T-T cell interactions are mediated by adhesion molecules.

The mechanism by which T cells signal other T cells is not well defined. This was investigated by studying the ability of circulating T cells to induce the proliferation of autologous T cell clones. Peripheral blood T cells activated by cross-linking of the CD3/T cell receptor complex, which increased the expression of cell adhesion molecules LFA-1, LFA-3 and ICAM-1, induced the proliferation of autologous T cell clones. Irradiated antigen-activated peripheral blood T cells could also induce the proliferation of T cell clones which could not recognize that antigen. T-T cell activation required cell contact, was not major histocompatibility complex (MHC) restricted and was blocked by monoclonal antibodies directed against adhesion molecules CD2 and LFA-3 but was not blocked by antibody to class II MHC determinants. As CD2 is the natural ligand for LFA-3, increased expression of T cell surface adhesion molecules LFA-1, ICAM-1 and particularly LFA-3 during an inflammatory response may rapidly recruit T cells that are activated through the CD2 pathway. These results allow a simplified model to explain how relatively few antigen/MHC-specific T cells can recruit large numbers of non-antigen-specific T cells in the generation of an inflammatory response and postulates a novel role of the CD2 molecule in T cell immune function.     

Induction of dendritic cell costimulator molecule expression is suppressed by T cells in the absence of antigen-specific signalling: role of cluster formation, CD40 and HLA-class II for dendritic cell activation.

Full activation of T lymphocytes by dendritic cells (DC) during antigen presentation is known to require the interaction of several inducible receptor-ligand pairs. We have postulated that the reciprocal activation of DC by T lymphocytes is also important. Potential signalling molecules that might increase the stimulatory capacity of DC during antigen presentation to T lymphocytes were tested using an in vitro model. Fresh human blood DC were cocultured with CD4+ and CD8+ allogeneic or with autologous T lymphocytes plus Staphylococcus superantigen A (SEA). Surprisingly, costimulator expression on DC cocultured with T lymphocytes was reduced in comparison to DC cultured alone. However, the minority (10-30%) of DC clustering with T lymphocytes showed antigen-specific up-regulation of the CD40, CD80 and CD86 costimulator molecules, whereas the non-clustered DC (70-90%) had less up-regulation than control DC cultured alone and did not respond to antigen-specific triggering. Monoclonal antibodies (mAb) to CD40 ligand (CD40L) and human leucocyte antigen (HLA)-DR, but not lymphocyte function-associated antigen-1 (LFA-1), LFA-3 or HLA-class I, significantly inhibited the T-lymphocyte induction of DC costimulator expression. Since HLA-class II, but not HLA-class I mAb, inhibited allogeneic T-lymphocyte-mediated activation of DC, CD4 T lymphocytes appear to be the main subset activating DC in the mixed lymphocyte reaction. Cross-linking of CD40, but not HLA-class II, up-regulated DC or B-cell costimulator expression. Although direct class II signalling does not appear to play a role in DC activation, antigen-specific T-cell recognition contributes via other mechanisms to regulate DC activation.     

The relation between T-cell expression of LFA-1 and immunological memory.

Antibodies against isotypes of the leucocyte common antigen (LCA, CD45) can be used to identify largely reciprocal subsets of human peripheral T cells, characterized by differential ability to respond to recall antigen in vitro. The transition from naive, unprimed T cells to memory cells capable of responding to recall stimulating has been associated with a switch in surface expression of CD45 from the CD45RA isotype to CD45RO. It has been proposed that this transition is accompanied by the coordinated up-regulation of a number of cell-surface molecules involved in cellular adhesion and/or activation, including the leucocyte function-associated antigens (LFA). In the present study we have examined the expression of LFA-1 on subsets of human peripheral T cells, and related it to the expression of markers of cellular activation and CD45 isotypes, and thus to immunological memory. Our results suggest that the intensity of LFA-1 expression on the surface membrane of human peripheral T cells is not tightly associated with maturation status as judged by LCA isotype expression, but rather reflects the degree of cellular activation. This conclusion is supported by data of T-cell function in vitro, showing similar antigen- and mitogen-induced proliferative responses in T-cell subsets characterized by low as well as high surface expression of LFA-1.     

