T-cell hybrids bear Fcgamma receptors and secrete suppressor immunoglobulin binding factor

Do immune complexes formed with autoantibodies have a role in the maintenance of immune homeostasis through interaction with FC receptors

Natural autoantibodies play an important regulatory role in the maintenance of immune homeostasis. They act as a first line of defense against environmental pathogens like toxins, bacteria and erythrocytes. In humans they are mainly produced by CD5+ B cells that are under the control of a regulatory T cell population. Fc-gamma receptors are involved in antigen recognition and signal transduction and tuning, and some of the members of the FcR family have structural similarity to MHC molecules; they may interact with multiple Ig ligands and with non-Ig ligands. We discuss the interactions between immune-complexes formed with natural autoantibodies and Fc-gamma receptors and suggest that such interactions may affect self-recognition in the thymus and regulate immune homeostasis.

In vitro inhibition of tumor B cell growth by IgG-BF-producing Fc gamma RII+ T cell hybridoma and by immunoglobulin G-binding factors

The growth-modulating effect on mouse hybridoma B cells of IgG-BF-producing Fc gamma RII+ mouse T cell hybridomas and of the IgG-BF isolated from the culture supernatants of these cells has been examined. Cocultures of IgG-secreting hybridoma B cells with IgG-BF-producing T hybridomas or with partially purified IgG-BF demonstrated a reproducible inhibition of the tumor B cell growth. The inhibition was due to a cytostatic and not to a cytotoxic effect. Hybridoma B cells cultured in liquid medium in the presence of soluble IgG-BF, or cocultured in semisolid agarose assays with IgG-BF-producing hybridoma T cells did not undergo immediate cytolysis but were prevented from proliferating. Thus, our data indicate that IgG-BF-producing Fc gamma RII+ T cells interfere with the proliferation of transformed B cells, possibly through soluble IgG-BF.

Possibilities of interference with the immune system of tumor bearers by non-lymphoid Fc gamma RII expressing tumor cells

The ectopic expression of Fc gamma RII by PyV transformed 3T3 cells derived from tumors of long latency has been established. It was suggested that this expression is one of several changes conferring upon the cells an increased capacity for survival. We found that in one case cells expressing a very high level of Fc gamma RII had also a very high metastatic phenotype as compared to FcR negative cells. Direct evidence that Fc gamma RIIbl functions as a progression factor was provided by transfection experiments. The transfected gene conferred an increased malignancy and invasive phenotype upon PyV or c-Ha-ras transformed cells. In the present study we tested the possibility that Fc gamma RII expressing tumor cells could interfere with the immune system. The following subjects were investigated: 1) The ability of Fc gamma R on the tumor cells to bind the ligand and/or release IBF. 2) The effect of a local accumulation of ligand and/or IBF (assumed to take place in situ in the tumor) on Fc gamma RII expressing T cells. It was found that both tumor-derived receptor positive and beta l transfected PyV transformed cells were capable of binding aggregated mouse IgG. The binding of bivalent ligand was followed by an increase in membrane Fc gamma RII expression. Also both types of cells were capable of releasing IBF. We then tested the possibility that a local accumulation of IgG within the tumor could effect Fc gamma R expressing T cells. It was found that aggregated mouse IgG (as well as IgGl) could stimulate the proliferation of the T cell hybridoma (T2D4) and other Fc gamma RII expressing T cells. We also found that the expression of beta Fc gamma RII specific mRNA peaked at the logarithmic phase of T2D4 cultures, in parallel with their maximal potential to release IBF. Several pathways for interference with the immune system are suggested.

Soluble Fc gamma R (sFc gamma R): detection in biological fluids and production of a murine recombinant sFc gamma R biologically active in vitro and in vivo

Soluble forms of receptors for the Fc portion of IgG (sFc gamma R) were detected in biological fluids from mice and humans. In mouse bearing tumors, circulating amounts of sFc gamma R increased concurrently with tumor growth. Tumors secreting IgG2a, IgG2b or IgG3 led to a 5- to 10-fold increase in serum sFc gamma R levels whereas tumors secreting IgG1, IgGA or other types of tumors (non Ig B cell tumors, T cell lymphoma and a melanoma) increased 2- to 3-fold the levels of circulating sFc gamma R. In the human, sFc gamma R were also detected in whole unstimulated saliva. Levels of sFc gamma RII and of sFc gamma RIII were variable and did not seem to depend on the dental status of the individuals. Finally, a murine recombinant sFc gamma R (rsFc gamma R) composed of the two extracellular domains of Fc gamma RII was produced by culture of transfected L cells in bioreactors. The purified rsFc gamma R was found to inhibit antibody production in vitro in anti-SRBC responses and by cultures of small B cells stimulated by anti-IgM antibodies in the presence of IL-4 and IL-5. Moreover, the i.p. injection of this material into adult mice immunized with SRBC led to a decrease of IgG antibody production by splenocytes, as measured by a hemolytic plaque assay, and in serum, as measured by antigen-specific ELISA.

