An autonomous CDR3δ is sufficient for recognition of the nonclassical MHC class I molecules T10 and T22 by γδ T cells

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Abstract
The molecular basis of ligand recognition by γδ TCRs remains vague. Adams and colleagues show that the CDR3δ loop, when grafted in place of a CDR3α loop, is sufficient to confer γδ TCR specificity on an αβ TCR. It remains unclear whether γδ T cell antigen receptors (TCRs) detect antigens in a way similar to antibodies or αβ TCRs. Here we show that reactivity between the G8 and KN6 γδ TCRs and the major histocompatibility complex class Ib molecule T22 could be recapitulated, with retention of wild-type ligand affinity, in an αβ TCR after grafting of a G8 or KN6 complementarity-determining region 3-δ (CDR3δ) loop in place of the CDR3α loop of an αβ TCR. We also found that a shared sequence motif in CDR3δ loops of all T22-reactive γδ TCRs bound T22 in energetically distinct ways, and that T10d, which bound G8 with weak affinity, was converted into a high-affinity ligand by a single point mutation. Our results demonstrate unprecedented autonomy of a single CDR3 loop in antigen recognition.