Soluble CD40 ligand induces expression of CD25 and CD23 in resting human tonsillar B lymphocytes

Identification of new phosphorylation sites of CD23 in B-cells of patients with chronic lymphocytic leukemia

B-cell chronic lymphocytic leukemia (B-CLL) is the most common lymphoproliferative disorder in adults. Patients with B-CLL strongly express the CD23 - C type of lectin (low affinity IgE receptor, Fc epsilon RII), which is linked to B cell activation and proliferation. Phosphorylation in lymphocytes is tightly associated with regulation of protein activities, functional regulation and cell signaling, and may thus affect initiation and/or progression of the disease. Here we report changes in the phosphorylation of CD23 on threonine (pThr314) and two serine residues (pSer254, pSer265) in B lymphocytes of B-CLL patients, using a flow cytometry approach. The majority of tested patients with active forms of B-CLL presented a notable overexpression of CD23 along with pThr314, pSer254, and pSer265 CD23 phosphorylation positivity. Moreover, we have experimentally stimulated the CD23 phosphorylations in a subset of peripheral blood lymphocytes of healthy controls by phorbol-12-myristate-13-acetate treatment. This affects the activation of competent phosphorylation mediating kinases, resulting in the enhanced phosphorylation pattern. Together, these data confirm that CD23 protein is phosphorylated in B cells of B-CLL patients, report the identification of new CD23 phosphorylation sites, and suggest a possible role(s) of such phosphorylations in the activation of CD23 during the process of lymphocytic activation in B-CLL.

Relative levels of the inflammatory cytokine TNFα and the soluble CD40 ligand profile in serum correlate with the thyrotoxic activity of Graves' disease

BACKGROUND

Interactions between CD40 and its ligand (CD40L) have important roles in T-cell-dependent activation of B cells, which may be related to the thyrotoxic activity of Graves' disease (GD). Soluble forms of CD40 ligand (sCD40L) are released from activated T cells and platelets, and several types of inflammatory cytokines are increased in patients with hyperthyroid GD. The aim of this study was to assess sCD40L and other cytokines as clinical indicators of disease activity or as possible markers of remission in GD.

METHODS

Serum levels of sCD40L, interleukin 18 (IL-18), tumor necrosis factor-alpha (TNFα), and TNFα receptors 1 and 2 (TNFR1 and TNFR2) were investigated in patients with active GD (GD-A), intractable GD (GD-IT), inactive GD (GD-IA), GD in remission (GD-R), and Hashimoto's thyroiditis (HT), and in control subjects (CON).

RESULTS

Serum concentrations of sCD40L were higher in the GD-A and GD-IT groups than in the HT and CON groups. Similarly, serum concentrations of IL-18, which induces Th1 cytokines, such as interferon-γ, were higher in the GD-A and GD-IT groups than in all other groups. Serum levels of TNFR1 and TNFR2 were also significantly higher in the GD-A than in all other groups. The mean serum concentration of TNFα was higher in the GD-R compared with the GD-A and GD-IT groups, although the difference was not significant. Serum sCD40L concentrations in the GD-R group were lower than in the GD-A and GD-IT groups. Finally, the ratio of serum TNFα to sCD40L was higher in the GD-R group than in the GD-A and GD-IT groups. This is the first report that serum sCD40L is increased in active GD, and that the serum TNFα:sCD40L ratio is a marker for remission in GD.

CONCLUSIONS

Our results suggest that not only thyrotoxicosis, but also the activity of the immunoreaction presenting as anti-thyrotropin receptor antibodies (TRAb) titer in GD, affects inflammatory cytokine serum profiles. Serum profiles of cytokines vary in patients with GD depending on disease activity. An elevated serum TNFα:sCD40L ratio indicates declining disease activity and reflects a shift from Th2 to Th1 dominance, suggesting that suppression of sCD40L or increased production of TNFα is required to initiate or maintain remission of GD.

