Minoru Hasegawa

CD22 regulates B lymphocyte function in vivo through both ligand-dependent and ligand-independent mechanisms.

The interaction of CD22 with α2,6-linked sialic acid ligands has been widely proposed to regulate B lymphocyte function and migration. Here, we generated gene-targeted mice that express mutant CD22 molecules that do not interact with these ligands. CD22 ligand binding regulated the expression of cell surface CD22, immunoglobulin M and major histocompatibility complex class II on mature B cells, maintenance of the marginal zone B cell population, optimal B cell antigen receptor–induced proliferation, and B cell turnover rates. However, CD22 negative regulation of calcium mobilization after B cell antigen receptor ligation, CD22 phosphorylation, recruitment of SHP-1 to CD22 and B cell migration did not require CD22 ligand engagement. These observations resolve longstanding questions regarding the physiological importance of CD22 ligand binding in the regulation of B cell function in vivo.

Complement Receptors CD21/35 Link Innate and Protective Immunity during Streptococcus pneumoniae Infection by Regulating IgG3 Antibody Responses

The CD21/35 receptor provides an important link between innate and adaptive immunity. Its importance during protective immune responses to encapsulated extracellular bacteria was assessed using a new line of mice completely deficient in CD21/35 expression (CD21/35(-/-)). CD21/35 expression was essential for the rapid trapping of C3dg-antigen complexes by B cells in vivo, especially in splenic marginal zones. Despite normal B cell development in CD21/35(-/-) mice, T cell-independent and -dependent antibody responses to low-dose antigens were significantly decreased, with a striking impairment in IgG3 responses. Accordingly, CD21/35(-/-) mice were more susceptible to acute lethal Streptococcus pneumoniae infection. Thus, CD21/35 expression is critical for early protective antibody responses to lethal pathogens that rapidly multiply and quickly overwhelm the immune system.

CD19 Can Regulate B Lymphocyte Signal Transduction Independent of Complement Activation

B lymphocytes are critically regulated by signals transduced through the CD19-CD21 cell surface receptor complex, where complement C3d binding to CD21 supplies an already characterized ligand. To determine the extent that CD19 function is controlled by complement activation, CD19-deficient mice (that are hyporesponsive to transmembrane signals) and mice overexpressing CD19 (that are hyperresponsive) were crossed with CD21- and C3-deficient mice. Cell surface CD19 and CD21 expression were significantly affected by the loss of CD21 and C3 expression, respectively. Mature B cells from CD21-deficient littermates had ∼36% higher cell surface CD19 expression, whereas CD21/35 expression was increased by ∼45% on B cells from C3-deficient mice. Negative regulation of CD19 and CD21 expression by CD21 and C3, respectively, may be functionally significant because small increases in cell surface CD19 overexpression can predispose to autoimmunity. Otherwise, B cell development and function in CD19-deficient and -overexpressing mice were not significantly affected by a simultaneous loss of CD21 expression. Although CD21-deficient mice were found to express a hypomorphic cell surface CD21 protein at low levels that associated with mouse CD19, C3 deficiency did not significantly affect B cell development and function in CD19-deficient or -overexpressing mice. These results, and the severe phenotype exhibited by CD19-deficient mice compared with CD21- or C3-deficient mice, collectively demonstrate that CD19 can regulate B cell signaling thresholds independent of CD21 engagement and complement activation.

CD19 Regulates Intrinsic B Lymphocyte Signal Transduction and Activation Through a Novel Mechanism of Processive Amplification

The fate of B lymphocytes is dependent on intrinsic and B cell antigen receptor (BCR)-induced signals. These signals are interpreted and modified by response regulators such as CD19 that govern mature B cell activation. The current understanding of how CD19 governs B lymphocyte signaling is outlined in this review. Primarily, CD 19 establishes a novel Src-family kinase amplification loop that regulates basal signal transduction thresholds in resting B cells. Moreover, CD19 amplifies Src-family kinase activation following BCR ligation. CD19 amplification of Lyn activity leads to processive phosphorylation of CD19 and downstream substrates including CD22. Phosphorylated CD19 recruits other effector molecules including Vav, Grb2, phosphoinositide 3-kinase, phospholipase C γ2, and c-Abl, which may contribute to CD19 regulation of B cell function. CD19/Lyn complex formation also regulates phosphorylation of CD22 and FcγRIIB, which inhibit B cell signal transduction through the recruitment of the SHP1 and SHIP phosphatases. These observations provide insight into how CD19 governs the molecular ordering and intensity of signals transduced in B cells, and how perturbations in CD19 expression or signaling function may contribute to autoimmunity.

