Cytotoxic T-lymphocyte activation involves a cascade of signalling and adhesion events

Three-dimensional isotropic imaging of live suspension cells enabled by droplet microvortices

Fast, nondestructive three-dimensional (3D) imaging of live suspension cells remains challenging without substrate treatment or fixation, precluding scalable single-cell morphometry with minimal alterations. While optical sectioning techniques achieve 3D live cell imaging, lateral versus depth resolution differences further complicate analysis. We present a scalable microfluidic method capable of 3D fluorescent isotropic imaging of live, nonadherent cells suspended inside picoliter droplets with high-speed single-cell volumetric readout (800 to 1,200 slices in 5 to 8 s) and near-diffraction limit resolution (~216 nm). The platform features a droplet trap array that leverages flow-induced droplet interfacial shear to generate intradroplet microvortices, which rotate single cells on their axis to enable optical projection tomography (OPT)-based imaging. This allows gentle (~1 mPa shear stress) observation of cells encapsulated inside nontoxic isotonic buffer droplets, facilitating scalable OPT acquisition by simultaneous spinning of hundreds of cells. We demonstrate 3D imaging of live myeloid and lymphoid cells in suspension, including K562 cells, as well as naive and activated T cells-small cells prone to movement in their suspended phenotype. Our fully suspended, orientation-independent cell morphometry, driven by isotropic imaging and spherical harmonic analysis, enabled the study of primary T cells across various immunological activation states. This approach unveiled six distinct nuclear content distributions, contrasting with conventional 2D images that typically portray spheroid and bean-like nuclear shapes associated with lymphocytes. Our arrayed-droplet OPT technology is capable of isotropic, single live-cell 3D imaging, with the potential to perform large-scale morphometry of immune cell effector function states while providing compatibility with microfluidic droplet operations.

Protective efficacy of Modified Vaccinia virus Ankara in preclinical studies

Modified Vaccinia virus Ankara (MVA) is a tissue culture-derived, highly attenuated strain of vaccinia virus (VACV) exhibiting characteristic defective replication in cells from mammalian hosts. In the 1960s MVA was originally generated as a candidate virus for safer vaccination against smallpox. Now, MVA is widely used in experimental vaccine development targeting important infectious diseases and cancer. Versatile technologies for genetic engineering, large-scale production, and quality control facilitate R&D of recombinant and non-recombinant MVA vaccines matching today's requirements for new biomedical products. Such vaccines are attractive candidates for delivering antigens from pathogens against which no, or no effective vaccine is available, including emerging infections caused by highly pathogenic influenza viruses, chikungunya virus, West Nile virus or zoonotic orthopoxviruses. Other directions are seeking valuable vaccines against highly complex diseases such as AIDS, malaria, and tuberculosis. Here, we highlight examples of MVA candidate vaccines against infectious diseases, and review the efforts made to assess both the efficacy of vaccination and immune correlates of protection in preclinical studies.

Protein Kinase C-θ (PKC-θ) in Natural Killer Cell Function and Anti-Tumor Immunity

The protein kinase C-θ (PKCθ), which is essential for T cell function and survival, is also required for efficient anti-tumor immune surveillance. Natural killer (NK) cells, which express PKCθ, play a prominent role in this process, mainly by elimination of tumor cells with reduced or absent major histocompatibility complex class-I (MHC-I) expression. This justifies the increased interest of the use of activated NK cells in anti-tumor immunotherapy in the clinic. The in vivo development of MHC-I-deficient tumors is much favored in PKCθ^−/− mice compared with wild-type mice. Recent data offer some clues on the mechanism that could explain the important role of PKCθ in NK cell-mediated anti-tumor immune surveillance: some studies show that PKCθ is implicated in signal transduction and anti-tumoral activity of NK cells elicited by interleukin (IL)-12 or IL-15, while others show that it is implicated in NK cell functional activation mediated by certain killer-activating receptors. Alternatively, the possibility that PKCθ is involved in NK cell degranulation is discussed, since recent data indicate that it is implicated in microtubule-organizing center polarization to the immune synapse in CD4⁺ T cells. The implication of PKC isoforms in degranulation has been more extensively studied in cytotoxic T lymphocyte, and these studies will be also summarized.

Initiation of TCR phosphorylation and signal transduction

Recent data with CD8+ T cells show that the initial phase of T cell receptor (TCR) binding to MHC–peptide (MHCp) is quickly followed by a second, stronger, binding phase representing the binding of CD8 to the MHCp. This second phase requires signaling by a Src-family kinase such as Lck. These data point out two aspects of the initial stage of TCR signaling that have not yet been clearly resolved. Firstly, how and by which Src-family kinase, is the initial phosphorylation of CD3ζ accomplished, given that the Lck associated with the co-receptors (CD4 or CD8) is not yet available. Secondly, what is the mechanism by which the co-receptor is brought close to the bound TCR before the co-receptor binds to MHCp?

Cell-in-cell structures formed between human cancer cell lines and the cytotoxic regulatory T-cell line HOZOT

We previously established a novel cell line, termed HOZOT, derived from umbilical cord blood mononuclear cells that is characterized as a human cytotoxic regulatory T (Treg) cell line with a FOXP3(+)CD4(+)CD8(+)CD25(+) phenotype. Here, we describe a new property of HOZOT cells: they actively penetrate into a variety of human cancer cell lines, but not into normal cell lines, and form apparent cell-in-cell structures. In the process of cell penetration, we observed that HOZOT cells adhered to target cells seemed to first insert their nuclei into the cytoplasm of target cells, distinct from the process of phagocytosis. In addition, blocking experiments showed that major histocompatibility complex class I is one of the target cell recognition molecules for HOZOT cells. Furthermore, we propose that cell-in-cell structures between HOZOT cells and target cancer cells could be one of the cytotoxic mechanisms of HOZOT cells.