TLR7/8 ligand, R-848, inhibits IgE synthesis by acting directly on B lymphocytes

TLRs are involved in the regulation of immune responses. R-848, a TLR7/8 ligand, has potent anti-viral and anti-tumour properties and has been used as a new immune response modifier for enhancing Th1 immune response. In this study, we found that R-848 significantly inhibited IgE synthesis from murine B cells at the single cell levels by anti-CD40 plus IL-4-stimulated splenocytes, in which R-848 acted on the early stage of B cell differentiation to modulate IgE synthesis. This inhibitory effect of R-848 on IgE synthesis was not isotype specific as it also inhibited IgG1 synthesis. Moreover, R-848 had no significant effect on the production of IgG2a by anti-CD40 plus IL-4-stimulated splenocytes. Further studies showed that R-848 markedly promoted murine B cell activation induced by anti-CD40 plus IL-4 by up-regulating the expression of B cell activation markers CD25, CD69 and co-stimulatory molecule CD80. In contrast, R-848 inhibited the proliferation and division of murine B cells in anti-CD40 plus IL-4-stimulated splenocytes. R-848 promoted the production of IFN-gamma and IL-12 that were partially responsible for its inhibitory effect on IgE production by anti-CD40 plus IL-4 because the addition of anti-IFN-gamma or anti-IL-12 mAbs to the cultures could significantly restore IgE production by splenocytes. Importantly, R-848 had a direct effect on purified B cells to inhibit IgE production induced by anti-CD40 plus IL-4. Taken together, these results demonstrate that R-848 markedly inhibits IgE synthesis, and suggest that R-848 could be used to treat allergic diseases.

The CD23a and CD23b proximal promoters display different sensitivities to exogenous stimuli in B lymphocytes

The single human CD23 gene encodes two protein products differing by six or seven amino acids in the extreme N-terminal cytoplasmic domain. The patterns of expression of CD23a and CD23b transcripts differs as a function of cell type and cell stimulation, with expression of CD23a being largely restricted to B cells and CD23b synthesis being inducible in a variety of haematopoietic cells by a range of exogenous stimuli. In this study, short defined sequences of the CD23a and CD23b proximal promoter regions were used to drive expression of exogenous reporter genes in transiently-transfected B cells exposed to a range of cellular stimuli. The CD23a promoter was activated only by IL-4, whereas the CD23b promoter was stimulated not only by IL-4, but also by stimulation with anti-mu, and anti-CD40. Deletion mutant analysis illustrated that of the two putative STAT6 binding sites present in the CD23a proximal promoter, deletion of the first site abrogated IL-4-driven transcriptional activation. Conversely, deletion of both STAT6 binding sites in the CD23b promoter was required before IL-4 sensitivity was lost. When the same CD23b promoter mutants were studied in the context of anti-CD40 and anti-mu stimulation of transfected cells, deletion of the NF-kappaB site abrogated anti-CD40-driven transcriptional activation, but not anti-mu-mediated effects which required additional deletion of putative AP1 sites lying close to the CD23b initiator methionine codon. The data of this report are consistent with the interpretation that the upstream regions of the CD23a and CD23b isoform coding sequences show distinct sensitivities to agents which induce CD23 protein expression at the plasma membrane, and that transcriptional activation by discrete stimuli reflects activation of particular transcriptional regulatory factors.

Epstein-Barr virus latent membrane protein-1 activates CD25 expression in lymphoma cells involving the NFkappaB pathway

