Single-molecule microscopy reveals plasma membrane microdomains created by protein-protein networks that exclude or trap signaling molecules in T cells

Membrane subdomains have been implicated in T cell signaling, although their properties and mechanisms of formation remain controversial. Here, we have used single-molecule and scanning confocal imaging to characterize the behavior of GFP-tagged signaling proteins in Jurkat T cells. We show that the coreceptor CD2, the adaptor protein LAT, and tyrosine kinase Lck cocluster in discrete microdomains in the plasma membrane of signaling T cells. These microdomains require protein-protein interactions mediated through phosphorylation of LAT and are not maintained by interactions with actin or lipid rafts. Using a two color imaging approach that allows tracking of single molecules relative to the CD2/LAT/Lck clusters, we demonstrate that these microdomains exclude and limit the free diffusion of molecules in the membrane but also can trap and immobilize specific proteins. Our data suggest that diffusional trapping through protein-protein interactions creates microdomains that concentrate or exclude cell surface proteins to facilitate T cell signaling.

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.

Visualization of CD2 interaction with LFA-3 and determination of the two-dimensional dissociation constant for adhesion receptors in a contact area

Many adhesion receptors have high three-dimensional dissociation constants (Kd) for counter-receptors compared to the KdS of receptors for soluble extracellular ligands such as cytokines and hormones. Interaction of the T lymphocyte adhesion receptor CD2 with its counter-receptor, LFA-3, has a high solution-phase Kd (16 microM at 37 degrees C), yet the CD2/LFA-3 interaction serves as an effective adhesion mechanism. We have studied the interaction of CD2 with LFA-3 in the contact area between Jurkat T lymphoblasts and planar phospholipid bilayers containing purified, fluorescently labeled LFA-3. Redistribution and lateral mobility of LFA-3 were measured in contact areas as functions of the initial LFA-3 surface density and of time after contact of the cells with the bilayers. LFA-3 accumulated at sites of contact with a half-time of approximately 15 min, consistent with the previously determined kinetics of adhesion strengthening. The two-dimensional Kd for the CD2/LFA-3 interaction was 21 molecules/microns 2, which is lower than the surface densities of CD2 on T cells and LFA-3 on most target or stimulator cells. Thus, formation of CD2/LFA-3 complexes should be highly favored in physiological interactions. Comparison of the two-dimensional (membrane-bound) and three-dimensional (solution-phase) KdS suggest that cell-cell contact favors CD2/LFA-3 interaction to a greater extent than that predicted by the three-dimensional Kd and the intermembrane distance at the site of contact. LFA-3 molecules in the contact site were capable of lateral diffusion in the plane of the phospholipid bilayer and did not appear to be irreversibly trapped in the contact area, consistent with a rapid off-rate. These data provide insights into the function of low affinity interactions in adhesion.

Assessment of costimulation and coinhibition in a triple parameter T cell reporter line: Simultaneous measurement of NF-κB, NFAT and AP-1

Engagement of the T cell receptor complex reprograms T cells for proliferation, cytokine production and differentiation towards effector cells. This process depends on activating costimulatory signals and is counteracted by coinhibitory molecules. Three transcription factors, namely NF-κB, NFAT and AP-1, have a major role in inducing the transcriptional program that is required for T cell activation and differentiation. Here we describe the generation of a triple parameter reporter based on the human Jurkat T cell line, where response elements for NF-κB, NFAT and AP-1 drive the expression of the fluorescent proteins CFP, eGFP and mCherry, respectively. The emission spectra of these proteins allow simultaneous assessment of NF-κB, NFAT and AP-1 activity in response to stimulation. Ligation of the TCR complex induced moderate reporter activity, which was strongly enhanced upon coengagement of the costimulatory receptors CD2 or CD28. Moreover, we have generated and tested triple parameter reporter cells that harbor costimulatory and inhibitory receptors not endogenously expressed in the Jurkat cells. In these experiments we could show that engagement of the costimulatory molecule 4-1BB enhances NF-κB and AP-1 activity, whereas coinhibition via PD-1 or BTLA strongly reduced the activation of NF-κB and NFAT. Engagement of BTLA significantly inhibited AP-1, whereas PD-1 had little effect on the activation of this transcription factor. Our triple parameter reporter T cell line is an excellent tool to assess the effect of costimulatory and coinhibitory receptors on NF-κB, NFAT and AP-1 activity and has a wide range of applications beyond the evaluation of costimulatory pathways.

