Fas receptor expression on B-lineage cells

The Fas/CD95 Receptor Regulates the Death of Autoreactive B Cells and the Selection of Antigen-Specific B Cells

Cell death receptors have crucial roles in the regulation of immune responses. Here we review recent in vivo data confirming that the Fas death receptor (TNFSR6) on B cells is important for the regulation of autoimmunity since the impairment of only Fas function on B cells results in uncontrolled autoantibody production and autoimmunity. Fas plays a role in the elimination of the non-specific and autoreactive B cells in germinal center, while during the selection of antigen-specific B cells different escape signals ensure the resistance to Fas-mediated apoptosis. Antigen-specific survival such as BCR or MHCII signal or coreceptors (CD19) cooperating with BCR inhibits the formation of death inducing signaling complex. Antigen-specific survival can be reinforced by antigen-independent signals of IL-4 or CD40 overproducing the anti-apoptotic members of the Bcl-2 family proteins.

ICOS expression by effector T cells influences the ability of regulatory T cells to inhibit anti-chromatin B cell responses in recipient mice

T regulatory cells are critical for the prevention of autoimmunity. Specifically, Treg cells can control anti-chromatin antibody production in vivo, and this correlates with decreased ICOS expression on CD4(+) T helper cells. Here we test the significance of high ICOS expression by T effector cells, firstly in terms of the anti-chromatin B cell response, and secondly on the ability of Treg cells to suppress T cell help. We bred CD4(+) T cell receptor transgenic mice with mice that carry the Roquin(san/san) mutation. The Roquin gene functions to limit ICOS mRNA such that CD4 T cells from mutant mice express elevated ICOS. Using an in vivo model, TS1.Roquin(san/san) Th cells were compared with wild-type TS1 Th cells with regard to their ability to help anti-chromatin B cells in the presence or absence of Treg cells. Both TS1 and TS1.Roquin(san/san) Th cells induced anti-chromatin IgM(a) antibodies, but the TS1.Roquin(san/san) Th cells resulted in the recovery of more class-switched and germinal center B cells. Neither source of Th cells were capable of inducing long-lived autoantibodies. Treg cells completely suppressed anti-chromatin IgM(a) antibody production and reduced anti-chromatin B cell recovery induced by TS1 Th cells. Importantly, this suppression was less effective when TS1.Roquin(san/san) Th cells were used. Thus, high ICOS levels on effector T cells results in autoimmunity by augmenting the autoreactive B cell response and by dampening the effect of Treg cell suppression.

B Cell Tolerance Checkpoints That Restrict Pathways of Antigen-Driven Differentiation

Autoreactive B cells can be regulated by deletion, receptor editing, or anergy. Rheumatoid factor (RF)-expressing B lymphocytes in normal mice are not controlled by these mechanisms, but they do not secrete autoantibody and were presumed to ignore self-Ag. Surprisingly, we now find that these B cells are not quiescent, but instead are constitutively and specifically activated by self-Ag. In BALB/c mice, RF B cells form germinal centers (GCs) but few Ab-forming cells (AFCs). In contrast, autoimmune mice that express the autoantigen readily generate RF AFCs. Most interestingly, autoantigen-specific RF GCs in BALB/c mice appear defective. B cells in such GCs neither expand nor are selected as efficiently as equivalent cells in autoimmune mice. Thus, our data establish two novel checkpoints of autoreactive B cell regulation that are engaged only after initial autoreactive B cell activation: one that allows GCs but prevents AFC formation and one that impairs selection in the GC. Both of these checkpoints fail in autoimmunity.

The influence of effector T cells and Fas ligand on lupus-associated B cells

Circulating autoantibodies against dsDNA and chromatin are a characteristic of systemic lupus erythematosus in humans and many mouse models of this disease. B cells expressing these autoantibodies are normally regulated in nonautoimmune-prone mice but are induced to secrete Abs following T cell help. Likewise, anti-chromatin autoantibody production is T cell-dependent in Fas/Fas ligand (FasL)-deficient (lpr/lpr or gld/gld) mice. In this study, we demonstrate that Th2 cells promote anti-chromatin B cell survival and autoantibody production in vivo. FasL influences the ability of Th2 cells to help B cells, as Th2-gld/gld cells support higher titers of anti-chromatin Abs than their FasL-sufficient counterparts and promote anti-chromatin B cell participation in germinal centers. Th1 cells induce anti-chromatin B cell germinal centers regardless of FasL status; however, their ability to stimulate anti-chromatin Ab production positively correlates with their level of IFN-gamma production. This distinction is lost if FasL-deficient T cells are used: Th1-gld/gld cells promote significant titers of anti-chromatin Abs regardless of IFN-gamma production levels. Thus, FasL from effector T cells plays an important role in determining the fate of anti-chromatin B cells.

