Rotational and lateral dynamics of I-A(k) molecules expressing cytoplasmic truncations

Immunological Functions of the Membrane Proximal Region of MHC Class II Molecules

Major histocompatibility complex (MHC) class II molecules present exogenously derived antigen peptides to CD4 T cells, driving activation of naïve T cells and supporting CD4-driven immune functions. However, MHC class II molecules are not inert protein pedestals that simply bind and present peptides. These molecules also serve as multi-functional signaling molecules delivering activation, differentiation, or death signals (or a combination of these) to B cells, macrophages, as well as MHC class II-expressing T cells and tumor cells. Although multiple proteins are known to associate with MHC class II, interaction with STING (stimulator of interferon genes) and CD79 is essential for signaling. In addition, alternative transmembrane domain pairing between class II α and β chains influences association with membrane lipid sub-domains, impacting both signaling and antigen presentation. In contrast to the membrane-distal region of the class II molecule responsible for peptide binding and T-cell receptor engagement, the membrane-proximal region (composed of the connecting peptide, transmembrane domain, and cytoplasmic tail) mediates these "non-traditional" class II functions. Here, we review the literature on the function of the membrane-proximal region of the MHC class II molecule and discuss the impact of this aspect of class II immunobiology on immune regulation and human disease.

Surface-anchored monomeric agonist pMHCs alone trigger TCR with high sensitivity

At the interface between T cell and antigen-presenting cell (APC), peptide antigen presented by MHC (pMHC) binds to the T cell receptor (TCR) and initiates signaling. The mechanism of TCR signal initiation, or triggering, remains unclear. An interesting aspect of this puzzle is that although soluble agonist pMHCs cannot trigger TCR even at high concentrations, the same ligands trigger TCR very efficiently on the surface of APCs. Here, using lipid bilayers or plastic-based artificial APCs with defined components, we identify the critical APC-associated factors that confer agonist pMHCs with such potency. We found that CD4⁺ T cells are triggered by very low numbers of monomeric agonist pMHCs anchored on fluid lipid bilayers or fixed plastic surfaces, in the absence of any other APC surface molecules. Importantly, on bilayers, plastic surfaces, or real APCs, endogenous pMHCs did not enhance TCR triggering. TCR triggering, however, critically depended upon the adhesiveness of the surface and an intact T cell actin cytoskeleton. Based on these observations, we propose the receptor deformation model of TCR triggering to explain the remarkable sensitivity and specificity of TCR triggering.

Using the T cell receptor (TCR) as a sensor, T cells of the immune system constantly migrate in lymphoid organs and probe the surface of antigen-presenting cells (APCs) for foreign antigens, a sign of pathogen infection. Antigen binding by TCRs leads to T cell activation and subsequent immune response to combat the pathogens. Interestingly, although T cells respond well to antigens on APCs, they do not recognize the same antigens in solution. What is it that makes antigens on APCs recognizable? To address this, we used lipid bilayers and plastic surfaces to construct artificial APCs with defined antigen number, composition, and configuration. We found that T cells respond to very few individual foreign antigens on artificial APCs, and contrary to some current opinion, formation of antigen clusters on APCs is not required for antigen recognition by T cells. TCR triggering, however, requires T cell adhesion to the APC surface and then occurs only if the T cells are able to move. We propose that at the dynamic T cell–APC interface, antigen on APCs activates T cells by applying force to the TCR and deforming its structure, which cannot be achieved by soluble antigens due to their lack of anchorage.

Why is it that T cells are blind to antigens in solution but highly sensitive to antigens anchored on a surface? The authors show that this is not due to antigen clustering, but could involve mechanical forces associated with cell locomotion.

T cell adhesion mechanisms revealed by receptor lateral mobility

Cell surface receptors mediate the exchange of information between cells and their environment. In the case of adhesion receptors, the spatial distribution and molecular associations of the receptors are critical to their function. Therefore, understanding the mechanisms regulating the distribution and binding associations of these molecules is necessary to understand their functional regulation. Experiments characterizing the lateral mobility of adhesion receptors have revealed a set of common mechanisms that control receptor function and thus cellular behavior. The T cell provides one of the most dynamic examples of cellular adhesion. An individual T cell makes innumerable intercellular contacts with antigen presenting cells, the vascular endothelium, and many other cell types. We review here the mechanisms that regulate T cell adhesion receptor lateral mobility as a window into the molecular regulation of these systems, and we present a general framework for understanding the principles and mechanisms that are likely to be common among these and other cellular adhesion systems. We suggest that receptor lateral mobility is regulated via four major mechanisms-reorganization, recruitment, dispersion, and anchoring-and we review specific examples of T cell adhesion receptor systems that utilize one or more of these mechanisms.

