Neutrophil Extracellular Traps Exert Potential Cytotoxic and Proinflammatory Effects in the Dental Pulp

INTRODUCTION

Neutrophil extracellular traps (NETs) are an important innate immune mechanism aimed at limiting the dissemination of bacteria within tissues and localizing antibacterial killing mechanisms. There is significant interest in the role of NETs in a range of infectious and inflammatory diseases; however, their role in diseased pulp has yet to be explored. Our aim was to determine their relevance to infected pulp and how their components affect human dental pulp cell (HDPC) responses.

METHODS

Diseased pulp tissue was stained for the presence of extracellular DNA and elastase to detect the presence of NETs. Bacteria known to infect pulp were also assayed to determine their ability to stimulate NETs. Coculture studies and NET component challenge were used to determine the effect of extracellular NET release on HDPC viability and inflammatory response. NET-stimulated HDPC secretomes were assessed for their chemotactic activity for lymphocytes and macrophages.

RESULTS

Data indicate that NETs are present in infected pulp tissue and whole NETs, and their histone components, particularly H2A, decreased HDPC viability and stimulated chemokine release, resulting in an attraction of lymphocyte populations.

CONCLUSIONS

NETs are likely important in pulpal pathogenesis with injurious and chronic inflammatory effects on HDPCs, which may contribute to disease progression. Macrophages are chemoattracted to NET-induced apoptotic HDPCs, facilitating cellular debris removal. NETs and histones may provide novel prognostic markers and/or therapeutic targets for pulpal diseases.

Effects of red light-emitting diode irradiation on dental pulp cells

Light irradiation activates a range of cellular processes in a variety of cell types, including stem cells, and can promote tissue repair. This study investigated the effects of light-emitting diode (LED) exposure on dental pulp cells (DPCs). Dose response analysis at 20-second intervals up to 120 seconds demonstrated that a LED array emitting 653-nm red light stimulated significantly increased cell growth at 3 and 7 days post-irradiation with 40 (149 mJ/cm(2)) and 60 (224 mJ/cm(2)) seconds of radiant exposure. Double-dosing cells at days 1 and 4 of a 7-day culture period with 60-second (224 mJ/cm(2)) LED exposure significantly increased cell growth compared with a single dosing regime. BrdU analysis demonstrated significantly increased proliferation rates associated with significantly increased ATP, nitric oxide (NO), and mitochondrial metabolic activity. LED-stimulated NO levels were not reduced by inhibition of NO-synthase activity. Light exposure also rescued the inhibition of mitochondrial dysfunction and increased levels of in vitro mineralization compared with control. Media exchange experiments indicated that autocrine signaling was not likely responsible for red-light-induced DPC activity. In conclusion, data analysis indicated that 653-nm LED irradiation promoted DPC responses relevant to tissue repair, and this is likely mediated by increased mitochondrial activity.

Enhancing the anti-lymphoma potential of 3,4-methylenedioxymethamphetamine ('ecstasy') through iterative chemical redesign: mechanisms and pathways to cell death

While 3,4-methylenedioxymethamphetamine (MDMA/'ecstasy') is cytostatic towards lymphoma cells in vitro, the concentrations required militate against its translation directly to a therapeutic in vivo. The possibility of 'redesigning the designer drug', separating desired anti-lymphoma activity from unwanted psychoactivity and neurotoxicity, was therefore mooted. From an initial analysis of MDMA analogues synthesized with a modified α-substituent, it was found that incorporating a phenyl group increased potency against sensitive, Bcl-2-deplete, Burkitt's lymphoma (BL) cells 10-fold relative to MDMA. From this lead, related analogs were synthesized with the 'best' compounds (containing 1- and 2-naphthyl and para-biphenyl substituents) some 100-fold more potent than MDMA versus the BL target. When assessed against derived lines from a diversity of B-cell tumors MDMA analogues were seen to impact the broad spectrum of malignancy. Expressing a BCL2 transgene in BL cells afforded only scant protection against the analogues and across the malignancies no significant correlation between constitutive Bcl-2 levels and sensitivity to compounds was observed. Bcl-2-deplete cells displayed hallmarks of apoptotic death in response to the analogues while BCL2 overexpressing equivalents died in a caspase-3-independent manner. Despite lymphoma cells expressing monoamine transporters, their pharmacological blockade failed to reverse the anti-lymphoma actions of the analogues studied. Neither did reactive oxygen species account for ensuing cell death. Enhanced cytotoxic performance did however track with predicted lipophilicity amongst the designed compounds. In conclusion, MDMA analogues have been discovered with enhanced cytotoxic efficacy against lymphoma subtypes amongst which high-level Bcl-2--often a barrier to drug performance for this indication--fails to protect.

