Environmental control of immunological synapse formation and duration

Pathobiology of HIV/SIV-associated changes in secondary lymphoid tissues

Acquired immunodeficiency syndrome (AIDS) is principally a disease of lymphoid tissues (LTs), due to the fact that the main target cell of human immunodeficiency virus (HIV) is the CD4(+) T lymphocyte that primarily resides within organs of the immune system. The impact of HIV infection on secondary LTs, in particular lymph nodes, is critical to delineate, as these immune organs are the principal sites for initiating and facilitating immune responses and are critical for lymphocyte homeostatic maintenance and survival. The underlying structural elements of LTs, fibroblastic reticular cell (FRC) network, not only form the architectural framework for these organs, but also play in integral role in the production and storage of cytokines needed for T-cell survival. There is an interdependent relationship between the FRC stromal network and CD4(+) T lymphocytes for their survival and maintenance that is progressively disrupted during HIV disease. HIV infection results in profound pathological changes to LTs induced by persistent chronic immune activation and inflammation that leads to progressive collagen deposition and fibrosis disrupting and damaging the important FRC network. In this review, I focus on the process, mechanisms, and the implications of pathological damage to important secondary LTs, combining what we have learned from HIV-infected individuals as well as the invaluable knowledge gained from studies in non-human primate simian immunodeficiency virus infection models.

The yin and yang of protein kinase C-theta (PKCθ): a novel drug target for selective immunosuppression

Protein kinase C-theta (PKCθ) is a protein kinase C (PKC) family member expressed predominantly in T lymphocytes, and extensive studies addressing its function have been conducted. PKCθ is the only T cell-expressed PKC that localizes selectively to the center of the immunological synapse (IS) following conventional T cell antigen stimulation, and this unique localization is essential for PKCθ-mediated downstream signaling. While playing a minor role in T cell development, early in vitro studies relying, among others, on the use of PKCθ-deficient (Prkcq(-/-)) T cells revealed that PKCθ is required for the activation and proliferation of mature T cells, reflecting its importance in activating the transcription factors nuclear factor kappa B, activator protein-1, and nuclear factor of activated T cells, as well as for the survival of activated T cells. Upon subsequent analysis of in vivo immune responses in Prkcq(-/-) mice, it became clear that PKCθ has a selective role in the immune system: it is required for experimental Th2- and Th17-mediated allergic and autoimmune diseases, respectively, and for alloimmune responses, but is dispensable for protective responses against pathogens and for graft-versus-leukemia responses. Surprisingly, PKCθ was recently found to be excluded from the IS of regulatory T cells and to negatively regulate their suppressive function. These attributes of PKCθ make it an attractive target for catalytic or allosteric inhibitors that are expected to selectively suppress harmful inflammatory and alloimmune responses without interfering with beneficial immunity to infections. Early progress in developing such drugs is being made, but additional studies on the role of PKCθ in the human immune system are urgently needed.

Age-related defects in the cytoskeleton signaling pathways of CD4 T cells

It has been postulated that the cytoskeleton controls many aspects of T cell function, including activation, proliferation and apoptosis. Recent advances in our understanding of F-actin polymerization and the Ezrin-Radixin-Moesin (ERM) family of cytoskeleton signal proteins have provided new insights into immunological synapse formation during T cell activation. During aging there is a significant decline of T cell function largely attributable to declines in activation of CD4 T cells and defects in the formation of the immunological synapse. Here we discuss recent progress in the understanding of how aging alters F-actin and ERM proteins in mouse CD4 T cells, and the implications of these changes for the T cell activation process.

Progress in mimetic studies of cell adhesion and the mechanosensing

Vesicle-substrate adhesion has been studied for over two decades with the motivation to understand and mimic cell adhesion. In recent years, with progress in theoretical modelling, the development of experimental techniques, and improved data-analysis procedures, considerable advances have been made in the understanding of the adhesion process. It is this progress which constitutes the focus of this review.