GAKIN, a novel kinesin-like protein associates with the human homologue of the Drosophila discs large tumor suppressor in T lymphocytes

Reorganization of the cortical cytoskeleton is a hallmark of T lymphocyte activation. Upon binding to antigen presenting cells, the T cells rapidly undergo cytoskeletal re-organization thus forming a cap at the cell-cell contact site leading to receptor clustering, protein segregation, and cellular polarization. Previously, we reported cloning of the human lymphocyte homologue of the Drosophila Discs Large tumor suppressor protein (hDlg). Here we show that a novel protein termed GAKIN binds to the guanylate kinase-like domain of hDlg. Affinity protein purification, peptide sequencing, and cloning of GAKIN cDNA from Jurkat J77 lymphocytes identified GAKIN as a novel member of the kinesin superfamily of motor proteins. GAKIN mRNA is ubiquitously expressed, and the predicted amino acid sequence shares significant sequence similarity with the Drosophila kinesin-73 motor protein. GAKIN sequence contains a motor domain at the NH(2) terminus, a central stalk domain, and a putative microtubule-interacting sequence called the CAP-Gly domain at the COOH terminus. Among the MAGUK superfamily of proteins examined, GAKIN binds to the guanylate kinase-like domain of PSD-95 but not of p55. The hDlg and GAKIN are localized mainly in the cytoplasm of resting T lymphocytes, however, upon CD2 receptor cross-linking the hDlg can translocate to the lymphocyte cap. We propose that the GAKIN-hDlg interaction lays the foundation for a general paradigm of coupling MAGUKs to the microtubule-based cytoskeleton, and that this interaction may be functionally important for the intracellular trafficking of MAGUKs and associated protein complexes in vivo.

Alternative roles for CD59

CD59 was first identified as a regulator of the terminal pathway of complement, which acts by binding to the C8/C9 components of the assembling membrane attack complex (MAC), to inhibit formation of the lytic pore. Structurally, CD59 is a small, highly glycosylated, GPI-linked protein, with a wide expression profile. Functionally, the role of CD59 in complement regulation is well-defined but studies have also shown clear evidence for signalling properties, which are linked to its glycophosphatidyl inositol (GPI) anchor and its location within lipid rafts. Cross-linking of CD59 using specific monoclonal antibodies drives both calcium release and activation of lipid-raft associated signalling molecules such as tyrosine kinases. These observations clearly show that CD59 exhibits roles independent of its function as a complement inhibitor. In this review, we examine the progression of research in this area and explore the alternative functions of CD59 that have recently been defined.

Stimulated human lamina propria T cells manifest enhanced Fas-mediated apoptosis

Lamina propria (LP) T cells respond poorly to a proliferative stimulus delivered via TCR/CD3 pathway, but retain considerable ability to respond to a stimulus delivered via CD2 costimulatory or accessory pathway. In the present study, we showed first that unstimulated LP T cells, as compared to unstimulated peripheral blood (PB) T cells, exhibit an increased level of apoptosis which is further increased following CD2 pathway stimulation, but not following via TCR/CD3 pathway stimulation. We next showed that IL-2 had a sparing effect on apoptosis of unstimulated LP T cells in that IL-2 decreased and anti-IL-2 increased apoptosis of these cells; in contrast, IL-2 had no effect on apoptosis of CD2-pathway stimulated cells. Finally, we showed that increased apoptosis of LP T cells induced by CD2-pathway stimulation is inhibited when Fas antigen is blocked by a nonstimulatory anti-Fas antibody. These studies suggest that LP T cells are characterized by increased susceptibility to Fas-mediated apoptosis most due to a downstream change in the Fas signaling pathway. Given that IFN-gamma secretion is significantly increased in LP T cells in which apoptosis is inhibited, this feature of LP T cells may represent a mechanism of regulating detrimental immune responses in the mucosal environment.

