Two hats too many

AbstractMy Word: Ronald N. Germain believes it's time scientists eliminated splits in their personality between researching and reviewing

Illuminating the Landscape of In Vivo Immunity Insights from Dynamic In Situ Imaging of Secondary Lymphoid Tissues

AbstractA central feature of the immune system is the migratory behavior of its cellular components. Thus, fully understanding the generation and maintenance of immune responses must include consideration of how hematopoietic cells home to, interact within, and exit from secondary lymphoid organs as well as peripheral tissues. Recent advances in in situ imaging techniques now permit direct observation of these events in their physiologic settings with high spatiotemporal resolution. This review summarizes progress in this area of investigation from a lymphocentric perspective. We highlight controversies, point out key unanswered questions, and briefly outline what we believe are some of the near-term directions that in situ microscopic analysis of the immune system will take

Cross-Antagonism of a T Cell Clone Expressing Two Distinct T Cell Receptors

AbstractInhibition of T cell activation can be mediated by analogs of the original antigenic peptide (TCR antagonists). Here, a T cell clone expressing two distinct TCR was used to investigate whether such inhibition involves an active mechanism by examining whether an antagonist for one TCR could influence responses stimulated by the other TCR engaging its agonist. Our results demonstrate functional cross-inhibition under these conditions involving the ability of antagonist:TCR interactions to diminish Lck enzymatic activity associated with the agonist-recognizing second TCR, apparently through enhancement of SHP-1 association with these receptors. Our findings reveal that inhibition of cellular responses by antagonists arises at least in part from active negative regulation of proximal TCR signaling and identify elements of the biochemical process

Revisiting Thymic Positive Selection and the Mature T Cell Repertoire for Antigen

To support effective host defense, the T cell repertoire must balance breadth of recognition with sensitivity for antigen. The concept that T lymphocytes are positively selected in the thymus is well established, but how this selection achieves such a repertoire has not been resolved. Here we suggest that it is direct linkage between self and foreign antigen recognition that produces the necessary blend of TCR diversity and specificity in the mature peripheral repertoire, enabling responses to a broad universe of unpredictable antigens while maintaining an adequate number of highly sensitive T cells in a population of limited size. Our analysis also helps to explain how diversity and frequency of antigen-reactive cells in a T cell repertoire are adjusted in animals of vastly different size scale to enable effective antipathogen responses and suggests a possible binary architecture in the TCR repertoire that is divided between germline-related optimal binding and diverse recognition

Constitutive Presentation of a Natural Tissue Autoantigen Exclusively by Dendritic Cells in the Draining Lymph Node

The major histocompatibility complex (MHC)-dependent presentation of processed tissue-specific self-antigens can contribute to either peripheral (extrathymic) tolerance or the differentiation of autoreactive T cells. Here, we have studied the MHC class II molecule presentation of gastric parietal cell (PC)-specific H+/K+-ATPase, which induces a destructive autoimmune gastritis in BALB/c mice lacking CD4+ CD25+ regulatory T cells. Immunofluorescence microscopy showed physical association of CD11c+ dendritic cells (DCs) with PCs in the gastric mucosa. H+/K+-ATPase protein was found within vesicular compartments of a few CD11c+ DCs only in the draining gastric lymph node (LN) and these antigen-containing DCs increased markedly in number with the onset of tissue destruction in autoimmune animals. Both CD8αhi and CD8αlo gastric DCs, but not peripheral or mesenteric DCs, showed evidence of constitutive in vivo processing and presentation of H+/K+-ATPase. These data provide direct support for a widely held model of local tissue antigen uptake and trafficking by DCs in normal animals and demonstrate that DCs in the draining LN can present a tissue-specific self-antigen under noninflammatory conditions without fully deleting autoreactive T cells or inducing active autoimmunity

Key Role of Local Regulation in Chemosensing Revealed by a New Molecular Interaction-Based Modeling Method

The signaling network underlying eukaryotic chemosensing is a complex combination of receptor-mediated transmembrane signals, lipid modifications, protein translocations, and differential activation/deactivation of membrane-bound and cytosolic components. As such, it provides particularly interesting challenges for a combined computational and experimental analysis. We developed a novel detailed molecular signaling model that, when used to simulate the response to the attractant cyclic adenosine monophosphate (cAMP), made nontrivial predictions about Dictyostelium chemosensing. These predictions, including the unexpected existence of spatially asymmetrical, multiphasic, cyclic adenosine monophosphate–induced PTEN translocation and phosphatidylinositol-(3,4,5)P(3) generation, were experimentally verified by quantitative single-cell microscopy leading us to propose significant modifications to the current standard model for chemoattractant-induced biochemical polarization in this organism. Key to this successful modeling effort was the use of “Simmune,” a new software package that supports the facile development and testing of detailed computational representations of cellular behavior. An intuitive interface allows user definition of complex signaling networks based on the definition of specific molecular binding site interactions and the subcellular localization of molecules. It automatically translates such inputs into spatially resolved simulations and dynamic graphical representations of the resulting signaling network that can be explored in a manner that closely parallels wet lab experimental procedures. These features of Simmune were critical to the model development and analysis presented here and are likely to be useful in the computational investigation of many aspects of cell biology

Natural killer cell behavior in lymph nodes revealed by static and real-time imaging

Natural killer (NK) cells promote dendritic cell (DC) maturation and influence T cell differentiation in vitro. To better understand the nature of the putative interactions among these cells in vivo during the early phases of an adaptive immune response, we have used immunohistochemical analysis and dynamic intravital imaging to study NK cell localization and behavior in lymph nodes (LNs) in the steady state and shortly after infection with Leishmania major. In the LNs of naive mice, NK cells reside in the medulla and the paracortex, where they closely associate with DCs. In contrast to T cells, intravital microscopy revealed that NK cells in the superficial regions of LNs were slowly motile and maintained their interactions with DCs over extended times in the presence or absence of immune-activating signals. L. major induced NK cells to secrete interferon-γ and to be recruited to the paracortex, where concomitant CD4 T cell activation occurred. Therefore, NK cells form a reactive but low mobile network in a strategic area of the LN where they can receive inflammatory signals, interact with DCs, and regulate colocalized T cell responses