Quantification of cell edge velocities and traction forces reveals distinct motility modules during cell spreading

Actin-based cell motility and force generation are central to immune response, tissue development, and cancer metastasis, and understanding actin cytoskeleton regulation is a major goal of cell biologists. Cell spreading is a commonly used model system for motility experiments -- spreading fibroblasts exhibit stereotypic, spatially-isotropic edge dynamics during a reproducible sequence of functional phases: 1) During early spreading, cells form initial contacts with the surface. 2) The middle spreading phase exhibits rapidly increasing attachment area. 3) Late spreading is characterized by periodic contractions and stable adhesions formation. While differences in cytoskeletal regulation between phases are known, a global analysis of the spatial and temporal coordination of motility and force generation is missing. Implementing improved algorithms for analyzing edge dynamics over the entire cell periphery, we observed that a single domain of homogeneous cytoskeletal dynamics dominated each of the three phases of spreading. These domains exhibited a unique combination of biophysical and biochemical parameters -- a motility module. Biophysical characterization of the motility modules revealed that the early phase was dominated by periodic, rapid membrane blebbing; the middle phase exhibited continuous protrusion with very low traction force generation; and the late phase was characterized by global periodic contractions and high force generation. Biochemically, each motility module exhibited a different distribution of the actin-related protein VASP, while inhibition of actin polymerization revealed different dependencies on barbed-end polymerization. In addition, our whole-cell analysis revealed that many cells exhibited heterogeneous combinations of motility modules in neighboring regions of the cell edge. Together, these observations support a model of motility in which regions of the cell edge exhibit one of a limited number of motility modules that, together, determine the overall motility function. Our data and algorithms are publicly available to encourage further exploration.

3D and 4D imaging of immune cells in vitro and in vivo

Analyzing the dynamics of cellular immune responses, although performed for decades in immunologic research, has seen an enormous increase in the number of studies using this approach since the development of intravital 2-photon microscopy. Meanwhile, new insights into the dynamics of cellular immunity are being published on a daily basis. This review gives a short overview of the currently most widely used techniques, both on the microscopy side as well as on the experimental part. Difficulties and promises will be discussed. Finally, a personal selection of the most interesting findings of the first 6 years of intravital 2-photon microscopy for immunological questions will be given. The overall aim is to get the reader interested into this fascinating way of investigating the immune response by means of "dynamic histology". This already has and will continue to broaden our view on how immune cells work in real life.

Migration, cell-cell interaction and adhesion in the immune system

Migration is an essential function of immune cells. It is necessary to lead immune cell precursors from their site of generation to the places of maturation or function. Cells of the adaptive immune system also need to interact physically with each other or with specialized antigen presenting cells in lymphatic tissues in order to become activated. Thereby a complex series of controlled migration events, adhesive interactions and signalling responses is induced. Finally cells must be able to leave the activating tissues and re-enter the bloodstream from which they extravasate into inflamed tissue sites. Cells of the innate immune system can function directly without the need for previous activation. However, these cells have to adapt their function to a panoply of pathogens and environmental niches which can be invaded. The current review highlights the central aspects of cellular dynamics underlying adaptive and innate cellular immunity. Thereby a focus will be put on recent results obtained by microscopic observation of live cells in vitro or by intravital 2-photon microscopy in live animals.

Chapter 15: Live-cell single-molecule force spectroscopy

We describe a method to measure the kinetics and micromechanical properties of individual receptor-ligand bonds formed between two living cells. Using living cells rather than recombinant proteins ensures that the orientation, surface density, and posttranslational modifications of the probed receptors are physiological and that their regulated attachment to the cytoskeleton can occur. A cell is tethered to a flexible cantilever and brought into contact with cells adherent to a substratum before being pulled at a controlled retraction velocity. Measurements of bond rupture forces and associated bond loading rates over an extended range of retraction velocities allow us to compute precisely the tensile strength, reactive compliance, lifetime, and dissociation rate of individual intercellular receptor-ligand bonds. We also describe tests of specificity and Monte Carlo simulations, which ensure that measurements obtained by this method correspond to a single type of intercellular adhesion bond. We illustrate this live-cell single molecule force spectroscopy assay by characterizing homotypic bonds composed of vascular endothelial -cadherin pairs formed between living endothelial cells. This versatile assay could be used to establish the molecular principles that drive a wide range of important physiological processes involving receptor-mediated intercellular adhesion, such as the immunological synapse between a lymphocyte and an antigen-presenting cell and synaptic interactions between neuron cells, and pathological processes resulting in altered intercellular adhesion.

