Peptide-Independent Folding and CD8αα Binding by the Nonclassical Class I Molecule, Thymic Leukemia Antigen

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Abstract
The nonclassical class I molecule, thymic leukemia (TL), has been shown to be expressed on intestinal epithelial cells and to interact with CD8+ intraepithelial T lymphocytes. We generated recombinant soluble TL (T18d) H chains in bacteria as inclusion bodies and refolded them with β2-microglobulin in the presence or absence of a random peptide library. Using a mAb, HD168, that recognizes a conformational epitope on native TL molecules, we observed that protein folds efficiently in the absence of peptide. Circular dichroism analysis demonstrated that TL molecules have structural features similar to classical class I molecules. Moreover, thermal denaturation experiments indicated that the melting temperature for peptide-free TL is similar to values reported previously for conventional class I-peptide complexes. Our results also show that CD8αα binding is not dependent on either TL-associated peptide or TL glycosylation.