Systematic review and meta-analysis of the epidemiology of polyautoimmunity in Sjögren's syndrome (secondary Sjögren's syndrome) focusing on autoimmune rheumatic diseases.

OBJECTIVE: The epidemiology of polyautoimmunity in Sjögren's syndrome (secondary Sjögren's syndrome - sSS) is not well defined and has not been investigated before using a systematic approach. We conducted a systematic review of the epidemiology of sSS associated with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), scleroderma, and myositis, assessing the prevalence rates (PRs) and clinical and serological features of sSS. METHOD: A systematic literature search of PubMed and Embase databases (updated to March 2016) was performed to identify all published data on PR, demographic profile, clinical manifestations, laboratory features, and causes of death associated with sSS. The PR's of sSS were summarized with PRs and 95% confidence intervals (CIs). RESULTS: The literature search identified 1639 citations, of which 42 fulfilled the inclusion criteria. Only 19 studies were of moderate to good quality and were selected for the meta-analysis. According to a random-effects model, the pooled PR for sSS associated with RA was 19.5% (95% CI 11.2 to 27.8) and the pooled PR for sSS associated with SLE was 13.96% (95% CI 8.88 to 19.04). The female/male ratio of sSS in the RA population was 14.7 (95% CI 7.09 to 256) and in the SLE population was 16.82 (95% CI 1.22 to 32.4). CONCLUSION: Prevalence rates of sSS vary widely in different populations. Both meta-analyses conducted in the RA and SLE populations were characterized by a high degree of study heterogeneity. The results of this meta-analysis highlight the need for better quality population studies.

Partitioning of Thy-1, GM1, and cross-linked phospholipid analogs into lipid rafts reconstituted in supported model membrane monolayers.

As shown earlier, raft-like domains resembling those thought to be present in natural cell membranes can be formed in supported planar lipid monolayers. These liquid-ordered domains coexist with a liquid-disordered phase and form in monolayers prepared both from synthetic lipid mixtures and lipid extracts of the brush border membrane of mouse kidney cells. The domains are detergent-resistant and are highly enriched in the glycosphingolipid GM1. In this work, the properties of these raft-like domains are further explored and compared with properties thought to be central to raft function in plasma membranes. First, it is shown that domain formation and disruption critically depends on the cholesterol density and can be controlled reversibly by treating the monolayers with the cholesterol-sequestering reagent methyl-beta-cyclodextrin. Second, the glycosylphosphatidylinositol-anchored cell-surface protein Thy-1 significantly partitions into the raft-like domains. The extent of this partitioning is reduced when the monolayers contain GM1, indicating that different molecules can compete for domain occupation. Third, the partitioning of a saturated phospholipid analog into the raft phase is dramatically increased (15% to 65%) after cross-linking with antibodies, whereas the distribution of a doubly unsaturated phospholipid analog is not significantly affected by cross-linking (approximately 10%). This result demonstrates that cross-linking, a process known to be important for certain cell-signaling processes, can selectively translocate molecules to liquid-ordered domains.

Molecular cloning of the rat TA1/LAT-1/CD98 light chain gene promoter.

The rat LAT-1 (L-amino acid transporter-1) gene is a CD98 light chain highly expressed in cancer and development. As an initial study of the molecular basis underlying regulation of its expression, we cloned 2 kb of the LAT-1 5' flanking region. Inverse RACE and primer extension methods were used to define the transcription initiation site at 80 bp upstream from the translational start site. Functional studies carried out in normal hepatic cells using constructs containing progressive 5' deletion from region -1958 to -185 showed 3-5-fold beta-galactosidase activities over control. The presence of an activator site(s) between -52 and -185 was indicated by low activities conferred by the construct spanning this region.

