Effect of B-cell receptor engagement on CD40-stimulated B cells

Activation of human B cells in vitro either by cross-linking of surface immunoglobulins (sIg) or by triggering CD40 antigen, in the presence of interleukin-10 (IL-10) and interleukin-2 (IL-2), may result in high levels of immunoglobulin secretion in vitro. We studied the combined effects of ligation of the B-cell receptor (BCR) and CD40 [with anti-CD40 monoclonal antibody (mAb)] on B-cell proliferation and production of human immunoglobulin. For this purpose highly purified splenic B cells were cultured with various combinations of anti-CD40 and IL-10/IL-2 or IL-4 in the presence of CD32-transfected L cells. Simultaneous cross-linking of the BCR was achieved by mAb held on CD32-L cells or Staphylococcus aureus (SA). We found that dual BCR and CD40 ligation with IL-10/IL-2 leads to reduced immunoglobulin G (IgG) secretion compared with B cells stimulated with either anti-CD40 and IL-10/IL-2, or compared with B cells stimulated with SA or anti-BCR mAb and IL-10/IL-2. Dual BCR and CD40 ligation with anti-immunoglobulin mAb (anti-kappa + anti-lambda light chains) but not with SA induced a similar reduction in IgM production. The reduced immunoglobulin secretion found during dual ligation is accompanied by increased proliferation. This was independent of cytokine stimulation but SA/CD40-induced proliferation was increased in the presence of IL-10/IL-2, although not with IL-4. The combination anti-kappa and anti-lambda with anti-CD40 showed a long-term suppression of IgG and IgM production (at least 14 days), while anti-kappa or anti-lambda alone, or SA, allowed a moderate recovery of immunoglobulin production by day 14. These results suggest that simultaneous B-cell antigen receptor cross-linking and CD40 engagement via CD40L on T cells induces strong initial proliferation. This may be followed later by antibody production depending on the strength of the BCR signal and the presence of the appropriate cytokines.     

Acquired CD40-ligand deficiency in chronic lymphocytic leukemia

Patients with B-cell chronic lymphocytic leukemia (CLL) acquire an immunodeficiency with many characteristics similar to those of persons with inherited defects in the gene encoding the CD40-ligand (CD154). We found that the blood and splenic CD4+ T cells of patients with CLL failed to express surface CD154 after CD3 ligation. However, using an enzyme-linked immunosorbent assay (ELISA)-based quantitative competitive polymerase chain reaction (PCR), we noted that CD3 ligation could induce such T cells to express CD154 messenger RNA at levels similar to that of CD3-activated T cells from normal donors. Moreover, addition of increasing numbers of CLL B cells to activated normal donor T cells rapidly resulted in progressively greater down-modulation of CD154. Such down-modulation of CD154 could be blocked by addition of CD40 monoclonal antibody to cultures in vitro. We propose that leukemia cell-mediated down-modulation of CD154 on activated T cells accounts for some of the acquired immune defects of patients with CLL.     

CD40 cross-linking inhibits specific antibody production by human B cells

Ligation of CD40 on B cells is a co-stimulatory signal for proliferation, antibody secretion, heavy chain switching and rescue from apoptosis after somatic mutation in the germinal centre. The importance of these manifold responses to CD40 activation for humoral immunity is exemplified by the inability of boys with X-linked hyper IgM syndrome to make IgG, IgE or IgA due to a mutation in in the gene coding for CD40 ligand (CD40L). In the present study, we have investigated the effect of CD40 ligation on specific antibody production by human B cells to influenza virus. The antibody response was T cell dependent and specific for the strain of influenza virus used as antigen. Addition of either CD40 mAb or recombinant trimeric CD40L profoundly inhibited specific antibody production. Antibody production by unseparated tonsillar mononuclear cells and by T-depleted B cells stimulated with antigen in the presence of T cell replacing factor were equally inhibited with CD40 antibody showing that the effect was due to ligation of CD40 on B cells rather than blocking of T cell help. The specific antibody detected in these experiments was mostly IgG with little or no IgM and was obtained from surface IgM B cells consistent with activation of a secondary (memory) response. Co-stimulation of tonsillar B cells with CD40 antibody and anti-IgG induced proliferation of IgG+ B cells. These results suggest that CD40 ligation can inhibit specific antibody responses and stimulate proliferation in the same IgG+ (memory) B cell subpopulation. Addition of CD40 antibody during the first 24-48 h of the response was required for inhibition, suggesting that the effect was on early B cell activation and/or proliferation required for antibody production. There was no correlation, however, between the ability of CD40 mAb to stimulate proliferation and inhibit antibody production. We suggest that early activation of CD40 in the specific antibody response inhibits the formation of plasma cells and promotes instead the generation of memory cells.     

