Isotype-specific cross-linking of select human Fc gamma R isoforms triggers release of IL-6

IL-6 increases B-cell IgG production in a feed-forward proinflammatory mechanism to skew hematopoiesis and elevate myeloid production

Src homology 2 domain-containing inositol 5-phosphatase (SHIP(-/-)) animals display an age-related increase in interleukin-6 (IL-6), a decrease in B lymphopoiesis, and an elevation in myelopoiesis. We investigated the origin of the IL-6 production and show that it is largely produced by peritoneal and splenic macrophages. IL-6 production by these macrophages is not a direct result of the loss of SHIP: IL-6 production is not spontaneous, is absent from bone marrow-derived macrophages, declines with prolonged culture of macrophages, and requires a stimulus present in vivo. The IL-6-rich peritoneal cavity of SHIP(-/-) mice shows more than 700-fold more immunoglobulin G (IgG) than wild-type, approximately 20% of which is aggregated or in an immune complex and contains B220(+) cells that secrete IgG. The SHIP-deficient peritoneal macrophages show evidence of IgG receptor stimulation. Animals lacking both the signal-transducing gamma-chain of IgG receptors and SHIP or Ig and SHIP produce less IL-6. The data indicate a feed-forward process in which peripheral macrophages, responding through IgG receptors to secreted IgG, produce IL-6, to support further B-cell production of IgG. Because of the proinflammatory phenotype of SHIP(-/-) animals, these findings emphasize the importance of IL-6-neutralizing strategies in autoimmune and proinflammatory diseases.

T-cell anergy induced by clonotype-specific antibodies: modulation of an autoreactive human T-cell clone in vitro

Monoclonal antibodies (mAb) specific for the clonotype of an autoreactive T cell may be useful reagents in the modulation of autoimmune disease. We have previously reported the generation of a set of mAb specific for the clonotypic structure of a human T-cell clone recognizing an epitope of human cartilage gp-39. This glycoprotein was recently identified as a candidate autoantigen in rheumatoid arthritis. Here, we demonstrate for the first time that small amounts of immobilized anticlonotype mAb can induce anergy in the autoreactive clone. Following the anergic stimulus, T cells failed to proliferate upon restimulation as a result of a lack of interleukin-2 (IL-2) gene transcription. In addition, a diminished interferon-gamma (IFN-gamma) production was found. Our data indicate that anergy was not a result of T-cell receptor (TCR) downmodulation or the absence of free TCR. The anergic state was induced independent of costimulation or the presence of IL-2 and no protein synthesis was required for the induction of anergy. Anticlonotype mAb-induced anergy was prevented by cyclosporin A, suggesting that active signalling via the calcium/calcineurin pathway was required for the induction of anergy. In coculture experiments, anergic T cells were found to suppress the response of reactive cells from the same clone. This bystander suppression led to 90% inhibition of peptide-induced proliferation. Together, these findings suggest that mAb to the clonotypic structure of autoreactive T cells may be suitable reagents for the functional inactivation of these T cells in autoimmune diseases.

Functional association between the human myeloid immunoglobulin A Fc receptor (CD89) and FcR gamma chain. Molecular basis for CD89/FcR gamma chain association

FcR gamma chain has previously been shown to interact with the TCR-CD3 complex, the IgE Fc receptor I (Fc epsilon RI), and the class I and IIIA IgG receptors (Fc gamma RI and Fc gamma RIIIa). Here, we demonstrate that the Fc receptor gamma chain associates with Fc alpha R in transfected IIA1.6 B lymphocytes. Fc alpha R could be expressed at the surface of IIA1.6 B cells by itself, but was devoid of signaling capacity. Upon co-expression of FcR gamma chain, a physical interaction with Fc alpha R could be demonstrated. This association proved crucial for the triggering of both proximal (intracellular calcium increase and tyrosine phosphorylation), as well as distal (IL-2 release), signal transduction responses. We next tested the hypothesis that a positively charged arginine residue (Arg209) within the transmembrane domain of Fc alpha R promotes association with FcR gamma chain. We therefore constructed Fc alpha R molecules where Arg209 was mutated to either a positively charged histidine, a negatively charged aspartic acid, or an uncharged leucine. A functional association between Fc alpha R and FcR gamma chain was observed only with a positively charged residue (Arg209 or His209) present within the Fc alpha R transmembrane domain. These data show that transmembrane signal transduction by the Fc alpha R is mediated via FcR gamma chain, and that Fc alpha R requires a positively charged residue within the transmembrane domain to promote functional association.

