Tumor necrosis factor (TNF) can induce a variety of cellular responses at low picomolar concentrations. This is in apparent conf lict with the published dissociation constants for TNF binding to TNF receptors in the order of 100-500 pM. To elucidate the mechanisms underlying the outstanding cellular sensitivity to TNF, we determined the binding characteristics of TNF to both human TNF receptors at 37°C. Calculation of the dissociation constant (K d ) from the association and dissociation rate constants determined at 37°C revealed a remarkable high affinity for TNF binding to the 60-kDa TNF type 1 receptor (TNF-R1; K d ‫؍‬ 1.9 ؋ 10 ؊11 M) and a significantly lower affinity for the 80-kDa TNF type 2 receptor (TNF-R2; K d ‫؍‬ 4.2 ؋ 10 ؊10 M). The high affinity determined for TNF-R1 is mainly caused by the marked stability of ligand-receptor complexes in contrast to the transient interaction of soluble TNF with TNF-R2. These data can readily explain the predominant role of TNF-R1 in induction of cellular responses by soluble TNF and suggest the stability of the TNF-TNF receptor complexes as a rationale for their differential signaling capability. In accordance with this reasoning, the lower signaling capability of homotrimeric lymphotoxin, compared with TNF, correlates with a lower stability of the lymphotoxin-TNF-R1 complex at 37°C.Tumor necrosis factor (TNF) is a pleiotropic cytokine that is a major mediator of immunological and pathophysiological reactions. As with many other cytokines, the concentration of TNF found in body fluids in pathophysiological situations is usually very low (1, 2). Various cellular responses of cultured cells can be initiated by low picomolar or even femtomolar concentrations of TNF (3-6). Because the binding of TNF to its cell surface receptors is a prerequisite for TNF responses, high-affinity binding sites for TNF with affinity constants in the same range should be assumed.Two distinct membrane receptors for TNF (TNF-Rs) with apparent molecular weights of 55-60 kDa (TNF-R1) and 70-80 kDa (TNF-R2) have been identified and molecularly cloned (for review, see ref. 7). Both TNF-Rs have been objects of intense physiological and biochemical investigations. Likewise, the ligand binding properties of the TNF-Rs have been extensively studied (8-11) although a detailed analysis became feasible only after the molecular cloning of the two individual receptor molecules. Equilibrium binding studies with 125 Ilabeled TNF ( 125 I-TNF) at 0°C defined high-affinity binding of TNF to both TNF-Rs with K d values of approximately 300-600 pM for TNF-R1 and 70-200 pM for TNF-R2 (12-18).Although most cell lines and primary tissues coexpress both receptor types, cellular responses to the soluble 17-kDa form of TNF seems to be dominated by the interaction with TNF-R1 (19-21). On the other hand, we have shown recently in different cellular systems that TNF-R2 can be strongly stimulated by the 26-kDa transmembrane form of TNF (transmembrane TNF) rather than by the soluble form, suggesting that transmemb...

Tumor necrosis factor (TNF) exists both as a membrane-integrated type II precursor protein and a soluble cytokine that have different bioactivities on TNFR2 (CD120b) but not on TNFR1 (CD120a). To identify the molecular basis of this disparity, we have investigated receptor chimeras comprising the cytoplasmic part of Fas (CD95) and the extracellular domains of the two TNF receptors. The membrane form of TNF, but not its soluble form, was capable of inducing apoptosis as well as activation of c-Jun N-terminal kinase and NF-B via the TNFR2-derived chimera. In contrast, the TNFR1-Fas chimera displayed strong responsiveness to both TNF forms. This pattern of responsiveness is identical to that of wild type TNF receptors, demonstrating that the underlying mechanisms are independent of the particular type of the intracellular signaling machinery and rather are controlled upstream of the intracellular domain. We further demonstrate that the signaling strength induced by a given ligand/receptor interaction is regulated at the level of adaptor protein recruitment, as shown for FADD, caspase-8, and TRAF2. Since both incidents, strong signaling and robust adapter protein recruitment, are paralleled by a high stability of individual ligand-receptor complexes, we propose that half-lives of individual ligand-receptor complexes control signaling at the level of adaptor protein recruitment.

