Fibronectin type III domains engineered to bind CD40L: cloning, expression, purification, crystallization and preliminary X-ray diffraction analysis of two complexes.

Tn3 proteins are a novel class of binding molecules based on the third fibronectin type III domain of human tenascin C. Target-specific Tn3 proteins are selected from combinatorial libraries in which three surface-exposed loops have been diversified. Here, the cocrystallization of two different Tn3 proteins in complex with CD40L, a therapeutic target for immunological disease, is reported. These crystal structures are the first to be reported of Tn3 proteins and will help to reveal how these engineered molecules achieve specific recognition of a cognate target.

Method for the validation of immunohistochemical staining using SCID mouse xenografts: expression of CD40 and CD154 in human non-small cell lung cancer.

This report proposes a concept for the standardization of immunohistochemical evaluation. Immunohistochemical staining has several problems associated with the sensitivity of the technical process and standardization of the assessment of potent staining. We provided data focusing on this concept through immunostaining for CD154 in non-small cell lung cancer (NSCLC). We used two types of anti-CD154 antibody as primary antibodies in immunohistochemical staining, as previously reported. Western blot analysis confirmed strong CD154 expression in the cultured cell line PC10, but not in LK2. We also assessed CD154 expression in SCID mouse xenografts of these cell lines. SCID xenograft data on western blot analysis were consistent with those of cultured cell lines. These xenografts could thus be used as positive or negative tissue controls for CD154 immunostaining. Primary antibodies should therefore be confirmed as recognizing target lesions, while control tissue specimens should be objectively confirmed as having target products using another experimental method. Our method would allow results to be unified at more than one laboratory and could act as an objective control assessment method in immunohistochemistry.

Small molecule inhibition of the TNF family cytokine CD40 ligand through a subunit fracture mechanism.

BIO8898 is one of several synthetic organic molecules that have recently been reported to inhibit receptor binding and function of the constitutively trimeric tumor necrosis factor (TNF) family cytokine CD40 ligand (CD40L, aka CD154). Small molecule inhibitors of protein-protein interfaces are relatively rare, and their discovery is often very challenging. Therefore, to understand how BIO8898 achieves this feat, we characterized its mechanism of action using biochemical assays and X-ray crystallography. BIO8898 inhibited soluble CD40L binding to CD40-Ig with a potency of IC(50) = 25 µM and inhibited CD40L-dependent apoptosis in a cellular assay. A co-crystal structure of BIO8898 with CD40L revealed that one inhibitor molecule binds per protein trimer. Surprisingly, the compound binds not at the surface of the protein but by intercalating deeply between two subunits of the homotrimeric cytokine, disrupting a constitutive protein-protein interface and breaking the protein's 3-fold symmetry. The compound forms several hydrogen bonds with the protein, within an otherwise hydrophobic binding pocket. In addition to the translational splitting of the trimer, binding of BIO8898 was accompanied by additional local and longer-range conformational perturbations of the protein, both in the core and in a surface loop. Binding of BIO8898 is reversible, and the resulting complex is stable and does not lead to detectable dissociation of the protein trimer. Our results suggest that a set of core aromatic residues that are conserved across a subset of TNF family cytokines might represent a generic hot-spot for the induced-fit binding of trimer-disrupting small molecules.

Expression and purification of soluble murine CD40L monomers and polymers in yeast Pichia pastoris.

The anti-murine CD40L monoclonal antibody MR1 has been widely used in immunology research to block the CD40-CD40L interaction for induction of transplantation tolerance and to abrogate autoimmune diseases. The availability of recombinant CD40L with high binding capacity for MR1 would provide a valuable immunologic research tool. In this study, we constructed the single chain murine soluble CD40L monomer, dimer, trimer and successfully expressed them in yeast Pichia pastoris under the control of the alcohol oxidase promoter. The secreted single chain murine soluble CD40L monomers, dimers, and trimers were initially enriched through histidine tag capture by Ni-Sepharose 6 fast flow resin and further purified on a cation exchange resin. Purity reached more than 95% for the monomer and dimer forms and more than 90% for the trimer. Protein yield following purification was 16 mg/L for the monomer and dimer, and 8 mg/L for the trimer. ELISA analysis demonstrated that the CD40L dimers and trimers correctly folded in conformations exposing the MR1 antigenic determinant.