A CD22-Shp1 phosphatase axis controls integrin ß7 display and B cell function in mucosal immunity.

The integrin α4ß7 selectively regulates lymphocyte trafficking and adhesion in the gut and gut-associated lymphoid tissue (GALT). Here, we describe unexpected involvement of the tyrosine phosphatase Shp1 and the B cell lectin CD22 (Siglec-2) in the regulation of α4ß7 surface expression and gut immunity. Shp1 selectively inhibited ß7 endocytosis, enhancing surface α4ß7 display and lymphocyte homing to GALT. In B cells, CD22 associated in a sialic acid-dependent manner with integrin ß7 on the cell surface to target intracellular Shp1 to ß7. Shp1 restrained plasma membrane ß7 phosphorylation and inhibited ß7 endocytosis without affecting ß1 integrin. B cells with reduced Shp1 activity, lacking CD22 or expressing CD22 with mutated Shp1-binding or carbohydrate-binding domains displayed parallel reductions in surface α4ß7 and in homing to GALT. Consistent with the specialized role of α4ß7 in intestinal immunity, CD22 deficiency selectively inhibited intestinal antibody and pathogen responses.

Crossing the barriers: Revisiting the gut feeling in rheumatoid arthritis.

To avoid autoimmunity, it is essential to keep the balance between the defence against pathogens and the maintenance of tolerance to self-antigens. Mucosal inflammation may lead to breakdown of tolerance and activation of autoreactive cells. Growing evidence suggests a major contribution of gut microbiota to the onset of chronic, autoimmune inflammatory diseases including rheumatoid arthritis (RA). RA patients show significant differences in the composition of gut microbiota compared to healthy controls, and in murine arthritis models certain bacteria can induce inflammatory Th17 responses or autoantibody production. The gut microbiota plays an important role in regulating the balance between immunogenic and tolerogenic immune responses. The intestinal barrier is the site of the body where most host-microbiota interaction takes place. Certain microbiota or their metabolites can cause a break in homeostasis by affecting the intestinal barrier integrity and permeability. However, an intact intestinal barrier is essential to separate the intestinal epithelium from toxins, microorganisms, and antigens in the gut lumen. This review will focus on the correlation between a leaky gut and the onset of arthritis. Furthermore, it will be discussed how targeting the intestinal barrier function by dietary changes might provide an opportunity to modulate the development of RA.

The B-cell inhibitory receptor CD22 is a major factor in host resistance to Streptococcus pneumoniae infection.

Streptococcus pneumoniae is a major human pathogen, causing pneumonia and sepsis. Genetic components strongly influence host responses to pneumococcal infections, but the responsible loci are unknown. We have previously identified a locus on mouse chromosome 7 from a susceptible mouse strain, CBA/Ca, to be crucial for pneumococcal infection. Here we identify a responsible gene, Cd22, which carries a point mutation in the CBA/Ca strain, leading to loss of CD22 on B cells. CBA/Ca mice and gene-targeted CD22-deficient mice on a C57BL/6 background are both similarly susceptible to pneumococcal infection, as shown by bacterial replication in the lungs, high bacteremia and early death. After bacterial infections, CD22-deficient mice had strongly reduced B cell populations in the lung, including GM-CSF producing, IgM secreting innate response activator B cells, which are crucial for protection. This study provides striking evidence that CD22 is crucial for protection during invasive pneumococcal disease.

B-cell antigen receptor expression and phosphatidylinositol 3-kinase signaling regulate genesis and maintenance of mouse chronic lymphocytic leukemia.