A fail-safe mechanism for negative selection of isotype-switched B cell precursors is regulated by the Fas/FasL pathway

In B lymphocytes, immunoglobulin (Ig)M receptors drive development and construction of naive repertoire, whereas IgG receptors promote formation of the memory B cell compartment. This isotype switching process requires appropriate B cell activation and T cell help. In the absence of T cell help, activated B cells undergo Fas-mediated apoptosis, a peripheral mechanism contributing to the establishment of self-tolerance. Using Igμ-deficient μMT mouse model, where B cell development is blocked at pro-B stage, here we show an alternative developmental pathway used by isotype-switched B cell precursors. We find that isotype switching occurs normally in B cell precursors and is T independent. Ongoing isotype switching was found in both normal and μMT B cell development as reflected by detection of IgG1 germline and postswitch transcripts as well as activation-induced cytidine deaminase expression, resulting in the generation of IgG-expressing cells. These isotype-switched B cells are negatively selected by Fas pathway, as blocking the Fas/FasL interaction rescues the development of isotype-switched B cells in vivo and in vitro. Similar to memory B cells, isotype-switched B cells have a marginal zone phenotype. We suggest a novel developmental pathway used by isotype-switched B cell precursors that effectively circumvents peripheral tolerance requirements. This developmental pathway, however, is strictly controlled by Fas/FasL interaction to prevent B cell autoimmunity.

Effect of IL-12 on TNF-alpha-mediated osteoclast formation in bone marrow cells: apoptosis mediated by Fas/Fas ligand interaction

Recently, it has been found that differentiation into osteoclasts is induced by TNF-alpha. In this study, we investigated the effect of IL-12 on TNF-alpha-mediated osteoclastogenesis. When mouse bone marrow cells were cultured with TNF-alpha, osteoclast-like cells were formed. When they were cultured with both TNF-alpha and IL-12, the number of adherent cells in the bone marrow cells decreased in an IL-12 dose-dependent manner. A combination of IL-12 and TNF-alpha was necessary to induce death of the adherent cells in this culture system. Apoptotic alterations, which were indicated by morphological changes such as cellular atrophy, nuclear and cellular fragmentation, and biochemical changes such as DNA fragmentation, were observed in the adherent cells. Apoptosis of the adherent cells was markedly inhibited by anti-Fas ligand (FasL) Ab. RT-PCR and FACS analyses revealed that TNF-alpha up-regulated Fas transcription to lead to Fas expression on the surfaces of the adherent cells, whereas IL-12 could not induce Fas on the cells. In contrast, IL-12 induced FasL transcription to lead to FasL expression on the surfaces of nonadherent bone marrow cells, whereas TNF-alpha could not induce FasL on the cells. These results implied that apoptosis of the adherent cells in bone marrow cells might be caused by interaction between TNF-alpha-induced Fas on the adherent cells and IL-12-induced FasL on the nonadherent cells.

Requirement of Fas expression in B cells for tolerance induction

Fas is a death receptor that belongs to the tumor necrosis factor receptor family and is expressed in various cell types, in particular, in lymphoid cells. A loss-of-function mutation in the Fas gene (lpr mutation) causes lymphadenopathy and splenomegaly, and accelerates autoimmune diseases in some strains of mice such as MRL. In this report, Fas cDNA driven by murine lck distal promoter was used to establish transgenic MRL-lpr mouse lines. The transgenic mice expressed functional Fas in mature T cells and B cells. The lymphadenopathy and splenomegaly caused by accumulation of abnormal T cells in the lpr mice were rescued in the transgenic mice. The number of B cells in the periphery as well as the serum IgG level were significantly reduced, and the autoimmune symptoms and mortality were ameliorated. These results indicate that both mature B cells and T cells must undergo Fas-mediated apoptosis to prevent the development of autoimmune diseases.

CD95 antigen mutations in hematopoietic malignancies

The CD95 receptor, also known as Fas/Apo-1, is a member of the Tumor Necrosis Factor receptor (TNF-R) family of death receptors. Apoptosis mediated by CD95 plays a central role in maintaining homeostasis of the immune system. Dysregulation of the CD95 apoptotic pathway has been proposed as a mechanism of oncogenesis by providing a survival advantage to potentially malignant cells. This extended lifespan could allow the accumulation of further mutations leading to malignant transformation. Several mechanisms of resistance to CD95 mediated apoptosis have been identified, including reduced surface expression of the receptor, overexpression of anti-apoptotic molecules, and loss of function mutations. This review will focus on the potential role of the CD95-CD95 ligand system in the pathogenesis of hematological malignancies, with particular emphasis on recent work from our laboratory examining the expression of CD95 in B cell lymphomas. We demonstrate that CD95 mutations occur at low frequency in NHL tumors, however, surface expression of the CD95 protein varies with the subtype of lymphoma. Loss of surface CD95 is more likely to occur in lymphomas of aggressive histology, and is unrelated to the detection of CD95 mutations.

