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Despite the initially reported high efficacy of vaccines directed against ancestral SARS-CoV-2, repeated infections in both unvaccinated and vaccinated populations remain a major global health challenge. Because of mutation-mediated immune escape by variants-of-concern (VOC), approved neutralizing antibodies (neutAbs) effective against the original strains have been rendered non-protective. Identification and characterization of mutation-independent pan-neutralizing antibody responses are therefore essential for controlling the pandemic. Here, we characterize and discuss the origins of SARS-CoV-2 neutAbs, arising from either natural infection or following vaccination. In our study, neutAbs in COVID-19 patients were detected using the combination of two lateral flow immunoassay (LFIA) tests, corroborated by plaque reduction neutralization testing (PRNT). A point-of-care neutAb LFIA, NeutraXpress™, was validated using serum samples from historical pre-COVID-19 negative controls, patients infected with other respiratory pathogens, and PCR-confirmed COVID-19 patients. Surprisingly, potent neutAb activity was mainly noted in patients generating both IgM and IgG against the Spike receptor-binding domain (RBD), in contrast to samples possessing anti-RBD IgG alone. We propose that low-affinity, high-avidity, germline-encoded natural IgM and subsequent generation of class-switched IgG may have an underappreciated role in cross-protection, potentially offsetting immune escape by SARS-CoV-2 variants. We suggest Reverse Vaccinology 3.0 to further exploit this innate-like defense mechanism. Our proposition has potential implications for immunogen design, and provides strategies to elicit pan-neutAbs from natural B1-like cells. Refinements in future immunization protocols might further boost long-term cross-protection, even at the mucosal level, against clinical manifestations of COVID-19.

TNF-like weak inducer of apoptosis (TWEAK), a tumor necrosis factor (TNF) family ligand with pleiotropic cellular functions, was originally described as capable of inducing tumor cell death in vitro. TWEAK functions by binding its receptor, Fn14, which is upregulated on many human solid tumors. Herein, we show that intratumoral administration of TWEAK, delivered either by an adenoviral vector or in an immunoglobulin Fc-fusion form, results in significant inhibition of tumor growth in a breast xenograft model. To exploit the TWEAK-Fn14 pathway as a therapeutic target in oncology, we developed an anti-Fn14 agonistic antibody, BIIB036. Studies described herein show that BIIB036 binds specifically to Fn14 but not other members of the TNF receptor family, induces Fn14 signaling and promotes tumor cell apoptosis in vitro. In vivo, BIIB036 effectively inhibits growth of tumors in multiple xenograft models, including colon (WiDr), breast (MDA-MB-231) and gastric (NCI-N87) tumors, regardless of tumor cell growth inhibition response observed to BIIB036 in vitro. The anti-tumor activity in these cell lines is not TNF-dependent. Increasing the antigen-binding valency of BIIB036 significantly enhances its anti-tumor effect, suggesting the contribution of higher order cross-linking of the Fn14 receptor. Full Fc effector function is required for maximal activity of BIIB036 in vivo, likely due to the cross-linking effect or tumor killing activity caused by antibody-dependent cell-mediated cytotoxicity. Taken together, the anti-tumor properties of BIIB036 validate Fn14 as a promising target in oncology and demonstrate its potential therapeutic utility in multiple solid tumor indications.

Recognition of viral antigenic peptides bound to major histocompatibility complex class I molecules (MHCI) by TCR is critical for initiating the responses of CD8(+) T cells that ultimately lead to elimination of virus-infected cells. This antigen recognition is enhanced by the CD8 coreceptor through its interaction with the peptide-MHCI complexes (pMHCI). Mouse CD8alphabeta can form two different complexes with pMHCI via either the CD8alpha- or CD8beta-dominated interaction. To understand the functional significance of these complexes in vivo, we generated Tg mice carrying a variant CD8alphabeta (CD8alpha(m3)beta) capable of forming only the CD8beta-dominated CD8alphabeta/pMHCI complex. These mice show sub-optimal thymic differentiation with reduced populations of CD8(+) single-positive thymocytes. Tg CD8(+) T cells exhibit a compromised developmental capacity when competing with CD8(+) T cells from B6 mice in mixed bone marrow chimera experiments. However, once these CD8(+) T cells have emigrated to the peripheral lymphoid organs, they exhibit normal effector function against viral infection. Our observations indicate that, in addition to the CD8 activity conferred by CD8beta-dominated CD8alphabeta/pMHCI complexes, full thymocyte differentiation requires additional coreceptor activities conferred by CD8alphaalpha and/or CD8alphabeta with CD8alpha-dominated CD8/pMHCI complexes.

Interaction of CD8 (CD8alphaalpha or CD8alphabeta) with the peptide-major histocompatibility complex (MHC) class I (pMHCI) is critical for the development and function of cytolytic T cells. Although the crystal structure of CD8alphaalpha.pMHCI complex revealed that two symmetric CD8alpha subunits interact with pMHCI asymmetrically, with one subunit engaged in more extensive interaction than the other, the details of the interaction between the CD8alphabeta heterodimer and pMHCI remained unknown. The Ig-like domains of mouse CD8alphabeta and CD8alphaalpha are similar in the size, shape, and surface electrostatic potential of their pMHCI-binding regions, suggesting that their interactions with pMHCI could be very similar. Indeed, we found that the CD8alpha variants CD8alpha(R8A) and CD8alpha(E27A), which were functionally inactive as homodimers, could form an active co-receptor with wild-type (WT) CD8beta as a CD8alpha(R8A)beta or CD8alpha(E27A)beta heterodimer. We also identified CD8beta variants that could form active receptors with WT CD8alpha but not with CD8alpha(R8A). This observation is consistent with the notion that the CD8beta subunit may replace either CD8alpha subunit in CD8alphaalpha.pMHCI complex. In addition, we showed that both anti-CD8alpha and anti-CD8beta antibodies were unable to completely block the co-receptor activity of WT CD8alphabeta. We propose that CD8alphabeta binds to pMHCI in at least two distinguishable orientations.

