Effects of hemizygous CD45 expression in the autoimmune Fasl(gld/gld) syndrome

Intracellular B Lymphocyte Signalling and the Regulation of Humoral Immunity and Autoimmunity

B lymphocytes are critical for effective immunity; they produce antibodies and cytokines, present antigens to T lymphocytes and regulate immune responses. However, because of the inherent randomness in the process of generating their vast repertoire of antigen-specific receptors, B cells can also cause diseases through recognizing and reacting to self. Therefore, B lymphocyte selection and responses require tight regulation at multiple levels and at all stages of their development and activation to avoid diseases. Indeed, newly generated B lymphocytes undergo rigorous tolerance mechanisms in the bone marrow and, subsequently, in the periphery after their migration. Furthermore, activation of mature B cells is regulated through controlled expression of co-stimulatory receptors and intracellular signalling thresholds. All these regulatory events determine whether and how B lymphocytes respond to antigens, by undergoing apoptosis or proliferation. However, defects that alter regulated co-stimulatory receptor expression or intracellular signalling thresholds can lead to diseases. For example, autoimmune diseases can result from altered regulation of B cell responses leading to the emergence of high-affinity autoreactive B cells, autoantibody production and tissue damage. The exact cause(s) of defective B cell responses in autoimmune diseases remains unknown. However, there is evidence that defects or mutations in genes that encode individual intracellular signalling proteins lead to autoimmune diseases, thus confirming that defects in intracellular pathways mediate autoimmune diseases. This review provides a synopsis of current knowledge of signalling proteins and pathways that regulate B lymphocyte responses and how defects in these could promote autoimmune diseases. Most of the evidence comes from studies of mouse models of disease and from genetically engineered mice. Some, however, also come from studying B lymphocytes from patients and from genome-wide association studies. Defining proteins and signalling pathways that underpin atypical B cell response in diseases will help in understanding disease mechanisms and provide new therapeutic avenues for precision therapy.

Sensitivity of leukemic T-cell lines to arsenic trioxide cytotoxicity is dependent on the induction of phosphatase B220/CD45R expression at the cell surface

Background

Arsenic trioxide (As₂O₃) is highly effective in treating acute promyelocytic leukemia (APL), but shows more variable therapeutic efficacy for other types of hematological malignancies. Previously, we reported that As₂O₃ selectively eliminates pathogenic B220-expressing T cells in autoimmune MRL/lpr mice. We investigated herein the relationship between As₂O₃ sensitivity of leukemic T-cell lines and the expression levels of the B220 isoform of transmembrane tyrosine phosphatase CD45.

Methods

GSH content, O2^- production, and B220, HSP70, Fas and FasL membrane expression was measured by flow cytometry. Subcellular localization of B220 was determined by imaging flow cytometry. Cell death was analyzed by morphological changes, annexin V and propidium iodide staining, and caspase 8 and 9 activation. B220 mRNA expression was analyzed by RT-PCR. Activated NF-κB p50 was quantified by a DNA binding ELISA.

Results

We selected human (Jurkat, Jurkat variant J45.01, HPB-ALL) and mouse (EL-4, BW5147, L1210) T-cell lines for their marked differences in As₂O₃ sensitivity over a large range of doses (1 to 20 μM). Differences in redox status cannot explain the dramatic differences in As₂O₃ sensitivity observed among the T-cell lines. Unexpectedly, we found that B220 is differentially induced on As₂O₃-treated T-cell lines. As₂O₃ treatment for 24 h induced low (HPB-ALL), intermediate (Jurkat) and high (EL-4, BW5147) levels of B220 membrane expression, membrane-bound HSP70 and cell death, but inhibited NF-κB p50 nuclear translocation. When high levels of B220 expression were achieved with low doses of As₂O₃, the T-cell lines died by apoptosis only. When high doses of As₂O₃ were required to induce B220 expression, leukemic T cells died by both apoptosis and necrosis.

Conclusions

Cellular redox status is not essential for As₂O₃ sensitivity of leukemic T cells, suggesting the existence of additional factors determining their sensitivity to As₂O₃ cytotoxicity. Phosphatase B220 could be such a factor of sensitivity. As₂O₃ treatment inhibits NF-κB p50 nuclear translocation, and induces B220 expression and cell death in a dose and time dependent manner. The levels of B220 induction on leukemic T cells strictly correlate with both the extent and form of cell death, B220 might therefore play a checkpoint role in death pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-251) contains supplementary material, which is available to authorized users.

Aberrant B-lymphocyte responses in lupus: inherent or induced and potential therapeutic targets

BACKGROUND

Lupus is a prototype autoimmune disease of unknown aetiology. The disease is complex; manifest diverse clinical symptoms and disease mechanisms. This complexity has provided many leads to explore: from disease mechanisms to approaches for therapy. B-lymphocytes play a central role in the pathogenesis of the disease. However, the cause of aberrant B-lymphocyte responses in patients and, indeed, its causal relationship with the disease remain unclear.