The biophysics of T lymphocyte activation in vitro and in vivo

T cell activation is crucial for the development of specific immune reactions. It requires physical contact between T cells and antigen-presenting cells (APC). Since these cells are initially located at distinct positions in the body, they have to migrate and find each other within secondary lymphoid organs. After encountering each other both cells have to maintain a close membrane contact sufficiently long to ensure successful signaling. Thus, there is the necessity to temporarily synchronize the motile behavior of these cells. Initially, it had been proposed that during antigen recognition, T cells receive a stop signal and maintain a stable contact with APC for several hours when an appropriate APC has been encountered. However, direct cell observation via time-lapse microscopy in vitro and in vivo has revealed a different picture. While long contacts can be observed, many interactions appear to be very short and sequential despite efficient signaling. Thus, two concepts addressing the biophysics of T cell activation have emerged. The single encounter model proposes that after a period of dynamic searching, a T cell stops to interact with one appropriately presenting APC until signaling is completed. The serial encounter model suggests that T cells are able to collect a series of short signals by different APC until a critical activation threshold is achieved. Future research needs to clarify the relative importance of short and dynamic versus long-lived T cell-APC encounters for the outcome of T cell activation. Furthermore, a thorough understanding of the molecular events underlying the observed complex motility patterns will make these phenomena amenable for intervention, which might result in the identification of new types of immune modulating drugs.

Application of in vivo microscopy: evaluating the immune response in living animals

The initiation of an immune response requires that professional antigen-presenting cells, such as dendritic cells, physically interact with antigen-specific T cells within the complex environment of the lymph node. Although the way in which antigen is presented to T cells and in particular the cellular associations involved in antigen-specific stimulation events have been extensively investigated, data on antigen presentation have come primarily from studies in vitro or examination of the late consequences of antigen presentation in vivo. However, there is increasing recognition that events defined in vitro might not correspond entirely to the physiological situation in vivo. Recent developments in imaging technology now allow real-time observation of single-cell and molecular interactions in intact lymphoid tissues and have already contributed to a more detailed picture of how cells coordinate the initiation or suppression of an immune response.

Clustering of T cell ligands on artificial APC membranes influences T cell activation and protein kinase C theta translocation to the T cell plasma membrane

T cell activation is associated with active clustering of relevant molecules in membrane microdomains defined as the supramolecular activation cluster. The contact area between these regions on the surface of T cells and APC is defined as the immunological synapse. It has been recently shown that preclustering of MHC-peptide complexes in membrane microdomains on the APC surface affects the efficiency of immune synapse formation and the related T cell activation. Disruption of such clusters may reduce the efficiency of stimulation. We describe here an entirely artificial system for Ag-specific, ex vivo stimulation of human polyclonal T cells (artificial APC (aAPC)). aAPC are based on artificial membrane bilayers containing discrete membrane microdomains encompassing T cell ligands (i.e., appropriate MHC-peptide complexes in association with costimulatory molecules). We show here that preclustering of T cell ligands triggered a degree of T cell activation significantly higher than the one achieved when we used either soluble tetramers or aAPC in which MHC-peptide complexes were uniformly distributed within artificial bilayer membranes. This increased efficiency in stimulation was mirrored by increased translocation from the cytoplasm to the membrane of protein kinase theta, a T cell signaling molecule that colocalizes with the TCR within the supramolecular activation cluster, thus indicating efficient engagement of T cell activation pathways. Engineered aAPC may have immediate application for basic and clinical immunology studies pertaining to modulation of T cells ex vivo.

Adhesion of human T cells to antigen-presenting cells through SIRPβ2-CD47 interaction costimulates T-cell proliferation

Signal-regulatory proteins (SIRPs) are transmembrane glycoproteins belonging to the immunoglobulin (Ig) superfamily that are expressed in the immune and central nervous systems. SIRPα binds CD47 and inhibits the function of macrophages, dendritic cells, and granulocytes, whereas SIRPβ1 is an orphan receptor that activates the same cell types. A recently identified third member of the SIRP family, SIRPβ2, is as yet uncharacterized in terms of expression, specificity, and function. Here, we show that SIRPβ2 is expressed on T cells and activated natural killer (NK) cells and, like SIRPα, binds CD47, mediating cell-cell adhesion. Consequently, engagement of SIRPβ2 on T cells by CD47 on antigen-presenting cells results in enhanced antigen-specific T-cell proliferation.

