Design of solubly expressed miniaturized SMART MHCs

Abstract

The precise recognition of specific peptide-MHC (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 (E. 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 {beta}2-microglobulin ({beta}2m) and the heavy chain 3 domain, and enable soluble expression of both H-2Db and A*02:01 in E. coli. 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. SignificanceDespite the critical role that TCR/pMHC interactions play in human health, it has remained difficult to produce reagents necessary to study them. Requirements for refolding or sequence optimization limit immunologists and biochemists ability to characterize diverse pMHC/TCR interactions. Here, we develop a de-novo designed protein domain that stabilizes the H-2Db and A*02:01 class I MHC allomorphs, allowing soluble expression in E. coli without the need for a stabilizing peptide, and improving display on the yeast surface, while maintaining peptide and TCR binding interactions. These features facilitate a wide range of experiments to more fully understand the nature of pMHC/TCR interactions, and pave the way for the development of stabilizing domains for all MHC allomorphs.