BAFF (B cell activating factor of the TNF family, also known as BlyS and TALL-1), a TNF family cytokine critical for the development and function of B cells, has been reported to bind to three receptors, BCMA (B cell maturation protein), TACI (transmembrane activator and CAML [calcium-modulator and cyclophilin ligand] interactor), and BAFFR (BAFF receptor), but with widely conflicting values for the affinity and selectivity of binding. BCMA and TACI additionally bind APRIL (a proliferation-inducing ligand), the TNF family ligand most homologous to BAFF. Using soluble, monomeric forms of the receptors, we demonstrate that BAFFR binds BAFF with K(D) approximately 16 nM, while BCMA binds with K(D) approximately 1.6 microM, indicating a approximately 100-fold selectivity for binding to BAFFR over BCMA. APRIL shows the opposite selectivity, binding to BCMA with K(D) approximately 16 nM while showing no detectable affinity for BAFFR (K(D)3 microM). The binding of BAFF or APRIL to these receptors is highly sensitive to assay-dependent avidity effects, likely explaining the widely ranging affinity values reported in the literature. Binding of BAFF to BCMA-Fc, a bivalent fusion protein consisting of the extracellular domain of BCMA fused to the hinge and CH1 and CH2 domains of human IgG1, in solution or coated onto an ELISA plate gave apparent binding affinities of approximately 0.63 and approximately 0.15 nM, respectively, compared to values of K(D(app))or= 30 and approximately 100 pM for the corresponding BAFFR/IgG1 fusion protein, BAFFR-Fc. The high selectivity of BAFF for BAFFR versus BCMA is thus partly obscured in these multivalent assays. The intrinsically high selectivity inferred from the measurements with monomeric receptor correlates well with in vivo data from knockout mice, providing a possible explanation for the observations that interruption of the BAFFR gene in the A/WySnJ mouse produces a phenotype similar to the BAFF knockout mouse, while the BCMA knockout mouse has no discernible B cell phenotype.

Peptidoglycan recognition proteins (PGRPs or PGLYRPs) are pattern recognition molecules that are found in insects and mammals and are critical for innate immune responses. PGRPs bind peptidoglycan, a ubiquitous component of bacterial cell walls, and are involved in killing bacteria, degrading peptidoglycan, and initiating host defense reactions. Relatively little is known about the four mammalian PGRPs. In this article, we report the sequences of mouse PglyrpIalpha and PglyrpIbeta and provide details of their expression in wild-type mouse tissues. PglyrpIalpha and PglyrpIbeta are encoded within the epidermal differentiation complex on mouse chromosome 3F. Both genes are expressed in epidermal and hematopoietic tissues. PglyrpIbeta is expressed in each of 16 tissues tested, while PglyrpIalpha expression is limited to fewer tissues, including the lung and spleen as well as several tissues of the digestive system. Both proteins are expressed in epithelial cells throughout the gut, and immunohistochemical staining shows expression in salivary glands, the squamous epithelium of the stomach, and the villi of the jejunum. Immunohistochemical staining further shows expression of both PglyrpIalpha and PglyrpIbeta in macrophages in the spleen. PglyrpIalpha is not expressed in resting RAW264.7 macrophage-like cells, but is induced by stimulation with lipopolysaccharide. PglyrpIbeta is constitutively expressed in RAW264.7 cells and is unaffected by lipopolysaccharide or peptidoglycan stimulation. Computational and experimental data suggest that these proteins are secreted. This work provides a step toward understanding the roles of PglyrpIalpha and PglyrpIbeta in host defense and chronic inflammatory conditions induced by bacteria or their components.

BAFF is considered a therapeutic target because dysregulated production of BAFF can induce systemic lupus erythematosus-like phenotype in mice, and elevated levels of BAFF are associated with disease severity in systemic lupus erythematosus and rheumatoid arthritis patients. Fc fusion decoy receptors, BCMA-Fc and BAFF-R-Fc, are therapeutic candidates for blocking BAFF. While studying their interactions with BAFF, we found that BAFF-R-Fc is more effective than BCMA-Fc for blocking BAFF binding to its receptors. We also found that a trimeric BAFF can bind more than one BAFF-R-Fc but only one BCMA-Fc. Moreover, we show that, in contrast to monovalent BAFF-R-Fc, monovalent BCMA does not form stable complexes with BAFF. Differences in their interaction with BAFF predict BAFF-R-Fc would be a better inhibitor. Indeed, we show BAFF-R-Fc is 10-fold more efficacious than BCMA-Fc for blocking BAFF-induced B cell proliferation in vitro and for blocking BAFF-mediated survival of mouse splenic B lymphocytes in vivo.

Tumor necrosis factor (TNF) ligand and receptor superfamily members play critical roles in diverse developmental and pathological settings. In search for novel TNF superfamily members, we identified a murine chromosomal locus that contains three new TNF receptor-related genes. Sequence alignments suggest that the ligand binding regions of these murine TNF receptor homologues, mTNFRH1, -2 and -3, are most homologous to those of the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors. By using a number of in vitro ligand-receptor binding assays, we demonstrate that mTNFRH1 and -2, but not mTNFRH3, bind murine TRAIL, suggesting that they are indeed TRAIL receptors. This notion is further supported by our demonstration that both mTNFRH1:Fc and mTNFRH2:Fc fusion proteins inhibited mTRAIL-induced apoptosis of Jurkat cells. Unlike the only other known murine TRAIL receptor mTRAILR2, however, neither mTNFRH2 nor mTNFRH3 has a cytoplasmic region containing the well characterized death domain motif. Coupled with our observation that overexpression of mTNFRH1 and -2 in 293T cells neither induces apoptosis nor triggers NFkappaB activation, we propose that the mTnfrh1 and mTnfrh2 genes encode the first described murine decoy receptors for TRAIL, and we renamed them mDcTrailr1 and -r2, respectively. Interestingly, the overall sequence structures of mDcTRAILR1 and -R2 are quite distinct from those of the known human decoy TRAIL receptors, suggesting that the presence of TRAIL decoy receptors represents a more recent evolutionary event.

The prevailing treatment strategies for autoimmune disorders employ global immunosuppressants that have harmful side effects with long-term use. A new vision for drug development relies on the generation of therapeutics that have specific and narrow targets, such as pathogenic cell populations. The cellular processes that initiate and maintain B cell dysregulation are not well understood and autoimmune disease results, in part, from the survival and activation of self-reactive B cells. Such B cells produce tissue-damaging pathogenic autoantibodies. BAFF (B cell-activating factor belonging to the TNF family), a member of the TNF family of ligands, may play a role in B cell-mediated diseases. BAFF is a survival factor for peripheral B cells. When BAFF is overexpressed in mice, B cell number and immunoglobulin production is increased and an autoimmune-like phenotype is observed. Mouse models of lupus-nephritis have been shown to exhibit increased serum BAFF levels correlating with disease severity, and many autoimmune patients were found to have higher levels of circulating BAFF than healthy volunteers. Thus, modulating the level and activity of BAFF in these patients may alleviate symptoms associated with their disease. Several potential therapeutic inhibitors targeting BAFF are under investigation, including an anti-BAFF antibody and receptor-Fc fusion proteins.