CD4-Ia interactions can occur in the absence of T-cell receptor/antigen-Ia recognition

The T-cell differentiation antigen, CD4, is expressed by major histocompatibility (MHC) class II restricted T lymphocytes. CD4+CD8- T cells use their T-cell receptor to recognize foreign antigens in association with MHC class II products (Ia). The association between CD4 expression and restriction by MHC class II products has led to the hypothesis that CD4 may interact with monomorphic determinants of MHC class II molecules. A large body of experimental evidence suggests that CD4 interaction with MHC class II molecules leads to an increase in the binding avidity of T cell-stimulator cell interactions. A direct test for a functional CD4-MHC class II interaction in T-cell activation requires a separate evaluation of CD4-Ia interactions from T-cell receptor (TcR)-antigen (Ag)/Ia recognition. However, a separate evaluation proves difficult since the T-cell receptor and CD4 may interact with the same MHC class II molecule. In this report, we use a T-cell activation protocol where TcR-Ag/Ia recognition is replaced by TcR complex-anti-CD3 antibody interactions. Therefore, the affinity of the TcR complex for its ligand (the anti-CD3 mAb) is independent from MHC expression on target cells and allows a separate evaluation of the role of accessory molecules in T-cell activation. We have analysed the effects of monoclonal anti-MHC class II antibodies on the activation of a CD4+ T-cell hybridoma in the absence of its TcR restricting MHC class II molecule (I-Ek) but in the presence of unrelated MHC class II molecules (I-Ed, I-Ad). The data obtained indicate a functional interaction between the CD4 molecule and a non-polymorphic region of the MHC class II product in T-cell triggering.     

Isolation of two CD50 (ICAM-3)-negative Jurkat T-cell clones and their application for analysis of CD50 function

Abstract: The leukocyte differentiation antigen CD50 (intercellular adhesion molecule-3, ICAM-3), mediates cell-cell adhesion through its ligand LFA-1 and is a transducting receptor molecule during T-cell activation. Since CD50 homologues in other species have not yet been identified, the role of this molecule can only be analyzed in human cell models. Thus, to better study CD50 function in T cells, we have obtained two CD50-negative T-cell clones, named CAMY.l and CAMY.2. These clones were derived from the Jurkat T-cell variant PPL.l. Data from analysis of protein expression, specific mRNA content and calcium mobilization assays have confirmed the absence of functional CD50 molecules on these two clones. Thus, CAMY.l and CAMY.2 show no CD50 expression by phenotypical and immunoprecipit-ation analysis. CD50-sperific mRNA content is undetectable by Northern blot analysis in these clones and, only, when RT-PCR was performed could specific mRNA be detected. Additionally, CD50 cross-linking on theses clones shows no increase in intracellular calcium. Transfection of CD50 cDNA on CAMY cells restores not only CD50 surface expression, but its functional ability to induce calcium mobilization, CD69 upregulation and cell morphological changes. The CAMY.l and CAMY.2 clones provide useful model systems to analyze CD50 function in T cells.     

Peptide-induced proliferation and lymphokine production in human T cells in the absence of antigen-presenting cells: role of T-cell activation state and costimulatory signals.

