Regulated expression and function of CD11c/CD18 integrin on human B lymphocytes. Relation between attachment to fibrinogen and triggering of proliferation through CD11c/CD18

Generation of circulating autoreactive pre-plasma cells fueled by naive B cells in celiac disease

Autoantibodies against the enzyme transglutaminase 2 (TG2) are characteristic of celiac disease (CeD), and TG2-specific immunoglobulin (Ig) A plasma cells are abundant in gut biopsies of patients. Here, we describe the corresponding population of autoreactive B cells in blood. Circulating TG2-specific IgA cells are present in untreated patients on a gluten-containing diet but not in controls. They are clonally related to TG2-specific small intestinal plasma cells, and they express gut-homing molecules, indicating that they are plasma cell precursors. Unlike other IgA-switched cells, the TG2-specific cells are negative for CD27, placing them in the double-negative (IgD-CD27-) category. They have a plasmablast or activated memory B cell phenotype, and they harbor fewer variable region mutations than other IgA cells. Based on their similarity to naive B cells, we propose that autoreactive IgA cells in CeD are generated mainly through chronic recruitment of naive B cells via an extrafollicular response involving gluten-specific CD4+ T cells.

Partial recovery of peripheral blood monocyte subsets in head and neck squamous cell carcinoma patients upon radio(chemo)therapy is associated with decreased plasma CXCL11

BACKGROUND

Head and neck squamous cell carcinoma (HNSCC) represents a common and heterogeneous malignancy of the oral cavity, pharynx and larynx. Surgery and radio(chemo)therapy are the standard treatment options and also have great influence on the composition of the tumor microenvironment and immune cell functions. However, the impact of radio(chemo)therapy on the distribution and characteristics of circulating monocyte subsets in HNSCC are not fully understood.

METHODS

Expression patterns of adhesion molecules and chemokine receptors CD11a (integrin-α L; LFA-1), CD11b (integrin-α M; Mac-1), CD11c (integrin-α X), CX3CR1 (CX3CL1 receptor) and checkpoint molecule PD-L1 (programmed cell death ligand-1) were investigated upon radio(chemo)therapeutic treatment using flow cytometry. Furthermore, comprehensive analysis of plasma cytokines was performed before and after treatment using ELISA measurements.

RESULTS

Our data reveal a partial recovery of circulating monocytes in HNSCC patients upon radio(chemo)therapeutic treatment, with differential effects of the individual therapy regimen. PD-L1 expression on non-classical monocytes significantly correlates with the individual plasma levels of chemokine CXCL11 (C-X-C motif chemokine 11).

CONCLUSIONS

Further comprehensive investigations on larger patient cohorts are required to elucidate the meaningfulness of peripheral blood monocyte subsets and chemokine CXCL11 as potential bioliquid indicators in HNSCC with regard to therapy response and the individual immunological situation.

CD11c is not required by microglia to convey neuroprotection after prion infection

Prion diseases are caused by the misfolding of a normal host protein that leads to gliosis, neuroinflammation, neurodegeneration, and death. Microglia have been shown to be critical for neuroprotection during prion infection of the central nervous system (CNS), and their presence extends survival in mice. How microglia impart these benefits to the infected host are unknown. Previous transcriptomics and bioinformatics studies suggested that signaling through the heterodimeric integrin receptor CD11c/CD18, expressed by microglia in the brain, might be important to microglial function during prion disease. Herein, we intracerebrally challenged CD11c-/- mice with prion strain RML and compared them to similarly infected C57BL/6 mice as controls. We initially assessed changes in the brain that are associated with disease such as astrogliosis, microgliosis, prion accumulation, and survival. Targeted qRT-PCR arrays were used to determine alterations in transcription in mice in response to prion infection. We demonstrate that expression of Itgax (CD11c) and Itgb2 (CD18) increases in the CNS in correlation with advancing prion infection. Gliosis, neuropathology, prion deposition, and disease progression in prion infected CD11c deficient mice were comparable to infected C57BL/6 mice. Additionally, both CD11c deficient and C57BL/6 prion-infected mouse cohorts had a similar consortium of inflammatory- and phagocytosis-associated genes that increased as disease progressed to clinical stages. Ingenuity Pathway Analysis of upregulated genes in infected C57BL/6 mice suggested numerous cell-surface transmembrane receptors signal through Spleen Tyrosine Kinase, a potential key regulator of phagocytosis and innate immune activation in the prion infected brain. Ultimately, the deletion of CD11c did not influence prion pathogenesis in mice and CD11c signaling is not involved in the neuroprotection provided by microglia, but our analysis identified a conspicuous phagocytosis pathway in the CNS of infected mice that appeared to be activated during prion pathogenesis.

Reduced IRF4 expression promotes lytic phenotype in Type 2 EBV-infected B cells

Humans are infected with two types of EBV (Type 1 (T1) and Type 2 (T2)) that differ substantially in their EBNA2 and EBNA 3A/B/C latency proteins and have different phenotypes in B cells. T1 EBV transforms B cells more efficiently than T2 EBV in vitro, and T2 EBV-infected B cells are more lytic. We previously showed that both increased NFATc1/c2 activity, and an NFAT-binding motif within the BZLF1 immediate-early promoter variant (Zp-V3) contained in all T2 strains, contribute to lytic infection in T2 EBV-infected B cells. Here we compare cellular and viral gene expression in early-passage lymphoblastoid cell lines (LCLs) infected with either T1 or T2 EBV strains. Using bulk RNA-seq, we show that T2 LCLs are readily distinguishable from T1 LCLs, with approximately 600 differentially expressed cellular genes. Gene Set Enrichment Analysis (GSEA) suggests that T2 LCLs have increased B-cell receptor (BCR) signaling, NFAT activation, and enhanced expression of epithelial-mesenchymal-transition-associated genes. T2 LCLs also have decreased RNA and protein expression of a cellular gene required for survival of T1 LCLs, IRF4. In addition to its essential role in plasma cell differentiation, IRF4 decreases BCR signaling. Knock-down of IRF4 in a T1 LCL (infected with the Zp-V3-containing Akata strain) induced lytic reactivation whereas over-expression of IRF4 in Burkitt lymphoma cells inhibited both NFATc1 and NFATc2 expression and lytic EBV reactivation. Single-cell RNA-seq confirmed that T2 LCLs have many more lytic cells compared to T1 LCLs and showed that lytically infected cells have both increased NFATc1, and decreased IRF4, compared to latently infected cells. These studies reveal numerous differences in cellular gene expression in B cells infected with T1 versus T2 EBV and suggest that decreased IRF4 contributes to both the latent and lytic phenotypes in cells with T2 EBV.

BCR activated CLL B cells use both CR3 (CD11b/CD18) and CR4 (CD11c/CD18) for adhesion while CR4 has a dominant role in migration towards SDF-1

Chronic lymphocytic leukaemia (CLL) is the most common leukaemia in the western world. In previous studies, various proportion of patients was found to carry CD11b+ or CD11c+ B cells whose presence was an unfavourable prognostic factor. The exact mechanism however, how these receptors contribute to the pathogenesis of CLL has not been revealed so far. Here we analysed the role of CD11b and CD11c on B cells of CLL patients in the adhesion to fibrinogen and in the migration towards stromal cell derived factor-1 (SDF-1) and studied the role of CR4 in the adherence of the CD11c+ B cell line BJAB. We observed that both CR3 and CR4 mediate adhesion of the malignant B cells. Moreover, we found, that CR4 was strongly involved in the migration of the leukemic cells towards the chemoattractant SDF-1. Our data suggest that CR3 and CR4 are not only passive markers on CLL B cells, but they might contribute to the progression of the disease. Since the role of SDF-1 is prominent in the migration of CLL cells into the bone marrow where their survival is supported, our findings help to understand how the presence of CD11c on leukemic B cells can worsen the prognosis of chronic lymphocytic leukaemia.