An essential role for Daxx in the inhibition of B lymphopoiesis by type I interferons

Interferon-alpha and -beta inhibit the interleukin-7-mediated growth and survival of T and B lymphoid progenitors via an unknown, STAT1-independent pathway. Gene expression profile analysis of interferon-beta-treated progenitor B cells revealed enhanced Daxx expression, with concomitant Daxx protein increase and nuclear body translocation. The interferon effects included downregulation of cell cycle regulating genes and cell cycle arrest, followed by Bcl-2 downregulation and apoptosis. Daxx antisense oligonucleotides rescued the interferon-treated pro-B cells from growth arrest and apoptosis in parallel with the reduction of nuclear Daxx. These findings implicate the gene repressor function of Daxx in interferon-induced apoptosis of lymphoid progenitors.

Natural killer cell-dependent apoptosis of peripheral murine hematopoietic progenitor cells in response to Fas cross-linking: involvement of tumor necrosis factor-alpha

Recently, a marked extramedullary myelopoiesis in Fas/CD95- or FasL/CD95L-deficient mice has been reported. In the present in vitro study, the mechanisms underlying Fas-induced apoptosis of normal peripheral colony-forming unit-C (CFU-C) progenitors in the spleen were analyzed. Surprisingly, it was found that clonogenic progenitors were protected from gammaIFN plus Fas-induced programmed cell death when Lin(+) cells were removed from cultured splenocytes. The cells that rendered CFU-C sensitive to the activation of the Fas pathway did not belong to the T or the myelocytic-monocytic lineage but comprised a non-B-cell subset expressing the activation marker B220. Among CD19(-) B220(+) splenocytes, nearly half were natural killer (NK) 1.1(+) cells whose in vivo depletion or deficiency in RAG2-gamma(c)(-/-) mice abrogated the effect of Fas cross-linking. NK cells exerted their accessory function, at least in part, through tumor necrosis factor-alpha (TNF-alpha), which they readily produced during pretreatment with the anti-Fas/CD95 monoclonal antibody and IFN-gamma and whose addition could compensate for the loss of sensitivity. In conclusion, this study provides evidence that peripheral clonogenic progenitors are not directly responsive to Fas cross-linking, even in the presence of IFN-gamma, but require NK cells as a source of TNF-alpha to make them susceptible to this death pathway.

Increased apoptosis in lamina propria B cells during polymicrobial sepsis is FasL but not endotoxin mediated

Recent studies from our laboratory demonstrated that mucosal lymphoid tissue such as Peyer's patch cells and lamina propria (LP) B lymphocytes from mice shows evidence of increased apoptosis after sepsis that is associated with localized inflammation/activation. The mechanism for this is poorly understood. Endotoxin as well as Fas/Fas ligand (FasL) have been shown to augment lymphocyte apoptosis; however, their contribution to the increase of apoptosis in LP B-cells during sepsis is not known. To study this, sepsis was induced by cecal ligation and puncture (CLP) in endotoxin-tolerant C3H/HeJ or FasL-deficient C3H/HeJ-FasL(gld) (FasL(-)) mice and LP lymphocytes were isolated 24 h later. Phenotypic, apoptotic, and functional indexes were assessed. The number of LP B cells decreased markedly in C3H/HeJ mice but not in FasL-deficient animals at 24 h after CLP. This was associated with comparable alteration in apoptosis and Fas antigen expression in the B cells of these mice. Septic LP lymphocytes also showed increased IgA production, which was absent in the FasL-deficient CLP mice. Furthermore, Fas ligand deficiency appeared to improve survival of septic challenge. These data suggest that the increase in B cell apoptosis in septic animals is partially due to a Fas/FasL-mediated process but not endotoxin.

Direct in vivo transfection of antisense Fas-ligand reduces tumor growth and invasion

The expression of Fas ligand (FasL) by tumor cells has been reported to have multiple, conflicting effects on tumor growth. The majority of the data support the theory that FasL expressing tumor cells evade immune surveillance by killing T cells expressing Fas. However, the role of the humoral immune-blockade by FasL expressing tumor cells has not been assessed. Using immune-competent mice, we observed that FasL expressing tumor cells reduced the antitumor antibody production together with the T and B cell content of the spleen in these mice. Further, to determine if the expression of FasL in the environment of the tumor suppresses the humoral antitumor immune response and influences tumor growth, a mouse model lacking T cells was used. To assess whether a local reduction of FasL could reduce tumor progression, a plasmid encoding antisense FasL cDNA was delivered directly into a growing tumor (SW620 colon carcinoma). Intratumoral delivery of the plasmid was able to transfect tumor cells, stromal cells, and peritumoral muscle cells. This antisense FasL tumor tissue transfection persisted for at least 25 days, produced a systemic decrease in soluble FasL, and resulted in a 50% reduction in the rate of tumor growth when compared with tumor tissue of the control groups. These results suggest that direct transfection of antisense FasL cDNA impairs FasL translation in tumor and stromal cells, and can inhibit tumor progression by impairing the FasL-mediated, stromal cell-assisted, tumor counter-attack.