Enhancement of IgG production elicited in mice by treatment with anti-CD8 antibody

Injection of an anti-CD8 monoclonal antibody into mice was followed by an increase in the production of immunoglobulins of the IgG1, IgG2a and IgG2b, but neither the IgG3 nor the IgM isotypes. A maximal effect was observed 6 days after administration of 100 micrograms antibody. However, no modification of spleen cell proliferation was elicited by the anti-CD8 treatment. Virally induced IgG2a and IgG2b secretion remained unchanged in mice infected with lactate dehydrogenase-elevating virus that received a concomitant injection of anti-CD8 antibody, whereas an enhancement of IgG1 was observed. Simultaneous treatment with an anti-CD4 antibody abrogated the immunoglobulin secretion triggered by anti-CD8. These results suggest that immunoglobulin production in unmanipulated mice is controlled by CD8+ lymphocytes.

21.1.1, a novel activation marker of T and B cells

A new rat mAb designated mAb 21.1.1 was raised against a T cell hybridoma of mouse origin, T2D4. This antibody, an IgG2b, immunoprecipitates from the membrane extracts of iodinated T2D4 cells a 56-kDa glycoprotein of apparent pI 4.6 which gives a 34-kDa polypeptide after treatment with endoglycosidase F. MAb 21.1.1 reacts with an antigen expressed on murine mitogen-activated thymocytes and T cells, and on B cells stimulated by anti-IgM antibodies. Cells isolated from the spleen, lymph nodes and bone marrow are negative, as are purified resting B cells or T cells. This antigen is strongly expressed on most day-16 fetal thymocytes whereas adult thymocytes are almost negative. mAb 21.1.1 may be useful for the study of activation and differentiation of T and B cells.

Detection of IgA binding site by flow cytometry with fluorescent microspheres

To establish a sensitive method to detect IgA Fc receptors (Fc alpha R) on human and murine lymphoid cells, fluorescent microspheres (FMS) were used in a flow cytometric assay. The following three assays with FMS were tested and compared with a conventional indirect Fc alpha R assay using FITC-labeled anti-IgA: (1) direct cell binding assay with murine myeloma IgA(MOPC315)-coated FMS(IgA-FMS assay), (2) indirect assay with TNP-BSA-coated FMS which bind to cells preincubated with MOPC315 IgA bearing anti-TNP activity (TNP-BSA-FMS assay), and (3) indirect assay with anti-IgA coated FMS after preincubation of the cells with IgA(anti-IgA-FMS assay). In these three assays for Fc alpha R using FMS, the binding of IgA to the cells was not affected by purified IgM or IgG preparations. In both indirect assays using TNP-BSA-FMS and anti-IgA-FMS, sharp and dose dependent IgA binding was obtained at lower IgA concentrations ranging from 4 to 125 micrograms/ml as compared with the conventional indirect assay. The background MFI levels in all these FMS assays remained as low as those in the conventional assay. These findings suggest that FMS coupled with TNP-BSA or anti-IgA antibodies is suitable for the detection of Fc alpha R on both murine and human T cells.

Immune complex-mediated inhibition of lymphocyte Fc-gamma receptors in the plasma of patients with type 1 (insulin-dependent) diabetes mellitus: association with anti-ssDNA antibodies

Sera of 43 patients with type 1 (insulin-dependent) diabetes and from 42 normal subjects were examined for the presence of Fc-gamma receptor-blocking IgG immune complexes, detected by EA rosette inhibition, and for the presence of anti-ssDNA antibodies by ELISA. Forty-four percent of diabetic patients had levels of inhibition above the upper limit of normality in comparison to 7% of normal controls. Anti-ssDNA antibodies were found in the sera of 14 out of 43 diabetic patients. Ten out of 14 anti-ssDNA positive patients (71.5%) had inhibition levels above the upper limit of normality in comparison to 9 out of 29 (31%) of the anti-ssDNA negative population. The difference was statistically significant (P less than 0.005). Levels of EA rosette inhibition were found to be high in patients with duration of diabetes of less than 2 years and correlated with high prevalence of anti-ssDNA antibodies. The percentage of EA rosette inhibition was found not to correlate with the levels of C1q-binding immune complexes, suggesting that immune complexes detected by EA rosette inhibition may belong to a pool of noncomplement-fixing immune complexes. The possible role of immune complexes with autoantibodies anti-ssDNA in the mechanism triggering and perpetuating autoimmune phenomena in diabetes is discussed.