Inhibition of secondary immune response in Cynomolgus monkeys. CDP7657 at 5 or 20 mg/kg (60 mg/kg was not evaluated in this study) was compared with hu5c8 at 20 mg/kg. Animals were administered a single dose of antibody or i.v saline. and challenged with tetanus toxoid (TT) on day 1; they then received a second dose of antibody and TT on day 30. Data are expressed as the mean anti-TT IgG titerâ Âąâ standard deviation; approximately 50 % inhibition was observed at 20 mg/kg CDP7657, although this was not statistically significant. ***P

Hu5c8, aglycosyl hu5c8 and CDP7657 are all inactive in the absence of sCD40L, as was sCD40L alone. In vitro platelet aggregation assay of human washed platelets. A Hu5c8 (blue), CDP7657 (black) and aglycosyl hu5c8 (red) did not cause platelet aggregation when introduced to platelets without recombinant human soluble CD40L (rhCD40L). B rhCD40L alone (10 Îźg/ml) was also inactive. (PDF 180 kb)

CDP7657 immune complex ( IC ) and aglycosyl hu5c8 IC do not induce platelet aggregation in rhesus monkey platelets both in the absence of presence of sub-aggregatory amounts of the platelet agonist (ADP). In vitro aggregation assay of rhesus monkey washed platelets. A CDP7657 IC did not aggregate rhesus monkey washed platelets in the absence (blue and red), or presence (black and green) of ADP. B Similarly, aglycosyl hu5c8 IC did not aggregate rhesus monkey washed platelets in the absence (blue and red), or presence (black and green) of ADP. (PDF 171 kb)

The cell surface co-stimulatory protein CD154 (CD40L) is a target for monoclonal antibody (mAb) inhibitors of T-cell mediated immune diseases. This protein, like most other members of the TNF ligand family, forms homotrimeric complexes on the cell surface and in solution, with a three-fold axis of symmetry. We find that several different anti-CD154 monoclonal antibodies form distinctive complexes with soluble CD154. These soluble complexes have been analyzed using size exclusion chromatography, static and dynamic light scattering, and electron microscopy and shown to consist of caged structures of various geometries. The cell surface complexes have been analyzed by confocal microscopy and, depending on the mAb, remain as small, separate complexes or form large aggregates. The formation of these complexes in solution is likely to have an impact on measures of affinity, while the cell surface complexes could affect binding potency and provoke other biological effects.

B cell activating factor (BAFF), a ligand belonging to the tumor necrosis factor (TNF) family, plays a critical role in regulating survival and activation of peripheral B cell populations and has been associated with autoimmune disease. BAFF is known to interact with three receptors, BCMA, TACI and BAFF-R, that have distant similarities with other receptors of the TNF family. We have determined the crystal structure of the TNF-homologous domain of BAFF at 2.8 A resolution. The structure reveals significant differences when compared to other TNF family members, including an unusually long D-E loop that participates in the formation of a deep, concave and negatively charged region in the putative receptor binding site. The BAFF structure was further used to generate a homology model of APRIL, a closely related TNF family ligand that also binds to BCMA and TACI, but not BAFF-R. Analysis of the putative receptor binding sites of BAFF and APRIL suggests that differences in the D-E loop structure and electrostatic surface potentials may be important for determining binding specificities for BCMA, TACI and BAFF-R.

Background: CD40 ligand (CD40L or CD154), a member of the tumor necrosis factor (TNF) family, plays a critical role in both humoral and cellular immune responses and has been implicated in biological pathways involving epithelial cells, fibroblasts, and platelets. Such a pathway is T cell-mediated B cell activation, a process that occurs through the interaction of CD40L with CD40 receptor expressed on B cells. It results in various B cell responses, including immunoglobulin isotype switching and B cell differentiation and proliferation. These responses can be inhibited by the monoclonal antibody 5c8, which binds with high affinity to CD40L. Results: To understand the structural basis of the inhibition, we determined the crystal structure of the complex of the extracellular domain of CD40L and the Fab fragment of humanized 5c8 antibody. The structure shows that the complex has the shape of a three-bladed propeller with three Fab fragments bound symmetrically to a CD40L homotrimer. To further study the nature of the antibody-antigen interface, we assessed the ability of 23 site-directed mutants of CD40L to bind to 5c8 and CD40 and analyzed the results in the context of the crystal structure. Finally, we observed via confocal microscopy that 5c8 binding to CD40L on the cell surface results in the formation of patches of clustered complexes. Conclusions: The structure reveals that 5c8 neutralizes CD40L function by sterically blocking CD40 binding. The antigenic epitope is localized in a region of the surface that is likely to be structurally perturbed as a result of genetic mutations that cause hyper-IgM syndrome. The symmetric trimeric arrangement of the Fab fragments in the complex results in a geometry that facilitates the formation of large clusters of complexes on the cell surface.

X-linked hyper-IgM (XHIM) syndrome is an immunological disorder resulting from mutations in the CD154 gene. Some mutations occur in splicing sites and result in transcripts encoding wild-type and mutant proteins. These mutants lack the tumor necrosis factor homologous (TNFH) domain and consequently fail to trimerize. Given that the TNFH domain is responsible for trimerization, one may predict that the TNFH mutant can not participate in the assembly of wild-type CD154. Thus, it was puzzling why these patients exhibit XHIM phenotype, presumably resulting from a lack of functional CD154. One possibility is that the TNFH mutant exhibits a dominant negative effect over the wild-type protein. To investigate this, we coexpressed the wild-type protein and a TNFH mutant and examined the biochemical and functional properties of the resulting CD154 products. We demonstrate that despite the lack of the TNFH domain, the TNFH mutant can associate with the wild-type protein. Furthermore, such an association compromises the ability of the wild-type protein to mature onto the cell surface. These results provide a mechanism for the defect of CD154 in XHIM patients producing both wild-type and TNFH variants and suggest that besides the TNFH domain, the stalk region participates in the assembly of CD154 trimers.

