Binding characteristics and isotype specificity of Fc receptors on K cells

Recombinant CD16A-Ig forms a homodimer and cross-blocks the ligand binding functions of neutrophil and monocyte Fcgamma receptors

Receptors for the Fc region of IgG (FcgammaR) are expressed as membrane-bound and soluble forms in inflammatory cells. These receptors mediate a variety of immunoregulatory functions. FcgammaR-bearing neutrophils and macrophages play a major role in mediating immune complex induced inflammatory diseases and antibody-mediated tissue injury in autoimmune diseases. To delineate the biological role of the soluble FcgammaR, a recombinant soluble CD16A (CD16A-Ig) was expressed and characterized. CD16A-Ig is secreted as a disulfide-linked homodimer of 70kDa. The purified CD16A-Ig bound to human IgG1 and IgG3 immobilized onto ELISA plates and on IgG1- and IgG3-coated erythrocytes. Saturation binding studies and Scatchard plot analysis showed that immune complex bound to the purified CD16A-Ig with high avidity. Moreover, CD16A-Ig competitively blocked the binding of cell surface-expressed CD16A-CHO cells to IgG-coated plates. The dimeric CD16A-Ig also efficiently cross-blocked the binding of IgG-coated target cells to other FcgammaR expressed on neutrophils and monocytes. These results demonstrate that CD16A-Ig forms a high avidity dimer and is capable of blocking cell-cell interactions mediated by inflammatory cells expressing FcgammaR and IgG-coated target cells. These findings suggest that the dimeric FcgammaR molecules could be used therapeutically for the intervention of immune complex-mediated inflammatory disease.

In vitro functional activity of IgG1 and IgG3 polyclonal and monoclonal anti-D

BACKGROUND AND OBJECTIVES

IgG anti-D is generally restricted to IgG1 and IgG3; it mediates red cell destruction through interactions with IgG Fc receptors (Fc gamma R) on effector cells. The relative ability of these two IgG subclasses of anti-D to mediate haemolysis in vitro by monocytes and K cells was investigated.

MATERIALS AND METHODS

Anti-D was affinity purified from 5 preparations of prophylactic anti-D immunoglobulin, and IgG subclasses quantified by ELISA; mean levels were 86.5% IgG1, 1.4% IgG2, 11.6% IgG3 and 0.4% IgG4. IgG1 and IgG3 polyclonal anti-D were further purified separately from some of the anti-D by removal of either IgG3 using magnetic beads coated with anti-IgG3, or of IgG1 using protein A. These preparations were compared with monoclonal anti-D (BRAD-3 and BRAD-5) for their ability to lyse red cells in antibody-dependent cell-mediated cytotoxicity (ADCC) assays.

RESULTS

Monocyte-mediated lysis of red cells coated with IgG3 anti-D was approximately twice that of cells coated with IgG1 anti-D at similar sensitization levels, and anti-D preparations containing 10% or more IgG3 gave similar lysis. By contrast, in the K cell ADCC, IgG1 anti-D was 2-4 times more haemolytic than IgG3 anti-D. Polyclonal IgG1 and IgG3 anti-D promoted about 20% more lysis than BRAD-5 (IgG1) and BRAD-3 (IgG3), respectively, in the K cell ADCC, although no difference was observed between polyclonal and monoclonal anti-D in the monocyte ADCC.

CONCLUSIONS

These experiments demonstrated a functional dichotomy between these two subclasses of anti-D; IgG3-coated red cells were lysed preferentially by monocytes mediated predominantly through Fc gamma R1 interactions, whereas haemolysis of IgG1-sensitized cells was mediated mainly by Fc gamma RIII on K cells.

Ligand binding and phagocytosis by CD16 (Fc gamma receptor III) isoforms. Phagocytic signaling by associated zeta and gamma subunits in Chinese hamster ovary cells

