The ligand-binding domain of CD22 is needed for inhibition of the B cell receptor signal, as demonstrated by a novel human CD22-specific inhibitor compound.

CD22 is a B cell-specific transmembrane protein of the Siglec family. It binds specifically to alpha2,6-linked sialic acid (Sia) residues, which are also present on glycoproteins on the B cell surface. CD22 acts as a negative regulator in B cell receptor-mediated signaling by recruitment of Src homology 2 domain-containing tyrosine phosphatase (SHP)-1 to its intracellular tail. To analyze how ligand-binding of CD22 influences its intracellular signaling domain, we designed synthetic sialosides as inhibitors for the lectin domain of CD22. One of these compounds inhibited binding of human CD22-Fc to target cells over 200-fold better than Sia and was highly selective for human CD22. When Daudi cells or primary B cells were stimulated with anti-immunoglobulin (Ig)M in presence of this sialoside inhibitor, a higher Ca(2+) response was observed, similar to CD22-deficient B cells. Accordingly, a lower tyrosine-phosphorylation of CD22 and SHP-1 recruitment was demonstrated in presence of the sialoside. Thus, by interfering with ligand binding of CD22 on the B cell surface, we have shown for the first time that the lectin domain of CD22 has a direct, positive influence on its intracellular inhibitory domain. Also, we have developed a novel low molecular weight compound which can enhance the response of human B cells.

A modified RMCE-compatible Rosa26 locus for the expression of transgenes from exogenous promoters.

Generation of gain-of-function transgenic mice by targeting the Rosa26 locus has been established as an alternative to classical transgenic mice produced by pronuclear microinjection. However, targeting transgenes to the endogenous Rosa26 promoter results in moderate ubiquitous expression and is not suitable for high expression levels. Therefore, we now generated a modified Rosa26 (modRosa26) locus that combines efficient targeted transgenesis using recombinase-mediated cassette exchange (RMCE) by Flipase (Flp-RMCE) or Cre recombinase (Cre-RMCE) with transgene expression from exogenous promoters. We silenced the endogenous Rosa26 promoter and characterized several ubiquitous (pCAG, EF1α and CMV) and tissue-specific (VeCad, αSMA) promoters in the modRosa26 locus in vivo. We demonstrate that the ubiquitous pCAG promoter in the modRosa26 locus now offers high transgene expression. While tissue-specific promoters were all active in their cognate tissues they additionally led to rare ectopic expression. To achieve high expression levels in a tissue-specific manner, we therefore combined Flp-RMCE for rapid ES cell targeting, the pCAG promoter for high transgene levels and Cre/LoxP conditional transgene activation using well-characterized Cre lines. Using this approach we generated a Cre/LoxP-inducible reporter mouse line with high EGFP expression levels that enables cell tracing in live cells. A second reporter line expressing luciferase permits efficient monitoring of Cre activity in live animals. Thus, targeting the modRosa26 locus by RMCE minimizes the effort required to target ES cells and generates a tool for the use exogenous promoters in combination with single-copy transgenes for predictable expression in mice.

Strong and ubiquitous expression of transgenes targeted into the beta-actin locus by Cre/lox cassette replacement.

Conventional approaches to produce transgenic mice recurrently yield unpredictable patterns and levels of transgene expression, a situation calling for the development of new techniques to overcome these drawbacks in the context of overexpression studies. Here we present an efficient method for rapid and reproducible transgenesis using the recombinase mediated cassette exchange (RMCE) (Bouhassira et al.: Blood 90:3332-3344, 1997) procedure. A lox511-EGFP-TK/neo-loxP cassette was placed under the control of the endogenous mouse beta-actin promoter. Heterozygous mice revealed strong and ubiquitous EGFP expression throughout embryogenesis and adulthood. Reproducibly, the same expression pattern was obtained with RMCE when it was used to replace the EGFP-harboring cassette by ECFP or placental alkaline phosphatase (PLAP) reporter genes (DePrimo et al.: Transgenic Res 5:459-466, 1996). Furthermore, the RMCE procedure proved efficient as well in embryonic stem (ES) cells as directly in zygotes. Our results demonstrate ubiquitous expression of floxed transgenes in the endogenous beta-actin locus and they support the general use of the beta-actin locus for targeted transgenesis.

Transitional and marginal zone B cells have a high proportion of unmasked CD22: implications for BCR signaling.

CD22, a B cell-specific member of the Siglec family, is an important inhibitor of B cell signaling. The first Ig-like domain of CD22 specifically binds to alpha2,6-linked sialic acids. Through these interactions CD22 can mediate adhesion to other cells in trans, but can also bind endogenous ligands on the B cell surface in cis. Cis binding of CD22 to sialylated ligands enhances the efficiency of inhibition and thereby reduces the BCR signaling strength. In this study we used a newly developed oligomeric streptavidin-based sialylated probe as an artificial CD22 ligand. We found that CD22 is bound to ligands in cis on most B cells. However, there is a proportion of B cells with unbound (unmasked) CD22. The subpopulation with unmasked CD22 is 2-fold increased in transitional and marginal zone B cells in the spleen and on B1 cells in the peritoneum, when compared to mature B cells. Also, B cells with unmasked CD22 have an activated phenotype. Unmasking of CD22 could be functionally involved in lowering the signaling threshold on developmental checkpoints such as transitional B cells and during B cell activation or could be a consequence of such activation processes.

Reduction of marginal zone B cells in CD22-deficient mice.

CD22 is a B cell-specific member of the immunoglobulin superfamily and binds to sialic acid. CD22 inhibits B cell receptor signaling. Mice deficient for CD22 show a largely normal B cell development. Here, we have performed a detailed analysis of the splenic B cell population and found that the subset of marginal zone (MZ) B cells was selectively reduced in CD22-deficient mice. CD22-deficient mice showed a lack of TNP-ficoll capturing cells in the MZ and a reduced response to TNP-ficoll, particularly when the antigen was applied intravenously. CD22-deficient B cells showed both enhanced motility as well as enhanced chemotaxis to certain chemokines. The altered chemokine responsiveness or the higher signaling capacity of CD22-deficient B cells may lead to the compromised MZ B cell compartment, as both processes have previously been shown to affect MZ composition.

Constitutively unmasked CD22 on B cells of ST6Gal I knockout mice: novel sialoside probe for murine CD22.

The interaction of CD22 with glycoprotein ligands bearing the Siaalpha2,6Gal-R sequence is believed to modulate its function as a regulator of B cell signaling. Although a commercial sialoside-polyacrylamide (PAA) probe, NeuAc- alpha2,6Gal-PAA, has facilitated studies on ligand binding by human CD22, murine CD22 binds instead with high affinity to NeuGcalpha2,6Gal-R. A multivalent probe with this sequence was constructed to facilitate investigations of ligand binding in CD22 function using genetically defined murine models. The probe is based on the sialoside-PAA platform, which is then biotinylated for easy detection. A series of sialoside probes were constructed with two different length linker arms between the sialoside and the backbone and three different sialoside to PAA molar ratios. The NeuGcalpha2,6Gal-PAA probe is specific for CD22: it binds to sialidase-treated B cells of wild-type mice but not B cells of CD22-null mice. Additionally, because the probe only binds to sialidase-treated wild-type cells, it confirms that CD22 is constitutively "masked" on most B cells from wild-type mice by binding to ligands in cis. In contrast, the probe bound equally well to native or sialidase-treated B cells from the immunocompromised ligand-deficient ST6Gal I knockout mice, demonstrating that CD22 is constitutively "unmasked" in these cells.