Studies on the induction of antigen-specific antibody in anti-CD40 cultured human B lymphocytes

Costimulatory signals provided by T cells are required for B cells to produce specific antibody to T-dependent antigen. We have investigated the suitability of using the CD40 culture system for the proliferation and differentiation of Ag-specific human B cells using cytomegalovirus (CMV) or tetanus toxoid (TT) as antigen. We modified the CD40 culture system (CD32-transfected L cells, anti-CD40, and IL-4) by applying a sequential cytokine stimulation and compared total B-cell cultures with antigen-specific B cells preselected by panning. The detection of specific antibody became possible when antigen-selected B cells were cultured for 7 days in the CD40 system to induce clonal expansion, followed by the addition of IL-2 and IL-10 for an additional 7 days to induce plasma-cell differentiation. We conclude that our initial inability to detect specific antibody in the CD40 system is due to overgrowth of nonspecific B-cell clones and that selection of antigen-specific B cells by panning overcomes this problem. Induction of antigen-specific antibody production was found to be optimal when the initial contact with antigen during panning was limited to between 1 to 24 hours.     

Autologous bone marrow support and bone disease in metastatic breast cancer

Production of IgM, IgG and IgA was induced from human blood B lymphocytes by culturing with a CD40 MoAb and IL-2 for 9 days. Replacement of IL-2 by IL-10 markedly enhanced production of all three isotypes. High levels of immunoglobulin production also occurred when activated irradiated autologous T cells replaced the CD40 MoAb, and when IL-10 replaced IL-2 in these cultures a spectacular increase in IgG production occurred. The effectiveness of the T cell stimulus depended on the mode of purification of the T cells and the nature of the stimulant used to activate them. Differences in the kinetics and level of expression of CD40L on the various T cell preparations were observed, but did not account for variations in immunoglobulin-inducing efficiency. Immunoglobulin production from sIgD+ and sIgD- B cells was investigated. IgG and IgA were found in sIgD+ cultures, indicating that some isotype switching had occurred, but the major part of the IgG and IgA secreted was from cells already committed to these isotypes. Anti-IgD or anti-IgM MoAbs enhanced the proliferation of B cells induced by anti-CD40 antibody, but immunoglobulin production was not enhanced. Factors affecting the balance of proliferation and differentiation are discussed.     

IL-12 up-regulates CD40 ligand (CD154) expression on human T cells

IL-12 is a heterodimeric cytokine produced by APC that promotes the development of CD4+ Th1 cells and their IFN-gamma production after TCR/CD3 triggering. We here investigated the capacity of IL-12 to modify the expression on T cells of CD40 ligand (CD40L or CD154), a molecule transiently expressed on activated T cells and known to be of utmost importance for cognate interaction with B cells and for activation of dendritic cells and macrophages. Our data demonstrate that IL-12 up-regulates CD40L expression on anti-CD3-activated human peripheral blood T cells. For optimal induction of CD40L, IL-12 synergizes with IL-2 as well as with other costimulatory interactions, such as B7/CD28. The effect of IL-12 was observed at both the protein and the mRNA level. T cells costimulated by IL-12 provided more efficient help for IL-4-dependent B cell proliferation and for IgG production than when activated in the absence of IL-12. This helper activity was blocked by an mAb against CD40L, indicating that the effect of IL-12 on B cells is mediated indirectly through CD40L. The data thus suggest that the effects of IL-12 on cellular and humoral immune responses are partly mediated through CD40L induction.     