The role of T-lymphocytes as antigen-presenting cells (APCs) for other T cells was investigated. Activated rabies-virus-specific human T-cell clones were shown to present peptide to class II major histocompatibility complex (MHC)-restricted T cells of a different fine specificity, resulting in lymphokine production and cell proliferation. Furthermore, purified and activated antigen-specific T cells could produce lymphokines and proliferate as a result of the addition of antigenic peptide in the absence of APC. The functional response of T cells to peptide in the absence of APC was amplified by the addition of phorbol ester (PMA) and was inhibited with antibodies specific to class II MHC or to the CD2 molecule. Experiments performed in single-cell suspension cultures using semisolid medium prepared with 1% agar demonstrate that T-cell proliferative and lymphokine responses to peptide both in the presence and absence of APC require the interaction of T-cell antigen receptor (TCR) molecules with class II MHC-peptide complexes on different cell surfaces (cell-cell contact). On the other hand, peptide self-presentation, which occurs by the binding of TCR with class II MHC-peptide complexes on the same cell surface (at the single-cell level), resulted in T-cell activation (i.e., high expression of surface CD2, CD25, and HLA-DR molecules), without proliferation or lymphokine secretion, a pattern observed in the induction of T-cell anergy by antigen. The results are discussed in terms of the role of class II MHC molecules on activated T-lymphocytes, which enable these cells to function as "professional APC" in the development of T-cell regulatory networks.     

CD4+ CD45R- suppressor-inducer T-cell clones: requirements for cellular interaction, proliferation and lymphokines for the induction of suppression in peripheral blood mononuclear cells.

Regulation of the induction of suppressive activity in peripheral blood mononuclear cells (PBMC) by human major histocompatibility complex (MHC) class II+ CD4+ CD45R+ suppressor-inducer T-cell clones has been investigated. Previously, it was shown that in this system, cyclosporin A-sensitive precursors gave rise to allo-indifferent MHC-unrestricted CD4+ suppressive cells. Their induction could be blocked by monoclonal antibodies (mAb) to multilocus MHC class II gene products (TU 39) but not by mAb preferentially reacting with HLA-DR, -DQ or -DP molecules. This product, functionally defined, was termed 'DY'. It is shown here that induction of suppression by DY follows established activation pathways: (i) cell adhesion was required because CD11a (LFA-1) mAb blocked suppressor-induction; (ii) CD4 mAb also blocked, consistent with the involvement of class II products in suppressor-induction; (iii) cell proliferation was required because mAb to transferrin receptors, or irradiation, inhibited induction; and (iv) such proliferation appeared to be interleukin (IL)-2-dependent because it was blocked by mAb to IL-2 receptor, and enhanced by exogenous IL-2 but not IL-4. It was also enhanced by exogenous IL-1 and IL-6, but not by IL-3, tumour necrosis factor-alpha (TNF alpha) or interferon-gamma (IFN-gamma). It therefore seems that the requirements for activation of suppression by CD4+ DY+ T-cell clones in this in vitro model bear many similarities to those for CD4+ helper T cells, namely, mediation by MHC class II with CD4 involvement, dependency on LFA-1-influenced cell interactions, and reliance on clonal expansion caused by IL-2 and possibly amplified by IL-1 and/or IL-6.     

The LFA-3 Adhesion Pathway is Differently Utilized by Superantigen-Activated Human CD4+ T-Cell Subsets

The superantigen SEA binds to MHC class II molecules and activates a large fraction of T cells as a result of interaction with particular TCR-V beta sequences. MHC class II transfected CHO cells induce a marginal CD4+ T-cell proliferation in the presence of SEA. CHO cells transfected with both MHC class II and LFA-3 (HLA-DR4/LFA-3 double transfectants) supported a vigorous T-cell proliferation and required 1000-fold lower SEA concentration than DR4-transfected cells. DR4/LFA-3 double transfectants presenting SEA to CD4+ T cells induced large amounts of IFN-gamma, while single DR4 transfectants failed to elicit IFN-gamma production. CD4+45RA+ naive T cells proliferated much more strongly compared with CD4+45R0+ memory T cells when SEA was presented by the DR4/LFA-3-transfected cells. In contrast, IFN-gamma production was only detected in CD4+45R0+ memory cells. The enhanced proliferation by the CD4+45RA+ naive T cells was not due to a stronger binding to the accessory DR4/LFA-3 cells. Human CD4+ T-cell lines mediated a low level of SEA-dependent cell-mediated cytotoxicity (SDCC) against DR4 target cells, whereas a strong SDCC was mediated against DR4/LFA-3-expressing target cells. These results demonstrate that superantigen-activated human CD4+ T cells require the adhesion molecule LFA-3 for optimal stimulation and that the CD4+ naive and memory T-helper cells are different in their response to LFA-3 as an accessory molecule.     