Regulation of IgG production by suppressor Fc gamma RII+ T hybridomas

In this work, we analyzed the immunoglobulin heavy (H) and light (L) chain production by two variant B hybridomas, UN2.C3 and UN2.C17.K1 co-cultured with cells from a Fc gamma RII+, IgG-binding factor (IgG-BF)-producer T hybridoma (T2D4.C1) or with cells of a Fc gamma RII-, IgG-BF-nonproducer variant (D10C5). We showed that only the Fc gamma RII+ hybridoma directly inhibits the IgG secretion by UN2.C3 through a soluble mediator. This inhibition affects the H and L chain synthesis as well as the H and L chain-encoding mRNA steady state. No apparent cytotoxic effect could be detected. In contrast, the production of kappa chain by an H chain-negative variant (UN2.C17.K1) was unaffected. This indicates that a complete IgG molecule is required to observe the inhibitory effect induced by T2D4.C1. The pattern of effector/target cell interactions observed in our work suggests that the soluble factor involved in the suppression of IgG production is IgG-BF, able to transiently modify the IgG gene expression in target cells.

Identification of the Fc gamma RII-related component of murine IgG-BF

Suppressor murine IgG-BF produced by the T cell hybrid (T2D4) expressing low affinity Fc gamma R (Fc gamma RII) contain four biologically active polypeptides of pI 5.2, 6.3, 7.7 and 8.3, respectively. They were fractionated by affinity chromatography on immunoadsorbents coupled with F(ab')2 fragments of the monoclonal anti-Fc gamma RII antibody 2.4G2 and by hydrophobic interaction chromatography. Both methods led to the identification of biologically active IgG-BF which react with 2.4G2 and of IgG-BF which do not react with 2.4G2. Molecules bearing the epitope recognized by 2.4G2 had an apparent pI of 5.3 while the pI of those which did not express this epitope were 6.3, 7.8 and 8.5, respectively. Therefore, one IgG-BF polypeptide of pI 5.3 is probably related to Fc gamma RII.

Human hybridoma suppressor factor (HSF) inhibits IL2 production in addition to suppressing immunoglobulin production

The human thymus cell hybridoma, 8E-24, secretes a potent immunosuppressive factor(s), HSF, which inhibits polyclonal immunoglobulin (Ig) production. Our current studies reveal that this suppression is monocyte dependent in that its suppressive activity for Ig production was not observed in monocyte-depleted lymphocyte cultures but was restored by addition of monocytes. The requirement for monocytes was equally satisfied by autologous or allogeneic monocytes or monocyte conditioned media. Although the suppression mediated by HSF required the presence of monocytes, the mechanism for monocyte participation appears to be different from that reported for suppression mediated by soluble immune response suppressor (SIRS) in that general oxygen free radical scavengers did not inhibit this activity. Further information on the mechanism of action of HSF was obtained from studies on its suppressive effect on the phytohemagglutinin(PHA)-induced proliferative response. In this system HSF significantly suppressed PHA induced interleukin 2(IL2) production of peripheral blood mononuclear cells (PBMC) in a dose dependent fashion without inhibiting the proliferative response of CTLL-20 target cells to IL2. Interleukin 1 (IL1) production by monocyte cultures was also not suppressed by HSF. These results indicate that HSF interferes with IL2 production and not its induction by IL1 or its interaction with the IL2 receptor. To investigate the role of HSF-induced IL2 suppression in the pokeweed mitogen (PWM) antibody synthesis assay, the time course of IgG, IgM, IL1, and IL2 production of PWM stimulated PBMC cultures was examined. Results showed that the peak of IL2 production occurred on the second day of culture and was significantly suppressed by HSF while IL1 production was not affected during the seven day culture period. Similar suppression of IL2 and IgM production was observed in cultures of B cells and T4+ cells. Reconstitution of the IL2 levels in these cultures with recombinant IL2 completely restored antibody production. These results suggest that HSF in the presence of monocytes modulates the function of T4+ cells by inhibiting IL2 production. The inhibition of IL2 production by HSF appears to be responsible for the suppression of antibody production.

