Introduction: Primary hemophagocytic lymphohistiocytosis (HLH) is a life-threatening inflammatory process that has been linked to abnormal cytotoxic T-cell and natural killer (NK) cell function. We report on the first case of severe combined immunodeficiency (SCID) caused by a CD3δ mutation presenting with HLH in a female of Mennonite descent. Case Description: A Low-German-speaking Mennonite female with past medical history of eczema, mouth sores, and refractory oral and diaper candidiasis presented at the age of 6 months with vomiting, diarrhea, and lethargy. The patient developed HLH that was refractory to treatment and led to multi-organ failure. Immunological evaluation was diagnostic for SCID and post-mortem genetic testing confirmed a homozygous mutation in CD3δ that was previously described in Mennonites. Method: Targeted molecular testing for CD3δ deficiency confirmed a homozygous C-to-T transition at nucleotide position 202, predicting a premature stop codon, with a truncation at residue 68 (R68X) in the extracellular domain of the protein. Discussion: Many primary immunodeficiency diseases (PID) that affect cytotoxic T cells and NK cells have presented with HLH. However, a growing number of PID with no obvious NK-cell defect have also been found to predispose patients to HLH, suggesting that failure of NK activity is not the only mechanism leading to this unusual form of inflammation. Conclusion: CD3δ is known to be critical for T-cell but not NK-cell development, which may suggest an alternate mechanism for overwhelming inflammation leading to HLH. Statement of novelty: This is the first case report of CD3δ deficiency presenting with HLH.

Background: Familial hemophagocytic lymphohistiocytosis (FHL) is a rare, genetic, immune dysregulation disorder of aberrant hyperactivation of lymphocytes causing inflammation and hemophagocytosis. We report on a 3-month old male who was evaluated for the possibility of FHL because of a positive family history. The patient was asymptomatic; however, levels of the soluble interleukin 2 (IL-2) receptor were elevated and the quantity and function of the natural killer (NK) cells were severely decreased. Methods: Purification of NK cells and evaluation of the cytotoxicity and IFNγ/TNFα secretion of NK cells after IL-2 activation relative to the patient's family members. Results: The patient's NK specific lysis was enhanced compared with his mother, and it was slightly higher than his sister. The IFNγ and TNFα secretion by the patient's NK cells after challenge with target 721 cells or anti-natural cytotoxicity receptors (NKp30 and NKp44) antibodies showed levels that are close to the mother's and sister's NK secretion levels. Owing to a low yield of NK cells from the patient's father the results for his NK cells are incomplete. The patient did not undergo HSCT and continued to be followed. He is now 7 years old and thriving without signs of FHL. His last examination was in August 2012 for functionality of isolated NK cells. The results showed normal cytotoxicity, cytokine secretion, and CD107a up-regulation to the NK cell surface. Conclusion: We propose that NK function assessment in patients with presumed FHL should be performed on isolated NK cell populations. This practice may reduce the number of false-negative results in NK function assays. Statement of novelty: In this case report we show that functional assessment of unpurified NK cells could lead to a false-negative assessment in 1 of the parameters in FHL. Assessment of NK function without NK purification may lead to an erroneous diagnosis of poor NK function.