Overexpression of LAT1/CD98 light chain is sufficient to increase system L-amino acid transport activity in mouse hepatocytes but not fibroblasts.

l-amino acid transporter-1 (LAT1) is a highly conserved gene identified as a light chain of the CD98 amino acid transporter and cellular activation marker. In our previous studies we found increased expression of LAT1 in primary human cancers. We have demonstrated also that LAT1 response to arginine availability is lost in transformed and tumorigenic cells such that expression is constitutively high. System l-amino acid transport activity correlates with changes in LAT1. To assess the functional relevance of increased LAT1 expression and the requirement for 4F2 heavy chain, we overexpressed these CD98 subunits together and separately in nontransformed hepatocytes and fibroblasts. Antigen tags in the expression constructs confirmed that expressed proteins were localized to both cytoplasmic and plasma membrane locations within the cells. Overexpression of LAT1 alone in mouse hepatocytes, but not fibroblasts, was sufficient to increase system l transport, and these cells displayed a growth advantage in conditions of limited arginine. Our results suggest that loss of regulation leading to constitutive expression of LAT1 can contribute to oncogenesis. We hypothesize that the altered LAT1 expression observed in hepatocarcinogenesis gives cells a growth or survival advantage through increased transport activity in a tumor microenvironment of limited amino acid availability.

Lipid rafts reconstituted in model membranes.

One key tenet of the raft hypothesis is that the formation of glycosphingolipid- and cholesterol-rich lipid domains can be driven solely by characteristic lipid-lipid interactions, suggesting that rafts ought to form in model membranes composed of appropriate lipids. In fact, domains with raft-like properties were found to coexist with fluid lipid regions in both planar supported lipid layers and in giant unilamellar vesicles (GUVs) formed from 1) equimolar mixtures of phospholipid-cholesterol-sphingomyelin or 2) natural lipids extracted from brush border membranes that are rich in sphingomyelin and cholesterol. Employing headgroup-labeled fluorescent phospholipid analogs in planar supported lipid layers, domains typically several microns in diameter were observed by fluorescence microscopy at room temperature (24 degrees C) whereas non-raft mixtures (PC-cholesterol) appeared homogeneous. Both raft and non-raft domains were fluid-like, although diffusion was slower in raft domains, and the probe could exchange between the two phases. Consistent with the raft hypothesis, GM1, a glycosphingolipid (GSL), was highly enriched in the more ordered domains and resistant to detergent extraction, which disrupted the GSL-depleted phase. To exclude the possibility that the domain structure was an artifact caused by the lipid layer support, GUVs were formed from the synthetic and natural lipid mixtures, in which the probe, LAURDAN, was incorporated. The emission spectrum of LAURDAN was examined by two-photon fluorescence microscopy, which allowed identification of regions with high or low order of lipid acyl chain alignment. In GUVs formed from the raft lipid mixture or from brush border membrane lipids an array of more ordered and less ordered domains that were in register in both monolayers could reversibly be formed and disrupted upon cooling and heating. Overall, the notion that in biomembranes selected lipids could laterally aggregate to form more ordered, detergent-resistant lipid rafts into which glycosphingolipids partition is strongly supported by this study.

Total internal reflection with fluorescence correlation spectroscopy: combined surface reaction and solution diffusion.

Total internal reflection with fluorescence correlation spectroscopy (TIR-FCS) is a method for measuring the surface association/dissociation rates and absolute densities of fluorescent molecules at the interface of solution and a planar substrate. This method can also report the apparent diffusion coefficient and absolute concentration of fluorescent molecules very close to the surface. An expression for the fluorescence fluctuation autocorrelation function in the absence of contributions from diffusion through the evanescent wave, in solution, has been published previously (N. L. Thompson, T. P. Burghardt, and D. Axelrod. 1981, Biophys. J. 33:435-454). This work describes the nature of the TIR-FCS autocorrelation function when both surface association/dissociation kinetics and diffusion through the evanescent wave contribute to the fluorescence fluctuations. The fluorescence fluctuation autocorrelation function depends in general on the kinetic association and dissociation rate constants, the surface site density, the concentration of fluorescent molecules in solution, the solution diffusion coefficient, and the depth of the evanescent field. Both general and approximate expressions are presented.

Interaction of mammalian mitochondrial elongation factor EF-Tu with guanine nucleotides.