Self-limiting systemic autoimmune disease during reconstitution of T cell-deficient mice with syngeneic T cells: support for a multifaceted role of T cells in the maintenance of peripheral B cell tolerance

The T cell compartment can be partially reconstituted in mice with targeted inactivation of the TCR C(beta) and C(delta) genes by injection of mature, syngeneic T cells. Surprisingly, during this reconstitution high titers of IgG anti-nuclear antibodies and symptoms of systemic autoimmune disease develop. However, this autoimmune response is transient and aged, reconstituted mice show no overt signs of disease. The autoantibody response appears to be derived from a pre-existing population of host self-reactive B cells and requires CD40 ligand-mediated co-stimulation from donor cells. Diminution of this response is coincident with a vigorous germinal center reaction and the disappearance of a subpopulation of activated B cells that expresses elevated levels of Fas. Collectively, our data support the idea that T cells play a multifaceted role in the maintenance of peripheral B cell tolerance that includes mediating the activation-induced death of autospecific B cells.

Unexpected autoantibody production in membrane Ig-mu-deficient/lpr mice

In the B lymphocyte lineage, Fas-mediated cell death is important in controlling activated mature cells, but little is known about possible functions at earlier developmental stages. In this study we found that in mice lacking the IgM transmembrane tail exons (muMT mice), in which B cell development is blocked at the pro-B stage, the absence of Fas or Fas ligand allows significant B cell development and maturation, resulting in high serum Ig levels. These B cells demonstrate Ig heavy chain isotype switching and autoimmune reactivity, suggesting that lack of functional Fas allows maturation of defective and/or self-reactive B cells in muMT/lpr mice. Possible mechanisms that may allow maturation of these B cells are discussed.

Alterations in splenic architecture and the localization of anti-double-stranded DNA B cells in aged mice

Aging is characterized by a decline in humoral immunity and a concommitant increased incidence of anti-DNA and other autoantibodies. To define how the regulation of autoreactive B cells is altered with age, we have used BALB/c mice with an Ig heavy H chain transgene to track the fate of anti-double-stranded (ds) DNA B cells in vivo. In young adult mice, anti-dsDNA B cells are developmentally arrested and excluded from the splenic B cell follicle, whereas in most aged mice they are mature and localize within the B cell follicle. Furthermore, we have detailed global changes in lymphoid architecture that accompany aging: CD4(+) T cells are found not only in the periarteriolar lymphoid sheath, but also in the B cell follicles. Strikingly, these disruptions are similar to those that precede serum anti-dsDNA antibody expression in autoimmune MRL-lpr/lpr mice.

The origin of anti-nuclear antibodies in bcl-2 transgenic mice

bcl-2 transgenic mice develop anti-double-stranded (ds) DNA antibodies similar to those present in systemic lupus erythematosus. To begin to understand where a breakdown in the regulation of autoreactive lymphocytes is occurring, we have used a bcl-2 transgene (Tg) in conjunction with an Ig Tg that allows us to identify and track anti-dsDNA B cells. Previously, we have shown that anti-dsDNA B cells are actively tolerized in BALB/c mice as manifested by their developmental arrest, follicular exclusion, increased in vivo turnover rate and lack of their antibody in the serum. The bcl-2 Tg mice increased the lifespan of anti-dsDNA B cells, but did not alter the other features of tolerance, indicating that the anergy of the anti-dsDNA B cells is independent of their reduced lifespan. Furthermore, these data suggest that the serum anti-dsDNA antibodies in bcl-2 transgenic mice are not due to a breakdown in the induction or maintenance of B cell anergy; rather they may originate from B cells that have transited through a germinal center.

Increased Fetal and Extramedullary Hematopoiesis in Fas-Deficient C57BL/6-lpr/lpr Mice

In this study, we examined the consequences of Fas deficiency on hematopoiesis in C57BL/6-lpr/lpr mice. We found a striking extramedullary increase in hematopoietic progenitor cells, comprising erythroid and nonerythroid lineages alike. These modifications preceded the lymphadenopathy, because early progenitors (colony-forming units-spleen [CFU-S] day 8) were already augmented in day-18 fetal livers of the lpr phenotype. Three weeks after birth, CFU-S increased in peripheral blood and spleen and colony-forming cells (CFU-C) began to accumulate 1 to 3 weeks later. Extramedullary myelopoiesis augmented progressively in Fas-deficient mice, reaching a maximum within 6 months. By then, mature and immature myeloid cells had infiltrated the spleen, the liver, and the peritoneal cavity. Similar changes occurred in C57BL/6-gld/gld mice, indicating that they resulted from Fas/FasL interactions. Medullary hematopoiesis was not significantly modified in adult mice of either strain. Yet, the incidence of CFU-S decreased after Fas cross-linking on normal bone marrow cells in the presence of interferon γ, consistent with a regulatory function of Fas/FasL interactions in early progenitor cell development. These data provide evidence that Fas deficiency can affect hematopoiesis both during adult and fetal life and that these modifications occur independently from other pathologies associated with the lpr phenotype.

