CD40 ligation impedes lymphoblastoid B cell proliferation and S-phase entry

Evidence from genome wide association studies implicates reduced control of Epstein-Barr virus infection in multiple sclerosis susceptibility

BACKGROUND

Genome wide association studies have identified > 200 susceptibility loci accounting for much of the heritability of multiple sclerosis (MS). Epstein-Barr virus (EBV), a memory B cell tropic virus, has been identified as necessary but not sufficient for development of MS. The molecular and immunological basis for this has not been established. Infected B cell proliferation is driven by signalling through the EBV produced cell surface protein LMP1, a homologue of the MS risk gene CD40.

METHODS

We have investigated transcriptomes of B cells and EBV-infected B cells at Latency III (LCLs) and identified MS risk genes with altered expression on infection and with expression levels associated with the MS risk genotype (LCLeQTLs). The association of LCLeQTL genomic burden with EBV phenotypes in vitro and in vivo was examined. The risk genotype effect on LCL proliferation with CD40 stimulation was assessed.

RESULTS

These LCLeQTL MS risk SNP:gene pairs (47 identified) were over-represented in genes dysregulated between B and LCLs (p < 1.53 × 10^-4), and as target loci of the EBV transcription factor EBNA2 (p < 3.17 × 10^-16). Overall genetic burden of LCLeQTLs was associated with some EBV phenotypes but not others. Stimulation of the CD40 pathway by CD40L reduced LCL proliferation (p < 0.001), dependent on CD40 and TRAF3 MS risk genotypes. Both CD40 and TRAF3 risk SNPs are in binding sites for the EBV transcription factor EBNA2, with expression of each correlated with EBNA2 expression dependent on genotype.

CONCLUSIONS

These data indicate targeting EBV may be of therapeutic benefit in MS.

Maintenance of the CD40-related immunodeficient response in hyper-IgM B cells immortalized with a LMP1-regulated mini-EBV

Our previous investigation of a patient (pt1) with non-X-linked hyper-immunoglobulin M syndrome revealed a CD40-mediated defect in B cell activation that resulted in low CD23 expression and absence of germ-line transcription and class-switch recombination. These deficiencies were complemented in vitro by a high threshold of sustained signaling through CD40. To further analyze the signaling defect in pt1 B cells, two types of Epstein-Barr virus lymphoblastoid cell lines (LCLs) were generated that either constitutively expressed the viral transforming protein latent membrane protein-1 (LMP1; pt1-LCL) or expressed it under the control of a tet-inducible promoter (pt1-LCL(tet)). Because LMP1 signals through the CD40 pathway, the pt1-LCL and pt1-LCL(tet) lines allow comparison of downstream functions in response to either constitutive LMP1 signals or regulated LMP1 and CD40 signals. Immortalized pt1-LCLs were initially CD23(lo)/CD38(hi) and reverted to a CD23(hi)/CD38(lo) phenotype upon extended growth in culture, suggesting that the CD40 defect was reversed by selection and/or constitutive expression of LMP1. In contrast, pt1-LCL(tet) cells retained the CD23(lo)/CD38(hi) phenotype after extended periods of culture and failed to up-regulate CD23 in response to CD40 signals. Analysis of pt1-LCL(tet) cells in response to the CD40 signals in the presence or absence of LMP1 revealed that mitogenic activation resulted only from LMP1 and not CD40, indicating a difference in the response of pt1 B cells to these two distinct signals. Together, these data demonstrate that the pt1-LCL(tet) cells maintain the CD40-related defect and provide a unique approach to study the independent effects of LMP1- and CD40-directed signals.

CD40 ligation downregulates EBNA-2 and LMP-1 expression in EBV-transformed lymphoblastoid cell lines

Epstein-Barr virus (EBV) drives the proliferation of human B cells in vitro and during primary infection in vivo. The transformed immunoblasts express nuclear proteins EBNA1-6, transcribed from the Cp/Wp promoter, and the membrane proteins LMP-1, -2A and -2B (lymphoblastoid type of latency). EBV persists through life in resting memory B cells with a restricted type of latency in the absence of the Cp/Wp promoter activity. Since CD40 crosslinking can reportedly inhibit the growth of EBV-transformed lymphoblastoid cell lines (LCLs), we have examined the effect of CD40 ligation on the expression of EBNAs and LMP-1 and on Cp EBV promoter activity together with several phenotypic markers. CD40 crosslinking led to a partial downregulation of EBNA-2, EBNA3-6 and LMP-1 in LCLs, paralleled by downregulation of Cp promoter activity. It also induced upregulation of the germinal center marker CD77 on the LCL cells. Our findings suggest that the encounter of proliferating EBV-transformed immunoblasts with CD40L, as would occur when normal B cells generate memory cells in germinal centers, may switch the viral transcription program from the full lymphoblastoid to a more restricted latency program in a proportion of cells. This would permit virus persistence in the B-cell memory compartment.

