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The pioneer transcription factor (TF) PU.1 controls hematopoietic cell fate by decompacting stem cell heterochromatin and allowing nonpioneer TFs to enter otherwise inaccessible genomic sites. PU.1 deficiency fatally arrests lymphopoiesis and myelopoiesis in mice, but human congenital PU.1 disorders have not previously been described. We studied six unrelated agammaglobulinemic patients, each harboring a heterozygous mutation (four de novo, two unphased) of SPI1, the gene encoding PU.1. Affected patients lacked circulating B cells and possessed few conventional dendritic cells. Introducing disease-similar SPI1 mutations into human hematopoietic stem and progenitor cells impaired early in vitro B cell and myeloid cell differentiation. Patient SPI1 mutations encoded destabilized PU.1 proteins unable to nuclear localize or bind target DNA. In PU.1-haploinsufficient pro-B cell lines, euchromatin was less accessible to nonpioneer TFs critical for B cell development, and gene expression patterns associated with the pro- to pre-B cell transition were undermined. Our findings molecularly describe a novel form of agammaglobulinemia and underscore PU.1's critical, dose-dependent role as a hematopoietic euchromatin gatekeeper., Competing Interests: Disclosures: D. Nguyen reported a patent to PCT/US2019/066079 with royalties paid. T. Olson reported personal fees from Bluebird Bio outside of the submitted work. J. Puck reported other from Invitae (spouse's employer) and other from UpToDate (royalties) outside the submitted work. J. Hajjar reported grants from Immune Deficiency Foundation, the US immunodeficiency network, Chao-physician Scientist award, the Texas Medical Center Digestive Diseases Center, and the Jeffrey Modell Foundation, other from Horizon, Pharming, Baxalta, CSL Behring, and the National Guard and Al-Faisal University Hospital outside the submitted work. J. Lupski reported grants from NIH/NINDS (R35 NS105078), and NIH/NIGMS (R01 GM106373), personal fees from Regeneron Genetics Center and Novartis, and other from 23andMe outside the submitted work. A. Marson reported personal fees from Arsenal Biosciences, Spotlight Therapeutics, PACT Pharma, Merck, Vertex, AlphaSights, ALDA, Amgen, Trizell, Juno Therapeutics, Health Advances, Lonza, Bernstein, Abbvie, Genentech, Illumina, Arcus, Jackson Laboratories, NanoString Technologies, GLG, and Rupert Case Management, grants from Anthem, Gilead, GlaxoSmithKline, Juno Therapeutics, Epinomics, Sanofi, and Parker Institute for Cancer Immunotherapy (PICI), non-financial support from Illumina, other from Parker Institute for Cancer Immunotherapy (PICI), ThermoFisher, and Third Rock Ventures outside the submitted work. In addition, A. Marson had a patent to WO 2016/123578 licensed (The identity of the licensee has not been made public) and a patent to PCT/US19/66079 licensed (The identity of the licensee has not been made public); and is an investor in and informal advisor to Offline Ventures. No other disclosures were reported., (© 2021 Le Coz et al.)

L-selectin ligands might be relevant for inflammatory cell trafficking into the small intestine in a spontaneous model of chronic ileitis (i.e., SAMP1/YitFc mice). Immunoblockade of peripheral node addressin or mucosal addressin cell adhesion molecule 1 failed to ameliorate ileitis, whereas P-selectin glycoprotein ligand 1 (PSGL-1) neutralization attenuated both the adoptively transferred and spontaneous disease. PSGL-1 was detected in venules of mesenteric lymph node and small intestine by immunohistochemistry and confirmed by real-time reverse transcription polymerase chain reaction and flow cytometry. In addition, reconstitution of wild-type mice with PSGL-1(-/-) bone marrow demonstrated that PSGL-1 messenger RNA and PSGL-1 protein expression remained on endothelium, localized within mesenteric lymph node and small intestine. Endothelial PSGL-1 bound P-selectin-IgG and its blockade or genetic deletion altered the recruitment of lymphocytes to the small intestine, as revealed by intravital microscopy and homing studies. Endothelial expression of PSGL-1 adds a new dimension to the various cellular interactions involved in small intestinal recruitment. Thus, the multiple roles of PSGL-1 may explain why targeting this single adhesion molecule results in attenuation of chronic murine ileitis, a disease previously resistant to antiadhesion molecule strategies.