New aspects in the regulation of human B cell functions by complement receptors CR1, CR2, CR3 and CR4

The involvement of complement in the regulation of antibody responses has been known for long. By now several additional B cell functions - including cytokine production and antigen presentation - have also been shown to be regulated by complement proteins. Most of these important activities are mediated by receptors interacting with activation fragments of the central component of the complement system C3, such as C3b, iC3b and C3d, which are covalently attached to antigens and immune complexes. This review summarizes the role of complement receptors interacting with these ligands, namely CR1 (CD35), CR2 (CD21), CR3 (CD11b/CD18) and CR4 (CD11c/CD18) expressed by B cells in health and disease. Although we focus on human B lymphocytes, we also aim to call the attention to important differences between human and mouse systems.

CD11c regulates hematopoietic stem and progenitor cells under stress

β2 integrins are well-known leukocyte adhesion molecules consisting of 4 members: CD11a-d. Their known biological functions range widely from leukocyte recruitment, phagocytosis, to immunological synapse formation, but the studies have been primarily focused on CD11a and CD11b. CD11c is 1 of the 4 members and is extremely homologous to CD11b. It has been well known as a dendritic cell marker, but the characterization of its function has been limited. We found that CD11c was expressed on the short-term hematopoietic stem cells and multipotent progenitor cells. The lack of CD11c did not affect the number of hematopoietic stem and progenitor cells (HSPCs) in healthy CD11c knockout mice. Different from other β2 integrin members, however, CD11c deficiency was associated with increased apoptosis and significant loss of HSPCs in sepsis and bone marrow transplantation. Although integrins are generally known for their overlapping and redundant roles, we showed that CD11c had a distinct role of regulating the expansion of HSPCs under stress. This study shows that CD11c, a well-known dendritic cell marker, is expressed on HSPCs and serves as their functional regulator. CD11c deficiency leads to the loss of HSPCs via apoptosis in sepsis and bone marrow transplantation.

Shared transcriptional profiles of atypical B cells suggest common drivers of expansion and function in malaria, HIV, and autoimmunity

Chronic infectious diseases have a substantial impact on the human B cell compartment including a notable expansion of B cells here termed atypical B cells (ABCs). Using unbiased single-cell RNA sequencing (scRNA-seq), we uncovered and characterized heterogeneities in naïve B cell, classical memory B cells, and ABC subsets. We showed remarkably similar transcriptional profiles for ABC clusters in malaria, HIV, and autoimmune diseases and demonstrated that interferon-γ drove the expansion of ABCs in malaria. These observations suggest that ABCs represent a separate B cell lineage with a common inducer that further diversifies and acquires disease-specific characteristics and functions. In malaria, we identified ABC subsets based on isotype expression that differed in expansion in African children and in B cell receptor repertoire characteristics. Of particular interest, IgD⁺IgM^lo and IgD^-IgG⁺ ABCs acquired a high antigen affinity threshold for activation, suggesting that ABCs may limit autoimmune responses to low-affinity self-antigens in chronic malaria.

Activated Human Memory B Lymphocytes Use CR4 (CD11c/CD18) for Adhesion, Migration, and Proliferation

Complement receptors CR3 (CD11b/CD18) and CR4 (CD11c/CD18) of myeloid cells are known for long to participate in actin linked functions like phagocytosis, adhesion, and migration. The expression and role of these two β₂-integrins however, in human B lymphocytes have only scarcely been studied so far, although it has been shown recently that CD11c⁺ B cells are mainly memory cells. In our systematic study we investigated B cells isolated from tonsils and peripheral blood of healthy donors. We found, that while only 5% of resting tonsillar B cells expressed CD11c, their number increased up to 26% after 3 days of BCR stimulation. Lower, but still remarkable percentage of B lymphocytes were positive for CD11c after stimulation via TLR9 alone or via TLR9 and BCR simultaneously. At the same time, we detected no significant expression of CD11b on resting or activated tonsillar B cells. Blood B lymphocytes showed a similar expression pattern of both β₂-integrins. We demonstrated that CD11c molecules appearing on the surface of B cells are newly synthesized, reaching the number of 9,500 per activated B cell. We found that CR4 expressing B cells belong to the memory pool and the increase of CD11c expression on tonsillar B cells upon BCR mediated activation occurs parallel with class switching. Analysis of the function of CD11c revealed, that this β₂-integrin contributes to the adhesion and migration of activated B lymphocytes. We also demonstrated that the CR4 mediated adhesion promotes the proliferation of the BCR activated cells. Our studies are the first to demonstrate that CD11c expressed on BCR-activated human B cells are not only passive markers but functional drivers of memory B cell responses.

Is Cd11c and Fmc7 Negativity in Chronic Lymphocytic Leukemia Poor Prognostic?

Chronic lymphocytic leukemia (CLL) is a common hematological malignancy. This study is aimed to investigate the prognostic effect of clinic, laboratory and flow cytometric analysis in CLL patients. Newly diagnosed 55 CLL cases were divided into two groups, as stable disease (Group 1) and progressive disease (Group 2). Group 1 included those who did not require any treatment since diagnosis and those who did not progress after receiving the first step anti CLL treatment. Group 2 included the patients who received ≥ 2 steps treatment. The relation between the two groups was analyzed statistically in terms of clinical, laboratory and flow cytometric findings. Twenty patients (36.3%) required treatment at the time of diagnosis, four patients (3.8%) received first-line treatment during follow-up and 31 (56.3%) patients were followed without any treatment. Thirteen patients required second step treatment after a median of 26.3 months. The risk of progression was found to be increased 5-fold (p = 0.015) in the CD38 positive patient group, 4.2-fold (p = 0.0147) in the FMC7 negative patient group and 2.8-fold in the CD11c negative patient group. FMC 7 negativity decreased total survival 5.9-fold (p = 0.051). Unlike similar publications, we found that patients with CD11c or FMC7 negativity were in a higher need for ≥ 2 step treatment. This suggests that CD11c or FMC7 negativity may be used as a poor prognostic marker in CLL.

Activation of dendritic cells by targeted DNA: a potential addition to the armamentarium for anti-cancer immunotherapy

In the past decade, remarkable progress has been made in immunotherapy against cancer. Specifically, the introduction of immune checkpoint inhibitors has revolutionized the field. However, many patients are unable to benefit significantly from this treatment option. One of the major reasons for this is most likely the absence of an adequate tumor-specific T cell response in these patients. A way to circumvent this problem might be to combine immune checkpoint inhibitor treatment with new strategies to activate tumor-specific T cells. One such strategy could be to activate and mature dendritic cells in situ. Dendritic cells carry an array of external and internal pattern recognition receptors that induce cell activation and maturation when interacting with their corresponding damage-associated or pathogen-associated molecular patterns (DAMPs or PAMPs). Targeting such molecular patterns directly to dendritic cells might be a way to evoke stronger immune responses. Here, we review our recent findings using antibody-targeted DNA. We summarize the results from our experiments showing that dendritic cells can be actively targeted in vivo through the α_Xβ₂ integrin subunit CD11c, and that DNA delivered through this receptor in vitro leads to maturation of dendritic cells via the cytosolic cGAS/STING DNA-sensing pathway.

T Cell-Dependent Affinity Maturation and Innate Immune Pathways Differentially Drive Autoreactive B Cell Responses in Rheumatoid Arthritis

OBJECTIVE

Rheumatoid arthritis (RA) is characterized by the activation of B cells that produce anti-citrullinated protein antibodies (ACPAs) and rheumatoid factors (RFs), but the mechanisms by which tolerance is broken in these B cells remain incompletely understood. We undertook this study to investigate whether ACPA+ and RF+ B cells break tolerance through distinct molecular mechanisms.

METHODS

We developed antigen-tetramers to isolate ACPA+ and RF+ B cells and performed single-cell RNA sequencing on 2,349 B cells from 6 RA patients and 1 healthy donor to analyze their immunoglobulin repertoires and transcriptional programs. Prominent immunoglobulins were expressed as monoclonal antibodies and tested for autoantigen reactivity.