Dynamic interplay between the neutral glycosphingolipid CD77/Gb3 and the therapeutic antibody target CD20 within the lipid bilayer of model B lymphoma cells

The centroblast-specific differentiation marker CD77 (Gb(3)), is the receptor for Shiga-like toxin (SLT). The dynamic relationship between Gb(3)/CD77 and key B-cell membrane proteins was studied in Burkitt's lymphoma cells with a focus on CD20. Engagement of Gb(3)/CD77 with SLT-B reduced the amount of CD20 and CXCR4 available, but levels of BCR, MHC Class II, CD21, CD27 and CD54 remained unchanged. Cholesterol depletion promoted a decrease in the number of sites accessed by CD20, CXCR4 and Gb(3)/CD77 antibodies. Constitutive localisation of Gb(3)/CD77 to lipid rafts was unperturbed by either SLT-B binding or cholesterol depletion, whereas the opposite was true for CD20. The effects were specific to SLT-B, highlighted by the inability of cholera toxin B-subunit to alter CD20 availability. Thus, the binding of Gb(3)/CD77 by its cognate ligand transmits information within the lipid bilayer of model lymphoma cells to impact the behaviour of selective proteins, most notably CD20, via a mechanism influenced by the level of cholesterol within the membrane.

Dopamine targets cycling B cells independent of receptors/transporter for oxidative attack: Implications for non-Hodgkin's lymphoma

Human B lymphocytes and derived lines from a spectrum of B cell malignancy were studied for expression of dopaminergic pathway components and for their cytostatic response to the catecholamine and related, potentially therapeutic compounds. Proliferating normal lymphocytes and dividing malignant clones rapidly arrested on exposure to dopamine in the low (</=10 muM) micromolar range. The antiparkinsonian drugs l-DOPA and apomorphine (particularly) were similarly antiproliferative. With the exception of D4, dopamine receptors D1-D5 were variably expressed among normal and neoplastic B cell populations, as was the dopamine transporter. Transcripts for D1 and D2 were frequently found, whereas D3 and D5 revealed restricted expression; dopamine transporter was detected in most cases. Nevertheless, pharmacological analysis disclosed that dopamine targeted cycling B cells independent of these structures. Rather, oxidative stress constituted the primary mechanism: the catecholamine's actions being mimicked by hydrogen peroxide and reversed by exogenous catalase, and evidence for the intracellular redox protein thioredoxin contributing protection. Among proliferating clones, growth arrest was accompanied by cell death in populations deplete in antiapoptotic Bcl-2: resting lymphocytes escaping low micromolar dopamine toxicity. Dysregulated bcl-2 expression, although preventing oxidative-induced caspase-dependent apoptosis, by itself conferred only minor protection against dopamine cytostasis. The selective impact of dopamine on lymphocytes that are in active cycle indicates an axis for therapeutic intervention not only in B cell neoplasia but also in lymphoproliferative disturbances generally. Rational tailoring of drug delivery systems already in development for Parkinson's disease could provide ideal vehicles for carrying the oxidative hit directly to the target populations.

Lymphoma cells protected from apoptosis by dysregulated bcl-2 continue to bind Annexin V in response to B-cell receptor engagement: A cautionary tale

Translocation of phosphatidylserine (PS) from the inner to outer leaflet of the surface membrane lipid bilayer, a characteristic early event of cells entering the apoptotic program, is routinely assessed by the Ca(2+)-dependent binding of Annexin V (AV). Here, we show that lymphoma cells protected from apoptosis by expression of a bcl-2 transgene or by virtue of the t(14;18)(q32;q21) translocation continue to register enhanced AV binding in response to BCR crosslinking. Induced AV binding appeared BCR-selective in that it did not proceed in Bcl-2(high) cells in response to calcium ionophore or the antidepressant fluoxetine, each of which activate the full apoptotic program in Bcl-2(low) equivalents. AV-positive cells did increase on crosslinking the BCR co-receptor CD19, despite it being a completely non-apoptotic signal. These findings advise caution when interpreting studies where Annexin V binding is used as a sole, or major, indicator of apoptotic death among lymphoma B cells.

