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Fc alpha RI/CD89 circulates in human serum covalently linked to IgA in a polymeric state
Paul J M van der Boog1, Ger van Zandbergen, Johan W de Fijter
1Department of Nephrology, Leiden University Medical Center, Albinusdreef 2, 2333 ZA Leiden, The Netherlands.
Insights
Soluble CD89 (Fc alpha RI) circulates in human serum, primarily complexed with IgA. This finding suggests a role for CD89 in the formation of polymeric IgA in vivo.
Area of Science:
- Immunology
- Glycoprotein research
- Myeloid cell biology
Background:
- The Fc receptor for IgA, CD89 (Fc alpha RI), is a type I receptor glycoprotein on myeloid cells with immune effector functions.
- While CD89 can be released in vitro, its in vivo existence as a soluble molecule is poorly understood.
Purpose of the Study:
- To investigate the presence and characteristics of soluble CD89 in human circulation.
- To determine the molecular form and binding partners of soluble CD89 in vivo.
Main Methods:
- Detection of soluble CD89 using ELISA, Western blotting, and dot-blot assays.
- Size fractionation of human serum and IgA complexes via gel filtration chromatography.
- Analysis of CD89-IgA interactions using anti-IgA affinity purification.
Main Results:
- Circulating CD89 was not detected by ELISA but was identified as a 30-kDa molecule in serum and plasma via Western blotting.
- Dot-blot analysis revealed CD89 in all tested human sera (mean concentration 1900 ng/ml), predominantly in high molecular mass fractions.
- CD89 was found to form direct, seemingly covalent complexes with IgA, specifically within high molecular mass IgA fractions, distinct from dimeric IgA.
Conclusions:
- Soluble CD89 circulates in human serum primarily complexed with IgA.
- These CD89-IgA complexes are associated with high molecular mass IgA.
- CD89 may play a role in the in vivo formation of polymeric serum IgA.
Abstract:
The FcR for IgA CD89/FcalphaRI, 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.
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