Elongation factor Tu (EF-Tu) promotes the binding of aminoacyl-tRNA (aa-tRNA) to the acceptor site of the ribosome. During the elongation cycle, EF-Tu interacts with guanine nucleotides, aa-tRNA and its nucleotide exchange factor (EF-Ts). Quantitative determination of the equilibrium dissociation constants that govern the interactions of mammalian mitochondrial EF-Tu (EF-Tu(mt)) with guanine nucleotides was the focus of the work reported here. Equilibrium dialysis with [3H]GDP was used to measure the equilibrium dissociation constant of the EF-Tu(mt) x GDP complex (K(GDP) = 1.0 +/- 0.1 microM). Competition of GTP with a fluorescent derivative of GDP (mantGDP) for binding to EF-Tu(mt) was used to measure the dissociation constant of the EF-Tu(mt) x GTP complex (K(GTP) = 18 +/- 9 microM). The analysis of these data required information on the dissociation constant of the EF-Tu(mt) x mantGDP complex (K(mGDP) = 2.0 +/- 0.5 microM), which was measured by equilibrium dialysis. Both K(GDP) and K(GTP) for EF-Tu(mt) are quite different (about two orders of magnitude higher) than the dissociation constants of the corresponding complexes formed by Escherichia coli EF-Tu. The forward and reverse rate constants for the association and dissociation of the EF-Tu(mt) x GDP complex were determined using the change in the fluorescence of mantGDP upon interaction with EF-Tu(mt). These values are in agreement with a simple equilibrium binding interaction between EF-Tu(mt) and GDP. The results obtained are discussed in terms of the recently described crystal structure of the EF-Tu(mt) x GDP complex.

Interaction of mitochondrial elongation factor Tu with aminoacyl-tRNA and elongation factor Ts.

Elongation factor (EF) Tu promotes the binding of aminoacyl-tRNA (aa-tRNA) to the acceptor site of the ribosome. This process requires the formation of a ternary complex (EF-Tu.GTP.aa-tRNA). EF-Tu is released from the ribosome as an EF-Tu.GDP complex. Exchange of GDP for GTP is carried out through the formation of a complex with EF-Ts (EF-Tu.Ts). Mammalian mitochondrial EF-Tu (EF-Tu(mt)) differs from the corresponding prokaryotic factors in having a much lower affinity for guanine nucleotides. To further understand the EF-Tu(mt) subcycle, the dissociation constants for the release of aa-tRNA from the ternary complex (K(tRNA)) and for the dissociation of the EF-Tu.Ts(mt) complex (K(Ts)) were investigated. The equilibrium dissociation constant for the ternary complex was 18 +/- 4 nm, which is close to that observed in the prokaryotic system. The kinetic dissociation rate constant for the ternary complex was 7.3 x 10(-)(4) s(-)(1), which is essentially equivalent to that observed for the ternary complex in Escherichia coli. The binding of EF-Tu(mt) to EF-Ts(mt) is mutually exclusive with the formation of the ternary complex. K(Ts) was determined by quantifying the effects of increasing concentrations of EF-Ts(mt) on the amount of ternary complex formed with EF-Tu(mt). The value obtained for K(Ts) (5.5 +/- 1.3 nm) is comparable to the value of K(tRNA).

TA1/LAT-1/CD98 light chain and system L activity, but not 4F2/CD98 heavy chain, respond to arginine availability in rat hepatic cells. Loss Of response in tumor cells.

Tumor associated gene-1/L amino acid transporter-1 (TA1/LAT-1) was recently identified as a light chain of the CD98 amino acid transporter and cellular activation marker. Our previous studies with primary rat hepatocyte cultures demonstrated that TA1 RNA levels were responsive to media amino acid concentrations, suggesting adaptive regulation. High level TA1 expression associated with transformed cells also suggested a role in tumor progression. The present study examined the relationship of TA1/CD98 expression, adaptive response, and associated amino acid transport to neoplastic transformation using a panel of well characterized rat hepatic cell lines. We found 1) increased expression of TA1 in response to amino acid depletion, specific for arginine but not glutamine; 2) loss of TA1 response to arginine in gamma-glutamyl transpeptidase-positive transformed and tumorigenic cells; 3) no appreciable response of 4F2/CD98 heavy chain to arginine levels; and 4) correlation of system L amino acid transport activity in response to arginine with changes in TA1/LAT-1 mRNA but not total immunoreacting protein. Our results suggest this CD98 light chain may act as an environmental sensor, responding to amino acid availability and that its regulation is complex. We hypothesize that altered TA1 expression is an early event in hepatocarcinogenesis giving neoplastic cells a growth or survival advantage, particularly under conditions of limited amino acid availability.

Rebinding of IgE Fabs at haptenated planar membranes: measurement by total internal reflection with fluorescence photobleaching recovery.