Epstein-Barr virus (EBV) is associated with several human malignancies including Burkitt's lymphoma (BL), Hodgkin's disease (HD) and nasopharyngeal carcinoma. A variety of cytokines and receptors have been described to be activated by EBV. Here we show that the IL-2 receptor (IL-2R) alpha-chain, which is weakly expressed on normal resting lymphoid cells, is activated by EBV. Comparison of EBV-negative BL cell lines and their EBV convertants showed an enhanced CD25 expression in EBV-positive BL cells. Transient expression of the oncogenic virus protein latent membrane protein-1 (LMP1) in L428 Hodgkin's lymphoma cells and in Burkitt's lymphoma cells (BL2, BL41, BL30) cells leads to enhanced CD25 expression. Both C-terminal activating regions (CTARs) of LMP1 are involved in CD25 activation. Inhibition of LMP1-mediated NFkappaB enhancement by a constitutive repressive form of IkappaB-alpha resulted in decreased CD25 surface expression, indicating that NFkappaB is involved in CD25 gene regulation. Furthermore, LMP1-mediated CD25 activation was associated with enhanced levels of the soluble form of CD25 (sCD25) in L428 Hodgkin's lymphoma cells but not in BL cells. LMP1 associated enhanced expression of membrane CD25 and soluble CD25 may have immunomodulatory functions and could be involved in biology of EBV-associated diseases.

CD4-independent infection of human B cells with HIV type 1: detection of unintegrated viral DNA

Although B lymphocytes are a major constituent of lymphoid organs and acquire a significantly altered phenotype and function in HIV-infected individuals, it remains unclear whether CD4-negative B cells are a susceptible host for viral entry and long-term productive infection. We screened a number of Epstein-Barr virus (EBV)-positive and-negative Burkitt's lymphoma (BL) B cell lines as well as subpopulations of normal B cells that include tonsillar naive and germinal center/memory B cells for the expression of HIV-1 receptors CD4, CXCR4, and CCR5. Cell lines and resting or activated normal B cells lacked CD4 and CCR5 but expressed CXCR4. We demonstrate HIV-1 infection of a CD4-negative, EBV-negative (BL) cell line, CA46, which remained productively infected yet noncytopathic for more than 36 months in culture. HIV-1 (HTLV-III(B)) infection of CA46 cells was mediated through CXCR4 in a CD4-independent manner and correlated with upregulation of the expression of B cell activation markers CD23 and CD95 (Fas receptor). Despite Fas receptor expression, HIV-1-infected CA46 cells remained resistant to Fas-mediated cell death. CA46-derived, CD4-independent viral isolates were proficient in infecting and causing syncytium formation in Molt4 T cells. The HIV-1 genomic organization in persistently infected CA46 clones was found to be predominantly unintegrated linear and circular DNA. Importantly, naive and germinal center/memory B cells could also be infected by HIV-1 in a CD4-independent manner. Although these B cell subpopulations expressed moderate to high levels of CXCR4, they required activation through CD40 and interleukin 4 receptor for infection. These findings point to B cells as an additional HIV-1 target and suggest a structural evolution of the HIV-1 genome responsible for CD4-independent and noncytopathic infections.

Decreased levels of CD54 (ICAM-1)-positive lymphocytes in the peripheral blood in untreated patients with active juvenile dermatomyositis

Significant abnormalities are observed in the peripheral blood of juvenile dermatomyositis (JDM) patients with active disease. In this study, we confirm that there is a significant increase in the relative percentage of B lymphocytes in the peripheral blood of a group of untreated children with newly diagnosed active JDM compared to healthy children (P < 0.0001). In order to investigate if properties intrinsic to B cells contributed to their relative increase in JDM, the percentage of B cells expressing activation markers (CD23, CD25, CD54, and CD69) was measured and compared to pediatric controls. Compared to healthy children less than 10 years of age (not significantly different from the JDM group), the JDM patients had an increase in the proportion of lymphocytes expressing CD19 (B cells; P = 0.0017) and decreases in the percentage of lymphocytes that were CD3(-) CD16(+) and/or CD56(+) (NK cells; P = 0. 01) and CD3(+) CD8(+) (T suppressor/cytotoxic cells; P = 0.02). There were no significant differences in any of the B-cell activation markers assessed. Of note, the percentage of CD54(+) non-B lymphocytes (i.e., T cells and NK cells expressing CD54) was significantly lower in the JDM patients (25% +/- 5%) than in the "age-related" healthy control group (43% +/- 4%; P = 0.013). These results suggest the following for untreated children with active JDM: (i) the increase in the percentage of peripheral blood B cells is not due to intrinsic B-cell activation, and (ii) CD54/ICAM-1(+) non-B cells, CD8(+) T cells, and NK cells are being removed from circulation and may be participating in the pathophysiology of the disease.