The receptor deformation model of TCR triggering

Through T cell receptors (TCRs), T cells can detect and respond to very small numbers of foreign peptides among a huge number of self-peptides presented by major histocompatibility complexes (pMHCs) on the surface of antigen-presenting cells (APCs). How T cells achieve such remarkable sensitivity and specificity through pMHC-TCR binding is an intensively pursued issue in immunology today; the key question is how pMHC-TCR binding initiates, or triggers, a signal from TCRs. Multiple competing models have been proposed, none of which fully explains the sensitivity and specificity of TCR triggering. What has been omitted from existing theories is that the pMHC-TCR interaction at the T cell/APC interface must be under constant mechanical stress, due to the dynamic nature of cell-cell interaction. Taking this condition into consideration, we propose the receptor deformation model of TCR triggering. In this model, TCR signaling is initiated by conformational changes of the TCR/CD3 complex, induced by a pulling force originating from the cytoskeleton and transmitted through pMHC-TCR binding interactions with enough strength to resist rupture. By introducing mechanical force into a model of T cell signal initiation, the receptor deformation model provides potential mechanistic solutions to the sensitivity and specificity of TCR triggering.

Delineation of the HLA-DR Region and the Residues Involved in the Association with the Cytoskeleton

Whereas the association of major histocompatibility complex (MHC) class II molecules with the cytoskeleton and their recruitment into lipid rafts play a critical role during cognate T/antigen-presenting cell interactions, MHC class II-induced signals, regions, and residues involved in their association and recruitment have not yet been fully deciphered. In this study, we show that oligomerization of HLA-DR molecules induces their association with the cytoskeleton and their recruitment into lipid rafts. The association of oligomerized HLA-DR molecules with the cytoskeleton and their recruitment into lipid rafts occur independently. Furthermore, the association with the cytoskeleton is HLA-DR-specific, since oligomerization of HLA-DP triggers its recruitment only into lipid rafts. HLA-DR molecules devoid of both alpha and beta cytoplasmic tails did not associate with the cytoskeleton, but their recruitment into lipid rafts was unimpeded. Deletion of either the alpha or beta cytoplasmic tail did not affect the association of HLA-DR with the cytoskeleton and/or recruitment into lipid rafts. HLA-DR molecules that were devoid of the alpha cytoplasmic chain and that had their beta cytoplasmic chain replaced with the HLA-DP beta chain or with a beta chain in which the residues at positions Gly(226)-His(227)-Ser(228) were substituted by alanine no longer associated with the cytoskeleton. They were, however, still recruited into lipid rafts. Together, these results support the involvement of different regions of the cytoplasmic tails in the association and the recruitment of HLA-DR into different compartments. The differential behavior of HLA-DP and -DR with respect to their association with the cytoskeleton may explain the previously described difference in their transduced signals.

Translational diffusion of individual class II MHC membrane proteins in cells

Single-molecule epifluorescence microscopy was used to observe the translational motion of GPI-linked and native I-E(k) class II MHC membrane proteins in the plasma membrane of CHO cells. The purpose of the study was to look for deviations from Brownian diffusion that might arise from barriers to this motion. Detergent extraction had suggested that these proteins may be confined to lipid microdomains in the plasma membrane. The individual I-E(k) proteins were visualized with a Cy5-labeled peptide that binds to a specific extracytoplasmic site common to both proteins. Single-molecule trajectories were used to compute a radial distribution of displacements, yielding average diffusion coefficients equal to 0.22 (GPI-linked I-E(k)) and 0.18 microm(2)/s (native I-E(k)). The relative diffusion of pairs of proteins was also studied for intermolecular separations in the range 0.3-1.0 microm, to distinguish between free diffusion of a protein molecule and diffusion of proteins restricted to a rapidly diffusing small domain. Both analyses show that motion is predominantly Brownian. This study finds no strong evidence for significant confinement of either GPI-linked or native I-E(k) in the plasma membrane of CHO cells.

