Plasma-membrane-bound macromolecules are dynamically aggregated to form non-random codistribution patterns of selected functional elements. Do pattern recognition processes govern antigen presentation and intercellular interactions?

Adaptive threshold-stochastic resonance (AT-SR) in MHC clusters on the cell surface

Highly conserved 2D receptor clusters (membrane rafts) of immunological signaling molecules with MHCI and MHCII antigens as their cores have been observed in the past on the surface of T- and B-cell lines of lymphoid origin, as well as on cells from patients with colon tumor and Crohn's disease. Conservativity is related to the ever presence of MHCI molecules. Although they are suspected to play a role in maintaining these clusters and facilitating transmembrane signaling, their exact role has been left largely enigmatic. Here we are suggesting stochastic resonance (SR), or "noise-assisted signal detection", as a general organizing principle for transmembrane signaling events evoked by processes like immune recognition and cytokine binding taking place in these clusters. In the conceptual framework of SR, in immune recognition as a prototype of transmembrane signaling, the sea of self-peptide-MHC complexes around a nonself-peptide presenting MHC is conceived as a source of quickly fluctuating unspecific signal ("athermal noise") serving the extra energy for amplifying the weak sub-threshold specific signal of the nonself-peptide presenting MHC. This same noise is also utilized for a readjustment of the threshold - and also the sensitivity and specificity - of detection by a closed loop feedback control of the TcR-CD8 (CD4) proximity on the detecting T-cell. The weak sub threshold specific signal of nonself-peptide presenting MHC is amplified by the superposing unspecific signals of the neighboring self peptide-MHC complexes towards the T-cell receptor as the detector. Because in a successful detection event both self- and nonself-peptides are detected simultaneously, the principle of coincidence (or lock-in) detection is also realized. The ever presence of MHC islands gets a natural explanation as a source of extra power - in a form of "athermal noise" - needed for coincidence detection and frequency encoding the evoked downstream signals. The effect is quite general, because the actual type of molecules surrounding a chief signaling molecule - like nonself-peptide holding MHC, interleukin-2 and -15 cytokine receptors (IL-2R/15R) - as the fluctuating interaction energy sources is immaterial. The model applies also for other types of signaling, such as those evoked by cytokine binding. The phenomenon of SR can also be interpreted as sampling of a low frequency, specific signal with a high frequency unspecific signal, the "noise". Recipes for identifying other forms of SR in membrane clusters with biophysical tools are recommended.

Fibrinogen receptor antagonists induce conformational changes of the human platelet glycoprotein IIb

BACKGROUND

Controversial results have been reported concerning the ability of fibrinogen receptor antagonists (fibans) to induce conformational changes in the fibrinogen receptor after binding to it as the initial step of fibrinogen binding and platelet activation.

METHODS

Platelets in citrated whole blood were stained with several pairs of anti-glycoprotein (anti-GP) IIb-directed monoclonal antibodies conjugated to phycoerythrin (PE) or indirectly labeled with Cy5. Pairs of monoclonal antibodies that induced a high-fluorescence resonance energy transfer (FRET) efficiency served as tools to detect activation-dependent changes of GP IIb after addition of adenosine diphosphate and several fibans.

RESULTS

Using the combination of the clones 5B12-PE and P2-biotin/SA-Cy5, a concentration-dependent alteration of the GP IIb conformation was observed after addition of tirofiban, eptifibatide, and lotrafiban. Magnitude and kinetics differed among the investigated substances.

CONCLUSION

The newly developed FRET assay allows the direct investigation of conformational changes of GP IIb after addition of platelet agonists or receptor ligands, as shown for three fibans.

Anomalous diffusion of major histocompatibility complex class I molecules on HeLa cells determined by single particle tracking

Single-particle tracking (SPT) was used to determine the mobility characteristics of MHC (major histocompatibility complex) class I molecules at the surface of HeLa cells at 22 degrees C and on different time scales. MHC class I was labeled using the Fab fragment of a monoclonal antibody (W6/32), covalently bound to either R-phycoerythrin or fluorescent microspheres, and the particles were tracked using high-sensitivity fluorescence imaging. Analysis of the data for a fixed time interval suggests a reasonable fit to a random diffusion model. The best fit values of the diffusion coefficient D decreased markedly, however, with increasing time interval, demonstrating the existence of anomalous diffusion. Further analysis of the data shows that the diffusion is anomalous over the complete time range investigated, 4-300 s. Fitting the results obtained with the R-phycoerythrin probe to D = D0talpha-1, where Do is a constant and t is the time, gave D0 = (6.7 +/- 4.5) x 10(-11) cm2 s-1 and alpha = 0.49 +/- 0.16. Experiments with fluorescent microspheres were less reproducible and gave slower anomalous diffusion. The R-phycoerythrin probe is considered more reliable for fluorescent SPT because it is small (11 x 8 nm) and monovalent. The type of motion exhibited by the class I molecules will greatly affect their ability to migrate in the plane of the membrane. Anomalous diffusion, in particular, greatly reduces the distance a class I molecule can travel on the time scale of minutes. The present data are discussed in relation to the possible role of diffusion and clustering in T-cell activation.

HLA class I and II antigens are partially co-clustered in the plasma membrane of human lymphoblastoid cells

Major histocompatibility complex (MHC) class II molecules displayed clustered patterns at the surfaces of T (HUT-102B2) and B (JY) lymphoma cells characterized by interreceptor distances in the micrometer range as detected by scanning force microscopy of immunogold-labeled antigens. Electron microscopy revealed that a fraction of the MHC class II molecules was also heteroclustered with MHC class I antigens at the same hierarchical level as described by the scanning force microscopy data, after specifically and sequentially labeling the antigens with 30- and 15-nm immunogold beads. On JY cells the estimated fraction of co-clustered HLA II was 0.61, whereas that of the HLA I was 0.24. Clusterization of the antigens was detected by the deviation of their spatial distribution from the Poissonian distribution representing the random case. Fluorescence resonance energy transfer measurements also confirmed partial co-clustering of the HLA class I and II molecules at another hierarchical level characterized by the 2- to 10-nm Förster distance range and providing fine details of the molecular organization of receptors. The larger-scale topological organization of the MHC class I and II antigens may reflect underlying membrane lipid domains and may fulfill significant functions in cell-to-cell contacts and signal transduction.