The molecular complexes involved in the nonhomologous end-joining process that resolves recombinationactivating gene (RAG)-induced double-strand breaks and results in V(D)J gene rearrangements vary during mammalian ontogeny. In the mouse, the first immunoglobulin gene rearrangements emerge during midgestation periods, but their repertoires have not been analyzed in detail. We decided to study the postgastrulation DJ H joints and compare them with those present in later life. The embryo DJ H joints differed from those observed in perinatal life by the presence of short stretches of nontemplated (N) nucleotides. Whereas most adult N nucleotides are introduced by terminal deoxynucleotidyl transferase (TdT), the embryo N nucleotides were due to the activity of the homologous DNA polymerase (Pol), which was widely expressed in the early ontogeny, as shown by analysis of Pol ؊/؊ embryos. Based on its DNA-dependent polymerization ability, which TdT lacks, Pol also filled in small sequence gaps at the coding ends and contributed to the ligation of highly processed ends, frequently found in the embryo, by pairing to internal microhomology sites. These findings show that Pol participates in the repair of early-embryo, RAG-induced double-strand breaks and subsequently may contribute to preserve the genomic stability and cellular homeostasis of lymphohematopoietic precursors during development.The adaptive immune system is characterized by the great diversity of its antigen receptors, which result from the activities of enzymatic complexes that cut and paste the genomic DNA of antigen receptor loci. The nonhomologous end-joining (NHEJ) machinery is then recruited to repair the doublestrand DNA breaks (DSBs) inflicted by the products of the recombination-activating genes (RAGs) (45,65). Within B cells, each immunoglobulin (Ig) receptor represents a singular shuffling of two heavy (H) and two light (L) chains, which are derived from the recombination of V, D, and J gene segments of the IgH locus and of V and J for IgL (71). Besides these combinatorial possibilities, most Ig variability derives from extensive processing of the coding ends, including exonucleolytic trimming of DNA ends, together with the addition of palindromic (P) nucleotides templated by the adjacent germ line sequence and of nontemplated (N) nucleotides secondary to the activity of the terminal deoxynucleotidyl transferase (TdT), a lymphoid-specific member of family X of DNA polymerases (reviewed in reference 56). During B-lineage differentiation, IgH rearrangements occur before those of the IgL locus, and D-to-J H rearrangements precede V-to-DJ H rearrangements (62). DJ H joints are formed in any of the three open reading frames (ORFs). ORF1 is predominantly used in mature Igs, ORF2 is transcribed as a D protein that provides negative signals to the B-cell precursors, and ORF3 frequently leads to stop codons (32,33,37). Germ line V, D, and J gene segments display short stretches of mutually homologous nucleotides (SSH), which are frequently used in gene rea...
Ab responses to selected Ags are produced by discrete B cell populations whose presence and functional relevance vary along the ontogeny. The earliest B lineage-restricted precursors in gestational day 11 mouse embryos display the CD19+CD45R/B220− phenotype. Phenotypically identical cells persist throughout gestation and in postnatal life, in parallel to the later-arising, CD19+CD45R+ B cells. Very early after birth, the CD19+CD45R− B cell subset included high frequencies of spontaneously Ig-secreting cells. In the adult spleen, a small subset of CD19highCD45R−/lowIgM+/−IgD−CD21/Cr2−/low cells, which was detected in perifollicular areas, displayed genetic and phenotypical traits of highly differentiated B cells, and was enriched in IgG- and IgA-secreting plasma cells. In vitro differentiation and in vivo adoptive transfer experiments of multipotent hemopoietic progenitors revealed that these CD19highCD45R−/low B cells were preferentially regenerated by embryo-, but not by adult bone marrow-, derived progenitors, except when the latter were inoculated into newborn mice. Both the early ontogenical emergence and the natural production of serum Igs, are shared features of this CD19highCD45R−/low B cell population with innate-like B lymphocytes such as B1 and marginal zone B cells, and suggest that the new population might be related to that category.
In the adult spleen, CD19+CD45R−/lo (19+45Rlo) lymphocytes of embryonic origin exist as a distinct population to that of the conventional B cell lineage. These cells display a plasmablast phenotype, and they spontaneously secrete IgG1 and IgA, whereas the bone marrow population of 19+45Rlo cells contains B1 progenitors. In this study, we show that 19+45Rlo cells are also present in Peyer’s patches and in the spleen throughout the life span of wild-type mice, beginning at postnatal day 7. Although this population is heterogeneous, the surface phenotype of most of these cells distinguishes them from follicular, transitional, marginal zone, and B1 cells. In CBA/CaHN mice, few 19+45Rlo cells were detected at postnatal day 7, and none was observed in the adult spleen. Splenic 19+45Rlo cells exhibited homeostatic BrdU uptake in vivo and actively transcribed cell cycle genes. When transferred to immunodeficient RAG2−/−γchain−/− recipient mice, 19+45Rlo cells survived and differentiated into IgG1– and IgA–plasma cells. Moreover, in vitro stimulation of splenic 19+45Rlo cells with LPS, CpG, BAFF/IL4, and CD40/IL4 induced cell proliferation, IgG1/IgA secretion and the release of IL-10, suggesting a potential immunoregulatory role for this subset of innate-like B cells.
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