Modulation of Fc gamma receptor-mediated early events by the tyrosine phosphatase CD45 in primary human monocytes. Consequences for interleukin-6 production

Modulation of the immune response in pristane-induced lupus by expression of activation and inhibitory Fc receptors

Altered homeostasis in Fcgamma receptor (FcgammaR) expression has been implicated in the induction of both immune complex-mediated glomerulonephritis and autoantibody production in systemic lupus erythematosus. FcgammaRI and III are required for immune complexes to activate inflammatory cells, thereby inciting tissue injury. In contrast, FcgammaRIIB functions as a negative regulator of immune complex-mediated inflammation and autoantibody production. We investigated the role of FcgammaRI/III versus FcgammaRIIB on pristane-induced lupus in mice. FcgammaRI/III and FcgammaRIIB-deficient ((-/-)) and control ((+/+)) BALB/c mice were injected with either pristane or PBS. Proteinuria and glomerular immune deposits were evaluated 9 months after treatment and serial sera were analysed for total IgG levels and lupus-specific autoantibodies. The incidence of nephritis was higher in pristane-treated FcgammaRIIB(-/-) mice than pristane-treated FcgammaRI/III(-/-) and (+/+) mice. Hypergammaglobulinaemia and spontaneous anti-DNA/chromatin autoantibody production were associated with interleukin (IL)-6 over-expression in FcgammaRIIB(-/-) mice and were augmented further by pristane treatment when compared to both FcgammaRI/III(-/-) and (+/+) mice. Lack of either FcgammaRIIB or FcgammaRI/III had little effect on both anti-nRNP/Sm and anti-Su production induced by pristane. Our results confirm that spontaneous autoimmunity occurs in the absence of FcgammaRIIB. Moreover, the lupus-like syndrome induced by pristane in BALB/c mice was regulated by opposing activating and inhibitory FcgammaRs. Activating FcgammaRs were required for significant proteinuria and unbridled activation in the absence of FcgammaRIIB dramatically exacerbated glomerular inflammatory responses. FcgammaRIIB may be a key modulator that suppresses cell activation in the inflammatory immune response in systemic lupus erythematosus in humans.

CD148 Is a Membrane Protein Tyrosine Phosphatase Present in All Hematopoietic Lineages and Is Involved in Signal Transduction on Lymphocytes

Evidence is presented showing that a protein tyrosine phosphatase different from CD45 is present on the membrane of human hematopoietic cells. The molecule recognized by the monoclonal antibody 143-41, which has been classified as CD148 in the VI International Workshop on Leukocyte Differentiation Antigens, was immunopurified and sequenced. The sequence obtained from N-terminus as well as from two different CNBr-digested peptides showed a close identity with a previously described tyrosine phosphatase named HPTP-η/DEP-1. CD148 is present on all hematopoietic lineages, being expressed with higher intensity on granulocytes than on monocytes and lymphocytes. Interestingly, whereas it is clearly present on peripheral blood lymphocytes, it is poorly expressed on different lymphoid cell lines of T and B origin. When this protein tyrosine phosphatase was cocrosslinked with CD3, an inhibition of the normally observed calcium mobilization was observed. This inhibition correlates with a decrease in phospholipase C-γ (PLC-γ) phosphorylation and is similar to the one observed with CD45. In addition, it is shown that the crosslinking of the CD148 alone is also able to induce an increase in [Ca2+]i. This increase is abolished in the presence of genistein and by cocrosslinking with CD45. These data, together with the induction of tyrosine phosphorylation on several substrates, including PLC-γ, after CD148 crosslinking, suggest the involvement of a tyrosine kinase-based signaling pathway in this process. In conclusion, the data presented show that CD148 corresponds to a previously described protein tyrosine phosphatase HPTP-η/DEP-1 and that this molecule is involved in signal transduction in lymphocytes.

The high-affinity Fc gamma RI on PMN: regulation of expression and signal transduction

