Variants in IGLL1 cause a broad phenotype from agammaglobulinemia to transient hypogammaglobulinemia

Background Agammaglobulinemia due to variants in IGLL1 has traditionally been considered an exceedingly rare form of severe B-cell deficiency, with only 8 documented cases in the literature. Surprisingly, the first agammaglobulinemic patient identified by newborn screening (NBS) through quantification of kappa-deleting recombination excision circles harbored variants in IGLL1 . Objective We comprehensively reviewed clinical and immunologic findings of patients with B-cell deficiency attributed to variants in IGLL1 . Methods NBS programs reporting the use of kappa-deleting recombination excision circle assays, the European Society for Immunodeficiencies Registry, and authors of published reports featuring patients with B-cell deficiency linked to IGLL1 variants were contacted. Only patients with (likely) pathogenic variants, reduced CD19 + counts, and no alternative diagnosis were included. Results The study included 13 patients identified through NBS, 2 clinically diagnosed patients, and 2 asymptomatic siblings. All had severely reduced CD19 + B cells (< 0.1 × 10 9 /L) at first evaluation, yet subsequent follow-up assessments indicated residual immunoglobulin production. Specific antibody responses to vaccine antigens varied, with a predominant reduction observed during infancy. Clinical outcomes were favorable with IgG substitution. Two patients successfully discontinued substitution therapy without developing susceptibility to infections and while maintaining immunoglobulin levels. The pooled incidence of homozygous or compound heterozygous pathogenic IGLL1 variants identified by NBS in Austria, Czechia, and Switzerland was 1.3:100,000, almost double of X-linked agammaglobulinemia. Conclusion B-cell deficiency resulting from IGLL1 variants appears to be more prevalent than initially believed. Despite markedly low B-cell counts, the clinical course in some patients may be milder than reported in the literature so far.