Regulation of autoreactive anti-IgG (rheumatoid factor) B cells in normal and autoimmune mice

In systemic autoimmune disease, autoantibodies target specific self-components in patterns that depend on the particular underlying disease. Therefore, in order to understand how tolerance to these self-components breaks down, it is important to study B cells with those specificities, rather than artificial autoantigens. We have been investigating the regulation of autoreactive B cells with specificity for self IgG2a (the rheumatoid factor or RF specificity) in order to understand how normal mice regulate RF autoantibodies and how this fails in autoimmune mice. A transgenic (Tgic) mouse based on an RF isolated from a diseased MRL/lpr/lpr mouse was constructed and studied in both normal and autoimmune-prone genetic backgrounds. Normal mice do not appear to regulate the RF clone negatively, nor do they appear to activate it substantially. Thus, a disease-related RF is "clonally indifferent." However, in a, Fas-deficient autoimmune-prone animal, these RF B cells are activated to divide and secrete in an antigen-specific manner. A high-affinity RF Tgic mouse was also constructed to determine whether RFs could be tolerized in normal mice. These B cells were deleted or edited in the presence of the autoantigen, which originated from maternal IgG in young mice. Interestingly, shortly after weaning, many mice began to produce autoreactive RF. Escape from tolerance could last for months and was most likely perpetuated by a positive feedback mechanism. Such a mechanism could exist in autoimmune animals and could have important implications for chronic autoimmune disease, as discussed.

Autoantigen-specific B cell activation in Fas-deficient rheumatoid factor immunoglobulin transgenic mice

In systemic autoimmune disease, self-tolerance fails, leading to autoantibody production. A central issue in immunology is to understand the origins of activated self-reactive B cells. We have used immunoglobulin (Ig) transgenic mice to investigate the regulation of autoreactive B cells with specificity for self-IgG2a (the rheumatoid factor [RF] specificity) to understand how normal mice regulate RF autoantibodies and how this fails in autoimmune mice. We previously showed that normal mice do not tolerize the AM14 RF clone, nor do they appear to activate it. Here we show that in Fas-deficient autoimmune mice, the picture is quite different. RF B cells are activated to divide and secrete, but only when the autoantigen is present. Thus, B cells that are ignored rather than anergized in normal mice can be stimulated to produce autoantibody in Fas-deficient mice. This demonstrates a novel developmental step at which intact Fas-Fas ligand signaling is required to regulate B cells in order to prevent autoimmunity. These data also establish the relevance of ignorant self-specific B cells to autoantibody production in disease and prove that in the case of the RF specificity, the nominal autoantigen IgG2a is the driving autoantigen in vivo.

Apoptosis, proliferation, and expression of Bcl-2, Fas, and Fas ligand in bronchial biopsies from asthmatics

The in situ apoptosis and the expression of molecules involved in this process, such as Bcl-2, Fas, and its ligand, Fas ligand (FasL), were examined in bronchial biopsies from healthy control subjects and from steroid-untreated or -treated asthmatics, using terminal transferase-mediated deoxyuridyltriphosphate nick-end labeling and immunohistochemical techniques, respectively. Bronchial submucosa from steroid- untreated asthmatics showed an increase in the number of eosinophils and a decrease in that of apoptotic cells compared with that of control subjects, but no significant changes in the number of T lymphocytes or in that of cells expressing Bcl-2, Fas, or FasL. Treatment with steroids reduced airway eosinophilia and augmented the proportion of apoptotic eosinophils. Compared with control subjects or untreated patients, steroid-treated asthmatics exhibited increased expression of Bcl-2, Fas, FasL, and of proliferating cell nuclear antigen (PCNA) in their bronchial epithelium, without changes in the number of apoptotic cells. Moreover, the intensity of the expression of Bcl-2, Fas, and FasL correlates well with that of PCNA. We conclude that steroids may reduce the inflammatory cell infiltrate in the bronchial submucosa in part by promoting eosinophil apoptosis and by inducing the expression of FasL on bronchial epithelial cells. Treatment with steroids may also augment survival and proliferation of epithelial cells, possibly via the expression of Bcl-2 and PCNA.