Interactions of the mast cell function-associated antigen with the type I Fcepsilon receptor

Clustering the mast cell function-associated antigen (MAFA), a membrane glycoprotein expressed on 2H3 cells, by its specific monoclonal antibody G63 substantially inhibits secretion normally triggered by aggregating these cells' Type I Fcepsilon receptor (FcepsilonRI). To explore possible MAFA-FcepsilonRI interactions giving rise to this inhibition, we have studied by time-resolved phosphorescence anisotropy the rotational behavior of both MAFA and FcepsilonRI as ligated by various reagents involved in FcepsilonRI-induced degranulation and MAFA-mediated inhibition thereof. From 4 to 37 degrees C the rotational correlation times (mean+/-S.D.) of FcepsilonRI-bound, erythrosin-conjugated IgE resemble those observed for MAFA-bound erythrosin-conjugated G63 Fab, 82+/-17 micros and 79+/-31 micros at 4 degrees C, respectively. Clustering the FcepsilonRI-IgE complex by antigen or by anti-IgE increases the phosphorescence anisotropy of G63 Fab and slows its rotational relaxation. Lateral diffusion of G63 Fab is also slowed by antigen clustering of the receptor. Taken together, these results suggest that unperturbed MAFA associates with clustered FcepsilonRI. They are also consistent with its interaction with the isolated receptor, a situation also suggested by FRET measurements on the system.

Molecular dynamics of point mutated I-A(k) molecules expressed on lymphocytes

We have recently reported the lateral and rotational diffusion parameters for I-A(k) molecules expressing various cytoplasmic truncations (Int. Immunol. 12 (2000) 1319). We now describe the membrane dynamics of I-A(k) with various mutations in the presumed contact region between alphabeta-heterodimers in an (alphabeta)2 dimer of dimers structure. Such mutations are known to strongly affect the antigen presentation ability of these molecules (Int. Immunol. 10 (1998) 1237-1249) but cause relatively small changes in the molecular dynamics of I-A(k). Lateral diffusion coefficients of I-A(k) wild-type molecules and mutants obtained via fringe fluorescence photobleaching recovery (FPR) ranged from 1.1 to 2.3x10(-10)cm2/s at room temperature while fractional mobilities averaged 75+/-6%. For all cell types examined, treatment with either hen egg lysozyme 46-61 peptide or db-cAMP reduced the I-A(k) mobile fraction by about 10% relative to untreated cells, suggesting that these treatments may increase lateral confinement of class II in lipid rafts or cytoskeletal interactions of the molecules. Wild-type I-A(k) and mutants capable of normal or partial antigen presentation exhibited, as a group, slightly longer rotational correlation times (RCT) at 4 degrees C than did mutants inactive in antigen presentation, 14+/-4 versus 10+/-1 micros, respectively. Moreover, peptide, cAMP and anti-CD40 mAb treatment all increased rotational correlation times for fully- and partially-functional I-A(k) but not for non-functional molecules. For example, 16 h peptide treatment yielded average RCTs of 28+/-12 and 10+/-1 micros for the groups of functional and non-functional molecules, respectively. Such modulation of the dynamics of functional class II molecules is consistent with these treatments' stabilization of class II or induction of new gene expression. Measurements of fluorescence resonant energy transfer between I-A(k), though complicated by cellular autofluorescence, averaged 6+/-7% over 15 cells or treatments, a result consistent with the presence of a small fraction of I-A(k) as a dimer of dimers species. In summary, our results suggest subtle changes in the molecular motions of class II molecules correlate with a significant impact on class II function. Molecules active in antigen presentation exhibit more restricted motion in the membrane, and thus presumably more extensive intermolecular interactions, than non-functional molecules. Further, treatments, such as db-cAMP and anti-CD40, which rescue antigen presentation by partially defective mutants, appear to increase such interactions, several types of which have already been reported for class II. A more detailed understanding of these phenomena will require both more sensitive biophysical tools and a more refined model of the role of class II intermolecular interactions in antigen presentation.

Calculations show substantial serial engagement of T cell receptors

The serial engagement model provides an attractive and plausible explanation for how a typical antigen presenting cell, exhibiting a low density of peptides recognized by a T cell, can initiate T cell responses. If a single peptide displayed by a major histocompatibility complex (MHC) can bind, sequentially, to different T cell receptors (TCR), then a few peptides can activate many receptors. To date, arguments supporting and questioning the prevalence of serial engagement have centered on the down-regulation of TCR after contact of T cells with antigen presenting cells. Recently, the existence of serial engagement has been challenged by the demonstration that engagement of TCR can down-regulate nonengaged bystander TCR. Here we show that for binding and dissociation rates that characterize interactions between T cell receptors and peptide-MHC, substantial serial engagement occurs. The result is independent of mechanisms and measurements of receptor down-regulation. The conclusion that single peptide-MHC engage many TCR, before diffusing out of the contact region between the antigen-presenting cell and the T cell, is based on a general first passage time calculation for a particle alternating between states in which different diffusion coefficients govern its transport.