Close encounters of the monoamine kind: immune cells betray their nervous disposition

Here we review the evidence for immune cells expressing multiple components of the serotonergic and dopaminergic systems that are more commonly associated with the central nervous system (CNS). We discuss where and how peripheral encounters with these biogenic monoamines occur and posit reasons as to why the immune system would wish to deploy these pathways. A full taxonomy of serotonergic and dopaminergic constituents and their workings in component cells of the immune system should facilitate the formulation of novel therapeutic approaches in diseases characterized by immune dysfunction and potentially provide a range of surrogate peripheral markers for registering and monitoring disturbances within the CNS.

The serotonin transporter (SLC6A4) is present in B-cell clones of diverse malignant origin: probing a potential anti-tumor target for psychotropics

Following our previous description of the serotonin transporter (SERT) acting as a conduit to 5-hydroxytryptamine (5-HT)-mediated apoptosis, specifically in Burkitt's lymphoma, we now detail its expression among a broad spectrum of B cell malignancy, while exploring additional SERT substrates for potential therapeutic activity. SERT was readily detected in derived B cell lines with origins as diverse as B cell precursor acute lymphoblastic leukemia, mantle cell lymphoma, diffuse large B cell lymphoma, and multiple myeloma. Concentration and timecourse kinetics for the antiproliferative and proapoptotic activities of the amphetamine derivatives fenfluramine (an appetite suppressant) and 3,4-methylenedioxymethamphetamine (MDMA; "Ecstasy") revealed them as being similar to the endogenous indoleamine. A tricyclic antidepressant, clomipramine, instead mirrored the behavior of the selective serotonin reuptake inhibitor fluoxetine, both being effective in the low micromolar range. A majority of neoplastic clones were sensitive to one or more of the serotonergic compounds. Dysregulated bcl-2 expression, either by t(14;18)(q32;q21) translocation or its introduction as a constitutively active transgene, provided protection from proapoptotic but not antiproliferative outcomes. These data indicate a potential for SERT as a novel anti-tumor target for amphetamine analogs, while evidence is presented that the seemingly more promising antidepressants are likely impacting malignant B cells independently of the transporter itself.

Improved access to CD20 following B cell receptor cross-linking at Burkitt's lymphoma cell surfaces

Here we report that B cell receptor (BCR) engagement rapidly improves the capacity of CD20 to be accessed by cognate antibody at model Burkitt's lymphoma cell surfaces. None of eight other surface molecules demonstrated such BCR-dependent enhancement of ligand binding while the quantity of accessible CD20 remained unchanged on either CD19 or CD40 engagement. Neither the actin-depolymerizing agent cytochalasin D nor inhibitors targeting signalling pathways associated with the BCR attenuated the CD20 increase that could be uncoupled from BCR endocytosis. Instead, a role for lipid rafts was indicated both from the inhibitory actions of cholesterol-sequestering methyl-beta-cyclodextrin and direct analysis of CD20 redistribution using sucrose density gradients and confocal microscopy. Whether such observations could find application in CD20-directed therapies where success can be compromised by otherwise low-level expression of target antigen is discussed.

Selective serotonin reuptake inhibitors directly signal for apoptosis in biopsy-like Burkitt lymphoma cells

Selective serotonin reuptake inhibitors (SSRIs) are the treatment of choice for clinical depression and a range of anxiety-related disorders. They are well tolerated over extended periods with more than 50 million people worldwide benefiting from their use. Here we show that 3 structurally distinct SSRIs--fluoxetine, paroxetine, and citalopram--act directly on Burkitt lymphoma (BL) cells to trigger rapid and extensive programmed cell death. SSRIs unexpectedly stimulated calcium flux, tyrosine phosphorylation, and down-regulation of the c-myc and nm23 genes in Burkitt lymphoma cells remaining faithful to the biopsy phenotype. Resultant SSRI-induced apoptosis was preceded by caspase activation, poly(ADP-ribose) polymerase-1 (PARP-1) cleavage, DNA fragmentation, a loss of mitochondrial membrane potential, and the externalization of phosphatidylserine, and reversed by the overexpression of bcl-2. Normal peripheral blood mononuclear cells and tonsil B cells, whether resting or stimulated into cycle, were largely resistant to SSRI-induced death as were 5 non-BL lymphoid cell lines tested. We discuss these findings within the context of whether the SSRI class of antidepressants could find future application as potential therapeutics for the highly aggressive and-because of its association with AIDS-increasingly more common Burkitt lymphoma.