CD4 and CD8: an inside-out coreceptor model for innate immune cells

CD8 and CD4 are expressed by several cell types that do not express TCR. These include DCs, macrophages, monocytes, and NK cells. CD8(+) monocytes and macrophages are abundant at the site of pathology in many rat disease models, particularly those involving immune complex-mediated pathology. Indeed, in some disease models, CD8(+) macrophages correlate with severity of pathology or directly cause pathology or tumor cell killing. Evidence suggests CD8 or CD4 can enhance FcgammaR-dependent responses of human monocytes. Building on data that key components of TCR and FcgammaR signaling can substitute one another efficiently, we postulate that CD4 and CD8 operate with FcgammaR and potentially other receptors to enhance responses of T cells and various innate immune cells. Our model suggests CD8 on myeloid cells may contribute directly to tumor killing and tissue pathology by enhancing FcgammaR responses. Moreover, the model suggests a role for CD8 in cross-presentation of antibody-associated antigen by DCs and a new mechanism to regulate TCR sensitivity.

TCR complex-activated CD8 adhesion function by human T cells

The CD8 receptor plays a central role in the recognition and elimination of virally infected and malignant cells by cytolytic CD8(+) T cells. In conjunction with the TCR, the CD8 coreceptor binds Ag-specific class I MHC (MHC-I) molecules expressed by target cells, initiating signaling events that result in T cell activation. Whether CD8 can further function as an adhesion molecule for non-Ag MHC-I is currently unclear in humans. In this study, we show that in human CD8(+) T cells, TCR complex signaling activates CD8 adhesion molecule function, resulting in a CD8 interaction with MHC-I that is sufficient to maintain firm T cell adhesion under shear conditions. Secondly, we found that while CD8 adhesive function was triggered by TCR complex activation in differentiated cells, including in vitro generated CTL and ex vivo effector/memory phenotype CD8(+) T cells, naive CD8(+) T cells were incapable of activated CD8 adhesion. Lastly, we examine the kinetics of, and signaling for, activated CD8 adhesion in humans and identify notable differences from the equivalent CD8 function in mouse. Activated CD8 adhesion induced by TCR signaling may contribute to the more rapid and robust elimination of pathogen-infected cells by differentiated CD8(+) T cells.

Multiscale analysis of T cell activation: correlating in vitro and in vivo analysis of the immunological synapse

Recently implemented fluorescence imaging techniques, such as total internal reflection fluorescence microscopy and two-photon laser scanning microscopy, have made possible multiscale analysis of the immune response from single molecules in an interface to cells moving in lymphoid tissues and tumors. In this review, we consider components of T cell sensitivity: the immunological synapse, the coordination of migration, and antigen recognition in vivo. Potency, dose, and detection threshold for peptide-MHC determine T cell sensitivity. The immunological synapse incorporates T cell receptor microclusters that initiate and sustain signaling, and it also determines the positional stability of the T cells through symmetry and symmetry breaking. In vivo decisions by T cells on stopping or migration are based on antigen stop signals and environmental go signals that can sometimes prevent arrest of T cells altogether, and thus can change the outcome of antigen encounters.

The CD8 T cell coreceptor exhibits disproportionate biological activity at extremely low binding affinities

T lymphocytes recognize peptides presented in the context of major histocompatibility complex (MHC) molecules on the surface of antigen presenting cells. Recognition specificity is determined by the alphabeta T cell receptor (TCR). The T lymphocyte surface glycoproteins CD8 and CD4 enhance T cell antigen recognition by binding to MHC class I and class II molecules, respectively. Biophysical measurements have determined that equilibrium binding of the TCR with natural agonist peptide-MHC (pMHC) complexes occurs with KD values of 1-50 microm. The pMHCI/CD8 and pMHCII/CD4 interactions are significantly weaker than this (KD >100 microm), and the relative roles of TCR/pMHC and pMHC/coreceptor affinity in T cell activation remain controversial. Here, we engineer mutations in the MHCI heavy chain and beta2-microglobulin that further reduce or abolish the pMHCI/CD8 interaction to probe the significance of pMHC/coreceptor affinity in T cell activation. We demonstrate that the pMHCI/CD8 coreceptor interaction retains the vast majority of its biological activity at affinities that are reduced by over 15-fold (KD > 2 mm). In contrast to previous reports, we observe that the weak interaction between HLA A68 and CD8, which falls within this spectrum of reduced affinities, retains substantial functional activity. These findings are discussed in the context of current concepts of coreceptor dependence and the mechanism by which TCR coreceptors facilitate T cell activation.

