Interleukin-2 self-association

Impact of product-related factors on immunogenicity of biotherapeutics

All protein therapeutics have the potential to be immunogenic. Several factors, including patient characteristics, disease state, and the therapy itself, influence the generation of an immune response. Product-related factors such as the molecule design, the expression system, post-translational modifications, impurities, contaminants, formulation and excipients, container, closure, as well as degradation products are all implicated. However, a critical examination of the available data shows that clear unequivocal evidence for the impact of these latter factors on clinical immunogenicity is lacking. No report could be found that clearly deconvolutes the clinical impact of the product attributes on patient susceptibility. Aggregation carries the greatest concern as a risk factor for immunogenicity, but the impact of aggregates is likely to depend on their structure as well as on the functionality (e.g., immunostimulatory or immunomodulatory) of the therapeutic. Preclinical studies are not yet capable of assessing the clinically relevant immunogenicity potential of these product-related factors. Simply addressing these risk factors as part of product development will not eliminate immunogenicity. Minimization of immunogenicity has to begin at the molecule design stage by reducing or eliminating antigenic epitopes and building in favorable physical and chemical properties.

Formulation of liposomes associated with recombinant interleukin-2: effect on interleukin-2 activity

Association of the cytokine interleukin-2 (rIL-2) within liposomes could prolong its circulating half-life and thus reduce side-effects and improve its efficacy in cancer and AIDS treatment. The effects of physical procedures used in liposome preparation on the biological activity of rIL-2 were determined. While heating to 50 degrees C reduced the activity of IL-2 in the CTLL-2 proliferation assay by 50%, sonication, either bath or probe, was less detrimental. The combination of all three treatments resulted in only 10% loss of activity. Probe sonication led to the appearance of dimers which were stable under reducing conditions. Small unilamellar and large unilamellar liposomes were formed, respectively, by probe sonication or extrusion of multilamellar vesicles of dipalmitoylphosphatidylcholine hydrated in the presence of rIL-2. A high proportion of the rIL-2 was associated with the vesicles. However, the biological activity of the liposome-associated rIL-2 was reduced 7- to 10-fold compared with control rIL-2. rIL-2 dimers were formed on contact with lipid, even without sonication. We can conclude that the association of rIL-2 with lipid masks its access to its cell-surface receptor at least under cell culture conditions.

The development of technetium-99m-labelled interleukin-2: a new radiopharmaceutical for the in vivo detection of mononuclear cell infiltrates in immune-mediated diseases

We describe here a new method for labelling interleukin-2 (IL-2) in high specific activity with 99mTc for in vivo studies in man. Labelling was performed via a two-step reaction using an N3S bifunctional chelating agent. To optimise the reaction, factors affecting the incorporation of 99mTc into the N3S ligand were studied. The conjugation of the preformed N3S chelate ligand to IL-2 was then similarly optimised. Various strategies for purifying the 99mTc-IL-2 were explored including size-exclusion, ion-exchange, and several modes of reversed-phase chromatography. The radiochemical purity of the purified protein was determined by HPLC, ITLC, TCA precipitation, and SDS-PAGE. The receptor binding capacity of 99mTc-IL-2 was studied. Biodistribution studies in normal mice were performed with 99mTc-IL-2 purified using different techniques or labelled after prolonged storage and compared to 125I-IL-2.

The effect of pH and temperature on the self-association of recombinant human interleukin-2 as studied by equilibrium sedimentation

The self-association of recombinant human interleukin-2 (rhIL-2) in solution was investigated as a function of pH and temperature using equilibrium sedimentation. Studies were performed at pH 3.6, 6.5 and 8.2, at 1 degree C and 20 degrees C. A model assuming an ideal single molecular species describes the data observed at pH 6.5 at both temperatures. At pH 8.2, the data from both temperatures can be better described by a weak monomer-dimer association equilibrium. The values of the association constants obtained indicate the presence of less than 10% dimer at a concentration of 1 mg/ml at both temperatures. At pH 3.6, aggregates with a Z average molecular weight of over 35 times that of monomeric rhIL-2 were formed. The smallest associating species present under these conditions corresponds to the monomer, which produces aggregates with a wide range of molecular weights. The monomer appears to be in equilibrium with the smallest aggregates, in that a model describing an indefinite association fits the data obtained at the highest centrifugal speed. No model was found to successfully describe the association of the monomer into the much larger aggregates observed at lower speeds. This may be the result of the lack of rapid thermodynamic reversibility of the larger aggregates. Temperature was found to have no significant effect on the largest aggregates that were formed at pH 3.6.

