Severe impairment of B cell function in lpr/lpr mice expressing transgenic Fas selectively on B cells

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

TRAIL and FasL Functions in Cancer and Autoimmune Diseases: Towards an Increasing Complexity

Tumor Necrosis Factor-Related Apoptosis Inducing Ligand (TRAIL/TNFSF10) and Fas Ligand (FasL/TNFSF6), two major cytokines of the TNF (Tumor Necrosis Factor) superfamily, exert their main functions from the immune system compartment. Mice model studies revealed that TRAIL and FasL-mediated signalling both control the homeostasis of the immune cells, mainly from the lymphoid lineage, and function on cytotoxic cells as effector proteins to eliminate the compromised cells. The first clues in the physiological functions of TRAIL arose from the analysis of TRAIL deficient mice, which, even though they are viable and fertile, are prone to cancer and autoimmune diseases development, revealing TRAIL as an important safeguard against autoimmunity and cancer. The naturally occurring gld (generalized lymphoproliferative disease) and lpr (lymphoproliferation) mutant mice develop lymphadenopathy and lupus-like autoimmune disease. The discovery that they are mutated in the fasl and the fas receptor gene, respectively, demonstrates the critical role of the FasL/Fas system in lymphocyte homeostasis and autoimmunity. This review summarizes the state of current knowledge regarding the key death and non-death immune functions that TRAIL and FasL play in the initiation and progression of cancer and autoimmune diseases.

Somatic mutagenesis in autoimmunity

Our laboratory investigates systemic autoimmune disease in the context of mouse models of systemic lupus erythematosus (SLE). SLE is associated with high titers of serum autoantibodies of the IgG class that are predominantly directed against nuclear antigens, with pathological manifestations that are considered by many to be characteristic of an immune-complex mediated disease. In this review, we focus on the known and potential roles of somatic mutagenesis in SLE. We will argue that anti-nuclear antibodies (ANA) arise predominantly from nonautoreactive B cells that are transformed into autoreactive cells by the process of somatic hypermutation (SHM), which is normally associated with affinity maturation during the germinal center reaction. We will also discuss the role of SHM in creating antigenic peptides in the V region of the B cell receptor (BCR) and its potential to open an avenue of unregulated T cell help to autoreactive B cells. Finally, we will end this review with new experimental evidence suggesting that spontaneous somatic mutagenesis of genes that regulate B cell survival and activation is a rate-limiting causative factor in the development of ANA.

TLR9-mediated signals rescue B-cells from Fas-induced apoptosis via inactivation of caspases

The death receptor, CD95/Fas, serves to eliminate potentially dangerous, self-reactive B cells. Engagement of B-cell receptors (BCR) on mature B-cells mediates the escape from cell death resulting in the activation and expansion of antigen specific clones. In addition to the antigen receptors, the receptors of B-cell activating factor belong to the tumor necrosis factor (TNF) family (BAFFR); moreover, the pattern recognition receptor, TLR9 may also deliver survival signals inhibiting Fas-mediated death of B-cells. Our aim was to compare the mechanism of BCR-induced and the BAFFR- or TLR9-stimulated rescue of B-cells from CD95/Fas-mediated apoptosis. We have found that BAFFR and TLR9 collaborate with BCR to protect B-cells from Fas-induced elimination and the rescue is independent of protein synthesis. The results revealed that the TLR9- and BCR-triggered rescue signals are transmitted through partially overlapping pathways; the protein kinase C (PKC) and the abl kinase induced phosphorylation may inactivate caspases in both CpG and anti-IgG stimulated cells. However, PI3-K activation is crucial upon the BCR driven anti-apoptotic effect, while p38 MAPK-mediated inactivation of caspases seems to play essential role in TLR9-mediated protection against Fas-induced programmed cell death.

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.

Clinical and immunological overlap between autoimmune lymphoproliferative syndrome and common variable immunodeficiency

Autoimmune lymphoproliferative syndrome (ALPS) is mainly caused by defects in the CD95 pathway. Raised CD3+TCRαβ+CD4-CD8- double negative T cells and impaired T cell apoptosis are hallmarks of the disease. In contrast, the B cell compartment has been less well studied. We found an altered distribution of B cell subsets with raised transitional B cells and reduced marginal zone B cells, switched memory B cells and plasma blasts in most of 22 analyzed ALPS patients. Moreover, 5 out of 66 ALPS patients presented with low IgG and susceptibility to infection revealing a significant overlap between ALPS and common variable immunodeficiency (CVID). In patients presenting with lymphoproliferation, cytopenia, hypogammaglobulinemia and impaired B cell differentiation, serum biomarkers were helpful in addition to apoptosis tests for the identification of ALPS patients. Our observations may indicate a role for apoptosis defects in some diseases currently classified as CVID.