In this paper we report that interferon-gamma (IFN-gamma) maintains and enhances functional properties of terminally differentiated polymorphonuclear cells (PMN). Such effects are obvious when compared with untreated PMN declining in their functional response. Culture for 18 hr with 100 IU/ml of recombinant IFN-gamma resulted in the expression of about 15,000 Fc gamma RI per PMN. IFN-gamma maintained viability of PMN and prevented reduction of Fc gamma RIIIb caused by apoptosis. No alterations were found in the level of Fc gamma RIIa. Next, functional properties of Fc gamma RI were evaluated. Calcium mobilization was detected in fura-2/acetoxymethylester (AM)-loaded PMN using a spectrophotometer and the respiratory burst was measured as luminol-dependent chemiluminescence. Cross-linking of an F(ab')2 fragment of monoclonal antibody (mAb) 22.2 to Fc gamma RI gave similar results to those obtained with intact monomeric human immunoglobulin G (mhIgG) when subsequently cross-linked. Moreover, after blocking Fc gamma RIIa and Fc gamma RIIIb with respective mAb fragments, mhIgG was still effective in triggering a calcium flux demonstrating that second messenger generation was caused by Fc gamma RI engagement. Even though Fc gamma RI expression was lower than that of Fc gamma RIIa, Fc gamma RI induced a higher increase of the respiratory burst. In addition, the protein tyrosine phosphatase CD45 was able to inhibit both responses when it was co-cross-linked with CD64, suggesting involvement of tyrosine phosphorylation in early signalling steps.

Cytokine responses of human blood monocytes stimulated with Igs

Using an in vitro system for stimulating human peripheral blood mononuclear cells (PBMC) with immobilized Ig, patterns of cytokine production as a function of different Ig classes and subclasses were elucidated. Wells were coated with IgA, IgG1, IgG2, IgG3 or IgG4. Equivalent protein content on surfaces of wells was demonstrated by a human kappa chain ELISA. Isolated human PBMC were added to Ig-coated wells and incubated for 24 hrs before supernatants were assayed for cytokines. The IgG subclasses showed differences in cytokine production stimulated from PBMC, with the relative stimulation for TNF alpha being IgG2 > or = IgG3 > or = IgG1 > IgG4 and for IL-6 production, IgG2 > or = IgG3 > IgG1 = IgG4. In contrast, the relative stimulation for IL-8 was IgG1 = IgG2 = IgG3 = IgG4. IgA caused less production of TNF alpha when compared to IgG2, but similar levels of IL-8. Such differences may have important implications in the pathogenesis of immune complex mediated diseases.

Fc receptor biology

This review deals with membrane Fc receptors (FcR) of the immunoglobulin superfamily. It is focused on the mechanisms by which FcR trigger and regulate biological responses of cells on which they are expressed. FcR deliver signals when they are aggregated at the cell surface. The aggregation of FcR having immunoreceptor tyrosine-based activation motifs (ITAMs) activates sequentially src family tyrosine kinases and syk family tyrosine kinases that connect transduced signals to common activation pathways shared with other receptors. FcR with ITAMs elicit cell activation, endocytosis, and phagocytosis. The nature of responses depends primarily on the cell type. The aggregation of FcR without ITAM does not trigger cell activation. Most of these FcR internalize their ligands, which can be endocytosed, phagocytosed, or transcytosed. The fate of internalized receptor-ligand complexes depends on defined sequences in the intracytoplasmic domain of the receptors. The coaggregation of different FcR results in positive or negative cooperation. Some FcR without ITAM use FcR with ITAM as signal transduction subunits. The coaggregation of antigen receptors or of FcR having ITAMs with FcR having immunoreceptor tyrosine-based inhibition motifs (ITIMs) negatively regulates cell activation. FcR therefore appear as the subunits of multichain receptors whose constitution is not predetermined and which deliver adaptative messages as a function of the environment.

The 'monocytic' cell line I937 displays typical B cell characteristics

The monocytic cell line I937 was derived from U937 by sorting for cells with high expression of MHC class II molecules. Further analysis of these class II molecules revealed the presence of the HLA-DR3 subtype suggesting that the cell line was a potential candidate for testing antigen presentation to T cells restricted by HLA-DR3. We found that the T cell clones CFTS4:2.80 and CFTS4:2.6 with the required restriction element responded to the house dust mite antigen DPT presented by I937 but not U937, whereas CFTS4:3.1, which is not HLA-DR restricted, did not respond to either cell line. Subsequent analysis of surface markers on I937, however, indicated that the cell line is of B cell origin. In contrast to the parental cell line U937, I937 was tested negative for CD4, CD31 and CD64 but expressed CD19, CD21 and CD40. Although neither surface nor cytoplasmic Ig molecules were detected in either I937 or U937, Southern blot analysis revealed IgH gene rearrangement in I937. In addition, a fragment specific for Epstein-Barr virus nuclear antigen (EBNA2) was amplified in I937 by PCR technique. Therefore, we conclude that I937 is an EBV-transformed B cell line, presumably derived from the same donor and not as reported originally as a subline of U937, which expresses high MHC class II levels.