Stamatis-Nick C. Liossis

Fce receptor type I γ chain replaces the deficient T cell receptor ζ chain in T cells of patients with systemic lupus erythematosus

OBJECTIVE T cells from the majority of patients with systemic lupus erythematosus (SLE) express significantly lower levels of T cell receptor zeta chain, a critical signaling molecule. However, TCR/CD3 triggering of SLE T cells shows increased phosphorylation of downstream signaling intermediates and increased [Ca2+]i response, suggesting the presence of alternative signaling mechanisms. We investigated whether Fcepsilon receptor type I gamma chain (FcepsilonRIgamma) could substitute for TCR zeta chain and contribute to T cell signaling in SLE. METHODS T cells were purified from the peripheral blood of 21 patients with SLE and 5 healthy volunteers. The expression of FcepsilonRIgamma was investigated using immunoblotting, reverse transcriptase-polymerase chain reaction, and flow cytometry methods. Involvement of the FcepsilonRIgamma in T cell signaling was studied by immunoprecipitation and/or immunoblotting after TCR/CD3 stimulation. RESULTS Western blotting and densitometric analysis showed that the expression of FcepsilonRIgamma in SLE T cells was 4.3-fold higher than in normal T cells (P < 0.001). Flow cytometric analyses of T lymphocyte subsets revealed that the proportions of FcepsilonRIgamma+,CD3+, FcepsilonRIgamma+,CD4+, and FcepsilonRIgamma+, CD8+ cells were significantly greater in SLE patients than in healthy controls (P < 0.001). Immunoprecipitation of SLE T cell lysates with an anti-FcepsilonRIgamma antibody showed that FcepsilonRIgamma associates with the tyrosine kinase Syk and the CD3epsilon chain, suggesting that FcepsilonRIgamma is functionally involved in TCR signaling. CONCLUSION These results demonstrate that the FcepsilonRIgamma chain is expressed at high levels in a large proportion of SLE T cells. The increased expression of FcepsilonRIgamma chain in SLE T cells may account in part for the aberrant antigen receptor-initiated signaling and contribute to the diverse cellular abnormalities found in this disease.

Immune cell signaling defects in lupus: activation, anergy and death

Recent studies have identified novel aberrations in antigen receptor-mediated signaling events in lymphocytes from patients with systemic lupus erythematosus. Here, we propose that in lupus lymphocytes, the receptor-mediated increase in protein tyrosine phosphorylation and cytoplasmic free Ca2+ responses, along with T-cell receptor zeta chain deficiency, might explain the previously described diverse and conflicting immunoregulatory defects in human lupus.

Increased expression of functional Fas-ligand in activated T cells from patients with systemic lupus erythematosus.

The Fas ligand induces apoptosis upon binding to Fas/APO-1 (CD95) bearing target cells. Activation induced cell death (AICD) in T cells is mediated by upregulation of Fas ligand on the cell surface membrane upon crosslinking of the TCR. AICD is considered to be essential for the elimination of autoreactive T cells in the peripheral blood. To elucidate possible abnormalities in the process of AICD in human SLE, we studied the expression and function of Fas ligand in polyclonal T cell lines from patients with SLE, patients with other rheumatic diseases and normal controls. SLE T cells expressed on their surface significantly higher amounts of Fas ligand compared to the two control groups. Stimulation of the cells with anti-CD3 mAb lead to further increase in surface membrane Fas ligand expression in all three groups with SLE expressing the highest amounts. The percentage of increase was though lower in SLE T cells than in normal T cells or disease control cells. The T cells were examined for Fas ligand-mediated cytotoxicity in a 51Cr release assay using Fas-expressing normal T cells as target cells. There was no difference in SLE and control T cells with regard to specific 51Cr lysis, indicating that the Fas ligand expressed by the SLE T cells is functional. Our data show that activated T cells from patients with SLE express high amounts of functional Fas ligand with intact TCR-mediated upregulation. This could account for the high apoptotic rates that have been observed in lymphocytes from patients with SLE.