CD2 sets quantitative thresholds in T cell activation

It has been proposed that CD2, which is highly expressed on T cells, serves to enhance T cell-antigen presenting cell (APC) adhesion and costimulate T cell activation. Here we analyzed the role of CD2 using CD2-deficient mice crossed with transgenic mice expressing a T cell receptor specific for lymphocytic choriomeningitis virus (LCMV)-derived peptide p33. We found that absence of CD2 on T cells shifted the p33-specific dose-response curve in vitro by a factor of 3-10. In comparison, stimulation of T cells in the absence of lymphocyte function-associated antigen (LFA)-1-intercellular adhesion molecule (ICAM)-1 interaction shifted the dose-response curve by a factor of 10, whereas absence of both CD2-CD48 and LFA-1-ICAM-1 interactions shifted the response by a factor of approximately 100. This indicates that CD2 and LFA-1 facilitate T cell activation additively. T cell activation at low antigen density was blocked at its very first steps, as T cell APC conjugate formation, TCR triggering, and Ca(2+) fluxes were affected by the absence of CD2. In vivo, LCMV-specific, CD2-deficient T cells proliferated normally upon infection with live virus but responded in a reduced fashion upon cross-priming. Thus, CD2 sets quantitative thresholds and fine-tunes T cell activation both in vitro and in vivo.

NK Cells Stimulate Proliferation of T and NK Cells through 2B4/CD48 Interactions

Few studies have addressed the consequences of physical interactions between NK and T cells, as well as physical interactions among NK cells themselves. We show in this study that NK cells can enhance T cell activation and proliferation in response to CD3 cross-linking and specific Ag through interactions between 2B4 (CD244) on NK cells and CD48 on T cells. Furthermore, 2B4/CD48 interactions between NK cells also enhanced proliferation of NK cells in response to IL-2. Overall, these results suggest that NK cells augment the proliferation of neighboring T and NK cells through direct cell-cell contact. These results provide new insights into NK cell-mediated control of innate and adaptive immunity and demonstrate that receptor/ligand-specific cross talk between lymphocytes may occur in settings other than T-B cell or T-T cell interactions.

Surface molecules that drive T cell development in vitro in the absence of thymic epithelium and in the absence of lineage-specific signals

Differentiation of immature double positive (DP) CD4+ CD8+ thymocytes into single positive (SP) CD4+ and CD8+ T cells is referred to as positive selection and requires physical contact with thymic cortical epithelium. We now have identified "coinducer" molecules on DP thymocytes that, together with TCR, signal DP thymocytes to differentiate into SP T cells in vitro in the absence of thymic epithelium. A remarkable number of different molecules on DP thymocytes possessed "coinducing" activity, including CD2, CD5, CD24, CD28, CD49d, CD81, and TSA-1. Interestingly, in vitro differentiation occurred in the absence of lineage-specific signals, yet resulted in the selective generation of CD4+CD8- T cells. Thus, the present study has identified surface molecules that can signal DP thymocytes to differentiate into SP T cells in the absence of thymic epithelium and has characterized a default pathway for CD4+ T cell differentiation.

CD2 is involved in maintenance and reversal of human alloantigen-specific clonal anergy

Induction and maintenance of a state of T cell unresponsiveness to specific alloantigen would have significant implications for human organ transplantation. Using human histocompatibility leukocyte antigen DR7-specific helper T cell clones, we demonstrate that blockade of the B7 family of costimulatory molecules is sufficient to induce alloantigen-specific T cell clonal anergy. Anergized cells do not respond to alloantigen and a variety of costimulatory molecules, including B7-1, B7-2, intercellular adhesion molecule-1 (ICAM-1), and lymphocyte function-associated molecule (LFA)-3. However, after culture in exogenous interleukin (IL)-2 for at least 7 d, anergized cells can respond to alloantigen in the presence of LFA-3. LFA-3 costimulation subsequently restores responsiveness to alloantigen in the presence of previously insufficient costimulatory signals. Expression of CD2R epitope is downregulated on anergic cells and is restored after 7 d of IL-2 culture. The loss of the CD2R is temporally associated with the inability of anergized cells to respond to LFA-3. These results suggest that in addition to blockade of B7 family members, inhibition of CD2 and, potentially, other costimulatory pathways that might reverse anergy will be necessary to maintain prolonged alloantigen-specific tolerance.