Beta 1/beta 3 integrin ligation is uncoupled from ERK1/ERK2 activation in cytotoxic T lymphocytes

beta 3 integrins mediate fibronectin binding and enhanced activation of cytotoxic T lymphocytes (CTL). The intracellular signals initiated by beta 3 integrins in lymphocytes are not well characterized, but in many cell types, beta 1 integrin ligation activates mitogen-activated protein (MAP) kinases. In the present study, we find that fibronectin can synergize with very low levels of CD3 stimulation to activate the extracellular signal-regulated kinase (ERK)1 and ERK2 MAP kinases but that fibronectin alone induces no detectable MAP kinase activation in CTL. Surprisingly, antibodies to beta1 or beta 3 integrins were also unable to stimulate MAP kinase activation, suggesting that although beta 1 integrins are capable of stimulating MAP kinase activation in other cells, they cannot do so in CTL. In CTL, phosphorylation of proline-rich tyrosine kinase 2 downstream of integrin stimulation did not result in recruitment of the adaptor protein Grb2. Additionally, we examined the role of MAP kinases in regulating integrin-mediated adhesion. Anti-CD3-triggered adhesion to fibronectin was largely insensitive to the MAP kinase kinase inhibitor PD98059. Triggered cell-spreading on fibronectin was inhibited by PD98059 but not by U0126. In summary, ligation of beta 3 integrin by antibodies or fibronectin or of beta1 integrin by monoclonal antibodies fails to activate ERK MAP kinases, but integrin ligation synergizes with T cell receptor stimulation upstream of MAP kinases.

Identification of CD226 ligand on colo205 cell surface

AIM: To confirm the existence of CD226 ligand and its distribution, which is a novel molecule that was cloned in 1996.

METHODS: The mRNA was extracted from TPA activated Jurkat cells and used as a template for reverse-transcription. After PCR amplification, the fragment including CD226 extracellular region and the splice donor sequence “ACTTACCTGT” was obtained and cloned into fusion expression vector pIG. The recombinant vector pCD226/Ig was transfected in COS-7 cells by DEAE-Dextran method, the secreting fusion protein was identified by Sandwich ELISA, and was purified by anti-CD226 affinity chromatography. This fusion protein was used as a probe in the investigation of CD226 ligand by immunohistochemistry. Existence of CD226 ligand was further identified by adhesion experiment.

RESULTS: Expression of a secreting fusion protein was identified by sandwich ELISA, indicating that both CD226 extracellular domain and IgGFc domain could be recognized respectively by anti-CD226 and anti-hIgFc mAb. About 130 μg CD226/Ig fusion protein could be obtained from 100 mL COS-7 culture supernatants by anti-CD226 affinity chromatography purification. SDS-PAGE showed that this fusion protein has a molecular mass of 83 ku. It was confirmed by immunohistochemistry that CD226 ligand expressed on the Colo205 cells, but not on Jurkat cell, U937 cell and mixed lymphocyte culture cells. In adhesive assay, resting Jurkat cells did not have significant adhesion to Colo205 cells. In contrast, activated Jurkat cells could bind to colon carcinoma Colo205 cells and this adhesive reaction could be blocked by CD226/Ig fusion protein or anti-CD226 mAb. Immunochemical experiment showed that Colo205 cells could be specifically stained by CD226/Ig, indicating that CD226 ligand exists on the surface of Colo205 cells.

CONCLUSION: Existence of CD226 ligand on the surface of Colo205 cells was identified by immunohistochemistry and adhesion blocking experiment. In addition, the secreting CD226/Ig fusion protein prepared in this study will be a potential tool for further investigation of CD226 ligand.

Role of phosphoinositide 3-kinase in TCR-signaled regulation of CD8-mediated adhesion to class I MHC protein

CD8 must be activated by signaling through the TCR in order to mediate CTL adhesion. Up-regulation of adhesion to class I protein is shown to be blocked by specific inhibitors of phosphoinositide 3-OH kinase (PI3-K), indicating a critical role for this enzyme in signaling for activation of CD8. A minimal TCR stimulus that activates CD8 does not result in a detectable increase in total cellular PI3-K activity, but an increase in PI3-K activity associated with p59(fyn) kinase can be detected. Genistein blocks this increase concomitantly with blocking the activation of adhesion, suggesting that activation of fyn-associated PI3-K is downstream of TCR-dependent activation of protein tyrosine kinase(s) in the signaling pathway that leads to up-regulation of CD8-dependent adhesion. Treatment of cells with phorbol ester also blocks the TCR-dependent increase in fyn-associated PI3-K and inhibits CD8-dependent adhesion. This suggests a feedback model for deactivation of CD8 adhesion to allow target cell release by CTL and recycling to kill additional targets. In contrast, phorbol ester treatment up-regulates integrin-mediated adhesions, suggesting complex cross-talk between the TCR and the different adhesion/cosignaling receptors during the binding and killing of antigen-bearing targets.

The human CD8 coreceptor effects cytotoxic T cell activation and antigen sensitivity primarily by mediating complete phosphorylation of the T cell receptor zeta chain

Recognition of antigen by cytotoxic T lymphocytes (CTL) is determined by interaction of both the T cell receptor and its CD8 coreceptor with peptide-major histocompatibility complex (pMHC) class I molecules. We examine the relative roles of these receptors in the activation of human CTL using mutations in MHC class I designed to diminish or abrogate the CD8/pMHC interaction. We use surface plasmon resonance to determine that point mutation of the alpha3 loop of HLA A2 abrogates the CD8/pMHC interaction without affecting the affinity of the T cell receptor/pMHC interaction. Antigen-presenting cells expressing HLA A2 which does not bind to CD8 fail to activate CTL at any peptide concentration. Comparison of CTL activation by targets expressing HLA A2 with normal, abrogated, or diminished CD8/pMHC interaction show that the CD8/pMHC interaction enhances sensitivity to antigen. We determine that the biochemical basis for coreceptor dependence is the activation of the 23-kDa phosphoform of the CD3zeta chain. In addition, we produce mutant MHC class I multimers that specifically stain but do not activate CTL. These reagents may prove useful in circumventing undesirable activation-related perturbation of intracellular processes when pMHC multimers are used to phenotype antigen-specific CD8+ lymphocytes.

