Major histocompatibility complex (MHC) proteins that present peptide ligands for recognition by T cell receptor form groups containing 2-20 of tightly packed molecules on the cell membrane of antigen presenting cells. While it is thought that MHC bunching could control antigen presentation and T cell reactions, accessible methodologies for examining the impact of MHC groups don’t take into account the deliberate assessment of how the group’s size, thickness, and arrangement inﬂuence T cell receptor commitment and activating of T cell receptor-mediated signalling.
To investigate membrane peptide–MHC (pMHC) groups, nanolipoprotein particles (NLPs) containing practical nickel chelating lipids (NiNLPs), permitting conjugation of His6-labelled proteins were utilized. NLPs are nanoscale discoidal membrane mimetics that are promptly self-gathered after mixing puriﬁed lipids and apolipoproteins and they give an adaptable stage to in-vitro and ex vivo applications while NiNLPs are a perfect stage to assess how pMHC grouping influence antigen acknowledgment and T cell flagging and the density of attached proteins can be dependably constrained by changing the level of nickel-chelating lipids in the NiNLPs.
Thomas Jefferson University scientists Dr. Nadia Anikeeva and Professor Yuri Sykulev together with Dr. Nicholas Fisher and Dr. Craig Blanchette at Lawrence Livermore National Laboratory analyzed how the density of stimulatory (cognate) and non-stimulatory (non-cognate or self) pMHC ligands within model membrane clusters influence their equilibrium binding and association kinetics with live CTL as well as the induction of TCR-induced Ca2+ signaling. Their research work is published in Journal of Immunology.
The research team observed that the enhancement factor for the binding of high versus low density non-cognate conjugates was much higher than that for the mutated and intact cognate conjugates. The difference was even more profound with temperature increase, with the amount of cell-bound low-density non-cognate pMHC/NiNLP being barely detectable at 37˚C as opposed to high-density pMHC/NiNLP. They also compared the time constants for high- and low-density cognate pMHC/NiNLPs and found that the latter bound to live T cells with slower kinetics, the time courses of both high- and low-density cognate pMHCmut/NiNLPs were monophasic, with very similar large time constants and low amplitudes. In contrast, the binding kinetics of low- and high-density non-cognate pMHC/NiNLPs were very different.
Furthermore, when evaluating the effects of pMHC ligand density and MHC mutation that weaken CD8–MHC interactions on TCR-mediated signaling, it was discovered that high-density cognate pMHC/NiNLPs were superior in the induction of robust Ca2+ signaling as opposed to low-density pMHC/NiNLP. The authors also discovered that high density of pMHC molecules capable of interacting with CD8 within model membrane MHC clusters facilitates the kinetics of clustered pMHC interactions with the T cell surface, which regulates the kinetics and magnitude of Ca2+ signaling.
The study by Nadia Anikeeva and her colleagues shows that the thickness of pMHCs, not simply their numbers, controls multivalent interactions of the model pMHC groups with T cell receptor and CD8 co-receptor on the T cells. Explaining the role of MHC grouping gave the helped in comprehending how variations in MHC bunching controls antigen acknowledgment and efﬁciency of T cell response against carcinogenic and virus-infected cells. These discoveries reveal a new insight into how MHC grouping inﬂuences multivalent interactions of pMHC ligands with CD8 and T cell receptor on live T cells that manage antigen acknowledgment, the kinetics of intracellular signalling, and the selectivity and efﬁciency of T cell reactions.
“CD8 co-receptor mediates two indispensable functions, namely, rapid scanning of pMHC ligands on target cells by CTL expediting finding of a rare antigen on target cell surface and, once “foreign” pMHC is identified, CD8 facilitates the induction of rapid kinetics of Ca2+ signaling required for swift granule release and efficient target cell destruction by CTL” Said Professor Yuri Sykulev, lead author in a statement to Medicine Innovates.
He then added “These indispensable CD8 functions are regulated by MHC clustering that influences multivalent interactions of pMHC ligands with CD8 and TCR occurring in the pseudo-two dimensional space at the CTL/target cell interface”.
Upon delivery to the surface, pMHC ligands appear as large patches containing about 100-150 pMHC molecules. These patches are relatively short-lived (half-lives of around 30 seconds), and pMHC molecules then diffuse away from the patches generating smaller clusters and individual molecules. This continuous process ensures sustained presentation of newly generated peptides providing information about what kind of proteins are made inside the cell.
Anikeeva, N., Fisher, N., O., Blanchette, C., D., Sykulev, Y., Extent of MHC Clustering Regulates Selectivity and Effectiveness of T Cell Responses, J Immunol 2019; 202:591-597;doi: 10.4049/jimmunol.1801196 .Go To J Immunol 2019