The suppression activity of Fc gamma receptors is not related to their T-cell origin

Recombinant soluble human Fc gamma RII: production, characterization, and inhibition of the Arthus reaction

A recombinant soluble form of human Fc gamma RII (rsFc gamma RII) was genetically engineered by the insertion of a termination codon 5' of sequences encoding the transmembrane domain of a human Fc gamma RII cDNA. Chinese hamster ovary cells were transfected with the modified cDNA and the secreted rsFc gamma RII purified from the tissue culture supernatant (to > 95%, assessed by SDS-PAGE) using heat aggregated human immunoglobulin G (IgG) immunoaffinity chromatography. The IgG-purified rsFc gamma RII was relatively homogeneous (approximately 31,000 M(r)) whereas the total unpurified rsFc gamma RII secreted into the tissue culture supernatant was heterogeneous relating to N-linked glycosylation differences. Functional in vitro activity of the rsFc gamma RII was demonstrated by: (a) ability to bind via the Fc portion of human IgG and mouse IgG (IgG2a > IgG1 > > IgG2b); (b) complete inhibition of binding of erythrocytes sensitized with rabbit IgG to membrane-bound Fc gamma RII on K562 cells; and (c) inhibition of the anti-Leu4-induced T cell proliferation assay. Blood clearance and biodistribution studies show the rsFc gamma RII was excreted predominantly through the kidney in a biphasic manner, with an alpha-phase (t1/2 approximately 25 min) and a beta-phase (t1/2 approximately 4.6 h); the kidneys were the only organs noted with tissue-specific accumulation. In vivo, the administration of rsFc gamma RII significantly inhibited the immune complex-mediated inflammatory response induced by the reversed passive Arthus reaction model in rats. There was a specific and dose-dependent relationship between the amount of rsFc gamma RII administered, and the reduction in the size and severity of the macroscopic inflammatory lesion. Histological analysis of the skin showed a diffuse neutrophil infiltrate in both control and rsFc gamma RII-treated rats, however the perivascular infiltrate and the red cell extravasation was less intense in the rsFc gamma RII-treated group. It is likely that complement activation leads to neutrophil chemotaxis, but neutrophil activation via Fc gamma RII, which results in inflammatory mediator release, is inhibited. The data indicate that rsFc gamma RII is a potential therapeutic agent for the treatment of antibody or immune complex-mediated tissue damage.

Regulation of antibody production mediated by Fc gamma receptors, IgG binding factors, and IgG Fc-binding autoantibodies

Fc receptors (FcRs) are immunoglobulin-binding structures that enable antibodies to perform a variety of functions by forming connections between specific recognition and effector cells. Besides eliciting cytotoxicity, inducing secretion of mediators and endocytosis of opsonized particles, FcRs are involved in the regulation of antibody production, both as integral membrane proteins and as soluble molecules released from the cell surface. Most FcRs belong to the same family of proteins as their ligands (immunoglobulin superfamily). This review contains recent data obtained by use of monoclonal antibodies and cloning studies on FcRs and FcR-like molecules. The importance of fine specificity of receptor binding site(s)--that of the conformation of FcRs and their ligands in triggering signaling mechanisms--is analyzed. The regulatory function of membrane-bound and -released FcRs; the correlation between cell cycle, FcR expression, and release; as well as the possible mechanisms of these phenomena are discussed.

Class-specific suppression of human B cell maturation by IgA-binding factors

IgA-binding factors (IgA-BFs) were prepared by chromatography on Sepharose 4B beads covalently linked to dimeric and polymeric monoclonal IgA1 from supernatants of peripheral blood mononuclear lymphocytes (PBMC) and human B cell lines incubated in serum-free medium. Receptors for IgA, as revealed by the binding of biotinylated monoclonal IgA1, were expressed on monocytes, T-enriched and T-depleted lymphocytes. IgA-BFs or control eluates were added to pokeweed mitogen (PWM)-stimulated PBMC cultures, and their effects on the terminal differentiation of polyclonally activated human B cells were assessed by enumeration of intracytoplasmic IgM-, IgG- or IgA-containing cells. A selective decrease of IgA-containing cells was observed in the presence of IgA-BFs whereas IgM- and IgG-containing cells remained unchanged. Differential counts of B blasts and plasma cells revealed that only the former were decreased following addition of IgA-BFs. Kinetic studies indicated that maximum inhibition of IgA-containing cell generation was achieved when IgA-BFs were added during the first 5 days of PWM-stimulated PBMC cultures, whereas no inhibition could be demonstrated when IgA-BFs were added 24 h before harvesting. IgA-BFs did not decrease [3H]thymidine incorporation in PWM-stimulated PBMC cultures. They diminished the proliferation of the surface IgA+ monoclonal human B cell line DAKIKI, but not that of the surface IgA- IM-9 cell line. Several control eluates obtained from the same cell supernatants absorbed on Sepharose 4B, Sepharose 4B-IgG or Sepharose 4B-beta 2-microglobulin had no effect. Finally, IgA-BFs prepared from supernatants of two human B cell lines bearing receptors for IgA selectively depressed the generation of intracytoplasmic IgA+ cells in PBMC cultures stimulated by PWM. Altogether the data indicate that IgA-BFs obtained by spontaneous release from heterogeneous mononuclear cell suspensions or from IgA receptor-positive human monoclonal B cell lines selectively depress the maturation of B cells into IgA plasma cells and the proliferation of a surface IgA+ B cell line.