Regulatory effects of IgG-BF on hybridoma B cells. molecular characterization of variant cell lines

Immunoglobulin G-binding factors (IgG-BF) produced by mouse T cells or hybridoma T cells (T2D4) were used to manipulate in vitro mouse hybridoma B cells. Both IgG production by, and proliferation of, these cells was inhibited by IgG-BF, or during co-cultures with IgG-BF-producing T2D4 cells. Thus, treatment of tumor B cells, besides its potential therapeutic use, represents an invaluable model for studying the regulation of Ig production by IgG-BF at a molecular level. To further analyze the molecular events induced by IgG-BF in B cell hybridomas, a set of variant clones of a hybridoma cell line (UN2) was isolated and variants were characterized for their Ig production and their Fc gamma R expression.

Increased expression of Fc gamma receptor in cancer patients and tumor bearing mice

In this study we report on some lines of ongoing research performed in our laboratory, in relation to the increased expression of FcR on tumor cells, as well as on cells present in the tumor-bearing host, and its possible role in tumor progression. In a previous study we have shown that a Polyoma virus (PyV)-induced anaplastic carcinoma (SEYF-a tumor) contained an FcR-expressing subpopulation of tumorigenic cells. We tested the effect of in vivo passaging of FcR-expressing and of non-FcR-expressing sub-populations of SEYF-a tumor cells on the expression of FcR, as revealed by the ability of these cells to bind the 2.4G2 monoclonal antibody, which is directed against mouse Fc gamma 2b/gamma 1R. It was found that upon in vivo passaging these two sub-populations became practically identical in their ability to bind anti-Fc gamma R antibody. On the other hand, in vitro passaging of FcR-expressing SEYF-a cells resulted in a gradual decrease in the expression of Fc gamma R. These results, indicating that the expression of Fc gamma R on tumor cells, per se, is dependent on a factor present in the in vivo environment were confirmed using 3T3 cells transformed in vitro by PyV (C) and forming tumors at first injection to mice (CTC). C cultures of various clones did not express Fc gamma R, while CTC cultures (cultures from tumors) became positive. We also detected an increase in the level of a soluble form of Fc gamma 2b/gamma 1R in the circulation of mice bearing PyV induced tumors. This increase paralleled the appearance of palpable tumors. A similar pattern of increase was observed in mice inoculated with the c-H-ras transformed tumorigenic clone 8/F/5, but not in mice inoculated with non-tumorigenic 3T3 cells. Data published by us show that metastatic breast cancer patients had significantly elevated Fc gamma R levels on their peripheral blood mononuclear cells (PBMC). Experiments presented here indicate a direct correlation between increased Fc gamma R levels on PBMC and tumor mass in colon, ovary and lung metastatic carcinoma patients. The possibility that malignantly transformed cells have the potential to cause proliferation of Fc gamma R expressing T cells was tested. It was found that extract derived from r-H-ras transformed 3T3 cells triggers the proliferation of a T cell hybridoma expressing Fc gamma R.

Subset of natural killer cells is induced by immune complexes to display Fc receptors for IgE and IgA and demonstrates isotype regulatory function

Expression of Fc receptors for IgE (FcER) or IgA (FcAR) on purified natural killer (NK) cells was investigated. No FcER+ and a few FcAR+ NK cells were detectable on freshly separated NK (NKH-1+) cells from normal donors. Incubation of NK cells with IgE-anti-IgE immune complexes or IgA-anti-IgA immune complexes induced up to 10 and 20% FcER+ or FcAR+ cells, respectively. These FcR were induced on CD3- but not on CD3+ NKH-1+ cells. In contrast, NK cells from patients with various dysgammaglobulinemias could not be induced to express FcER or FcAR corresponding to their abnormal circulating IgE and/or IgA levels. Enriched FcER+ or FcAR+ induced NK cell supernatants from normals enhanced IgE or IgA synthesis from Ig secreting B cell lines in an isotype-specific fashion without increasing proliferation. Thus NK cells, after interaction with specific Ig isotypes in complexes, express FcR and produce differentiation factors for that isotype.

Software for the quantitative evaluation of in vitro monoclonal antibody production from ELISA data

Software has been developed which permits the quantitation of monoclonal antibodies secreted by B cell hybridomas. This program does not require the user to enter a large number of complex parameters and can be easily used without any previous computer experience. It fits all the experimental and standard curves by determining overlapping linear domains using the linear least-squares method. The program is based on logarithmic interpolations for determining Ig concentrations comparing experimental samples to Ig concentrations in standards. It provides a complete print-out of the data with editing options and is written in BASIC EDEX 4.0 Commodore computer language. It permits the accurate quantification of minute amounts of monoclonal antibodies and can be used to detect the inhibitory or enhancing effects of lymphokines or cytokines on Ig secretion by hybridoma B cells.