Pattern formation during T-cell adhesion

T cells form intriguing patterns during adhesion to antigen-presenting cells. The patterns are composed of two types of domains, which either contain short TCR/MHCp receptor-ligand complexes or the longer LFA-1/ICAM-1 complexes. The final pattern consists of a central TCR/MHCp domain surrounded by a ring-shaped LFA-1/ICAM-1 domain, whereas the characteristic pattern formed at intermediate times is inverted with TCR/MHCp complexes at the periphery of the contact zone and LFA-1/ICAM-1 complexes in the center. Several mechanisms have been proposed to explain the T-cell pattern formation. Whereas biologists have emphasized the role of active cytoskeletal processes, previous theoretical studies suggest that the pattern evolution may be caused by spontaneous self-assembly processes alone. Some of these studies focus on circularly symmetric patterns and propose a pivot mechanism for the formation of the intermediate inverted pattern. Here, we present a statistical-mechanical model which includes thermal fluctuations and the full range of spatial patterns. We confirm the observation that the intermediate inverted pattern may be formed by spontaneous self-assembly. However, we find a different self-assembly mechanism in which numerous TCR/MHCp microdomains initially nucleate throughout the contact zone. The diffusion of free receptors and ligands into the contact zone subsequently leads to faster growth of peripheral TCR/MHCp microdomains and to a closed ring for sufficiently large TCR/MHCp concentrations. At smaller TCR/MHCp concentrations, we observe a second regime of pattern formation with characteristic multifocal intermediates, which resemble patterns observed during adhesion of immature T cells or thymozytes. In contrast to other theoretical models, we find that the final T-cell pattern with a central TCR/MHCp domain is only obtained in the presence of active cytoskeletal transport processes.

Dual Function of the Extracellular Matrix: Stimulatory for Cell Cycle Progression of Naive T Cells and Antiapoptotic for Tissue-Derived Memory T Cells

Tissue T cells encounter Ag in a distinct microenvironment, where they are embedded in the interstitial extracellular matrix (ECM). In contrast, while naive T cells are exposed to Ag in the lymph node, immediately after naive T cells are activated they must extravasate into the ECM to function effectively. Because integrin-mediated adhesion to the ECM modulates cell cycle progression and survival in adherent nonimmune cells, we hypothesize that blood and tissue-derived T cells have similarly adapted their behavior to their first or continued encounter with ECM. T cells from peripheral blood (PBT) and tissue (the intestinal lamina propria T cell (LPT)) were stimulated with anti-CD3-coated beads in the presence or absence of native ECM derived from intestinal fibroblasts, plate-immobilized fibronectin, or collagen type I. Native ECM and collagen, but not fibronectin, induced in anti-CD3 activated PBT a 4- to 5-fold increase in the entry, progression, and completion of the cell cycle over that triggered by anti-CD3 alone. Neutralizing beta1 integrin Abs abrogated this increase. None of these ECM proteins stimulated cell cycle progression in LPT. In contrast, anti-CD3 activation of LPT in the presence of native ECM and fibronectin reduced activation-induced cell death by 40%. These results demonstrate that naive and effector/memory T cells respond differently upon exposure to specific ECM components. When naive PBT encounter Ag in the context of ECM, their progression through the cell cycle is enhanced, favoring clonal expansion; while tissue T cell longevity may be mediated by interactions with the ECM.

Stop and Go Traffic to Tune T Cell Responses

Adaptive immune responses are initiated by interactions of T cells with antigen-presenting cells, but the basic nature of these interactions during an immune response in vivo has been a matter of speculation. While some in vitro systems provide evidence for stable interactions, referred to as immunological synapses, compelling evidence supports T cell activation through serial transient interactions. Deep tissue intravital and organ culture microscopy studies suggest that both modes of interaction are employed, but new issues have emerged. This review will discuss in vitro results that framed the hypotheses that are currently being tested in vivo. I present a model in which TCR stop signals compete with chemokine-mediated go signals to adjust the duration of immunological synapse formation and tune the immune response between tolerance and full activation.

A spectrum of biophysical interaction modes between T cells and different antigen-presenting cells during priming in 3-D collagen and in vivo

For activation T cells engage antigen-presenting cells (APCs) in lymphatic tissues. The contact duration and kinetics (static versus dynamic) vary considerably in different model systems; however, it is unclear whether T cells, APCs, or the environment are responsible for the observed discrepancies. Using 3-D collagen matrices as structural scaffold, we directly compared the kinetics of T-cell engagement and activation by functionally major APC types, ie, dendritic cells (DCs) and resting or activated B cells. Resting B cells engaged T cells in long-lived (several hours), adhesive, and leukocyte function-associated antigen-1 (LFA-1)-dependent conjugates in 3-D collagen as well as in intact lymph nodes in vivo. DCs and preactivated B cells, however, supported predominantly dynamic, short-lived (minutes), and sequential contacts to T cells that were dependent on high cytoskeletal activity of the APCs but could not be inhibited by anti-LFA-1 treatment. Naive T cells were most strongly activated by DCs and activated B cells, whereas resting B cells were 100-fold less efficient to induce T-cell proliferation. Thus, in the same 3-D environment, naive T cells respond with a spectrum of different interaction modes dependent on the type and activation state of the APCs. Thereby, more dynamic interaction kinetics is positively correlated with higher T-cell priming efficiency.