Inhibition of a secondary human alloimmune response via the soluble active component of CD154 (CD40L) in severe combined immune-deficient mice engrafted with human lymphocytes

BACKGROUND

Alloimmunization requires a process known as co-stimulation. An important co-stimulatory pathway for most immune responses is mediated by the interaction of CD40 on antigen-presenting cells with CD154 (CD40L) on host T cells. Blockade of this co-stimulatory pathway simultaneous with exposure to challenge with HLA-incompatible cells is hypothesized to inhibit alloimmunization.

STUDY DESIGN AND METHODS

Severe combined immune-deficient (SCID) mice were reconstituted with human peripheral blood lymphocytes (Hu-PBL-SCID mice) from a subject primed to HLA antigens and challenged with HLA-incompatible lymphocytes. Mice were challenged in the presence or absence of an 18-kDa soluble recombinant active form of human CD154 (18-kDa CD154). Human IgG production, alloimmunization, and in vitro T-cell responsiveness were assessed.

RESULTS

There was no significant effect of 18-kDa CD154 on human IgG levels in these mice, but it inhibited the development of HLA-specific alloantibody in this model to five subsequent untreated white cell challenges. In vitro T-cell proliferation in a mixed lymphocyte culture was also prevented by 18-kDa CD154.

CONCLUSION

The recombinant protein 18-kDa CD154 inhibited the ability of the Hu-PBL-SCID mice to mount a secondary immune response to allostimulation. This implies that transfusion-induced alloimmunization utilizes CD40-CD154 co-stimulation and that blockade of this pathway can inhibit T-cell function and interfere with the development of alloimmunization.

Elevated levels and functional capacity of soluble CD40 ligand in systemic lupus erythematosus sera

OBJECTIVE

To measure soluble CD40 ligand (sCD40L) in sera from patients with systemic lupus erythematosus (SLE) and to study the functional capacity of sCD40L in mediating B cell activation.

METHODS

A 2-site enzyme-linked immunosorbent assay (ELISA) was used to measure sCD40L in the sera of 66 SLE patients, 30 disease control patients, and 23 healthy subjects. Induction of B cell activation antigen expression was used to assess the functional capacity of sCD40L in SLE sera.

RESULTS

The mean concentration of sCD40L was statistically significantly higher (P < 0.0001) in SLE patients than in disease controls or healthy subjects, and segregation of SLE patients by severe, moderate, or mild extent of disease showed a relationship between disease severity and sCD40L concentration. Western blot analysis demonstrated the presence of the 18-kd band of sCD40L in SLE sera, and the results of a 1-site ELISA protocol suggested that some of the product in SLE sera was present in dimer or trimer form. Functional studies showed that 10 ng/ml of recombinant CD40L, a level present in some SLE sera, induced increased expression of CD95 on B cells. Several SLE sera also induced CD95 or CD86 on Ramos B cells, a result that was inhibited by anti-CD40L monoclonal antibodies.

CONCLUSION

The soluble form of CD40L is present in the sera of most patients with SLE and may have the capacity to mediate B cell activation. Aberrant expression of CD40L might be predicted to result in activation of bystander B cells, including those that have encountered self antigens, and to contribute to autoantibody secretion.