CD40 Ligand (CD40L) is transiently expressed on the surface of T-cells and binds to CD40, which is expressed on the surface of B-cells. This binding event leads to the differentiation, proliferation, and isotype switching of the B-cells. The physiological importance of CD40L has been demonstrated by the fact that expression of defective CD40L protein causes an immunodeficiency state characterized by high IgM and low IgG serum levels, indicating faulty T-cell dependent B-cell activation. To understand the structural basis for CD40L/CD40 association, we have used a combination of molecular modeling, mutagenesis, and X-ray crystallography. The structure of the extracellular region of CD40L was determined by protein crystallography, while the CD40 receptor was built using homology modeling based upon a novel alignment of the TNF receptor superfamily, and using the X-ray structure of the TNF receptor as a template. The model shows that the interface of the complex is composed of charged residues, with CD40L presenting basic side chains (K143, R203, R207), and CD40 presenting acidic side chains (D84, E114, E117). These residues were studied experimentally through site-directed mutagenesis, and also theoretically using electrostatic calculations with the program Delphi. The mutagenesis data explored the role of the charged residues in both CD40L and CD40 by switching to Ala (K143A, R203A, R207A of CD40L, and E74A, D84A, E114A, E117A of CD40), charge reversal (K143E, R203E, R207E of CD40L, and D84R, E114R, E117R of CD40), mutation to a polar residue (K143N, R207N, R207Q of CD40L, and D84N, E117N of CD40), and for the basic side chains in CD40L, isosteric substitution to a hydrophobic side chain (R203M, R207M). All the charge-reversal mutants and the majority of the Met and Ala substitutions led to loss of binding, suggesting that charged interactions stabilize the complex. This was supported by the Delphi calculations which confirmed that the CD40/CD40L residue pairs E74-R203, D84-R207, and E117-R207 had a net stabilizing effect on the complex. However, the substitution of hydrophilic side chains at several of the positions was tolerated, which suggests that although charged interactions stabilize the complex, charge per se is not crucial at all positions. Finally, we compared the electrostatic surface of TNF/TNFR with CD40L/CD40 and have identified a set of polar interactions surrounded by a wall of hydrophobic residues that appear to be similar but inverted between the two complexes.

Certain monoclonal antibodies (MAbs) directed against CD4 can efficiently block HIV-1 replication in vitro. To explore CD4-directed passive immunotherapy for prevention or treatment of AIDS virus infection, we previously examined the biological activity of a nondepleting CD4-specific murine MAb, mu5A8. This MAb, specific for domain 2 of CD4, blocks HIV-1 replication at a post-gp120-CD4 binding step. When administered to normal rhesus monkeys, all CD4+ target cells were coated with antibody, yet no cell clearance or measurable immunosuppression occurred. However, strong anti-mouse Ig responses rapidly developed in all monkeys. In the present study, we report a successfully humanized form of mu5A8 (hu5A8) that retains binding to both human and monkey CD4 and anti-AIDS virus activity. When administered intravenously to normal rhesus monkeys, hu5A8 bound to all target CD4+ cells without depletion and showed a significantly longer plasma half-life than mu5A8. Nevertheless, an anti-hu5A8 response directed predominantly against V region determinants did eventually appear within 2 to 4 weeks in most animals. However, when hu5A8 was administered to rhesus monkeys chronically infected with the simian immunodeficiency virus of macaques, anti-hu5A8 antibodies were not detected. Repeated administration of hu5A8 in these animals resulted in sustained plasma levels and CD4+ cell coating with humanized antibody for 6 weeks. These studies demonstrate the feasibility of chronic administration of CD4-specific MAb as a potential means of treating or preventing HIV-1 infection.

CD40 ligand (CD40L), a 33-kDa type II membrane glycoprotein expressed primarily on activated CD4+ T lymphocytes, is responsible for the helper function of T cells on resting B cells in a non-antigen-dependent, non-major histocompatability complex-restricted fashion. Interaction of CD40L with its receptor CD40 induces proliferation of and isotype switching in B lymphocytes. Recently we solved the x-ray structure of recombinant soluble CD40L and showed that, similar to other members of the tumor necrosis factor family, CD40L indeed exists as a trimer. We now report that, under normal physiological conditions, CD40L molecules exist as heteromultimeric complexes. These CD40L complexes, made of the full length and smaller fragments of CD40L, are present on the cell surface of T lymphocytes and are capable of interacting with CD40 molecule. A prominent fragment with a mass of 31 kDa accounts for as much as half of the CD40L on the surface of Jurkat cells. N-terminal sequence data revealed that this fragment lacks the cytoplasmic tail. A minor 18-kDa fragment of CD40L was also characterized which lacks the cytoplasmic tail, transmembrane region, and stalk region of the extracellular domain. The presence of CD40L heteromultimeric variants implies an additional regulation of the functional activity of this ligand complex.

In indoor location environment, shadowing, multipath and reflection phenomena make the position estimation inaccurate. Large number of Access Points (APs) equipped with single antenna could obtain enough Receive Signal Strength (RSS) information to describe signal characteristics of the target space. However, it results in a large investment costs. This paper provides a novel indoor positioning solution using the multi-antennas architecture to increase the accuracy of location estimation. The system obtains enough RSS to compute the user location and provides a reasonable cost of deployments in indoor location environment. Experimental and numerical results show that the proposed indoor location estimation demonstrates a high accuracy of positioning and outstanding performance when RSS is collected by smart antenna technique. More specifically, the accuracy is increased from 1.34% to 92.4% and the error variance is reduced by 96% as compared to the traditional location fingerprint method.

Anti-CD40L immunotherapy in systemic lupus erythematosus patients was associated with thromboembolism of unknown cause. We previously showed that monoclonal anti-CD40L immune complexes (ICs) activated platelets in vitro via the IgG receptor (FcγRIIa). In this study, we examined the prothrombotic effects of anti-CD40L ICs in vivo. Because mouse platelets lack FcγRIIa, we used FCGR2A transgenic mice. FCGR2A mice were injected i.v. with preformed ICs consisting of either anti-human CD40L mAb (M90) plus human CD40L, or a chimerized anti-mouse CD40L mAb (hMR1) plus mouse CD40L. ICs containing an aglycosylated form of hMR1, which does not bind FcγRIIa, were also injected. M90 IC caused shock and thrombocytopenia in FCGR2A but not in wild-type mice. Animals injected with hMR1 IC also experienced these effects, whereas those injected with aglycosylated-hMR1 IC did not, demonstrating that anti-CD40L IC-induced platelet activation in vivo is FcγRIIa-dependent. Sequential injections of individual IC components caused similar effects, suggesting that ICs were able to assemble in circulation. Analysis of IC-injected mice revealed pulmonary thrombi consisting of platelet aggregates and fibrin. Mice pretreated with a thrombin inhibitor became moderately thrombocytopenic in response to anti-CD40L ICs and had pulmonary platelet-thrombi devoid of fibrin. In conclusion, we have shown for the first time that anti-CD40L IC-induced thrombosis can be replicated in mice transgenic for FcγRIIa. This molecular mechanism may be important for understanding thrombosis associated with CD40L immunotherapy. The FCGR2A mouse model may also be useful for assessing the hemostatic safety of other therapeutic Abs.

