Nuclear Factor Kappa B ( NF ‐ κB )1 and NF ‐ κB2 Deficiency: An Unpredictable Defect With Wide Clinical and Immunologic Phenotype

The Nuclear Factor Kappa B (NF-kB) signalling pathway is essential for regulating inflammation, cell survival, and adaptive immune responses through its canonical (NF-κB1-associated) and noncanonical (NF-κB2-associated) branches. The canonical pathway mediates rapid and broad reactions to immune stimuli, whereas the noncanonical pathway is activated by specific signals and contributes to lymphoid organ development, B-cell maturation including germinal centre activity, T-cell differentiation, thymic selection and antiviral defence. NF-κB transcription factors consist of five subunits: NF-kB1 (p105/p50), NF-kB2 (p100/p52), RelA, RelB and c-Rel-and heterozygous variants in NFKB1 or NFKB2 genes result in a wide range of immune dysregulation. NFKB1 mutations have been associated with common variable immunodeficiency, autoinflammatory or rheumatologic manifestations, Epstein-Barr virus-related lymphoproliferation, gastrointestinal involvement and susceptibility to recurrent or opportunistic infections. Clinical features of NFKB2 deficiency show considerable variability depending on mutation location, but recurrent infections and hypogammaglobulinemia remain the most frequently reported findings. Despite growing recognition of NF-κB-related inborn errors of immunity, the phenotypic spectrum continues to expand and genotype-phenotype correlations remain challenging. In this study, we describe seven individuals from five families with NFKB1 or NFKB2 variants and demonstrate considerable clinical and immunological heterogeneity, including marked intrafamilial variability.