Does a quorum sensing mechanism direct the behavior of immune cells?

Adaptive discrimination between harmful and harmless antigens in the immune system by predictive coding

The immune system discriminates between harmful and harmless antigens based on past experiences; however, the underlying mechanism is largely unknown. From the viewpoint of machine learning, the learning system predicts the observation and updates the prediction based on prediction error, a process known as "predictive coding." Here, we modeled the population dynamics of T cells by adopting the concept of predictive coding; conventional and regulatory T cells predict the antigen concentration and excessive immune response, respectively. Their prediction error signals, possibly via cytokines, induce their differentiation to memory T cells. Through numerical simulations, we found that the immune system identifies antigen risks depending on the concentration and input rapidness of the antigen. Further, our model reproduced history-dependent discrimination, as in allergy onset and subsequent therapy. Taken together, this study provided a novel framework to improve our understanding of how the immune system adaptively learns the risks of diverse antigens.

In vivo imaging of EVs in zebrafish: New perspectives from "the waterside"

To harmoniously coordinate the activities of all its different cell types, a multicellular organism critically depends on intercellular communication. One recently discovered mode of intercellular cross-talk is based on the exchange of "extracellular vesicles" (EVs). EVs are nano-sized heterogeneous lipid bilayer vesicles enriched in a variety of biomolecules that mediate short- and long-distance communication between different cells, and between cells and their environment. Numerous studies have demonstrated important aspects pertaining to the dynamics of their release, their uptake, and sub-cellular fate and roles in vitro. However, to demonstrate these and other aspects of EV biology in a relevant, fully physiological context in vivo remains challenging. In this review we analyze the state of the art of EV imaging in vivo, focusing in particular on zebrafish as a promising model to visualize, study, and characterize endogenous EVs in real-time and expand our understanding of EV biology at cellular and systems level.

Quorum Sensing by Monocyte-Derived Populations

Quorum sensing is a type of cellular communication that was first described in bacteria, consisting of gene expression regulation in response to changes in cell-population density. Bacteria synthesize and secrete diffusive molecules called autoinducers, which concentration varies accordingly with cell density and can be detected by the producing cells themselves. Once autoinducer concentration reaches a critical threshold, all bacteria within the autoinducer-rich environment react by modifying their genetic expression and adopt a coordinated behavior (e.g., biofilm formation, virulence factor expression, or swarming motility). Recent advances highlight the possibility that such type of communication is not restricted to bacteria, but can exist among other cell types, including immune cells and more specifically monocyte-derived cells (1). For such cells, quorum sensing mechanisms may not only regulate their population size and synchronize their behavior at homeostasis but also alter their activity and function in unexpected ways during immune reactions. Although the nature of immune autoinducers and cellular mechanisms remains to be fully characterized, quorum sensing mechanisms in the immune system challenge our traditional conception of immune cell interactions and likely represent an important mode of communication at homeostasis or during an immune response. In this mini-review, we briefly present the prototypic features of quorum sensing in bacteria and discuss the existing evidence for quorum sensing within the immune system. Mainly, we review quorum sensing mechanisms among monocyte-derived cells, such as the regulation of inflammation by the density of monocyte-derived cells that produce nitric oxide and discuss the relevance of such models in the context of immune-related pathologies.

The Potential Role of Quorum Sensing in Clonal Growth and Subsequent Expansion of Bone Marrow Stromal Cell Strains in Culture

Clonal development (clonogenicity) is an inherent property of a subset of postnatal bone marrow (BM) adherent stromal mesenchymal stem cells (MSCs) from which a multipotent progeny develops in culture. Our data suggest that clonogenicity and BM-MSC expansion are two distinct biological events. This hypothesis is based on the following observations: (1) the beginning of clonal growth is a property strictly dependent on serum and independent of the social context, (2) the expansion of individual clone is influenced by events deriving from a social context during initial growth, (3) clonogenic cells grown in a social context in presence of serum can emancipate themselves to generate a secondary different progeny, and (4) the ability of socially generated clones to develop an inherent potential for further growth suggests that quorum sensing may operate in BM-MSC cultures and determine the potential growth of clonal strains.

Abrupt and gradual onset of synchronized oscillations due to dynamical quorum sensing in the single-cathode multi-anode nickel electrodissolution system

The nonlinear dynamics of an oscillatory Ni electrodissolution-hydrogen ion reduction system are explored in a multi-electrode anode-single cathode system. A mathematical analysis of the charge balance equations reveals that the coupling scheme is similar to dynamical quorum sensing, where the number of anode wires affects a parameter related to the population density. In a parameter region where the large population exhibits stationary behavior, with sufficiently strong coupling (with small individual resistances attached to the anode wires), synchronized oscillations emerge abruptly with decreasing the number of anodes. Therefore, an "inverse" dynamical quorum sensing takes place. With weak coupling the transition is gradual. The experiments are supported by numerical simulation of a kinetic model of the process. The results thus show that the description of nontrivial cathode-anode interactions in the form of dynamical quorum sensing provides an efficient way of analyzing the dynamical response of complex, interacting electrochemical reactions.

AFM study shows prominent physical changes in elasticity and pericellular layer in human acute leukemic cells due to inadequate cell-cell communication

Biomechanical properties of single cells in vitro or ex vivo and their pericellular interfaces have recently attracted a lot of attention as a potential biophysical (and possibly prognostic) marker of various diseases and cell abnormalities. At the same time, the influence of the cell environment on the biomechanical properties of cells is not well studied. Here we use atomic force microscopy to demonstrate that cell-cell communication can have a profound effect on both cell elasticity and its pericellular coat. A human pre-B p190^BCR/ABL acute lymphoblastic leukemia cell line (ALL3) was used in this study. Assuming that cell-cell communication is inversely proportional to the distance between cells, we study ALL3 cells in vitro growing at different cell densities. ALL3 cells demonstrate a clear density dependent behavior. These cells grow very well if started at a relatively high cell density (HD, >2 × 10⁵ cells ml^-1) and are poised to grow at low cell density (LD, <1 × 10⁴ cells ml^-1). Here we observe ∼6× increase in the elastic (Young's) modulus of the cell body and ∼3.6× decrease in the pericellular brush length of LD cells compared to HD ALL3 cells. The difference observed in the elastic modulus is much larger than typically reported for pathologically transformed cells. Thus, cell-cell communication must be taken into account when studying biomechanics of cells, in particular, correlating cell phenotype and its biophysical properties.

Effect of purified fractions from cell culture supernate of high-density pre-B acute lymphoblastic leukemia cells (ALL3) on the growth of ALL3 cells at low density

The mechanisms underlying the aberrant growth and interactions between cells are not understood very well. The pre-B acute lymphoblastic leukemia cells directly obtained from an adult patient grow very poorly or do not grow at all at low density (LD), but grow better at high starting cell density (HD). We found that the LD ALL3 cells can be stimulated to grow in the presence of diffusible, soluble factors secreted by ALL3 cells themselves growing at high starting cell density. We then developed a biochemical purification procedure that allowed us to purify the factor(s) with stimulatory activity and analyzed them by nanoliquid chromatography-tandem mass spectrometry (nanoLC-MS/MS). Using nanoLC-MS/MS we have identified several proteins which were further processed using various bioinformatics tools. This resulted in eight protein candidates which might be responsible for the growth activity on non-growing LD ALL3 cells and their involvement in the stimulatory activity are discussed.