Supramolecular attack particles are autonomous killing entities released from cytotoxic T cells

Cytotoxic T lymphocytes (CTLs) kill infected and cancerous cells. We detected transfer of cytotoxic multiprotein complexes, called supramolecular attack particles (SMAPs), from CTLs to target cells. SMAPs were rapidly released from CTLs and were autonomously cytotoxic. Mass spectrometry, immunochemical analysis, and CRISPR editing identified a carboxyl-terminal fragment of thrombospondin-1 as an unexpected SMAP component that contributed to target killing. Direct stochastic optical reconstruction microscopy resolved a cytotoxic core surrounded by a thrombospondin-1 shell of ~120 nanometer diameter. Cryo-soft x-ray tomography analysis revealed that SMAPs had a carbon-dense shell and were stored in multicore granules. We propose that SMAPs are autonomous extracellular killing entities that deliver cytotoxic cargo targeted by the specificity of shell components.

CX3CR1+ interstitial dendritic cells form a contiguous network throughout the entire kidney

Dendritic cells (DCs) interface innate and adaptive immunity in nonlymphoid organs; however, the exact distribution and types of DC within the kidney are not known. We utilized CX3CR1GFP/+ mice to characterize the anatomy and phenotype of tissue-resident CX3CR1+ DCs within normal kidney. Laser-scanning confocal microscopy revealed an extensive, contiguous network of stellate-shaped CX3CR1+ DCs throughout the interstitial and mesangial spaces of the entire kidney. Intravital microscopy of the superficial cortex showed stationary interstitial CX3CR1+ DCs that continually probe the surrounding tissue environment through dendrite extensions. Flow cytometry of renal CX3CR1+ DCs showed significant coexpression of CD11c and F4/80, high major histocompatibility complex class II and FcR expression, and immature costimulatory but competent phagocytic ability indicative of tissue-resident, immature DCs ready to respond to environment cues. Thus, within the renal parenchyma, there exists little immunological privilege from the surveillance provided by renal CX3CR1+ DCs, a major constituent of the heterogeneous mononuclear phagocyte system populating normal kidney.

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.

The immunological synapse and CD28-CD80 interactions

According to the two-signal model of T cell activation, costimulatory molecules augment T cell receptor (TCR) signaling, whereas adhesion molecules enhance TCR-MHC-peptide recognition. The structure and binding properties of CD28 imply that it may perform both functions, blurring the distinction between adhesion and costimulatory molecules. Our results show that CD28 on naïve T cells does not support adhesion and has little or no capacity for directly enhancing TCR-MHC-peptide interactions. Instead of being dependent on costimulatory signaling, we propose that a key function of the immunological synapse is to generate a cellular microenvironment that favors the interactions of potent secondary signaling molecules, such as CD28.

The immunological synapse

The adaptive immune response is initiated by the interaction of T cell antigen receptors with major histocompatibility complex molecule-peptide complexes in the nanometer scale gap between a T cell and an antigen-presenting cell, referred to as an immunological synapse. In this review we focus on the concept of immunological synapse formation as it relates to membrane structure, T cell polarity, signaling pathways, and the antigen-presenting cell. Membrane domains provide an organizational principle for compartmentalization within the immunological synapse. T cell polarization by chemokines increases T cell sensitivity to antigen. The current model is that signaling and formation of the immunological synapse are tightly interwoven in mature T cells. We also extend this model to natural killer cell activation, where the inhibitory NK synapse provides a striking example in which inhibition of signaling leaves the synapse in its nascent, inverted state. The APC may also play an active role in immunological synapse formation, particularly for activation of naïve T cells.

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.

Reprogramming T cells: the role of extracellular matrix in coordination of T cell activation and migration

The stable immunological synapse between a T cell and antigen-presenting cell coordinates migration and activation. Three-dimensional collagen gels transform this interaction into a series of transient hit-and-run encounters. Here we integrate these alternative modes of interaction in a model for primary T cell activation and effector function in vivo.

Environmental control of immunological synapse formation and duration

The coordination of T-cell migration and antigen recognition is crucial for an effective immune response. We have proposed that this coordination is achieved by formation of an immunological synapse between the T cell and the antigen-presenting cell (APC). Our view contrasts with the serial encounter model also proposed in this issue of Trends in Immunology, which is based on transient T cell-APC interactions when surrounded by collagen. Here, we propose a model that reconciles immunological synapse formation and serial encounters based on environmental control of immunological synapse formation.

A supramolecular basis for CD45 tyrosine phosphatase regulation in sustained T cell activation

Transmembrane protein tyrosine phosphatases, such as CD45, can act as both positive and negative regulators of cellular signaling. CD45 positively modulates T cell receptor (TCR) signaling by constitutively priming p56lck through the dephosphorylation of the C-terminal negative regulatory phosphotyrosine site. However, CD45 can also exert negative effects on cellular processes, including events triggered by integrin-mediated adhesion. To better understand these opposing actions of tyrosine phosphatases, the subcellular compartmentalization of CD45 was imaged by using laser scanning confocal microscopy during functional TCR signaling of live T lymphocytes. On antigen engagement, CD45 was first excluded from the central region of the interface between the T cell and the antigen-presenting surface where CD45 would inhibit integrin activation. Subsequently, CD45 was recruited back to the center of the contact to an area adjacent to the site of sustained TCR engagement. Thus, CD45 is well positioned within a supramolecular assembly in the vicinity of the engaged TCR, where CD45 would be able to maintain src-kinase activity for the duration of TCR engagement.

