T cell receptor-independent CD2 signal transduction in FcR+ cells

Signals Controlling Lytic Granule Polarization at the Cytotoxic Immune Synapse

Cytotoxic immunity relies on specialized effector T cells, the cytotoxic T cells, which are endowed with specialized cytolytic machinery that permits them to induce death of their targets. Upon recognition of a target cell, cytotoxic T cells form a lytic immune synapse and by docking the microtubule-organizing center at the synaptic membrane get prepared to deliver a lethal hit of enzymes contained in lytic granules. New insights suggest that the directionality of lytic granule trafficking along the microtubules represents a fine means to tune the functional outcome of the encounter between a T cell and its target. Thus, mechanisms regulating the directionality of granule transport may have a major impact in settings characterized by evasion from the cytotoxic response, such as chronic infection and cancer. Here, we review our current knowledge on the signaling pathways implicated in the polarized trafficking at the immune synapse of cytotoxic T cells, complementing it with information on the regulation of this process in natural killer cells. Furthermore, we highlight some of the parameters which we consider critical in studying the polarized trafficking of lytic granules, including the use of freshly isolated cytotoxic T cells, and discuss some of the major open questions.

Transient cytokine-induced liver injury following administration of the humanized anti-CD3 antibody visilizumab (HuM291) in Crohn's disease

OBJECTIVES

Monoclonal antibodies to CD3 and CD4 T-cell receptors are evolving for Crohn's disease (CD) and ulcerative colitis. Their administration is often associated with a cytokine release syndrome (CRS).

METHODS

We evaluated data from two prospective clinical trials (NCT00267709 and NCT00267722) of visilizumab (HuM291), a novel humanized anti-CD3 antibody, in 34 patients with CD who received 10 microg/kg intravenously on 2 consecutive days. Serum hepatic tests including bilirubin, alkaline phosphatase (AP), gamma-glutamyltransferase (GGT), alanine aminotransferase (ALT) and aspartate aminotransferase (AST), visilizumab concentrations, and a panel of 16 cytokines were measured pre- and postadministration of visilizumab.

RESULTS

Patients experienced CRS symptoms at a median of 45 min postinfusion. The cytokine profile was characterized by interferon-inducible protein-10 (IP-10), interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-alpha), interferon-gamma, monocyte chemotactic protein 1 (MCP-1), interleukin-8 (IL-8), interleukin-6 (IL-6), interleukin-2 (IL-2), and interleukin 1 receptor antagonist (IL-1Ra), which were elevated between 6 (IL-1Ra) and 870 (IP-10) times their baseline concentrations. TNF-alpha and IL-2 peaked at the first day 1 h post infusion, whereas all others peaked at 6 h post infusion. Eighty-six percent of patients experienced an elevation above the upper limit of normal in hepatic enzymes (GGT 73%, AST 73%, ALT 64%, and AP 42% of patients), but not bilirubin, within 6 h postinfusion.

CONCLUSIONS

Transient elevation of hepatic enzymes occurred frequently in patients with CD treated with visilizumab and was associated with CRS. CD patients could be predisposed due to an aberrant expression of adhesion molecules in the liver that promotes CRS upon engagement of the T-cell receptor and may relate to extraintestinal disease manifestations such as primary sclerosing cholangitis.

Expression, epitope analysis, and functional role of the LFA-2 antigen detectable on neoplastic mast cells

Recent data suggest that mast cells (MCs) in patients with systemic mastocytosis or mast cell leukemia express a CD2-reactive antigen. To explore the biochemical nature and function of this antigen, primary MCs as well as the MC line HMC-1 derived from a patient with mast cell leukemia were examined. Northern blot experiments revealed expression of CD2 messenger RNA in HMC-1, whereas primary nonneoplastic MCs did not express transcripts for CD2. In cell surface staining experiments, bone marrow (BM) MCs in systemic mastocytosis (n = 12) as well as HMC-1 cells (30%-80%) were found to express the T11-1 and T11-2 (but not T11-3) epitopes of CD2. By contrast, BM MCs in myelodysplastic syndromes and nonhematologic disorders (bronchiogenic carcinoma, foreskin phimosis, uterine myeomata ) were consistently CD2(-). All MC species analyzed including HMC-1 were found to express LFA-3 (CD58), the natural ligand of CD2. To study the functional role of CD2 on neoplastic MCs, CD2(+) and CD2(-) HMC-1 cells were separated by cell sorting. CD2(+) HMC-1 cells were found to form spontaneous aggregates and rosettes with sheep erythrocytes in excess over CD2(-) cells, and a T11-1 antibody inhibited both the aggregation and rosette formation. Moreover, exposure of CD2(+) HMC-1 cells to T11-1 or T11-2 antibody was followed by expression of T11-3. In addition, stimulation of neoplastic MCs through T11-3 and a second CD2 epitope resulted in histamine release. These data show that neoplastic MCs express functionally active CD2. It is hypothesized that expression of CD2 is associated with pathologic accumulation and function of MCs in systemic mastocytosis.