Fc receptor-bearing T cells and Ig binding factors as class-specific suppressors of polyclonally activated human B cells

By studying the model of polyclonal activation of PBMC from healthy adult humans, selective suppression of the generation of cIgG+ or cIgA+ cells could be achieved by T gamma and T alpha cells activated by Agg-IgG and Agg-IgA, respectively. Furthermore a comparable class-specific suppression was achieved by addition of IgG-BF or IgA-BF released by various cell types including T-enriched PBMC suspensions, B cells or monocytes. The latter effect required the presence of radiosensitive T cells. Whereas T gamma and T alpha cells activated by Agg-Ig inhibited the generation of cIg+ and Ig-secreting cells of the matching class, Ig-BFs were shown to act at a transitional stage of B cell maturation by blocking cIg+ generation and/or proliferation, without impairing Ig secretion by fully differentiated plasma cells. Yet another lectin-like factor, termed BMIF, released by FcR- as well as by FcR+ lymphoid or nonlymphoid cells (e.g. polymorphonuclear neutrophils), could block the maturation of cIg+ into Ig-secreting plasma cells. Unlike Ig-BF, BMIF was not isotype specific. Cells and lymphokines which control the initial stages of B cell activation and differentiation have been extensively investigated but little is known at present about the regulation of the progression from cIg+B blasts to fully differentiated plasma cells. Sequential determination of cIg+ blasts, plasma cells, PFC, and Ig secretion in polyclonally activated PBMC cultures shows an orderly sequence of appearance and decrease of cells at these different stages, suggesting that up and down regulatory signals control each step. Furthermore the demonstration of suppressor pathways which affect B cell maturation at precise transitional stages provides further indirect evidence towards a sequential regulation of each successive differentiation event. In view of the heterogeneity of FcRs with respect to subclass specificity, affinity, cell type distribution and structure, much remains to be done to elucidate the precise regulatory functions of those molecules in the late stages of B cell maturation. From our studies it would appear that some types of Ig-BF would ensure the recognition of Fc determinants on B cell sIg, but still require T cell, and possibly other factors produced by those cells, to alter B cell maturation. This is in keeping with several models in which isotype specific T cells, but not the Ig-BFs thereof, were shown to regulate B cell differentiation.

The murine Fc receptor for immunoglobulin: purification, partial amino acid sequence, and isolation of cDNA clones

The murine Fc receptor for IgG (Fc gamma R) was purified to homogeneity by immunoaffinity chromatography from detergent lysates of the macrophage cell line J774. Microsequencing of intact protein yielded a single amino-terminal sequence, which was confirmed and extended to 20 residues by the isolation of an overlapping peptide. The isolation of additional proteolytic fragments obtained by using Staphylococcus aureus V8 protease, cyanogen bromide, and lysine C proteinase, facilitated sequence analysis of a total of 119 amino acid residues. Codon usage charts were used to construct oligonucleotide probes based on the amino acid sequences of three nonoverlapping peptides. These probes were used to screen a cDNA library derived from the WEHI-3B myelomonocytic cell line, and a single cDNA clone (pFc24) to which all three probes hybridized was isolated. This clone, containing a 1.02-kilobase cDNA insert, has been characterized by restriction mapping and partial DNA sequencing, and it has been shown to encode the Fc gamma R. The sequence at the 5' end of the clone contained the coding information for the amino-terminal sequence of the Fc gamma R as well as a putative 13-amino acid signal sequence. The 3' end of the clone encoded a peptide identified in purified receptor preparations. Thus, the presence of coding information at the 5' and 3' ends of this clone suggests that full-length Fc receptor cDNA spans greater than 1 kilobase.

Only T alpha or T gamma cells can be triggered by IgG or IgA to suppress the production of the matching Ig class

A short exposure of human peripheral blood mononuclear cells (PBMC) to heat-aggregated IgG (or IgA) was found to inhibit pokeweed mitogen (PWM)-induced human B cell differentiation into IgG (or IgA)-producing cells. The present study was designed to investigate the nature of the cells inducible by aggregated (Agg)-IgG or Agg-IgA into isotype-specific suppressors. When T cell-enriched suspensions were exposed to Agg-IgG or Agg-IgA, then thoroughly washed, and cultured with autologous nonrosetting cells in the presence of PWM, a selective suppression of the generation of cells synthesizing and secreting IgG or IgA, respectively, was observed. Conversely, no inhibition was induced by T-depleted PBMC treated in exactly the same way. Unlike Agg-IgG, Agg-F(ab')2 fragments did not trigger suppression. Finally, in vitro X-irradiation of T cells pretreated with Agg-IgG or Agg-IgA abrogated their suppressive activity. It is concluded that among cells bearing Fc gamma or Fc alpha receptors, only T cells were inducible into suppressor cells by the binding of the matching immunoglobulin class.

