Inflammatory macrophages exploit unconventional pro-phagocytic integrins for phagocytosis and anti-tumor immunity.

Blockade of the inhibitory checkpoint SIRPα-CD47 promotes phagocytosis of cancer cells by macrophages and is a promising avenue in anti-cancer therapy. Productive phagocytosis is strictly predicated on co-engagement of pro-phagocytic receptors-namely, Fc receptors (FcRs), integrin CD11b, or SLAMF7-by their ligands on cancer cells. Here, we examine whether additional pro-phagocytic receptors could be harnessed to broaden the scope of phagocytosis. Inflammatory stimuli, including multiple cytokines and Toll-like receptor (TLR) ligands, augment phagocytosis efficiency and fully alleviate the requirement of FcRs, CD11b, and SLAMF7 for phagocytosis. These effects are mediated by the unconventional pro-phagocytic integrins CD11a and CD11c, which act with CD18 to initiate actin polarization, leading to phagocytosis. Some inflammatory stimuli enable phagocytosis even in the absence of SIRPα-CD47 blockade. Higher CD11c expression in macrophage-enriched tumors correlates with improved survival in clinical studies. Thus, inflammatory macrophages exploit unconventional pro-phagocytic integrins for improved phagocytosis and anti-tumor immunity.

Epigenetic dysregulation of ACE2 and interferon-regulated genes might suggest increased COVID-19 susceptibility and severity in lupus patients.

Infection caused by SARS-CoV-2 can result in severe respiratory complications and death. Patients with a compromised immune system are expected to be more susceptible to a severe disease course. In this report we suggest that patients with systemic lupus erythematous might be especially prone to severe COVID-19 independent of their immunosuppressed state from lupus treatment. Specifically, we provide evidence in lupus to suggest hypomethylation and overexpression of ACE2, which is located on the X chromosome and encodes a functional receptor for the SARS-CoV-2 spike glycoprotein. Oxidative stress induced by viral infections exacerbates the DNA methylation defect in lupus, possibly resulting in further ACE2 hypomethylation and enhanced viremia. In addition, demethylation of interferon-regulated genes, NFκB, and key cytokine genes in lupus patients might exacerbate the immune response to SARS-CoV-2 and increase the likelihood of cytokine storm. These arguments suggest that inherent epigenetic dysregulation in lupus might facilitate viral entry, viremia, and an excessive immune response to SARS-CoV-2. Further, maintaining disease remission in lupus patients is critical to prevent a vicious cycle of demethylation and increased oxidative stress, which will exacerbate susceptibility to SARS-CoV-2 infection during the current pandemic. Epigenetic control of the ACE2 gene might be a target for prevention and therapy in COVID-19.

Truncation of CXCL8 to CXCL8(9-77) enhances actin polymerization and in vivo migration of neutrophils.

CXCL8 is the principal human neutrophil-attracting chemokine and a major mediator of inflammation. The chemokine exerts its neutrophil-chemotactic and neutrophil-activating activities via interaction with glycosaminoglycans (GAGs) and activation of the G protein-coupled receptors (GPCRs) CXCR1 and CXCR2. Natural CXCL8 displays an exceptional degree of amino (NH2 )-terminal heterogeneity. Most CXCL8 forms result from proteolytic processing of authentic CXCL8(1-77). Here, we compared the potencies to activate and recruit neutrophils of the 3 most abundant natural CXCL8 forms: full-length 77 amino acid CXCL8 and the 2 major natural truncated forms lacking 5 or 8 NH2 -terminal amino acids. NH2 -terminal truncation hardly affected the capacity of CXCL8 to induce shedding of CD62L or to up-regulate the expression of the adhesion molecules CD11a, CD11b, or CD15 on human neutrophils. In addition, the potency of CXCL8 to induce neutrophil degranulation and its effect on phagocytosis remained unaltered upon removal of 5 or 8 NH2 -terminal residues. However, NH2 -terminal truncation strongly potentiated CXCL8-induced actin polymerization. CXCL8(6-77) and CXCL8(9-77) showed a comparable capacity to induce Ca2+ signaling in human neutrophils and to direct in vitro neutrophil migration. Strikingly, the ability of CXCL8(9-77) to recruit neutrophils into the peritoneal cavity of mice was significantly enhanced compared to CXCL8(6-77). These results suggest that NH2 -terminal truncation influences specific biological activities of CXCL8 and indicate that CXCL8(9-77) may be the most potent neutrophil-attracting CXCL8 form in vivo.