A p56lck-independent pathway of CD2 signaling involves Jun kinase

The p56 Src family non-receptor tyrosine kinase has been shown to be critical for T lymphocyte differentiation and activation. Hence in the absence of p56, T cell receptor triggered activation does not occur. We now provide evidence for a CD2-based signaling pathway which, in contrast to that of the T cell receptor, is independent of p56. CD2-mediated interleukin-2 production occurs via activation of Jun kinase in cell lines lacking p56. Jun kinase then facilitates the binding of c-Jun/c-Fos heterodimers to the AP-1 consensus site and the subsequent transcriptional activity of the interleukin-2 promoter. These data elucidate differences between TCR and CD2 signaling pathways in the same T cells.

Identification of a proline-rich sequence in the CD2 cytoplasmic domain critical for regulation of integrin-mediated adhesion and activation of phosphoinositide 3-kinase

The CD2 molecule is one of several lymphocyte receptors that rapidly initiates signaling events regulating integrin-mediated cell adhesion. CD2 stimulation of resting human T cells results within minutes in an increase in beta1-integrin-mediated adhesion to fibronectin. We have utilized the HL60 cell line to map critical residues within the CD2 cytoplasmic domain involved in CD2 regulation of integrin function. A panel of CD2 cytoplasmic domain mutants was constructed and analyzed for their ability to upregulate integrin-mediated adhesion to fibronectin. Mutations in the CD2 cytoplasmic domain implicated in CD2-mediated interleukin-2 production or CD2 avidity do not affect CD2 regulation of integrin activity. A proline-rich sequence, K-G-P-P-L-P (amino acids 299 to 305), is essential for CD2-mediated regulation of beta1 integrin activity. CD2-induced increases in beta1 integrin activity could be blocked by two phosphoinositide 3-kinase (PI 3-K) inhibitors or by overexpression of a dominant negative form of the p85 subunit of PI 3-K. In addition, CD2 cytoplasmic domain mutations that abrogate CD2-induced increases in integrin-mediated adhesion also ablate CD2-induced increases in PI 3-K enzymatic activity. Surprisingly, CD2 cytoplasmic domain mutations that inhibit CD2 regulation of adhesion do not affect the constitutive association of the p85 subunit of PI 3-K association with CD2. Mutation of the proline residues in the K-G-P-P-L-P motif to alanines prevented CD2-mediated activation of integrin function and PI 3-K activity but not mitogen-activated protein (MAP) kinase activity. Furthermore, the MEK inhibitor PD 098059 blocked CD2-mediated activation of MAP kinase but had no effect on CD2-induced adhesion. These studies identify a proline-rich sequence in CD2 critical for PI 3-K-dependent regulation of beta1 integrin adhesion by CD2. In addition, these studies suggest that CD2-mediated activation of MAP kinase is not involved in CD2 regulation of integrin adhesion.

The SH3 domain of p56lck binds to proline-rich sequences in the cytoplasmic domain of CD2

CD2, a cell surface glycoprotein expressed on T cells and natural killer cells, can couple to signaling pathways that result in T cell proliferation. An Src-like protein tyrosine kinase, p56lck, coprecipitates with CD2, and perturbation of CD2 by monoclonal antibodies results in an increase in the activity of p56lck, suggesting that an interaction with p56lck contributes to CD2-mediated signaling. Herein, we investigate the mechanism by which CD2 associates with p56lck. We demonstrate that CD2 and p56lck associate when coexpressed in nonlymphoid cells, that this association requires the cytoplasmic domain of CD2, and that the SH3 domain of p56lck mediates its interactions with CD2. Using truncation mutants of CD2, we identify two regions in the cytoplasmic domain of CD2 involved in binding p56lck. Each region contains a proline-rich sequence that, in the form of a synthetic peptide, directly binds p56lck. Thus, proline-rich sequences in the cytoplasmic domain of CD2 allow this transmembrane receptor to bind to the SH3 domain of p56lck.