CD4+ T-cell population mediates development of inflammatory bowel disease in T-cell receptor alpha chain-deficient mice

BACKGROUND & AIMS: Increase of T cells expressing CD4 and T-cell receptor (TCR) alpha- beta+ (beta[dim]) was observed in the mucosal and peripheral lymphoid tissues of TCR alpha-/- mice with inflammatory bowel disease (IBD). The aim of this study was to characterize the CD4+ TCR alpha-beta+ T cells. METHODS: Cytokine production, TCR V beta usage, and helper function for Peyer's patch B cells by the CD4+ TCR alpha-beta+ T cells were assessed. RESULTS: The CD4+ TCR alpha-beta+ T cells purified from mesenteric lymph nodes and lamina propria of the intestine of IBD mice exclusively produced interleukin 4, used selected subsets (V beta6, V beta8, V beta14, and V beta15) of TCR, and massively proliferated after stimulation with staphylococcal enterotoxin B. Addition of the CD4+ TCR alpha-beta+ T cells to Peyer's patch B-cell cultures markedly enhanced immunoglobulin (Ig) A, IgG, and IgM antibody responses. Furthermore, depletion of the TCR alpha-beta+ T cells with monoclonal antibody against TCR beta chain completely suppressed the onset of IBD and polyclonal B-cell activation in the TCR alpha-/- mice. CONCLUSIONS: These findings suggest the CD4+ TCR alpha-beta+ T cells-mediated development of IBD in TCR alpha-/- mice.     

Preclinical antitumor activity of an antibody against the leukocyte antigen CD48

We have evaluated the antitumor activity of a murine antibody (IgG2a) against the leukocyte antigen CD48. CD48 is expressed on T and B lymphocytes, monocytes, and a wide range of lymphoid malignancies. To assess the therapeutic potential of an anti-CD48 antibody, we established a reproducible model of human B-cell (Raji) leukemia/lymphoma in C.B17/scid mice, where untreated mice develop hind leg paralysis due to tumor engraftment. Using this model, the murine anti-CD48 antibody HuLy-m3 was shown to mediate a strong in vivo antitumor effect. Long-term survival (>1 year) of scid mice was obtained after treatment with three 200-microg i.v. doses of anti-CD48 antibody on days 0, 2, and 4 after i.v. injection of tumor cells. In contrast, mice treated with an isotype control antibody developed hind leg paralysis after 34 +/- 3 days. A strong antitumor response was still observed when a dose of 20 microg of HuLy-m3 antibody was used. During preclinical investigations, we also examined a number of properties of the CD48 antigen. CD48 is present at high levels on the surface of T and B cells, but most (>95%) CD34-positive cells do not express CD48. Anti-CD48 antibodies are maintained on the surface of antigen-positive cells for extended periods (>24 h). These properties suggest that anti-CD48 antibodies may be useful in the treatment of a number of diseases including lymphoid leukemias and lymphomas.     

Cross-linking of the CD40 ligand on human CD4+ T lymphocytes generates a costimulatory signal that up-regulates IL-4 synthesis