Immunoglobulin G-binding factors (IgG-BF) inhibit IgG secretion by, as well as proliferation of, hybridoma B cells

Mouse immunoglobulin G-binding factors (IgG-BF) produced either by activated T cells (ATC) or by hybridoma T cells (T2D4) directly inhibit the in vitro IgG secretion by hybridoma B cells. This inhibition affects IgG1, IgG2a and IgG2b and can be detected as early as after 2 h incubation of the cells with IgG-BF eluted from non-equilibrium pH gradient electrophoresis gels. Moreover, IgG-BF also exert a strong growth-inhibitory effect on hybridoma B cells without any detectable immediate cytotoxicity. These results provide an experimental basis to analyze the molecular and biological effects induced by IgG-BF on B cells.

Complement factor H-binding protein of Raji cells and tonsil B lymphocytes

Several reports have indicated that Factor H has specific effects on certain cell populations, suggesting that Factor H receptors may exist. Lambris & Ross [(1982) J. Exp. Med. 155, 1400-1411] purified a protein from Raji B-lymphoblastoid cell culture supernatants, using Factor H-Sepharose affinity chromatography. This species appeared to consist of two disulphide-linked components each of Mr 50,000, with an additional 50,000-Mr chain attached non-covalently. The existence of cell-surface Factor H-binding proteins has now been re-investigated with 125I surface-labelled Raji and tonsil B cells. Non-ionic-detergent extracts of the cells, in 0.1% Nonidet P40/10 mM-sodium phosphate buffer, pH 7.4, were incubated with Factor H-Sepharose in the presence of proteinase inhibitors. After the beads had been washed, bound components were eluted with 50 mM-NaCl. A single radioactive species was eluted from the resin, which migrates identically with Factor H (apparent Mr 170,000) in SDS/polyacrylamide-gel electrophoresis under reducing and non-reducing conditions. Biosynthetic radiolabelling studies confirmed that this species was synthesized by Raji cells. Examination of culture supernatants from biosynthetically radiolabelled Raji cells showed again the presence of a single soluble species that bound to Factor H-Sepharose, but this species was of lower Mr (approx. 105,000) than the membrane-derived protein. The soluble form may be produced by proteolysis of the membrane form, or may be of separate origin. The similarity in size of the cell-surface protein to Factor H was initially confusing, but it is distinct from cell-surface Factor H on the basis of three criteria: (1) it is not recognized by anti-(Factor H) monoclonal antibodies MRC OX23 and MRC OX24, nor by polyclonal F(ab')2 anti-(Factor H); (2) it does not bind to Zn2+-chelate resin, whereas Factor H does; (3) cell-surface Factor H present on U937 cells does not bind to Factor H-Sepharose.

A sensitive method for testing the effect of immunoglobulin binding factor on Ig secretion by hybridoma B cells

A microtiter plate assay to detect the effect of immunoglobulin binding factor (IBF) on Ig secretion by hybridoma B cells is described. This technique permits the analysis of Ig secretion by only 200-400 hybridoma B cells using a PFC assay and an ELISA, and therefore increases the IBF/cell number ratio. This increase allows the detection of a strong IBF-mediated inhibitory effect on Ig secretion by hybridoma B cells, which is otherwise difficult to obtain reproducibly. The technique is simple, does not require any transferring step and is convenient, since it permits large numbers of samples to be tested. It can be extended to test IBF for all isotypes or other lymphokines that affect Ig secretion.

IgA binding factors and Fc receptors for IgA: comparative studies between IgA and IgE Fc receptor systems

The expression of Fc receptors (FcR) for IgA (Fc alpha R) as well as for IgE (Fc epsilon R) on T lymphocytes (T cells) is enhanced or up regulated by the corresponding class of immunoglobulins (Ig). The production of class-specific regulatory factors binding to IgA and IgE (IgA binding factor [IgA-BF]; IgE binding factor [IgE-BF]) is also induced by these respective ligands. Murine IgA-BFs produced by a T hybridoma T2D4 and concanavalin A-activated spleen cells suppressed the in vitro IgA antibody responses of pokeweed mitogen-stimulated mouse spleen cells class-specifically. Human IgA antibody response was also suppressed by the murine IgA-BF. Similar suppressive IgA-BF is also produced by a human natural killer (NK)-like cell line (YT), which has no rearrangement of the T cell receptor beta-chain gene, indicating that non-T non-B/LGL cells may also be involved in the regulation of the class-specific antibody responses. It appears that, in human as well as murine systems, T- and NK-cells have the capacity to co-express multiple class-specific FcRs and to produce the corresponding immunoglobulin binding factors. While the Fc epsilon R expression is abnormally enhanced in the diseases with hyperimmunoglobulinemia E, disregulation of Fc alpha R is associated with certain human diseases involving the altered IgA regulation. In IgA nephropathy, which is characterized by increased serum IgA level and IgA deposition in the mesangium, there is an enhancement of the expression of Fc alpha R. In contrast, IgA failed to induce Fc alpha R significantly on the lymphocytes from the patients with selective IgA deficiency, indicating that Fc alpha R plays an important role in the IgA regulation in vivo.