The cytoplasmic tail of the T cell receptor zeta chain is required for signaling via CD26

The protease dipeptidylpeptidase IV (CD26) provides an alternative activation pathway for T lymphocytes and is involved in several aspects of T cell function. Activation via CD26 requires the expression of the T cell receptor (TcR)/CD3 complex. Here we have investigated the role of the TcR zeta chain for T cell activation via CD26. T cell hybridomas expressing TcR with various deletions in the CD3 zeta chain were transfected with a CD26 cDNA and the response of the transfected cells to anti-CD26 monoclonal antibodies was tested. Our data show that the zeta chain is essential and that at least one YXXL motif in the cytoplasmic tail of the zeta chain is required for CD26-mediated signaling. Other TcR components do not replace the zeta chain.

Interaction between human CD2 and CD58 involves the major beta sheet surface of each of their respective adhesion domains

The CD58 binding site on human CD2 was recently shown by nuclear magnetic resonance structural data in conjunction with site-directed mutagenesis to be a highly charged surface area covering approximately 770A2 on the major AGFCC'C" face of the CD2 immunoglobulin-like (Ig-like) NH2-terminal domain. Here we have identified the other binding surface of the CD2-CD58 adhesion pair by mutating charged residues shared among CD2 ligands (human CD58, sheep CD58, and human CD48) that are predicted to be solvent exposed on a molecular model of the Ig-like adhesion domain of human CD58. This site includes beta strand residues along the C strand (E25, K29, and K30), in the middle of the C' strand (E37) and in the G strand (K87). In addition, several residues on the CC' loop (K32, D33, and K34) form this site. Thus, the interaction between CD2 and CD58 involves the major beta sheet surface of each adhesion domain. Possible docking orientations for the CD2-CD58 molecular complex are offered. Strict conservation of human and sheep CD58 residues within the involved C and C' strands and CC' loop suggests that this region is particularly important for stable formation of the CD2-CD58 complex. The analysis of this complex offers molecular insight into the nature of a receptor-ligand pair involving two Ig family members.

CD26: a surface protease involved in T-cell activation

CD26 is a proteolytic enzyme (dipeptidylpeptidase IV) with a wide tissue distribution and a unique specificity. Recent developments indicate that CD26 is a multifunctional molecule that may have important functions in the immune system. Here, Bernhard Fleischer reviews the current knowledge of CD26 and discusses the possible functions of this molecule in T lymphocytes.

An autoimmune MRL/Mp-Ipr/Ipr mouse-derived monoclonal IgG antibody stimulates cytokine production in bone-marrow-derived cell line by cross-linking of a cell surface antigen and Fc receptor

An IgG1 mAb 1G10 derived from an autoimmune MRL/Mp-Ipr/Ipr (MRL/Ipr) mouse has previously been shown to induce IL-3, TNF-alpha and IL-6 production, and autocrine growth in an IL-3-dependent myeloid cell line, FDC-P2/185-4. In the present study, we have attempted to further define the molecular mechanism responsible for the 1G10-induced activation of FDC-P2/185-4 cells. We have shown that 1G10 lacked anti-IgG1 rheumatoid factor activity, failing to generate self-associated immune complexes. Since 1G10 stimulated cells in an Fc gamma R-dependent manner, it seems likely that cross-linking of a cell surface antigen and Fc gamma R by 1G10 antibody is responsible for the stimulation of FDC-P2/185-4 cells. Among several mAb specific to surface antigens expressed on FDC-P2/185-4 cells (MHC class I, LFA-1, and Fc gamma R), only a mAb specific to the alpha chain of LFA-1 alpha was able to induce the IL-3 and Fc gamma R-dependent proliferation of FDC-P2/185-4 cells, similar to that induced by 1G10. Immunoprecipitation analysis revealed that 1G10 recognized a polypeptide with a molecular mass of 140 kilodaltons (p140), which differed from Fc gamma R and from LFA-1 alpha chain. These results suggest that cross-linking of not general but particular cell surface antigens and Fc gamma R stimulates FDC-P2/185-4 cells to produce cytokines resulting in their proliferation.

The CD58 (LFA-3) binding site is a localized and highly charged surface area on the AGFCC'C" face of the human CD2 adhesion domain

Using site-directed mutagenesis in conjunction with NMR structural data on the adhesion domain of human CD2, we have defined the binding region for CD58. Previous structural studies of rat and human CD2 indicate that this adhesion domain is immunoglobulin-like. Here we report that the CD58 binding site is a well-circumscribed, charged surface area covering approximately 770 A2 on the AGFCC'C" face of the CD2 beta barrel. This site contains beta-strand residues in the carboxyl-terminal half of the F strand (including Lys-82 and Tyr-86), the top of the C strand (Asp-32 and Lys-34), and the C' strand (Gln-46), which are all solvent exposed. In addition, several exposed residues on the FG loop (Gly-90, Lys-91, Asn-92, and Val-93), the CC' loop (Lys-41 and Lys-43), and the C'C" loop (Arg-48 and Lys-51) form this site. In contrast, neither residues on the more peripheral G and C" strands of the same CD2 surface nor residues on B, E, and D strands of the opposite face are involved in CD58 binding. This CD58 binding site is predicted to lie most distal to the T-lymphocyte surface membrane, with ready access to CD58 on the surface of the opposing antigen-presenting cell.

