Soluble peptide–MHC monomers cause activation of CD8⁺T cells through transfer of the peptide to T cell MHC molecules

Abstract

T cell receptor (TCR)-mediated activation of CD4 ؉ T cells is known to require multivalent engagement of the TCR by, for example, oligomeric peptide-MHC complexes. In contrast, for CD8 ؉ T cells, there is evidence for TCR-mediated activation by univalent engagement of the TCR. We have here compared oligomeric and monomeric L d and K b peptide-MHC complexes and free peptide as stimulators of CD8 ؉ T cells expressing the 2C TCR. We found that the monomers are indeed effective in activating naïve and effector CD8 ؉ T cells, but through an unexpected mechanism that involves transfer of peptide from soluble monomers to T cell endogenous MHC (K b ) molecules. The result is that T cells, acting as antigenpresenting cells, are able to activate other naïve T cells.CD8 ϩ T lymphocytes ͉ T cell receptor ͉ major histocompatibility complex ͉ antigen presentation ͉ lymphocyte activation A ntigen-driven activation of cells of the immune system generally involves multivalent antigen engagement that leads to clustering or ''crosslinking'' of cell-surface receptors. This concept arose largely from observations of antigen-induced capping of surface immunoglobulins on B cells (1, 2) and multivalent antigen triggering of histamine release from mast cells (3). Whether multivalent engagement of T cell antigen receptors (TCR) is required to initiate T cell activation has been difficult to determine, primarily because the antigens are peptide-MHC complexes normally found as integral membrane proteins on the surfaces of other cells. However, experiments using soluble recombinant MHC molecules lacking transmembrane domains and carrying defined synthetic peptides have shown clearly that peptide-MHC oligomers, but not monomers, can activate CD4 ϩ T cells (4-7). Thus, for CD4 ϩ T cells, multivalent engagement of TCR is sufficient to initiate cellular activation, whereas monovalent engagement is apparently nonproductive.For CD8 ϩ T cells, however, the issue remains unclear. Several studies have found that multivalent engagement is required to activate these cells (4, 8-10), but some evidence suggests the sufficiency of monovalent TCR engagement to trigger a cytolytic response, such as the requirement for such a small number of peptide-MHC complexes per target cell in cytolytic assays, to imply that a single complex can trigger the cytolytic response in CD8 T cells (11). Activation of CD8 ϩ T cells by soluble peptide-MHC monomers has been clearly observed in the presence of CD8 molecules (12, 13). It was suggested from that result that monovalent engagement of TCR can serve as the initiating event when it results in TCR association with CD8 (12). However, in another study, TCR-CD8 heterodimerization induced by peptide-MHC monomers was observed but was found not to activate the cells (9).In this study, we investigated the requirements for initiating signaling processes in naïve and activated CD8 ϩ T cells carrying the 2C TCR. The 2C TCR recognizes various peptide-MHC complexes, including (i) the potent allogenic activator Materials and Methods...

T cell activation correlates with an increasedproportion of antigen among the materials acquiredfrom target cells

“…Thus, CTL acquire covalent pepMHC complexes in a signallingdependent manner. This observation also rules out the possibility that fluorescent peptide transfer results from peptide dissociation followed by reassociation on the MHC molecules of the CTL, a process that was proposed to account for CTL activation by soluble pepMHC monomers [41,42]. Detectable acquisition of covalent pepMHC complexes and IFN-c production require higher levels of TCR engagement than does triggering of biotin capture or cytotoxicity…”

Human Major Histocompatibility Complex (MHC) Class I Molecules with Disulfide Traps Secure Disease-related Antigenic Peptides and Exclude Competitor Peptides

“…Cross-presentation is a major pathway by which DNA vaccines elicit T cell responses, but the inefficiency of cross-presentation can be a contributing factor to relatively weak T cell responses after DNA vaccination. In addition, recent studies have documented the cross-presentation of synthetic peptides from recombinant MHC-I tetramers used to activate T cells (22,23). Thus biochemical stability of the SCT is a requirement for improved DNA vaccination or MHC-I tetramer approaches.…”

CD8 T Cells, Like CD4 T Cells, Are Triggered by Multivalent Engagement of TCRs by MHC-Peptide Ligands but Not by Monovalent Engagement

“…This type of study in CD4 T cells showed that dimers and higher oligomers of cognate class II MHC-peptide complexes were able to stimulate many T cell markers such as TCR down-regulation (4), CD69 up-regulation (5), and CD25 up-regulation (6); conversely, monomeric MHC-peptide complexes were not able to induce such markers (7)(8)(9)(10). When similar studies have been undertaken treating CD8 T cells with soluble, recombinant class I MHC-peptide complexes, conflicting studies reported activation in response to oligomeric complexes (10 -15), to monomeric complexes (13)(14)(15), and even to free peptide (14,15). A partial explanation for this discrepancy may be "cross-presentation" of peptide by T cells, wherein peptide adventitiously released from soluble MHC monomers can be loaded onto T cell endogenous class I MHC molecules with resultant T-T presentation and activation (14,15).…”

“…When similar studies have been undertaken treating CD8 T cells with soluble, recombinant class I MHC-peptide complexes, conflicting studies reported activation in response to oligomeric complexes (10 -15), to monomeric complexes (13)(14)(15), and even to free peptide (14,15). A partial explanation for this discrepancy may be "cross-presentation" of peptide by T cells, wherein peptide adventitiously released from soluble MHC monomers can be loaded onto T cell endogenous class I MHC molecules with resultant T-T presentation and activation (14,15). When T cells that do not express endogenous MHC are used, neither peptide nor class I MHC monomers caused down-regulation of TCR or up-regulation of activation markers (14,15).…”

“…A partial explanation for this discrepancy may be "cross-presentation" of peptide by T cells, wherein peptide adventitiously released from soluble MHC monomers can be loaded onto T cell endogenous class I MHC molecules with resultant T-T presentation and activation (14,15). When T cells that do not express endogenous MHC are used, neither peptide nor class I MHC monomers caused down-regulation of TCR or up-regulation of activation markers (14,15).…”