CD16, the low affinity Fc gamma receptor III for IgG (Fc gamma RIII), exists as a polypeptide-anchored form (Fc gamma RIIIA or CD16A) in human natural killer cells and macrophages and as a glycosylphosphatidylinositol-anchored form (Fc gamma RIIIB or CD16B) in neutrophils. CD16A requires association of the gamma subunit of Fc epsilon RI or the zeta subunit of the TCR-CD3 complex for cell surface expression. The CD16B is polymorphic and the two alleles are termed NA1 and NA2. In this study, CD16A and the two alleles of CD16B have been expressed in Chinese hamster ovary (CHO) cells and their ligand binding and phagocytic properties analyzed. The two allelic forms of CD16B showed a similar affinity toward human IgG1. However, the NA1 allele showed approximately 2-fold higher affinity for the IgG3 than the NA2 allele. Although all three forms of CD16 efficiently bound rabbit IgG-coated erythrocytes (EA), only CD16A coexpressed with the gamma subunit phagocytosed EA. The phagocytosis mediated by CD16A expressed on CHO cells was independent of divalent cations but dependent on intact microfilaments. CHO cells expressing CD16A-gamma and CD16A-zeta chimeras also phagocytosed EA. The phagocytosis was specifically inhibited by tyrphostin-23, a tyrosine kinase inhibitor. In summary, our results demonstrate that glycosylphosphatidylinositol-anchored CD16B alleles differ from CD16A in their ability to mediate phagocytosis. Furthermore, since studies with other Fc gamma Rs have shown that CHO cells lack the phagocytic pathway mediated by the cytoplasmic domain of Fc gamma Rs, the phagocytosis of EA by CHO cells stably transfected with CD16A and CD16A-subunit chimera provides an ideal system to dissect the phagocytic signaling pathways mediated by these Fc gamma R-associated subunits.

Functional assessment of therapeutic anti-D immunoglobulin using Fc-mediated assays

Therapeutic anti-D immunoglobulin preparations issued by the Scottish National Blood Transfusion Service, between 1980 and 1986, were evaluated using in-vitro Fc-mediated functional tests that reflect potential in-vivo mechanisms of specific red cell destruction and clearance. All batches tested were found to: (a) contain anti-D of mainly IgG1 subclass and lesser amounts of IgG3; (b) mediate lymphocyte and monocyte rosetting; and (c) produce lytic activity in both K cell and monocyte ADCC. The functional activity of the therapeutic immunoglobulin preparations over this period of production had not altered despite increased plasma contributions latterly to the pool from deliberately immunized male donors. This is the first in-vitro study of the Fc-mediated function of therapeutic polyclonal anti-D preparations. As these preparations were clinically effective in the prophylactic anti-D programme, such bioassays of FcRI/II and FcRIII activity are justified for the future evaluation of immune plasma before blending for fractionation and production of therapeutic anti-D immunoglobulin.

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.

Functional studies on Fc receptors

Fc receptors (FcRs) are cell surface molecules that recognize and bind to the constant domains of immunoglobulins. In doing so, they enable antibodies to perform several biological functions, by forming a link between specific antigen recognition and FcR-bearing cells. Here, Gabriella Sármay provides an overview of recent studies on FcRs in Hungarian laboratories, concentrating on their role in selected biological systems.

Mapping and comparison of the interaction sites on the Fc region of IgG responsible for triggering antibody dependent cellular cytotoxicity (ADCC) through different types of human Fc gamma receptor

In the present study 3-iodo-4-hydroxy-5-nitrophenacetyl (NIP)-specific antibodies were compared for induction of antibody dependent lysis of NIP-derivatised red blood cells effected by pre-stimulated U937 or HL-60 cells and by K cells. The chimaeric antibodies have heavy chains corresponding to human IgG subclasses 1-4, and include site-directed mutants of IgG3 as well as the aglycosylated form of IgG3; a mouse IgG2b antibody and a site-directed mutant IgG2b were also examined. rIFN stimulated U937 or HL-60 cells express increased levels of Fc gamma R1 compared to unstimulated cells; PMA stimulated HL-60 and U937 cells express an increased level of Fc gamma R11 compared to unstimulated cells; K cells express Fc gamma R111. Using these effector cell populations and the target cells mentioned above, we have compared anti-NIP antibodies with different heavy chain constant domains for their ability to induce ADCC through human Fc gamma R1, Fc gamma R11 and Fc gamma R111. The results suggest that all three human Fc gamma receptors appear to recognise a binding site on IgG within the lower hinge (residues 234-237) and trigger ADCC via this site, but that each receptor sees this common site in a different way. The possibility that other amino acid residues also participate in the binding/triggering site(s) cannot be excluded.

