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4 However, NK-cell CD2 expression did not vary between CD16A-expressing and CD16A-deficient family members (see Fig E5 in this article’s Online Repository at in multiple blood samples analyzed over an 18-month period. These observations closely match the phenotype of NK cells from FcγR3A knockout mice, 3 but were surprising because CD16A has been suggested to participate in processes of natural cytotoxicity, via association with CD2. However, natural cytotoxicity against K562 cells or after stimulation of specific receptors including NKG2D and 2B4 was comparable to that mediated by CD16-expressing family members and unrelated healthy donors ( Fig 2, B). NK cells from the CD16A-deficient individuals did not mediate antibody-dependent cellular cytotoxicity (ADCC) ( Fig 2, A).
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Although not detected in cDNA from mononuclear cells, low levels of CD16B/A mRNA could be found in granulocyte mRNA ( Fig 1, D), consistent with the 5′ regulatory elements being those of CD16B. The FcγR2/3 locus at chromosome 1q23.3 is known to be prone to recombination and unequal crossing over 2 and detailed genetic analyses showed that the CD16A-deficient individuals had inherited a maternal chromosome where the FCGR3A gene had been deleted and a paternal chromosome that encoded a hybrid FCGR3B/A gene ( Fig 1, C). Sequencing of patient CD16 cDNA clones identified both transmembrane and GPI-linked receptors (see Fig E4 in this article’s Online Repository at However, analysis of the sequences revealed that the 5′ region of the cDNA clones encoding a transmembrane domain contained residues more typical of CD16B. It may also be relevant that 1 of the 3 affected siblings was female. However, because the marked divergence in clinical features between the twins occurred at age 14 to 15 years, it might be possible that at least some of these features were age-related. Differences in the severity of disease between patients harboring similar mutations are not uncommon and as yet we have no definitive explanation as to why the clinical presentation of the index patient was more severe. Moreover, CD16 protein was undetectable in western blot analysis of purified PBMCs (see Fig E1 in this article’s Online Repository at Granulocytes of this individual did express the GPI-linked CD16B receptor, but at reduced levels compared with healthy controls (see Fig E2 in this article’s Online Repository at Analysis of the immediate family identified 2 other siblings (a twin brother and 1 sister) who also lacked CD16A expression on peripheral blood NK cells and monocytes ( Fig 1, B a family tree is shown in Fig E3 in this article’s Online Repository at Although these CD16A-deficient siblings did not present such severe clinical symptoms, detailed studies ( Case Report and Table E1) have revealed frequently elevated levels of EBV DNAemia and their evolution is being monitored. NK-cell CD16 expression was not observed with either the B73 or the 3G8 mAbs that bind different epitopes on CD16 1. MFI, Mean Fluorescence intensity PMN, polymorphonuclear leukocyte. D, Low levels of CD16BA mRNA are found only in granulocytes of family members with this gene. C, The structure of the low/medium- affinity FcγR locus on chromosome 1, compared with the same genomic region in the various members of the family studied. B, CD16 expression on NK cells and monocytes of healthy, unrelated controls and family members. A, Representative cytometry plots showing the lack of CD16 expression by patient NK cells.