Inborn errors of immunity (IEI), also known as primary immunodeficiency diseases, are a heterogenous group of disorders which can present at any age with recurrent, unusual or severe infections, immune dysregulation and other clinical features. Severe combined immunodeficiency (SCID) is a particularly severe, life-threatening IEI which typically presents in infancy and affects both cellular and humoral immunity. Infants with SCID have low or absent naïve T cells, and variably affected B and NK cells, depending on the underlying genetic defect.
Given that the clinical outcomes of SCID are significantly improved with early diagnosis and treatment, newborn screening for this condition has been implemented in many countries worldwide, including New Zealand, and will hopefully be established in Australia in the near future. Infants are screened for SCID using a DNA-based method where qPCR is used to measure TREC and KREC levels (surrogate markers for naïve T cells and B cells, respectively) from Guthrie card dried blood spot samples.
Flow cytometric assays are a critical component of the laboratory assessment of infants with suspected SCID, or those identified to have low TREC and/or KREC levels on their newborn screening test. These assays, including lymphocyte subset analysis (including assessment of naïve T cells), other special stains to determine key protein expression and functional lymphocyte analysis will be discussed. In addition, flow cytometry-based methods assessing clonality of T cells and STAT phosphorylation pathways are also helpful in the investigation of patients with SCID. The role of STAT phosphorylation studies in other immunodeficiencies will also be discussed.