The many roles of FAS receptor signaling in the immune system

FAS belongs to the subgroup of the tumor necrosis factor receptor (TNF-R) family that contains an intracellular "death domain" and triggers apoptosis. Its physiological ligand FASL is a member of the TNF cytokine family. Studies with mutant mice and cells from human patients have shown that FAS plays critical roles in the immune system, including the killing of pathogen-infected cells and the death of obsolete and potentially dangerous lymphocytes. Fas thereby functions as a guardian against autoimmunity and tumor development. FAS triggers apoptosis through FADD-mediated recruitment and activation of caspase-8. In certain cells such as hepatocytes, albeit not lymphocytes, FAS-induced apoptosis requires amplification through proteolytic activation of the proapoptotic BCL-2 family member BID. Curiously, several components of the FAS signaling machinery have been implicated in nonapoptotic processes, including cellular activation, differentiation, and proliferation. This review describes current understanding of Fas-induced apoptosis signaling and proposes experimental strategies for future advances.

Autoantibody production in lpr/lpr gld/gld mice reflects accumulation of CD4+ effector cells that are resistant to regulatory T cell activity

In Fas/FasL-deficient mice anti-chromatin Ab production is T cell dependent and is not apparent until after 10 weeks of age. Early control of anti-chromatin antibodies may be due to the counterbalancing influence of Treg cells. Here we show that Treg cells block lpr/lpr gld/gld Th cells from providing help to anti-chromatin B cells in an in vivo transfer system. Interestingly, the percentage and absolute numbers of Foxp3+ Treg cells is elevated in BALB/c-lpr/lpr gld/gld mice and increases with age compared to BALB/c mice. The majority of Foxp3 expression is found in the B220- CD4+ T cell population, and Foxp3-expressing cells are localized in the splenic PALS (periarteriolar lymphocyte sheath). Strikingly, although the lack of functional Fas/FasL does not affect the ability of Treg cells to block Th cell proliferation, Treg cells can block the IFN-gamma differentiation of Th cells from BALB/c or young BALB-lpr/lpr gld/gld mice but not of pre-existing Th1 cells from older BALB/c-lpr/lpr gld/gld mice. Thus, we suggest autoantibody production is not caused by the lack of Treg cells but by a defect in activation-induced cell death that leads to the accumulation of T effector cells that are resistant to regulatory T cell activity.

Development of Immunocompetent Lymphocytes In Vivo From Murine Umbilical Cord Blood Cells

BACKGROUND

Reports concerning the immunological functions of lymphocytes derived from umbilical cord blood cell (UCBC) have been limited.

METHODS

In murine syngeneic transplantation system using green fluorescent protein transgenic donors, UCBC-derived lymphocytes were studied for their immunological competence.

RESULTS

Hematopoietic stem cells (HSC) among UCBC differentiated in the recipients into phenotypically mature T and B lymphocytes. The T lymphocytes were capable of specific recognition of major histocompatibility complex/peptide complex and of the subsequent activation. The antigen-induced CD4(+) T cells produced lymphokine upon in vitro antigen stimulation. CD8(+) T cells simulated in the mixed lymphocyte culture could lyze specific target cells. Furthermore, RAG2(-/-) mice reconstituted with UCBC mounted specific antibody responses to T-dependent antigen comparable to those by bone marrow chimeras and also rejected allogeneic skin grafts.

CONCLUSION

Collectively the data indicated that T and B lymphocytes derived from UCBC-HSC are fully competent in immunological terms.