In previous work, a general analytical theory for ligand rebinding at cell surfaces was developed for a reversible bimolecular reaction between ligands in solution and receptors on a membrane surface [Lagerholm, B. C., and Thompson, N. L. (1998) Biophys. J. 74, 1215-1228]. This theory can be used to predict theoretical forms for data obtained by using total internal reflection with fluorescence photobleaching recovery (TIR-FPR) [Thompson, N. L., Burghardt, T. P., and Axelrod, D. (1981) Biophys. J. 33, 435-454]. Thus, one method by which the rebinding theory can be tested is to use TIR-FPR. In the work described herein, the reversible kinetics of mouse monoclonal anti-dinitrophenyl (DNP) IgE Fabs at substrate-supported planar membranes composed of 25 mol % DNP-conjugated phosphatidylethanolamine and 75 mol % dipalmitoylphosphatidylcholine have been examined by using TIR-FPR. Data were obtained as a function of the Fab solution concentration. Higher Fab concentrations reduce rebinding (and increase the fluorescence recovery rate) because different Fab molecules compete for the same surface-binding sites. Data were also obtained for solutions containing different volume fractions of glycerol. In these measurements, higher glycerol concentrations increase rebinding (and decrease the fluorescence recovery rate) because the solution viscosity is increased and the Fab diffusion coefficient in solution is decreased. The TIR-FPR data were quantitatively compared with theoretical predictions which follow from the general theory for rebinding at the membrane surface. The data were consistent with the theoretical predictions and, therefore, provide experimental verification of the previously developed theory.

The cytoplasmic region of mouse Fc gamma RIIb1, but not Fc gamma RIIb2, binds phospholipid membranes.

The cytoplasmic regions of the mouse low-affinity Fc gamma RII isoforms, Fc gamma RIIb1 and Fc gamma RIIb2, play key roles in signal transduction by mediating different cellular functions. The Fc gamma RIIb1 (94 residues) and Fc gamma RIIb2 (47 residues) cytoplasmic regions are generated by differential mRNA splicing in which a single aspartic acid residue in Fc gamma RIIb2 is replaced by a 48-residue insert in Fc gamma RIIb1. In previous work, quantities of the mFc gamma RIIb1 and mFc gamma RIIb2 cytoplasmic regions were generated, and their secondary structures were examined in different solutions with circular dichroism [Chen, L., Thompson, N. L., and Pielak, G. J. (1997) Protein Sci. 6, 1038-1046]. In the work described here, steady-state light scattering was used to investigate possible interactions of the two isolated cytoplasmic regions with phospholipid vesicles. Three phospholipid compositions were examined: phosphatidylserine/phosphatidylcholine (PS/PC) (25/75, mol/mol); phosphatidylinositol bisphosphate/phosphatidylcholine (PIP2/PC) (25/75, mol/mol); and pure phosphatidylcholine (PC). Binding was examined in the presence and absence of Ca2+. The mFc gamma RIIb1 cytoplasmic peptide binds PS/PC vesicles weakly in the absence of Ca2+ and more strongly in the presence of Ca2+. For PIP2/PC vesicles, the behavior is reversed; binding is weak in the presence of Ca2+ and stronger in its absence. The mFc gamma RIIb1 peptide also weakly binds pure PC vesicles, in a Ca2+-independent manner. The mFc gamma RIIb2 cytoplasmic peptide does not bind, in the presence or absence of Ca2+, to PS/PC, PIP2/PC, or PC vesicles. The implications of these results for the mechanisms of signal transduction mediated by the two mFc gamma RII cytoplasmic regions are discussed.

TA1 oncofetal rat liver cDNA and putative amino acid permease: temporal correlation with c-myc during acute CCl4 liver injury and variation of RNA levels in response to amino acids in hepatocyte cultures.

TA1 is a rat liver oncofetal cDNA and a member of an emerging family of evolutionarily conserved molecules with homology to amino acid transporters and permeases. The aim of these studies was to characterize the regulation and role of TA1 in acute rat liver injury by examining its relation to regeneration and metabolic stress. Following a single dose of CCl4, TA1 message was expressed 3-48 h. The major 3.3-kb TA1 transcript correlated temporally with c-myc expression. A novel 2.9-kb TA1 transcript was expressed more variably 24-48 h. TA1 protein was restricted to hepatocytes in G0 and G1 phases of the cell cycle. Relative to CCl4, a much smaller increase in TA1 was noted after partial hepatectomy and TA1 preceded the peak of c-myc expression. In vitro TA1 was not induced in hepatocytes by EGF or the acute-phase cytokines IL-6 and TNF-alpha, but was found to be modulated in response to amino acid availability. TA1 expression increased in media without arginine and glutamine and was repressed by total amino acid levels 5-fold over basal MEM. Together, these results contrast with the constitutive expression observed in transformed cells and suggest an adaptive role for TA1 during liver injury.