Delivery of interleukin 2 for immunotherapy

The local production of interleukin 2 (IL-2) by T lymphocytes acts to enhance the immune response by inducing growth and differentiation of a variety of immune cells. In clinical situations that require immunostimulation, such as vaccination and enhancement of tumor immunity, IL-2 therapy has been considered; however, the extraordinary toxicity of the drug inoculated systemically has greatly limited its application. Since the most serious toxic consequences of the drug are related to its systemic delivery, alternative strategies have been developed. Local delivery of the cytokine has been successfully used in some circumstances, and this form of delivery does not result in the life-threatening complications that limit systemic use. Liposome encapsulated IL-2 represents a mechanism to accentuate local delivery by causing a depot effect. Finally, the use of IL-2 has been predicated on the conception of the cytokine as an absolute monomer. Nevertheless, IL-2 spontaneously forms noncovalent and covalent self-associations. Because covalent dimers have been shown to initiate differential signalling in target cells, it is necessary to account for this property in devising and evaluating therapeutic protocols; moreover, it seems possible to use this property for modifying and regulating the therapeutic response.

Detection of intracellular interleukin 2: evidence for novel immunologically related forms of the lymphokine

At present, few data are available on intracellular interleukin 2 (IL-2) and its posttranscriptional regulation. Unlike other lymphokines, IL-2 does not accumulate within the cell, but is rapidly secreted following its production. The process of detection and biochemical characterization of intracellular IL-2 involved using a high producer subclone of the Jurkat T-lymphoma line as a source for IL-2, in combination with a two-step separation protocol and a sensitive detection method. Following phytohemagglutinin (PHA)/4 beta-phorbol 12-myristate 13 acetate (TPA) stimulation, a 14 kDa molecule could be visualized in Western blots by means of two monoclonal anti-IL-2 antibodies possessing different epitope specificities. This molecule exhibited biological activity of IL-2 as determined by a murine cytotoxic T-cell proliferation assay. In addition to this biologically active form of the lymphokine, a strongly immunoreactive protein with a molecular weight of 54 kDa (P54) was found in Jurkat cell lysates. Further biochemical characterization of this intracellular variant revealed an isoelectric point similar to that of secreted forms of IL-2. All attempts to split the 54 kDa molecule into smaller subunits failed, and no biological IL-2 activity could be measured in response to P54. However, the appearance of this high molecular weight variant followed clear-cut time kinetics. The highest concentration of P54 was found to occur after 2 h of stimulation. Thereafter its concentration decreased continuously, while the amount of the biologically active 14 kDa variant increased under ongoing stimulation. One possible explanation for these results is that P54 may represent an immature form of IL-2 that is tightly linked to a carrier molecule.

Solid phase interleukin 2

Interleukin 2 was bound to Sepharose beads in order to assess its potential bioactivity in vitro. Although the cytokine was attached to the solid phase via a standard chemical reaction for covalent coupling, it was spontaneously released into culture medium in an active form during short-term incubation. Release was absolutely dependent on the availability of soluble protein in the incubation medium, and it was greater at 37 than at 4 degrees C. Approximately 80% of the cytokine attached to the Sepharose beads was recovered in the medium after appropriate incubation for several days. The release of interleukin 2 from the solid phase resulted from the dissolution of aggregated cytokine. Noncovalent self-aggregation of interleukin 2 onto solid surfaces represents an unusual biochemical property that has implications for the molecular interactions of the cytokine with the solid phase, for the use of bound interleukin 2 in preparative or analytical modes, and for the development of immunotherapy for patients with solid tumors.

Monitoring of protein conformation by high-performance size-exclusion liquid chromatography and scanning diode array second-derivative UV absorption spectroscopy

Genetic methods now allow the rapid production of mutant proteins for structure-function analysis. To properly interpret any change in biologic activity resulting from modification in primary sequence, it is essential to monitor conformational changes resulting from mutations. Several methods allow low-resolution protein conformational analysis. One method, second-derivative UV absorption spectroscopy, is particularly useful for proteins containing tyrosine and/or tryptophan residues. Using high-performance size-exclusion liquid chromatography and scanning diode array detection we have demonstrated that it is possible to monitor the degree of aggregation as well as conformational perturbation for a series of interleukin-2 structural mutants. Furthermore, the combination of high-performance liquid chromatography and second-derivative UV absorption spectroscopy avoids a potential artifactual contribution in non-chromatographic analysis due to protein aggregation.

