The molecular makeup and function of regulatory and effector synapses

Summary:  Physical interactions between T cells and antigen-presenting cells (APCs) form the basis of any specific immune response. Upon cognate contacts, a multimolecular assembly of receptors and adhesion molecules on both cells is created, termed the immunological synapse (IS). Very diverse structures of ISs have been described, yet the functional importance for T-cell differentiation is largely unclear. Here we discuss the principal structure and function of ISs. We then focus on two characteristic T-cell–APC pairs, namely T cells contacting dendritic cells (DCs) or naive B cells, for which extremely different patterns of the IS have been observed as well as fundamentally different effects on the function of the activated T cells. We provide a model on how differences in signaling and the involvement of adhesion molecules might lead to diverse interaction kinetics and, eventually, diverse T-cell differentiation. We hypothesize that the preferred activation of the adhesion molecule leukocyte function-associated antigen-1 (LFA-1) and of the negative regulator for T-cell activation, cytotoxic T-lymphocyte antigen-4 (CTLA-4), through contact with naive B cells, lead to prolonged cell–cell contacts and the generation of T cells with regulatory capacity. In contrast, DCs might have evolved mechanisms to avoid LFA-1 overactivation and CTLA-4 triggering, thereby promoting more dynamic contacts that lead to the preferential generation of effector cells.     

Nonparametric Block-Structured Modeling of Lung Tissue Strip Mechanics

Very large amplitude pseudorandom uniaxial perturbations containing frequencies between 0.125 and 12.5 Hz were applied to five dog lung tissue strips. Three different nonlinear block-structured models in nonparametric form were fit to the data. These models consisted of (1) a static nonlinear block followed by a dynamic linear block (Hammerstein model); (2) the same blocks in reverse order (Wiener model); and (3) the blocks in parallel (parallel model). Both the Hammerstein and Wiener models performed well for a given input perturbation, each accounting for greater than 99% of the measured stress signal variance. However, the Wiener and parallel model parameters showed some dependence on the strain amplitude and the mean stress. In contrast, a single Hammerstein model accounted for the data at all strain amplitudes and operating stresses. A Hammerstein model featuring a fifth-order polynomial static nonlinearity and a linear impulse response function of 1 s duration accounted for the most output variance (99.84% ± 0.13%, mean ± standard deviations for perturbations of 50% strain at 1.5 kPa stress). The static nonlinear behavior of the Hammerstein model also matched the quasistatic stress–strain behavior obtained at the same strain amplitude and operating stress. These results show that the static nonlinear behavior of the dog lung tissue strip is separable from its linear dynamic behavior. © 1998 Biomedical Engineering Society. PAC98: 8745Bp, 8710+e     

Finite element analysis of imposing femtonewton forces with micropipette aspiration

A novel technique of imposing femtonewton forces with micropipette aspiration [i.e., the extended micropipette aspiration technique (EMAT)] is proposed, and an axisymmetric finite element analysis of this technique is provided. The EMAT is experimentally based upon a micropipette manipulation system and is theoretically based upon hydrodynamics. Any spherical object such as a human neutrophil or a latex bead can be employed as the force transducer, so cell–cell interactions can be directly studied. Our computational analysis shows that femtonewton forces can indeed be imposed. The force magnitude is sensitive to the radius of the micropipette and the micropipette-transducer distance, but it is much less sensitive to other parameters including the radius of the transducer, the substrate curvature, and the thickness of the micropipette wall. Combining the EMAT and the previously developed micropipette aspiration technique will allow us to impose an unprecedented range of forces, from a few femtonewtons to a few hundred piconewtons on single molecules or receptor-ligand bonds. © 2002 Biomedical Engineering Society. PAC2002: 8780Fe, 8715La, 0270Dh, 8715Aa     

A Computational Fluid Dynamic Model of Antigen–Antibody Surface Adsorption on a Piezoelectric Immunosensor