Adeno-associated virus (AAV) vectors currently represent the most attractive platform for viral gene therapy and are also valuable research tools to study gene function or establish disease models. Consequently, many academic labs, core facilities, and biotech/pharma companies meanwhile produce AAVs for research and early clinical development. Whereas fast, universal protocols for vector purification (downstream processing) are available, AAV production using adherent HEK-293 cells still requires time-consuming passaging and extensive culture expansion before transfection. Moreover, most scalable culture platforms require special equipment or extensive method development. To tackle these limitations in upstream processing, this study evaluated frozen high-density cell stocks as a ready-to-seed source of producer cells, and further investigated the multilayered CELLdisc culture system for upscaling. The results demonstrate equal AAV productivity using frozen cell stock–derived cultures compared to conventionally cultured cells, as well as scalability using CELLdiscs. Thus, by directly seeding freshly thawed cells into CELLdiscs, AAV production can be easily upscaled and efficiently standardized to low-passage, high-viability cells in a timely flexible manner, potentially dismissing time-consuming routine cell culture work. In conjunction with a further optimized iodixanol protocol, this process enabled supply to a large-animal study with two high-yield AAV2 capsid variant batches (0.6–1.2 × 1015 vector genomes) in as little as 4 weeks.

The inflammatory and proapoptotic cytokine TNF possesses a compelling potential as an antitumoral therapeutic agent. Possible target cells include the malignant cells themselves, the tumor vasculature, or the immune system. As the clinical use of TNF is limited by systemic toxicity, targeting strategies using TNF-based fusion proteins are currently used. A major obstacle, however, is that homotrimeric TNF ligands are prone to activity loss due to dissociation into their monomers. In this study, we report the construction of single-chain TNF molecule, a TNF mutant consisting of three TNF monomers fused by short peptide linkers. In comparison to wild-type TNF, single-chain TNF was found to possess increased stability in vitro and in vivo, displayed reduced systemic toxicity yet slightly enhanced antitumoral activity in mouse models. Creation of single-chain variants is a new approach for improvement of functional activity of therapeutics based on TNF family ligands.

Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine for which a receptor has not been identified. That MIF has intracellular functions has been suggested by its enzymatic activity and constitutive expression profile. The discovery of functional MIF-c-Jun activation domain binding protein 1 (JAB1) binding has confirmed this notion and indicated that nonreceptor-based signaling mechanisms are important for MIF function. Here, we have generated and tested several biologically active labeled MIF derivatives to further define target protein binding by MIF and its cellular uptake characteristics. (35)S-MIF, biotinylated MIF, and fluoresceinated MIF were demonstrated to exhibit full biologic activity. Neither by applying a standard iodinated MIF preparation nor by using the biologically active (35)S-MIF derivative in receptor-binding studies were we able to measure any receptor-binding activity on numerous cells, confirming that uptake of MIF into target cells and MIF signaling can occur by receptor-independent pathways. When MIF derivatives were applied in cellular uptake studies, MIF was found to be endocytosed into both immune and nonimmune cells and targeted to the cytosol and lysosomes. The entry of MIF was temperature and energy dependent and was inhibited by monodansylcadaverine but not by ouabain. Endocytosed biotin-MIF bound JAB1 not only in macrophages, as shown previously, but also in nonimmune cells. A tagged MIF construct, MIF-enhanced green fluorescent protein (EGFP), was shown to be a valuable tool, as EGFP constructs of critical MIF cysteine mutants exhibited identical cellular localization properties to those of wild-type MIF (wtMIF). Our results indicate that MIF membrane receptors are not widely expressed, if at all, and suggest that the cellular uptake of MIF occurs by nonreceptor-mediated endocytosis rather than penetration. All the derivatives investigated, except for iodinated MIF, represent valuable tools for further MIF target protein and cellular studies.