DESIGN

This article provides a synopsis of current knowledge of immunological abnormalities in lupus with an emphasis on abnormalities in the B-lymphocyte compartment.

RESULTS

There is evidence for abnormalities in most compartments of the immune system in animal models and patients with lupus including an ever expanding list of abnormalities within the B-lymphocyte compartment. In addition, recent genome-wide linkage analyses in large cohorts of patients have identified new sets of genetic association factors some with potential links with defective B-lymphocyte responses although their full pathophysiological effects remain to be determined. The accumulating knowledge may help in the identification and application of new targeted therapies for treating lupus disease.

CONCLUSIONS

Cellular, molecular and genetic studies have provided significant insights into potential causes of immunological defects associated with lupus. Most of this insight relate to defects in B- and T-lymphocyte tolerance, signalling and responses. For B-lymphocytes, there is evidence for altered regulation of inter and intracellular signalling pathways at multiple levels. Some of these abnormalities will be discussed within the context of potential implications for disease pathogenesis and targeted therapies.

B-lymphocyte signalling abnormalities and lupus immunopathology

Lupus is a complex autoimmune rheumatic disease of unknown aetiology. The disease is associated with diverse features of immunological abnormality in which B-lymphocytes play a central role. However, the cause of atypical B-lymphocyte responses remains unclear. In this article, we provide a synopsis of current knowledge on intracellular signalling abnormalities in B-lymphocytes in lupus and their potential effects on the response of these cells in mouse models and in patients. There are numerous reported defects in the regulation of intracellular signalling proteins and pathways in B-lymphocytes in lupus that, potentially, affect critical biological responses. Most of the evidence for these defects comes from studies of disease models and genetically engineered mice. However, there is also increasing evidence from studying B-lymphocytes from patients and from genome-wide linkage analyses for parallel defects to those observed in mice. These studies provide molecular and genetic explanations for the key immunological abnormalities associated with lupus. Most of the new information appears to relate to defects in intracellular signalling that impact B-lymphocyte tolerance, cytokine production and responses to infections. Some of these abnormalities will be discussed within the context of disease pathogenesis.

Loss of P2X7 receptor plasma membrane expression and function in pathogenic B220+ double-negative T lymphocytes of autoimmune MRL/lpr mice

Lupus is a chronic inflammatory autoimmune disease influenced by multiple genetic loci including Fas Ligand (FasL) and P2X7 receptor (P2X7R). The Fas/Fas Ligand apoptotic pathway is critical for immune homeostasis and peripheral tolerance. Normal effector T lymphocytes up-regulate the transmembrane tyrosine phosphatase B220 before undergoing apoptosis. Fas-deficient MRL/lpr mice (lpr mutation) exhibit lupus and lymphoproliferative syndromes due to the massive accumulation of B220⁺ CD4^–CD8^– (DN) T lymphocytes. The precise ontogeny of B220⁺ DN T cells is unknown. B220⁺ DN T lymphocytes could be derived from effector CD4⁺ and CD8⁺ T lymphocytes, which have not undergone activation-induced cell death due to inactivation of Fas, or from a special cell lineage. P2X7R is an extracellular ATP-gated cell membrane receptor involved in the release of proinflammatory cytokines and TNFR1/Fas-independent cell death. P2X7R also regulate early signaling events involved in T-cell activation. We show herein that MRL/lpr mice carry a P2X7R allele, which confers a high sensitivity to ATP. However, during aging, the MRL/lpr T-cell population exhibits a drastically reduced sensitivity to ATP- or NAD-mediated stimulation of P2X7R, which parallels the increase in B220⁺ DN T-cell numbers in lymphoid organs. Importantly, we found that this B220⁺ DN T-cell subpopulation has a defect in P2X7R-mediated responses. The few B220⁺ T cells observed in normal MRL^+/+ and C57BL/6 mice are also resistant to ATP or NAD treatment. Unexpectedly, while P2X7R mRNA and proteins are present inside of B220⁺ T cells, P2X7R are undetectable on the plasma membrane of these T cells. Our results prompt the conclusion that cell surface expression of B220 strongly correlates with the negative regulation of the P2X7R pathway in T cells.