A novel computational fluid dynamic model describing the antigen–antibody binding on an electrode surface is presented. It was assumed that the adsorption rate of the antibody sample is dependent upon the flow field in the vicinity of the electrode. Numerical solution of the steady flow in a two-dimensional triangular cell using the Navier–Stokes equations was carried out for predicting mass adsorption on the surface of the crystal. The relationships between the mass adsorbed over the area surface of the electrode, the kinetics of the binding process, and the flow field were determined. The effect of the inlet conditions (location, velocity magnitude, and direction) on the time constant of the mass adsorption process was investigated. It was found that the time constant was decreased by moving the inlet near the edge of the crystal or increasing the normal to the boundary component of the velocity. These changes may significantly reduce the time needed to conduct the test. © 2000 Biomedical Engineering Society. PAC00: 8780-y, 8550+k, 8710+e     

Reduction of myocardial cross-bridge turnover rate in presence of EMD 53998, a novel Ca2+-sensitizing agent

We have studied the effect of EMD 53998 (5-(1-(3, 4-dimethoxybenzoyl)-1,2,3,4-tetrahydrochinolin-6-yl)-6-methyl-3, 6-dihydro-2H-1,3,4-thiadiazin-2-one) on cross-bridge turnover rate at varying Ca²⁺ concentrations. Cross-bridge cycling rate was estimated both by adenosine triphosphatase measurements and determination of mechanical characteristics of constantly activated fibres, which is assumed to reflect cross-bridge kinetics. The results indicate that the turnover rate of myocardial cross-bridges was reduced in the presence of EMD 53998 at low Ca²⁺ concentrations (pCa6.25), but not at higher Ca²⁺ concentrations (pCa5.85).     

Annual stability in the levels of lymphocyte subpopulations identified by monoclonal antibodies in blood of healthy individuals

The absolute numbers and percentages of lymphocytes, monocytes, and lymphocyte subpopulations in the blood mononuclear cells were examined monthly in two healthy individuals over a 22-month period. The object of this study was to determine whether the levels of lymphocyte subpopulations identified by monoclonal antibodies, Leu4⁺ (T), Leu3⁺ (helper T), Leu2⁺ (suppressor/cytotoxic T), and Leu7⁺ (natural killer) cells, were stable during the year for healthy donors. The results were analyzed by the cosiner method to estimate the rhythmicity of these subpopulations. The number of lymphocytes varied, showing a moderate circannual rhythm with a peak in early summer, whereas the number of monocytes also varied but its variation did not show a specific rhythm. The absolute numbers of T-lymphocyte subpopulations and Leu3⁺ and Leu2⁺ cells showed a covariation, with a peak in early summer in parallel with the circannual rhythm of total lymphocyte counts. A subpopulation of granular lymphocytes with natural killer function, Leu7⁺ cells, also showed a significant variation during the year. Of particular interest is that Leu3/Leu2 ratios were considerably stable during the year. The two-time point examination of these lymphocyte markers including HB-2⁺ B cells in August and January in 15 normal donors did not show any significant differences, although the mean values were slightly higher in summer. The stability and variability of these lymphocyte markers are displayed graphically and the details of these variations are listed.     

Dynamic changes in the tuning of striate neurons to the shapes of cross-shaped figures

Time slice analysis was used to study the dynamics of tuning to the shapes of cross-shaped figures flashing in the receptive fields of 83 neurons in the primary visual cortex (field 17) of the cat brain. Tuning was assessed in terms of the numbers of spikes in the overall response and its sequential 20-msec fragments. Only 11.7% of neurons produced reproducibly developing spike responses to a given shape (defined as the angle between the lines), i.e., had a preferred cross-shaped figure. In the remaining cases (88.3%), tuning of neurons to the shape of the cross showed dynamic changes. In 7.2% of cases, changes in the preferred shape of the cross occurred monophasically; changes were biphasic in 27.0% of cases, while in the remaining 54.1% of cases, the dynamics in changes in the preferred cross shape were undulatory. The tuning of receptive field zones is assessed as the cause of these effects and their difference from the previously observed dynamics of preferred orientations of single bars and cross-shaped figures; the functional significance of these effects is also discussed.Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 89, No. 10, pp. 1216–1225, October, 2003.     