Early lymphocyte activation in elderly humans: impaired T and T-dependent B cell responses

Immunosenescence is characterized by an increase in autoantibody production. Because both T and B cell stimulation are key events for producing antibodies, we investigated early T and B cell activation by means of CD23 and CD40L (two very early activation antigens). PBMC from elderly humans (EH) were studied following culture with either medium, anti-CD3mAb, rIL-4, or PMA + ionomycin. CD23 expression on elderly B cells after anti-CD3 challenge of PBMC, a reflect of T-dependent B cell activation, was clearly defective. Conversely, CD23 expression on EH B cells following activation with soluble factors as rIL-4 was preserved. CD40L expression was also impaired in EH T cells following anti-CD3 challenge. However, activation by means of PMA and/or ionomycin was preserved both in T cells (CD40L expression) and in B cells (CD23 expression). These results indicate that a defective T-dependent B cell activation related to defective T cell activation located between surface membrane and PKC/ionomycin function is an intrinsic characteristic of immunosenescence. We have not found intrinsic B-cell defects, and we conclude that the characteristically impaired early B cell activation in EH is mostly due to T cell defects.

Differential Responses to CD40 Ligation Among Burkitt Lymphoma Lines That Are Uniformly Responsive to Epstein-Barr Virus Latent Membrane Protein 1

Ligation of CD40 on the surface of B cells induces multiple phenotypic effects, many of which are mimicked by the EBV latent membrane protein 1 (LMP1) through its interaction with downstream components of the CD40 signaling pathway. Because the effects of LMP1 have been most closely studied in human Burkitt Lymphoma (BL) cell lines retaining a tumor biopsy-like phenotype in vitro, we have examined the response of a panel of such lines to CD40 ligation. Two distinct patterns of response were observed that were unrelated to the surface level of CD40 or to the EBV genome status of the lines. Following exposure to either CD40-specific mAbs or the soluble trimeric ligand (sCD40L), high responder (HR) lines showed rapid aggregation, activation of NF-κB, up-regulation of cell surface markers ICAM-1/CD54 and Fas/CD95, and growth inhibition. Aggregation was seen at lower doses than those required to elicit the other effects. By contrast, low responder (LR) lines showed no detectable response to CD40 mAbs, while their responses to sCD40L were limited to activation of NF-κB and up-regulation of CD95 only. However, in transfection experiments, LMP1 uniformly induced the full spectrum of phenotypic effects in both HR and LR lines. We conclude that some BL cell lines show a highly restricted response to CD40 ligation but remain fully susceptible to LMP1.

Two differently regulated nuclear factor kappaB activation pathways triggered by the cytoplasmic tail of CD40

CD40 signaling modulates the immune response at least in part by activation of nuclear factor kappaB (NFkappaB). It has been shown that two distinct domains in the CD40 cytoplasmic tail (cyt), namely cyt-N and cyt-C, independently activate NFkappaB. Although four members of the tumor necrosis factor receptor-associated factor (TRAF) family, including TRAF2, TRAF3, TRAF5, and TRAF6, bind to the CD40 cyt, how each TRAF protein contributes to the NFkappaB activation by CD40 is not clear. Here we report that TRAF2, TRAF3, and TRAF5 bind cyt-C, whereas TRAF6 binds cyt-N. cyt-N is conserved poorly between human and mouse CD40, while cyt-C is highly conserved. However, single aa substitution of Glu-235 in cyt-N of human CD40 with Ala abolishes the binding of TRAF6 to cyt-N and NFkappaB activation by cyt-N. Conservation of this Glu between mouse and human CD40 strongly suggests that TRAF6 could link cyt-N to signals essential for CD40-mediated immune response. Furthermore, NFkappaB activation by cyt-C is inhibited by a kinase-negative form of NFkappaB-inducing kinase more efficiently than that by cyt-N, consistent with the result that NFkappaB activation by TRAF2 and TRAF5 is inhibited by a kinase-negative form of NFkappaB-inducing kinase more efficiently than that by TRAF6. These results indicate that NFkappaB activating signals emanating from cyt-N and cyt-C are mediated by the different members of the TRAF family and could be regulated in a distinct manner.