Receptors for IgA on rabbit lymphocytes. II. Characterization of their binding parameters for IgA

The specificity of rabbit receptors for IgA (RFc alpha) was investigated on lymphocytes isolated from the Peyer's patches and spleen. Specifically was determined by the inhibition of IgA rosette formation with intact and proteolytic fragments of rabbit and human secretory IgA (SIgA), human myeloma proteins, and secretory component. RFc alpha bound human SIgA as efficiently as rabbit SIgA. Of the human IgA tested, RFc alpha preferentially bound to the IgA2 subclass and most avidity to a multimeric IgA molecule. In addition., the site of the RFc alpha-IgA interactions was localized to the C alpha 2 domain in the Fc portion of IgA. RFc alpha on spleen lymphocytes showed the same pattern of binding specificity toward the human IgA subclasses as the RFc alpha on lymphocytes from Peyer's patches. However, differences between the spleen and Peyer's patches were observed in the interaction of RFc alpha and the rabbit IgA subclasses. RFc alpha on lymphocytes from the Peyer's patches bound the rabbit IgA-g subclass more efficiently than the rabbit IgA-f subclass, whereas the converse was true for the spleen RFc alpha. Human secretory component inhibited IgA but not IgG or IgM rosette formation as a result of its interaction with lymphocytes. These studies suggest: 1) that there may be two types of RFc alpha expressed by lymphocytes in certain tissues, and 2) that there are two closely associated receptors on the lymphocyte--one specific for secretory component and the other for structures within the C alpha 2 domain of IgA.