Combinations of CD45 isoforms are crucial for immune function and disease

Expression of the CD45 Ag in hemopoietic cells is essential for normal development and function of lymphocytes, and both mice and humans lacking expression exhibit SCID. Human genetic variants of CD45, the exon 4 C77G and exon 6 A138G alleles, which alter the pattern of CD45 isoform expression, are associated with autoimmune and infectious diseases. We constructed transgenic mice expressing either an altered level or combination of CD45 isoforms. We show that the total level of CD45 expressed is crucial for normal TCR signaling, lymphocyte proliferation, and cytokine production. Most importantly, transgenic lines with a normal level, but altered combinations of CD45 isoforms, CD45(RABC/+) and CD45(RO/+) mice, which mimic variant CD45 expression in C77G and A138G humans, show more rapid onset and increased severity of experimental autoimmune encephalomyelitis. CD45(RO/+) cells produce more TNF-alpha and IFN-gamma. Thus, for the first time, we have shown experimentally that it is the combination of CD45 isoforms that affects immune function and disease.

CD45 regulates apoptosis in peripheral T lymphocytes

Programmed cell death (apoptosis) is a key mechanism for regulating lymphocyte numbers. Murine lymph node lymphocytes cultured in vitro without added stimuli show significant levels of apoptosis over 24 h, detectable by staining with Annexin V. CD4 and CD8 T lymphocytes from transgenic (Tg) mice expressing single CD45RABC or CD45RO isoforms show increased apoptosis and the extent of apoptosis is inversely correlated with the level of CD45 expression. CD45 Tg cells exhibit phosphatidyl serine translocation and DNA oligonucleosome formation, and can be partially rescued from apoptosis by culture in caspase inhibitors or common gamma-chain-binding cytokines. We conclude that CD45 is an important regulator of spontaneous apoptosis in T lymphocytes and this mechanism may contribute to the disease associations reported for individuals expressing CD45 variant alleles.

Altered CD45 expression and disease

CD45, the leucocyte common antigen, is a haemopoietic cell-specific tyrosine phosphatase. Many isoforms are generated by alternative splicing, but their function remains obscure. The extracellular domain of CD45 is highly polymorphic in all vertebrates. Importantly, human polymorphic variants that alter CD45 isoform expression are associated with autoimmune and infectious diseases, establishing CD45 as an important immunomodulator with a significant influence on disease burden. Here, we discuss the new opportunities provided by the human variants for investigating and understanding how CD45 regulates antigen receptor signalling, cytokine responses and apoptosis.

CD45: a critical regulator of signaling thresholds in immune cells

Regulation of tyrosine phosphorylation is a critical control point for integration of environmental signals into cellular responses. This regulation is mediated by the reciprocal actions of protein tyrosine kinases and phosphatases. CD45, the first and prototypic receptor-like protein tyrosine phosphatase, is expressed on all nucleated hematopoietic cells and plays a central role in this process. Studies of CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient humans initially demonstrated the essential role of CD45 in antigen receptor signal transduction and lymphocyte development. It is now known that CD45 also modulates signals emanating from integrin and cytokine receptors. Recent work has focused on regulation of CD45 expression and alternative splicing, isoform-specific differences in signal transduction, and regulation of phosphatase activity. From these studies, a model is emerging in which CD45 affects cellular responses by controlling the relative threshold of sensitivity to external stimuli. Perturbation of this function may contribute to autoimmunity, immunodeficiency, and malignancy. Moreover, recent advances suggest that modulation of CD45 function can have therapeutic benefit in many disease states.

A role for the Fas/Fas ligand apoptotic pathway in regulating myeloid progenitor cell kinetics

Bone marrow from wild-type mice and mice with mutated Fas (lpr) or mutated Fas ligand (gld) was used to investigate the role of the Fas/FasL system in the regulation of myeloid progenitor cell kinetics.Granulocyte-macrophage colony-forming cells (CFU-GM) were measured by a standard colony assay and the proliferative activity of CFU-GM was measured by replating primary colonies and observing secondary colony formation. Fas expression was restored to lpr mouse bone marrow cells by retrovirus-mediated gene transfer and gld mouse marrow cells were treated with soluble FasL. Wild-type marrow cells were treated with YVAD (a caspase inhibitor) or anti-Fas monoclonal antibodies. There were greater frequencies of myeloid progenitor cells (CFU-GM) in lpr and gld mouse marrow compared to wild-type (WT) marrow (p = 0.0008). The proliferative capacity of CFU-GM was also significantly greater for lpr and gld CFU-GM compared to WT CFU-GM (p = 0.0003 and 0.0001, respectively). Retrovirus-mediated restoration of Fas into lpr marrow, and provision of soluble FasL (sFasL) to gld CFU-GM reduced CFU-GM proliferation to WT levels. Treatment of WT CFU-GM with YVAD or anti-FasL monoclonal antibody increased CFU-GM proliferation to the levels found in lpr and gld CFU-GM. YVAD significantly increased and anti-Fas significantly reduced the proliferative capacity of human CFU-GM (p = 0.015 and 0.04, respectively).Fas, FasL, and caspase activation may play an important role in regulating myeloid progenitor cell kinetics.