Epitope affinity for MHC class I determines helper requirement for CTL priming

We show here that priming and memory generation of antigen-specific CD8+ cytotoxic T lymphocytes (CTL) does not require help if the immunogen binds major histocompatibility complex (MHC) class I molecules with high affinity. This conclusion was based on the study of three chemically distinct optimal length CTL epitopes with high affinity for the restriction element Kb. In contrast, when two subdominant epitopes with intermediate MHC binding affinity were studied, either a class II MHC-restricted T helper cell epitope or administration of antibody to CD40 was required to obtain significant CTL priming. Depending on the epitope, one source of help was much more efficient than the other.

Critical role for CD8 in T cell receptor binding and activation by peptide/major histocompatibility complex multimers

Recent data using MHC/peptide tetramers and dimers suggests that the T cell coreceptors, CD4 and CD8, although important for T cell activation, do not play a direct role in facilitating T cell receptor (TCR) binding to multivalent MHC/peptide ligands. Instead, a current model proposes that coreceptors are recruited only after a stable TCR-MHC/peptide complex has already formed and signaled. In contrast, we show using multimeric class I MHC/peptide ligands that CD8 plays a critical (in some cases obligatory) role in antigen-specific TCR binding. T cell activation, measured by calcium mobilization, was induced by multimeric but not monomeric ligands and also showed CD8 dependency. Our analysis using anti-CD8 antibodies revealed that binding to different epitopes of CD8 can either block or augment TCR-MHC/peptide interaction. These effects on TCR binding to high-affinity agonist ligands were even more pronounced when binding to multimeric low-affinity ligands, including TCR antagonists, was studied. Our data have important implications for the role of CD8 in TCR binding to MHC/peptide ligands and in T cell activation. In addition, our results argue against the view that multimeric MHC/peptide ligands bind directly and solely to the TCR; rather, our data highlight a pivotal contribution of CD8 for this association.

Extracellular matrix-induced signaling in Entamoeba histolytica: its role in invasiveness

Entamoeba histolytica trophozoites can invade the mucosa of the host large intestine to cause disease. Here, Isaura Meza describes recent work indicating that the trophozoites respond to the binding of extracellular matrix proteins by activating receptor-mediated signal transduction pathways. Activation of phosphokinase C or adenylyl cyclase induces protein phosphorylation, actin gene expression and reorganization of the actin cytoskeleton to form adhesive structures that promote interaction with the substrate and the release of proteases. The release of proteases at the sites of contact, with the subsequent degradation of the substrate and generation of chemotactic peptides, facilitates locomotion and dissemination of the invading trophozoites.

Adhesion to fibronectin promotes the activation of the p125(FAK)/Zap-70complex in human T cells

The beta1 integrins are a family of heterodimeric adhesion receptors involved in cell-to-cell contacts and cell-to-extracellular matrix interactions. Through their adhesive role, integrins participate in transduction of outside/inside signals and contribute to trigger a multitude of cellular events such as differentiation, cell activation, and motility. The fibronectin integrin receptors, alpha4beta1 and alpha5beta1, can function as costimulatory molecules in T-cell receptor (TCR)-dependent T-cell activation. In the current study the Jurkat T-cell line was used as a model system to investigate the TCR-independent role of cell adhesion to fibronectin in the activation of Zap-70, a central molecule in the signalling events in T cells. Upon adhesion to plastic immobilized fibronectin but not to bovine serum albumin (BSA) the phosphorylation of p125FAK, a protein kinase that localizes to focal adhesion sites, was induced. Moreover, clustering of fibronectin receptors led to the detection of a p125FAK/Zap-70 complex. Finally, while the complex between fak-B, another protein kinase localized to focal adhesion sites, and Zap-70 was detected in cells plated either on BSA or on fibronectin, the formation of the p125FAK/Zap-70 complex appeared specifically induced following fibronectin-mediated integrin clustering. These data suggest the existence of a high degree of specificity when the members of the beta1 integrin family mediate signalling pathways in T cells.

Antagonism of cytotoxic T-lymphocyte activation by soluble CD8

The CD8 co-receptor is important in the differentiation and selection of class I MHC-restricted T cells during thymic development, and in the activation of mature T lymphocytes in response to antigen. Here we show that soluble CD8alphaalpha receptor, despite an extremely low affinity for MHC, inhibits activation of cytotoxic lymphocytes by obstructing CD3 zeta-chain phosphorylation. We propose a model for this effect that involves interference of productive receptor multimerization at the T-cell surface. These results provide new insights into the mechanism of T-cell activation and evidence that CD8 function is exquisitely sensitive to disruption, an effect that might be exploited by molecular therapeutics.

T cell receptor and coreceptor CD8 alphaalpha bind peptide-MHC independently and with distinct kinetics

The T cell surface glycoprotein CD8 enhances T cell antigen recognition by binding to MHC class I molecules. We show that human CD8 alphaalpha binds to the MHC class I molecule HLA-A2 with an extremely low affinity (Kd approximately 0.2 mM at 37 degrees C) and with kinetics that are between 2 and 3 orders of magnitude faster than reported for T cell receptor/peptide-MHC interactions. Furthermore, CD8 alphaalpha had no detectable effect on a T cell receptor (TCR) binding to the same peptide-MHC class I complex. These binding properties provide an explanation as to why the CD8/MHC class I interaction is unable to initiate cell-cell adhesion and how it can enhance TCR recognition without interfering with its specificity.

