Localization of the binding site for the monocyte immunoglobulin (Ig) A-Fc receptor (CD89) to the domain boundary between Calpha2 and Calpha3 in human IgA1.

The influence of regions, other than the hinge, on the susceptibility of human IgA1 to cleavage by diverse bacterial IgA1 proteases, was examined using IgA1 mutants bearing amino acid deletions, substitutions, and domain swaps. IgA1 lacking the tailpiece retained its susceptibility to cleavage by all of the IgA1 proteases. The domain swap molecule α1α2γ3, in which the CH3 domain of IgA1 was exchanged for that of human IgG1, was resistant to cleavage with the type 1 and 2 serine IgA1 proteases of Neisseria meningitidis, Neisseria gonorrhoeae, and Haemophilus influenzae, but remained sensitive to cleavage with the metallo-IgA1 proteases of Streptococcus pneumoniae, Streptococcus oralis, Streptococcus sanguis, and Streptococcus mitis. Substitution of the IgA1 Cα3 domain motif Pro440-Phe443 into the corresponding position in the Cγ3 domain of α1α2γ3 resulted now in sensitivity to the type 2 IgA1 protease of N. meningitidis, indicating the possible requirement of these amino acids for sensitivity to this protease. For the H. influenzae type 2 protease, resistance of an IgA1 mutant in which the CH3 domain residues 399–409 were exchanged with those from IgG1, but sensitivity of mutant HuBovα3 in which the Cα3 domain of bovine IgA replaces that of human IgA1, suggests that CH3 domain residues Glu403, Gln406, and Thr409 influence sensitivity to this enzyme. Hence, unlike the situation with the metallo-IgA1 proteases of Streptococcus spp., the sensitivity of human IgA1 to cleavage with the serine IgA1 proteases of Neisseria and Haemophilus involves their binding to different sites specifically in the CH3 domain.

Section: Discussionmentioning

confidence: 99%

Sites in the CH3 Domain of Human IgA1 That Influence Sensitivity to Bacterial IgA1 Proteases

Senior

Woof

2006

“…The "docking site" on IgA for Fc␣RI has been mapped to the boundary of CH2 and CH3 (38,39), a location occupied by SC (40). To evaluate the effect of SC on IgA-Fc␣RI binding, we fractionated IgA1 isolated from supernatants of IgA1-J-SC and that of IgA2-J-transfected BHK cells.…”

Section: Discussionmentioning

confidence: 99%

Activity of Human IgG and IgA Subclasses in Immune Defense Against Neisseria meningitidis Serogroup B

Vidarsson¹,

Pol²,

Elsen

et al. 2001

122

125

Both IgG and IgA Abs have been implicated in host defense against bacterial infections, although their relative contributions remain unclear. We generated a unique panel of human chimeric Abs of all human IgG and IgA subclasses with identical V genes against porin A, a major subcapsular protein Ag of Neisseria meningitidis and a vaccine candidate. Chimeric Abs were produced in baby hamster kidney cells, and IgA-producing clones were cotransfected with human J chain and/or human secretory component. Although IgG (isotypes IgG1–3) mediated efficient complement-dependent lysis, IgA was unable to. However, IgA proved equally active to IgG in stimulating polymorphonuclear leukocyte respiratory burst. Remarkably, although porin-specific monomeric, dimeric, and polymeric IgA triggered efficient phagocytosis, secretory IgA did not. These studies reveal unique and nonoverlapping roles for IgG and IgA Abs in defense against meningococcal infections.

“…CD89 binds both IgA1 and IgA2 with similar affinity (K a ϳ 10 6 M Ϫ1 ). The site of interaction between CD89 and IgA was identified in the first extracellular domain of CD89 (4,5) and the C␣2/C␣3 junction of IgA (6). CD89 is constitutively expressed as a 50-to 70-kDa protein on neutrophils and monocytes/macrophages, or as a 70-to 100-kDa glycoprotein on eosinophils due to increased glycosylation (7).…”

Section: Fc␣ri/cd89 Circulates In Human Serum Covalently Linked Tomentioning

confidence: 99%

FcαRI/CD89 Circulates in Human Serum Covalently Linked to IgA in a Polymeric State

Boog

Zandbergen

Fijter

et al. 2002

The FcR for IgA CD89/FcαRI, is a type I receptor glycoprotein, expressed on myeloid cells, with important immune effector functions. In vitro CD89 can be released from CD89-expressing cells upon activation. Little information is available on the existence of this soluble molecule in vivo. Using specific and sensitive ELISA techniques (detection limit 50 pg/ml), we were not able to detect circulating CD89 in human sera. However, using Western blotting, a 30-kDa soluble CD89 molecule was demonstrated in both serum and plasma. Moreover, using a specific semiquantitative dot-blot system, we found CD89 in all human sera tested (mean concentration 1900 ng/ml). Size fractionation of human serum using gel filtration chromatography showed that the CD89 molecule was predominantly present in larger molecular mass fractions. Direct complexes between IgA and CD89 were demonstrated by anti-IgA affinity purification, and when analyzed under nonreducing conditions appeared to be covalently linked. Size fractionation of affinity-purified IgA showed the presence of soluble CD89 only in the high molecular mass fractions of IgA, but not in monomeric IgA. High molecular mass complexes of CD89-IgA could be distinguished from J chain containing dimeric IgA. These data show that CD89 circulates in complex with IgA, and suggest that CD89 might contribute to the formation of polymeric serum IgA.