RESULTS

ACPA+ and RF+ B cells were enriched in the peripheral blood of RA patients relative to healthy controls. Characterization of patient-derived monoclonal antibodies confirmed ACPA and RF targeting of tetramer-specific B cells at both antigen-inexperienced and affinity-matured B cell stages. ACPA+ B cells used more class-switched isotypes and exhibited more somatic hypermutations relative to RF+ B cells, and these differences were accompanied by down-regulation of CD72 and up-regulation of genes that promote class-switching and T cell-dependent responses. In contrast, RF+ B cells expressed transcriptional programs that stimulate rapid memory reactivation through multiple innate immune pathways. Coexpression analysis revealed that ACPA+ and RF+ B cell-enriched genes belong to distinct transcriptional regulatory networks.

CONCLUSION

Our findings suggest that ACPA+ and RF+ B cells are imprinted with distinct transcriptional programs, which suggests that these autoantibodies associated with increased inflammation in RA arise from 2 different molecular mechanisms.

Non-identical twins: Different faces of CR3 and CR4 in myeloid and lymphoid cells of mice and men

Integrins are cell membrane receptors that are involved in essential physiological and serious pathological processes. Their main role is to ensure a closely regulated link between the extracellular matrix and the intracellular cytoskeletal network enabling cells to react to environmental stimuli. Complement receptor type 3 (CR3, αMβ₂, CD11b/CD18) and type 4 (CR4, αXβ₂, CD11c/CD18) are members of the β₂-integrin family expressed on most white blood cells. Both receptors bind multiple ligands like iC3b, ICAM, fibrinogen or LPS. β₂-integrins are accepted to play important roles in cellular adhesion, migration, phagocytosis, ECM rearrangement and inflammation. Several pathological conditions are linked to the impaired functions of these receptors. CR3 and CR4 are generally thought to mediate overlapping functions in monocytes, macrophages and dendritic cells, therefore the potential distinctive role of these receptors has not been investigated so far in satisfactory details. Lately it has become clear that a functional segregation has evolved between the two receptors regarding phagocytosis, cellular adhesion and podosome formation. In addition to their tasks on myeloid cells, the expression and function of CR3 and CR4 on lymphocytes have also gained interest recently. The picture is further complicated by the fact that while these β₂-integrins are expressed by immune cells both in mice and humans, there are significant differences in their expression level, functions and the pathological consequences of genetic defects. Here we aim to summarize our current knowledge on CR3 and CR4 and highlight the functional differences between these receptors, involving their expression in myeloid and lymphoid cells of both men and mice.

The versatile functions of complement C3-derived ligands

The complement system is a major component of immune defense. Activation of the complement cascade by foreign substances and altered self-structures may lead to the elimination of the activating agent, and during the enzymatic cascade, several biologically active fragments are generated. Most immune regulatory effects of complement are mediated by the activation products of C3, the central component. The indispensable role of C3 in opsonic phagocytosis as well as in the regulation of humoral immune response is known for long, while the involvement of complement in T-cell biology have been revealed in the past few years. In this review, we discuss the immune modulatory functions of C3-derived fragments focusing on their role in processes which have not been summarized so far. The importance of locally synthesized complement will receive special emphasis, as several immunological processes take place in tissues, where hepatocyte-derived complement components might not be available at high concentrations. We also aim to call the attention to important differences between human and mouse systems regarding C3-mediated processes.

CD11c+ T-bet+ memory B cells: Immune maintenance during chronic infection and inflammation?

CD11c+ T-bet+ B cells have now been detected and characterized in different experimental and clinical settings, in both mice and humans. Whether such cells are monolithic, or define subsets of B cells with different functions is not yet known. Our studies have identified CD11c+ IgM+ CD19^hi splenic IgM memory B cells that appear at approximately three weeks post-ehrlichial infection, and persist indefinitely, during low-level chronic infection. Although the CD11c+ T-bet+ B cells we have described are distinct, they appear to share many features with similar cells detected under diverse conditions, including viral infections, aging, and autoimmunity. We propose that CD11c+ T-bet+ B cells as a group share characteristics of memory B cells that are maintained under conditions of inflammation and/or low-level chronic antigen stimulation. In some cases, these cells may be advantageous, by providing immunity to re-infection, but in others may be deleterious, by contributing to aged-associated autoimmune responses.

Development of conventional dendritic cells: from common bone marrow progenitors to multiple subsets in peripheral tissues

Our understanding of conventional dendritic cell (cDC) development and the functional specializations of distinct subsets in the peripheral tissues has increased greatly in recent years. Here, we review cDC development from the distinct progenitors in the bone marrow through to the distinct cDC subsets found in barrier tissues, providing an overview of the different subsets described in each location. In addition, we detail the transcription factors and local signals that have been proposed to control this developmental process. Importantly, despite these significant advances, numerous questions remain to be answered regarding cDC development. For example, it remains unclear whether the different subsets described, such as the CD103+CD11b+ and CD103-CD11b+ cDCs in the intestines, truly represent different populations or rather distinct developmental or activation stages. Furthermore, whether distinct progenitors exist for these cDC subsets remains to be determined. Thus in the last part of this review we discuss what we believe will be the main questions facing the field for the coming years.

Functional studies of chronic lymphocytic leukemia B cells expressing β2-integrin type complement receptors CR3 and CR4

The expression and role of CR3 (CD11b/CD18) and CR4 (CD11c/CD18) in B cells are not yet explored in contrast to myeloid cells, where these β₂-integrin type receptors are known to participate in various cellular functions, including phagocytosis, adherence and migration. Here we aimed to reveal the expression and role of CR3 and CR4 in human B cells. In B cells of healthy donors CR3 and CR4 are scarcely expressed. However, two patients with chronic lymphocytic leukemia (CLL) characterized by a peculiar immune-phenotype containing both CD5-positive and CD5-negative B cell populations made possible to study these molecules in distinct B cell subsets. We found that CD11b and CD11c were expressed on both CD5-positive and CD5-negative B cells, albeit to different extents. Our data suggest that these receptors are involved in spreading, since this activity of CpG-activated B cells on fibrinogen could be partially blocked by monoclonal antibodies specific for CD11b or CD11c. CpG-stimulation lead to proliferation of both CD5-positive and CD5-negative B cells of the patients with a less pronounced effect on the CD5-positive cells. In contrast to normal B cells, CLL B cells of both patients reacted to CpG-stimulation with robust IL-10 production. The concomitant, suboptimal stimulus via the BCR and TLR9 exerted either a synergistic enhancing effect or resulted in inhibition of proliferation and IL-10 production of patients' B cells. Our data obtained studying B cells of leukemic patients point to the role of CR3 and probably CR4 in the interaction of tumor cells with the microenvironment and suggest the involvement of IL-10 producing B cells in the pathologic process.

Leukocyte arrest: Biomechanics and molecular mechanisms of β2 integrin activation

Integrins are a group of heterodimeric transmembrane receptors that play essential roles in cell-cell and cell-matrix interaction. Integrins are important in many physiological processes and diseases. Integrins acquire affinity to their ligand by undergoing molecular conformational changes called activation. Here we review the molecular biomechanics during conformational changes of integrins, integrin functions in leukocyte biorheology (adhesive functions during rolling and arrest) and molecules involved in integrin activation.

Cutting Edge: IL-4, IL-21, and IFN-γ Interact To Govern T-bet and CD11c Expression in TLR-Activated B Cells

T-bet and CD11c expression in B cells is linked with IgG2c isotype switching, virus-specific immune responses, and humoral autoimmunity. However, the activation requisites and regulatory cues governing T-bet and CD11c expression in B cells remain poorly defined. In this article, we reveal a relationship among TLR engagement, IL-4, IL-21, and IFN-γ that regulates T-bet expression in B cells. We find that IL-21 or IFN-γ directly promote T-bet expression in the context of TLR engagement. Further, IL-4 antagonizes T-bet induction. Finally, IL-21, but not IFN-γ, promotes CD11c expression independent of T-bet. Using influenza virus and Heligmosomoides polygyrus infections, we show that these interactions function in vivo to determine whether T-bet(+) and CD11c(+) B cells are formed. These findings suggest that T-bet(+) B cells seen in health and disease share the common initiating features of TLR-driven activation within this circumscribed cytokine milieu.