Gap junctions and connexin-mediated communication in the immune system

Gap junctions and connexins are present in the immune system. In haematopoiesis, connexin 43, the most widely distributed gap junction protein, appears to be a key player in the development of progenitor cells and their communication with stromal cells. Connexin 43 is expressed by macrophages, neutrophils and mast cells. Lymphocytes also express connexin 43, and inhibition of gap junction channels in these cells by using highly specific connexin mimetic reagents has profound effects on immunoglobulin secretion and synthesis of cytokines. Lymphocytes and leukocytes also communicate directly in vitro with endothelial cells via gap junctions. Connexins are implicated in inflammatory reactions in a range of tissues. Their involvement in atherosclerotic plaque formation in the vascular system is also a current growth point in research, and could lead to the development of therapeutic interventions.

Integrin alpha2beta1 on rat myeloma cells modulates interaction of alpha4beta1 integrin with vascular cell adhesion molecule-1 but not fibronectin

It is well established that alpha2beta1 integrin functions as a receptor for collagen and laminin; whereas alpha4beta1 integrin binds fibronectin and vascular cell adhesion molecule-1 (VCAM-1). In the present study, we showed that rat myeloma YB2/0 cells constitutively expressed alpha4beta1 but not alpha2beta1 integrin. Transfection of cDNA of mouse a2 integrin subunit resulted in the expression of heterologous alpha2beta1 integrin on YB2/0 cells (YBmalpha2). The expression of alpha2beta1 conferred YBmalpha2 cells the ability to interact with collagen and laminin. In comparison with mock transfected YB2/0 cells (YBpF), YBmalpha2 cells exhibited increases in the binding and migration on VCAM-1; in contrast, both YBpF and YBmalpha2 were similar in their interactions with fibronectin or fibronectin fragment FN-40 that contains the binding site for alpha4beta1 integrin. The interaction of alpha4beta1 with VCAM-1 was further stimulated upon ligation with alpha2beta1-specific mAb. The use of specific inhibitory mAb demonstrated the role of alpha4beta1 in mediating the observed interactions with fibronectin and VCAM-1. Therefore, results show that expression of alpha2beta1 differentially regulated alpha4alpha1 integrin function by stimulating its interactions with VCAM-1 but not fibronectin. The in vivo significance of alpha2beta1 integrin expression was demonstrated by intravital videomicroscopy showing that ligation of alpha2beta1 enhanced alpha4beta1-mediated extravasation of YBmalpha2 cells in the liver.

Real-time imaging of lymphocytes in vivo

New preparations, fluorescent probes and imaging techniques are providing the means to observe the behavior of cells in the tissue environment of lymphoid organs. In particular, when combined with two-photon laser microscopy, intravital imaging of surgically exposed lymph nodes provides a unique view of lymphocyte migration and antigen presentation as it occurs within the living animal. The view is emerging that lymphocytes migrate randomly within lymphoid organs, and that lymphocyte contact with antigen-presenting cells may be a stochastic process rather than one guided by chemokine gradients.

Cutting edge: T lymphocyte activation by repeated immunological synapse formation and intermittent signaling

The activation of biological T cell responses requires prolonged contact with APCs and sustained signaling. We investigated whether signaling must be uninterrupted to commit T cells to cytokine production or whether T cell activation may also result from summation of interrupted signals. Upon periodic addition and removal of a src kinase inhibitor, human CD4(+) T cells destroyed and re-formed immunological synapses while aborting and restarting signal transduction. Remarkably, under these conditions, T cells were eventually activated to IFN-gamma production and the amount of IFN-gamma produced was directly related to the total signaling time despite the repeated interruptions. Our results illustrate that T cell activation does not require a stable immunological synapse and can be achieved by interrupted signaling. It is implied that T cells can add activation signals, possibly collected on multiple APCs.

Extracellular matrix conditions T cells for adhesion to tissue interstitium

The activation and differentiation of peripheral blood T cells (PBT) are known to correlate with increased surface expression and adhesive capacity of beta(1) integrins, which mediate adhesion to the extracellular matrix (ECM). However, little is known about the regulation of integrin expression, affinity, and avidity on tissue T cells after they are embedded in the interstitial ECM. In this study we show that tissue T cells, freshly isolated from their residence in the interstitial ECM of the intestinal lamina propria, express a distinct subset of functionally active integrins that contribute to enhanced adhesion to purified collagen, fibronectin, and cell-derived ECM when compared with freshly isolated, short term activated, and long term cultured PBT. Furthermore, integrin usage is distinct between circulating and tissue-derived T cells, in that lamina propria T cells prefer to bind to collagen, while PBT lymphoblasts choose fibronectin when presented with a complex, three-dimensional, cell-derived matrix. To identify the extrinsic factors that regulate the conversion from a nonadhesive PBT to highly adhesive tissue T cell, we demonstrate that activation of PBT in the presence of fibronectin or collagen rapidly generates a surface integrin expression profile, an integrin usage pattern, and adhesive capacity mirroring that of a tissue T cell. These results indicate that the tissue ECM microenvironment instructs newly arrived T cells for further interactions with the underlying matrix and thereby imprints them with a signature tissue adhesive phenotype.