Immunoglobulin-specific suppressor T cells

This article has considered evidence that supports the occurrence and functional importance of suppressor T cells that are directed to B cell targets. Cells with these features have been demonstrated in experimental animals and in humans. The designation "suppressor" comes from the serologic phenotype of these cells as well as from their functional property of noncytotoxic inhibition of B cell function. Distinct suppressor T cells with these properties have been identified that effect antigen-, idiotype-, isotype-, and allotype-specific suppression of B cell function. While such cells had been suspected from earlier studies of normal immune responses, the development of monoclonal B cell models using tumor cells has provided a means to readily detect these suppressor T cells and to investigate the mechanisms by which they mediate their effects. Tumor models have proved to be powerful tools in the effort to identify and analyze the elements that underlie the complexity of immune responses. Combined with the insights provided by molecular genetic approaches and flow cytometry, functional and responsive lymphoid tumor cells are being used with increasing frequency to address basic immunoregulatory issues. An important family of suppressor T cells with B cell targets are those that express surface Fc receptors, elaborate immunoglobulin-binding factors, and appear to participate in the regulation of immunoglobulin heavy chain class expression. In addition to their importance in the regulation of heavy chain class expression during normal immune responses, alterations in FcR+ T cells in a number of disease states may provide clues that will lead to a better understanding of disorders of immune regulation.

2.4G2, a monoclonal antibody to macrophage Fc gamma receptors, reacts with murine T cell Fc gamma receptors and IgG-binding factors

In order to determine whether T cell Fc gamma receptors (Fc gamma R) and IgG-binding factors (IgG-BF) are structurally related, we searched for common antigens on these molecules. We found that the anti-macrophage Fc gamma 1/gamma 2bR monoclonal antibody 2.4G2 binds to similar determinant(s) on the macrophage-like J774 cells and on the Fc gamma R+ hybridoma T cells T2D4. On the T cell membrane, these determinants are associated with Fc gamma 1/gamma 2bR. They are absent on Fc gamma R- hybridoma T cells or the FcR- BW5147 thymoma cells. 2.4G2-reactive molecules were also detected in soluble material either extracted or released in the supernatant of Fc gamma R+ hybridoma T cells. 2.4G2-reactive molecules released in the supernatant of T2D4 cells could be absorbed on Sepharose beads coupled to rabbit IgG and they were recovered by acid elution. Conversely, Sepharose beads coupled to 2.4G2 retained molecules which had affinity for rabbit IgG, which suppressed an in vitro antibody response and which had the same molecular weight as IgG-BF. These results indicate that T cell Fc gamma R and IgG-BF share common epitopes and that 2.4G2 can serve as an anti-IgG-BF antibody.

Role of infection in the modulation of mouse circulating soluble cell-free Fc gamma 2b/gamma 1 receptor

We have recently demonstrated the occurrence of functional circulating soluble cell-free Fc gamma 2b/gamma 1 receptor in normal mouse serum by means of the 2.4G2 rat monoclonal antibody. With the solid phase radioimmunoassay described previously, we report here a dramatic increase in the level of circulating soluble cell-free Fc gamma 2b/gamma 1R in Schistosoma mansoni-infected mice individually tested before and on days 22, 41, 62, and 82 after infection. This increase was statistically highly significant when the five measurements from each mouse were compared, and also when the entire group of infected mice was compared with the stable level of Cf-Fc gamma 2b/gamma 1R measured in strain-, sex-, and age-matched control mice individually tested on the same days. This increase was commensurate with a rise in the amount of IgG detected in the sera of the S. mansoni-infected mice. Thus, infection seems to be one of the factors that modulate the expression of such soluble Cf-Fc gamma 2b/gamma 1R in mouse serum. Furthermore, the experimental device reported here for the production of high levels of Cf-Fc gamma 2b/gamma 1R by infection (in this case by parasitic infection) could serve as a model for obtaining large quantities of serum Cf-Fc gamma 2b/gamma 1R for further purification. Lastly, we point out that, by establishing a functional relationship with circulating IgG, such soluble Cf-Fc gamma 2b/gamma 1R may modulate some of the functions in which the Fc portion of immunoglobulins is involved.