Integrin-mediated interactions influence the tissue specificity of CD8+ cytolytic T lymphocytes

We previously reported that CD8+ cytotoxic T lymphocytes (CTL) elicited in response to allogeneic renal epithelial cells (anti-REC CTL) preferentially lyse REC targets as compared to conventional lymphoid cell (LC) targets. It is often tacitly assumed that such cell type specificity results from CTL recognition of tissue-restricted MHC/peptide complexes. However, we herein report that anti-REC CTL uniquely express CD103, an integrin with known specificity for the epithelial cell-restricted ligand E-cadherin, and are deficient in expression of CD11a (LFA-1), an integrin known to play a critical accessory role in promoting lysis of LC targets. We demonstrate that CD8+ CTL clones with disparate CD103/CD11a phenotypes but identical specificities for allo-MHC/peptide can exhibit marked differences in cell type specificity. Antibody blocking studies provided direct evidence that CD103 serves as an accessory molecule that promotes lysis of REC targets. Taken together, these data indicate that integrin-mediated accessory interactions can influence the capacity of CD8+ CTL to discriminate between different cell types.

Alloreactive T Cells That Do Not Require TCR and CD8 Coengagement Are Present in Naive Mice and Contribute to Graft Rejection

Class I alloreactive CTL populations have been defined as either CD8 dependent or CD8 independent, based upon their ability to kill target cells in the presence of Ab to CD8. The CD8-dependent population uses CD8 in a coreceptor role with the TCR, and mutations in the class I molecule that destroy the CD8 binding site abrogate CTL killing, even if the target cell expresses other allelic forms of class I molecules with an intact binding site for CD8. The CD8-independent population apparently does not require CD8, as Ab to CD8 has no effect on the ability of these cells to kill appropriate target cells. We have isolated a third population of CTL that is inhibited by the addition of CD8 Ab yet can kill target cells that express the alloantigenic molecule incapable of binding CD8, provided that the target cells also express non antigenic class I molecules that contain an intact binding site for CD8. We refer to these cells as CD8 bystander-dependent CTL. Many (10 of 12) of these CTL were able to kill H-2Kb-expressing transfectants of T2 cells, consistent with the idea that they recognize a peptide-independent determinant that may be expressed at a high density on the cell surface. These CD8 bystander-dependent CTL are only readily detectable in vitro when spleen cells from mice primed in vivo with a skin graft are used.

The TCR-Binding Region of the HLA Class I α2 Domain Signals Rapid Fas-Independent Cell Death: A Direct Pathway for T Cell-Mediated Killing of Target Cells?

TCR binding to an MHC class I/peptide complex is a central event in CTL-mediated elimination of target cells. In this study, we demonstrate that specific activation of the TCR-binding region of the HLA-A2 class I α2 domain induces apoptotic cell death. mAbs to this region rapidly induced apoptosis of HLA-A2-expressing Jurkat E11 cells, as determined by morphologic changes, phosphatidylserine exposure on the cell surface, and propidium iodide uptake. In contrast, apoptosis was not induced following culture with mAbs directed to other regions of the class I molecule. Death signaling by class I molecules is apparently dependent on coreceptor activation, as apoptosis is also signaled by HLA-A2 molecules, where the intracytoplasmic residues were deleted. HLA class I α2-mediated cell death appeared to proceed independent of the Fas pathway. Compared with apoptotic signaling by Fas ligation, HLA class I α2-mediated responses displayed a faster time course and could be observed within 30 min. Furthermore, class I α2-induced cell death did not involve observable DNA fragmentation. The apoptotic response was not affected significantly by peptide inhibitors of IL-1β converting enzyme (ICE)-like proteases and CPP32. Taken together, activation of the TCR-binding domain of the class I α2 helix may result in apoptotic signaling apparently dependent on a novel death pathway. Thus, target HLA class I molecules may directly signal apoptotic cell death following proper ligation by the TCR.

Intercellular and Intracellular Events Following the MHC-Unrestricted TCR Recognition of a Tumor-Specific Peptide Epitope on the Epithelial Antigen MUC1

We examined the functional and molecular parameters involved in direct TCR recognition of a tumor-specific peptide epitope on the tumor Ag MUC1. This peptide epitope is tandemly repeated and recognized on the native molecule rather than processed and bound to the MHC. Even though the TCR was not MHC restricted, intercellular interactions found to facilitate this recognition included intercellular adhesion molecule-1/LFA-1, LFA-3/CD2, and class I/CD8. Intracellular parameters of MHC-unrestricted CTL activation were examined to compare the recognition of the MUC1 epitope presented on synthetic microspheres, with the recognition of the native epitope in the context of other molecules on the target cells. The epitope on microspheres induced a transient influx of Ca2+ that was not accompanied by detectable tyrosine phosphorylation of the ζ-associated protein ZAP-70, whereas recognition of MUC1 epitopes on tumor cells caused a sustained Ca2+ influx and ZAP-70 phosphorylation. The transient influx of Ca2+ was not sufficient to cause translocation of the nuclear factor of activated T cells (NF-AT) into the nucleus or CTL proliferation. In contrast, recognition of the MUC1 epitope on tumor cells resulted in full activation of the CTL, nuclear translocation of NF-AT, and proliferation. MHC-unrestricted TCR triggering, therefore, involves similar intercellular and intracellular events that participate in the conventional, MHC-restricted Ag recognition. Direct recognition of the MUC1 peptide epitope by the TCR in the absence of presentation by the MHC induces a partial signal that is completed by further interactions of other receptor/ligand pairs on the surface of the CTL and their target cells.