Mice deficient in fas ligand (gld) or fas (lpr) show few alterations in granulopoiesis

Evidence exists that the life span of mature, circulating neutrophils is influenced by apoptosis induced by interactions between Fas ligand (FasL) and Fas (CD95). However, the role of FasL/Fas-mediated apoptosis in granulopoiesis has not been explored. The present study assessed differences in granulopoiesis between normal (B6) mice and mice carrying mutations in the genes for FasL (B6/gld) or Fas (B6/lpr). Granulopoiesis was examined by quantitating mature granulocytes in the blood, committed myeloid progenitor cells (or colony-forming units; CFU) in the bone marrow (BM), and granulocyte lineage cells in the BM. The present study also characterized through flow cytometry the ability of GR-1(+) granulocyte lineage cells from B6, B6/gld, and B6/lpr mice to undergo spontaneous apoptosis in vitro. In comparison to B6 mice, B6/gld mice (but not B6/lpr mice) showed small, but significant increases in the number and percentage of blood granulocytes and in the number of myeloid CFU. However, the number and percentage of GR-1(+) granulocyte lineage cells in the BM were similar in the three strains. The rate of spontaneous apoptosis of GR-1(+) granulocyte lineage cells also did not differ between B6, B6/gld, and B6/lpr mice. In B6 and B6/gld mice, Fas expression on granulocyte lineage cells was downregulated in conjunction with a decrease in forward-angle light scatter (fsc) and externalization of phosphatidylserine (PS), two measures of apoptosis. These results suggest that FasL-Fas interactions play only a minor role in modulating numbers of committed myeloid progenitor cells and the size of the peripheral pool of mature granulocytes. Interactions between FasL and Fas do not influence the size of the BM pool of granulocyte lineage cells or the ability of those cells to undergo spontaneous apoptosis.

Antigen receptor-initiated signals for B cell development and selection

The B cell antigen receptor or signaling components of this receptor play varying roles in determining the fate of the B cell at different stages of B cell development. Signals generated through this receptor complex or its various components determine the survival, progression, expansion, and activation of the B cell. Factors that determine the fate of the B cell following antigen receptor engagement include the developmental stage, coreceptor expression, degree and stability of ligand-receptor engagement, and the availability of T cell-derived secondary signals. The author's laboratory is interested in defining the molecular processes linking B cell antigen receptor signaling to these specific cellular responses in the context of antigen-independent and dependent B cell development. Here our current progress and thinking with regard to the determination between negative selection and activation as it pertains to the transition from the immature- to the mature-stage B cell will be discussed.

Self-reactive B cells in nonautoimmune and autoimmune mice

The defining feature of autoimmune disease is the presence of specific autoreactive lymphocytes. Systemic lupus erythematosus (SLE), for example, is characterized by a discrete set of antibodies directed to nuclear antigens; these include autoantibodies to DNA and snRNPs that are diagnostic for SLE. The murine model of SLE, the MRL-lpr/lpr mouse, likewise, has a similar autoantibody profile. To understand how SLE-associated autoantibodies are regulated in healthy individuals and to identify mechanisms underlying their expression in autoimmunity, we have developed a transgenic (tg) model system using multiple sets of tgs. The development of B cells bearing these tgs has been studied in BALB/c and MRL-lpr/lpr autoimmune backgrounds, and the relative fates of anti-ssDNA and anti-dsDNA tg B cells when they are a part of a diverse as well as monoclonal B cell repertoire have been evaluated.

Mutations in the Fas Antigen in Patients With Multiple Myeloma

Programmed cell death, or apoptosis, is well documented as a physiological means of eliminating activated lymphocytes and maintaining immune homeostasis. Apoptosis has also been implicated in the targeting of tumor cells by cytotoxic T lymphocytes and natural killer cells. One of the two primary mechanisms used in cell-mediated cytotoxicity is the Fas/FasLigand system. Activated or transformed cells expressing the Fas antigen on their surface are susceptible to killing by immune effector cells that express the Fas ligand. Many neoplastic cells, including those derived from patients with multiple myeloma, express Fas antigen on their surface, but do not undergo apoptosis in response to antigen crosslinking. One possibility for the lack of Fas-mediated apoptosis includes mutations in the Fas antigen. Loss of function mutations in the Fas antigen have been associated with congenital autoimmune disease in humans, and have been defined as the genetic defect the in lpr mice. Mutations in the Fas antigen have not been previously described in cancer patients. In this study, we show that mutations occur in the Fas antigen which may cause loss of function and contribute to the pathogenesis of the neoplastic disease, multiple myeloma. Using reverse transcriptase-polymerase chain reaction (RT-PCR), single-stranded conformation polymorphism (SSCP) analysis, and DNA sequencing, we examined the cDNA structure of the Fas antigen in 54 bone marrow (BM) specimens obtained from myeloma patients. Six patient specimens (11%) did not express detectable levels of Fas antigen mRNA. Of the 48 BM specimens which did express Fas antigen, 5 (10%) displayed point mutations. All of the mutations identified were located in the cytoplasmic region of the Fas antigen known to be involved in transduction of an apoptotic signal. Two separate individuals demonstrated an identical mutation at a site previously shown to be mutated in the congenital autoimmune syndrome, ALPS. One patient exhibited a point mutation at a site only two amino acids removed from the documented lesion of the lprcg mouse. Although the functional status of these point mutations remains to be determined, we propose that Fas antigen mutations may contribute to the pathogenesis and progression of myeloma in some patients.