Controlling immune responses by targeting antigens to dendritic cell subsets and B cells

Delivering antigens in vivo by coupling them to mAbs specific for unique receptors on antigen-presenting cells (APCs) is a promising approach for modulating immune responses. Antigen delivery to receptors found on myeloid dendritic cell (DC) subsets, plasmacytoid DCs and B cells has shown them all to be viable targets to stimulate either the cellular or humoral arms of the immune system. It is now evident that antigen-targeting approaches can also be used to invoke antigen-specific inhibition of immune responses. The outcome of activation versus inhibition is determined by a combination of factors that include the choice of APC, the receptor that is targeted, whether to include an adjuvant and, if so, which adjuvant to employ. In addition to their use as a means to modulate immune responses, antigen-targeting systems are also a useful method to investigate the function of DC subsets and the early mechanistic events that underlie the initiation of both cellular and humoral immune responses. In this review, we focus on the literature surrounding the control of B-cell responses when antigen is delivered to various APC subsets.

Extracellular transport of cell-size particles and tumor cells by dendritic cells in culture

Many particulate materials of sizes approximating that of a cell disseminate after being introduced into the body. While some move about within phagocytic inflammatory cells, others appear to move about outside of, but in contact with, such cells. In this report, we provide unequivocal photomicroscopic evidence that cultured, mature, human dendritic cells can transport in extracellular fashion over significant distances both polymeric beads and tumor cells. At least in the case of polymeric beads, both fibrinogen and the β2-integrin subunit, CD18, appear to play important roles in the transport process. These discoveries may yield insight into a host of disease-related phenomena, including and especially tumor cell invasion and metastasis.

T cell-dependent IgM memory B cells generated during bacterial infection are required for IgG responses to antigen challenge

Immunological memory has long considered to be harbored in B cells that express high-affinity class-switched IgG. IgM-positive memory B cells can also be generated following immunization, although their physiological role has been unclear. In this study, we show that bacterial infection elicited a relatively large population of IgM memory B cells that were uniquely identified by their surface expression of CD11c, CD73, and programmed death-ligand 2. The cells lacked expression of cell surface markers typically expressed by germinal center B cells, were CD138 negative, and did not secrete Ab ex vivo. The population was also largely quiescent and accumulated somatic mutations. The IgM memory B cells were located in the region of the splenic marginal zone and were not detected in blood or other secondary lymphoid organs. Generation of the memory cells was CD4 T cell dependent and required IL-21R signaling. In vivo depletion of the IgM memory B cells abrogated the IgG recall responses to specific Ag challenge, demonstrating that the cell population was required for humoral memory, and underwent class-switch recombination following Ag encounter. Our findings demonstrate that T cell-dependent IgM memory B cells can be elicited at high frequency and can play an important role in maintaining long-term immunity during bacterial infection.

Brain dendritic cells: biology and pathology

Dendritic cells (DC) are the professional antigen-presenting cells of the immune system. In their quiescent and mature form, the presentation of self-antigens by DC leads to tolerance; whereas, antigen presentation by mature DC, after stimulation by pathogen-associated molecular patterns, leads to the onset of antigen-specific immunity. DC have been found in many of the major organs in mammals (e.g. skin, heart, lungs, intestines and spleen); while the brain has long been considered devoid of DC in the absence of neuroinflammation. Consequently, microglia, the resident immune cell of the brain, have been charged with many functional attributes commonly ascribed to DC. Recent evidence has challenged the notion that DC are either absent or minimal players in brain immune surveillance. This review will discuss the recent literature examining DC involvement within both the young and aged steady-state brain. We will also examine DC contributions during various forms of neuroinflammation resulting from neurodegenerative autoimmune disease, injury, and CNS infections. This review also touches upon DC trafficking between the central nervous system and peripheral immune compartments during viral infections, the new molecular technologies that could be employed to enhance our current understanding of brain DC ontogeny, and some potential therapeutic uses of DC within the CNS.

Experimental inoculation of juvenile rhesus macaques with primate enteric caliciviruses

BACKGROUND

Tissue culture-adapted Tulane virus (TV), a GI.1 rhesus enteric calicivirus (ReCV), and a mixture of GII.2 and GII.4 human norovirus (NoV)-containing stool sample were used to intrastomacheally inoculate juvenile rhesus macaques (Macaca mulatta) in order to evaluate infection caused by these viruses. METHODOLOGY & FINDINGS: Two of the three TV-inoculated macaques developed diarrhea, fever, virus-shedding in stools, inflammation of duodenum and 16-fold increase of TV-neutralizing (VN) serum antibodies but no vomiting or viremia. No VN-antibody responses could be detected against a GI.2 ReCV strain FT285, suggesting that TV and FT285 represent different ReCV serotypes. Both NoV-inoculated macaques remained asymptomatic but with demonstrable virus shedding in one animal. Examination of duodenum biopsies of the TV-inoculated macaques showed lymphocytic infiltration of the lamina propria and villous blunting. TV antigen-positive (TV+) cells were detected in the lamina propria. In most of the TV+ cells TV co-localized perinuclearly with calnexin--an endoplasmic reticulum protein. A few CD20+TV+ double-positive B cells were also identified in duodenum. To corroborate the authenticity of CD20+TV+ B cells, in vitro cultures of peripheral blood mononuclear cells (PBMCs) from healthy macaques were inoculated with TV. Multicolor flow cytometry confirmed the presence of TV antigen-containing B cells of predominantly CD20+HLA-DR+ phenotype. A 2-log increase of viral RNA by 6 days post inoculation (p<0.05) suggested active TV replication in cultured lymphocytes.

CONCLUSIONS/SIGNIFICANCE

Taken together, our results show that ReCVs represent an alternative cell culture and animal model to study enteric calicivirus replication, pathogenesis and immunity.

CD11c-expressing cells reside in the juxtavascular parenchyma and extend processes into the glia limitans of the mouse nervous system

Recent studies demonstrated that primary immune responses can be induced within the brain depending on vessel-associated cells expressing markers of dendritic cells (DC). Using mice transcribing the green fluorescent protein (GFP) under the promoter of the DC marker CD11c, we determined the distribution, phenotype, and source of CD11c+ cells in non-diseased brains. Predilection areas of multiple sclerosis (MS) lesions (periventricular area, adjacent fibre tracts, and optical nerve) were preferentially populated by CD11c+ cells. Most CD11c+ cells were located within the juxtavascular parenchyma rather than the perivascular spaces. Virtually all CD11c+ cells co-expressed ionized calcium-binding adaptor molecule 1 (IBA-1), CD11b, while detectable levels of major histocompatibility complex II (MHC-II) in non-diseased mice was restricted to CD11c+ cells of the choroid plexus. Cellular processes project into the glia limitans which may allow transport and/or presentation of intraparenchymal antigens to extravasated T cells in perivascular spaces. In chimeric mice bearing CD11c-GFP bone marrow, fluorescent cells appeared in the CNS between 8 and 12 weeks after transplantation. In organotypic slice cultures from CD11c-GFP mice, the number of fluorescent cells strongly increased within 72 h. Strikingly, using anti-CD209, an established marker for human DC, a similar population was detected in human brains. Thus, we show for the first time that CD11c+ cells can not only be recruited from the blood into the parenchyma, but also develop from an intraneural precursor in situ. Dysbalance in their recruitment/development may be an initial step in the pathogenesis of chronic (autoimmune) neuroinflammatory diseases such as MS.