Dynamics of T-cell antagonism: enhanced viral diversity and survival

In rapidly evolving viruses the detection of virally infected cells can possibly be subverted by the production of altered peptides. There are peptides with single amino acid changes that can dramatically change T-cell responses, e.g. a loss of cytotoxic activity. They are still recognized by the T cell, but the signals required for effector function are only partially delivered. Thus, altered peptide presenting cells can act as decoy targets for specific immune responses. The existence of altered peptides in vivo has been demonstrated in hepatitis B and HIV. Using a mathematical model we address the question of how these altered peptides can affect the virus-immune system dynamics, and demonstrate that virus survival is enhanced. If the mutation rate of the virus is sufficient, one observes complex dynamics in which the antagonism acts so as to maintain the viral diversity, possibly leading to the development of a mutually antagonistic network or a continual turnover of escape mutants. In either case the pathogen is able to outrun the immune system. Indeed, sometimes the enhancement is so great that a virus that would normally be cleared by the immune system is able to outrun it.

Enrichment of antigen-specific T lymphocytes by panning on immobilized MHC-peptide complexes

Numerous studies have focused on characterizing and monitoring antigen-specific T cells during the course of an immune response. Mostly indirect methods were used to circumvent the low frequency of T cell precursors and the inherent complexity of T cell receptor (TcR)-MHC-peptide interactions. Here, we took advantage of peptide-specific adhesion induced by immobilized MHC-peptide complexes. We describe a simple technique which allows enrichment in antigen-specific T lymphocytes among a heterogeneous CD8+ T cell population. Enrichment of T cells according to their specificity should facilitate their characterization and provide an attractive tool for immunotherapy.

Cytotoxic T-lymphocyte interaction with fibronectin and vitronectin: activated adhesion and cosignalling

Stimulation of cloned cytotoxic T lymphocytes (CTL) with anti-T-cell receptor (TCR) monoclonal antibody (mAb) in solution resulted in rapid and sustained activation of adhesion to immobilized fibronectin (FN) but did not initiate degranulation. Addition of a second antibody (Ab) to further cross-link the TCR substantially increased the level of adhesion and also activated degranulation, as measured by release of serine esterase, in the presence of immobilized FN but not in its absence. Thus, binding to FN can provide a costimulatory signal to activate degranulation. TCR cross-linking also activated CD8-dependent adhesion to class I, and CD8 provided a costimulatory signal upon binding to class I. However, the requirements for activating adhesion and generating the costimulatory signal differed significantly for FN versus class I ligand, suggesting that these two receptor-ligand systems do not share a common mechanism of action. Co-immobilizing FN and alloantigen resulted in increased serine esterase release in comparison with that stimulated by antigen alone, and required the FN and class I be on the same surface. Peptide and antibody blocking demonstrated that CTL binding to FN, and to vitronectin (VN), was mediated by the alpha V beta 3 vitronectin receptor (VNR). Thus, VNR is activated by a signal from the TCR to mediate adhesion to FN or VN, and delivers a costimulatory signal for degranulation via a different mechanism than costimulation by CD8 binding to class I.

Inhibition of protein tyrosine kinases or protein kinase C prevents nonspecific killer T lymphocyte-mediated tumoricidal activity

The signal transduction events which govern major histocompatibility complex-unrestricted tumour cell destruction by nonspecific killer T lymphocytes induced with anti-CD3 antibody have not yet been determined. In this study we used pharmacologic inhibitors to investigate the role of protein tyrosine kinases (PTK) and protein kinase C (PKC) in this process. The PTK-inhibitors herbimycin A, genistein, and methyl 2,5-dihydroxycinnamate blocked anti-CD3-activated killer T (AK-T) lymphocyte-mediated killing of tumour target cells. The PKC-inhibitors staurosporine, calphostin C, and myristoylated PKC pseudosubstrate peptide, as well as PKC desensitization by phorbol 12-myristate 13-acetate pretreatment, also suppressed the cytolytic effector function of AK-T lymphocytes. Lack of tumoricidal activity was not due to reduced AK-T lymphocyte binding to tumour target cells but was associated with the abrogation of granule exocytosis, indicating that PTK and PKC are involved in the postbinding process which results in delivery of the 'lethal hit' by AK-T lymphocytes.

Involvement of p130(Cas) and p105(HEF1), a novel Cas-like docking protein, in a cytoskeleton-dependent signaling pathway initiated by ligation of integrin or antigen receptor on human B cells

The Crk-associated substrate p130(Cas) (Cas) and the recently described human enhancer of filamentation 1 (HEF1) are two proteins with similar structure (64% amino acid homology), which are thought to act as "docking" molecules in intracellular signaling cascades. Both proteins contain an N-terminal Src homology (SH), three domain and a cluster of SH2 binding motifs. Here we show that ligation of either beta1 integrin or B cell antigen receptor (BCR) on human tonsillar B cells and B cell lines promoted tyrosine phosphorylation of HEF1. In contrast, Cas tyrosine phosphorylation was observed in certain B cell lines but not in tonsillar B cells, indicating a more general role for HEF1 in B cell signaling. Interestingly, pretreatment of tonsillar B cells with cytochalasin B dramatically reduced both integrin- and BCR-induced HEF1 phosphorylation, suggesting that some component of the BCR-mediated signaling pathway is closely linked with a cytoskeletal reorganization. Both HEF1 and Cas were found to complex with the related adhesion focal tyrosine kinase (RAFTK), and when tyrosine phosphorylated, with the adapter molecule CrkL. In addition, the two molecules were detected in p53/56(Lyn) immunoprecipitates, and Lyn kinase was found to specifically bind the C-terminal proline-rich sequence of Cas in an in vitro binding assay. These associations implicate HEF1 and Cas as important components in a cytoskeleton-linked signaling pathway initiated by ligation of beta1 integrin or BCR on human B cells.

