Dimensions and Interactions of Large T-Cell Surface Proteins

Abstract

The first step of the adaptive immune response involves the interaction of T cells that express T-cell receptors (TCRs) with peptide-loaded major histocompatibility complexes expressed by antigen-presenting cells (APCs). Exactly how this leads to activation of the TCR and to downstream signaling is uncertain, however. Recent findings suggest that one of the key events is the exclusion of the large receptor-type tyrosine phosphatase CD45, from close contacts formed at sites of T-cell/APC interaction. If this is true, a full understanding of how close contact formation leads to signaling would require insights into the structures of, and interactions between, large membrane proteins like CD45 and other proteins forming the glycocalyx, such as CD43. Structural insights into the overall dimensions of these proteins using crystallographic methods are hard to obtain, and their conformations on the cell surface are also unknown. Several imaging-based optical microscopy techniques have however been developed for analyzing protein dimensions and orientation on model cell surfaces with nanometer precision. Here we review some of these methods with a focus on the use of hydrodynamic trapping, which relies on liquid flow from a micropipette to move and trap membrane-associated fluorescently labeled molecules. Important insights that have been obtained include (i) how protein flexibility and coverage might affect the effective heights of these molecules, (ii) the height of proteins on the membrane as a key parameter determining how they will distribute in cell-cell contacts, and (iii) how repulsive interactions between the extracellular parts of the proteins influences protein aggregation and distribution.

Membrane domains and phosphatase exclusion produce robust signaling responses in B cells engaged with natural ligands and artificial cross-linkers presented on lipid bilayers

“…The simulations presented in Fig 7 mimic the key aspect of the kinetic segregation model, by including a repulsive interaction at sites of receptor engagement felt only by phosphatases. As described by others (Davis and van der Merwe, 2006;Choudhuri et al, 2009;Schmid et al, 2016;Junghans et al, 2018), we find this mechanism alone is capable of inducing receptor activation upon engagement, by lowering the concentration and therefore the activity of phosphatase. Our simulations additionally demonstrate that the sensitivity and dynamic range of this signaling system is enhanced when it is also coupled to membrane domains.…”

A cell topography-based mechanism for ligand discrimination by the T cell receptor

“…In this context, cognate pMHC binding, which can slow or even halt TCR diffusion (30, 31), is expected to promote signaling simply by increasing the TCR’s dwell time inside the close contact, increasing the probability of receptor triggering. Depletion of the phosphatases is considered to be a passive process, driven by differences in the size of CD45 versus that of signaling and adhesive molecular complexes that form at the T cell/APC contact (24, 32–34).…”

Phagocytic Integrins: Activation and Signaling

“…Kindlin-3-induced clustering is reported to activate Src family kinases (SFKs) (109,110) by the exclusion of tyrosine phosphatases such as CD45 (68). Size exclusion of these membrane-bound phosphatases with large extracellular domains seems to be a common feature of integrin-mediated close-contact immune processes, such as Dectin-1 and FcγRIII phagocytosis and immune synapse formation (68,111,112). This process does not exclude SFKs but favors their activation due to removing the inhibitory effect of these phosphatases (109,110).…”