Conformational perturbation of interleukin-2: a strategy for the design of cytokine analogs

Interleukin-2 (IL-2) is a representative of a growing family of small proteins termed lymphokines which are responsible for mediating cell differentiation, growth and function in the immune system. Many of these proteins are being evaluated for their clinical potential. From the perspective of drug development, structure-function analysis of these molecules and their receptors require the use methodologies different than those traditionally employed for small peptides and other natural products. However, similar pharmacologic principles apply and an understanding of ligand-receptor interactions and the associated responses is required in order to efficiently pursue agonist and antagonist design. Although IL-2 is a protein of only 133 amino acid residues for which a low resolution X-ray structure does exist, the complexity of its receptor system has provided an added challenge to structure-function studies. Consequently, little is known concerning the receptor contact residues for this protein. We have attempted to utilize established principles of protein and peptide structure to manipulate the conformation of IL-2 in a manner which has provided analogs helpful for receptor interaction studies. These proteins have not only providing useful information on the nature of the IL-2 receptor but have also revealed potential strategies for the design of IL-2 agonists and antagonists.

Macromolecular association of ADP-ribosyltransferase and its correlation with enzymic activity

The macromolecular self-association of ADP-ribosyltransferase protein in solution was studied by several experimental techniques: quantitative gel filtration, electrophoretic analyses in non-denaturing gels, and cross-linking the enzyme protein with glutaraldehyde, dimethyl pimelimidate, dimethyl suberimidate, dimethyl 3,3'-dithiobisproprionimidate and tetranitromethane. The self-association of the polypeptide components obtained by plasmin digestion was also determined by using the above cross-linking agents. Monomers and cross-linked dimers of the enzyme protein, possessing enzymic activity, were separated in non-denaturing gels by electrophoresis. The basic polypeptide fragments, exhibiting molecular masses of 29 kDa and 36 kDa, self-associated, whereas the polypeptides with molecular masses of 56 kDa and 42 kDa associated only to a negligible extent, indicating that the peptide regions that also bind DNA and histones are probable sites of self-association in the intact enzyme molecule. Macromolecular association of the enzyme was indicated by a protein-concentration-dependent red-shift in protein fluorescence. The specific enzymic activity of the isolated ADP-ribosyltransferase depended on the concentration of the enzyme protein, and at 2.00 microM concentration the enzyme was self-inhibitory. Dilution of the enzyme protein to 30-40 nM resulted in a large increase in its specific activity. Further dilution to 1-3 nM coincided with a marked decrease of specific activity. Direct enzymic assays of electrophoretically separated monomers and cross-linked dimers demonstrated that the dimer appears to be the active molecular species that catalyses poly(ADP-ribose) synthesis. The NAD+ glycohydrolase activity of the enzyme was also dependent on protein concentration and was highest at 1-3 nM enzyme concentration, when polymerase activity was minimal, indicating that the monomeric enzyme behaved as a glycohydrolase, whereas poly(ADP-ribosyl)ation of enzyme molecules was maximal when the enzyme tends to be self-associated to the dimeric form.

Measurement of serum interleukin-2 activity

The accurate measurement of serum interleukin-2 activity is crucial for assessing the efficacy and toxicity of systemic immunotherapy with recombinant interleukin-2. Incubation of serum at 56 degrees C for 30 minutes facilitates the bioassay for interleukin-2 activity by destroying the interleukin-2 inhibitory activity native to human serum. As this report will demonstrate, however, 30% to 50% of interleukin-2 activity in serum taken from patients or normal volunteers was destroyed by heating at 56 degrees C. No loss of recombinant interleukin-2 activity occurred during heating in serum-free media. The percentage of interleukin-2 activity lost at 56 degrees C varied from patient to patient and also varied with the time of exposure. Native serum interleukin-2 inhibitory activity can be removed, and interleukin-2 activity can be assessed accurately in serial dilutions of the serum beyond 1:64.

Mutant proteins of human interleukin 2. Renaturation yield, proliferative activity and receptor binding

Muteins, i.e. proteins altered by mutation of their genes, of interleukin 2 (Il2) were generated by oligonucleotide-directed mutagenesis in vitro. All acidic and basic residues conserved between man and mouse were exchanged as well as four lipophilic residues contained within four hydrophobic segments of the protein. The muteins were produced in Escherichia coli and submitted to a renaturation and purification protocol, before bioactivity and receptor binding of each of them was determined. All muteins besides two (K44/T125 and Q110/T125) could be renatured and purified. One mutein (K94/T125) exhibited a more than tenfold-improved renaturation yield. One amino exchange (Asp-20 to Asn) resulted in an about 20-fold reduction in proliferative activity and high-affinity receptor binding. The binding to the low-affinity Il2-binding protein (Tac antigen) was unimpaired. A second exchange (Arg-38 to Gln) had no effect on proliferative activity. The binding to both the high- and the low-affinity receptor, however, was reduced about 20-fold. Preliminary trials on the stability of these muteins by guanidinium hydrochloride denaturation studies detected no differences between wild-type interleukin 2 and muteins. It is suggested that Asp-20 forms part of the binding site for the large receptor subunit whereas Arg-38 is involved in the contact site to the small subunit.