Chronic lymphocytic leukemia (CLL) is a frequent lymphoproliferative disorder of B cells. Although inhibitors targeting signal proteins involved in B-cell antigen receptor (BCR) signaling constitute an important part of the current therapeutic protocols for CLL patients, the exact role of BCR signaling, as compared to genetic aberration, in the development and progression of CLL is controversial. In order to investigate whether BCR expression per se is pivotal for the development and maintenance of CLL B cells, we used the TCL1 mouse model. By ablating the BCR in CLL cells from TCL1 transgenic mice, we show that CLL cells cannot survive without BCR signaling and are lost within 8 weeks in diseased mice. Furthermore, we tested whether mutations augmenting B-cell signaling influence the course of CLL development and its severity. The phosphatidylinositol-3-kinase (PI3K) signaling pathway is an integral part of the BCR signaling machinery and its activity is indispensable for B-cell survival. It is negatively regulated by the lipid phosphatase PTEN, whose loss mimics PI3K pathway activation. Herein, we show that PTEN has a key regulatory function in the development of CLL, as deletion of the Pten gene resulted in greatly accelerated onset of the disease. By contrast, deletion of the gene TP53, which encodes the tumor suppressor p53 and is highly mutated in CLL, did not accelerate disease development, confirming that development of CLL was specifically triggered by augmented PI3K activity through loss of PTEN and suggesting that CLL driver consequences most likely affect BCR signaling. Moreover, we could show that in human CLL patient samples, 64% and 81% of CLL patients with a mutated and unmutated IgH VH, respectively, show downregulated PTEN protein expression in CLL B cells if compared to healthy donor B cells. Importantly, we found that B cells derived from CLL patients had higher expression levels of the miRNA-21 and miRNA-29, which suppresses PTEN translation, compared to healthy donors. The high levels of miRNA-29 might be induced by increased PAX5 expression of the B-CLL cells. We hypothesize that downregulation of PTEN by increased expression levels of miR-21, PAX5 and miR-29 could be a novel mechanism of CLL tumorigenesis that is not established yet. Together, our study demonstrates the pivotal role for BCR signaling in CLL development and deepens our understanding of the molecular mechanisms underlying the genesis of CLL and for the development of new treatment strategies.

Human CD22-Transgenic, Primary Murine Lymphoma Challenges Immunotherapies in Organ-Specific Tumor Microenvironments.

Targeted immunotherapies have greatly changed treatment of patients with B cell malignancies. To further enhance immunotherapies, research increasingly focuses on the tumor microenvironment (TME), which differs considerably by organ site. However, immunocompetent mouse models of disease to study immunotherapies targeting human molecules within organ-specific TME are surprisingly rare. We developed a myc-driven, primary murine lymphoma model expressing a human-mouse chimeric CD22 (h/mCD22). Stable engraftment of three distinct h/mCD22+ lymphoma was established after subcutaneous and systemic injection. However, only systemic lymphoma showed immune infiltration that reflected human disease. In this model, myeloid cells supported lymphoma growth and showed a phenotype of myeloid-derived suppressor cells. The human CD22-targeted immunotoxin Moxetumomab was highly active against h/mCD22+ lymphoma and similarly reduced infiltration of bone marrow and spleen of all three models up to 90-fold while efficacy against lymphoma in lymph nodes varied substantially, highlighting relevance of organ-specific TME. As in human aggressive lymphoma, anti-PD-L1 as monotherapy was not efficient. However, anti-PD-L1 enhanced efficacy of Moxetumomab suggesting potential for future clinical application. The novel model system of h/mCD22+ lymphoma provides a unique platform to test targeted immunotherapies and may be amenable for other human B cell targets such as CD19 and CD20.

Epratuzumab modulates B-cell signaling without affecting B-cell numbers or B-cell functions in a mouse model with humanized CD22.

Treatment of systemic lupus erythematosus patients with epratuzumab (Emab), a humanized monoclonal antibody targeting CD22, leads to moderately reduced B-cell numbers but does not completely deplete B cells. Emab appears to induce immunomodulation of B cells, but the exact mode of action has not been defined. In the present study, we aimed to understand the effects of Emab on B cells using a humanized mouse model (Huki CD22), in which the B cells express human instead of murine CD22. Emab administration to Huki CD22 mice results in rapid and long-lasting CD22 internalization. There was no influence on B-cell turnover, but B-cell apoptosis ex vivo was increased. Emab administration to Huki CD22 mice had no effect on B-cell numbers in several lymphatic organs, nor in blood. In vitro exposure of B cells from Huki CD22 mice to Emab resulted in decreased B-cell receptor (BCR) induced Ca(2+) mobilization, whereas B-cell proliferation after Toll-like receptor (TLR) stimulation was not affected. In addition, IL-10 production was slightly increased after TLR and anti-CD40 stimulation, whereas IL-6 production was unchanged. In conclusion, Emab appears to inhibit BCR signaling in a CD22-dependent fashion without strong influence on B-cell development and B-cell populations.