A novel function for chemokines: downregulation of neutrophil migration

Migration is a key function of stem cells during ontogenesis, of fibroblasts in wound healing and of immune cells in host defence. The signals that initiate migration are as important as signals that terminate migration, once the destination has been reached. We now show that formyl-methionyl-leucyl-phenylalanine (fMLP)-induced migration of neutrophils was inhibited by increasing concentrations of interleukin-8 (IL-8). IL-8 dose dependently increased the frequency and the duration of stop-periods, whereas the percentage of cells of a population that was locomotory active remained constant. The stop-signal delivered by IL-8 was intracellularly transduced by a dichotomic pathway: (i) the activation of the adenylyl cyclase leads to an increase of cytosolic cyclic adenosine monophosphate, which results in an activation of the sarcoplasmatic/endoplasmatic reticulum calcium ATPase pump and a calcium sequestration; (ii) the activation of the phospholipase Cbeta (PLCbeta) generates inositol-1,4,5-phosphate (IP3) and diacylglycerol (DAG), which results in IP3-mediated release of intracellularly stored calcium in the endoplasmatic reticulum and DAG-mediated activation of protein kinase C. Thus, we show for the first time that a chemokine, IL-8, in concert with fMLP, downregulates the neutrophil migration through the regulation of the intracellular calcium concentration via the adenylyl cyclase and the PLCbeta2.

Coordination of T cell activation and migration through formation of the immunological synapse

T cell activation is based on interactions of T cell antigen receptors with MHC-peptide complexes in a specialized cell-cell junction between the T cell and antigen-presenting cell-the immunological synapse. The immunological synapse coordinates naïve T cell activation and migration by stopping T cell migration with antigen-presenting cells bearing appropriate major histocompatibility complex (MHC) peptide complexes. At the same time, the immunological synapse allows full T cell activation through sustained signaling over a period of several hours. The immunological synapse supports activation in the absence of continued T cell migration, which is required for T cell activation through serial encounters. Src and Syk family kinases are activated early in immunological synapse formation, but this signaling process returns to the basal level after 30 min; at the same time, the interactions between T cell receptors (TCRs) and MHC peptides are stabilized within the immunological synapse. The molecular pattern of the mature synapse in helper T cells is a self-stabilized structure that is correlated with cytokine production and proliferation. I propose that this molecular pattern and its specific biochemical constituents are necessary to amplify signals from the partially desensitized TCR.

Autonomous T cell trafficking examined in vivo with intravital two-photon microscopy

The recirculation of T cells between the blood and secondary lymphoid organs requires that T cells are motile and sensitive to tissue-specific signals. T cell motility has been studied in vitro, but the migratory behavior of individual T cells in vivo has remained enigmatic. Here, using intravital two-photon laser microscopy, we imaged the locomotion and trafficking of naive CD4(+) T cells in the inguinal lymph nodes of anesthetized mice. Intravital recordings deep within the lymph node showed T cells flowing rapidly in the microvasculature and captured individual homing events. Within the diffuse cortex, T cells displayed robust motility with an average velocity of approximately 11 microm x min(-1). T cells cycled between states of low and high motility roughly every 2 min, achieving peak velocities >25 microm x min(-1). An analysis of T cell migration in 3D space revealed a default trafficking program analogous to a random walk. Our results show that naive T cells do not migrate collectively, as they might under the direction of pervasive chemokine gradients. Instead, they appear to migrate as autonomous agents, each cell taking an independent trafficking path. Our results call into question the role of chemokine gradients for basal T cell trafficking within T cell areas and suggest that antigen detection may result from a stochastic process through which a random walk facilitates contact with antigen-presenting dendritic cells.

Progressive differentiation and selection of the fittest in the immune response

T cells are stimulated by stochastic exposure to antigen-presenting cells and cytokines. We review evidence that the level of signal that is accumulated determines progression through hierarchical thresholds for proliferation and differentiation, leading to the generation of various intermediates and effector T cells. These cells are then selected to enter the memory pool according to their fitness--that is, their capacity to access and use survival signals. We suggest that the intermediates that are generated by antigenic stimulation of T and B cells persist as central memory cells, which can mount secondary responses to antigen and maintain appropriate levels of effector cells and antibodies throughout the lifetime of an individual.