Cutting edge: hierarchy of chemokine receptor and TCR signals regulating T cell migration and proliferation

Chemokines play an important role in establishing the distribution of lymphocyte subpopulations in primary and secondary lymphoid tissues and in the recruitment of leukocytes to sites of inflammation. However, the potential of chemokines to down-regulate immune responses has not been demonstrated. We now show that certain chemokine gradients have the potential to suppress T cell activation by preventing formation of the immunological synapse, the specialized cell-cell junction that forms before a T cell can be fully activated. Our data reveals an immunosuppressive potential of chemokines engaging the CXCR3 and CCR7 receptors, but not the CXCR4, CCR2, CCR4, or CCR5 receptors. These results suggest a novel mechanism for T cell ignorance of agonist MHC-peptide complexes based on dominant chemokine gradients.

The immunological synapse and the actin cytoskeleton: molecular hardware for T cell signaling

The actin cytoskeleton seems to play two critical roles in the activation of T cells. One of these roles is T cell shape development and movement, including formation of the immunological synapse. The other is the formation of a scaffold for signaling components. This review focuses on the recent convergence of cell biology and immunology studies to explain the role of the actin cytoskeleton in creating the molecular basis for immunological synapse formation and T cell signaling.

Cytoskeletal polarization and redistribution of cell-surface molecules during T cell antigen recognition

T cell antigen recognition is accompanied by cytoskeletal polarization towards the APC and large-scale redistribution of cell surface molecules into 'supramolecular activation clusters' (SMACs), forming an organized contact interface termed the 'immunological synapse' (IS). Molecules are arranged in the IS in a micrometer scale bull's eye pattern with a central accumulation of TCR/peptide-MHC (the cSMAC) surrounded by a peripheral ring of adhesion molecules (the pSMAC). We propose that segregation of cell surface molecules on a much smaller scale initiates TCR triggering, which drives the formation of the IS by active transport processes. IS formation may function as a checkpoint for full T cell activation, integrating information on the presence and quality of TCR ligands and the nature and activation state of the APC.

Relative contribution of LFA-1 and Mac-1 to neutrophil adhesion and migration

To differentiate the unique and overlapping functions of LFA-1 and Mac-1, LFA-1-deficient mice were developed by targeted homologous recombination in embryonic stem cells, and neutrophil function was compared in vitro and in vivo with Mac-1-deficient, CD18-deficient, and wild-type mice. LFA-1-deficient mice exhibit leukocytosis but do not develop spontaneous infections, in contrast to CD18-deficient mice. After zymosan-activated serum stimulation, LFA-1-deficient neutrophils demonstrated activation, evidenced by up-regulation of surface Mac-1, but did not show increased adhesion to purified ICAM-1 or endothelial cells, similar to CD18-deficient neutrophils. Adhesion of Mac-1-deficient neutrophils significantly increased with stimulation, although adhesion was lower than for wild-type neutrophils. Evaluation of the strength of adhesion through LFA-1, Mac-1, and CD18 indicated a marked reduction in firm attachment, with increasing shear stress in LFA-1-deficient neutrophils, similar to CD18-deficient neutrophils, and only a modest reduction in Mac-1-deficient neutrophils. Leukocyte influx in a subcutaneous air pouch in response to TNF-alpha was reduced by 67% and 59% in LFA-1- and CD18-deficient mice but increased by 198% in Mac-1-deficient mice. Genetic deficiencies demonstrate that both LFA-1 and Mac-1 contribute to adhesion of neutrophils to endothelial cells and ICAM-1, but adhesion through LFA-1 overshadows the contribution from Mac-1. Neutrophil extravasation in response to TNF-alpha in LFA-1-deficient mice dramatically decreased, whereas neutrophil extravasation in Mac-1-deficient mice markedly increased.

Congenital nephrotic syndrome in mice lacking CD2-associated protein

CD2-associated protein (CD2AP) is an 80-kilodalton protein that is critical for stabilizing contacts between T cells and antigen-presenting cells. In CD2AP-deficient mice, immune function was compromised, but the mice died at 6 to 7 weeks of age from renal failure. In the kidney, CD2AP was expressed primarily in glomerular epithelial cells. Knockout mice exhibited defects in epithelial cell foot processes, accompanied by mesangial cell hyperplasia and extracellular matrix deposition. Supporting a role for CD2AP in the specialized cell junction known as the slit diaphragm, CD2AP associated with nephrin, the primary component of the slit diaphragm.

