Undetectable NK Cells due to the FCGR3A Variant, L66H, Which May Not Be Directly Disease-Causing.

A baby girl born at 38 weeks of gestation with no perinatal complications was brought to our attention for possible severe combined immunodeficiency (SCID) because of low T cell receptor excision circles (TRECs) (11/mcL, reference range > 18) on newborn screen. Blood was collected for confirmatory flow cytometry and the absolute number of T cells was borderline low (2116/mcL, reference range 2500–5500) but sufficient to rule out SCID. However, her NK cell enumeration showed absent (< 1%) NK cells (CD3− CD56+ CD16+ by BD Biosciences 6-color TBNK, San Jose, CA). Subsequent testing at a local laboratory identified normal numbers of NK cells (26%), but at this laboratory, NK cells were defined as (CD3− CD56+ by locally derived reagents). Complete laboratory values can be found in Supplementary Table E1. We therefore pursued further testing to determine if CD16 was absent from the patient’s cells or if there was a lack of targeting by the specific anti-CD16 monoclonal antibody used.