Single-chain antigen recognition receptors that costimulate potent rejection of established experimental tumors

Tumor cells are usually weakly immunogenic as they largely express self-antigens and can down-regulate major histocompatability complex/peptide molecules and critical costimulatory ligands. The challenge for immunotherapies has been to provide vigorous immune effector cells that circumvent these tumor escape mechanisms and eradicate established tumors. One promising approach is to engineer T cells with single-chain antibody receptors, and since T cells require 2 distinct signals for optimal activation, we have compared the therapeutic efficacy of erbB2-reactive chimeric receptors that contain either T-cell receptor zeta (TCR-zeta) or CD28/TCR-zeta signaling domains. We have demonstrated that primary mouse CD8(+) T lymphocytes expressing the single-chain Fv (scFv)-CD28-zeta receptor have a greater capacity to secrete Tc1 cytokines, induce T-cell proliferation, and inhibit established tumor growth and metastases in vivo. The suppression of established tumor burden by cytotoxic T cells expressing the CD28/TCR-zeta chimera was critically dependent upon their interferon gamma (IFN-gamma) secretion. Our study has illustrated the practical advantage of engineering a T-cell signaling complex that codelivers CD28 activation, dependent only upon the tumor's expression of the appropriate tumor associated antigen.

Two-photon tissue imaging: seeing the immune system in a fresh light

Many lymphocyte functions, such as antigen recognition, take place deep in densely populated lymphoid organs. Because direct in vivo observation was not possible, the dynamics of immune-cell interactions have been inferred or extrapolated from in vitro studies. Two-photon fluorescence excitation uses extremely brief (<1 picosecond) and intense pulses of light to 'see' directly into living tissues, to a greater depth and with less phototoxicity than conventional imaging methods. Two-photon microscopy, in combination with newly developed indicator molecules, promises to extend single-cell approaches to the in vivo setting and to reveal in detail the cellular collaborations that underlie the immune response.

T-cell activation: a multidimensional signaling network

Naïve T cell activation requires the interactions of antigen receptors, adhesion molecules and co-stimulatory molecules. Antigen receptors and adhesion molecules are involved in spatio-temporal movement to form a stable immunological synapse. This stable junction interrupts T cell migration, and provides a platform for temporally regulated co-stimulatory receptor signaling spanning a period of days.

Tumor escape from killing: role of killer inhibitory receptors and acquisition of tumor resistance to cell death

Immunotherapy of cancer has always been a very attractive fourth-modality therapeutic approach. Over the past few years, advances in the identification of tumor antigens have offered new perspectives and provided new opportunities for more accurate immunotherapy for cancer. However, when applied to patients with established tumors, it rarely leads to an objective response. This is partly due to the fact that tumors evade host immunity at both the induction and effector phases. Thus, understanding tumor escape mechanisms may be the key to successful immunotherapy for cancer. In the present review, we will focus on how the expression of killer Ig receptors (KIR) on tumor infiltrating lymphocytes can compromise their function and how tumors evade apoptotic death - two additional mechanisms of tumor escape.

Chemokines and dendritic cells: a crucial alliance

Dendritic cells (DC) are bone marrow-derived professional antigen-presenting cells that function as sentinels of the immune system. Their importance in immunity resides in their unique ability to prime or tolerize T lymphocytes, thereby initiating or inhibiting immune responses. They reside in all tissues and organs and upon appropriate activation, migrate to secondary lymphoid organs to present antigen to T lymphocytes in the T cell zones. Because of this central role in T cell activation, there is a great deal of interest in using DC therapeutically to deliver positive or negative signals to the immune system. The DC system is critically dependent on the ability of DC at different stages of maturation to respond to a range of soluble and cell-bound signals, including members of the chemokine gene superfamily. This review will describe the interactions between DC and the chemokine system.

Cell adhesion and polarity during immune interactions

Intercellular interactions are critical for a coordinated function of different cell types involved in the immune response. Here we review the cellular and molecular events occurring during cell-cell immune contacts. Cognate naïve CD4+ T lymphocyte-dendritic cell (DC) and primed T cell-antigen-presenting B lymphocyte interactions are discussed. The engagement of cytotoxic T lymphocytes (CTL) or natural killer cells (NK) with their targets is analyzed and compared to the process of T cell-antigen-presenting cell (APC) conjugate formation. The immunological synapse, a complex cluster of molecules organized at the contact area of cell conjugates, exhibits common features but shows some differences depending on cell types involved. Cellular interactions occur in sequential stages that involve dramatic changes in cell polarity and dynamic redistribution of cell membrane receptors. The role of membrane microdomains, adaptor molecules and the cytoskeleton in the regulation of the molecular reorganization at cell-cell contacts is also discussed.