Accessory Signals in T-T Cell Interactions Between Antigen- and Alloantigen-Specific, Human Memory T Cells Generated in Vitro

The potential of activated HLA class II-positive T cells as antigen-/alloantigen-presenting cells remains controversial. In our model system we use in vitro-primed, HLA class II-specific T cells of the memory T-cell phenotype, CD4+, CD29+ (4B4+), and CD45RO+ (UCHL-1). We have previously shown that alloactivated, HLA class II-positive T cells (Ta) are unable to stimulate proliferative responses in naive and primed allospecific T cells when 'back-stimulation' is avoided. The explanation of this feature of Ta is unknown, but it is due neither to suppression nor to insufficient HLA class II expression. Accordingly, we investigated the possibility that Ta have a deficient expression of accessory signals critical for the induction of proliferative T-cell responses. We found that (1) non-mitogenic concentrations of phorbol myristate acetate (PMA) in combination with either rIL-4, a CD28-reactive MoAb (Kolt-2), or a calcium ionophore (A23187) enabled Ta to elicit alloantigen-specific memory T-cell responses and to present purified protein derivative (PPD) to PPD-specific T-cell lines. The addition of irradiated, Epstein-Barr virus-transformed B-cell lines (EBV-LCL) (but not their supernatants) had a similar but less pronounced effect; (2) MoAb directed against HLA class II, CD25 (IL-2R), CD2, CD4, CD11a (LFA-1), or CD45RO molecules inhibited these responses; (3) PMA was required within the first hour of culture in order to induce optimal alloantigen-specific T-cell activation, while rIL-4 was fully effective when added after 20-44 h of culture; (4) incubation for 20 h of Ta with rIL-4 plus PMA markedly up-regulated CD54 expression on the Ta, and IL-4 seemed to potentiate the effect of PMA on the CD54 expression. In conclusion, the present data indicate that the inability of Ta to elicit (allo)antigen-specific, proliferative T-cell response is due to a lack of critical accessory signals. Up-regulation of CD54 was not sufficient for Ta to stimulate proliferative responses. Neither cytokines (IL-1, IL-6, and others) nor triggering of CD2 epitopes (T11.2 and T11.3) by soluble MoAb or solid phase support by MoAb against a number of accessory molecules provided the necessary signals. Thus, our data indicate that other, as yet unknown, signalling pathways play a key role in antigen- and alloantigen-specific T-T interactions. These pathways still need to be identified.     

(BALB/c x C57BL/6)F1 mice are tolerant to undetectable mixed haplotype A beta dA alpha b and mixed isotype A beta dE alpha d self class II molecules.

The existence of mixed haplotype A beta dA alpha b and mixed isotype A beta dE alpha d molecules was demonstrated in A beta d gene introduced C57BL/6 (B6) transgenic and in A beta dE alpha d double gene introduced B6 transgenic mice, respectively. Using alloreactive and antigen reactive T-cell clones, these mixed class II molecules were shown to function as mixed lymphocyte culture reaction (MLR) stimulating determinants and restriction elements for antigen recognition by T cells. The amounts of expression of these mixed class II molecules were sufficient for the stimulation of alloreactive T cells in primary MLR culture. (BALB/c x B6)F1 (CBF1) mice were shown not to express functional mixed haplotype and mixed isotype class II molecules using alloreactive and antigen reactive T-cell clones. Interestingly, however, CBF1 mice did not respond to such mixed haplotype and mixed isotype class II molecules. These results show that CBF1 mice do not respond to undetectably-expressed class II molecule and suggest that they are tolerant to undetectable self class II molecules. The possible interpretations for this are discussed.     