Performance of CD3xCD19 bispecific monoclonal antibodies in B cell malignancy

Bispecific monoclonal antibodies, with a dual specificity for tumor associated antigens on target cells and for surface markers on immune effector cells, have been shown (in vitro) to be effective in directing and triggering effector cells to kill target cells resulting in target cell lysis. Bispecific monoclonal antibodies (BsAb) against the CD3 antigen on T cells and the CD19 antigen on B cell were developed. Data obtained by in vitro experiments might indicate that clinical responses in BsAb immunotherapy, will only be obtained in patients with minimal tumor load, and may need additional T cell stimulation via cytokines such as IL-2. Although these experiments have shown us their limitations, they also include the promise of BsAb-directed immunotherapy in B cell malignancy as further demonstrated during a Phase I trail, showing little toxicity. Clearly, much remains to be done before this BsAb is routinely used for therapy, but, the results presented show that the CD3xCD19 BsAb has a potential as a therapeutic agent in B cell malignancy. This report describes the experiments performed to test a new immunotherapeutic approach for the treatment of B cell malignancy. Bispecific antibodies are described that can target cytotoxic T cells to tumor cells and elicit a cytolytic action towards these cancer cells.

The interaction between human Fc gamma RI and the gamma-chain is mediated solely via the 21 amino acid transmembrane domain of Fc gamma RI

We have established a biological assay to investigate the nature of the non-covalent interaction between two integral type I membrane proteins, Fc gamma RI and gamma-chain. Fc gamma RI, the human high affinity receptor for immunoglobulin G (IgG), is expressed on the surface of macrophages and monocytes and mediates a broad range of important immunological functions. Fc gamma RI relies on a functional interaction with a second integral type I membrane protein, gamma-chain, to mediate many of these functions. For example, Fc gamma RI can only mediate phagocytosis of IgG-coated particles in COS cells when co-expressed with gamma-chain. We have previously shown that the cytoplasmic domain of Fc gamma RI is not necessary for this functional interaction. In this study we have used the phagocytosis assay to investigate the role of the transmembrane region of Fc gamma RI in mediating this functional interaction with gamma-chain by using mutant and chimeric forms of the receptor. Three mutants, which introduce or remove charged residues from a conserved 10 amino acid stretch of amino acids in the proximal transmembrane region of Fc gamma RI, were able to mediate phagocytosis of IgG-coated particles. In contrast, two chimeric receptors, In which 21 of the amino acids in the distal transmembrane region of Fc gamma RI were replaced with the transmembrane region of the related receptors CD2 or LFA3, were expressed but failed to interact functionally with gamma-chain to mediate phagocytosis. Thus, these mutants demonstrate that the interaction between human Fc gamma RI and gamma-chain is mediated solely via these 21 amino acids in the transmembrane domain of Fc gamma RI.

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

Stimulation of primary human monocytes from several donors by cross-linking of Fc gamma receptor type I (Fc gamma RI) and Fc gamma RII gave rise to calcium mobilization and protein tyrosine phosphorylation. These early events were not observed without cross-linking. CD45, a transmembrane tyrosine phosphatase, when co-cross-linked with either Fc gamma RI or Fc gamma RII, could prevent Fc gamma RI and Fc gamma RII-mediated calcium mobilization and protein tyrosine phosphorylation. When interleukin (IL)-6 production was measured, we noted a strong IL-6 production after activation of primary human monocytes by cross-linking of Fc gamma RI or Fc gamma RII. In contrast to calcium mobilization and tyrosine phosphorylation of proteins, IL-6 production was not affected by co-cross-linking of CD45 with either Fc gamma RI or Fc gamma RII. Interestingly, cross-linking of the CD45 itself was sufficient to induce IL-6 production. Our results show that the CD45 molecule is important in modulating early events following stimulation of primary human monocytes by cross-linking of Fc gamma RI or Fc gamma RII. However, triggering of CD45 alone can also induce IL-6 production, indicating that CD45 ligation itself can give signals and may have an important role in cytokine induction pathways.