Clonal B cells in patients with hepatitis C virus-associated mixed cryoglobulinemia contain an expanded anergic CD21low B-cell subset

Hepatitis C virus (HCV) is associated with the B-cell lymphoproliferative disorders mixed cryoglobulinemia (MC) and non-Hodgkin lymphoma. We have previously reported that HCV(+)MC(+) patients have clonal expansions of hypermutated, rheumatoid factor-bearing marginal zone-like IgM(+)CD27(+) peripheral B cells using the V(H)1-69 gene. Here we coupled transcriptional profiling with immunophenotypic and functional studies to ascertain these cells' role in MC pathogenesis. Despite their fundamental role in MC disease, these B cells have overall transcriptional features of anergy and apoptosis instead of neoplastic transformation. Highly up-regulated genes include SOX5, CD11C, galectin-1, and FGR, similar to a previously described FCRL4(+) memory B-cell subset and to an "exhausted," anergic CD21(low) memory B-cell subset in HIV(+) patients. Moreover, HCV(+)MC(+) patients' clonal peripheral B cells are enriched with CD21(low), CD11c(+), FCRL4(high), IL-4R(low) memory B cells. In contrast to the functional, rheumatoid factor-secreting CD27(+)CD21(high) subset, the CD27(+)CD21(low) subpopulation exhibits decreased calcium mobilization and does not efficiently differentiate into rheumatoid factor-secreting plasmablasts, suggesting that a large proportion of HCV(+)MC(+) patients' clonally expanded peripheral B cells is prone to anergy and/or apoptosis. Down-regulation of multiple activation pathways may represent a homeostatic mechanism attenuating otherwise uncontrolled stimulation of circulating HCV-containing immune complexes.

Targeted regulation of self-peptide presentation prevents type I diabetes in mice without disrupting general immunocompetence

Peptide loading of MHC class II (MHCII) molecules is directly catalyzed by the MHCII-like molecule HLA-DM (DM). Another MHCII-like molecule, HLA-DO (DO), associates with DM, thereby modulating DM function. The biological role of DO-mediated regulation of DM activity in vivo remains unknown; however, it has been postulated that DO expression dampens presentation of self antigens, thereby preventing inappropriate T cell activation that ultimately leads to autoimmunity. To test the idea that DO modulation of the MHCII self-peptide repertoire mediates self tolerance, we generated NOD mice that constitutively overexpressed DO in DCs (referred to herein as NOD.DO mice). NOD mice are a mouse model for type 1 diabetes, an autoimmune disease mediated by the destruction of insulin-secreting pancreatic beta cells. Our studies showed that diabetes development was completely blocked in NOD.DO mice. Similar to NOD mice, NOD.DO animals selected a diabetogenic T cell repertoire, and the numbers and function of Tregs were normal. Indeed, immune system function in NOD.DO mice was equivalent to that in NOD mice. NOD.DO DCs, however, presented an altered MHCII-bound self-peptide repertoire, thereby preventing the activation of diabetogenic T cells and subsequent diabetes development. These studies show that DO expression can shape the overall MHCII self-peptide repertoire to promote T cell tolerance.

Functional role of CD11c+ monocytes in atherogenesis associated with hypercholesterolemia

BACKGROUND

Monocyte activation and migration into the arterial wall are key events in atherogenesis associated with hypercholesterolemia. CD11c/CD18, a beta2 integrin expressed on human monocytes and a subset of mouse monocytes, has been shown to play a distinct role in human monocyte adhesion on endothelial cells, but the regulation of CD11c in hypercholesterolemia and its role in atherogenesis are unknown.

METHODS AND RESULTS

Mice genetically deficient in CD11c were generated and crossbred with apolipoprotein E (apoE)-/- mice to generate CD11c-/-/apoE-/- mice. Using flow cytometry, we examined CD11c on blood leukocytes in apoE-/- hypercholesterolemic mice and found that compared with wild-type and apoE-/- mice on a normal diet, apoE-/- mice on a Western high-fat diet had increased CD11c+ monocytes. Circulating CD11c+ monocytes from apoE-/- mice fed a high-fat diet exhibited cytoplasmic lipid vacuoles and expressed higher levels of CD11b and CD29. Deficiency of CD11c decreased firm arrest of mouse monocytes on vascular cell adhesion molecule-1 and E-selectin in a shear flow assay, reduced monocyte/macrophage accumulation in atherosclerotic lesions, and decreased atherosclerosis development in apoE-/- mice on a high-fat diet.

CONCLUSIONS

CD11c, which increases on blood monocytes during hypercholesterolemia, plays an important role in monocyte recruitment and atherosclerosis development in an apoE-/- mouse model of hypercholesterolemia.

CD11c expression identifies a population of extrafollicular antigen-specific splenic plasmablasts responsible for CD4 T-independent antibody responses during intracellular bacterial infection

Although T-independent immunity is known to be generated against bacterial capsular and cell wall polysaccharides expressed by a number of bacterial pathogens, it has not been studied in depth during intracellular bacterial infections. Our previous study demonstrated that Ehrlichia muris, an obligate intracellular tick-borne pathogen, generates protective classical TI responses in CD4 T cell-deficient C57BL/6 mice. We found that E. muris T-independent immunity is accompanied by the expansion of a very large extrafollicular spleen population of CD11c(low)-expressing plasmablasts that exhibit characteristics of both B-1 and marginal zone B cells. The plasmablasts comprised up to 15% of the total spleen lymphocytes and approximately 70% of total spleen IgM(high)IgD(low) cells during peak infection in both wild-type and MHC class II-deficient mice. The CD11c(low) cells exhibited low surface expression of B220, CD19, and CD1d, high expression of CD11b, CD43, but did not express CD5. Approximately 50% of the CD11c(low) cells also expressed CD138. In addition to CD11b and CD11c, the plasmablasts expressed the beta(1) (CD29) and alpha4 (CD49d) integrins, as well as the chemokine receptor CXCR4, molecules which may play roles in localizing the B cells extrafollicular region of the spleen. During peak infection, the CD11c(low) cells accounted for the majority of the IgM-producing splenic B cells and nearly all of the E. muris outer membrane protein-specific IgM-secreting cells. Thus, during this intracellular bacterial infection, CD11c expression identifies a population of Ag-specific spleen plasmablasts responsible for T-independent Ab production.

Discriminating gene expression profiles of memory B cell subpopulations

Morphologically and functionally distinct subpopulations of human memory B (B_Mem) cells are identifiable by either their expression of CD27 or Fc receptor–like 4 (FCRL4), an immunoglobulin domain containing a receptor with strong inhibitory potential. We have conducted comparative transcriptome and proteome analyses of FCRL4⁺ and FCRL4⁻ B_Mem cells and found that these two subsets have very distinctive expression profiles for genes encoding transcription factors, cell-surface proteins, intracellular signaling molecules, and modifiers of the cell-cycle status. Among the differentially expressed transcription factors, runt-related transcription factor 1 (RUNX1) transcript levels were up-regulated in FCRL4⁻ cells, whereas RUNX2 transcripts were preferentially detected in FCRL4⁺ cells. In vitro evidence for FCRL4 promoter responsiveness and in vivo promoter occupancy suggested that RUNX transcription factors are involved in the generation of these B_Mem cell subpopulations. A distinctive signature profile was defined for the FCRL4⁺ B_Mem cells by their expression of CD11c, receptor activator for nuclear factor κB ligand, and FAS cell-surface proteins, in combination with increased levels of SOX5, RUNX2, DLL1, and AICDA expression. We conclude that this recently identified subpopulation of B_Mem cells, which normally resides in epithelial tissue-based niches, may serve a unique role in mucosal defense and, conversely, as a target for neoplastic transformation events.

Differential expression of beta 2-integrins and cytokine production between gammadelta and alphabeta T cells in experimental autoimmune encephalomyelitis

The expression of beta 2-integrins on gammadelta T cells in naïve mice or those with experimental autoimmune encephalomyelitis (EAE) remains poorly characterized. We compared beta 2-integrin expression and cytokine production between gammadelta and alphabeta T cells over the acute course of EAE. We observed that unlike in alphabeta T cells, beta 2-integrin expression on gammadelta T cells increased significantly from baseline, peaked at Day 10, and remained unchanged in the draining lymph nodes or declined in the spleen and CNS by Day 15. In addition, IFN-gamma- and TNF-alpha-producing gammadelta T cells infiltrated the CNS rapidly and produced significantly more of these cytokines than alphabeta T cells throughout the course of EAE. These results suggest unique roles for beta 2-integrins in the trafficking of gammadelta versus alphabeta T cells during EAE and that gammadelta T cells infiltrate the CNS rapidly, producing cytokines, which modulate acute disease.