TGFbeta regulates the CD4+CD25+ T-cell pool and the expression of Foxp3 in vivo

Factors influencing the development of CD4+CD25+ T-cells in vivo are poorly understood. In order to investigate the contribution of TGFbeta1 to the development and function of CD4+CD25+ T-cells, we generated a gain of function mutation resulting in the overexpression of an active form of TGFbeta1 in T-cells under control of the human CD2 promoter. In peripheral lymphoid organs and in the thymus, the frequency of CD4+CD25+ T-cells was increased in transgenic mice. This appeared to be due to an autocrine effect of TGFbeta on T-cells, since concomitant impairment of TGFbeta-signaling in double transgenic mice resulted in a phenotype similar to wild type. In contrast, in single transgenic mice with impaired TGFbeta-signaling in T-cells, CD4+CD25+ T-cell numbers were reduced in peripheral lymphoid organs but not in the thymus. In addition, TGFbeta was found to regulate the expression of Foxp3 in vivo, a transcription factor essential for the generation and function of regulatory T-cells. In CD4+CD25+ T-cells, TGFbeta1 increased the expression of Foxp3, whereas a decreased expression was seen in CD4+CD25+ T-cells with impaired TGFbeta-signaling. TGFbeta1 induced the expression of IL-10 in transgenic T-cells, but the increased in vitro suppressive capacity observed in transgenic CD4+CD25+ T-cells was due to the secretion of TGFbeta and not IL-10. Therefore, our study provides in vivo evidence for a role of TGFbeta in the homeostasis of CD4+CD25+ T-cells.

Involvement of CD2 and CD3 in galectin-1 induced signaling in human Jurkat T-cells

Galectin-1 (gal-1) a member of the mammalian beta-galactoside-binding proteins recognizes preferentially Galbeta1-4GlcNAc sequences of oligosaccharides associated with several cell surface glycoconjugates. In the present work, gal-1 has been identified to be a ligand for the CD3-complex as well as for CD2 as detected by affinity chromatography of Jurkat T-cell lysates on gal-1 agarose and by binding of the biotinylated lectin to CD3 and CD2 immunoprecipitates on blots. In CD45(+)Jurkat E6.1 cells, the lectin stimulates a sustained increase in the intracytoplasmic calcium concentration ([Ca(2+)](i)) consisting of both the release of calcium from intracellular stores and the calcium influx from the extracellular space. This effect of gal-1 on [Ca(2+)](i)is completely inhibited by lactose at 10 mM and was absent in CD45(-)Jurkat J45.01 cells. Preincubation of Jurkat E6.1 cells with cholera toxin or with the protein tyrosine kinase inhibitor herbimycin A reduced the gal-1 induced calcium response whereas the increase in [Ca(2+)](i)stimulated by CD2 or CD3 monoclonal antibodies (mAbs) was completely inhibited. Depolarization of E6.1 cells in a high-potassium buffer, a standard method to activate voltage-operated calcium channels, was without effect on [Ca(2+)](i). Membrane depolarization with gramicidin or by a high-potassium buffer was without effects on the lectin-mediated calcium release from intracellular stores but inhibited the gal-1 induced receptor-operated calcium influx. In Jurkat E6.1 cells the lectin stimulates the transient generation of inositol-1,4,5-trisphosphate and the tyrosine phosphorylation of phospholipase Cgamma1. The results suggest that the ligation of CD2 and CD3 by gal-1 induces early events in T-cell activation comparable with that elicited by CD2 or CD3 mAbs.

Differentiation of regulatory T cells 1 is induced by CD2 costimulation

Induction and maintenance of peripheral tolerance is an important phenomenon for the control of homeostasis in the immune system. There is now compelling evidence for CD4(+) T cells that prevent immune pathology, both in autoimmunity and in transplantation. However, the mechanisms involved in the specific differentiation of these T cells are unknown. We had previously shown that repetitive stimulations of naive T cells in the presence of IL-10 induce the differentiation of T regulatory cells 1. We further dissected the mechanism of IL-10 function and demonstrated that IL-10 acts by the down-regulation of most costimulatory molecules without modifying the expression of CD58. Using artificial APCs expressing various costimulatory molecules, we demonstrated that, in contrast to other costimulation patterns, costimulation via CD2 alone, in the absence of costimulations through CD28- or LFA-1, induced T cell anergy in an IL-10-independent pathway along with the differentiation of Ag-specific regulatory T cells. T regulatory cell-1 differentiation via CD2 was very efficient as both high IL-10 secretion and regulatory function were observed after the first stimulation of naive T cells with CD32-CD58 L cells. The possibility to rapidly induce the differentiation of Ag-specific regulatory T cells will certainly accelerate their characterization and their potential use as regulators of T cell-mediated diseases.

