Design of solubly expressed miniaturized SMART MHCs

The precise recognition of specific peptide-major histocompatibility complex (pMHC) complexes by T cell receptors (TCRs) plays a key role in infectious disease, cancer, and autoimmunity. A critical step in many immunobiological studies is the identification of T cells expressing TCRs specific to a given pMHC antigen. However, the intrinsic instability of empty class-I MHCs limits their soluble expression in Escherichia coli and makes it very difficult to characterize even a small fraction of possible pMHC/TCR interactions. To overcome this limitation, we designed small proteins which buttress the peptide binding groove of class I MHCs, replacing β2-microglobulin (β2m) and the heavy chain α3 domain, and enable soluble and partially soluble expression in E. coli of H-2D^b and A*02:01, respectively. We demonstrate that these soluble, monomeric, antigen-receptive, truncated (SMART) MHCs retain both peptide- and TCR-binding specificity and that peptide-bound structures of both allomorphs are similar to their full-length, native counterparts. With extension to the majority of HLA alleles, SMART MHCs should be broadly useful for probing the T cell repertoire in approaches ranging from yeast display to T cell staining.