B-Lymphopoiesis in Fetal Liver, Guided by Chemokines

[Why the regeneration of immunological tolerance by vaccination is difficult]

Autoimmunity, including that involved in chronic inflammatory rheumatic diseases, seems to be the price we have to pay for our efficient immune system. It has the ability to precisely recognize pathogens and tumor cells, to efficiently fight them, to adapt to their alterations and provide specific immunity for a lifetime. "Inoculation", and more specifically "vaccination" takes advantage of this, either by transfer of protective antibodies (passive vaccination) or by using attenuated pathogens or parts of them by which a specific protective immunity is induced (active vaccination). The idea to use vaccination to reduce undesired (auto)immunity and chronic inflammation is nothing new in rheumatology. Many biologicals are antibodies, which specifically block the mediators of inflammation and in the broader sense are similar to a passive vaccination. The active vaccination with autoantigens using the recent mRNA/liposome technology, has shown in experimental animal models that they can prevent the formation of chronic inflammatory immune reactions, in that they strengthen the physiological tolerance and deviate the immune system to noninflammatory immune reactions against the antigen; however, there is still a long way to go to achieve the actual goals of a permanent suppression of established undesired immune reactions and the regeneration of immunological tolerance.

Disruption of cholangiocyte-B cell crosstalk by blocking the CXCL12-CXCR4 axis alleviates liver fibrosis

B cells can promote liver fibrosis, but the mechanism of B cell infiltration and therapy against culprit B cells are lacking. We postulated that the disruption of cholangiocyte-B-cell crosstalk could attenuate liver fibrosis by blocking the CXCL12-CXCR4 axis via a cyclooxygenase-2-independent effect of celecoxib. In wild-type mice subjected to thioacetamide, celecoxib ameliorated lymphocytic infiltration and liver fibrosis. By single-cell RNA sequencing and flow cytometry, CXCR4 was established as a marker for profibrotic and liver-homing phenotype of B cells. Celecoxib reduced liver-homing B cells without suppressing CXCR4. Cholangiocytes expressed CXCL12, attracting B cells to fibrotic areas in human and mouse. The proliferation and CXCL12 expression of cholangiocytes were suppressed by celecoxib. In CXCL12-deficient mice, liver fibrosis was also attenuated with less B-cell infiltration. In the intrahepatic biliary epithelial cell line HIBEpiC, bulk RNA sequencing indicated that both celecoxib and 2,5-dimethyl-celecoxib (an analog of celecoxib that does not show a COX-2-dependent effect) regulated the TGF-β signaling pathway and cell cycle. Moreover, celecoxib and 2,5-dimethyl-celecoxib decreased the proliferation, and expression of collagen I and CXCL12 in HIBEpiC cells stimulated by TGF-β or EGF. Taken together, liver fibrosis can be ameliorated by disrupting cholangiocyte-B cell crosstalk by blocking the CXCL12-CXCR4 axis with a COX-2-independent effect of celecoxib.

Role of Paralogue of XRCC4 and XLF in DNA Damage Repair and Cancer Development

Non-homologous end joining (cNHEJ) is a major pathway to repair double-strand breaks (DSBs) in DNA. Several core cNHEJ are involved in the progress of the repair such as KU70 and 80, DNA-dependent protein kinase catalytic subunit (DNA-PKcs), Artemis, X-ray repair cross-complementing protein 4 (XRCC4), DNA ligase IV, and XRCC4-like factor (XLF). Recent studies have added a number of new proteins during cNHEJ. One of the newly identified proteins is Paralogue of XRCC4 and XLF (PAXX), which acts as a scaffold that is required to stabilize the KU70/80 heterodimer at DSBs sites and promotes the assembly and/or stability of the cNHEJ machinery. PAXX plays an essential role in lymphocyte development in XLF-deficient background, while XLF/PAXX double-deficient mouse embryo died before birth. Emerging evidence also shows a connection between the expression levels of PAXX and cancer development in human patients, indicating a prognosis role of the protein. This review will summarize and discuss the function of PAXX in DSBs repair and its potential role in cancer development.

An XRCC4 mutant mouse, a model for human X4 syndrome, reveals interplays with Xlf, PAXX, and ATM in lymphoid development

We developed an Xrcc4M61R separation of function mouse line to overcome the embryonic lethality of Xrcc4-deficient mice. XRCC4M61R protein does not interact with Xlf, thus obliterating XRCC4-Xlf filament formation while preserving the ability to stabilize DNA ligase IV. X4M61R mice, which are DNA repair deficient, phenocopy the Nhej1-/- (known as Xlf -/-) setting with a minor impact on the development of the adaptive immune system. The core non-homologous end-joining (NHEJ) DNA repair factor XRCC4 is therefore not mandatory for V(D)J recombination aside from its role in stabilizing DNA ligase IV. In contrast, Xrcc4M61R mice crossed on Paxx-/-, Nhej1-/-, or Atm-/- backgrounds are severely immunocompromised, owing to aborted V(D)J recombination as in Xlf-Paxx and Xlf-Atm double Knock Out (DKO) settings. Furthermore, massive apoptosis of post-mitotic neurons causes embryonic lethality of Xrcc4M61R -Nhej1-/- double mutants. These in vivo results reveal new functional interplays between XRCC4 and PAXX, ATM and Xlf in mouse development and provide new insights into the understanding of the clinical manifestations of human XRCC4-deficient condition, in particular its absence of immune deficiency.