Cytotoxic T lymphocyte (CTL) adherence assay (CAA): a non-radioactive assay for murine CTL recognition of peptide-MHC class I complexes

Cytotoxic T lymphocytes (CTL) form an important immune surveillance system against intracellular pathogens. Here we describe a simple, visual assay for identifying peptides specifically recognized by CTL, based on the discovery that CTL develop increased adhesive properties upon TCR triggering. Several CTL lines were shown to pellet to the bottom of a round bottom 96-well plate in the absence of peptide. In contrast, these same CTL lines incubated with their cognate peptide, allowing them to present peptide to each other, adhered to the sides of the well and were readily distinguished by macroscopic visual examination of the plate after 4-5 h or overnight incubation. This CTL adherence assay (CAA) demonstrated peptide specificity and MHC restriction, and was titratable with peptide concentration. With this technique, a minimal-sized, malaria CTL epitope was correctly identified from a panel of overlapping nonamers, although the adherence pattern of two mono-substituted, variant peptides was less predictive of lytic activity. Also, substitutions in an HIV-1 envelope CTL epitope that reduced lytic activity were correctly predicted. Inhibitors of RNA and protein synthesis, upon preincubation, abrogated the adherence, indicating, at minimum, a need for live cells. Wortmannin, a PI-3 kinase inhibitor, inhibited the peptide specific adherence, consistent with a role for TCR or integrin signal transduction in CAA. Other cytoskeletal and metabolic inhibitors had no effect. Adherence of the T cells may involve low affinity, nonspecific interactions since wells coated with FCS, BSA or milk powder all produced an effective CAA in the presence of peptide under serum free conditions. Consequently, CAA may represent a rapid, simple method for screening large numbers of peptides to find cytolytic epitopes for a given CTL line and may identify additional epitopes causing T cell activation and adherence but not cytolytic activity.

Cytotoxic T lymphocytes express a beta3 integrin which can induce the phosphorylation of focal adhesion kinase and the related PYK-2

Fibronectin has been shown to stimulate tyrosine phosphorylation of a number of proteins in the 115-125 kDa range and facilitate degranulation by alloantigen-specific cytotoxic T lymphocyte (CTL) clones in response to substimulatory amounts of anti-CD3 or anti-T cell receptor (TCR). The current study was initiated to further characterize integrin expression and usage by these CTL clones. We demonstrate that vitronectin and fibrinogen, but not laminin or collagen, are also able to both facilitate degranulation in the presence of substimulatory anti-CD3 and stimulate tyrosine phosphorylation of these 115-125-kDa proteins, with a 115-kDa protein being the most prominently phosphorylated. These results implicate the expression and usage of the vitronectin receptor, alpha beta3 integrin, by these CTL clones. We demonstrate by both flow cytometry and immunoprecipitation that CTL clones do in fact express beta3 integrin. Immobilized antibody to beta3 stimulates the phosphorylation of the 115-125-kDa proteins, suggesting that engagement of beta3 transmits the same signal into these cells as fibronectin or vitronectin. The fibronectin and vitronectin-induced phosphorylation as well as adhesion to either fibronectin or vitronectin can be significantly inhibited with antibodies to beta3 integrins. Finally, we are able to immunoprecipitate 115-kDa proteins with antiserum to focal adhesion kinase and a related kinase, called PYK-2, that becomes phosphorylated in response to vitronectin or immobilized anti-beta3. Taken together, these results demonstrate that CTL express and use beta3-integrins as signaling molecules which can augment TCR-mediated stimulation.

T cell receptor and CD8-dependent tyrosine phosphorylation events in cytotoxic T lymphocytes: activation of p56lck by CD8 binding to class I protein

Tyrosine phosphorylation of proteins plays a central role in T cell activation. Mitogens or anti-receptor antibodies have been employed to study these signaling events, but the extent to which these mimic receptor interactions with native ligands is unclear. Cytotoxic T lymphocytes can be activated for functional responses using purified, native class I ligands presented on a surface. Previous work showed that stimulation with fluid-phase anti-T cell receptor (TCR) monoclonal antibody (mAb) activates CD8 to mediate adhesion to class I proteins and that activated CD8 generates a co-stimulatory signal upon binding to class I. Changes in tyrosine phosphorylation of substrates and activity of the p56lck kinase have now been examined in this two-step process. The observed changes are small in comparison to those found using more potent nonphysiological stimuli, but may more accurately reflect the events required for activation of functional responses. Fluid-phase anti-TCR mAb caused increased tyrosine phosphorylation of a discrete subset of cellular substrates. Increased phosphorylation of additional substrates occurred upon CD8 binding to class I, resulting in a phosphorylation pattern comparable to that found in cells stimulated with class I alloantigen. Anti-TCR mAb alone caused increased tyrosine phosphorylation of p56lck. When CD8 bound to class I, phosphorylation of p56lck decreased to below the basal level found in unstimulated cells, accompanied by a substantial increase in kinase activity. These results are consistent with the two-step model for TCR activation of CD8/class I interactions and directly demonstrate that CD8 binding to class I leads to up-regulation of p56lck activity.