CD11cloB220+ interferon-producing killer dendritic cells are activated natural killer cells

Interferon-producing killer dendritic cells (IKDCs) are a recently described subset of CD11c^loB220⁺ cells that share phenotypic and functional properties of DCs and natural killer (NK) cells (Chan, C.W., E. Crafton, H.N. Fan, J. Flook, K. Yoshimura, M. Skarica, D. Brockstedt, T.W. Dubensky, M.F. Stins, L.L. Lanier, et al. 2006. Nat. Med. 12:207–213; Taieb, J., N. Chaput, C. Menard, L. Apetoh, E. Ullrich, M. Bonmort, M. Pequignot, N. Casares, M. Terme, C. Flament, et al. 2006. Nat. Med. 12:214–219). IKDC development appears unusual in that cytokines using the interleukin (IL)-2 receptor β (IL-2Rβ) chain but not those using the common γ chain (γ_c) are necessary for their generation. By directly comparing Rag2^−/−γ_c^−/y, Rag2^−/−IL-2Rβ^−/−, Rag2^−/−IL-15^−/−, and Rag2^−/−IL-2^−/− mice, we demonstrate that IKDC development parallels NK cell development in its strict IL-15 dependence. Moreover, IKDCs uniformly express NK-specific Ncr-1 transcripts (encoding NKp46), whereas NKp46⁺ cells are absent in Ncr1^gfp/+γ_c^−/y mice. Distinguishing features of IKDCs (CD11c^loB220⁺MHC-II⁺) were carefully examined on developing NK cells in the bone marrow and on peripheral NK cells. As B220 expression was heterogeneous, defining B220^lo versus B220^hi NK1.1⁺ NK cells could be considered as arbitrary, and few phenotypic differences were noted between NK1.1⁺ NK cells bearing different levels of B220. CD11c expression did not correlate with B220 or major histocompatibility complex (MHC) class II (MHC-II) expression, and most MHC-II⁺ NK1.1⁺ cells did not express B220 and were thus not IKDCs. Finally, CD11c, MHC-II, and B220 levels were up-regulated on NK1.1⁺ cells upon activation in vitro or in vivo in a proliferation-dependent fashion. Our data suggest that the majority of CD11c^loB220⁺ “IKDC-like” cells represent activated NK cells.

p150/95 (CD11c/CD18) expression is required for the development of experimental autoimmune encephalomyelitis

p150/95 (CD11c/CD18, CR4) is a member of the beta(2)-integrin family of adhesion molecules and is considered an important phagocytic receptor. The role of p150/95 in the development of central nervous system demyelinating diseases, including multiple sclerosis, remains unexplored. To determine p150/95-mediated mechanisms in experimental autoimmune encephalomyelitis (EAE), we performed EAE using CD11c-deficient (CD11c(-/-)) mice. EAE in CD11c(-/-) mice was significantly attenuated and characterized by markedly reduced spinal cord T-cell infiltration and interferon-gamma production by these cells. Adoptive transfer of antigen-restimulated T cells from wild-type to CD11c(-/-) mice produced significantly attenuated EAE, whereas transfer of CD11c(-/-) antigen-restimulated T cells to control mice induced a very mild, monophasic EAE. T cells from MOG(35-55) peptide-primed CD11c(-/-) mice displayed an unusual cytokine phenotype with elevated levels of interleukin (IL)-2, IL-4, and IL-12 but reduced levels of interferon-gamma, tumor necrosis factor-alpha, IL-10, IL-17, and transforming growth factor-beta compared with control mice. Overall, CD11c(-/-) T cells from primed mice proliferated comparably to that of control T cells on MOG(35-55) restimulation. Our results indicate that expression of p150/95 is critical on both T cells as well as other leukocytes for the development of demyelinating disease and may represent a novel therapeutic target for multiple sclerosis.

Targeting antigens to dendritic cells in vivo

Dendritic cells (DCs) play a key role in antigen-specific immune regulation. DCs take up and process antigens and present these as peptides through MHC molecules to T cells. Recent pre-clinical and clinical studies have exploited DCs as a means to improve vaccine efficiency. In these studies, monocyte-derived autologous DCs are loaded ex vivo with antigens and re-administered to the patient. These tailor-made vaccines are costly and labor intensive, and therefore less suitable for large-scale immunization programs. As a next step in the development of DC vaccines, it is proposed to load DCs with antigens in vivo. Drug delivery systems harboring antigens have been targeted to DCs via specific surface receptors preferentially expressed by DCs, resulting in priming of humoral and cellular immune responses. The present review focuses on the various antigen delivery systems that are currently in use and the DC surface receptors they target.

CD11b-mediated migratory property of peripheral blood B cells

BACKGROUND

CD11b belongs to the integrin family and is expressed on neutrophils, monocytes, natural killer cells, and a subset of lymphocytes. Although CD11b expressed on neutrophils and monocytes has been extensively investigated and has been reported to play an important role in the migration of these subsets of leukocytes, the function of CD11b expressed on a subset of B cells has not yet been clarified.

OBJECTIVE

To elucidate the functional activity of CD11b expressed on B cells, we characterized the CD11b-expressing cells among the B-cell population and investigated their migratory ability.

METHODS

Isolated peripheral blood CD19 + B cells were analyzed by flow cytometry. The migratory ability of B cells was evaluated by the transwell assay, and the contribution of CD11b to this ability was investigated by using an anti-CD11b blocking mAb.

RESULTS

The majority of CD27 - IgD + naive B cells were CD11b - , whereas most CD27 + memory cells were CD11b +. Among the CD27 + memory cells, expression of CD11b was stronger on the IgD - cells than on the IgD + cells. In the transwell assay, the migrating cells were predominantly CD27 + IgD - cells, most of which expressed CD11b. The addition of an anti-CD11b blocking mAb resulted in the significant reduction of the number of migrating B cells.

CONCLUSION

Memory B cells express CD11b and, in contrast with naive B cells, have high migratory ability. CD11b plays an essential role in the homing process of memory cells.

IL-10-Producing B220+CD11c−APC in Mouse Spleen

APC acting at the early stages of an immune response can shape the nature of that response. Such APC will include dendritic cells (DCs) but may also include populations of B cells such as marginal zone B cells in the spleen. In this study, we analyze APC populations in mouse spleen and compare the phenotype and function of B220(+)CD11c(-) populations with those of CD11c(+) spleen DC subsets. Low-density B220(+) cells had morphology similar to DCs and, like DCs, they could stimulate naive T cells, and expressed high levels of MHC and costimulatory molecules. However, the majority of the B220(+) cells appeared to be of B cell lineage as demonstrated by coexpression of CD19 and surface Ig, and by their absence from RAG-2(-/-) mice. The phenotype of these DC-like B cells was consistent with that of B cells in the marginal zone of the spleen. On bacterial stimulation, they preferentially produced IL-10 in contrast to the DCs, which produced IL-12. Conventional B cells did not produce IL-10. The DC-like B cells could be induced to express low levels of the DC marker CD11c with maturational stimuli. A minority of the B220(+)CD11c(-) low-density cells did not express CD19 and surface Ig and may be a DC subset; this population also produced IL-10 on bacterial stimulation. B220(+) APC in mouse spleen that stimulate naive T cells and preferentially produce IL-10 may be involved in activating regulatory immune responses.