Intestinal Epithelial Cells Express Immunomodulatory ISG15 During Active Ulcerative Colitis and Crohn's Disease.

BACKGROUND AND AIMS: Intestinal epithelial cells [IECs] secrete cytokines that recruit immune cells to the mucosa and regulate immune responses that drive inflammation in inflammatory bowel disease [IBD]. However, experiments in patient-derived IEC models are still scarce. Here, we aimed to investigate how innate immunity and IEC-specific pattern recognition receptor [PRR] signalling can be involved in an enhanced type I interferon [IFN] gene signature observed in colon epithelium of patients with active IBD, with a special focus on secreted ubiquitin-like protein ISG15. METHODS: Gene and protein expression in whole mucosa biopsies and in microdissected human colonic epithelial lining, in HT29 human intestinal epithelial cells and primary 3D colonoids treated with PRR-ligands and cytokines, were detected by transcriptomics, in situ hybridisation, immunohistochemistry, western blots, and enzyme-linked immunosorbent assay [ELISA]. Effects of IEC-secreted cytokines were examined in human peripheral blood mononuclear cells [PBMCs] by multiplex chemokine profiling and ELISA. RESULTS: The type I IFN gene signature in human mucosal biopsies was mimicked in Toll-like receptor TLR3 and to some extent tumour necrosis factor [TNF]-treated human IECs. In intestinal biopsies, ISG15 expression correlated with expression of the newly identified receptor for extracellular ISG15, LFA-1 integrin. ISG15 was expressed and secreted from HT29 cells and primary 3D colonoids through both JAK1-pSTAT-IRF9-dependent and independent pathways. In experiments using PBMCs, we show that ISG15 releases IBD-relevant proinflammatory cytokines such as CXCL1, CXCL5, CXCL8, CCL20, IL1, IL6, TNF, and IFNγ. CONCLUSIONS: ISG15 is secreted from primary IECs upon extracellular stimulation, and mucosal ISG15 emerges as an intriguing candidate for immunotherapy in IBD.

miR-34a as hub of T cell regulation networks.

BACKGROUND: Micro(mi)RNAs are increasingly recognized as central regulators of immune cell function. While it has been predicted that miRNAs have multiple targets, the majority of these predictions still await experimental confirmation. Here, miR-34a, a well-known tumor suppressor, is analyzed for targeting genes involved in immune system processes of leucocytes. METHODS: Using an in-silico approach, we combined miRNA target prediction with GeneTrail2, a web tool for Multi-omics enrichment analysis, to identify miR-34a target genes, which are involved in the immune system process subcategory of Gene Ontology. RESULTS: Out of the 193 predicted target genes in this subcategory we experimentally tested 22 target genes and confirmed binding of miR-34a to 14 target genes including VAMP2, IKBKE, MYH9, MARCH8, KLRK1, CD11A, TRAFD1, CCR1, PYDC1, PRF1, PIK3R2, PIK3CD, AP1B1, and ADAM10 by dual luciferase assays. By transfecting Jurkat, primary CD4+ and CD8+ T cells with miR-34a, we demonstrated that ectopic expression of miR-34a leads to reduced levels of endogenous VAMP2 and CD11A, which are central to the analyzed subcategories. Functional downstream analysis of miR-34a over-expression in activated CD8+ T cells exhibits a distinct decrease of PRF1 secretion. CONCLUSIONS: By simultaneous targeting of 14 mRNAs miR-34a acts as major hub of T cell regulatory networks suggesting to utilize miR-34a as target of intervention towards a modulation of the immune responsiveness of T-cells in a broad tumor context.

The down-regulation of hsa_circ_0012919, the sponge for miR-125a-3p, contributes to DNA methylation of CD11a and CD70 in CD4+ T cells of systemic lupus erythematous.