LFA-3 co-stimulates cytokine secretion by cytotoxic T lymphocytes by providing a TCR-independent activation signal

T cell activation is known to depend not only on efficient antigen recognition and subsequent signaling through TCR, but also on interactions involving multiple adhesion and accessory molecules such as CD28/B7, LFA-1/ICAM-1 and LFA-3/CD2. The present study dissects the role of LFA-3/CD2 interactions in the activation of melanoma-specific CD8⁺ T cell clones. To this end we analyzed the influence of LFA-3 density on melanoma cells on lysis and cytokine production (TNF, IL-2, IFN-γ) by T cells following activation by various amounts of antigenic peptides. Our results indicate that increasing LFA-3 density on melanoma cells variably affects their lysis susceptibility, but systematically and considerably enhances cytokine production by melanoma-specific cytotoxic T lymphocyte (CTL) clones. At any stimulatory antigen density, LFA-3 increased the fraction of responding cells and/or cytokine amounts produced by individual cells, without affecting TCR down-regulation. These results show that CD2 engagement increases cytokine gene activation essentially by providing to T cells a TCR-independent co-activation signal. From a practical point of view, our data demonstrate that the level of LFA-3 expressed on tumors critically affects cytokine production by specific CTL and thus the efficiency of specific immune reactions mediated by these cells.     

Human Naive and Memory T-Helper Cells Display Distinct Adhesion Properties to ICAM-1, LFA-3 and B7 Molecules

In this paper the contribution of different accessory molecules to the adhesion of resting, naive and memory CD4⁺ T cells was examined utilizing a panel of CHO cell transfectants as model antigen-presenting cells (APCs). CD4⁺ T lymphocytes demonstrated strong adhesion to HLA-DR4 transfected CHO cells co-expressing B7, ICAM-I or LFA-3 molecules, suggesting that all three adhesion pathways is utilized by resting CD4⁺ cells. Monoclonal antibodies (MoAbs) against the corresponding receptors on T cells, e.g. anti-CD28, anti-LFA-1β and anti-CD2, inhibited completely T-cell adhesion to natural ligands expressed on transfected CHOcells. Pretreatment of CD4⁺ T cells with NKI-L16 MoAb, which interact with an activation epitope on LFA-loc chain, enhanced adhesion to ICAM-1 but not B7 or LFA-3 expressing CHO cells. Analysis of T helper-cell subsets revealed that memory T cells bound several fold stronger to ICAM-1 expressing transfectants compared to the CD4⁺ 45RA⁺ naive T cells, whereas adhesion to B7, LFA-3- and B7/LFA-3-expressing CHO cells was similar in both T-cell subsets. The kinetics of adhesion of naive and memory CD4⁺ T cells to ICAM-1 was rapid and similar in both subsets. The NKI-L16 MoAb multiplied several times ICAM-1-dependent adhesion in naive compared to memory cells, which enabled the naive cells to reach a similar adhesion level as memory cells. The results suggest that resting naive CD4⁺ T cells utilize preferentially the CD2/LFA-3 or CD28/B7 adhesion pathways upon adhesion to APCs, while memory CD4⁺ T cells utilize the CD2/LFA-3, CD28/B7 and LFA-l/ICAM-1 adhesion pathways. The NK.I-L16 MoAb-induced upregulation of adhesion involves an increased affinity of LFA-1 for its ligand and not a change in the number of LFA-1 molecules. This is compatible with a view that naive cells express a large number of inactive LFA-1 molecules, whereas memory cells express preferentially activated LFA-1 molecules. The inherent low number of active LFA-I molecules on naive CD4⁺ T cells may be important in keeping these cells in a resting state.     