The glycoprotein P-selectin is a cell adhesion molecule of stimulated platelets and endothelial cells, which mediates the interaction of these cells with neutrophils and monocytes. It is a membrane component of cell storage granules, and is a member of the selectin family which includes E-selectin and L-selectin. P-selectin recognizes both lineage-specific carbohydrate ligands on monocytes and neutrophils, including the Lewis x antigen, sialic acid, and a protein component. In inflammation and thrombosis, P-selectin may mediate the interaction of leukocytes with platelets bound in the region of tissue injury and with stimulated endothelium. To evaluate the role of P-selectin in platelet-leukocyte adhesion in vivo, the accumulation of leukocytes within an experimental thrombus was explored in an arteriovenous shunt model in baboons. A Dacron graft implanted within an arteriovenous shunt is thrombogenic, accumulating platelets and fibrin within its lumen. These bound platelets express P-selectin. Here we show that antibody inhibition of leukocyte binding to P-selectin expressed on platelets immobilized on the graft blocks leukocyte accumulation and inhibits the deposition of fibrin within the thrombus. These results indicate that P-selectin is an important adhesion molecule on platelets, mediating platelet-leukocyte binding in vivo, that the presence of leukocytes in thrombi is mediated by P-selectin, and that these leukocytes promote fibrin deposition.

Studies in mice and humans have revealed that the T cell, immunoglobulin, mucin (TIM) genes are associated with several atopic diseases. TIM-1 is a type I membrane protein that is expressed on T cells upon stimulation and has been shown to modulate their activation. In addition to a recently described interaction with dendritic cells, TIM-1 has also been identified as a phosphatidylserine recognition molecule, and several protein ligands have been proposed. Our understanding of its activity is complicated by the possibility that TIM-1 possesses multiple and diverse binding partners. In order to delineate the function of TIM-1, we generated monoclonal antibodies directed to a cleft formed within the IgV domain of TIM-1. We have shown here that antibodies that bind to this defined cleft antagonize TIM-1 binding to specific ligands and cells. Notably, these antibodies exhibited therapeutic activity in a humanized SCID model of experimental asthma, ameliorating inflammation, and airway hyperresponsiveness. Further experiments demonstrated that the effects of the TIM-1–specific antibodies were mediated via suppression of Th2 cell proliferation and cytokine production. These results demonstrate that modulation of the TIM-1 pathway can critically influence activated T cells in a humanized disease model, suggesting that TIM-1 antagonists may provide potent therapeutic benefit in asthma and other immune-mediated disorders.

Background— Tumor necrosis factor–like weak inducer of apoptosis (TWEAK), a member of the tumor necrosis factor superfamily, is a multifunctional cytokine known to regulate cellular functions in contexts of injury and disease through its receptor, fibroblast growth factor–inducible molecule 14 (Fn14). Although many of the processes and downstream signals regulated by the TWEAK/Fn14 pathway have been implicated in the development of cardiac dysfunction, the role of TWEAK in the cardiovascular system is completely unknown. Methods and Results— Herein, we demonstrate that mouse and human cardiomyocytes express the TWEAK receptor Fn14. Furthermore, we determine that elevated circulating levels of TWEAK, induced via transgenic or adenoviral-mediated gene expression in mice, result in dilated cardiomyopathy with subsequent severe cardiac dysfunction. This phenotype was mediated exclusively by the Fn14 receptor, independent of tumor necrosis factor-α, and was associated with cardiomyocyte elongation and cardiac fibrosis but not cardiomyocyte apoptosis. Moreover, we find that circulating TWEAK levels were differentially upregulated in patients with idiopathic dilated cardiomyopathy compared with other forms of heart disease and normal control subjects. Conclusions— Our data suggest that TWEAK/Fn14 may be important in regulating myocardial structural remodeling and function and may play a role in the pathogenesis of dilated cardiomyopathy.

Rats maintained on a low potassium diet develop hypokalemia, which is associated with an approximately 80% decrease in the number of (Na+,K+)-ATPase molecules in skeletal muscle sarcolemma (Norgaard, A., Kjeldsen, K., and Clausen, T. (1981) Nature 293, 739-741); the skeletal muscles of the hypokalemic rats become paralyzed after exposure to insulin in low [K+] media (Otsuka, M., and Ohtsuki, I. (1970) Am. J. Physiol. 219, 1178-1182). We have been interested in the interactions between the insulin receptor and the alpha 2 isoform of the (Na+,K+)-ATPase as a mechanism for the insulin activation of (Na+,K+)-pumping and decided to use the hypokalemic rats to obtain additional information on this question. We show here that the amount of the alpha 2 isoform in the skeletal muscles of hypokalemic rats is greatly decreased as determined by immunoblotting and (Na+,K+)-ATPase activity; the effect of hypokalemia on the amount of the alpha 1 isoform is small. The mechanism of the decrease in the alpha 2 isoform is not known, but it is not due to transcriptional regulation of the alpha 2 gene because the amounts of the transcripts for this polypeptide are increased in the rats on the low potassium diet. The (Na+,K+)-pump that remains in the skeletal muscles of rats on a low potassium diet for a period of 2 weeks is still activated by insulin; under these conditions, however, insulin does not bring about a decrease in the intracellular [Na+] in contrast to the situation with normal muscle.