Population depletion activates autonomous CD154-dependent survival in biopsylike Burkitt lymphoma cells

Population size is governed through cells reacting to a variety of intrinsic and extrinsic cues. Tumors, while liberated from many of the homeostatic constraints placed on physiologic counterparts, can nonetheless remain subject to both social and environmental control. Burkitt lymphoma cells faithful to the biopsy phenotype were used to model the reliance of the colony, if any, on an inbuilt population sensor. Below a normally suicidal threshold number of cells, low picomolar quantities of exogenous CD40 ligand (CD40L/CD154) were found to sustain the clone without the discernible shift in phenotype that accompanies high CD40L encounter. Although CD154 was undetectable in populous cultures, message was induced as numbers became limiting. Correspondingly, attempts to neutralize endogenous CD40L activity failed to perturb cells at optimal densities but resulted in their marked decline as the critical threshold was approached. These data reveal an auto-inducible survival mechanism seemingly regulated through the monitoring of population size, a process somewhat akin to that of "quorum sensing" among gram-negative bacteria in which diffusible molecules provide a means of communication to coordinate gene expression with population density. This process could be activated as cells discern depletions in their community or when deprived of signals otherwise furnished within an appropriate environmental niche.

The extrafollicular-to-follicular transition of human B lymphocytes: induction of functional globotriaosylceramide (CD77) on high threshold occupancy of CD40

Amongst lymphocytes, expression of CD77 (globotriaosylceramide, Gb3) is exclusive to B cells of the germinal center (GC). Its acquisition by extrafollicular B cells may thus herald their commitment to a follicular response. Here we show that high threshold occupancy of CD40 by its cognate ligand (CD40L) promotes rapid induction of CD77 expression in non-GC (CD38(lo)) B cells. The kinetics of CD77 acquisition mirrored those of GC-related markers CD95 and CD86 but contrasted with the more delayed increase in CD38 expression. Induction of CD77 was not a simple consequence of cell cycle entry: other conditions of stimulation equally capable of driving proliferation failed to promote CD77 expression. CD77 was functional in that cells were now sensitive to Verotoxin-1, an Escherichia coli-derived ligand of Gb3. These data indicate that acquisition by extrafollicular B cells of CD77 results from high threshold occupancy of CD40, a situation that should be reached physiologically only once a critical level of T cell priming has been achieved.

Minimal cross-linking and epitope requirements for CD40-dependent suppression of apoptosis contrast with those for promotion of the cell cycle and homotypic adhesions in human B cells

Eight different CD40 mAb shared with soluble trimeric CD40 ligand (sCD40LT) the capacity to rescue germinal center (GC) B cells from spontaneous apoptosis and to suppress antigen receptor-driven apoptosis in group I Burkitt's lymphoma cells. Three mAb (G28-5, M2 and M3) mimicked sCD40LT in its ability to promote strong homotypic adhesion in resting B cells, whereas others (EA5, BL-OGY/C4 and 5C3) failed to stimulate strong clustering. Binding studies revealed that only those mAb that promoted strong B cell clustering bound at, or near to, the CD40L binding site. While all eight mAb and sCD40LT were capable of synergizing with IL-4 or phorbol ester for promoting DNA synthesis in resting B cells, co-stimulus-independent activation of the cells into cycle through CD40 related directly to the extent of receptor cross-linking. Thus, mAb which bound outside the CD40L binding site synergized with sCD40LT for promoting DNA synthesis; maximal levels of stimulation were achieved by presenting any of the mAb on CD32 transfectants in the absence of sCD40LT or by cross-linking bound sCD40LT with a second antibody. Monomeric sCD40L, which was able to promote rescue of GC B cells from apoptosis, was unable to drive resting B cells into cycle. These studies demonstrate that CD40-dependent rescue of human B cells from apoptosis requires minimal cross-linking and is essentially epitope independent, whereas the requirements for promoting cell cycle progression and homotypic adhesion are more stringent. Possible mechanisms underlying these differences and their physiological significance are discussed.