The initiating etiologic factor in Crohn's disease (CD) remains unclear. SAMP1/YitFc (SAMP) mice develop chronic ileitis similar to human CD. We used bone marrow chimeras to determine if SAMP ileitis results from a primary immunological defect or from dysregulated mucosal immunity secondary to intrinsic, nonhematopoietic (e.g., epithelial) dysfunction. SAMP mice receiving wild-type (AKR) BM developed severe ileitis, whereas SAMP BM did not confer ileitis to WT recipients. WT lymphocytes from reconstituted SAMP mice resembled native SAMP populations in regard to surface phenotype and cytokine production. Ilea from native SAMP mice and SAMP recipients of wild-type BM displayed decreased epithelial barrier resistance ex vivo and increased epithelial permeability in vivo compared to native WT mice and AKR recipients of SAMP BM. This permeability defect preceded the development of ileal inflammation, was present in the absence of commensal bacteria, and was accompanied by altered ileal mRNA expression of the tight junction proteins claudin-2 and occludin. Our results provide evidence that the primary defect conferring ileitis in SAMP mice originates from a nonhematopoietic source. Generation of pathogenic lymphocytes is a consequence of this defect and does not reflect intrinsic proinflammatory leukocyte properties. Decreased barrier function suggests that defects in the epithelium may represent the primary source of SAMP ileitis susceptibility.

The signaling events leading to the activation of integrins and firm arrest of rolling neutrophils in inflamed venules have yet to be elucidated. In vitro assays suggest that both E-selectin and chemokines can trigger arrest of rolling neutrophils, but E-selectin(-/-) mice have normal levels of adherent neutrophils in inflamed venules. To test whether chemokine-induced neutrophil arrest in vivo can be unmasked by blocking E-selectin, we investigated neutrophil adhesion in inflamed cremaster muscle venules in tumor necrosis factor (TNF)-alpha-treated CXCR2(-/-) or wild-type (WT) mice injected with E-selectin blocking monoclonal antibody (mAb) 9A9. To block chemokine receptor signaling, we investigated E-selectin(-/-) or WT mice treated with pertussis toxin (PTx) intravenously. Neutrophil adhesion was unchanged in CXCR2(-/-), E-selectin(-/-), PTx-treated WT, or mAb 9A9-treated WT mice. However, TNF-alpha-induced neutrophil adhesion was almost completely abrogated in E-selectin(-/-) mice treated with PTx and significantly reduced in CXCR2(-/-) mice treated with the E-selectin blocking mAb. In thioglycollate-induced peritonitis, PTx treatment blocked neutrophil recruitment into the peritoneum of E-selectin(-/-) mice, but had only a partial effect in WT animals. These data show that E-selectin- and chemokine-mediated arrest mechanisms are overlapping in this model and identify CXCR2 as an important neutrophil arrest chemokine in vivo.

Leukocyte rolling in postcapillary venules of inflamed tissues is reduced in L-selectin-deficient mice and mice treated with L-selectin blocking antibodies, but the glycoprotein ligand for L-selectin in inflamed venules is unknown. Here, we show that L-selectin-dependent rolling after P-selectin blockade is completely absent in P-selectin glycoprotein ligand-1 (PSGL-1)-/- mice or wild-type mice treated with a PSGL-1 blocking monoclonal antibody. Immunohistochemistry and flow cytometry failed to show PSGL-1 expression on resting or inflamed endothelium or on platelets. To investigate whether leukocyte-expressed PSGL-1 is mediating L-selectin-dependent rolling, we reconstituted lethally irradiated wild-type mice with PSGL-1-/- bone marrow cells. These chimeric mice showed no L-selectin-dependent rolling, suggesting that leukocyte-expressed PSGL-1 mediates L-selectin-dependent rolling. Frame-to-frame video analysis of L-selectin-dependent rolling in wild-type mice showed that the majority of observed L-selectin-dependent leukocyte rolling was between free flowing leukocytes and already adherent leukocytes or possibly leukocyte fragments, followed by E-selectin-dependent leukocyte rolling along the endothelium. Leukocyte rolling was significantly slower for leukocyte-endothelial than leukocyte-leukocyte interactions. We conclude that leukocyte-expressed PSGL-1 serves as the main L-selectin ligand in inflamed postcapillary venules. L-selectin binding to PSGL-1 initiates tethering events that enable L-selectin-independent leukocyte-endothelial interactions. These findings provide a molecular mechanism for the inflammatory defects seen in L-selectin-deficient mice.