CD89: the human myeloid IgA Fc receptor

CD89 (Fc alphaRI) is the human myeloid IgA Fc receptor expressed on cells, such as neutrophils, eosinophils and monocytes/macrophages. Cross-linking of CD89 on these cells, by IgA-opsonised particles (e.g. bacteria, viruses) or anti-CD89 monoclonal antibodies, can trigger various immunological effector functions which are generally protective but may also cause harm to the body. CD89 is a transmembrane glycoprotein that binds both subclasses of IgA in all its molecular forms (i.e. monomeric, dimeric and secretory IgA) via a region of its membrane-distal EC1 domain. DNA studies have shown that the CD89 gene is located within the newly described leukocyte receptor cluster (LRC) on chromosome 19. CD89 is more closely related to the KIR and MIR proteins, whose genes are also found in the LRC, than to other human Fc receptors (FcRs). On myeloid cells, CD89 is able to associate with the immunoreceptor tyrosine-based activation motif (ITAM)-containing the FcR gamma chain, which is responsible for intracellular signaling via CD89. Recently, it has been suggested that some cells express CD89 in a form that does not associate with the FcR gamma chain. Although the biological relevance of this observation is not yet clear, it may explain certain anti-inflammatory/inhibitory effects attributed to IgA. Here we review current knowledge concerning the genetics, structure and biological function of CD89.