TCR triggering on the move: diversity of T-cell interactions with antigen-presenting cells

Polarized T cells are mobile cells optimized for migration, receptor scanning, and signaling. When in contact with antigen-presenting cells (APCs), polarized T cells can develop a spectrum of biophysical interaction modes ranging from adhesive sticking to dynamic crawling. Both static and dynamic contacts support sustained triggering of the T-cell receptor (TCR), leading to signal induction, T blast formation, and proliferation. In dynamic interactions, T cells crawl across the surface of the APC at speeds of 2-6 micro m/min and simultaneously establish an asymmetric tight yet mobile junction plane, representing a dynamic immunological synapse. In dynamic synapses three functional compartments of the polarized T cell are in close contact with the APC surface, i.e. leading edge, cell body and uropod. Through its mobility, the asymmetric junction is topographically suited for receptor scanning and engagement at the leading edge, retrograde receptor movement along the junction, and exit from the uropod. Herein we develop a model on scanning encounters between T cells and APCs that includes the simultaneous engagement of T-cell leading edge and uropod and implicates a serial receptor triggering mode in cell-cell recognition.

Identification of self through two-dimensional chemistry and synapses

Cells in the immune and nervous systems communicate through informational synapses. The two-dimensional chemistry underlying the process of synapse formation is beginning to be explored using fluorescence imaging and mechanical techniques. Early analysis of two-dimensional kinetic rates (k(on) and k(off)) and equilibrium constants (K(d)) provides a number of biological insights. First, there are two regimes for adhesion-one disordered with slow k(on) and the other self-ordered with 10(4)-fold faster k(on). Despite huge variation in two-dimensional k(on), the two-dimensional k(off) is like k(off) in solution, and two-dimensional k(off) is more closely related to intrinsic properties of the interaction than the two-dimensional k(on). Thus difference in k(off) can be used to set signaling thresholds. Early signaling complexes are compartmentalized to generate synergistic signaling domains. Immune antigen receptor components have a role in neural synapse editing. This suggests significant parallels in informational synapse formation based on common two-dimensional chemistry and signaling strategies.

Intercellular transfer and supramolecular organization of human leukocyte antigen C at inhibitory natural killer cell immune synapses

After accumulation of target cell human leukocyte antigen (HLA)-C at inhibitory natural killer (NK) cell immune synapses, some HLA-C transfers from target cells to NK cell plasma membranes and cytoplasm. This unexpected intercellular transfer of HLA-C is dependent on NK receptor recognition, since HLA-Cw6 or -Cw4 but not -Cw3 transfer to an NK transfectant expressing killer Ig-like receptor (KIR)2DL1. Strikingly, live-cell time-lapse laser scanning confocal microscopy shows vesicles containing target cell green fluorescent protein-tagged HLA-C migrating away from immune synapses into NK cells. Unlike clustering of HLA-C at the immune synapse, intercellular transfer of HLA-C is dependent on NK cell ATP, but not target cell ATP. However, the intercellular transfer of HLA-C is not dependent on active polymerization of the actin cytoskeleton. In addition, different arrangements of HLA-C are seen at inhibitory NK immune synapses, and these alter as NK synapses mature, but in a fashion distinct from that seen upon T cell activation.

Knockout mice: a paradigm shift in modern immunology

In the past decade, advances in genetic engineering and mouse knockout technology have transformed our understanding of the immune system. In particular, new perspectives on T-cell development, co-stimulation and activation have emerged from the study of single and multiple gene-knockout animals, as well as from conditional knockout and 'knock-in' mutants. Analysis of these animals has clarified important intracellular signalling pathways and has shed light on the regulatory mechanisms that govern normal immune responses and autoimmunity.

Role of adhesion molecules in activation signaling in T lymphocytes

The T cell and antigen-presenting cell communicate to initiate an immune response through formation of an immunological synapse. This specialized cell-cell junction is compartmentalized into adhesion molecule and T cell receptor enriched regions or SMACs. Distinct signals seem to be generated in the T cell receptor and adhesion molecule-dominated regions. This review focuses on how these distinct signaling pathways may be integrated within the T cell to set thresholds for T cell activation, proliferation, and survival.