CD22 ligand-binding and signaling domains reciprocally regulate B-cell Ca2+ signaling.

A high proportion of human B cells carry B-cell receptors (BCRs) that are autoreactive. Inhibitory receptors such as CD22 can downmodulate autoreactive BCR responses. With its extracellular domain, CD22 binds to sialic acids in α2,6 linkages in cis, on the surface of the same B cell or in trans, on other cells. Sialic acids are self ligands, as they are abundant in vertebrates, but are usually not expressed by pathogens. We show that cis-ligand binding of CD22 is crucial for the regulation of B-cell Ca(2+) signaling by controlling the CD22 association to the BCR. Mice with a mutated CD22 ligand-binding domain of CD22 showed strongly reduced Ca(2+) signaling. In contrast, mice with mutated CD22 immunoreceptor tyrosine-based inhibition motifs have increased B-cell Ca(2+) responses, increased B-cell turnover, and impaired survival of the B cells. Thus, the CD22 ligand-binding domain has a crucial function in regulating BCR signaling, which is relevant for controlling autoimmunity.

Epistatic effects of Siglec-G and DNase1 or DNase1l3 deficiencies in the development of systemic lupus erythematosus.

Systemic lupus erythematosus (SLE) is a severe autoimmune disease that displays considerable heterogeneity not only in its symptoms, but also in its environmental and genetic causes. Studies in SLE patients have revealed that many genetic variants contribute to disease development. However, often its etiology remains unknown. Existing efforts to determine this etiology have focused on SLE in mouse models revealing not only that mutations in specific genes lead to SLE development, but also that epistatic effects of several gene mutations significantly amplify disease manifestation. Genome-wide association studies for SLE have identified loci involved in the two biological processes of immune complex clearance and lymphocyte signaling. Deficiency in an inhibitory receptor expressed on B lymphocytes, Siglec-G, has been shown to trigger SLE development in aging mice, as have mutations in DNA degrading DNase1 and DNase1l3, that are involved in clearance of DNA-containing immune complexes. Here, we analyze the development of SLE-like symptoms in mice deficient in either Siglecg and DNase1 or Siglecg and DNase1l3 to evaluate potential epistatic effects of these genes. We found that germinal center B cells and follicular helper T cells were increased in aging Siglecg -/- x Dnase1 -/- mice. In contrast, anti-dsDNA antibodies and anti-nuclear antibodies were strongly increased in aging Siglecg-/- x Dnase1l3-/- mice, when compared to single-deficient mice. Histological analysis of the kidneys revealed glomerulonephritis in both Siglecg -/- x Dnase1 -/- and Siglecg-/- x Dnase1l3-/- mice, but with a stronger glomerular damage in the latter. Collectively, these findings underscore the impact of the epistatic effects of Siglecg with DNase1 and Dnase1l3 on disease manifestation and highlight the potential combinatory effects of other gene mutations in SLE.

Similar inflammatory DC maturation signatures induced by TNF or Trypanosoma brucei antigens instruct default Th2-cell responses.