Spatial dynamics of receptive fields in cat primary visual cortex related to the temporal structure of thalamocortical feedforward activity

We investigated how changes in the temporal firing rate of thalamocortical activity affect the spatiotemporal structure of receptive field (RF) subunits in cat primary visual cortex. Spike activity of 67 neurons (48 simple, 19 complex cells) was extracellulary recorded from area 17/18 of anesthetized and paralyzed cats. A total of 107 subfields (on/off) were mapped by applying a reverse correlation technique to the activity elicited by bright and dark rectangles flashed for 300 ms in a 20x10 grid. We found that the width of the (suprathreshold) discharge fields shrank on average by 22% during this 300-ms-long stimulus presentation time. Fifty-eight subfields (54%) shrank by more than 20% of peak width and only ten (less than 10%) showed a slight increase over time. The main size reduction took place 40-60 ms after response onset, which corresponded to the transition from transient peak firing to tonic visual activity in thalamocortical relay cells (TC). The experimentally obtained RFs were then fitted with the aid of a neural field model of the primary visual pathway. Assuming a Gaussian-shaped spatial sensitivity profile across the RF subfield width, the model allowed us to estimate the subthreshold RF (depolarization field, D-field) from the minimal discharge field (MDF). The model allowed us to test to what degree the temporal dynamics of thalamocortical activity contributes to the spatiotemporal changes of cortical RFs. To this end, we performed the fitting procedure either with a pure feedforward model or with a field model that also included intracortical feedback. Spatial and temporal parameters obtained from fits of the experimental RFs matched closely to those achieved by simulating a pure feedforward system with the field model but were not compatible with additional intracortical feedback. Thus, our results show that dot stimulation, which optimally excites thalamocortical cells, leads to a shrinkage with respect to the size of the RF subfield at the first transient response of visual cortical RFs which seems mainly due to a change in the thalamic firing pattern. In these experiments little or no influence from intracortical sources was observed, which, however, may play a role when using more complex visual stimuli.     

Influence of pleural pressure variations on cardiovascular system dynamics: a model study

A mathematical model of the human cardiovascular system (CVS) is used to study the effect of different respiratory manoeuvres on the circulation. The model simulates the normal CVS and the interaction between the heart and the intrathoracic pressure. The vascular system is represented by resistive, capacitive and inertial elements whereas the ventricles are assumed to function according to the time-varying elastance concept based on their transmural pressures. The model predicts that normal inspiratory effeort effects an increase in the venous return, an increase in the pulmonary flow and a slight decrease in the left ventricular stroke volume (LVSV), which represents a decrease in ejection due to the increased LV transmural pressure. A step decrease in pleural pressure to −40 mm Hg, representing the Müller manoeuvre (MM), accentuates these findings, showing a decrease in LVSV in spite of an increase in the LV end-diastolic volume (LVEDV), LV end-systolic volume (LVESV) and the LV filling pressure, expressed as the mean left atrial transmural pressure. Simulating intermittent positive pressure ventilation (IPPV) with added positive end expiratory pressure (PEEP) shows an 18·6 per cent decrease in the cardiac output compared with quiet respiration. The calculated results of the model are in good agreement with available experimental data, suggesting that most of these findings may be explained by basic haemodynamic principles in the uncontrolled CVS.     

计算流体动力学在叶轮式心脏泵中的应用研究

造成溶血、血栓等血液破坏现象的内在原因之一是血液的动力学行为。研究表明，不规则的流动模式尤其是切变流中产生的机械切应力直接导致血液的破坏。计算机技术的迅速发展使得微观动力学的数值模拟成为可能。本文针对基于流线型设计的叶轮心脏泵和直叶片叶轮心脏泵，应用计算流体动力学对其内部的流动行为进行了数值模拟。分析两种心脏泵的内流场和切应力分布，认为在相同的边界条件下，流线型设计的叶轮心脏泵要比直叶片心脏泵更符合血液动力学的要求，对血液的破坏较小。