LAUSR

To the editor, We report the case of a 5-year-old Caucasian girl with very early-onset inflammatory bowel disease (VOIBD) who had inadequate response to initial therapy with azathioprine. Immune system evaluation revealed an isolated complete natural killer cell (NK cell) deficiency, which resolved with discontinuation of the drug. The patient first presented at age 4 with a history of angular cheilitis and oral ulcerations of the soft palate. She had notable findings of microcytic anemia (Hb 10.7 g/dL, MCV 73.2 fL), elevated sedimentation rate of 72 mm/hr, and elevated fecal calprotectin of 396 mcg/g (normal less than 50 mcg/g). Colonoscopy showed segmental moderate inflammation with congestion, erosions, friability, and scarring throughout the entire colon with pathology demonstrating focal active ileitis and non-necrotizing granulomas in the terminal ileum compatible with Crohn’s disease. Due to the young age of onset, additional immunologic screening was performed including enumeration of T, B, and natural killer cells, immunoglobulins, pneumococcal and tetanus titers, and testing for chronic granulomatous disease which was normal or negative, respectively. She was subsequently treated with corticosteroids and azathioprine, with testing demonstrating wildtype thiopurine methyltransferase genotype, predictive of normal azathioprine metabolism. Over the ensuing 12 months, she continued to have active disease and azathioprine was incrementally increased to 3 mg/kg. Drug levels at this dose showed sub-therapeutic 6-TGN at 229 pmol/RBC (range 230–400), below risk levels for myelotoxicity with no elevation of the hepatotoxic metabolite 6-MMPN. Additional immunologic evaluation at that time revealed normal T cell phenotyping and lymphocyte proliferation to mitogens. Repeat lymphocyte subpopulations revealed an isolated absence of NK cells by clinical testing in two separate samples 2 days apart (Fig. 1a). Additional research-based phenotyping did not reveal the presence of any CD56-negative NK cell-like population expressing markers such as CD122, CD16, or NK cell receptors including killer immunoglobulin-like (KIR) or NKG2 molecules, confirming an absence of peripheral blood NK cells. Functional NK cell testing showed absent NK cell cytotoxicity of K562 target cells (Fig. 1b). Infectious testing included negative blood PCR for EBVandCMVwith no serologic evidence of infection for EBV, CMV, HSV, and VZV. Azathioprine was discontinued due to inadequate disease control and concern about its effects on NK cell compartment. After 3 weeks, NK cells returned and by 6 weeks had normalized in number (Fig. 1a and c), and NK cell cytotoxicity against K562 target cells was normal based on lytic units (Fig. 1b). Phenotyping of the patient’s NK cells after stopping azathioprine revealed a relative decrease in the percentage of mature NK cells, defined as CD56CD16 cells expressing CD57, while there was a corresponding increased percentage of immature CD56CD57NKG2A cells compared to a healthy agematched control (Fig. 1d and e). The CD56 NK cells had high expression of NKG2A, a receptor typically associated with the less mature CD56 subset, but did express KIR (not shown) * Megan A. Cooper [email protected]