Summary Distinct differences in the capacity of monocyte FcyRII of different individuals to bind or not bind mouse IgGl defines a polymorphism ot FcyRIIa and has previously been defined as the high responder (HR) or low responder (LR) polymorphism of FcyRII. The precise definition of the molecular basis of the human HR/LR polymorphism of FcyRIIa from the peripheral blood mononuclear cells of normal individuals has been determined by anti-CD3 induction of T cell proliferation, the polymerase chain reaction {PCR), nucleotide sequencing, transfection and IgG binding. Amplification of first strand cDNA from mRNA isolated from mononuclear cells was performed by PCR using primers specific for the sequences encoding the leader and cytoplasmic sequences of FcyRUa, which is normally expressed in monocytes. Sequencing of the PCR products and transfection of these to FcyR cells indicated that in FcyRIIa of HR or LR individuals; (i) three nucieotide substitutions (CA to TG and G to A) resulted in the change of glutamine to tryptophan at position 27 {first extracellular domain) and arginine to histidine at position 131 (second extracellular domain); (ii) expression of cDNA encoding the various combinations of these indicated that arginine at position 131 was essential for IgGl binding whereas the amino acid changes at position 27 had no effect; and (iii) IgGl at high concentration bound to al! allomorphic forms of FcyRIIa. These results indicate that position 131 in the second extracellular domain of FcyRIIa is intimately involved in immunoglobulin binding. Furthermore, the apparent inability oi IgGl to bind to FcyRIIa of non-responder individuals is not absolute in that immune complexes sensitized with a high concentration of MlgGl will bind to non-responder-type FcyRIIa.

No abstract

Full length cDNA clones encoding the mouse Fc gamma RI were isolated by using redundant oligonucleotide probes based on previously determined amino acid sequence of protein bound to an IgG2a antibody column. Sequence analysis of cDNA clones indicates that mouse Fc gamma RI is a transmembrane glycoprotein that is composed of three disulfide bonded extracellular Ig binding domains unlike Fc gamma RII of man and mouse. These extracellular domains contain five potential sites of N-linked glycosylation; three sites in the first domain and one in each of the second and third domains. In addition a transmembrane region is present followed by a cytoplasmic tail of 84 amino acids. Analysis of the amino acid sequence of the first two extracellular domains of Fc gamma RI indicate that these are highly homologous to the extracellular domains of Fc gamma RII; the third domain is different and shows a lower level of homology to other FcR domains but is clearly related to the Ig super-family. Transfected cells expressing Fc gamma RI were shown to bind immune complexes of rabbit IgG; and monomeric IgG2a bound to transiently transfected cells with an affinity of approximately 5 x 10(7) M-1, i.e. the receptor was of high affinity and therefore was by definition Fc gamma RI. Northern analysis demonstrated that Fc gamma RI mRNA could be detected in the Fc gamma RI+ myeloid cell lines WEH1 3B and J774. Finally, Southern analysis indicated that Fc gamma RI is likely to be encoded by a single copy gene of approximately 9 kb.

We have recently mapped the human FCGR2 gene to chromosome 1 bands q23-q24. In situ hybridization of FCGR2 cDNA with a cell line containing a t(1:19)(q23;p13) derived from a patient with pre-B ALL has allowed a more accurate localization of this gene to chromosome 1 band q23. Furthermore, this study indicated a splitting of the FCGR2 gene or gene cluster by the t(1;19). However, Southern analysis showed no genetic rearrangement when compared with a karyotypically normal Epstein-Barr virus (EBV)-transformed cell line from the same patient. This suggests that the translocation breakpoint does not occur within the coding region of this gene.