Adhesion receptors in lymphocyte activation

The past several years have seen significant progress in understanding the role of T lymphocyte coreceptors in adhesion and activation. New insights have been gained in several areas: the avidity regulation of beta 1 and beta 2 integrins and their role in signal transduction; the regulation of CD8 avidity; the role of Lck in CD4 coreceptor activity; and the novel role for CD2 adhesion in the T cell antigen response.

CD2 regulates responsiveness of activated T cells to interleukin 12

Interleukin (IL) 12 is a 70-kD heterodimeric cytokine produced by antigen-presenting cells (APCs) such as macrophages in response to infectious pathogens and interferon (IFN) gamma. The varied immunomodulatory effects of IL-12 include the stimulation of proliferation and IFN-gamma production by T cells, and it also has a central role in the development of the T helper cell type 1 immune phenotype. We undertook the production of antibodies capable of modulating the response of T cells to IL-12, and in the process we discovered two antibodies that inhibited the ability of IL-12 to stimulate T cell proliferation. In this report, we demonstrate that these anti-bodies recognize CD2, and we show how antibodies directed toward either the adhesion domain of CD2 or its ligand, CD58, specifically inhibit IL-12 induced proliferation and IFN-gamma production by phytohemagglutinin-activated T cells, leaving the response to IL-12 unaffected. A three-to fourfold reduction in proliferation and IFN-gamma production was observed at IL-12 concentrations as high as 1 nM, with complete inhibition occurring at < or = 1 pM. This novel effect is not directly mediated at the level of the IL-12 receptor, as shown by the inability of these antibodies to block IL-12 binding to activated T cells. Furthermore, by using activating pairs of CD2 antibodies, we show that CD2 stimulation strongly synergizes with IL-12, even at 0.1 pM, in inducing both T cell proliferation and IFN-gamma production. Cytolytic T lymphocyte-associated antigen 4-immunoglobulin-mediated inhibition of the B7/CD28 interaction did not affect the T cell response to either IL-12 or IL-2, but the removal of APCs selectively diminished the proliferative response to IL-12. Based on this data, we hypothesize that CD2 has a central role in an IL-12/IFN-gamma positive feedback loop between T cell and APC, providing the key functional link via a CD2/CD58 interaction that controls T cell responsiveness to IL-12. This model provides a basis for future investigations aimed at defining the signaling mechanisms that mediate this cytokine-specific regulatory effect of CD2, and it offers insight into how a cytokine receptor and distinct adhesion molecule can interact to modulate responsiveness to that cytokine. In addition, it underscores the possibility that the clinical potential of an immunomodulatory drug like IL-12 may be governed by the presence or absence of specific costimulation.

Enforced expression of Runx2 perturbs T cell development at a stage coincident with beta-selection

The development of T cells in the thymus is regulated by a series of stage-specific transcription factors. Deregulated expression of these factors can lead to alterations in thymocyte development with the production of aberrant cell subsets and predispose to tumor formation. The three genes of the Runx family are multilineage regulators of differentiation that have been reported to be expressed in the T cell lineage. However, their roles in thymocyte development and T cell function are largely unknown. While the Runx2/Cbfa1/AML3/Pebp2alphaa gene plays a primary role in osteogenesis and regulates a number of key bone regulatory genes, we show here that Runx2 is also expressed during the earliest phase of thymic development, in the double-negative subset. Furthermore, enforced expression of Runx2 in transgenic mice under the CD2 promoter was found to affect T cell development at a stage coincident with beta-selection, resulting in an expansion of double-negative CD4 and CD8 immature single-positive cells. Unlike wild-type controls this preselection population (CD4-CD8+heat-stable Ag+TCR-) is in a nonproliferative state, but appears to be primed for further transformation events. Overall the data suggest that Runx2 accelerates development to the CD8 immature single-positive stage, but retards subsequent differentiation to the double-positive stage. Thus, Runx2 joins a small group of transcription factors that can interfere with early T cell development, cause an expansion of a specific subset, and predispose to lymphoma.