Although there is good evidence that the induction of IL-4 synthesis in CD4+ T lymphocytes is favored by Ag presentation by B cells and not macrophages, the precise molecular signals provided by B cells to T cells that enhance IL-4 synthesis are not clear. To examine this issue, we established an APC-independent system to activate highly purified T cells and induce cytokine synthesis, using immobilized mAbs against several T cell surface molecules, including CD3, CD28, and the CD40 ligand (CD40L). The counter-receptors for all three of these molecules are expressed on B cells, and include CD40, which is expressed primarily on B cells, but also on dendritic cells and thymic epithelium. We found that IL-4 synthesis was greatly enhanced by triggering of CD40L on the T cell surface in conjunction with ligation of CD3/TCR and CD28, whereas ligation of CD3/TCR and CD28 in the absence of CD40L triggering resulted in little or no IL-4 synthesis. CD40L costimulation greatly enhanced IL-4 synthesis both in T cells from normal nonallergic adult subjects as well as in naive T cells from cord blood. Furthermore, we demonstrated that IL-4 synthesis was optimally enhanced when the strength of the CD3/TCR signal was limiting, while IL-4 synthesis was inhibited when CD3/TCR stimulation was maximal. These studies confirm that IL-4 synthesis can be induced in normal T lymphocytes in the absence of exogenous IL-4, and demonstrate that CD40L costimulation is of fundamental importance in regulation of IL-4 production. In addition, these findings provide a mechanism by which B cells preferentially enhance IL-4 synthesis in T cells at low Ag concentrations.     

Effects of type I/type II interferons and transforming growth factor-beta on B-cell differentiation and proliferation. Definition of costimulation and cytokine requirements for immunoglobulin synthesis and expression

In this report, we sought to determine the role of selected type I interferons [interferon-alpha (IFN-alpha) and interferon-tau (IFN-tau)], IFN-gamma and transforming growth factor-beta (TGF-beta) in the regulation of bovine antibody responses. B cells were stimulated via CD40 in the presence or absence of B-cell receptor (BCR) cross-linking. IFN-alpha enhanced IgM, IgG2 and IgA responses but did not enhance IgG1 responses. BCR signalling alone was more effective at inducing IgG2 responses with IFN-alpha than dual cross-linking with CD40. Recombinant ovine IFN-tau was less effective at inducing IgG2 responses when compared with IFN-alpha, though IgA responses were similar in magnitude following BCR cross-linking. At higher concentrations, IFN-tau enhanced IgA responses greater than twofold over the levels observed with IFN-alpha. Previous studies have shown that addition of IFN-gamma to BCR or pokeweed mitogen-activated bovine B cells stimulates IgG2 production. However, following CD40 stimulation alone, IFN-gamma was relatively ineffective at stimulating high-rate synthesis of any non-IgM isotype. Dual cross-linking via CD40 and the BCR resulted in decreased synthesis of IgM with a concomitant increase in IgA and similar levels of IgG2 production to those obtained via the BCR alone. We also assessed the effects of endogenous and exogenous TGF-beta on immunoglobulin synthesis by bovine B cells. Exogenous TGF-beta stimulates both IgG2 and IgA production following CD40 and BCR cross-linking in the presence of IL-2. Blocking endogenous TGF-beta did not inhibit the up-regulation of IgG2 or IgA by interferons.     

Hepatobiliary effects of tertiary-butylhydroperoxide (tBOOH) in isolated rat hepatocyte couplets

CD40 ligand (CD40L) is involved in the T-cell-dependent regulation of B-cell growth and survival and can rescue normal germinal centre B cells and several types of malignant B cells from apoptosis in vitro. We have previously reported that serum of patients with chronic lymphocytic leukaemia contained elevated levels of biologically active soluble CD40L (sCD40L). Whether an augmented CD40L pathway exists in patients with other types of B-cell lymphoid malignancies and the source of native sCD40L in these patients is currently unknown. Using a sensitive ELISA assay, soluble CD40L (sCD40L) was detected in the sera of both healthy individuals and patients with haematological malignancies; however, its level was significantly elevated only in patients with B-cell lymphomas (P<0.0001). Several types of malignant B cells coexpressed CD40 and CD40L proteins, and CD40L mRNA was detected in purified resting malignant B cells. The dual expression of CD40 and CD40L in B cells and the presence of native sCD40L in human serum suggest that a direct T-B-cell contact may not be required for CD40L delivery to B cells. This data raises the possibility that an autocrine cytokine loop involving CD40L may contribute to the growth regulation of benign and malignant B cells in vivo.     