Suppression of polyclonal immunoglobulin production by a soluble factor produced by a human thymus hybridoma

A human thymus cell hybridoma was established using thymus cells obtained from a patient with common variable hypogammaglobulinemia and associated thymoma. This hybridoma secreted a suppressor factor for polyclonal antibody synthesis. Supernatants of this hybrid showed 40-80% suppression of both IgM and IgG synthesis by pokeweed mitogen-stimulated human peripheral blood lymphocytes. Hybridoma supernatants were suppressive for immunoglobulin production only if added within the initial 48 h of the seven-day culture period. Suppression of antibody production by the hybridoma supernatant was prevented by preabsorption with T lymphocytes. Further, the suppressor factor was shown to inhibit antibody production in reconstructed cultures containing T4+ cells and B cells, yet the suppression could be abrogated by increasing the number of T4+ cells. The hybrid supernatant had no affect on the proliferation of human mononuclear cells in response to pokeweed mitogen, lipopolysaccharide, concanavalin A or alloantigen but inhibited phytohemagglutinin-induced proliferation. The target cell population for the inhibition of phytohemagglutinin responsiveness was shown to be a T4+ lymphocyte (helper inducer T cell). These results suggest that thymus hybridoma cells can produce immunoregulatory products that act through the modulation of T4+ lymphocyte function. To our knowledge this is the first human thymus cell hybridoma to be reported. Studies on such cell lines may provide important information on immunoregulatory thymic factors.

Human monocyte cell line (U937) releases suppressive IgG-binding factor(s)

Fc gamma receptor-bearing U937 cells, when incubated in serum-free buffer, were found to release spontaneously a suppressor material for pokeweed mitogen-drigen IgG synthesis which could be retained on Sepharose 4B-IgG immunosorbents. Immunosorbents coupled with IgM or F (ab')2 fragments of IgG were unable to retain the inhibitory activity of U937-derived material, suggesting a binding specificity for the Fc gamma fragment of IgG. This suppressor material corresponds therefore by definition to an IgG-binding factor (IgG-BF). The mechanism for in vitro suppression of the antibody response by U937-derived IgG-BF was investigated. It did not interfere with cell proliferation and displayed maximum effect when added at day 3 of the culture period. Tested for its effect on IgG, IgM and IgA synthesis, IgG-BF suppressed antibody production following a pattern specific for IgG. Finally gel filtration of the suppressor material gave rise to two peaks of inhibitory activity with an apparent molecular mass comprised between 30 to 40 kDa and 60 to 73 kDa, respectively.

Receptors for immunoglobulin isotypes (FcR) on murine T cells. I. Multiple FcR expression on T lymphocytes and hybridoma T cell clones

T cell receptors for the Fc portion of the various isotypes of mouse immunoglobulins (FcR) were examined by rosette formation, using as indicator cells erythrocytes coated with monoclonal antibodies of all known isotypes of serum immunoglobulins. Three populations of mouse T cells were studied: normal thymocytes, activated T cells (ATC), generated by educating thymocytes in lethally irradiated allogeneic hosts, and hybridoma T cells, derived from somatic hybridization of ATC with the FcR-negative thymoma BW.5147. We found that many different FcR could be distinguished by their specificity for a single isotype or for a combination of several isotypes; ATC and hybridoma T cells expressed several such receptors that, at least in cloned cells, could be demonstrated to be borne by individual cells; hybridoma T cells of independent origin bore indistinguishable receptors whereas ATC expressed markedly different FcR and upon overnight incubation at 37 degrees C, immunoglobulins were found to bind onto the cell surface, even though no corresponding constitutive FcR was detected. The same was observed with hybridoma T cells and with thymocytes. It follows that a single T cell can express several FcR. Altogether, these FcR are capable of binding all known isotypes of serum immunoglobulins. They differ from one T cell to another.