Cerebrospinal fluid lymphocyte subpopulations in multiple sclerosis

T3+ (all-T) and T8+ (suppressor/cytotoxic) cells were studied in cerebrospinal fluid (CSF) from 24 patients with multiple sclerosis (MS) and from 24 subjects with various "non-immunological" disease (NID). MS patients were classed as (a) during the acute phase of the 1st episode of the disease, (b) in acute relapse, (c) with chronic progressive disease, (d) with increased or (e) normal CSF IgG content or (f) with neurological impairment (Kurtzke scale) less than or equal to 3 or (g) greater than 3. In MS cases considered as a whole a significant decrease in CSF T3+ cells was found compared to NID patients. When single groups were considered, T3+ cells decrease was significant in classes (b), (d) and (f). Significantly lower percentages of T8+ cells, compared to NID, were found in MS classes (a), (d) and (f).

Suppression of polyclonal B cell activation by IgG-binding factors. Requirement for T cells

IgG-binding factors (IgG-BF) prepared from cell-free supernatant of human peripheral blood mononuclear cells interfere with the polyclonal activation of peripheral B cells by decreasing the numbers of IgG-containing cells and Ig plaque-forming cells. Using Nocardia opaca delipidated cell mitogen (NDCM), a T helper cell-independent polyclonal B cell activator, it was found that the suppressive effect of IgG-BF was no longer demonstrable after removal of T cells. In pokeweed mitogen-stimulated cultures, the suppression by IgG-BF required the presence of radiosensitive T cells. Selective depletion of OKT4+ or OKT8+ subsets in NDCM-stimulated cultures showed that IgG-BF required the presence of OKT4+ lymphocytes to induce suppression. It is concluded that the effect of human IgG-BF was mediated by one or several subsets of T cells.

Fc receptor--more answers, more questions

Fc receptors, belonging to the most important surface structures of a number of cells participating in the immune processes, have been intensely studied during the past decade. The present review summarizes the contemporary knowledge of the specificity and heterogeneity of Fc receptors and of factors influencing their expression, and includes some views on their function. In addition, it mentions their relationship to other cell surface structures, expression of Fc receptors during ontogeny of the organism and in certain diseases. Finally, data concerning the isolation and biochemical characterization of the Fc receptor molecule are presented.

Suppression of polyclonal human B cell activation by IgG binding factors: interference with the maturation of Ig-containing cells into Ig-secreting cells

Human IgG binding factors (IgG BF) were prepared by immunopurification on IgG immunosorbents from cell-free supernatants of unstimulated peripheral blood mononuclear cells (PB MNC). The suppressive effects of IgG BF was studied using PB MNC stimulated by pokeweed mitogen or by nocardia delipidated cell mitogen. At the end of the culture three parameters of B cell activation were measured: (1) the numbers of IgM-, IgG-, or IgA-containing cells (CC) using direct immunofluorescence, (2) the numbers of IgM, IgG, or IgA plaque-forming cells (PFC) using a Protein A hemolytic plaque assay, and (3) the concentrations of IgM, IgG, or IgA in culture supernatants using an enzyme-linked immunosorbent assay. Addition of IgG BF at the third day of culture resulted in a selective decrease of IgG CC, while IgM CC and IgA CC were increased or unchanged. Conversely, IgG BF induced a nonselective diminution of the number of PFC and of the amount of secreted Ig of the three major Ig classes. Therefore the results demonstrate two distinct effects of IgG BF: (1) an isotype-specific suppression of cells producing IgG, demonstrated by the parallel decrease of IgG CC and IgG PFC, and (2) a blocking of the late stages of B cell maturation evidenced by the discrepancy between normal or elevated Ig CC and decreased Ig PFC of the IgM and IgA classes.

Surface markers on lymphocytes from human cerebrospinal fluid. Identification by monoclonal antibodies

Lymphocyte subpopulations in cerebrospinal fluid (CSF) and peripheral blood (PB) were studied using monoclonal antibodies and the common membrane markers. The results in three groups of patients were compared: 36 subjects with 'non-immunological disorders' (NID), 14 subjects with multiple sclerosis (MS) and 6 with subacute sclerosing panencephalitis (SSPE). It was found that, in patients with NID, (1) 90% of cells were T lymphocytes, reactive with OKT3; (2) the helper/suppressor (T4/T8) ratios were the same in the CSF and the PB; (3) the OKIa1 percentage was lower in the CSF than in the PB; and (4) only a few cells were 'immature', reacting with OKT10. Using the membrane markers (E rosettes, Fc IgG receptors and surface immunoglobulins), on the other hand, it was noted that the majority of cells in the CSF were identified as suppressor T lymphocytes and surface immunoglobulin-positive B cells were less common than the Ia1 marker suggested. There were no significant differences between the CSF results in patients with NID and MS but the OKT3 lymphocytes were reduced in CSF samples from patients with SSPE.