The Influence of LFA 1 on Human T Cells Stimulated by Solid-Phase Immobilized HLA Class II-Peptide Complexes

Purified HLA class II-peptide complexes immobilized to a solid support induce proliferation of human T-cell clones, indicating that human T-cell clones can proliferate in the absence of secondary signals from accessory cells. We hypothesized that T cells can provide co-stimulatory signals to T cells. LFA-1 molecules play an important role in homotypic interactions of T cells and murine monoclonal antibodies reactive with LFA 1 can inhibit T cell T cell interactions. LFA-1 reactive monoclonal antibodies inhibited cytolysis of peptide-pulsed T cells by T cells and partially inhibited T-cell proliferation. To study the direct effect of the LFA 1 molecule on T-cell activation, we co-immobilized HLA-class II-peptide complexes with LFA-1 reactive MoAbs. Co-immobilization resulted in an enhanced proliferative response of the T-cell clones. This could indicate that the LFA 1 molecule on T cells is not a passive adhesion molecule, but is capable of transducing a signal that synergizes with the stimulatory signal via the T-cell receptor.     

Antigen-independent adhesion of CD45RA (naive) and CD45RO (memory) CD4 T cells to B cells.

We found that naive (CD45RA+) CD4 T cells have a lower capacity of adhesion to Epstein-Barr virus (EBV) immortalized B cells than memory (CD45RO+) CD4 T cells, as judged by conjugate formation. This would appear to be due to differences in the expression of adhesion molecules [lymphocyte function-associated antigen (LFA)-1, CD2]. However, kinetic studies showed that the degree of adhesion of naive T cells to B cells was stable over 60 min while that of memory T cells, like that of unseparated CD4 T cells, was characterized by a rapid formation and rapid dissociation of conjugates. This could be explained by a difference in the sensitivity of naive and memory CD4 T cells to down-regulation of antigen-independent adhesion by CD4-MHC class II interaction. Indeed, memory T cells also adhered stably to MHC class II(-) B cells. The adhesion of memory T cells, but not naive T cells, to MHC class II(+) B cells was sensitive to inhibition by OKT4a an anti-CD4 antibody, human immunodeficiency (HIV) gp160 (env) protein and a 12-mer peptide encompassing the 35-46 sequence of the HLA, DR beta 1 domain and previously shown to inhibit activation of HLA class II-restricted CD4 T cell responses. Since MHC class II expression did not influence the degree of conjugate formation by naive or memory CD4 T cells with B cells, CD4-MHC class II interaction does not appear to be involved in binding itself, but may down-regulate the adhesion of memory but not naive CD4 T cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of cell-cell communication in inhibiting butyric acid-induced T-cell apoptosis

We have previously demonstrated that human gingival fibroblasts rescue butyric acid-induced T-cell apoptosis via proinflammatory cytokines such as interleukin 6 (IL-6) and IL-11, which are produced by fibroblasts stimulated with butyric acid. In this study, we determined if T-cell adhesion to human gingival fibroblasts influenced the susceptibility of T cells to butyric acid-induced apoptosis. We have shown that the number of Jurkat T cells adherent to gingival fibroblasts (Gin-1 cells) was significantly increased by the addition of butyric acid. All Jurkat cells that adhered to Gin-1 cells remained viable, while the nonadherent Jurkat cells dropped into apoptosis. The increase in T-cell adhesion to fibroblasts was also observed when Jurkat cells, but not Gin-1 cells, were pretreated with butyric acid. The expression levels of CD44, very late antigen 2 (VLA-2) and VLA-5 but not of leukocyte function-associated antigen 1 (LFA-1) and VLA-4 on Jurkat cells were increased following treatment with butyric acid. Furthermore, pretreatment of butyric acid-sensitized Jurkat cells with monoclonal antibodies against CD44, VLA-2, and VLA-5, but not LFA-1 and VLA-4, followed by coculture with Gin-1 cells inhibited T-cell adhesion to fibroblasts and increased apoptosis of nonadherent T cells after coculture of gingival fibroblasts and Jurkat cells. These results indicate that T-cell adherence to fibroblasts is enhanced by butyric acid and that butyric acid-induced T-cell apoptosis is down-regulated by T-cell adhesion to gingival fibroblasts through an interaction with the adhesion molecules CD44, VLA-2, and VLA-5 expressed on T cells stimulated with butyric acid.     