Function of human Fc gamma RIIA and Fc gamma RIIIB

Fc gamma RIIA (CD32), a conventional type I transmembrane protein, and Fc gamma RIIIB (CD16B), which has a glycan phosphatidylinositol (GPI) membrane anchor, are both expressed on human neutrophils. Although some details remain to be elucidated, signaling following crosslinking of Fc gamma RIIA requires the activation of tyrosine kinases of both Src-family kinases and Syk, resulting in tyrosine phosphorylation of Shc, phospholipase C gamma isozymes, and a [Ca2+]i transient. Ligation of neutrophil Fc gamma RIIIB triggers a [Ca2+]i transient, and degranulation, although probably not ADCC or an oxidative burst. However, the mechanism for signal transduction by Fc gamma RIIIB, which lacks a transmembrane domain, is not known. Fc gamma RIIA and Fc gamma RIIIB appear to synergize with each other, leading to suggestions that the GPI-anchored Fc gamma RIIIB utilizes the Fc gamma RIIA signaling apparatus. The relevance of proposed specialized membrane domains enriched in GPI-anchored proteins, sphingomyelin and glycolipids to the signaling properties of Fc gamma RIIIB likewise remains to be explored.

Pretreatment with divided doses of steroids strongly decreases side effects of OKT3

The aim of this study was to attenuate side effects of OKT3 by variation of the time interval between administration of corticosteroids and OKT3 in renal allograft recipients. In view of a maximal lymphocytopenia at six hours following MPNS, we postulated a greater preventive action on side effects from administration of methylprednisolone (MPNS) at six hours preceding the first dose of OKT3 compared to administration immediately before. Two groups of renal transplant patients treated for acute rejection with 5 mg OKT3 were studied. Ten patients received 500 mg MPNS six hours and ten patients one hour before administration of OKT3. We measured clinical side effects, body temperature, TNF and IL-6. There were no differences between the two groups regarding clinical side effects and peak body temperatures. However, MPNS administered six hours before administration of OKT3 diminished TNF release; MPNS one hour before decreased IL-6 release. We studied an additional group of six patients receiving 250 mg MPNS six hours before, followed by 250 mg one hour before OKT3. This group experienced significantly less side effects and lower body temperature. In addition, IL-6 levels were significantly decreased. We conclude that two times 250 mg MPNS administered six hours and one hour before the first administration of OKT3 effectively attenuates adverse reactions following administration of OKT3.

Current topic: human placental Fc receptors

Human immunoglobulin G (IgG) Fc receptors are important in the materno-fetal relationship. Three classes of IgG Fc receptors are recognized which generate multiple isoforms, most of which are expressed in different cellular components of human placenta at different times during pregnancy. Although the distinct biological functions of Fc gamma R phenotypes expressed in human placenta are still unknown, recent data provide evidence for an important association between the Fc gamma R phenotype and transcytosis of IgG in the placenta. Selective transfer of maternal IgG across the placenta provides passive immunity to the fetus during the period when its own immune system is gaining protective potential. Furthermore, placenta-specific macrophages may contribute through Fc gamma R-mediated phagocytosis to the protection of the fetus from either infection or maternal immune attack against paternally inherited fetal antigens. Ontogeny and expression of various isoforms of Fc gamma R subtypes may be the key to the elucidation of the transport mechanism of maternal IgG to the fetus, in addition to the determination of the mechanisms of placental protection of the fetus against the maternal immune system.