The light chain of CD98 is identified as E16/TA1 protein.

The 80/40-kDa CD98 protein complex was purified using an anti-CD98 heavy chain monoclonal antibody coupled to Sepharose beads. Eluted proteins were subjected to preparative SDS-polyacrylamide gel electrophoresis, and protein corresponding to the 40-kDa CD98 light chain was excised. Following proteolysis with trypsin, a peptide fragment was sequenced by mass spectrometry. The nine residues obtained were identical to established C-terminal sequences of the human E16 and rat TA1 proteins, suggesting that TA1/E16 protein is the CD98 light chain. Consistent with this, anti-TA1/E16 antibodies specifically immunoblotted the approximately 35-40-kDa light chain present upon immunoprecipitation of the human CD98 complex. Furthermore, anti-CD98 heavy chain antibody specifically co-immunoprecipitated hemagglutinin-tagged light chain from cells transfected with hemagglutinin-tagged E16 cDNA. In conclusion, the CD98 light chain is identical to the TA1/E16 protein, based on partial amino acid sequence identity, antibody cross-reactivity, genetic reconstitution evidence, similar molecular size, and comparable cell distribution.

Theory for ligand rebinding at cell membrane surfaces.

Conditions for which a ligand reversibly bound to a cell surface dissociates and then rebinds to the surface have been theoretically examined. The coupled differential equations that describe reaction at the interface between sites on a plane and three-dimensional solution have been described previously (Thompson, N. L., T. P. Burghardt, and D. Axelrod. 1981. Biophys. J. 33:435-454). Here, we use this theoretical formalism to provide an analytical solution for the spatial and temporal dependence of the probabilities of finding a molecule on the surface or in the solution, given initial placement on the surface at the origin. This general analytical solution is used to derive a simple expression for the probability that a molecule rebinds to the surface at a given position and time after release at the origin and time zero. The probability expressions provide fundamental equations that form a basis for subsequent modeling of ligand-receptor interactions in specific geometries.

Decreased IgG-Fc gamma RII dissociation kinetics in the presence of a protein antigen.

Total internal reflection fluorescence microscopy has been used to examine the interaction of a mouse monoclonal IgG2b, in the absence and presence of its protein antigen, with mouse Fc gamma RII in substrate-supported planar membranes. Equilibrium association and kinetic dissociation constants were measured for the antibody S6-34.11, which is specific for bovine prothrombin fragment 1 (BF1). These measurements showed that BF1 induces a statistically significant decrease (30-40%) in the IgG-Fc gamma RII dissociation kinetics. A corresponding increase in the equilibrium association constant was not observed, perhaps because the statistical accuracy of the equilibrium measurements is lower than that for the kinetic measurements. The consequences of these results for understanding the mechanism by which macrophages recognize and ingest opsonized targets are discussed.

Design, synthesis, expression, and characterization of the genes for mouse Fc gamma RIIb1 and Fc gamma RIIb2 cytoplasmic regions.

The cytoplasmic regions of the mouse low-affinity Fc gamma RII isoforms, mFc gamma RIIb1, and mFc gamma RIIb2, play a key role in signal transduction by mediating different cellular functions. mFc gamma RIIb1 has a 94-residue cytoplasmic region, whereas mFc gamma RIIb2 has a 47-residue cytoplasmic region. Genes encoding the cytoplasmic regions of mFc gamma RIIb1 (b1-94) and mFc gamma RIIb2 (b2-47) were designed, synthesized, and expressed as fusion proteins in Escherichia coli. A sequence-specific protease, thrombin, was used to release the b1-94 peptide, which was purified by using HPLC. The b2-47 peptide was synthesized chemically. CD spectropolarimetry was employed to examine the secondary structures of b1-94 and b2-47. These studies were conducted in aqueous solution, in mixtures of water and trifluoroethanol or methanol, and as a function of temperature. The results indicate that the b1-94 and b2-47 structures are sensitive functions of the solvent environment, and that nonaqueous solvents induce significant alpha-helical structure.

