Comment on Risueño et al, page 601
Risueño and colleagues report that APA1/1 monoclonal antibody can be used to detect TCRs that have interacted with antigenic MHC-peptide complexes in vivo.
Along-standing controversy in T-cell activation is whether the T-cell receptor (TCR) undergoes conformational changes upon engaging agonist major histocompatibility complex (MHC)–peptide complexes. In 2002, Gil et al1 demonstrated that TCR ligation by agonist MHC-peptide complexes or activating antibodies induced binding of the cytoplasmic adapter protein Nck to a polyproline motif in the cytoplasmic domain of the CD3ϵ chain. This unmasking provided the best evidence for a ligand-induced conformational change in the TCR. While the functional necessity of this change for TCR signaling has not been proved, there is no question that this change is biochemically useful for identification of TCR complexes that have been engaged by agonist MHC-peptide complexes.
The utility of Nck recruitment to this site in situ is limited because Nck is also recruited to a downstream signaling complex including Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76) and Vav, which, due perhaps to amplification, appear to account for most of the Nck recruited to the immunologic synapse.2 Therefore, another way to detect this conformational change would greatly increase its accessibility in intact cells or in vivo.
In this issue of Blood, Risueño and colleagues show that the commercially available anti-CD3ϵ antibody APA1/1 detects the same conformational change in the TCR that is detected by Nck and can do so in fixed tissues. As with Nck binding, the APA1/1 epitope exposure did not require metabolic activity or signaling through the TCR, so this appears to be a direct response of the TCR machinery to agonist MHC-peptide complexes without intervening events. The APA1/1 epitope thus provides a new reporter for productive TCR engagement that is not subject to the interference issues that faced the biologic ligand Nck. The APA1/1 antibody detected TCR accumulated in the immunologic synapse formed by agonist MHC-peptide complexes, but not by antagonist MHC-peptide complexes. The APA1/1 antibody reacts with both mouse and human TCR complexes, indicating that it has broad potential applications for in vivo experimental immunology and perhaps clinical applications to detect concurrent T-cell activation in short-term cultures or in biopsy specimens. Thus, the use of APA1/1 should greatly extend the potential applications of the original finding by allowing detection of engaged TCR in vitro and in vivo and should speed a deeper understanding of the biologic importance of this conformational change. ▪