CD40 ligation alters the cell cycle of differentiating keratinocytes

CD40 is expressed in normal human keratinocytes, especially in the basal cell layer. We have recently reported that CD40 ligation strongly inhibits keratinocyte proliferation and induces their differentiation. In this study, the CD40 pathway that prevents keratinocyte growth was investigated. We first reported that interferon-gamma treatment potentiated the CD40-mediated inhibition of keratinocyte proliferation. CD40-CD40 ligand interactions, in the presence or absence of interferon-gamma, neither enhanced spontaneous keratinocyte apoptosis, nor did it enhance apoptosis induced by various agents. More importantly, we showed that CD40 signaling altered the keratinocyte cell cycle, as demonstrated by a decreasing number of cells in the G1 and S phases and an accumulation in G2/M phase of the cell cycle. Furthermore, western blot analysis of cell cycle regulatory proteins, showed a decrease in cyclin A and E expression in CD40-activated keratinocytes. Collectively, these results indicate that CD40 ligation inhibits keratinocyte renewal by a mechanism independent of cell apoptosis and that modulation of the keratinocyte cell cycle is an additional outcome of CD40 signaling.

Proliferative response of mantle cell lymphoma cells stimulated by CD40 ligation and IL-4

Mantle cell lymphoma (MCL) is a tumor of intermediate-size, IgM+, IgD+ B cells derived from the mantle zone of the germinal center. Little is known about its specific immunologic features or responsiveness to T cell-derived signals. In this work, we evaluated the proliferation and cell cycle properties of freshly isolated MCL cells after CD40 ligation, in the absence and presence of interleukin 4 (IL-4). In each MCL case examined, there was a marked growth-enhancing effect of these two stimuli characterized by improved viability, augmented expression of Ki-67, and induction of the proliferating cell nuclear antigen (PCNA). Cyclin D1 was expressed throughout the cell cycle in MCL cells induced to enter S phase. From these investigations, we conclude that the biology of MCL B lymphocytes is affected by CD154 (CD40 ligand) and IL-4, two signals usually provided by CD4+ T cells. The capacity to manipulate the activation and cell cycle state of MCL cells by these specific immunological stimuli may be exploited to confer susceptibility to chemotherapy agents and develop novel therapies in this disease.

Bryostatin and CD40-ligand enhance apoptosis resistance and induce expression of cell survival genes in B-cell chronic lymphocytic leukaemia

Modulating signal transduction pathways represents a promising approach for altering the biological behaviour of haemopoietic malignancies. B-cell chronic lymphocytic leukaemia (B-CLL) cells were treated in vitro with CD40-ligand (CD40L) (CD154) or the protein kinase C modulator Bryostatin-1, exploring the effects on: (a) sensitivity to apoptosis induction by chemotherapeutic drugs (fludarabine, dexamethasone) or anti-Fas antibody; (b) expression of apoptosis-regulatory proteins (Bcl-2, Bcl-X, Mcl-1, Bax, Bak, BAG-1, Flip, XIAP); (c) expression of cell surface co-stimulatory antigens (CD80 [B7.1]; CD54 [ICAM-1]; CD70); and (d) expression of immune modulatory receptors (CD27, CD40, CD95 [Fas]). CD40L and Bryostatin decreased both spontaneous and drug-induced apoptosis in most B-CLL specimens tested. Apoptosis resistance was associated with CD40L- and Bryostatin-induced elevations in the anti-apoptotic Bcl-2 family protein Mcl-1. CD40L also induced striking increases in the levels of the anti-apoptotic protein Bcl-XL in B-CLLs. CD40L stimulated increases in the surface expression of CD40, CD54, CD69, CD70, CD80 and CD95, whereas Bryostatin induced expression of CD40, CD54, CD69 and CD95 but not the co-stimulatory molecules CD70 and CD80. Despite elevations in the expression of CD95 (Fas), anti-Fas antibodies failed to induce apoptosis of CD40L- and Bryostatin-treated B-CLL cells. This Fas-resistance was associated with increased expression of the Fas-antagonist Flip in CD40L-treated, and with elevations in the caspase inhibitor XIAP in Bryostatin-treated B-CLLs. The potential anti-apoptotic properties of CD40L and Bryostatin should be taken into consideration when employing these agents in clinical trials involving patients with B-CLL.