Insights into the ontogeny of the human fetal adaptive immune system are of great value for understanding immunocompetence of the developing fetus. However, to date, this has remained largely uncharted territory, in large part because blood samples from healthy, early gestation fetuses have been hard to come by. In a comprehensive study, we analyzed levels of T cell receptor excision circles (TRECs), signal-joint κ receptor excision circles (sjKRECs), and intron recombination signal sequence-K-deleting element (iRSS-Kde) rearrangement, and T and B lymphocyte repertoire clonality in human fetuses from 12 to 26 weeks of gestational age. Using next-generation sequencing, we analyzed the diversity and complexity of T cell receptor β (TRB) and immunoglobulin heavy chain (IGH) repertoires in four fetuses at 12, 13, 22, and 26 weeks of gestation and in healthy full-term infants. We report the progressive increase of TREC, sjKREC, and iRSS-Kde levels over time and confirm that B cell development precedes T cell development in the human fetus. Temporally and spatially regulated maturation of B and T cell repertoire diversity and complexity during human fetal development was observed, including evidence that immunoglobulin somatic hypermutation and class switch recombination occur already during intrauterine life. Our results help define physiological levels of immunodeficiency in premature infants and may serve as a reference for future studies aimed at investigating the impact of intrauterine pathologies on fetal immune development and function.

Background Neutrophils are the predominant phagocytes that provide protection against bacterial and fungal infections. Genetically determined neutrophil disorders confer a predisposition to severe infections and reveal novel mechanisms that control vesicular trafficking, hematopoiesis, and innate immunity. Methods We clinically evaluated seven children from five families who had neutropenia, neutrophil dysfunction, bone marrow fibrosis, and nephromegaly. To identify the causative gene, we performed homozygosity mapping using single-nucleotide polymorphism arrays, whole-exome sequencing, immunoblotting, immunofluorescence, electron microscopy, a real-time quantitative polymerase–chain-reaction assay, immunohistochemistry, flow cytometry, fibroblast motility assays, measurements of apoptosis, and zebrafish models. Correction experiments were performed by transfecting mutant fibroblasts with the nonmutated gene. Results All seven affected children had homozygous mutations (Thr224Asn or Glu238Lys, depending on the child's ethnic origin) in VPS45, which encodes a protein that regulates membrane trafficking through the endosomal system. The level of VPS45 protein was reduced, as were the VPS45 binding partners rabenosyn-5 and syntaxin-16. The level of β1 integrin was reduced on the surface of VPS45-deficient neutrophils and fibroblasts. VPS45-deficient fibroblasts were characterized by impaired motility and increased apoptosis. A zebrafish model of vps45 deficiency showed a marked paucity of myeloperoxidase-positive cells (i.e., neutrophils). Transfection of patient cells with nonmutated VPS45 corrected the migration defect and decreased apoptosis. Conclusions Defective endosomal intracellular protein trafficking due to biallelic mutations in VPS45 underlies a new immunodeficiency syndrome involving impaired neutrophil function. (Funded by the National Human Genome Research Institute and others.)