BACKGROUND: Systemic lupus erythematous (SLE) is an autoimmune disease characterized by the production of autoantibodies directed against various autoantigens. But the expression profiles and functions of circular RNAs (circRNAs) in SLE are still scarce. OBJECTIVES: To explore the roles of circRNA in SLE and its potential diagnostic potential in SLE. METHODS: SLE patients and healthy control subjects were recruited. CD4+ T cells were isolated, circRNA microarray analysis were used to screen for circRNA candidate in CD4+ T cells. Expression of DNMT1, CD11a and CD70, and methylation level of CD11a and CD70 were detected after transfecting hsa_circ_0012919-targetted siRNA. The network analysis of hsa_circ_0012919 was used by bioinformatics. Luciferase reporter assay and fluorescence in situ hybridization (FISH) assay were used for screening for which miRNAs could bind with hsa_circ_0012919. RESULTS: Twelve circRNAs were up-regulated and two circRNAs were down-regulated in SLE patients group after circRNA microarray analysis. Hsa_circ_0012919 was further confirmed to be significantly different between healthy control and SLE patients (P<0.05) and associated with SLE characters (P<0.05). Down-regulation of hsa_circ_0012919 (i) increased the expression of DNMT1 and reduced the expression of CD70, CD11a, (ii) reversed the DNA hypomethylation of CD11a and CD70 in CD4+ T cells of SLE, but it could be reversed by down-regulation of DNMT1. Hsa_circ_0012919 regulated KLF13 and RANTES by miR-125a Conclusion: Hsa_circ_0012919 could be regarded as a biomarker for SLE and hsa_circ_0012919 was the competitive endogenous RNA (ceRNA) for miR-125a-3p.

Extracellular ISG15 Signals Cytokine Secretion through the LFA-1 Integrin Receptor.

ISG15 is a ubiquitin-like protein that functions in innate immunity both as an intracellular protein modifier and as an extracellular signaling molecule that stimulates IFN-γ secretion. The extracellular function, important for resistance to mycobacterial disease, has remained biochemically uncharacterized. We have established an NK-92 cell-based assay for IFN-γ release, identified residues critical for ISG15 signaling, and identified the cell surface receptor as LFA-1 (CD11a/CD18; αLß2 integrin). LFA-1 inhibition blocked IFN-γ secretion, splenocytes from CD11a-/- mice did not respond to ISG15, and ISG15 bound directly to the αI domain of CD11a in vitro. ISG15 also enhanced secretion of IL-10, indicating a broader role for ISG15 in cytokine signaling. ISG15 engagement of LFA-1 led to the activation of SRC family kinases (SFKs) and SFK inhibition blocked cytokine secretion. These findings establish the molecular basis of the extracellular function of ISG15 and the initial outside-in signaling events that drive ISG15-dependent cytokine secretion.

A novel intracellular pool of LFA-1 is critical for asymmetric CD8+ T cell activation and differentiation.

The integrin lymphocyte function-associated antigen 1 (LFA-1; CD11a/CD18) is a key T cell adhesion receptor that mediates stable interactions with antigen-presenting cell (APC), as well as chemokine-mediated migration. Using our newly generated CD11a-mYFP knock-in mice, we discovered that naive CD8+ T cells reserve a significant intracellular pool of LFA-1 in the uropod during migration. Intracellular LFA-1 quickly translocated to the cell surface with antigenic stimulus. Importantly, the redistribution of intracellular LFA-1 at the contact with APC was maintained during cell division and led to an unequal inheritance of LFA-1 in divided T cells. The daughter CD8+ T cells with disparate LFA-1 expression showed different patterns of migration on ICAM-1, APC interactions, and tissue retention, as well as altered effector functions. In addition, we identified Rab27 as an important regulator of the intracellular LFA-1 translocation. Collectively, our data demonstrate that an intracellular pool of LFA-1 in naive CD8+ T cells plays a key role in T cell activation and differentiation.

Mutation characterization and heterodimer analysis of patients with leukocyte adhesion deficiency: Including one novel mutation.

BACKGROUND AND AIM: Leukocyte adhesion deficiency type 1 (LAD-I) is a rare, autosomal recessive disorder of neutrophil migration, characterized by severe, recurrent bacterial infections, inadequate pus formation and impaired wound healing. The ITGB2 gene encodes the ß2 integrin subunit (CD18) of the leukocyte adhesion cell molecules, and mutations in this gene cause LAD-I. The aim of the current study was to investigate the mutations in patients diagnosed with LAD-I and functional studies of the impact of two previously reported and a novel mutation on the expression of the CD18/CD11a heterodimer. MATERIALS AND METHODS: Blood samples were taken from three patients who had signed the consent form. Genomic DNA was extracted and ITGB2 exons and flanking intronic regions were amplified by polymerase chain reaction. Mutation screening was performed after Sanger sequencing of PCR products. For functional studies, COS-7 cells were co-transfected with an expression vector containing cDNA encoding mutant CD18 proteins and normal CD11a. Flow cytometry analysis of CD18/CD11a expression was assessed by dimer-specific IB4 monoclonal antibody. RESULTS: Two previously reported mutations and one novel mutation,p. Cys562Tyr, were found. All mutations reduced CD18/CD11 heterodimer expression. CONCLUSION: Our strategy recognized the p.Cys562Tyr mutation as a pathogenic alteration that does not support CD18 heterodimer formation. Therefore, it can be put into a panel of carrier and prenatal diagnosis programs.