Mutations in the Fas Antigen in Patients With Multiple Myeloma

Programmed cell death, or apoptosis, is well documented as a physiological means of eliminating activated lymphocytes and maintaining immune homeostasis. Apoptosis has also been implicated in the targeting of tumor cells by cytotoxic T lymphocytes and natural killer cells. One of the two primary mechanisms used in cell-mediated cytotoxicity is the Fas/FasLigand system. Activated or transformed cells expressing the Fas antigen on their surface are susceptible to killing by immune effector cells that express the Fas ligand. Many neoplastic cells, including those derived from patients with multiple myeloma, express Fas antigen on their surface, but do not undergo apoptosis in response to antigen crosslinking. One possibility for the lack of Fas-mediated apoptosis includes mutations in the Fas antigen. Loss of function mutations in the Fas antigen have been associated with congenital autoimmune disease in humans, and have been defined as the genetic defect the in lpr mice. Mutations in the Fas antigen have not been previously described in cancer patients. In this study, we show that mutations occur in the Fas antigen which may cause loss of function and contribute to the pathogenesis of the neoplastic disease, multiple myeloma. Using reverse transcriptase-polymerase chain reaction (RT-PCR), single-stranded conformation polymorphism (SSCP) analysis, and DNA sequencing, we examined the cDNA structure of the Fas antigen in 54 bone marrow (BM) specimens obtained from myeloma patients. Six patient specimens (11%) did not express detectable levels of Fas antigen mRNA. Of the 48 BM specimens which did express Fas antigen, 5 (10%) displayed point mutations. All of the mutations identified were located in the cytoplasmic region of the Fas antigen known to be involved in transduction of an apoptotic signal. Two separate individuals demonstrated an identical mutation at a site previously shown to be mutated in the congenital autoimmune syndrome, ALPS. One patient exhibited a point mutation at a site only two amino acids removed from the documented lesion of the lprcg mouse. Although the functional status of these point mutations remains to be determined, we propose that Fas antigen mutations may contribute to the pathogenesis and progression of myeloma in some patients.

IL-7 sensitizes human pre-B cells but not pro-B cells to Fas/APO-1 (CD95)-mediated apoptosis

Homeostasis of human B cell development is maintained by a complex network of cytoplasmic and surface expressed molecules. Abnormalities in this process may result in the expansion of malignant B cell precursors in B lineage acute lymphoblastic leukaemia (ALL). ALL cells share surface antigens with normal early precursor B cells. We have studied here the role of Fas/APO-1 (CD95) antigen on leukaemic precursor B cell line growth and survival, and the modulation of its effects by signals involved in normal early B cell development. Four ALL cell lines representative of the early steps of B cell differentiation are shown to express surface Fas/APO-1 (CD95) antigen and to undergo apoptosis in the presence of anti-Fas cross-linking antibodies. This effect is strongly enhanced when pre-B, but not pro-B cells, are pretreated with IL-7 but not with IL-2, IL-3, IL-4 or IL-10. Furthermore, pre-B cell death induced by anti-Fas antibodies in combination with IL-7 is increased upon pre-B receptor but not CD19 cross-linking. Bcl-2 and Bax protein expression is not influenced by IL-7 or pre-BR stimulation in either pro-B or pre-B cell lines. These results indicate that signals involved in normal early B cell development can modulate the Fas (CD95)-mediated apoptosis of leukaemic precursor B cells.

Unique site of IgG2a and rheumatoid factor production in MRL/lpr mice

MRL/lpr (Fas-deficient) mice develop an autoimmune syndrome associated with excessive production of autoantibodies. A significant portion of these autoantibodies are IgG2a molecules specific for many of the autoantigens recognized by the sera of patients with systemic lupus erythematosus. In addition, MRL/lpr mice make exceedingly high titers of IgG or IgA rheumatoid factors (RF) specific for autologous IgG2a. The microenvironment of the IgG2a-producing B cells as well as the prototypic RF autoantibodies was determined by a combination of immunohistochemical and in situ hybridization techniques. In contrast to the antibody-producing cells present in mice responding to conventional foreign antigens, both IgG2a+ and RF+ B cells were found to be densely clustered in the T-cell-rich inner periarteriolar lymphatic sheath of the spleen. These results suggest that conventional antibody and autoantibody production in MRL/lpr mice may be mechanistically distinct processes.

