FcγRIIIB Gene Duplication: Evidence for Presence and Expression of Three Distinct FcγRIIIB Genes in NA(1+,2+)SH(+) Individuals

Abstract

Abstract Recently, a new alloantigen on IgG Fc receptor type IIIb (FcγRIIIb), SH, was described (Bux et al, Blood 89:1027, 1997). We identified three healthy individuals whose neutrophils reacted positively with the SH antiserum. The neutrophil antigen (NA) phenotype of all three donors was NA(1+,2+). Analysis of genomic DNA showed that the three donors were positive for the described SH-encoding mutation in the NA2-FcγRIIIB gene, 266C→A. However, NA(1,2) genotyping and nucleotide sequencing of an NA2-specific fragment amplified from the genomic DNA fragment showed that these individuals carried three FcγRIIIBgenes, namely, NA1-FcγRIIIB, NA2-FcγRIIIB, andSH-FcγRIIIB, encoding NA1-FcγRIIIb, NA2-FcγRIIIb, and SH-FcγRIIIb, respectively. Southern blot analysis confirmed these findings. Furthermore, all three transcripts were isolated from neutrophil mRNA. To investigate whether the presence of threeFcγRIIIB genes resulted in a higher membrane expression of FcγRIIIb, we measured the reactivity of neutrophils from NA(1+,2+)SH(+) individuals with a panel of CD16 monoclonal antibodies (MoAbs) in comparison with neutrophils from NA(1+,2+)SH(−) controls. Reactivity of four different anti–pan-FcγRIII MoAbs and NA2-specific MoAb GRM1 was higher with SH(+) neutrophils compared with controls, whereas that of NA1-specific MoAbs was similar, which is in concordance with the results from the genomic analysis. We observed that reactivity with NA2-specific CD16 MoAb PEN1 was sixfold higher in SH(+) individuals compared with controls. Apparently, the 60Ala→Asp substitution in SH-FcγRIIIb influences the epitope recognized by PEN1. In conclusion, we identified three NA(1+,2+)SH(+) individuals carrying three FcγRIIIB genes and observed a clear gene-dosage effect on the level of expression of neutrophil FcγRIIIb.