DCs represent the major cell type leading to polarized T-helper (Th) cell responses in vivo. Here, we asked whether the instruction of murine Th2 responses by DCs matured with the proinflammatory cytokine TNF is qualitatively different from maturation by different types of TLR4/MyD88-dependent variant-specific surface glycoproteins (VSGs) of Trypanosoma brucei (T. brucei). The results obtained by analyzing DC surface markers, Notch ligand mRNA, cytokines, asthma, and experimental autoimmune encephalomyelitis (EAE) models as well as performing microarrays indicate that both types of stimuli induce similar inflammatory, semi-mature DC profiles. DCs matured by TNF or VSG treatment expressed a common inflammatory signature of 24 genes correlating with their Th2-polarization capacity. However, the same 24 genes and 4498 additional genes were expressed by DCs treated with LPS that went on to induce Th1 cells. These findings support the concept of a default pathway for Th2-cell induction in DCs matured under suboptimal or inflammatory conditions, independent of the surface receptors and signaling pathways involved. Our data also indicate that quantitative differences in DC maturation might direct Th2- vs Th1-cell responses, since suboptimally matured inflammatory DCs induce default Th2-cell maturation, whereas fully mature DCs induce Th1-cell maturation.

B Cell Siglecs-News on Signaling and Its Interplay With Ligand Binding.

CD22 and Siglec-G are members of the Siglec family. Both are inhibitory co-receptors on the surface of B cells and inhibit B-cell receptor induced signaling, characterized by inhibition of the calcium mobilization and cellular activation. CD22 functions predominantly as an inhibitor on conventional B cells, while Siglec-G is an important inhibitor on the B1a-cell subset. These two B-cell Siglecs do not only inhibit initial signaling, but also have an important function in preventing autoimmunity, as double deficient mice develop a lupus-like phenotype with age. Siglecs are characterized by their conserved ability to bind terminal sialic acid of glycans on the cell surface, which is important to regulate the inhibitory role of Siglecs. While CD22 binds α2,6-linked sialic acids, Siglec-G can bind both α2,6-linked and α2,3-linked sialic acids. Interestingly, ligand binding is differentially regulating the ability of CD22 and Siglec-G to control B-cell activation. Within the last years, quite a few studies focused on the different functions of B-cell Siglecs and the interplay of ligand binding and signal inhibition. This review summarizes the role of CD22 and Siglec-G in regulating B-cell receptor signaling, membrane distribution with the importance of ligand binding, preventing autoimmunity and the role of CD22 beyond the naïve B-cell stage. Additionally, this review article features the long time discussed interaction between CD45 and CD22 with highlighting recent data, as well as the interplay between CD22 and Galectin-9 and its influence on B-cell receptor signaling. Moreover, therapeutical approaches targeting human CD22 will be elucidated.

B7-H1-deficiency enhances the potential of tolerogenic dendritic cells by activating CD1d-restricted type II NKT cells.

BACKGROUND: Dendritic cells (DC) can act tolerogenic at a semi-mature stage by induction of protective CD4(+) T cell and NKT cell responses. METHODOLOGY/PRINCIPAL FINDINGS: Here we studied the role of the co-inhibitory molecule B7-H1 (PD-L1, CD274) on semi-mature DC that were generated from bone marrow (BM) cells of B7-H1(-/-) mice and applied to the model of Experimental Autoimmune Encephalomyelitis (EAE). Injections of B7-H1-deficient DC showed increased EAE protection as compared to wild type (WT)-DC. Injections of B7-H1(-/-) TNF-DC induced higher release of peptide-specific IL-10 and IL-13 after restimulation in vitro together with elevated serum cytokines IL-4 and IL-13 produced by NKT cells, and reduced IL-17 and IFN-gamma production in the CNS. Experiments in CD1d(-/-) and Jalpha281(-/-) mice as well as with type I and II NKT cell lines indicated that only type II NKT cells but not type I NKT cells (invariant NKT cells) could be stimulated by an endogenous CD1d-ligand on DC and were responsible for the increased serum cytokine production in the absence of B7-H1. CONCLUSIONS/SIGNIFICANCE: Together, our data indicate that BM-DC express an endogenous CD1d ligand and B7-H1 to ihibit type II but not type I NKT cells. In the absence of B7-H1 on these DC their tolerogenic potential to stimulate tolerogenic CD4(+) and NKT cell responses is enhanced.