Association of Src-like protein tyrosine kinases with the CD2 cell surface molecule in rat T lymphocytes and natural killer cells

The cell surface molecule CD2 has a signaling role in the activation of T lymphocytes and natural killer cells. Because perturbation of CD2 leads to the appearance of tyrosine-phosphorylated proteins, we investigated the possibility that CD2 associates with cytoplasmic protein tyrosine kinases. As determined by in vitro kinase assays and phosphoamino acid analysis, protein tyrosine kinase activity coprecipitated with CD2 from rat T lymphocytes, T lymphoblasts, thymocytes, interleukin-2-activated natural killer cells, and RNK-16 cells (a rat natural killer cell line). In each case, both p56lck and p59fyn were identified in the CD2 immunoprecipitate. In the thymus, the association between CD2 and these kinases occurred predominately in a small subset of thymocytes that had the cell surface phenotype of mature T cells, indicating that the association is a regulated event and occurs late in T-cell ontogeny. The finding that CD2 is associated with p56lck and p59fyn in detergent lysates suggests that interactions with these Src-like protein kinases play a critical role in CD2-mediated signal transduction.

Association of Src-like protein tyrosine kinases with the CD2 cell surface molecule in rat T lymphocytes and natural killer cells

The cell surface molecule CD2 has a signaling role in the activation of T lymphocytes and natural killer cells. Because perturbation of CD2 leads to the appearance of tyrosine-phosphorylated proteins, we investigated the possibility that CD2 associates with cytoplasmic protein tyrosine kinases. As determined by in vitro kinase assays and phosphoamino acid analysis, protein tyrosine kinase activity coprecipitated with CD2 from rat T lymphocytes, T lymphoblasts, thymocytes, interleukin-2-activated natural killer cells, and RNK-16 cells (a rat natural killer cell line). In each case, both p56lck and p59fyn were identified in the CD2 immunoprecipitate. In the thymus, the association between CD2 and these kinases occurred predominately in a small subset of thymocytes that had the cell surface phenotype of mature T cells, indicating that the association is a regulated event and occurs late in T-cell ontogeny. The finding that CD2 is associated with p56lck and p59fyn in detergent lysates suggests that interactions with these Src-like protein kinases play a critical role in CD2-mediated signal transduction.

The CD3 zeta cytoplasmic domain mediates CD2-induced T cell activation

CD2-mediated T lymphocyte activation requires surface expression of CD3- Ti, the T cell receptor (TCR) for antigen major histocompatibility complex protein. Given the importance of CD3 zeta in TCR signaling, we have directly examined the ability of the CD3 zeta cytoplasmic domain to couple CD2 to intracellular signal transduction pathways. A cDNA encoding a chimeric protein consisting of the human CD3 zeta cytoplasmic domain (amino acid residues 31-142) fused to the CD8 alpha extracellular and transmembrane domains (amino acid residues 1-187) was transfected into a CD2+CD3-CD8- variant of the human T cell line Jurkat. The resulting transfectants expressed the CD8 alpha/CD3 zeta chimeric receptor at the cell surface in the absence of other TCR subunits. Stimulation of these transfectants with anti-T11(2) + anti- T11(3) monoclonal antibodies (mAbs) initiated both a prompt cytosolic free calcium ([Ca2+]i) rise and protein tyrosine kinase activation. Stimulation with either intact anti-T11(2) + anti-T11(3) mAbs or purified F(ab')2 fragments resulted in interleukin 2 (IL-2) secretion. In contrast, control cell lines transfected with a cDNA encoding wild- type CD8 alpha, and thus lacking surface expression of the CD3 zeta cytoplasmic domain, failed to show any [Ca2+]i rise, protein tyrosine kinase activation, or IL-2 secretion after identical stimulation. These data directly establish the CD3 zeta cytoplasmic domain as a necessary and sufficient component of the CD3-Ti complex involved in T lymphocyte activation through CD2. Moreover, they show that CD2 signaling can function in the absence of Fc receptors.

