Common Variable Immunodeficiency Disorders: A perspective from New Zealand.

Common Variable Immunodeficiency Disorders (CVID) are the most frequent symptomatic Primary Immunodeficiency (PID) in adults and children. Patients with CVID present with predominant antibody deficiency with varying degrees of impaired cellular immunity. CVID was previously a diagnosis of exclusion, which led to considerable uncertainty about which patients would qualify for subcutaneous or intravenous immunoglobulin (SCIG/IVIG) replacement. Over the last twelve years, several sets of diagnostic criteria have been published which identify these disorders with greater precision. These new CVID diagnostic criteria assist with decisions on treatment, particularly SCIG/IVIG replacement. With the advent of massively parallel genome sequencing technologies, it has become apparent that a significant proportion of individuals with a CVID phenotype have an underlying causative genetic defect. If such a pathogenic variant is identified, these individuals are removed from the overarching diagnosis of CVID and are deemed to have a CVID-like disorder caused by a specific Inborn Error of Immunity (IEI). New Zealand has had a long-standing customized PID genetic testing program. Two novel autosomal dominant pathogenic variants causing CVID-like disorders, consequent to haploinsufficiency of Nuclear Factor kappa-light-chain-enhancer of activated B cells (NFKB1) and Transcription Factor 3 (TCF3), were identified in New Zealand families. The latter pathogenic variant was shown to have an epistatic interaction with TNFRSF13B (TACI) in a patient with a digenic CVID-like disorder. Epistasis is the synergistic, non-linear interaction between two or more genetic loci, leading to much more severe (or much milder) disease. This perspective reviews the current understanding of these disorders with contributions from three New Zealand-based studies: The Prospective NZ CVID and the NZ hypogammaglobulinemia sub-studies as well as a large retrospective case series of Transient Hypogammaglobulinemia of Infancy (THI). These clinical and genomic studies have offered insights into the complexities of these rare PIDs. This review examines current areas of uncertainty in the diagnosis of of these disorders.