Suppression of apoptosis in normal and neoplastic human B lymphocytes by CD40 ligand is independent of Bc1-2 induction

The tendency of isolated germinal center (GC) B cells to undergo apoptosis was suppressed by recombinant cell-bound CD40 ligand (CD40L): after 2 days at 37 degrees C, > 80% of cells remained viable in the presence of CD40L as compared to < 1% in control cultures. CD40L sustained a high rate of DNA synthesis in GC cells and was more effective than monoclonal antibody to CD40 in this regard. Group I Burkitt lymphoma (BL) cell lines induced to undergo apoptosis with anti-immunoglobulin or calcium ionophore were also protected by CD40L. In BL cells, this route of rescue was not accompanied by induction of Bc1-2 protein, the expression of which has been linked to hemopoietic cell survival. Bc1-2 was induced in GC cells responding to CD40L, but its appearance was a relatively late event not reaching significant levels over controls until day 2 of culture. Thus induction of Bc1-2 appears to be secondary to the survival signal imparted by CD40L. These findings are discussed in relation to a potential role for CD40L in supporting B cell tumors in vivo and the discovery that the molecular defect in the X-linked Hyper-IgM syndrome is targeted to the CD40L gene.

IL-2 expands and maintains IgM plasmablasts from a CD5+ subset contained within the germinal centre cell-enriched (surface IgD-/CD39- buoyant) fraction of human tonsil

IL-2 was found to promote the rapid growth of a minority population contained within the germinal centre (GC) cell-enriched (CD39- and/or IgD- buoyant) fraction of human tonsillar B lymphocytes. The cells emerging in response to IL-2 had a high mitotic index and morphologically resembled plasmablasts. Cultures could be maintained in the absence of feeder cells for up to 3 weeks in IL-2 and were characterized by large amounts of IgM in their supernatants: approximately 40% of the cells contained readily detectable cytoplasmic IgM by day 10 of culture. Negligible quantities of IgG and IgA were found. The target population for IL-2-driven expansion and IgM secretion was smIg+/CD38+ and was subject to suppression by anti-IgM antibody. While only 8% of cells within the GC cell-enriched fraction were CD5+ (compared with 15% of high density resting B cells), their removal led to an 83% reduction in the amount of IgM produced in response to IL-2, IL2 selectively expanded this minor CD5+ subset such that by day 6 of culture they comprised 57% of all viable cells. Cultures established with IL-2 showed increasing expression of cytoplasmic Bcl-2 and withdrawal of growth factor resulted in cell death via apoptosis. We discuss these results in relation to CD5+ B cells and their potential role in antibody responses to TD antigens.

A subset of anti-CD21 antibodies promote the rescue of germinal center B cells from apoptosis

Germinal center cells (GCC) are programmed to die by apoptosis unless they receive a positive signal for rescue. The primary signal in vivo is believed to be dependent on interaction with antigen held as immune complexes on follicular dendritic cells (FDC), a subset of which express large amounts of CD23, a low-affinity receptor for IgE. Recombinant soluble CD23 (sCD23) and interleukin-1 have been found to potentiate the survival of GCC in vitro. Recently, CD23 was shown to interact specifically with a ligand other than IgE, namely CD21 (CR2/Epstein-Barr virus receptor). In the present study, we show that a subset of anti-CD21 monoclonal antibodies behave similarly to soluble CD23 in their effect on GCC inasmuch as they: (i) diminish the occurrence of apoptosis; (ii) promote a plasmacytoid appearance in rescued cells; (iii) up-regulate expression of the Bcl-2 proto-oncogene. These findings indicate that FDC-derived CD23 exerts its effects on GCC via CD21.

Stimulation of tyrosine phosphorylation without inositol lipid hydrolysis in human B lymphocytes on engaging CD72

Occupancy of CD72 on resting tonsillar B cells by monoclonal antibodies (mAb) promotes entry into the G1 phase of the cell cycle with an accompanying increase in MHC Class II expression and provides a co-stimulus to immobilized anti-mu for driving DNA synthesis. We now report that engagement of CD72 by mAb stimulates tyrosine phosphorylation in B cells with a peak of activity seen at 5-10 min. Two major substrates of 29 and 57 kDa showed a basal level of phosphorylation which increased with time, while a 40 kDa protein and several other minor components were phosphorylated de novo on the addition of mAb to CD72. Inositol lipid hydrolysis was found to be unperturbed, although a shallow rise in the basal level of intracellular free Ca2+ was provoked on engaging CD72. Receptor cross-linking was not a requirement for signaling human B cells through CD72: simple occupancy by univalent antibody was sufficient both to trigger the rise in basal [Ca2+]i and to promote DNA synthesis.