Preferential interaction of the CD4+29+/45RA-subset of human CD4+ T lymphocytes with an antibody against the cell-membrane ganglioside GD3

This study shows that the two major subpopulations of CD4+ T lymphocytes defined on the basis of differential expression of the CD29 and CD45RA antigens, show significant differences in reactivity with a monoclonal antibody against the GD3 ganglioside. Double staining studies showed that the GD3 ganglioside is predominantly expressed on cells from the CD4+29+/45RA subsets. The preferential interaction of the CD4+29+/45RA cell subset with the anti-GD3 MoAb was further confirmed by the proliferative and calcium-flux studies. Accordingly, the reciprocal, CD4+(29-)/45RA+ subset was unable to proliferate in response to the anti-GD3 MoAb alone. Although it did show a significant mobilization of calcium ions and proliferation to IL-2 when stimulated with the anti-GD3 MoAb, these response were much less pronounced than the responses of the CD4+29+/45RA- subset. Finally, when two T-cell stimulating monokines, IL-1 and IL-6, were tested for the ability to modulate the anti-GD3 mediated proliferation, only the former, but not the latter was able to enhance the proliferation. Although the natural ligand for the GD3 ganglioside remains unknown, the data presented here provide further evidence in support of the notion that the T-cell surface molecules different from the T-cell receptor MHC-antigen complex may contribute to the preferential activation of one of the CD4+ T lymphocyte subsets.

CD3 zeta dependence of the CD2 pathway of activation in T lymphocytes and natural killer cells

In T lymphocytes, signal transduction through the CD2 adhesion molecule requires surface expression of the CD3-Ti T-cell receptor (TCR) complex. In contrast, in natural killer (NK) cells, CD2 functions in the absence of a TCR. Because the TCR on T lymphocytes and the CD16 low-affinity IgG Fc receptor (Fc gamma receptor type III) complex on NK cells share a common CD3 zeta subunit, we reasoned that CD3 zeta may be critical for CD2 signaling in T lymphocytes and NK cells. Here we show that transfection of a cDNA encoding a transmembrane form of CD16 into TCR- variants of the Jurkat T-cell line results in CD16 expression in association with endogenous CD3 zeta homodimers and restores CD2 signaling function. To test directly the role of CD3 zeta in CD2 triggering, we then transfected human CD2 into each of two murine T-T hybridomas that express TCRs containing either a full-length CD3 zeta subunit or a truncated CD3 zeta subunit incapable of transducing signals. Despite expression of equivalent surface levels of TCR, CD2-mediated signaling is seen only in the T cells containing wild-type CD3 zeta. These findings show that (i) CD16 on NK cells is functionally equivalent to the TCR on T lymphocytes for coupling CD2 to signaling pathways and (ii) CD2 signal transduction depends upon the CD3 zeta subunit of the TCR complex and, by inference, the CD3 zeta subunit of the CD16 complex.

Signal transduction by the CD2 antigen in T cells and natural killer cells: requirement for expression of a functional T cell receptor or binding of antibody Fc to the Fc receptor, Fc gamma RIIIA (CD16)

Crosslinking of CD2 antigen on T lymphocytes and natural killer (NK) cells leads to a rise in cytoplasmic-free Ca2+ concentration ([Ca2+]i). However, CD2 seems unlikely to interact directly with the second messenger pathways since signaling via CD2 is poor in T cells that lack the T cell receptor (TCR) and is absent in L cells or insect cells that express CD2. In contrast, NK cells that are also TCR- can be triggered via CD2, but it is unclear as to whether the CD16 Fc receptor (FcR) may facilitate this effect. The CD16 transmembrane molecule is expressed in a complex with the zeta homodimer or the zeta/gamma heterodimer and these dimers are also associated with the TCR complex. Thus, it seemed that zeta chains may provide the link between signaling on NK cells and T cells. This could be tested on TCR- cells since when CD16 is transfected into T cells it is expressed in a complex with TCR zeta homodimer or the zeta/gamma heterodimer. At first, potentiation of CD2 signaling was seen on TCR- Jurkat cells expressing CD16, but this was found to be dependent on trace levels (1%) of IgG in F(ab')2 antibody preparations. With pure F(ab')2, the effect was lost. Signaling on a rat NK cell line was also re-examined with F(ab')2 antibodies that had no IgG contamination, and again no signal transduction via CD2 was seen. We thus conclude that there is no clear evidence for potent signaling via CD2 on cells that lack a TCR complex and that TCR zeta chain expressed at the cell surface is not sufficient to potentiate signaling via CD2 as measured by an increase in [Ca2+]i.