Inflammation participates in tissue repair through multiple mechanisms including directly regulating the cell fate of resident progenitor cells critical for successful regeneration. Upon surveying target cell types of the TNF ligand TWEAK, we observed that TWEAK binds to all progenitor cells of the mesenchymal lineage and induces NF-kappaB activation and the expression of pro-survival, pro-proliferative and homing receptor genes in the mesenchymal stem cells, suggesting that this pro-inflammatory cytokine may play an important role in controlling progenitor cell biology. We explored this potential using both the established C2C12 cell line and primary mouse muscle myoblasts, and demonstrated that TWEAK promoted their proliferation and inhibited their terminal differentiation. By generating mice deficient in the TWEAK receptor Fn14, we further showed that Fn14-deficient primary myoblasts displayed significantly reduced proliferative capacity and altered myotube formation. Following cardiotoxin injection, a known trigger for satellite cell-driven skeletal muscle regeneration, Fn14-deficient mice exhibited reduced inflammatory response and delayed muscle fiber regeneration compared with wild-type mice. These results indicate that the TWEAK/Fn14 pathway is a novel regulator of skeletal muscle precursor cells and illustrate an important mechanism by which inflammatory cytokines influence tissue regeneration and repair. Coupled with our recent demonstration that TWEAK potentiates liver progenitor cell proliferation, the expression of Fn14 on all mesenchymal lineage progenitor cells supports a broad involvement of this pathway in other tissue injury and disease settings.

B cell-activating factor of the tumor necrosis factor family (BAFF/BLys) plays a critical role in B cell survival and immune responses through its three receptors: BAFF receptor (BAFF-R/BR3), transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) and B cell maturation antigen (BCMA). Using specific antibodies, we have investigated the expression of BAFF-R on human tonsillar B cells and their functional roles in naive and germinal center (GC) B cell differentiation. Our studies show that BAFF-R is the dominant receptor on naive B cells. However, three receptors are differentially modulated during in vitro GC-B cell differentiation. BAFF-R expression increased initially and then decreased with a corresponding induction of TACI and BCMA expression during differentiation to plasma cells (PCs). Consistently, blocking of BAFF-R alone with specific mAb inhibited GC-B cell proliferation and PC generation in the early period of their differentiation, whereas depletion of BAFF with TACI-Ig exhibited consistent inhibition throughout the differentiation. Finally, histological and molecular analyses of human tonsil tissue revealed that follicular dendritic cells produce BAFF. In conclusion, BAFF in the GC plays an important role through more than one receptor, and the three known receptors are differentially modulated as GC-B cells differentiate to PCs.

Members of the tumor necrosis factor (TNF) superfamily regulate cell survival and proliferation and have been implicated in cancer. Tweak (TNF-related weak inducer of apoptosis) has pleiotropic biological functions including proapoptotic, proangiogenic and proinflammatory activities. We explored a role for Tweak in mammary gland transformation using a three-dimensional model culture system. Tweak stimulates a branching morphogenic phenotype, similar to that induced by pro-oncogenic factors, in Eph4 mammary epithelial cells cultured in matrigel. Increased proliferation and invasiveness are observed, with a concomitant inhibition of functional differentiation. Levels of matrix metalloproteinase-9 (MMP-9) are significantly increased following Tweak treatment. Notably, MMP inhibitors are sufficient to block the branching phenotype induced by Tweak. The capacity to promote proliferation, inhibit differentiation and induce invasion suggests a role for Tweak in mammary gland tumorigenesis. Consistent with this, we have observed elevated protein levels of the Tweak receptor, Fn14, in human breast tumor cell lines and xenograft models as well as in primary human breast tumors. Together, our results suggest that the Tweak/Fn14 pathway may be protumorigenic in human breast cancer.

The tumor necrosis factor (TNF) family of related receptors and ligands contains a rich collection of molecules that are important players in a broad spectrum of biological systems. While several family members are critical for development and function of the immune system, providing both activation and death signals, other members are involved in nonimmunological functions as diverse as hair follicle formation. TNF homology searches during the past several years have led to the discovery of numerous novel ligands, two of which will be the focus of this review. BAFF, a cytokine responsible for B cell survival, has recently been the subject of intense investigation that has expanded our understanding of mature B cell genesis, and mechanisms involved in developing B cell pathologies. APRIL is a close relative of BAFF and while its biological roles are less well understood, it may have both immune and non-immune functions. Herein we will discuss the discovery, structure, cognate receptors and functions of these two proteins.

Blockade of the CD154-CD40 co-stimulatory pathway with anti-CD154 mAbs has shown impressive efficacy in models of autoimmunity and allotransplantation. Clinical benefit was also demonstrated in systemic lupus erythematosus (SLE) and idiopathic thrombocytopenia patients with the humanized anti-CD154 mAb, 5C8 (hu5C8). However, thromboembolic complications that occurred during the course of the hu5C8 clinical trials have proven to be a major setback to the field and safe alternative therapeutics targeting the CD154-CD40 pathway are of great interest. Recently, effector mechanisms have been shown to play a part in anti-CD154 mAb-induced transplant acceptance in murine models, while this issue remains unresolved for humoral-mediated models. Herein, aglycosyl anti-CD154 mAbs with reduced binding to FcgammaR and complement were used as a novel means to test the role of effector mechanisms in non-human primate and murine models not amenable to gene knockout technology. While aglycosyl hu5C8 mAb was relatively ineffective in rhesus renal and islet allotransplantation, it inhibited primary and secondary humoral responses to a protein immunogen in cynomolgus monkeys. Moreover, an aglycosyl, chimeric MR1 mAb (muMR1) prolonged survival and inhibited pathogenic auto-antibody production in a murine model of SLE. Thus, the mechanisms required for efficacy of anti-CD154 mAbs depend on the nature of the immune challenge.

The tumor necrosis factor (TNF) family of related receptors and ligands contains a rich collection of molecules that are important players in a broad spectrum of biological systems. While several fa

Human monocytes displayed increased expression of CD40 following infection with virulentMycobacterium tuberculosis. Nevertheless, soluble CD40 ligand (CD40L; also designated CD154) had no effect on the intracellular growth of the organism. Restriction of the intracellular growth ofM. tuberculosisby peripheral blood lymphocytes and antigen-specific CD4+T-cell lines likewise was not reduced by blocking anti-CD40L monoclonal antibody 5c8.