Immune cell signaling aberrations in human lupus

A large array of heterogeneous aberrations of the immune system have been described in systemic lupus erythematosus (SLE). Since the function and the fate of the immune system cells are governed principally by the biochemical events that follow ligation of specialized cell-surface receptors, we will review in this article recent developments in our understanding of abnormalities in the biochemistry of signals generated either by the antigen-receptor complex or by systems of costimulatory cell-surface molecules, like the CD28/CTLA4:CD80/CD86 and the CD40:CD40L pairs found on the surface membrane of lupus immune cells.

B Cells in Systemic Lupus Erythematosus

The principal function of B cells is the production of antibodies. In addition, B cells are also cytokine producers and efficient antigen-presenting cells (APCs). The overactivity of B cells and the production of a variety of autoantibodies comprise hallmarks in the immunopathology of systemic lupus erythematosus (SLE) (1–4). Some of the produced autoantibodies clearly contribute to the induction of tissue injury, whereas others represent markers of disease manifestations, or they are innocent bystanders. The study of human B cells has been hampered so far by their small numbers in the circulation, which are even fewer in the commonly lymphopenic lupus patients, their restricted life-span in vitro, and the inability to establish B-cell lines without transforming them and altering their biology. Data obtained from the study of lupus-prone mice provide us with potentially useful information, even though questions always arise whether the derived conclusions can be transferred unmodified to human disease. This chapter focuses on the contribution of B cells to the pathogenesis of human SLE, and makes reference to the murine models of lupus to clarify our understanding of the autoimmune B cell.

Crosslinking of Fas/CD95 suppresses the CD3-mediated signaling events in Jurkat T cells by inhibiting the association of the T-cell receptor ζ chain with src-protein tyrosine kinases and ZAP70

Crosslinking of Fas (APO-1/CD95) on the surface of T cells initiates a biochemical cascade leading to programmed cell death. We have previously shown that crosslinking of Fas with an apoptosis-inducing IgM anti-Fas mAb results in suppression of the CD3-initiated cell signaling including Ca2+ mobilization and protein tyrosine phosphorylation. We conducted experiments to decipher the mechanisms whereby the cross talk between the Fas- and CD3 signaling pathways occur. We used lysates from Jurkat T and examined the composition of the TCR ζ chain-precipitated immune complexes using immunoblots. While crosslinking of Fas affected the association of p59fyn and p56lck tyrosine kinases with the TCR ζ chain to a limited degree, it dramatically inhibited the association of the protein tyrosine kinase ZAP70 with the ζ chain. In cells that were preincubated with an apoptosis-inducing anti-Fas mAb, the binding of the protein tyrosine phosphatases SHP-1 to the TCR ζ chain was increased. These experiments indicate that crosslinking of Fas interferes with early T cell signaling events by promoting the recruitment of SHP-1 and decreasing the association of protein tyrosine kinases with TCR ζ chain. Therefore, crosslinking of Fas antigen may regulate the antigen-induced T cell response and play an active role in the T cell anergy.

Heat-shock of normal T-cells and T-cell lines downregulates the TCR/CD3-mediated cytoplasmic Ca2+ responses and the production of inositol trisphosphate

The application of heat shock to different types of cells is known to cause multiple biochemical and metabolic changes. The predominant event though is an increased synthesis and expression of a family of proteins, the heat-shock proteins (Hsp). Some of these proteins are currently considered to be involved in the signal transduction pathway. We investigated for any possible effects of heating fresh normal peripheral T-cells and T-cell lines, on the early signal transduction events which follow the antigen (Ag) receptor-complex (TCR/CD3) crosslinking. More specifically, the Ag-receptor-initiated free cytoplasmic Ca2+ ([Ca2+]i) responses and the production of inositol 1,4,5-trisphosphate (IP3) were evaluated. Heating fresh unmanipulated peripheral T-cells 8 hours before the TCR/CD3 stimulation resulted in decreased [Ca2+]i responses. This was also true for cells of normal short-term T-cell lines as well. The TCR/CD3-mediated production of IP3, which is a mediator of the [Ca2+]i response, was also decreased in heat-shocked T-cells.