The T cell-B cell interaction via OX40-OX40L is necessary for the T cell-dependent humoral immune response

Recent in vitro studies have established that activated B cells express OX40 ligand (L), a member of the tumor necrosis factor/nerve growth factor family of cytokines, and become stimulated to proliferate and secrete immunoglobulin (Ig) after cross-linking of OX40L by its counterreceptor OX40, which is expressed on activated T cells. In the present study we investigated the in vivo role of this receptor-ligand pair for the interaction of T and B cells in the course of the T-dependent B cell response against 2,4,6 trinitro-phenyl-keyhole limpet hemocyanin. First, we showed that OX40 is maximally expressed by T cells in the periarteriolar lymphoid sheath (PALS) 3 d after primary immunization. These OX40+ cells are located in close proximity to antigen-specific, activated B cells. Second, we demonstrated that blocking of OX40-OX40L interaction with polyclonal anti-OX40 antibody or with antibodies against certain peptide sequences within its extracellular domain resulted in a profound decrease of the anti-hapten IgG response, whereas the antihapten IgM response was grossly unchanged. Third, we showed that this antibody treatment leads to an inhibition of the development of PALS-associated B cell foci, whereas the formation of germinal centers remained intact. Finally, our data suggest that, whereas B cell memory development was not impaired by anti-OX40 administration, OX40-OX40L interaction seems to be crucial in the secondary immune response. We conclude from these data that the OX40-OX40L interaction in vivo is necessary for the differentiation of activated B cells into highly Ig-producing cells, but is not involved in other pathways of antigen-driven B cell differentiation such as memory cell development in the germinal centers.     

Utility of bolus dynamic CT for the detection of hypervascular malignant hepatic tumors: mainly referring to the comparison with delayed phase contrast-enhanced CT

We have studied the proliferation and CD40 antigen expression of lymphocytes, and the cytotoxicity to monocytes, of antisense phosphorothioate oligodeoxynucleotides complementary to the SP II promoter of HBV mRNA (sequence I) and the X gene (sequence II) in patients with chronic hepatitis B. The oligo sequence I stimulated proliferation of both T and, to a lesser extent, B cells. The percentage of cells expressing CD40 in T and B cell co-cultures increased from 4.2% to 13.8% after oligo stimulation in patients, while it increased form 4.7% to 48.6% in healthy controls. The sense sequence (sequence III) of the X gene also enhanced the expression of CD40 antigen in patients with hepatitis B. The proportion of CD40 cells (26%) in a resting B-cell preparation from hepatitis B patients decreased to zero after a 5-day culture with sequence I, but IgG levels in the culture supernatant increased. The cytotoxic properties of monocytes were not influenced by the oligos. These findings indicate that antisense oligos against hepatitis B virus (HBV) have mitogenic effects on the proliferation of human lymphocytes in a non-specific manner and may activate T cells to express CD40 antigen.     

Heterogeneity of CD44 expression among human B-cell subpopulations

CD44 is a widely distributed cell surface glycoprotein that participates in a number of cellular adhesion and signal transduction processes. Germinal center B cells express very low levels of CD44, whereas their precursors and differentiation products express much higher levels. In immunofluorescence studies comparing 20 antibodies classified as being against the hematopoietic isoform of CD44, one antibody, A1G3, was unreactive with germinal center B cells, whereas the other antibodies showed low intensity but definite reactivity. Western blotting and sequential immunoprecipitation studies of lysates from density-separated lymphocyte fractions showed two bands that were differentially expressed and reacted differently with A1G3 compared with the other CD44 antibodies. These results suggest that germinal center B cells and non-germinal center B cells express different forms of CD44. Of 21 malignant B-cell populations examined, 5 showed reactivity with a "standard" CD44 reagent and significantly reduced reactivity with A1G3, while one sample showed the opposite pattern and the remainder were positive for both reagents. Of 10 cell lines studied, 5 were differentially stained by A1G3 and a standard CD44 antibody. PCR amplification of reverse transcribed mRNA from sorted human tonsil B-cell subpopulations and Southern blotting showed that B cells express a number of splice isoforms of CD44. These results demonstrate that B cells express multiple forms of CD44; both splice insert isoforms and variants distinguished by A1G3; the form of CD44 expressed depends on B-cell differentiation state.     