Alefacept, an immunomodulatory recombinant fusion protein composed of the first extracellular domain of LFA-3 fused to the human IgG1 hinge, CH2, and CH3 domains, has recently been shown in phase II and III clinical trials to safely reduce disease expression in patients with chronic plaque psoriasis. Alefacept modulates the function of and selectively induces apoptosis of CD2+ human memory-effector T cells in vivo. We have sought to gain further understanding of the mechanisms of action that influence the biological activity of alefacept and may contribute to its efficacy and patient responsiveness. Specifically evaluated is the ability of alefacept to activate intracellular signals mediated via CD2 and/or FcγRIII (CD16). Experimentation using isoforms of alefacept engineered to have amino acid substitutions in the IgG1 CH2 domain that impact FcγR binding indicate that alefacept mediates cognate interactions between cells expressing human CD2 and CD16 to activate cells, e.g., increase extracellular signal-regulated kinase phosphorylation, up-regulate cell surface expression of the activation marker CD25, and induce release of granzyme B. In the systems used, this signaling is shown to require binding to CD2 and CD16 and be mediated through CD16, but not CD2. Experimentation using human CD2-transgenic mice and isoforms of alefacept confirmed the requirement for FcγR binding for detection of the pharmacological effects of alefacept in vivo. Thus alefacept acts as an effector molecule, mediating cognate interactions to activate FcγR+ cells (e.g., NK cells) to induce apoptosis of sensitive CD2+ target cells.

Angiogenic regulators modulate endothelial cell functions, including proliferation, migration, secretion, and adhesion, through their action on endothelial cells or other cell types. TWEAK, a novel member of the tumor necrosis factor family, appears to be a pro-angiogenic agent on the basis of previous studies demonstrating its ability to induce interleukin-8 production by epithelial tumor lines, stimulate proliferation of human vascular cell types and neovascularization in rat corneas. Here, we further characterized the angiogenic potential of TWEAK, revealing a dual role for TWEAK as an angiogenic regulator. We demonstrate that TWEAK is a potent inducer of endothelial cell survival and cooperates with basic fibroblast growth factor to induce the proliferation and migration of human endothelial cells and morphogenesis of capillary lumens. In contrast, TWEAK antagonizes the morphogenic response of endothelial cells to vascular endothelial growth factor (VEGF) without inhibiting VEGF-induced survival or proliferation. Thus, our observations suggest that TWEAK may differentially regulate microvascular growth, remodeling and/or maintenance in vivo, depending upon the angiogenic context.

X-linked hyper-IgM syndrome is a rare immunodeficiency disorder resulting from mutations in the gene encoding the CD40 ligand (CD154) molecule. These mutations are very heterogeneous, ranging from a single point mutation to a large deletion in the open reading frame. To investigate the molecular mechanisms that are responsible for the functional defect of these mutants, we examined the biochemical properties of 14 hyper-IgM-related CD154 mutant proteins produced by transient expression in COS7 cells. We show that deletion mutants lacking a significant portion of the tumor necrosis factor homologous domain cannot be stably produced. In contrast, point mutants can be detected as oligomers. Surprisingly, gene products of two point mutants, Thr-211 → Asp and Met-36 → Arg, can bind to the receptor, CD40. For Thr-211 → Asp variant, it is comparable to the wild-type protein in its surface expression level, biochemical structure, and functional activities. Thus, it appears that this mutation is a polymorphism of CD154 gene. For Met-36 → Arg variant, although it is interactive with CD40, it has a much lower surface expression level than wild-type protein. We propose that Met-36 → Arg mutant represents a prototype of a defective CD154 family whose low cell surface expression of intrinsically active protein is simply insufficient to trigger productive signals through CD40.

Backgound The CD40 ligand (CD40L) is a member of the tumor necrosis factor (TNF) family of proteins and is transiently expressed on the surface of activated T cells. The binding of CD40L to CD40, which is expressed on the surface of B cells, provides a critical and unique pathway of cellular activation resulting in antibody isotype switching, regulation of apoptosis, and B cell proliferation and differentiation. Naturally occurring mutations of CD40L result in the clinical hyper-IgM syndrome, characterized by an inability to produce immunoglobulins of the IgG, IgA and IgE isotypes. Results We have determined the crystal structure of a soluble extracellular fragment of human CD40L to 2 a resolution and with an R factor of 21.8%. Although the molecule forms a trimer similar to that found for other members of the TNF family, such as TNF α and lymphotoxin- α , and exhibits a similar overall fold, there are considerable differences in several loops including those predicted to be involved in CD40 binding. Conclusion: The structure suggests that most of the hyper-IgM syndrome mutations affect the folding and stability of the molecule rather than the CD40-binding site directly. Despite the fact that the hyper-IgM syndrome mutations are dispersed in the primary sequence, a large fraction of them are clustered in space in the vicinity of a surface loop, close to the predicted CD40-binding site.

Abstract 5128 Objectives: Hu5c8, a whole IgG1 anti-CD40L antibody, is known to cause thrombotic side effects in patients with Systemic Lupus Erythematosus (SLE). Anti-CD40L-mediated activation of platelets in vitro was shown to be FcγRIIa-dependent1, suggesting that the adverse effects of hu5c8 could be due to this mechanism, and that anti-CD40L antibodies lacking a functional Fc region would not activate platelets. CDP7657 is a monovalent Fab' PEG anti-CD40L antibody being developed for the treatment of SLE. The aim of the present work was to study a range of anti-CD40L antibody formats in isolated platelet assays and to evaluate the role of the Fc region and of valency. Methods: Isolated human platelets from healthy donors were used in the platelet aggregometry and serotonin release assay systems. Antibodies were added either alone, or pre-complexed with soluble CD40L trimer (sCD40L) at optimal stoichiometry. Antibody IV.3 was used to block FcγRIIa. The antibodies and antibody fragments tested included hu5c8 in IgG1, aglycosyl IgG, Fab′, & (Fab′)2 formats, CDP7657, and the CDP7657 variable regions in di-Fab' PEG, tri-Fab' PEG and aglycosyl formats. Results: Hu5c8 in complex with sCD40L caused strong aggregation (in 2–3 minutes) and >85% serotonin release, both alone and in complex with sCD40L. This activation was completely abrogated by blocking FcγRIIa. In contrast, platelet activation did not occur with CDP7657 or other antibody formats, in any assay, regardless of the valency or presence of sCD40L. Conclusions: The data support the hypothesis that platelet activation by anti-CD40L antibodies is dependent on the presence of an active Fc moiety, via interactions with FcγRIIa, and indicate that CDP7657 has the potential to deliver the therapeutic effect of anti-CD40L blockade in SLE without thrombotic toxicity. References:(1) Langer F et al. 2005; Thromb Haemost 93: 1137. Disclosures: Amirkhosravi: UCB: Research Funding.