Receptors for immunoglobulin isotypes (FcR) on murine T cells. II. Multiple FcR induction on hybridoma T cell clones

In the preceding report (Eur. J. Immunol. 1985. 15: 662), we described a variety of receptors for the Fc portion of the different isotypes of mouse immunoglobulins (FcR), that were found to be expressed on hybridoma T cell clones. In the present work, we wondered whether the expression of these T cell FcR would be regulated by environmental influences such as the presence of corresponding ligands. We found that exposing the cells to the bulk of serum immunoglobulins in vivo, or to purified monoclonal immunoglobulins in vitro both resulted in FcR induction. The expression of all constitutive receptors, i.e. Fcgamma 1/2bR, Fcgamma3R, FcalphaR and FcepsilonR, could be increased upon incubation with IgG1, IgG2b, IgG3, IgA and IgE, respectively. After induction, the specificity of FcR was not modified. Two FcR were detectable only upon induction. These were Fcgamma2a/2b/1R, induced by IgG2a and FcmuR, induced by IgM. Interestingly, FcR detectable after induction only were short-lived at the membrane. Ten to 15 h after induction they were not detected any more, whereas the expression of constitutive FcR remained elevated for at least 24 h following induction. Therefore, depending on the concentration of immunoglobulins in the environment, and depending on whether they are short lived or long lived, FcR can modulate their expression on the membrane of T cells. Such a versatility might be an efficient means to contribute to isotypic regulation through the release of regulatory immunoglobulin-binding factors. factors.

cDNA clones encoding IgE-binding factors from a rat-mouse T-cell hybridoma

cDNA clones encoding rodent IgE-binding factors (IgE-BF) were isolated from cDNA libraries of a rat-mouse T hybridoma that secretes IgE-suppressive factor (IgE-SF) upon incubation with rat IgE. COS7 cells transfected with two of the cDNAs expressed IgE-BF, which selectively potentiate an in vitro IgE response. IgE-BF expressed in COS7 cells are glycoproteins of approximately equal to 60 and approximately equal to 11 kDa. DNA sequence analysis of an IgE-BF cDNA revealed a 556-amino acid (62 kDa) protein coding region. The results suggest that IgE-potentiating and IgE-suppressive factors share common precursor polypeptides and that the 11-kDa IgE-BF is derived from a 60-kDa precursor.

Isotype-specific immunoregulation. IgA-binding factors produced by Fc alpha receptor-positive T cell hybridomas regulate IgA responses

T-T hybridomas, produced by fusions between R1.1 T lymphoma and cloned T helper cells that promote IgA responses (Th A cells) were characterized in this study. A total of 85 cloned cell lines were produced, and their supernatants were assessed for support of antigen-dependent IgA (and IgM and IgG) responses. 16 of 85 culture fractions supported IgA anti-sheep red blood cell, -horse red blood cell, or -trinitrophenyl responses in either lipopolysaccharide-triggered splenic B cell, or normal Peyer's patch B cell cultures, and the responses were specific for the antigen used for in vitro immunization. None of the supernatants from the cell lines induced significant polyclonal responses in these B cell cultures. Interestingly, the 16 hybridomas that produced supernatants with IgA-promoting properties had Fc receptors for IgA (Fc alpha R), but did not express Fc mu R or Fc gamma R. When supernatants from Fc alpha R+ T cell lines were subjected to IgA affinity chromatography, the IgA-promoting activity bound to IgA (IBF alpha) and was recovered in the eluate. No binding of active fractions occurred when supernates were passed through IgM or IgG immunoadsorbent columns. High concentrations of purified IBF alpha suppressed T-dependent IgA responses, while an optimal level was required for enhancement of this isotype response. These results suggest that Fc alpha R+ hybridomas derived from Th A cells release IBF alpha into the culture medium, and that these molecules regulate IgA responses to various T-dependent antigens.

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.

Identification of soluble Fc receptors in mouse serum and the conditioned medium of stimulated B cells

We have evaluated the expression of surface Fc gamma 2b/gamma 1R by lipopolysaccharide (LPS)-activated murine spleen cells, the release of soluble Fc gamma 2b/gamma 1R by activated spleen cells, and the presence of circulating Fc gamma 2b/gamma 1R in mouse serum. LPS activation of murine spleen cells and a cloned B cell line, BCL-1 CW 13.20-3B3, resulted in a 5-10-fold increase in surface Fc gamma 2b/gamma 1R and the concominant appearance in the culture medium of a soluble molecule that is antigenically related to the Fc gamma 2b/gamma 1R. The increase in cell-associated and soluble Fc gamma 2b/gamma 1R after LPS activation is attributable primarily to B cells. Circulating Fc gamma 2b/gamma 1R was also detected in normal mouse serum at a concentration of 10(-9) to 10(-8) M. Levels of circulating Fc gamma 2b/gamma 1R increase with the age of the animals, and were low in adult germ-free mice and elevated in young mice with certain autoimmune diseases. The circulating Fc gamma 2b/gamma 1R bound to IgG-Sepharose, and was partially purified by affinity chromatography on 2.4G2 Fab-Sepharose. After radiolabeling and immunoprecipitation with rabbit anti-Fc gamma 2b/gamma 1R serum, one component of Mr 48,000, was detected.