Differential dependence of TH-0, TH-1 and TH-2 CD4 4+ T cells on co-stimulatory activity provided by the accessory molecule LFA-1

Background The adhesion molecule LFA-1 contributes to the activation response of peripheral blood human CD4+ T cells. Less is known of its contribution to stimulation of long-term CD4+ T cell lines and clones or of its potential to co-stimulate CD4+ T cells of different functional phenotype. Objective This study was therefore performed to investigate co-stimulatory properties of the LFA-1 (CD11a/CD 18) complex in the activation of human CD4+ T cell lines and clones of TH-0. TH-1 and TH-2 subsets. Methods Co-stimulatory activity was measured by cross-linking antibodies to CD 11a or CD18 with anti-CD3 antibodies to plastic and then measuring the proliferative response of CD4+ T cells to these antibodies. Results A house duct mite allergen-specific CD4+ T cell line (TH-2) demonstrated much greater dependence on both C'DI la and CD IK than a mycobacterial antigen-specific CD4+ T cell line (TH-1). Co-stimulatory activity through LFA-1 was also provided to a house dust mite-specific CD4+ T cell clone (DE-9; TH-2) but not to an influenza haemagglutinin-specific CD4+ T cell clone (HA 1.7: TH-0). In contrast, soluble antibodies to CD 18 inhibited proliferativc responses of both DE-9 and HA1.7 to an immunogenic challenge of antigen and to stimulation by unti-CD3 antibodies. However, the allergen-specific T cells were more susceptible to inhibition. Signal transduction was also observed from the T-cell receptor to LFA-1. Ligation of the T-cell receptor modulated the phenotypic expression of LFA-1 and ICAM-1 on both HA1-7 and DE-9). Phenotypic modulation was observed as a result of both activation and the induction of non-responsiveness. Conclusion These experiments indicate that CD4+ T cells of TH-2 functional phenotype may have a greater requirement for the co-stimulatory activity of LFA-1 than CD4+ T cells of TH-0 or TH-1 phenotypes.     

CD4 binding to major histocompatibility complex class II antigens induces LFA-1-dependent and -independent homotypic adhesion of B lymphocytes.

T helper cells recognize processed antigen (Ag) in the context of major histocompatibility complex (MHC) class II antigens present on the surface of B cells and other Ag-presenting cells. This interaction is mediated through the T cell receptor complex with associate recognition of class II molecules by the CD4 molecule. In this study, the binding of a soluble recombinant CD4/Ig heavy chain fusion protein (CD4-gamma 3) or monoclonal antibody (mAb) to class II antigens on human B cells was shown to induce rapid and specific homotypic adhesion of B cells and most B lymphoblastoid cell lines. mAb reactive with CD4 inhibited CD4-gamma 3-induced adhesion and a mutant B lymphoblastoid cell line deficient in class II antigens failed to respond. Induction of homotypic adhesion was dependent on energy metabolism and a functional cytoskeleton, and class II+ pre-B cells did not exhibit adhesion in response to these stimuli, suggesting that cross-linking of class II molecules generated a transmembrane signal and did not simply aggregate cells. In addition, MHC class II-induced adhesion was Fc receptor independent, as 15 mAb of different Ig isotypes reactive with HLA-D or HLA-DQ gene products induced adhesion. Anti-class II mAb and CD4-gamma 3 were able to induce adhesion at concentrations as low as 10 ng/ml and 100 ng/ml, respectively. Suboptimal stimulation of B cell lines through HLA-D antigens induced homotypic adhesion that was dependent on the activation of LFA-1 (CD11a/CD18), and which could be blocked by specific mAb. However, at greater signal strengths, adhesion was not blocked by mAb against the known adhesion receptors, suggesting the induction of a novel adhesion pathway. Consistent with this, homotypic adhesion induced by engagement of MHC class II antigens was observed with LFA-1-deficient B cell lines, and was independent of CD49d or CD18 expression. Thus, the direct engagement of B cell class II antigens by CD4 is likely to generate transmembrane signals which trigger both LFA-1-dependent and LFA-1-independent adhesion pathways.     