Monoclonal antibodies are increasingly used as therapeutics, and some (anti-VEGF, anti-CD40L) have produced unexpected thrombotic side effects. For example, clinical trials with the humanized monoclonal CD40L antibody, hu5c8, in patients with systemic lupus erythematosus (SLE, a chronic inflammatory autoimmune disease) were prematurely terminated due to thromboembolic events. In SLE, elevated soluble CD40L (sCD40L) antigen levels may indicate in vivo platelet activation, putting patients at increased risk for thrombosis. We therefore hypothesized that anti-CD40L immunotherapy may represent an additional prothrombotic hit, leading to clinical manifestation of thromboembolic disease in patients with SLE. In this report, we provide experimental evidence that hu5c8, when combined with homotrimeric recombinant human sCD40L (rhsCD40L) in 1:3 molar ratio to allow formation of higher-order immune complexes (IC), activates platelets in an FcγRIIA-dependent manner. In a washed platelet system, IC formed of rhsCD40L (5 μg/mL) with hu5c8 (15 μg/mL) strongly induced platelet aggregation, while IC of rhsCD40L with hu5c8-Fab or hu5c8-F(ab’)2 did not. Similarly, IC of rhsCD40L with hu5c8-agly, an aglycosylated hu5c8 variant with reduced Fc domain binding to the low affinity platelet IgG receptor, FcγRIIA (CD32), failed to induce aggregation. FcγRIIA dependency was further demonstrated with the monoclonal CD32 antibody, IV.3. These results suggest that hu5c8-mediated platelet activation requires both antigen (CD40L) and the IgG Fc domain. Flow cytometric analysis of P-selectin (CD62P) confirmed that IC of rhsCD40L with hu5c8, but not with Fab, F(ab’)2, or hu5c8-agly, strongly activated platelets via FcγRIIA. When washed platelets were first sensitized with ADP, resulting in surface exposure of endogenous CD40L and sub-maximal aggregation of 5–25%, hu5c8 (10 μg/mL) induced strong platelet aggregation of up to 80–100% in the absence of exogenous rhsCD40L, indicating that physiologic levels of CD40L were sufficient for hu5c8-mediated platelet activation. ADP-sensitized platelets did not respond to equimolar concentrations of Fab, F(ab’)2, or hu5c8-agly. In the serotonin release assay (SRA), which is used for the detection of FcγRIIA-activating heparin-platelet factor 4 (PF4) autoantibodies in patients with heparin-induced thrombocytopenia (HIT), the experimental procedure generates close platelet-platelet contacts and causes release of α-granule proteins such as CD62P, PF4, and CD40L. In the absence of rhsCD40L, hu5c8, but not Fab, F(ab’)2, or hu5c8-agly, induced significant dense granule release, suggesting that exogenous rhsCD40L was not an absolute requirement for hu5c8-mediated platelet activation. In summary, our results show that the humanized monoclonal CD40L antibody, hu5c8, can strongly activate platelets in an Fc domain-dependent manner and that physiologically released CD40L is sufficient for this activity. This mechanism of activation is similar to that of HIT, in which homotetrameric PF4 released from prestimulated platelets provides the molecular basis for clustering of, and signaling through, FcγRIIA, resulting in a vicious cycle of platelet activation. Because thrombosis is a serious complication of HIT, it is reasonable to speculate that hu5c8-mediated platelet activation contributed to the pathophysiology of thromboembolic events in SLE patients receiving anti-CD40L immunotherapy.

Multiple forms of the catalytic subunit of the (Na+,K+)ATPase have been identified in rat brain. While two of them (alpha 1 and alpha 2) have been well characterized, the third form (alpha 3) of these catalytic subunits only recently has been described by cDNA cloning; the corresponding polypeptide has not been isolated. In this paper it is shown that rat brain contains the alpha 3 chain. The catalytic subunits of the (Na+, K+)ATPase from rat brain axolemma were purified by SDS-PAGE and subjected to formic acid cleavage. Amino acid sequence analysis of the resulting fragments revealed that axolemma has the alpha 3 form of the catalytic subunit. In addition, alpha 3-specific antiserum was raised in rabbits immunized with a synthetic peptide. Immunoblotting with this antiserum revealed that the alpha 3 form of the (Na+,K+)ATPase is present also in whole brain microsomes. In SDS-PAGE, the mobilities of the three catalytic subunits of brain (Na+, K+)ATPase follow the order alpha 1 greater than alpha 2 greater than alpha 3. Determination of the ouabain-inhibitable ATPase activity indicates that if the alpha 3 form of the (Na+,K+)ATPase is able to hydrolyze ATP, it is present in a form of the enzyme with a high affinity for this cardiac glycoside and is similar to the alpha 2 form in this respect.

A long-term research project toward Mandarin speech recognition techniques for very large vocabulary and unlimited text is considered. By carefully examining the special structures of Chinese language, the first-stage goal is set to be the design of efficient techniques to recognize the finals of Mandarin syllables. In this paper, three special approaches to do this are proposed. The Segmental Model Approach defines the final models by dividing the finals into several segments according to the acoustic structures of the speech signals. The Three-pass Approach uses three consecutive passes to classify the finals into small sets and improve the recognition efficiency. The Multi-section Vector Quantization (MSVQ) Approach, on the other hand, significantly reduces the necessary computation time by incorporating the branch-and-bound algorithm and common codebook concept with the MSVQ techniques. Extensive computer simulations are performed first to optimize each approach by choosing the best set of parameters then to compare the performance of the three approaches. It was found that all the three approaches are very efficient in terms of relatively high recognition rate and short computation time, and the MSVQ Approach provides the highest recognition rate at the shortest computation time, thus it is most attractive.

Endogenous inhibitors of cell proliferation have been purified from several sources, including normal tissues and conditioned medium from epithelial and fibroblast cell lines. Structural and functional studies on these molecules and their receptors will provide new insights into the significance of growth inhibitors in cellular homeostasis.

The mouse fibroblast line 3T3 (1) exhibits a form of growth control in vitro in that it reaches only a very low saturation density and can remain for long periods of time in a viable but nondividing state. This phenomenon, termed density-dependent inhibition of growth (2), was first described and most extensively studied in the 3T3 cell line. In this paper we review some recent evidence indicating that endogenous growth inhibitors may account for at least part of the growth inhibition observed in 373 fibroblast cultures and that such inhibitors may be isolated and characterized in terms of their functional interactions with target cells. The results of these studies suggest several useful refinements in our analysis of the mechanisms of growth control in cultured cells.