Coordinate regulation of T cell activation by CD2 and CD28

T cell activation requires co-engagement of the TCR with accessory and costimulatory molecules. However, the exact mechanism of costimulatory function is unknown. Mice lacking CD2 or CD28 show only mild deficits, demonstrating that neither protein is essential for T cell activation. In this paper we have generated mice lacking both CD2 and CD28. T cells from the double-deficient mice have a profound defect in activation by soluble anti-CD3 Ab and Ag, yet remain responsive to immobilized anti-CD3. This suggests that CD2 and CD28 may function together to facilitate interactions of the T cell and APC, allowing for efficient signal transduction through the TCR.

Granzyme B leakage-induced cell death: a new type of activation-induced natural killer cell death

Activation-induced natural killer (NK) cell death is very rapid compared to activation-induced T or B cell death. Here we show that NK cell activation is accompanied by the leakage of granzymeB from intracellular granules into the cytoplasm. Evidence for granzyme B leakage includes the formation of granzyme B/serine proteinase inhibitor 9 (PI-9) complexes that are detected by immunoprecipitation as well as colocalization of granzyme B and PI-9 detected by immunocytochemistry. The pro-apoptotic molecule Bid, a specific substrate for granzyme B, was cleaved within 2 min following CD2-induced NK cell activation, suggesting that granzyme B triggers apoptosis by directing Bid to mitochondrial membranes. The granzyme B/PI-9 protein ratio was found to mirror the percentage of CD2-induced NK cell death, suggesting that an excess of leaked granzyme B over its inhibitor is a major determinant of cell death. We suggest that granzyme B leakage-induced cell death is an important determinant of activation-induced NK cell death and that this process may be important for the fate of NK cells which encounter malignant or virus-infected cells.

Thiol-mediated inhibition of FAS and CD2 apoptotic signaling in activated human peripheral T cells

Fas and CD2 receptors can transduce apoptotic signals through two independent biochemical pathways. In this study, we first evaluated the role of intracellular GSH in these signaling pathways by inducing variations in the GSH pool of activated peripheral T lymphocytes. Increasing the concentration of intracellular GSH by means of N-acetyl-L-cysteine (NAC) and GSH ethyl ester (OEt) resulted in total protection against cell death, while inhibiting GSH synthesis with buthionine sulfoximine (BSO) greatly enhanced cell sensitivity to Fas and CD2 apoptotic signaling. The protection exerted by NAC and GSH OEt was essentially based on their capacity to establish an intracellular reducing environment as it still occurred in BSO-treated cells. Thiol-containing compounds (cysteine, captopril, D-penicillamine and 2-mercaptoethanol) inhibited apoptosis while a series of non-thiol antioxidants (including catalase and vitamin E) failed to do so, suggesting that protection was secondary to thiols/disulfides exchange reactions at the level of cysteine residues in proteins and not to detoxification of reactive oxygen intermediates. This conclusion was further supported by the finding that no enhanced generation of O.-2 and H2O2 could be detected in cells experiencing early stages of apoptosis such as a decreased concentration of intracellular GSH and cell shrinkage. Also, protection occurred in the presence of protein synthesis inhibitors, indicating that it was due to post-translational sulfhydryl redox regulation of critical molecules involved in the apoptotic cascade. These data suggest that GSH, the most abundant intracellular thiol antioxidant, may be important in counteracting Fas- and CD2-mediated apoptosis of T lymphocytes.

Efficient virus transmission from dendritic cells to CD4+ T cells in response to antigen depends on close contact through adhesion molecules

Monocyte-derived cultured dendritic cells (DCs) are potent antigen-presenting cells (APCs) and are susceptible to HIV-1Lai infection. Compared to the low level of virus production by HIV-1-infected DCs alone, a level of virus two to three orders of magnitude higher was produced by cocultivation of HIV-1-infected DCs with autologous resting CD4+ T cells in the presence of a nominal antigen. In this coculture system, direct contact of HIV-1-infected DCs with T cells was crucial for efficient virus transmission and subsequent virus production. Blocking of the LFA-1/ICAM-1 or LFA-3/CD2 interaction between these cells substantially reduced virus production, without influence or IL-2 production by activated T cells. In contrast, cell-cell transmission of HIV between non-APCs and activated T cells was not blocked by an antibody against LFA-3. Since a low level of virus production by HIV-infected DCs was upregulated by cross-linking of CD40, it was suggested that not only focal adhesion, but also mutual activation of HIV-infected DCs and T cells through adhesion molecules, may potentiate virus transmission and production and that such activation signals to HIV may be distinct from signals responsible for IL-2 production in activated T cells.

