Quantification of the nonspecific intercellular transfer of fluorescent molecules between labeled and unlabeled rat thymocytes

Intercellular transfer of antigen-presenting cell determinants onto T cells: molecular mechanisms and biological significance

Upon physiological stimulation, receptors with tyrosine kinase activity (RTK) are rapidly internalized together with their soluble ligands. T cell activation is the consequence of recognition by the T cell receptor (TCR) of specific peptide-major histocompatibility protein complexes (peptide-MHC) present at the membrane of antigen-presenting cells (APC). The TCR belongs to the RTK family and is known to be endocytosed upon ligand recognition. It differs from most other RTK in that its ligand, the peptide-MHC complex, is membrane bound and the TCR-ligand interaction is quite weak. Recent experiments have shown that the TCR ligand becomes internalized by T cells upon stimulation. Here we review current knowledge on the molecular mechanisms by which the membrane-bound MHC molecules can be transferred onto T cells, and propose hypotheses on the role this phenomenon could play in physio-pathological situations involving T cells.

Interest of image processing in cell biology and immunology

Microscopy is a basic tool for cell biologists. Recent progress of electronics and computer science made powerful methodologies for digital processing of microscopic images easily available. These methods allowed impressive increase of the power of conventional microscopy. Dramatic image enhancement may be achieved by combination of filtering techniques, computer-based deblurring and contrast enhancement. Quantitative treatment of digitized images allows absolute determination of the density of different components of the observed sample, including antigens, intracellular calcium and pH. Morphometric studies are also greatly facilitated by image processing techniques. The capture of fast phenomena may be performed by transfer of small portion of microscopic images into computer memory as well as particular use of confocal microscopy. Finally, improved display of experimental data through coded colors or other procedures may enhance the amount of information that can be conveyed by visual examination of microscopical images. The purpose of the present review is to describe the basic principles of image processing and exemplify the power of this approach with a variety of illustrated applications to conventional, fluorescence or electron microscopy as well as confocal microscopy.

F-actin content and spatial distribution in resting and chemoattractant-stimulated human polymorphonuclear leucocytes. Which role for intracellular free calcium?

Intracellular free calcium concentration ([Ca2+]i) plays a pivotal role for many responses in polymorphonuclear leucocytes (PMNs) stimulated by chemoattractants such as N-formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe). The importance of [Ca2+]i in the morphological polarization was investigated by using calcium-manipulated PMNs. We loaded human PMNs with BAPTA/AM to buffer or chelate [Ca2+]i in the presence or the absence of extracellular calcium by using fluo-3/AM as calcium indicator. The shape changes of PMNs were determined by microscopic examination, and membrane ruffling by right-angle light-scatter changes. Actin polymerization and F-actin distribution were recorded by staining PMNs with bodipy-phallacidin and quantified by quantitative fluorescence microscopy. We found that calcium-free incubation of PMNs loaded or not with 50 microM BAPTA/AM did not modify morphological polarization, membrane ruffling, actin assembly and F-actin distribution of PMNs stimulated with fMet-Leu-Phe, suggesting that these responses were probably functionally linked. It should be noted that incubation of PMNs in calcium-free conditions resulted in a radial distribution of F-actin and a moderate polymerization of actin, but not in morphological polarization of PMNs. Moreover, both calcium-sensitive and calcium-insensitive mechanisms of actin polymerization were additive, and inhibitable by 5 micrograms/ml cytochalasin B.

Analysis of the topological changes induced on cells exposed to adhesive or mechanical stimuli

Fluorescent probes are widely used to study cell structure and function. However, few reports were devoted to a quantitative analysis of the intracellular distribution of fluorescent markers. In the present work, we describe the topographical changes of surface and cytoskeletal markers on individual cells subjected to adhesive or mechanical interaction. Conjugates were prepared with a cytotoxic T-lymphocyte clone and target cells. Specific antigens, membrane phospholipids, surface glycoconjugates, and polymerized actin were labeled with fluorescent antibodies or biochemical probes. The analysis of fluorescence distributions in conjugates demonstrated a selective reorganization of the plasma membrane with a gathering of some molecular species in the intercellular adhesion area. Furthermore, individual phagocytic cells were sucked into glass micropipets, then stained with fluorescent phallacidin to analyze the effect of mechanical efforts on the cytoskeleton organization. The concentration of polymerized actin was found to be similar in mechanically-induced protrusions and whole cells. It is concluded that adhesive interactions may result in marked cell polarization and formation of membrane zones with a particular biochemical composition. The submembranar cytoskeleton might play a role in this process.