Rap1 Is Essential for B-Cell Locomotion, Germinal Center Formation and Normal B-1a Cell Population

Integrin regulation by Rap1 is indispensable for lymphocyte recirculation. In mice having B-cell-specific Rap1a/b double knockouts (DKO), the number of B cells in lymph nodes decreased to approximately 4% of that of control mice, and B cells were present in the spleen and blood. Upon the immunization with NP-CGG, DKO mice demonstrated the defective GC formation in the spleen, and the reduced NP-specific antibody production. In vitro, Rap1 deficiency impaired the movement of activated B cells along the gradients of chemoattractants known to be critical for their localization in the follicles. Furthermore, B-1a cells were almost completely absent in the peritoneal cavity, spleen and blood of adult DKO mice, and the number of B-cell progenitor/precursor (B-p) were reduced in neonatal and fetal livers. However, DKO B-ps normally proliferated, and differentiated into IgM+ cells in the presence of IL-7. CXCL12-dependent migration of B-ps on the VCAM-1 was severely impaired by Rap1 deficiency. Immunostaining study of fetal livers revealed defects in the co-localization of DKO B-ps and IL-7-producing stromal cells. This study proposes that the profound effects of Rap1-deficiency on humoral responses and B-1a cell generation may be due to or in part caused by impairments of the chemoattractant-dependent positioning and the contact with stromal cells.

The changing mouse embryo transcriptome at whole tissue and single-cell resolution

During mammalian embryogenesis, differential gene expression gradually builds the identity and complexity of each tissue and organ system1. Here we systematically quantified mouse polyA-RNA from day 10.5 of embryonic development to birth, sampling 17 tissues and organs. The resulting developmental transcriptome is globally structured by dynamic cytodifferentiation, body-axis and cell-proliferation gene sets that were further characterized by the transcription factor motif codes of their promoters. We decomposed the tissue-level transcriptome using single-cell RNA-seq (sequencing of RNA reverse transcribed into cDNA) and found that neurogenesis and haematopoiesis dominate at both the gene and cellular levels, jointly accounting for one-third of differential gene expression and more than 40% of identified cell types. By integrating promoter sequence motifs with companion ENCODE epigenomic profiles, we identified a prominent promoter de-repression mechanism in neuronal expression clusters that was attributable to known and novel repressors. Focusing on the developing limb, single-cell RNA data identified 25 candidate cell types that included progenitor and differentiating states with computationally inferred lineage relationships. We extracted cell-type transcription factor networks and complementary sets of candidate enhancer elements by using single-cell RNA-seq to decompose integrative cis-element (IDEAS) models that were derived from whole-tissue epigenome chromatin data. These ENCODE reference data, computed network components and IDEAS chromatin segmentations are companion resources to the matching epigenomic developmental matrix, and are available for researchers to further mine and integrate.

Gαi Signaling Promotes Marginal Zone B Cell Development by Enabling Transitional B Cell ADAM10 Expression

The follicular (FO) versus marginal zone (MZ) B cell fate decision in the spleen depends upon BCR, BAFF, and Notch2 signaling. Whether or how G_i signaling affects this fate decision is unknown. Here, we show that direct contact with Notch ligand expressing stromal cells (OP9-Delta-like 1) cannot promote normal MZ B cell development when progenitor B cells lack Gα_i proteins, or if Gi signaling is disabled. Consistent with faulty ADAM10-dependent Notch2 processing, Gα_i-deficient transitional B cells had low ADAM10 membrane expression levels and reduced Notch2 target gene expression. Immunoblotting Gα_i-deficient B cell lysates revealed a reduction in mature, processed ADAM10. Suggesting that Gα_i signaling promotes ADAM10 membrane expression, stimulating normal transitional B cells with CXCL12 raised it, while inhibiting Gα_i nucleotide exchange blocked its upregulation. Surprisingly, inhibiting Gα_i nucleotide exchange in transitional B cells also impaired the upregulation of ADAM10 that occurs following antigen receptor crosslinking. These results indicate that Gα_i signaling supports ADAM10 maturation and activity in transitional B cells, and ultimately Notch2 signaling to promote MZ B cell development.

PAXX and Xlf interplay revealed by impaired CNS development and immunodeficiency of double KO mice

The repair of DNA double-stranded breaks (DNAdsb) through non-homologous end joining (NHEJ) is a prerequisite for the proper development of the central nervous system and the adaptive immune system. Yet, mice with Xlf or PAXX loss of function are viable and present with very mild immune phenotypes, although their lymphoid cells are sensitive to ionizing radiation attesting for the role of these factors in NHEJ. In contrast, we show here that mice defective for both Xlf and PAXX are embryonically lethal owing to a massive apoptosis of post-mitotic neurons, a situation reminiscent to XRCC4 or DNA Ligase IV KO conditions. The development of the adaptive immune system in Xlf^-/-PAXX^-/- E18.5 embryos is severely affected with the block of B- and T-cell maturation at the stage of IgH and TCRβ gene rearrangements, respectively. This damaging phenotype highlights the functional nexus between Xlf and PAXX, which is critical for the completion of NHEJ-dependent mechanisms during mouse development.