The alternatively spliced alpha(E)C domain of human fibrinogen-420 is a novel ligand for leukocyte integrins alpha(M)beta(2) and alpha(X)beta(2)

The interaction of human plasma fibrinogen with leukocyte integrins alpha(M)beta(2) (CD11b/CD18, Mac-1) and alpha(X)beta(2) (CD11c/CD18, p150,95) is an important component of the inflammatory response. Previously, it was demonstrated that binding of fibrinogen to these integrins is mediated by gammaC, the globular C-terminal domain of the gamma chain. In this study, evidence was found of another fibrinogen domain that can serve as a ligand for the 2 leukocyte integrins: alpha(E)C, a homologous domain that extends the alpha chains in a recently discovered subclass of fibrinogen known as fibrinogen-420. Recombinant alpha(E)C supported strong adhesion and migration of cells expressing alpha(M)beta(2) and alpha(X)beta(2), including nonactivated and activated U937 and THP-1 monocytoid cells, and neutrophils. Cells transfected with complementary DNA for these integrins also bound alpha(E)C. The specificity of interaction was substantiated by inhibition of cell adhesion with antibodies against alpha(M), alpha(X), and beta(2) subunits. Also, neutrophil inhibitory factor, a specific inhibitor of alpha(M)beta(2) and alpha(X)beta(2) function, efficiently blocked cell adhesion to alpha(E)C. In alpha(M)beta(2) and alpha(X)beta(2), the I domain is the binding site for alpha(E)C, since alpha(E)C bound to recombinant alpha(M) I and alpha(X)I domains in a dose-dependent and saturable manner. Synthetic peptides that duplicated sequences gamma190 to 202 and gamma377 to 395, previously considered putative binding sites in gammaC, effectively inhibited alpha(M)beta(2)- and alpha(X)beta(2)-mediated adhesion to alpha(E)C, suggesting that recognition of alpha(E)C by the I domain involves structural features in common with those of gammaC. These findings identify alpha(E)C as a second domain in fibrinogen-420 that binds alpha(M)beta(2) and alpha(X)beta(2) and can mediate leukocyte adhesion and migration.

Apoptosis and altered dendritic cell homeostasis in lupus nephritis are limited by anti-CD154 treatment

Autoimmunity results from a failure in central and/or peripheral tolerance; however, the events that initiate and maintain this dysfunction remain unclear. To better understand the mediators involved in autoimmunity, we investigated the cellular mechanisms maintaining disease in the (SWR x NZB)F(1) (SNF(1)) mouse model of systemic lupus erythematosus. Previously, we have shown that autoimmunity in this model is dependent on CD40-CD154 interactions. Herein, our studies reveal that the severity of disease in SNF(1) mice correlates with a marked increase in the frequency of apoptotic splenocytes, including a higher proportion of apoptotic dendritic cells (DC) in vivo. In addition, we demonstrate a significant disease-related increase in the absolute number of splenic CD11c(high) DC. The increased DC number appears to be attributable to DC proliferation and enhanced migration to the spleen, most likely induced by elevated splenic expression of secondary lymphoid chemokine. Importantly, these imbalances in apoptosis, secondary lymphoid chemokine expression, and DC homeostasis were reduced or normalized by anti-CD154 treatment. Thus, our data demonstrate CD154-dependent regulation of apoptosis and DC homeostasis in mice with lupus-like autoimmune disease. We suggest that these mechanisms comprise an autostimulatory loop, maintaining the cascade of autoimmunity by DC presentation of self-Ags derived from apoptotic cells and CD154-mediated costimulation.

Amino acid residues in the PSI domain and cysteine-rich repeats of the integrin beta2 subunit that restrain activation of the integrin alpha(X)beta(2)

The leukocyte integrin alpha(X)beta(2) (p150,95) recognizes the iC3b complement fragment and functions as the complement receptor type 4. alpha(X)beta(2) is more resistant to activation than other beta(2) integrins and is inactive in transfected cells. However, when human alpha(X) is paired with chicken or mouse beta(2), alpha(X)beta(2) is activated for binding to iC3b. Activating substitutions were mapped to individual residues or groups of residues in the N-terminal plexin/semaphorin/integrin (PSI) domain and C-terminal cysteine-rich repeats 2 and 3. These regions are linked by a long range disulfide bond. Substitutions in the PSI domain synergized with substitutions in the cysteine-rich repeats. Substitutions T4P, T22A, Q525S, and V526L gave full activation. Activation of binding to iC3b correlated with exposure of the CBR LFA-1/2 epitope in cysteine-rich repeat 3. The data suggest that the activating substitutions are present in an interface that restrains the human alpha(X)/human beta(2) integrin in the inactive state. The opening of this interface is linked to structural rearrangements in other domains that activate ligand binding.

Expression of the CD11c antigen in B-cell chronic lymphoproliferative disorders

This study analyzed the expression of the beta2 integrin CD11c in 155 patients with well-characterized B-cell chronic lymphoproliferative disorders: 106 B-cell chronic lymphocytic leukemias (B-CLL), 21 hairy cell leukemias (HCL), 9 B-cell prolymphocytic leukemias (PLL) and 19 low grade non-Hodgkin's lymphomas (NHL) in leukemic phase. CD11c was expressed in 100% of patients with HCL and B-PLL, while in B-CLL and NHL it was expressed in only 49 and 57%, respectively. Furthermore, in B-CLL the expression of CD11c was found mainly in patients with early stage of disease. In addition, when the fluorescence intensity of CD11c, calculated by MFI, was evaluated, it proved significantly higher in HCL and B-PLL compared to the values recorded in B-CLL and NHL (325 and 387 vs 34 and 56, respectively) (p < 0.05). Our results demonstrate that the evaluation of CD11c, both in terms of overall positivity and of fluorescence intensity, represents an additional useful parameter for a more precise differential diagnosis within the spectrum of B-cell chronic lymphoproliferative disorders.

Role of beta2 integrins in the prevention of apoptosis induction in chronic lymphocytic leukemia B cells

Immunologically committed lymphocytes, especially mature, leukemic B cells, proliferate then accumulate without further cell division in chronic lymphocytic leukemia patients (CLL). These mature, leukemic B cells often produce autoantibodies. Under normal circumstances, immunologically committed lymphocytes that are autoreactive are deleted by a programmed cell death mechanism. In CLL cells, these mechanisms appear to be inhibited; therefore, cells accumulate rather than be destroyed. To understand the mechanism by which cell survival is selected over death in CLL cells, we studied the role of beta2 integrins and their ligands in the regulation of apoptosis. CLL cells were treated with monoclonal antibodies directed against beta2 integrins. Antibodies directed against the I-domain of the alpha chain of CD11b/CD18 inhibited apoptosis. The identity of the physiological ligand or counter-receptor for beta2 integrins that was required for the inhibition of apoptosis induction was sought. The ligand iC3b, but not ICAM-1 or fibrinogen, was identified as a ligand that could prevent apoptosis of CLL B cells. Free iC3b levels were elevated in CLL patients indicating that this ligand is available in vivowhere it may interact with beta2 integrins on CLL B cells and sustain their viability by preventing activation of the programmed cell death pathway. Leukemia (2000) 14, 34-39.

Matrix valency regulates integrin-mediated lymphoid adhesion via Syk kinase

Lymphocytes accumulate within the extracellular matrix (ECM) of tumor, wound, or inflammatory tissues. These tissues are largely comprised of polymerized adhesion proteins such as fibrin and fibronectin or their fragments. Nonactivated lymphoid cells attach preferentially to polymerized ECM proteins yet are unable to attach to monomeric forms or fragments of these proteins without previous activation. This adhesion event depends on the appropriate spacing of integrin adhesion sites. Adhesion of nonactivated lymphoid cells to polymeric ECM components results in activation of the antigen receptor-associated Syk kinase that accumulates in adhesion-promoting podosomes. In fact, activation of Syk by antigen or agonists, as well as expression of an activated Syk mutant in lymphoid cells, facilitates their adhesion to monomeric ECM proteins or their fragments. These results reveal a cooperative interaction between signals emanating from integrins and antigen receptors that can serve to regulate stable lymphoid cell adhesion and retention within a remodeling ECM.

