Primary Antibody Deficiencies: Scientific Discoveries Leading to Challenging Paradigms in a Complex Disease Landscape
Authors: Durandy Anne , Kracker Sven , Cavazzana Marina 
Published: April 2021

Abstract
Primary antibody deficiencies (PADs) are the most common primary immunodeficiencies in humans that affect both children and adults. The patients display a decrease in IgG and/or IgA serum levels associated with lower, normal or higher IgM levels, which cause an increased susceptibility to bacterial but also viral infections. Main other complications are auto-immunity, lymphoproliferation and increased susceptibility to various types of cancers. PADs can be caused by B intrinsic and/or extrinsic defects leading to impaired early B cell development, B cell migration, B cell survival, B cell activation or defects in Immunoglobulin class switch recombination (CSR). Among the latter (B cell activation- and CSR-deficiencies), some PADs are linked to unbalanced signaling pathways, in which both gain of function and loss of function variations lead to different immune defects, pinpointing the requirement of a tightly controlled activity for proper immune response.
1) Activated PI3K-d Syndromes (APDS) are due to dominant mutations in either the PIK3CD gene encoding the p110d protein, the catalytic subunit of phosphoinositide 3 kinase d (PI3Kd) or the PIK3R1 gene encoding the regulatory subunit of phosphoinositide 3 kinase p85a. Both gene defects lead to hyperactivation of PI3K through an increased p110δ function. A frequent hyper-IgM phenotype is observed, likely due to both a T and a B cell defect. Of note, impaired or loss of function of p110δ or p85α subunit of PI3K leads to a completely different phenotype characterized by an autosomal recessive agammaglobulinemia. The main life-threatening complication of APDS is B lymphoma occurrence. Targeted therapy aiming to decrease PI3K activation is currently on trial.
2) The importance of the canonical NF-kB signaling for B cell function was firstly highlighted by the description that missense hemizygous mutations in the IKBKG gene, encoding NEMO, the NF-kB essential modulator, cause an X-linked CSR-deficiency associated with hyphohydrotic ectodermal dysplasia. Recently, heterozygous mutations in either the NFKB1 or the NFKB2 genes (encoding respectively for p105 processed to p50 and p100 processed to p52), have been reported to be causative for common variable immunodeficiency (CVID) due to haploinsufficiency of NF-êkB1 (p52) or NF-kB2 (p50), with less nuclear NF-kB accumulation and less transcriptional regulation. Some carriers are asymptomatic, pinpointing to incomplete penetrance, that suggests the role of gene modifyers or environmental factors. Although NF-κB1-deficiency affects mostly B cells, NF-kB2- deficiency combines a T, B and NK cell defect and is frequently associated to pituitary hormone deficiency.
CARD11 (caspase recruitment domain containing protein 11)-deficiency (caused by loss of function bi-allelic mutations) also indicated the importance of CARD11 mediated canonical NF-kB1 signaling for B and T cell function since CARD11-deficient patients have a CVID phenotype. Interestingly, heterozygous gain of function mutations in CARD 11 lead to a different phenotype, the so-called BENTA syndrome ("B cell expansion with NF-kB and T cell anergy") characterized by B cell lymphocytosis, lymphoproliferation and immunodeficiency.
These examples of PADs show that gain of function, haploinsufficiencies and loss of function mutations can be deleterious by disturbing the delicate balance required for adequate signaling pathways during the immune response.

Disclosures
No relevant conflicts of interest to declare.