Mechanisms for induction of acquired host immunity by neutrophil peptide defensins

Human neutrophil peptide (HNP) defensins were studied to determine their potential effects on adaptive mucosal immunity. Intranasal delivery of HNPs plus ovalbumin (OVA) enhanced OVA-specific serum IgG antibody (Ab) responses. However, OVA-specific IgA Abs were not induced in mucosal secretions or in serum. CD4(+) T cells of intranasally immunized mice displayed higher OVA-specific proliferative responses and elevated production of interferon gamma, interleukin (IL) 5, IL-6, and IL-10 when compared with control groups receiving OVA alone. In vitro, HNPs also enhanced both proliferative responses and T helper (Th) cytokine secretion profiles of CD3epsilon-stimulated spleen- and Peyer's patch-derived naive CD4(+) T cells. HNPs modulated the expression of costimulatory molecules by lipopolysaccharide- or CD3epsilon-stimulated splenic and Peyer's patch B or T cell populations, respectively. These studies show that defensins enhance systemic IgG, but not IgA, Ab responses through help provided by CD4(+) Th1- and Th2-type cytokines and foster B and T cell interactions to link innate immunity with the adaptive immune system.     

CD40 ligand and autoantigen are involved in the pathogenesis of low-grade B-cell lymphomas of mucosa-associated lymphoid tissue

Low-grade MALT-type lymphomas are malignancies of mucosal marginal-zone B cells and preceded by reactive inflammatory lymphoid tissue. Experimental observations suggest that antigen and CD40 Ligand act during cognate T/B cell interaction and are crucial for germinal center B-cell maturation generating marginal-zone B cells. To investigate the mechanisms underlying the development of extranodal MALT-type lymphomas, the immunoglobulin receptor was sequenced and analyzed for antigen specificity using heterohybridoma technology. Furthermore, CD40 ligand expression was evaluated by immunohistochemistry and by semiquantitative RT-PCR, and ligand binding to the CD40 of tumor B cells was studied using the CD40 system. Hypermutations were found in low-grade lymphomas throughout CDR1-CDR3 suggestive of positive selection through their antigen receptor. Different VH families were used and more than 69% of tumor immunoglobulins bound different mucosal antigens. CD40L expression was found in the tumor marginal zone in substantial amounts. The in vitro proliferation response of all low-grade MALT-type lymphomas was dependent on anti-CD40-mediated signals and cytokines. Our data provide evidence that autoantigen as well as the CD40L expressed by activated nonneoplastic T cells may drive the evolution of low-grade MALT-type lymphomas either directly or by paracrine mechanisms and that antigen may contribute to lymphoma pathogenesis.     

Expression of mdr1 and mrp in the normal B-cell homologue of B-cell chronic lymphocytic leukaemia

B-cell chronic lymphocytic leukaemia (CLL) cells commonly express the multidrug resistance phenotype. The aim of this study was to establish whether the normal homologue in B-cell ontogeny of B-CLL also expressed the multidrug resistance (mdr) phenotype. Human tonsillar lymphocytes were sorted to yield two B-cell subsets based on the expression of CD19, CD5 and CD10. The normal homologue was represented by a population of B cells that was CD19 positive, CD10 negative and weakly expressed CD5. Based upon functional analysis and the detection of mdr1 mRNA by semi-quantitative PCR, these cells expressed the mdr phenotype. In contrast, functional multidrug resistance could not be demonstrated in CD19-positive CD10-positive cells with strong expression of CD5, nor could mdr1 mRNA be found in these cells. MRP was variably expressed in both B-cell subsets with no discernable differences in the pattern of expression. We conclude that normal B cells with a phenotype resembling that of B-CLL cells express the multidrug resistance phenotype.     