G1/S cell cycle arrest provoked in human T cells by antibody to CD26

CD26 is T cell costimulatory molecule with dipeptidyl peptidase IV (DPPIV) enzyme activity located in its extracellular region. The expression of CD26 is enhanced after activation of T cells, while it is preferentially expressed on a subset of CD4+ memory T cells in the resting state. In this paper, we demonstrate that binding of the soluble anti-CD26 monoclonal antibody (mAb) 1F7 inhibits human T-cell growth and proliferation in both CD26-transfected Jurkat T-cell lines and human T-cell clones by inducing G1/S arrest, which is associated with enhancement of p21Cip1 expression. This effect depends on the DPPIV enzyme activity of the CD26 molecule. Moreover, we show that expression of p21Cip1 after treatment with the anti-CD26 mAb 1F7 appears to be induced through activation of extracellular signal-regulated kinase (ERK) pathway. These data thus suggest that anti-CD26 treatment may have potential use in the clinical setting involving activated T cell dysregulation, including autoimmune disorders and graft-vs.-host disease.     

Human duodenal epithelial cells constitutively express molecular components of antigen presentation but not costimulatory molecules

Constitutive expression of major histocompatibility complex (MHC) class II molecules by duodenal epithelial cells (EC) suggests that they can present antigen to CD4(+) T cells. However, other molecular components including invariant chain (Ii), HLA-DM, and costimulatory molecules CD80, CD86 and CD40, are required for efficient T-cell activation. We have investigated whether normal human duodenal EC possess these molecules and whether they can mediate MHC class II antigen presentation. EC were isolated from duodenal biopsies from patients in whom pathology was excluded. Freshly-isolated duodenal EC did not stimulate autologous T-cell proliferation against purified protein derivative of tuberculin. Flow cytometry and immunoblot analysis revealed that duodenal EC constitutively express HLA-DR, Ii, and HLA-DM. Surface MHC class II associated invariant chain peptide (CLIP) was not detectable, suggesting that HLA-DM functions normally in CLIP removal. Duodenal EC expressed SDS-stable HLA-DR alphabeta heterodimers, indicating that peptide binding had occurred. Surface expression of CD80, CD86 or CD40 was not detected although mRNA for these costimulatory molecules was present in all samples. These results suggest that nondiseased human duodenal EC can process and present antigen by the MHC class II pathway, but that they may induce anergy, rather than activation, of local T cells.     

The negative regulatory function of the lymphocyte-activation gene-3 co-receptor (CD223) on human T cells

Accumulating evidence indicates that the CD4 homologue lymphocyte activation gene-3 (LAG-3) plays a down-regulatory role on T-cell responses. However, the role of LAG-3/major histocompatibility complex (MHC) class II interactions on primary human T-cell responses, as well as the mechanism by which down-regulation occurs, are not clear. Here, we show that LAG-3 colocalized with CD3, CD4 or CD8 in areas of cholesterol-rich raft aggregation during this primary response, as well as in the clustered raft region formed between T cells and antibody-coated beads. Addition of a blocking LAG-3-specific monoclonal antibody to both CD4 and CD8 primary resting T cells activated under conditions of antigen-presenting cell-driven stimulation and low antigen concentrations augments CD69 activation antigen expression, T-cell expansion and T helper 1 (Th1, but not Th2) cytokine production. Blocking LAG-3/MHC class II interactions leads to an increase in the number of cells entering division at these low concentrations of antigen and to more rounds of divisions with an accumulation of cells in the S-phase of the cell cycle. These results indicate that LAG-3 signalling inhibits early events in primary activation of human CD4 and CD8 T cells and further support a role for LAG-3 signalling in regulating the expansion of activated effector or memory T cells, either directly or indirectly through Treg suppressor activity.