Activation of cell-surface receptors stimulates generation of intracellular signals that, in turn, direct the cellular response. However, mechanisms that ensure combinatorial control of these signaling events are not well understood. We show here that the Ca2+ and reactive oxygen intermediates generated upon BCR activation rapidly engage in a cooperative interaction that acts in a feedback manner to amplify the early signal generated. This cooperativity acts by regulating the concentration of the oxidant produced. The latter exerts its influence through a pulsed inactivation of receptor-coupled phosphatases, where the amplitude of this pulse is determined by oxidant concentration. The extent of phosphatase inhibition, in turn, dictates what proportion of receptor-proximal kinases are activated and, as a result, the net strength of the initial signal. It is the strength of this initial signal that finally determines the eventual duration of BCR signaling and the rate of its transmission through downstream pathways.
Here we report a novel regulatory mechanism for autophagy-mediated degradation of Mycobacterium tuberculosis (Mtb) and specific strategy exploited by the virulent Mtb to evade it. We show while both avirulent (H37Ra) and virulent (H37Rv) mycobacteria could readily localize to autophagosomes, their maturation into autolysosomes (flux) was significantly inhibited by the latter strain. The inhibition of autophagy flux by the virulent strain was highly selective, as it did not perturb the basal autophagy flux in the macrophages. Selective inhibition of flux of Mtb-containing autophagosomes required virulence regulators PhoP and ESAT-6. We show that the maturation of Mtb-containing autophagosomes into autolysosomes required recruitment of the late endosome marker RAB7, forming the intermediate compartment amphisomes. Virulent Mtb selectively evaded their targeting to the amphisomes. Thus we report a crosstalk between autophagy and phagosome maturation pathway and highlight the adaptability of Mtb, manifested by selective regulation of autophagy flux.
The success of Mycobacterium tuberculosis as a pathogen derives from its facile adaptation to the intracellular milieu of human macrophages. To explore this process, we asked whether adaptation also required interference with the metabolic machinery of the host cell. Temporal profiling of the metabolic flux, in cells infected with differently virulent mycobacterial strains, confirmed that this was indeed the case. Subsequent analysis identified the core subset of host reactions that were targeted. It also elucidated that the goal of regulation was to integrate pathways facilitating macrophage survival, with those promoting mycobacterial sustenance. Intriguingly, this synthesis then provided an axis where both host- and pathogen-derived factors converged to define determinants of pathogenicity. Consequently, whereas the requirement for macrophage survival sensitized TB susceptibility to the glycemic status of the individual, mediation by pathogen ensured that the virulence properties of the infecting strain also contributed towards the resulting pathology.
In this study, germline Abs were used to select clones from a random dodecapeptide phage-display library. This revealed a much greater heterogeneity of binders than could be obtained with mutated daughter Abs that presumably had been selected in vivo by nominal Ag during active immune responses. We demonstrate that the pluripotency of germline Abs can subsequently be optimized by binding interactions that correlate with thermodynamic changes indicative of structural adaptations at the interface. This singular feature confers on each Ab a distinct window of Ag specificities, where the entropic space explored constitutes a thermodynamic signature of that particular Ab. Combining site plasticity may facilitate overlaps in such windows, with independent Abs converging onto common determinants with near identical binding affinities. In addition to providing for an amplified recognition potential, this networking of individual spectra of Ag specificities simultaneously facilitates the rapid recognition of Ag. Importantly, it also ensures that the primary response is composed of Abs with a high degree of “evolvability.”
Survival of Mycobacterium tuberculosis (Mtb) within the host macrophage is mediated through pathogen-dependent inhibition of phagosome-lysosome fusion, which enables bacteria to persist within the immature phagosomal compartment. By employing ultrastructural examination of different field isolates supported by biochemical analysis, we found that some of the Mtb strains were in fact poorly adapted for subsistence within endocytic vesicles of infected macrophages. Instead, through a mechanism involving activation of host cytosolic phospholipase A2, these bacteria rapidly escaped from phagosomes, and established residence in the cytoplasm of the host cell. Interestingly, by facilitating an enhanced suppression of host cellular autophagy, this translocation served as an alternate virulence acquisition mechanism. Thus, our studies reveal plasticity in the adaptation strategies employed by Mtb, for survival in the host macrophage.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.