Histone demethylase JMJD3 regulates CD11a expression through changes in histone H3K27 tri-methylation levels in CD4+ T cells of patients with systemic lupus erythematosus.

Aberrant CD11a overexpression in CD4+ T cells induces T cell auto-reactivity, which is an important factor for systemic lupus erythematosus (SLE) pathogenesis. Although many studies have focused on CD11a epigenetic regulation, little is known about histone methylation. JMJD3, as a histone demethylase, is capable of specifically removing the trimethyl group from the H3K27 lysine residue, triggering target gene activation. Here, we examined the expression and function of JMJD3 in CD4+ T cells from SLE patients. Significantly decreased H3K27me3 levels and increased JMJD3 binding were detected within the ITGAL (CD11a) promoter locus in SLE CD4+ T cells compared with those in healthy CD4+ T cells. Moreover, overexpressing JMJD3 through the transfection of pcDNA3.1-JMJD3 into healthy donor CD4+ T cells increased JMJD3 enrichment and decreased H3K27me3 enrichment within the ITGAL (CD11a) promoter and up-regulated CD11a expression, leading to T and B cell hyperactivity. Inhibition of JMJD3 via JMJD3-siRNA in SLE CD4+ T cells showed the opposite effects. These results demonstrated that histone demethylase JMJD3 regulates CD11a expression in lupus T cells by affecting the H3K27me3 levels in the ITGAL (CD11a) promoter region, and JMJD3 might thereby serve as a potential therapeutic target for SLE.

Identification of Stage-Specific Surface Markers in Early B Cell Development Provides Novel Tools for Identification of Progenitor Populations.

Whereas the characterization of B lymphoid progenitors has been facilitated by the identification of lineage- and stage-specific surface markers, the continued identification of differentially expressed proteins increases our capacity to explore normal and malignant B cell development. To identify novel surface markers with stage-specific expression patterns, we explored the reactivity of CD19(+) B cell progenitor cells to Abs targeted to 176 surface proteins. Markers with stage-specific expression were identified using a transgenic reporter gene system subdividing the B cell progenitors into four surface IgM(-) stages. This approach affirmed the utility of known stage-specific markers, as well as identifying additional proteins that selectively marked defined stages of B cell development. Among the stage-specific markers were the cell adhesion proteins CD49E, CD11A, and CD54 that are highly expressed selectively on the most immature progenitors. This work identifies a set of novel stage-specific surface markers that can be used as a complement to the classical staining protocols to explore B lymphocyte development.

Transcription factor RFX1 is ubiquitinated by E3 ligase STUB1 in systemic lupus erythematosus.

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease caused by complex interactions between genes and the environment. The expression level of transcription factor regulatory factor X 1 (RFX1) is reduced in T cells from SLE patients. RFX1 can regulate epigenetic modifications of CD70 and CD11a and plays an important role in the development of SLE. However, the mechanisms that mediate reduction of RFX1 in SLE are unclear. Here, we demonstrate that RFX1 protein expression can be tightly regulated by polyubiquitination-mediated proteosomal degradation via STIP1 homology and U-box containing protein 1 (STUB1). The E3 ligase STUB1 is upregulated in CD4(+)T cells of SLE patients compared to healthy subjects. Overexpression of STUB1 in CD4(+)T cells leads to upregulation of levels of CD70 and CD11a in T cells. The modulation of STUB1 activity may provide a novel therapeutic approach for SLE.

Macrophages Are Involved in Gut Bacterial Translocation and Reversed by Lactobacillus in Experimental Uremia.