Prominent dominant negative effect of a mutant Fas molecule lacking death domain on cell-mediated induction of apoptosis

Using a panel of transfectant B lymphoma cells expressing varying amounts of the mutant Fas together with the endogenous wild type Fas, semi-quantitative studies on the dominant negative effect of a murine mutant Fas molecule lacking death domain were carried out. In anti-Fas antibody-mediated induction of apoptosis, the mutant molecules exerted significant dominant-negative effect only when their expression level was comparable to or higher than that of wild type molecules, or when exposed to low amounts of the antibody. The inhibitory effect was accompanied by the failure in DISC formation in spite of Fas aggregation. When they were subjected to T cell-mediated Fas-based induction of apoptosis, however, the dominant negative effect was prominent such that the expression of even a small amount of the mutant molecules resulted in significant inhibition. Such a strong inhibitory effect explains the dominant phenotype of this type of mutant Fas molecules in ALPS heterozygous patients and also implies that the physiological effectors for Fas in vivo are cells, i.e., FasL-expressing activated T cells.

Cell-mediated fas-based lysis of dendritic cells which are apparently resistant to anti-Fas antibody

In this report, the controversy concerning the sensitivity of dendritic cells (DCs) to Fas-dependent induction of apoptosis was examined using murine DCs. Although DCs could not be lysed when exposed to an anti-Fas antibody, Jo2, the observed resistance turned out to reflect their lack of the expression of Fc(gamma)R necessary for crosslinking the antibody, rather than their intrinsic resistance. Thus, at least a fraction of DCs was sensitive to Jo2 in the presence of Fc(gamma)R-expressing by-standers. Consistently, a significant fraction of DCs was sensitive to Fas-dependent lysis mediated by T cells including the antigen-specific killing by CD4+ T cells. Both immature (class II MHClow) and mature (class II MHChigh) DCs were sensitive to the Fas-based induction of apoptosis.

Antibody production in early life supported by maternal lymphocyte factors

To examine the influence of maternal lymphocyte factors on the immune responses in offspring in early life, antibody production in neonates born to either normal or lymphocyte-deficient mothers was analyzed. Recombination activating gene (Rag)-2(+/-) mouse neonates born to Rag-2(+/+), Rag-2(+/-)or Rag-2(-/-)mothers were injected with goat anti-mouse IgD antiserum, and IgE and IgG(1) production was evaluated. The levels of IgE and IgG(1) were higher in the pups born to Rag-2(+/+)and Rag-2(+/-) dams than to lymphocyte-deficient Rag-2(-/-) dams. The enhanced antibody production in the former compared with the latter neonates was also found following immunization with ovalbumin or TNP-Ficoll. Thus, the presence of maternal lymphocyte factors was suggested in neonates that augmented antigen-specific antibody production in both T cell-dependent and -independent pathways. A reduction in antibody production was observed in normal neonates when they were foster-nursed by Rag-2(-/-) mothers. Thus, the maternal lymphocyte factors enhancing the immune responses in newborns were shown to be present in breast-milk.

Activation and differentiation of autoreactive B-1 cells by interleukin 10 induce autoimmune hemolytic anemia in Fas-deficient antierythrocyte immunoglobulin transgenic mice

The Fas (CD95) gene is among critical genetic factors in some autoimmune diseases, which are characterized by autoantibody (autoAb) productions. In mice, mutations in the Fas gene cause lymphoproliferation (lpr) which predominantly develops glomerulonephritis, whereas the mutations in human cause autoimmune lymphoproliferative syndrome (ALPS) characterized by autoimmune hemolytic anemia (AIHA) and thrombocytopenia. Although the mechanism of antinuclear Ab in Fas-deficient background has been well characterized, that of antierythrocyte Ab production in ALPS has been still unclear. To investigate this mechanism, we developed a mouse line by crossing the antierythrocyte antibody transgenic mice (H+L6 mice) and Fas-deficient mice. Although Fas deficiency did not break tolerance of autoreactive B-2 cells in H+L6 mice, autoreactive B-1 cells in Fas-deficient H+L6 homozygous mice became activated and differentiated into autoAb-producing cells in mesenteric lymph nodes and lamina propria of intestine, resulting in severe anemia. In addition, serum levels of interleukin (IL)-10 significantly increased in Fas-/- x H+L6 homozygous mice and administration of anti-IL-10 Ab prevented exacerbation of autoAb production and AIHA. These results suggest that activation of B-1 cells is responsible for induction of AIHA in Fas-deficient condition and that IL-10 plays a critical role in terminal differentiation of B-1 cells in these mice.

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.