Integrin regulation by RhoA in thymocytes

A survival guide to early T cell development

The survival of immature T cell precursors is dependent on both thymus-derived extrinsic signals and self-autonomous pre-TCR-mediated signals. While the role of cytokines and the pre-TCR in promoting thymocyte survival has been well established, the relationship between pro- and anti-apoptotic signaling cascades remains poorly defined. Recent studies have established a link between cell survival and growth factor-mediated maintenance of cellular metabolism. In this regard, the Notch signaling pathway has emerged as more than an inducer of T lineage commitment and differentiation, but also as a potent trophic factor, promoting the survival and metabolic state of pre-T cells. In this review, we describe current concepts of the intracellular signaling pathways downstream of cell intrinsic and extrinsic factors that dictate survival versus death outcomes during early T cell development.

Signature patterns revealed by microarray analyses of mice infected with influenza virus A and Streptococcus pneumoniae

We used cDNA microarrays to identify differentially expressed genes in mice in response to infections with influenza virus A/PR/8/34 (H1N1) and Streptococcus pneumoniae. Expression microarray analysis showed up-regulation and down-regulation of many genes involved in the defense, inflammatory response and intracellular signaling pathways including chemokine, apoptosis, MAPK, Notch, Jak-STAT, T-cell receptor and complement and coagulation cascades. We have revealed signature patterns of gene expression in mice infected with two different classes of pathogens: influenza virus A and S. pneumoniae. Quantitative real-time RT-PCR results confirmed microarray results for most of the genes tested. These studies document clear differences in gene expression profiles between mice infected with influenza virus A and S. pneumoniae. Identification of genes that are differentially expressed after respiratory infections can provide insights into the mechanisms by which the host interacts with different pathogens, useful information about stage of diseases and selection of suitable targets for early diagnosis and treatments. The advantage of this novel approach is that the detection of pathogens is based on the differences in host gene expression profiles in response to different pathogens instead of detecting pathogens directly.

Two distinct cytoplasmic regions of the beta2 integrin chain regulate RhoA function during phagocytosis

α_Mβ₂ integrins mediate phagocytosis of opsonized particles in a process controlled by RhoA, Rho kinase, myosin II, Arp2/3, and actin polymerization. α_Mβ₂, Rho, Arp2/3, and F-actin accumulate underneath bound particles; however, the mechanism regulating Rho function during α_Mβ₂-mediated phagocytosis is poorly understood. We report that the binding of C3bi-opsonized sheep red blood cells (RBCs) to α_Mβ₂ increases Rho-GTP, but not Rac-GTP, levels. Deletion of the cytoplasmic domain of β₂, but not of α_M, abolished Rho recruitment and activation, as well as phagocytic uptake. Interestingly, a 16–amino acid (aa) region in the membrane-proximal half of the β₂ cytoplasmic domain was necessary for activating Rho. Three COOH-terminal residues (aa 758–760) were essential for β₂-induced accumulation of Rho at complement receptor 3 (CR3) phagosomes. Activation of Rho was necessary, but not sufficient, for its stable recruitment underneath bound particles or for uptake. However, recruitment of active Rho was sufficient for phagocytosis. Our data shed light on the mechanism of outside-in signaling, from ligated integrins to the activation of Rho GTPase signaling.

Differential requirement for RhoA GTPase depending on the cellular localization of protein kinase D

This study explores the links between the GTPase RhoA and the serine kinase protein kinase D (PKD) during thymocyte development. The rationale is that RhoA and PKD regulate common biological responses during T cell development, but there is nothing known about their interdependence. In fibroblasts, Rho function is required for activation of PKD catalytic activity. However, the data show that activation of Rho is neither sufficient nor essential for PKD activation in T cells. One alternative explanation for the apparent convergence of PKD and Rho signaling in T cells is that PKD responses might be Rho-dependent. To address this latter possibility, we probed the Rho requirements for the actions of constitutively active PKD mutants in pre-T cells of transgenic mice. Active PKD can localize to either the plasma membrane or the cytosol, and we therefore compared the Rho requirements for the actions of membrane- or cytosol-localized PKD. Here we show that membrane-localized PKD regulation of pre-T cell differentiation is Rho-dependent, but the actions of cytosol-localized PKD are not. These studies demonstrate that a Rho requirement for PKD activation is not ubiquitous. Moreover, links between PKD and Rho are determined by the cellular location of PKD.

T lymphocytes on the move: chemokines, PI 3-kinase and beyond

The ordered, directional migration of T lymphocytes is a key process in development, immune surveillance and the immune response. Chemokines have an important role in the guidance of T lymphocytes and activate several members of the phosphoinositide 3-kinase (PI 3-kinase) family, which contribute to various aspects of the migratory machinery in many cell systems. However, the role of PI 3-kinase in T-cell movement is unclear, and its importance has been largely dismissed. Over the past two years, there has been exciting progress in our appreciation not only of the finer details of PI 3-kinase involvement in T-cell migration, but also of other signalling events that probably influence T-cell migration in response to recognized chemoattractants. These aspects of T-cell migration are the subject of this review.

Tracking thymocyte migration in situ

The dynamic process of thymocyte migration can now be visualized in real-time and in the context of the native thymic environment. With improved computational resources, key information can be extracted from real-time imaging data and the migratory behaviors of developing thymocytes can be quantitated. The extraction and exploitation of three dimensional data through time is providing new insight into the nature and regulation of intrathymic migration. In this review we discuss this interdisciplinary approach and the promise it holds for the study of thymocyte migration in situ.