The immunological synapse: a molecular machine controlling T cell activation

The specialized junction between a T lymphocyte and an antigen-presenting cell, the immunological synapse, consists of a central cluster of T cell receptors surrounded by a ring of adhesion molecules. Immunological synapse formation is now shown to be an active and dynamic mechanism that allows T cells to distinguish potential antigenic ligands. Initially, T cell receptor ligands were engaged in an outermost ring of the nascent synapse. Transport of these complexes into the central cluster was dependent on T cell receptor-ligand interaction kinetics. Finally, formation of a stable central cluster at the heart of the synapse was a determinative event for T cell proliferation.

Making a little affinity go a long way: a topological view of LFA-1 regulation

Lymphocytes utilize adhesion to navigate in the body and to transiently interact with a variety of potential antigen presenting cells. Interactions of adhesion molecules are governed by the law of mass action and the less understood rules of apposed biological membranes. Biochemical parameters such as adhesion molecule affinity only tell part of the story. Factors such as lateral mobility, membrane alignment and cytoskeletal interactions are equally important in determining the final outcome. Therefore it is important to determine mechanisms by which the properties of cell membranes and the cytoskeleton reinforce or hinder adhesion molecule interactions. Work from my lab has shown that one mechanism by which lymphocyte adhesion molecules cooperate is to align adhering membranes with nanometer precision. Here, I discuss a model for LFA-1 regulation that is dependent on three independent processes: LFA-1 lateral mobility, ligand induced generation of a small amount of high affinity LFA-1 and local membrane alignment. I propose that coordination of these processes allows rapid interconversion between stable adhesion and detachment.

A novel adaptor protein orchestrates receptor patterning and cytoskeletal polarity in T-cell contacts

Recognition of antigen by T cells requires the formation of a specialized junction between the T cell and the antigen-presenting cell. This junction is generated by the recruitment and the exclusion of specific proteins from the contact area. The mechanisms that regulate these events are unknown. Here we demonstrate that ligand engagement of the adhesion molecule, CD2, initiates a process of protein segregation, CD2 clustering, and cytoskeletal polarization. Although protein segregation was not dependent on the cytoplasmic domain of CD2, CD2 clustering and cytoskeletal polarization required an interaction of the CD2 cytoplasmic domain with a novel SH3-containing protein. This novel protein, called CD2AP, is likely to facilitate receptor patterning in the contact area by linking specific adhesion receptors to the cytoskeleton.

Low affinity interaction of human or rat T cell adhesion molecule CD2 with its ligand aligns adhering membranes to achieve high physiological affinity

The mechanism by which low affinity adhesion molecules function to produce stable cell-cell adhesion is unknown. In solution, the interaction of human CD2 with its ligand CD58 is of low affinity (500 mM-1) and the interaction of rat CD2 with its ligand CD48 is of still lower affinity (40 mM-1). At the molecular level, however, the two systems are likely to be topologically identical. Fluorescently labeled glycosylphosphatidylinositol-anchored CD48 and CD58 were prepared and incorporated into supported phospholipid bilayers, in which the ligands were capable of free lateral diffusion. Quantitative fluorescence imaging was used to study the binding of cell surface human and rat CD2 molecules to the fluorescent ligands in contact areas between Jurkat cells and the bilayers. These studies provide two major conclusions. First, CD2/ligand interactions cooperate to align membranes with nanometer precision leading to a physiologically effective two-dimensional affinity. This process does not require the intact cytoplasmic tail of CD2. Second, the degree of membrane alignment that can be achieved by topologically similar receptors deteriorates with decreasing affinity. This suggests an affinity limit for the ability of this mode of cooperativity to achieve stable cell-cell adhesion at approximately 10 mM-1.

Survival of FimH-expressing enterobacteria in macrophages relies on glycolipid traffic

Strains of Escherichia coli persist within the human gut as normal commensals, but are frequent pathogens and can cause recurrent infection. Here we show that, in contrast to E. coli subjected to opsonic interactions stimulated by the host's immune response, E. coli that bind to the macrophage surface exclusively through the bacterial lectin FimH can survive inside the cell following phagocytosis. This viability is largely due to the attenuation of intracellular free-radical release and of phagosome acidification during FimH-mediated internalization, both of which are triggered by antibody-mediated internalization. This different processing of non-opsonized bacteria is supported by morphological evidence of tight-fitting phagosomes compared with looser, antibody-mediated phagosomes. We propose that non-opsonized FimH-expressing E. coli co-opt internalization of lipid-rich microdomains following binding to the FimH receptor, the glycosylphosphatidylinositol-linked protein CD48, because (1) the sterol-binding agents filipin, nystatin and methyl beta-cyclodextrin specifically block FimH-mediated internalization; (2) CD48 and the protein caveolin both accumulate on macrophage membranes surrounding bacteria; and (3) antibodies against CD48 inhibit FimH-mediated internalization. Our findings bring the traditionally extracellular E. coli into the realm of opportunistic intracellular parasitism and suggest how opportunistic infections with FimH-expressing enterobacteria could occur in a setting deprived of opsonizing antibodies.