Recipient polyclonal B cell activation and immunoglobulin production induced by priming with a retroviral superantigen

The superantigen vSAG-7 (or MIs 1a) is a membrane glycoprotein encoded by the endogenous retrovirus mouse mammary tumor virus 7 (MMTV-7) and is highly stimulatory for V beta 6/CD4+ T cells. Priming of adult MMTV-7-negative mice with vSAG-7-expressing cells initially results in the activation of the peripheral V beta 6/CD4+ T cell compartment and is followed by T cell tolerance to the superantigen. During the course of tolerance induction the number of recipient B lymphocytes increases in the lymph nodes, but not the spleen, of vSAG-7-primed recipients. These B cells also express increased levels of class II MHC and present passively acquired superantigen. In this study we asked if these effects on the host B cell compartment are followed by the production of immunoglobulin. Priming of MMTV-7-negative BALB/c or CB.17 mice with vSAG-7-expressing cells from DBA/2 mice induced increases of both IgM and IgG2a in the serum. Use of Igh congenic CB.17 (IgMb) mice as recipients of the vSAG-7-presenting cells from DBA/2 (IgMa) donors indicated that the IgM and IgG produced were entirely of host origin. Priming with vSAG-7 also amplified (four- to fivefold) the antibody-producing cell response induced to a suboptimal dose of sheep RBC. Priming with purified B cells from vSAG-7 donors resulted in recipient V beta 6/CD4+ T cell activation and increased numbers of recipient B cells in the lymph nodes, but did not induce immunoglobulin production. In contrast, priming with purified CD8+ T cells resulted in increased quantities of serum IgM but not vSAG-7-reactive T cell activation or increased numbers of recipient B cells in the lymph nodes. These results indicate that the T cell activation and increased B cell number with upregulated class II MHC expression initially observed following vSAG-7 priming of adult MMTV-7-negative recipients are not linked to the induction of the recipient-derived immunoglobulin production.     

CD40/CD40 ligand interactions in normal, reactive and malignant lympho-hematopoietic tissues

CD40 is a 48 Kd integral membrane protein expressed by cells of B cells, origin, dentritic cells, monocytes, epithelial cells, endothelial cells and tumor cells including carcinomas, B cell lymphomas/leukemias and Hodgkin and Reed-Sternberg (HRS) cells of Hodgkin's disease (HD). CD40 has been clustered as a member of the nerve growth factor (NGF)/tumor necrosis factor (TNF) receptor superfamily with the corresponding counterstructure, the CD40 ligand (L) being mainly expressed by activated CD4+ T cells, but also some activated CD8+ T cells, basophils, eosinophils, mast cells and stromal cells. CD40L shares significant amino acid homology with TNF particularly in its extracellular domain ("TNF homology region") and is therefore viewed as a member of the TNF ligand superfamily. Binding of CD40L+ T cells to CD40+ B cells is thought to play a major role in T cell-dependent B cell activation, B cell proliferation, Ig isotype switching, memory B cell formation and rescue of B cells from apoptotic death in germinal centers. Mutations of the CD40L gene have been associated with the X-linked hyper-IgM immunodeficiency syndrome, pointing to the critical role of the CD40/CD40L interaction in the T cell-B cell interplay. Accordingly, expression of CD40 by human lympho-hematopoietic tumors has been shown in most of the B cell neoplasias, H-RS cells and HD and some carcinomas. In contrast, CD40L+ tumor cells are almost invariably restricted to CD4+/CD8- T cell lymphomas. Overall, functional CD40/CD40L interactions appear to be critical for cellular activation signals during immune responses and neoplastic tumor cell growth. The understanding of the biology of CD40L has improved our diagnostic and therapeutic repertoire in the management of several human diseases, including CD40+ tumors.