Increased levels of soluble low-affinity Fc gamma receptors (IgG-binding factors) in the sera of tumour-bearing mice

Soluble forms of low affinity Fc gamma receptors (Fc gamma R), also called IgG-binding factors (IgG-BF), have been shown to play a regulatory role in immune responses. By using an immunodot assay with the anti-mouse Fc gamma R MoAb, 2.4G2, the levels of IgG-BF have been measured in the sera of mice bearing syngeneic tumours of lymphoid or non-lymphoid origin or in mice injected with high doses of murine IgG. These sera contained large amounts of IgG-BF as compared with controls. In the case of mice bearing IgG2a- or IgG2b-secreting hybridomas or lymphomas, serum IgG-BF increased progressively with tumour size and serum monoclonal IgG concentration, reaching 4-12 times the normal levels. A less than three-fold increase was found in mice bearing an IgG1-secreting hybridoma or tumours which do not secrete IgG (IgA-secreting hybridoma, non-immunoglobulin-secreting lymphoid tumours or melanoma) or in mice injected with 9 mg of monoclonal IgG2a. The enhancement of serum IgG-BF levels was independent of the expression of Fc gamma R by the tumour cells, suggesting that the majority of IgG-BF secreted in response to tumours was produced by the host rather than by the tumour. The increased production of IgG-BF may participate in the control of tumour growth and in the modulation of the host immune responses in tumour-bearing animals.

Fc-receptor-mediated phagocytosis: abnormalities associated with diabetes mellitus

The family of Fc receptors (FcR) for IgG play pivotal roles in affector, effector, and regulatory functions of cells of the immune system. Thus, changes in expression and activation of FcRs may contribute to a variety of disease manifestations that are the consequence of abnormalities in immune system function. Patients with diabetes mellitus are often plagued with recurrent bacterial and mycotic infections, as well as large and small vessel injury which may in part be immune mediated and which lead to organ dysfunction. Hormone-mediated changes in immune system function have been postulated to contribute to a variety of the complications experienced by patients with diabetes mellitus. It is the purpose of this review to summarize current knowledge regarding abnormalities in immune system function in diabetes mellitus with special emphasis on classical hormonal modulation of Fc receptor-mediated phagocytosis.

Rosette formation between immobilised human lymphocytes and erythrocytes sensitised with monoclonal anti-D

A rapid, reproducible and sensitive assay was developed to investigate the ability of human lymphocytes to form rosettes with erythrocytes sensitised with human monoclonal anti-D. Erythrocytes sensitised with a known number of anti-D molecules per cell were incubated with lymphocytes immobilised on plastic by poly(L-lysine), the resulting rosettes fixed, unbound erythrocytes removed by washing and the cell preparation stained. IgG1 and IgG3 anti-D-coated erythrocytes gave similar rosette formation at sensitisation levels in the range of 5000-15,000 molecules per cell, although at lower sensitisation levels IgG3 gave greater rosette formation than IgG1. A minimum of 500 IgG3 and 1000 IgG1 anti-D molecules per erythrocyte were required for rosetting.

Human monoclonal anti-D antibodies. II. The relationship between IgG subclass, Gm allotype and Fc mediated function

Eight monoclonal antibodies (mabs) to the Rh antigen D produced by Epstein-Barr virus transformed B-lymphoblastoid cell lines from two individuals have been compared for their behaviour in in vitro cell-mediated assays. Three IgG1 Glm(1,17) and two IgG3 G3m(21) mabs from one donor and three IgG1 Glm(3) mabs from another were used. IgG3 anti-D mabs induced greater adherence and phagocytosis of sensitized red cells by U937 monocytes than IgG1 anti-D mabs or the polyclonal anti-D. Minimum sensitization levels for rosetting and phagocytosis by U937 monocytes were 2,000 molecules IgG/cell for IgG3 and 5,000 molecules/cell for IgG1 mabs; maximum rosetting mediated by both IgG1 and IgG3 mabs was obtained at 15,000-20,000 molecules/cell. The IgG3 anti-D mabs were comparable to polyclonal anti-D in mediating binding of sensitized red cells to gamma-interferon stimulated monocyte-derived cultured macrophages and were markedly more effective than the IgG1 anti-D mabs. However, in lymphocyte ADCC assays, only anti-D mabs which were IgG1 Glm(3) were effective in mediating high levels of lysis of sensitized red cells, unlike the IgG1 Glm (1,17) or IgG3 G3m(21) mabs. Minimum sensitization levels required for this lymphocyte-mediated red cell lysis were found to be approximately 5,000 molecules/cell with one IgG1 Glm(3) mab; maximum lysis with this mab was obtained at 10,000 molecules/cell. Polyclonal anti-D containing both IgG1 and IgG3 was effective in all three assays. These observations suggest that different isotypes and allotypes of anti-D antibodies mediate red cell removed or destruction by monocyte or lymphocyte effector cell through functionally dissimilar Fc receptor interactions.