Factors modifying survival pathways of germinal center B cells. Glucocorticoids and transforming growth factor-beta, but not cyclosporin A or anti-CD19, block surface immunoglobulin-mediated rescue from apoptosis

The tendency for germinal center (GC) B cells to enter apoptosis is suppressed on engaging antigen receptor with immobilized anti-immunoglobulin; cross-linking of surface CD40 by monoclonal antibodies provides an additional signal for rescuing GC cells from programmed death. These observations are believed to reflect events that, in vivo, would allow for the selection of centrocytes which have undergone somatic mutation on Ig V-region genes to generate antigen receptor of high affinity. The purpose of the present study was to identify factors capable of modifying the survival pathways of GC cells. Transforming growth factor-beta, at an optimal concentration of 1 ng/ml, was found to inhibit surface immunoglobulin (sIg)-mediated rescue of GC cells but had no influence on survival promoted through CD40. Both routes of rescue were blocked by the glucocorticoid prednisolone at pharmacological concentrations (ID50 = 10(-7) M). Cyclosporin A, an antagonist of sIg-mediated signaling in resting B cells, failed to block rescue of GC cells through either of the receptor-activated pathways. Antibody to CD19--which also suppresses the activation of resting B cells--not only left GC cell rescue undiminished, but rather provided a modest survival signal of its own; interferon-alpha behaved similarly while interferon-gamma failed to influence GC cell survival in either direction.

Growth factor requirements for the stimulation of germinal center B cells: evidence for an IL-2-dependent pathway of development

Germinal center (GC) B cells readily undergo apoptosis, a tendency which can be suppressed in vitro by immobilized anti-Ig; mAb to CD40 and soluble CD23 (in synergy with IL-1 alpha) also effect rescue of GC cells from programmed cell death. In the present study, the signals which stimulate rescued GC populations to DNA synthesis have been examined and compared to those established for the activation of follicular mantle (FM) B cells. On co-culture with anti-Ig, optimal responses in FM B cells can be achieved with a combination of IL-4 and CD40 antibody; these activities also provided a modest stimulus to GC cells but, for this population, anti-Ig was ineffective at augmenting the response further. Stimulations of GC B cells were enhanced, however, when performed on a support of primary fetal lung fibroblasts; a major influence of stroma was to promote, by direct cell-cell contact, the CD40-dependent survival of GC B cells. FM B cells were relatively independent of such stromal support. In marked contrast to FM cells, GC B cells were found to respond by enhanced DNA synthesis to IL-2 even when quite low concentrations of the factor were present (IC50 = 2 U/ml). Stimulation of GC cells via this pathway was augmented almost 2-fold on the inclusion of anti-Ig whereas neither fibroblasts, IL-4, nor CD40 antibody made any additional contribution to the IL-2-dependent response. The requirements found for stimulating GC cells in vitro are discussed with reference to the signals that this population may encounter in appropriate microenvironments in vivo: the variety of options apparently available could reflect changing priorities at different stages of a developing GC response.

Recombinant 25-kDa CD23 and interleukin 1 alpha promote the survival of germinal center B cells: evidence for bifurcation in the development of centrocytes rescued from apoptosis

Germinal centers contain a proliferating pool of centroblasts which give rise to non-dividing centrocyte. Centrocytes are programmed to die by apoptosis unless they receive a positive signal for rescue. Rescue, in vivo, is likely to be dependent, initially, on interaction with antigen held on follicular dendritic cells (FDC). A subset of FDC located in that part of the germinal center furthest from centroblasts is particularly rich in CD23. Supernatants containing high levels of soluble CD23 were found not only to encourage the survival of germinal center B cells but also to promote their differentiation toward a plasmacytoid morphology; these activities were diminished following removal of CD23 from the supernatants. Recombinant 25-kDa CD23 was initially found to be incapable of providing the signal for germinal center cell development but on the addition of interleukin 1 alpha which, by itself, was inactive, rescue and differentiation of germinal center B cells were now achieved. Apoptosis in germinal center cells could also be prevented by the ligation of surface CD40 with monoclonal antibody: however, rescue via this pathway was not accompanied by plasmacytoid differentiation. These findings provide a functional rationale to the high level expression of CD23 found within a discrete subset of FDC and indicate a bifurcation in the development of germinal center B cells following their rescue from apoptosis.