Several new rat class III FcγR isoforms are described here, extending the genetic complexity of this receptor family and further distinguishing rat CD16 from mouse CD16, represented by only one receptor isoform, and human CD16, represented by only two isoforms. RNase protection assays reveal that three rat tumor cell lines - RBL-1 basophilic leukemia cells, RM-SV1 macrophages, and CRNK-16 NK cells - all coordinately express multiple and probably identical rtFcγRIII-related transcripts in similar relative proportions but at significantly different levels. These results indicate that no single isoform predominates in these cell types but that the overall level of rtFcγRIII-related transcripts is differentially regulated. Two of the rtFcγRIII isoforms found to have extensive amino acid sequence differences in their second extracellular (EC2) domains are shown to bind rat and mouse IgG subclasses differently. This result suggests that the receptor isoform diversity in this species may function as a mechanism for extending the IgG-binding capacity of rat leukocytes. Cloned cDNA for the rat CD3ζ protein was also isolated in this study and its ability to augment surface expression of class III FcγR was tested by rosetting of cDNA-transfected COS cells. Like the structurally homologous mouse CD3ζ, rat CD3ζ fails to promote surface expression of FcγRIII, sharply contrasting the efficient receptor expression produced by human CD3ζ. Variations in the transmembrane amino acid sequences correlate with the divergent capacities of these CD3ζ molecules to augment receptor expression. The high levels of CD3ζ message expressed in rat NK cells may indicate that other unidentified heterosubunits are required for assembly of rat CD3ζ into functional CD16 receptors.
|Number of pages||12|
|Journal||Journal of Immunology|
|Publication status||Published - May 15 1993|
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