Characterization of Human Fibroleukin, a Fibrinogen-Like Protein Secreted by T Lymphocytes

We have recently cloned the human homologue of the murine pT49 cDNA (hpT49h), a transcript encoding a protein homologous to the β- and γ-chains of fibrinogen. Here, we report the identification of the hpT49h gene product using mAbs generated against a peptide corresponding to the carboxyl-terminal end of the deduced protein and a recombinant protein fragment expressed in Escherichia coli. mAbs 23A6, 7B12, and 3F4 specifically recognized a protein of 70 kDa in reducing SDS-PAGE in the culture supernatant of 293T cells transiently transfected with the full length hpT49h cDNA and freshly isolated PBMC. Under nonreducing conditions, the material migrated with a molecular mass of 250 to 300 kDa, indicating that the 70-kDa protein forms a disulfide bonded complex. Because of its homology with fibrinogen, we have termed this protein fibroleukin. Fibroleukin is spontaneously secreted in vitro by freshly isolated CD4+ and CD8+ T lymphocytes. RT-PCR analysis revealed preferential expression of fibroleukin mRNA in memory T lymphocytes (CD3+/CD45R0+) compared with naive T lymphocytes (CD3+/CD45RA+). Fibroleukin production by PBMC was rapidly lost in culture. Production could be partially maintained in the presence of IFN-γ, while T lymphocyte activation had no effect. To demonstrate fibroleukin production in vivo, we analyzed colon mucosa by immunohistology. Fibroleukin staining was detected in the extracellular matrix of the T lymphocyte-rich upper portion of the lamina propria mucosa. While the exact function of fibroleukin remains to be defined, these data suggest that fibroleukin may play a role in physiologic lymphocyte functions at mucosal sites.

Focussing human B cell specificity by immunoselection via antigen-presenting cells in vitro

We present a protocol for in vitro immunization of B cells using monocyte-derived accessory cells (MoAC). MoAC are developed from human peripheral blood monocytes in culture and represent functionally competent inducers of antigen-specific immune responses. Using MoAC, we attempted to immunoselect TT-specific lymphocytes by rosetting. Adherent human MoAC were pulsed with tetanus toxoid (TT) and allowed to form clusters with autologous lymphocytes, followed by removal of non-adherent cells. After one week of culture, a specific anti-TT antibody response emerged on a low background of unspecific Ig. In comparison, cultures which had not been selected for adherent cells produced a high polyclonal background. Our results demonstrate that from peripheral blood cells, previously not a favourable source for in vitro immunization, in a majority of tests antigen-specific B cells could efficiently be immunoselected via adherence to autologous antigen-presenting cells, leading to a high-titre in vitro immunization.

CCAAT-enhancer-binding proteins (C/EBP) regulate the tissue specific activity of the CD11c integrin gene promoter through functional interactions with Sp1 proteins

The CD11c/CD18 integrin binds lipopolysaccharide, fibrinogen, and heparin, and mediates leukocyte adhesion, spreading, and migration. CD11c/CD18 is primarily found on myeloid cells and its expression is regulated during myeloid differentiation by transcriptional mechanisms acting on the CD11c gene promoter. We now describe that CCAAT/enhancer-binding proteins (C/EBP) contribute to the basal, tissue-specific and developmentally regulated activity of the CD11c promoter. A C/EBP-binding site within the CD11c promoter (CEBP-80) is bound by CEBPalpha in undifferentiated U937 cells and by C/EBPalpha- and C/EBPbeta-containing dimers in phorbol 12-myristate 13-acetate-differentiating cells, and its disruption decreased the CD11c promoter activity in a cell type-dependent manner. C/EBPalpha transactivated the CD11c promoter through the CEBP-80 element, and C/EBPalpha transactivation was also dependent on the Sp1-70- and Sp1-120 Sp1-binding sites. The -90/-50 fragment from the CD11c promoter, containing the adjacent CEBP-80, Sp1-70, and AP1-60 sites, differentially enhanced the activity of the minimal prolactin promoter in hematopoietic and epithelial cells. Altogether, these results demonstrate that C/EBP factors participate in the tissue-restricted and regulated expression of the CD11c/CD18 integrin through functional interactions with Sp1, suggest that Sp1-related factors modulate C/EBPalpha transcriptional activity on the CD11c promoter, and demonstrate the existence of a composite regulatory element recognized by C/EBP, Sp1, and AP-1 factors and whose enhancing effects are cell-type dependent.

A mitogenic action for fibrinogen mediated through intercellular adhesion molecule-1

Intercellular adhesion molecule-1 (ICAM-1) is a cell surface ligand for alphaLbeta2 and alphaMbeta2 integrins and has a key role in leukocyte adhesion to the vascular endothelium. The plasma protein fibrinogen has also been shown to interact with ICAM-1. We have investigated the effect of fibrinogen binding to ICAM-1-expressing cells on cell proliferation. The inclusion of 200-800 nM fibrinogen but not fibronectin to the culture medium of Raji induced a 2-4-fold increase in [3H]thymidine incorporation after 8 h. Cell proliferation in cultures containing fibrinogen was also confirmed by direct cell counting. The proliferative response in Raji was abrogated by an anti-ICAM-1 mAb 84H10 which maps to the first Ig domain of ICAM-1. A purified truncated form of ICAM-1 containing the first two Ig-like domains and a peptide with amino acid sequence corresponding to ICAM-1 (8-22) was also able to block the proliferative action of fibrinogen on Raji. 200 nM fibrinogen induced a 3-fold increase in [3H]thymidine incorporation by 293 cells transfected with ICAM-1 cDNA but not control non-transfected 293 cells. Comparable mitogenic effects were achieved with fibrinogen fragments X and D100, and with a synthetic peptide with an amino acid sequence matching fibrinogen gamma chain (117-133). These results indicate that interaction between discrete sequences within ICAM-1 and fibrinogen result in cellular proliferation.

CD11c integrin gene promoter activity during myeloid differentiation

The integrin CD11c/CD18 functions as a cell surface receptor for numerous soluble factors and proteins (LPS, fibrinogen, iC3b), mediates leukocyte interactions with other cell types and is a signal transducing receptor. CD11c/CD18 is found primarily on myeloid cells, where its expression is regulated both during differentiation and during monocyte maturation into tissue macrophages. To determine the transcription factors and cis-acting elements driving the developmentally-regulated expression of CD11c/CD18 the proximal regulatory region of the CD11c gene has been structurally and functionally characterized using the U937 and HL-60 cell lines as myeloid differentiation models. The tissue-specific activity of the CD11c promoter is conferred by two Sp1-binding sites and an adjacent C/EBP-binding element, with a likely contribution from other transcription factors with a more limited tissue distribution (PU.1, Oct-2, Myb). The participation of Sp1 in the transcription of the CD11c gene strongly suggests that CD11c/CD18 expression is dependent on the proliferative state of the cell, thus establishing a first level of control for the regulated expression of CD11c/CD18 during myeloid differentiation. The differentiation responsiveness of the CD11c promoter has been mapped to an AP-1-binding site whose mutation greatly decreases the inducibility of the promoter during the PMA-triggered differentiation of U937 cells. Although AP-1 mediates the responsiveness to several other differentiating agents including GM-CSF, additional elements are required for induction of the CD11c promoter activity upon Sodium Butyrate-triggered differentiation. In fact, the Sodium Butyrate-responsiveness and the presence of both AP-1- and C/EBP-binding sites suggests that the proximal regulatory region of the CD11c promoter might include an extracellular matrix-response element. As a whole, the transcription of the CD11c gene appears to be controlled by the proliferative state of the cell and is tightly coupled to progression along the myeloid differentiation pathway. The differentiation inducibility of the CD11c promoter has been further demonstrated after stable transfection into U937 cells, where the -361/+43 fragment retains the capacity to drive luciferase expression upon PMA-, GM-CSF- or Sodium Butyrate-triggered myeloid differentiation. Thus, while the characterization of the transcription factors regulating CD11c expression is still in progress, the CD11c promoter has been shown to constitute a very useful tool for the identification of myeloid-differenting agents which might be of potential therapeutical interest.