Human Fc receptors for IgG

Human lymphocytes bear at least three different classes of Fc gamma R, which are different in cellular distribution, structure, and function. This review deals with recent advances in understanding the complexity of huFc gamma R. In particular, studies demonstrating the presence of alternate phosphatidyl inositol glycan-anchored and transmembrane-anchored forms of huFc gamma RIII (CD16) are discussed.

The Fc gamma receptor of natural killer cells is a phospholipid-linked membrane protein

Three types of receptor for the Fc (constant) region of human immunoglobulin G have been described; FcRI, a high-affinity (Ka approximately equal to 10(8) M-1) receptor expressed on monocytes; FcRII (CD32), a low-affinity (Ka approximately equal to 10(6) M-1) receptor expressed on B cells, granulocytes, macrophages and platelets; and FcRIII (CD16, FcRIo), a low-affinity receptor expressed on macrophages, neutrophils, eosinophils, natural killer cells and a subset of T cells believed to comprise the suppressor cells. Anti-CD16 antibodies block natural killer-cell mediated antibody dependent cellular cytotoxicity (ADCC). Binding of aggregated IgG to CD16 on natural killer cells leads to the expression of lymphocyte activation antigens, mediator release, morphological changes and lytic activity. We report here the isolation of a complementary DNA clone encoding CD16 determinants which gave rise to IgG binding of the expected affinity and subtype specificity in COS cells, and which proved to encode a phospholipid anchored protein. A single messenger RNA transcript was found in all positive RNA samples, and N-glycanase treatment showed the form found in COS cells was identical to the form present on peripheral blood mononuclear cells (PBMCs). We also show that CD16 is most closely related to the alpha-form of the murine IgG 2b/1 receptor and propose that extracellular contacts mediate the signal initiated by IgG binding.

Isolation of cDNAs for two distinct human Fc receptors by ligand affinity cloning

Two cDNA clones encoding different but related receptors for immunoglobulin G constant domains were isolated from cDNA expression libraries by a ligand-mediated selection procedure ('affinity cloning'). Because both of the receptors encoded by the cDNAs react with CDw32 monoclonal antibodies, and both show the appropriate IgG binding affinity, both appear to be forms of the receptor formerly thought to be a single species called FcRII. The extracellular domains encoded by the isolated clones are closely related to the murine IgG2b/1 beta receptor extracellular domains, but the intracellular domains are unrelated. The receptors expressed in COS cells show a preference for IgG1 among IgG subtypes and no affinity for IgM, IgA or IgE. Abundant expression of the RNAs was detected in myeloid cell lines and placenta.

The effect of synthetic peptides corresponding to Fc sequences in human IgG1 on various steps in the B cell activation pathway

The influence of synthetic peptides comprising sequences in the exposed positions of the Fc region of human IgG 1 was tested on B lymphocyte activation. CH 2 domain peptides having an inhibitory effect on antibody-dependent cellular cytotoxicity, as well as the whole Fc fragment, induced the appearance of the early signs of activation on resting B lymphocytes such as increase in cell volume and HLA-DR antigen expression or leukocyte migration inhibitory factor production. The peptides did not induce proliferation of resting B cells even when B cell growth factor (BCGF)-containing supernatants were added. Exposure to Fc fragment, however, induced a weak proliferation which was significantly enhanced by BCGF. On the other hand, both the Fc fragment and the CH 2 or CH 3 domain peptides enhanced the IgM synthesis of human blood mononuclear cells when a suboptimal dose of pokeweed mitogen was present. This effect was lost when Fc fragment or the peptides were added on the third day of culture. These results suggest that the early steps of B cell activation can be induced by Fc fragment and by small molecular weight Fc peptides, which are potential ligands of Fc receptors. The Fc fragment activates B cells to the state where they respond to BCGF, but the peptides do not possess this activity. Furthermore, both Fc fragment and Fc peptides are able to enhance the IgM synthesis, when accessory cells and the appropriate differentiating factors are present.