BACKGROUND: Uremia causes gut microbiome dysbiosis, which is characterized by a reduction in beneficial bacteria. Intestinal bacterial translocation (BT) contributes to microinflammation in uremia, which is associated with adverse outcomes. Whether macrophages are involved in BT remains unclear. AIMS: We investigated the involvement of macrophages in BT and microinflammation in uremic rats and whether Lactobacillus LB can influence macrophage activity. METHODS: Male Sprague-Dawley rats were randomly divided into three groups: sham, uremia, and uremia + probiotic. Macrophages and GFP-labeled tracer bacteria in intestinal and extraintestinal tissues were observed by fluorescence microscopy. The macrophage ultrastructure was examined by transmission electron microscopy. Immunochemistry was used to analyze the expression of cluster of differentiation 11a (CD11a), inducible nitric oxide synthase (iNOS), and intercellular adhesion molecule-1 (ICAM-1). RT-PCR and Western blot were employed to assess the mRNA and protein expression of early growth response gene 1 (EGR1) and toll-like receptor 4 (TLR4). RESULTS: In uremic rats, the colocalization of GFP-labeled tracer bacteria and macrophages was visible in intestinal and extraintestinal tissues. Compared with the sham group, the uremic macrophages showed fewer cytoplasmic protrusions and pseudopodia. Administration of Lactobacillus LB restored the protrusions and pseudopodia. Compared with the sham group, the uremia group exhibited macrophages with higher staining intensities for CD11a, iNOS, and ICAM-1, and higher mRNA and protein expression of TLR4 and EGR1. CONCLUSIONS: Intestinal macrophages in the uremic rats are polarized toward a proinflammatory phenotype, resulting in microinflammation. Macrophages with impaired phagocytic function are associated with BT. Lactobacillus LB reduces BT by enhancing macrophage phagocytosis.

Correlation between miR-126 expression and DNA hypomethylation of CD4+ T cells in rheumatoid arthritis patients.

It has been known that the occurrence of rheumatoid arthritis (RA) was closely correlated with DNA hypomethylation in CD4+ T cells, in which DNA methyltransferase plays a certain role. This study therefore investigated the effect of miR-126 on CD4+ T cell subgroup in RA patients and the alternation of DNA hypomethylation, in an attempt to provide new sights into the pathogenesis and treatment of RA. CD4+ T cells separated from RA patients were transfected with miRNA (miR)-126 expression vector or miR-126 inhibitor expression vector. The expression levels of CD11a, CD70 and DNMT1 mRNA were examined by real-time PCR. Protein levels of CD11a and CD70 were tested by flow cytometry while DNMT1 protein level was quantified by Western blotting. DNA was modified by sodium bisulfite and was sequenced for the methylation status of promoters of CD11a and CD70 genes. Both mRNA and protein expressions of CD11a and CD70 genes in CD4+ T cells were elevated by miR-126 transfection, along with decreased DNMT1 protein level but not mRNA level. The methylation degree of promoters of both CD11a and CD70 genes were significantly depressed after miR-126 transfection. The transfection by miR-126 inhibitor effectively reversed such effects. In RA patients, elevated miR-126 may promote the expression of CD11a and CD70 via the induction of hypomethylation of gene promoters by depressing DNMTI1 protein levels.

A small-molecule screen for enhanced homing of systemically infused cells.

Poor homing of systemically infused cells to disease sites may limit the success of exogenous cell-based therapy. In this study, we screened 9,000 signal-transduction modulators to identify hits that increase mesenchymal stromal cell (MSC) surface expression of homing ligands that bind to intercellular adhesion molecule 1 (ICAM-1), such as CD11a. Pretreatment of MSCs with Ro-31-8425, an identified hit from this screen, increased MSC firm adhesion to an ICAM-1-coated substrate in vitro and enabled targeted delivery of systemically administered MSCs to inflamed sites in vivo in a CD11a- (and other ICAM-1-binding domains)-dependent manner. This resulted in a heightened anti-inflammatory response. This represents a new strategy for engineering cell homing to enhance therapeutic efficacy and validates CD11a and ICAM-1 as potential targets. Altogether, this multi-step screening process may significantly improve clinical outcomes of cell-based therapies.

Integrin alpha L controls the homing of regulatory T cells during CNS autoimmunity in the absence of integrin alpha 4.