LIGHT Is Constitutively Expressed on T and NK Cells in the Human Gut and Can Be Induced by CD2-Mediated Signaling

The TNF superfamily cytokine, lymphotoxin-like inducible protein that competes with glycoprotein D for binding herpesvirus entry mediator on T cells (LIGHT; TNFSF14), can augment T cell responses inducing IFN-gamma production and can drive pathological gut inflammation when expressed as a transgene in mouse T cells. LIGHT expression by human intestinal T cells suggests the possibility that LIGHT may play a key role in regulation of the mucosal immune system. A nonenzymatic method was developed for the isolation of T cells from the human lamina propria, permitting analysis of native cell surface protein expression. Cell surface LIGHT was constitutively expressed on mucosal T and NK cells and a subpopulation of gut-homing CD4(+) T cells in the periphery. In addition, CD2-mediated stimulation induced efficient LIGHT expression on intestinal CD4(+) T cells, but not on peripheral blood T cells, suggesting a gut-specific, Ag-independent mechanism for LIGHT induction. By contrast, herpesvirus entry mediator expression on gut T cells was unperturbed, implicating the transcriptional regulation of LIGHT as a mechanism modulating signaling activity in the gut. Quantitative analysis of LIGHT mRNA in a cohort of inflammatory bowel disease patients indicated elevated expression in biopsies from small bowel and from inflamed sites, implicating LIGHT as a mediator of mucosal inflammation.

IL-2-activated CD8+CD44high cells express both adaptive and innate immune system receptors and demonstrate specificity for syngeneic tumor cells

CD8(+) T cells depend on the alphabeta TCR for Ag recognition and function. However, Ag-activated CD8(+) T cells can also express receptors of the innate immune system. In this study, we examined the expression of NK receptors on a population of CD8(+) T cells expressing high levels of CD44 (CD8(+)CD44(high) cells) from normal mice. These cells are distinct from conventional memory CD8(+) T cells and they proliferate and become activated in response to IL 2 via a CD48/CD2-dependent mechanism. Before activation, they express low or undetectable levels of NK receptors but upon activation with IL-2 they expressed significant levels of activating NK receptors including 2B4 and NKG2D. Interestingly, the IL-2-activated cells demonstrate a preference in the killing of syngeneic tumor cells. This killing of syngeneic tumor cells was greatly enhanced by the expression of the NKG2D ligand Rae-1 on the target cell. In contrast to conventional CD8(+) T cells, IL-2-activated CD8(+)CD44(high) cells express DAP12, an adaptor molecule that is normally expressed in activated NK cells. These observations indicate that activated CD8(+)CD44(high) cells express receptors of both the adaptive and innate immune system and may play a unique role in the surveillance of host cells that have been altered by infection or transformation.

CD2BP1 modulates CD2-dependent T cell activation via linkage to protein tyrosine phosphatase (PTP)-PEST

Human CD2 regulates T cell activation and adhesion via mechanisms yet to be fully understood. This study focuses on CD2BP1, a CD2 cytoplasmic tail-binding protein preferentially expressed in hematopoetic cells. Structural and functional analyses suggest that CD2BP1 acts as a scaffold protein, participating in regulation of the actin cytoskeleton. In this study, using a murine Ag-specific primary T cell transduction system to assess CD69, IL-2, and IFN-gamma expression, we provide evidence that CD2BP1 directly and negatively impacts T cell activation via isolated CD2 triggering or TCR stimulation dependent on coordinate CD2 engagement. Disruption of protein tyrosine phosphatase-PEST and/or CD2BP1 association with the CD2 signalsome rescues T cells from the inhibitory effect of CD2 crosslinking. The overexpression of CD2BP1 selectively attenuates phospholipase Cgamma1, ERK1/2, and p38 phosphorylation without abrogating CD2-independent TCR stimulation. This study provides new insight on the regulation of T cell activation and may have implications for autoimmune processes known to be associated with CD2BP1 mutations.