A CD19-Dependent Signaling Pathway Regulates Autoimmunity in Lyn-Deficient Mice

CD19 and the Src family protein tyrosine kinases (PTKs) are important regulators of intrinsic signaling thresholds in B cells. Regulation is achieved by cross-talk between Src family PTKs and CD19; Lyn is essential for CD19 phosphorylation, while CD19 establishes an Src family PTK activation loop that amplifies kinase activity. However, CD19-deficient (CD19−/−) B cells are hyporesponsive to transmembrane signals, while Lyn-deficient (Lyn−/−) B cells exhibit a hyper-responsive phenotype resulting in autoimmunity. To identify the outcome of interactions between CD19 and Src family PTKs in vivo, B cell function was examined in mice deficient for CD19 and Lyn (CD19/Lyn−/−). Remarkably, CD19 deficiency suppressed the hyper-responsive phenotype of Lyn−/− B cells and autoimmunity characterized by serum autoantibodies and immune complex-mediated glomerulonephritis in Lyn−/− mice. Consistent with Lyn and CD19 each regulating conventional B cell development, B1 cell development was markedly reduced by Lyn deficiency, with further reductions in the absence of CD19 expression. Tyrosine phosphorylation of Fyn and other cellular proteins induced following B cell Ag receptor ligation was dramatically reduced in CD19/Lyn−/− B cells relative to Lyn−/− B cells, while Syk phosphorylation was normal. In addition, the enhanced intracellular Ca2+ responses following B cell Ag receptor ligation that typify Lyn deficiency were delayed by the loss of CD19 expression. BCR-induced proliferation and humoral immune responses were also markedly inhibited by CD19/Lyn deficiency. These findings demonstrate that while the CD19/Lyn amplification loop is a major regulator of signal transduction thresholds in B lymphocytes, CD19 regulation of other Src family PTKs also influences B cell function and the development of autoimmunity.

Elevated serum L-selectin levels and abnormal regulation of L-selectin expression on leukocytes in atopic dermatitis: Soluble L-selectin levels indicate disease severity

BACKGROUND L-selectin mediates leukocyte rolling on endothelium at sites of inflammation, suggesting that L-selectin may be involved in the development of cutaneous lesions of atopic dermatitis (AD). After leukocyte activation, L-selectin is rapidly shed from the cell surface. OBJECTIVE The purpose of this study was to assess leukocyte L-selectin expression and quantitate levels of serum soluble L-selectin (sL-selectin) in patients with AD. METHODS Serum sL-selectin levels in patients with AD (n = 70), contact dermatitis (n = 18), and psoriasis (n = 23), as well as normal control subjects (n = 30), were examined by using an ELISA. The L-selectin expression on leukocytes in heparinized blood samples from patients with AD (n = 18) and normal control subjects (n = 10) was also examined by flow cytometry. RESULTS Serum levels of sL-selectin in patients with AD were significantly higher than those found in normal control subjects. Furthermore, sL-selectin levels correlated positively with disease severity and total serum IgE levels in AD. The expression of L-selectin on B cells, monocytes, and neutrophils was significantly decreased in patients with AD compared with normal control subjects, although those on CD4(+) or CD8(+) T cells from patients with AD were similar to those from normal control subjects. CONCLUSION Elevated sL-selectin levels and the abnormal expression of L-selectin on some leukocyte subsets in patients with AD may correlate with inflammation associated with AD. Furthermore, the level of sL-selectin may be a useful immunologic indicator for disease activity in AD.