502 and IgA's Ch2 C311. While SC unfolds upon IgA binding, this binding imparts no change on the structure of IgA <ref name="nineseven"/>.  

==sIgA1 and sIgA2==  

[[Image:SIgA.jpg|thumb|Adapted from Bonner, et al 2009 and Bonner, et al 2008.]]

:Binding of the secretory component to the convex edge of the Fc region of dimeric IgA1 maintains <scene name='Rebecca_Martin/Sandbox1/Siga1def/1'>Secretory IgA1</scene> in a near planar conformation, <ref name="nineten" />, <ref name="eight" />. The Fc regions align end to end without overlap, and the fab fragments remain in alignment with the Fc plane. In contrast, <scene name='Rebecca_Martin/Sandbox1/Siga1/1'>Secretory IgA2</scene> fab fragments remain out of alignment with the Fc plane. Because the secretory component resides at the convex region of the Fc portion, the D1 and D5 impart steric hindrance on the fab fragments, which are forced out of alignment. Consequently, IgA2 assumes a nonplanar conformation. The longer hinge region of IgA1 allows it to maintain its planar conformation.  

'''Structure and the Mucosal Environment'''

:'''Glycosylation''' of the IgA, the J chain, and the secretory component lends to protection against proteolytic attack in the harsh mucosal environment. '''Dimerization''' allows transcytosis.  In addition, glycosylation on the secretory chain is thought to assist with anchoring to the mucosa. Through steric hindrance, the secretory component assists in preventing the binding of microorganisms to gut mucosa, impeding their entry into the mucosa <ref name="ten"/>, <ref name="two">PMID: 12768205</ref>.  

:The Fc portion is more susceptible to intestinal proteases than other regions of the IgA.  This region of increased susceptibility is the precise region to which the secretory component remains after the pIgR is cleaved <ref name="seven"/>. So, the secretory component offers the antibody additional '''protection against proteolytic cleavage'''. Binding to Fc region reduces flexibility at the hinge and between the 2 Fc regions. The loss in flexibility correlates with a decrease in the likelihood that the IgA will be in the correct conformation for cleavage to occur <ref name=" nineseven" /> and prevents large bacterial matrix metalloproteases from cleaving the Fc and hinge regions. So, the secretory component and dimeric IgA synergize to create a protected protein fit for the harsh mucosal environment.

<applet load='1ow0' size='300' frame='true' align='right' caption='Fc portion of IgA bound to FcalphaR' />

:The FcalphaR binding sites are located one per heavy chain at each Ch2-Ch3 interface. Both both domains contribute one binding site. So, the stoichiometry between monomeric IgA and the FcalphaR is <scene name='Rebecca_Martin/Sandbox1/Fc/3'>2:1</scene> <ref name="five"/>. The Fc portion is shown in red, and the receptor is in blue. Dimerization would increase this stoichiometry 4:1; however, 2 of the binding sites will be <scene name='Rebecca_Martin/Sandbox1/Siga1_sites_covered/1'>covered by the secretory component</scene>. Because of <scene name='Rebecca_Martin/Sandbox1/Fc/4'>orientation</scene> constraints, only 1 of the 2 remaining binding sites will be available to bind receptor. Therefore, physiologic stoichiometry is 1:1.  

:The binding of IgA to the Fcalpha receptor does not elicit a structural change in the antibody <ref name="two"/>. Effector function is elicited when multiple receptors bind and resultant clustering triggers signaling events. The 1:1 stoichiometry greatly limits Fcalpha clustering and consequent effector functions by effectively '''limiting the concentration of available antibody binding sites''' in the local environment, favoring neutralization in the absence of cytotoxic and inflammatory responses upon antigen recognition. Additional modulation occurs through internal signaling events. For example, cytokines strigger changes in cytoskeletal arrangments that result in clustering of the FcalphaR at the cell surface.  In effect, the interaction of the '''secretory component limits the effector and inflammatory responses''' upon antigen binding without limiting the ability of the antibody to neutralize pathogens or exclude commensals from breeching the mucosal barrier.