Binding of the soluble, truncated form of an Fc receptor (mouse Fc gamma RII) to membrane-bound IgG as measured by total internal reflection fluorescence microscopy.

Total internal reflection fluorescence microscopy has been used to investigate the binding of the soluble extracellular domain of mouse Fc gamma RII (sFc gamma RII) to an anti-trinitrophenyl monoclonal mouse IgG2b (GK14.1) specifically bound to substrate-supported planar membranes composed of dipalmitoylphosphatidylcholine (DPPC) and trinitrophenylaminocaproyldipalmitoylphosphatidylethanolamine (TNP-cap-DPPE). The equilibrium dissociation constants for sFc gamma RII at GK14.1-coated TNP-cap-DPPE/DPPC planar membranes containing 0.5-25 mol% TNP-cap-DPPE were approximately 1 microM. Total internal reflection with fluorescence photobleaching recovery was used to examine the dissociation kinetics. The fluorescence recovery curves were better described as a sum of two exponentials rather than by one exponential; the rates and fractional recoveries were approximately 1 s-1 (65%) and approximately 0.1 s-1 (35%). The similarity between the values of these equilibrium and kinetic parameters to those previously measured for the binding of IgG in solution to intact mouse Fc gamma RII reconstituted into planar membranes suggests that conformational changes which may occur when IgG is constrained to a membrane surface do not significantly affect the equilibrium or kinetics of IgG-mouse Fc gamma RII binding. The stoichiometry of sFc gamma RII-GK14.1 binding was 1:4, indicating that a significant fraction of the membrane-bound antibodies were not accessible for receptor binding. Possible mechanisms that might underlay the observed heterogeneity in sFc gamma RII-IgG binding kinetics are discussed.

Total internal reflection fluorescence: applications in cellular biophysics.

Molecular interactions occurring on or near cell membrane surfaces are expected to have different properties from those occurring in bulk solutions. One particularly useful technique for studying surface-associated processes at the molecular level is total internal reflection fluorescence. In this method, the evanescent field from an internally reflected excitation source selectively excites fluorescent molecules on or near a surface. Evanescent excitation has been used recently with a variety of techniques in fluorescence microscopy and spectroscopy to probe the fundamental physicochemical properties of biochemical reactions at natural or model biological surfaces. These studies are providing enhanced understanding of cellular function. Several recent developments in total internal reflection fluorescence methodology from other fields are likely to find future application in cellular biophysics.

Synthetic peptides from mouse Fc receptor (MoFc gamma RII) that alter the binding of IgG to MoFc gamma RII.

Fc receptors are transmembrane proteins, found on the surfaces of immune cells, that aid in the removal of foreign pathogens by binding to antibody-coated targets via the Fc regions of the antibodies. Using peptides synthesized on pins, overlapping dodecapeptides (170) were synthesized to cover the extracellular region of the mouse Fc receptor for IgG, moFc gamma RII. The peptides were screened for antibody binding activity by using multivalent immune complexes composed of anti-dinitrophenyl monoclonal mouse IgG1 (ANO6) and dinitrophenyl conjugated to human serum albumin (DNP-HSA). Assays were also carried out with an anti-moFc gamma RII monoclonal rat IgG (2.4G2). The peptides that interacted with these antibodies prompted the synthesis of two soluble peptides: peptide A [Fc gamma RII-(108-119), RCHSWRNKLLNRamide] and peptide B [Fc gamma RII-(153-165), CKGSLGRTLHQSKamide]. Monomeric S-alkylated (A, B), homodimeric (AA, BB), heterodimeric (AB), and scrambled homodimeric (CC, DD) forms of these peptides were synthesized and examined for their ability to inhibit immune-complex binding to immobilized soluble Fc gamma RII. Peptides AA and CC completely inhibited immune-complex binding while each of the other peptides partially inhibited binding (AB, 80%; A, 80%; BB, 65%; DD, 64%; B, 52%). The pair of monomeric moFc gamma RII peptides and the set of five dimeric peptides showed the same increase in binding inhibition with increasing net positive charge per residue. These results suggest that the Fc region of IgG binds to the solvent-exposed B/C and F/G loops of the moFc gamma RII receptor through predominantly electrostatic forces.