Graft-versus-host-disease-associated lymphoid hypoplasia and B cell dysfunction is dependent upon donor T cell-mediated Fas-ligand function, but not perforin function

Allogeneic bone marrow transplant recipients often exhibit a graft-versus-host-disease (GVHD)-associated immune deficiency that can be prolonged and lead to life-threatening infections. We have examined the role of donor T cell-mediated cytotoxic function in the development of GVHD-associated immune deficiency. A major histocompatibility complex-matched model of allogeneic bone marrow transplantation was employed in which lethally irradiated C3H.SW mice received a nonlethal dose of T cells from either perforin-deficient (B6-perforin 0/0), Fas-ligand (FasL)-defective (B6-gld), or normal (B6) allogeneic donor mice. T cell-depleted marrow from B6-Ly-5.1 congenic donor mice was transplanted along with the donor T cell populations to determine the effects of donor T cell-mediated cytotoxicity on engraftment. Our results demonstrate that recipients of perforin-deficient or normal allogeneic T cells exhibit profound lymphoid hypoplasia and severely reduced splenic proliferative responses to lipopolysaccharide in vitro. In contrast, GVHD-associated lymphoid hypoplasia is dramatically reduced and in vitro B cell function is intact in recipients of FasL-defective allogeneic T cells. Engraftment of myeloid and erythroid lineage cells occurs irrespective of donor T cell cytotoxic function. Although recipients of perforin-deficient or normal allogeneic T cells exhibited hematopoietic engraftment exclusively of donor origin, recipients of FasL-defective donor T cells exhibited significant mixed chimerism (Ly-5.1/Ly-5.2). Because only marrow of donor origin was transplanted, this finding suggests that Fas-mediated antirecipient cytotoxicity is required for clearance of residual hematopoietic stem cells of host origin that persist following lethal irradiation.

Peripheral deletion of rheumatoid factor B cells after abortive activation by IgG

Rheumatoid factor (RF) B cells proliferate during secondary immune responses to immune complexed antigen and antigen specific T cells, but higher affinity RFs are not detected except in patients with rheumatoid arthritis and other autoimmune diseases. Consequently, there must exist highly efficient mechanisms for inactivation of these higher-affinity RF B cell clones under normal circumstances. Exposure of transgenic mice expressing a human IgM RF to soluble human IgG in the absence of T cell help causes antigen specific B cell deletion in 2-3 days. The deletion is independent of the Fas/Fas ligand (FasL) pathway of apoptosis and is preceded by a phase of partial activation involving increase in cell size and expression of B7 and ICAM-1, and transient release of low levels of immunoglobulin. Complete B cell activation involving the formation of germinal centers and sustained high level RF secretion only occurs if T cell help is provided simultaneously. RF B cells exposed to tolerogen remain competent to secrete RF in vitro if provided with an appropriate antigenic stimulus and T cell help. Consequently, death of these cells is not preceded by anergy. Abortive activation/deletion of B cells by antigen in the absence of T cell-derived survival signals may represent the major mechanism for maintaining peripheral tolerance in B cells expressing higher affinity RF. The lack of anergy, and the potential for reactivation before death, provide a means for maintaining RF production under pathologic circumstances, such as may occur in the inflamed rheumatoid synovium.

CD40: a pivotal receptor in the determination of life/death decisions in B lymphocytes

CD40 is a 48 kDa glycoprotein predominantly expressed on B cells in both mouse and man, which interacts with a counterligand (CD40L), expressed on activated CD4+ T cells. CD40/CD40L interactions are now known to be essential for the initiation of antibody responses to T-dependent antigens. In this review we discuss the immunobiology of CD40, with a special emphasis on our own studies in the mouse. These have focused on signal transduction via CD40, the role of cytokines (both T cell-derived and B cell-derived) in CD40-mediated B cell activation, and the role of CD40 in protecting B cells from apoptotic cell death. The available data indicate clearly that this protein is a pivotal receptor on B cells, both for the delivery of activating signals and for promoting B cell survival.

B cells in systemic autoimmune disease: recent insights from Fas-deficient mice and men

In mice functionally deficient for either Fas or Fas ligand expression, the failure of Fas-ligand-expressing cytotoxic T cells to eliminate autoreactive B cells can result in excessive autoantibody production. Recent in vitro studies have shown that B cells activated by CD40 ligand become extremely sensitive to Fas-mediated apoptosis while IL-4 and/or surface IgM receptor engagement protects B cells from Fas ligand cytolysis. Potential in vivo sites for Fas ligand regulation of self-reactive B cells have been suggested and implications for human disease have been investigated.