Experimental autoimmune encephalomyelitis (EAE), the animal model of multiple sclerosis (MS), results from an autoimmune attack of the central nervous system (CNS) by effector T helper (Th) 1 and Th17 cells. Regulatory T cells (Treg) can control effector T cells and limit the progression of CNS autoimmunity. Integrin alpha 4 (Itga4) is critical for the entry of Th1 but not Th17 cells into the CNS during EAE. Whether Itga4 controls the homing of Tregs in the CNS and whether Tregs can limit Th17-mediated EAE has, however, not been addressed. Through selective elimination of Itga4 in Foxp3-expressing cells, we show here that Tregs can suppress Th17-mediated EAE and enter into the CNS independently of Itga4. Furthermore, similarly to Th17 cells and in contrast to Th1 cells, Tregs depend on LFA-1 for their entry into the CNS in the absence of Itga4. Therefore, these data suggest that the efficacy of Itga4 neutralization on MS progression may be associated with the prevention of Th1 cells and the maintenance of Tregs migration into the CNS.

Upregulation of CD11A on hematopoietic stem cells denotes the loss of long-term reconstitution potential.

Small numbers of hematopoietic stem cells (HSCs) generate large numbers of mature effector cells through the successive amplification of transiently proliferating progenitor cells. HSCs and their downstream progenitors have been extensively characterized based on their cell-surface phenotype and functional activities during transplantation assays. These cells dynamically lose and acquire specific sets of surface markers during differentiation, leading to the identification of markers that allow for more refined separation of HSCs from early hematopoietic progenitors. Here, we describe a marker, CD11A, which allows for the enhanced purification of mouse HSCs. We show through in vivo transplantations that upregulation of CD11A on HSCs denotes the loss of their long-term reconstitution potential. Surprisingly, nearly half of phenotypic HSCs (defined as Lin-KIT(+)SCA-1(+)CD150(+)CD34-) are CD11A(+) and lack long-term self-renewal potential. We propose that CD11A(+)Lin-KIT(+)SCA-1(+)CD150(+)CD34- cells are multipotent progenitors and CD11A-Lin-KIT(+)SCA-1(+)CD150(+)CD34- cells are true HSCs.

CD18 deficiency evolving to megakaryocytic (M7) acute myeloid leukemia: case report.

Leukocyte adhesion deficiency type 1 (LAD 1 - CD18 deficiency) is a rare disease characterized by disturbance of phagocyte function associated with less severe cellular and humoral dysfunction. The main features are bacterial and fungal infections predominantly in the skin and mucosal surfaces, impaired wound healing and delayed umbilical cord separation. The infections are indolent, necrotic and recurrent. In contrast to the striking difficulties in defense against bacterial and fungal microorganisms, LAD 1 patients do not exhibit susceptibility to viral infections and neoplasias. The severity of clinical manifestations is directly related to the degree of CD18 deficiency. Here, a 20 year-old female presenting a partial CD18 deficiency that developed a megakaryocytic (M7) acute myeloid leukemia is described for the first time. The clinical features of the patient included relapsing oral thrush due to Candida, cutaneous infections and upper and lower respiratory tract infections, followed by a locally severe necrotic genital herpetic lesion. The patient's clinical features improved for a period of approximately two years, followed by severe bacterial infections. At that time, the investigation showed a megakaryocytic acute myeloid leukemia, treated with MEC without clinical improvement. The highly aggressive evolution of the leukemia in this patient suggests that adhesion molecules could be involved in the protection against the spread of neoplastic cells.

CD11a is essential for normal development of hematopoietic intermediates.

The process of lymphopoiesis begins in the bone marrow (BM) and requires multiple cellular intermediates. For T cell production, lymphoid progenitors exit the BM and home to the thymus where maturation and selection ensue. These processes are dependent on a number of factors, including chemokines and adhesion molecules. Although the ß2 integrin CD11a plays an important role in the migration of lymphocytes to lymph nodes, the role of CD11a in T cell development is largely undefined. Our studies now show that, in CD11a(-/-) mice, thymic cellularity was decreased and early T cell development was partially impaired. Remarkably, CD11a was critical for generation of common lymphoid progenitors (CLPs) and lymphoid-primed multipotent progenitors. However, in intact CD11a(-/-) mice, peripheral B and T cell subsets were only modestly altered, suggesting that compensatory mechanisms were operating. In contrast, competitive BM-reconstitution assays revealed an essential role for CD11a in the generation of thymocytes and mature T and B cells. This defect was linked to the requirement for CD11a in the development of CLPs. Furthermore, our results identified CLPs, and not lymphoid-primed multipotent progenitors, as the requisite CD11a-dependent precursor for lymphocyte development. Thus, these findings established a key role for CD11a in lymphopoiesis.