Quantum dot/peptide-MHC biosensors reveal strong CD8-dependent cooperation between self and viral antigens that augment the T cell response

Abstract

Cytotoxic T lymphocytes (CTL) can respond to a few viral peptide-MHC-I (pMHC-I) complexes among a myriad of virus-unrelated endogenous self pMHC-I complexes displayed on virus-infected cells.To elucidate the molecular recognition events on live CTL, we have utilized a self-assembled biosensor composed of semiconductor nanocrystals, quantum dots, carrying a controlled number of virus-derived (cognate) and other (noncognate) pMHC-I complexes and examined their recognition by antigen-specific T cell receptor (TCR) on anti-virus CD8 ؉ T cells. The unique architecture of nanoscale quantum dot͞pMHC-I conjugates revealed that unexpectedly strong multivalent CD8 -MHC-I interactions underlie the cooperative contribution of noncognate pMHC-I to the recognition of cognate pMHC-I by TCR to augment T cell responses. The cooperative, CD8-dependent spread of signal from a few productively engaged TCR to many other TCR can explain the remarkable ability of CTL to respond to virus-infected cells that present few cognate pMHC-I complexes.CD8 coreceptor ͉ peptide-MHC clustering ͉ sensitivity of T cell responses ͉ antivirus immunity R ecognition of peptide-MHC (pMHC) complexes presented on the surface of infected cells by T cell receptors (TCR) initiates the T cell response against these cells. While TCR specifically recognizes peptide and polymorphic helices of MHC class I and class II proteins (1), the nonpolymorphic domain of these MHC proteins also interacts with the CD8 and CD4 coreceptors that mark distinct populations of T lymphocytes, i.e., CD8 ϩ and CD4 ϩ T cells. The MHC-coreceptor interactions occur regardless of the nature of MHC-bound peptide (2). Because MHC-I is expressed on every nucleated cell, CD8 ϩ T cells (also known as cytotoxic T lymphocytes or CTL), can recognize and, if necessary, respond to virtually any infected cell in the body, and thus play a key role in anti-virus immunity.Unraveling molecular recognition events as they occur at the cell membrane of T cells is essential for understanding mechanisms responsible for the unique sensitivity and selectivity of the T cell response. Soluble MHC proteins and their oligomers (3) have been extensively used for this purpose. However, the majority of these oligomers, MHC tetramers in particular, may not mimic the orientation and the typical distances between MHC monomers in their natural environment, and these limitations may influence the results of experiments in which these oligomers are used to explore TCR-mediated molecular events on the T cell surface. Indeed, the architecture of a multivalent ligand that binds to clustered receptors can strongly influence the ligand's biological activity (4).Luminescent semiconductor nanocrystals (quantum dots, QDs) provide excellent nanoscaffolds to array several receptor molecules on their surfaces by metal-affinity-driven self-assembly, providing aggregate-free solutions of QD conjugates that specifically recognize their respective ligands (5-9).In this report, we use self-assembled QD͞pMHC-I conjugates (in which the MHC...

How a T Cell Receptor-like Antibody Recognizes Major Histocompatibility Complex-bound Peptide

“…Meanwhile, it is thought that CD8/ MHC-I interactions, which occur prior to pMHC recognition by TCR, could influence TCR association with MHC and, consequently, TCR orientation over MHC (22,30). That 25-D1.16 assumes the same orientation over the pMHC ligand as TCR supports the notion that TCR has an intrinsic propensity for MHC (31) and that MHC, not CD8, determines selection of TCRs that bind with canonical orientation.…”

Stage-dependent reactivity of thymocytes to self-peptide–MHC complexes

“…Aside from its significance for T cell development, the responses of DN thymocytes to self-pMHC is particularly interesting in light of evidence that TCR-mediated signaling is greatly enhanced by CD4-and CD8-dependent cooperation between self-pMHC and agonist pMHC (47)(48)(49). Nevertheless, mature 2C T cells that lack CD4 and CD8 can make robust pMHCspecific responses to APC that present highly potent pMHC agonists (Fig.…”

Sensitivity of T cells to antigen and antagonism emerges from differential regulation of the same molecular signaling module

“…3). Experiments have also provided evidence for cooperative interactions between agonist and endogenous ligands in triggering CD8 ϩ T cells (17,27).…”

“…Contrary to the assumption in ref. 18, experiments support a role for endogenous ligands in triggering CD8 ϩ T cells (17,27), but the evidence for feedback regulation of Lck in modulating membraneproximal signaling is strong (12).…”