The p56lck SH2 domain mediates recruitment of CD8/p56lck to the activated T cell receptor/CD3/zeta complex

The CD4 or CD8 co-receptors and the T cell receptor (TCR) are though to interact with the same antigen-presenting major histocompatibility complex molecule in a stable ternary complex. Therefore, the TCR and its co-receptor need to come into close proximity on the surface of the T cell. We have previously shown that the interaction of the p56lck SH2 domain with zeta-associated, tyrosine phosphorylated ZAP-70 and Syk kinases leads to an enhanced association of CD4 with TCR/CD3/zeta complex after CD3 stimulation of Jurkat cells. In this report, we analyzed whether a similar mechanism can mediate recruitment of the CD8 alpha alpha and CD8 alpha beta isoforms to the TCR. We demonstrate in vivo in association of CD8 alpha alpha/p56lck with the tyrosine kinase ZAP-70 after CD3 stimulation of Jurkat cells. A phosphopeptide competing in vitro for the binding of tyrosine phosphorylated proteins to the SH2 domain of p56lck specifically impedes the association of ZAP-70 with CD8 alpha alpha/p56lck without affecting the zeta/ZAP-70 interaction. The same peptide is able to compete for the activation-dependent association of the CD8 alpha alpha or CD8 alpha beta isoform with the TCR/CD3/zeta complex. Moreover, co-precipitation of the TCR with both CD8 isoforms was observed after CD3 stimulation. These findings strongly suggest that the p56lck SH2 domain mediates recruitment of CD8/p56lck to the activated TCR/CD3/zeta complex.

Dominance of tissue-restricted cytotoxic T lymphocytes in the response to allogeneic renal epithelial cell lines

Our current knowledge of cytotoxic T lymphocytes (CTL) is largely derived from studies of effector populations generated in allogeneic mixed leukocyte cultures (MLC) and assayed for lytic activity to lymphoid cell (LC) targets. We herein report that the CTL response to allogeneic renal epithelial cell lines (REC) is dominated by effectors that efficiently lyse REC targets but show little cross-reactivity with LC targets. In contrast, CTL generated against allogeneic spleen cell stimulators (ie., in MLC) lysed REC and LC targets at comparable levels. Lytic activity in both types of cultures was mediated by CD8+TCRalpha/beta+ cells directed to classical H2 class I alloantigens. Anti-REC effectors cross-reacted with fibroblast and macrophage targets but not with targets commonly used to detect alloreactive CTL, such as lipopolysaccharide- or Con A-stimulated lymphoblasts or lymphoid tumor lines, whereas MLC-elicited effectors efficiently lysed all targets. CTL clones propagated from anti-REC cultures exhibited the same allospecificity and tissue specificity as bulk anti-REC effectors. Individual CTL clones were highly heterogeneous in their capacity to recognize the same class I alloantigen expressed on cells derived from different tissues. These data demonstrate that the cellular environment in which CD8 precursors encounter class I alloantigens can have a profound effect on the cell-type specificity of CTL populations. An important implication of these data is that conventional assays of CTL lytic activity may fail to reveal a significant component of the host response to allogeneic tissues.

DNAM-1, a novel adhesion molecule involved in the cytolytic function of T lymphocytes

Intercellular adhesion molecules play an important role in the generation of T lymphocyte-mediated immune responses. Here, we describe a novel accessory molecule, DNAX accessory molecule-1 (DNAM-1), that is constitutively expressed on the majority of peripheral blood T lymphocytes. DNAM-1 is a 65 kDa transmembrane glycoprotein consisting of 318 aa including two immunoglobulin-like domains. Anti-DNAM-1 monoclonal antibody (MAb) inhibits T and NK cell-mediated cytotoxicity against a variety of tumor cell targets and blocks cytokine production by alloantigen-specific T cells. In addition, DNAM-1 is a tyrosine-phosphorylated signal-transducing molecule that participates in primary adhesion during cytotoxic T lymphocyte (CTL)-mediated cytotoxicity.

The immune response: the efferent arm

Once naive T cells encounter antigen, they become primed effector cells. The scope of effector functions mediated by these cells defines the efferent arm of the immune response. The change from naive to primed effector cell is known as adaptive immunity and takes 2 forms: cell mediated, in which T cells mediate effector function, and humoral, in which antibodies are the effector molecules. There are 3 types of effector T cells: inflammatory CD4 T cells, which activate macrophages; helper CD4 T cells, which help B lymphocytes produce antibody; and cytotoxic CD8 T cells, which kill their target cells. The interaction of primed effector cells with their targets results in phenotypic changes in the cells and the secretion of cytokines. These cytokines may be secreted by the primed effector T cell, the target cell, or both. Cytokines function in either autocrine (secreted and used by the same cell) or paracrine (secreted by 1 cell and used by a different cell) circuits and have marked regulatory effects on cells in both the immune and skeletal systems. Many of these cytokines, which were once thought to be products exclusively of immune cells, are now known to be produced by cells of the skeletal system. Both the specific and nonspecific components of the immune response have profound effects on remodeling of the musculoskeletal system during normal and pathologic states.

Hepatic expression of macrophage inflammatory protein-1 alpha and macrophage inflammatory protein-1 beta after liver transplantation

Two local events that are crucial for T cell emigration into tissue are (1) activation of T cell integrins to permit binding to endothelial counter-receptors and (2) directed migration through the endothelium and into tissue in response to chemotactic factors. Because the chemokines macrophage inflammatory protein-1 alpha (MIP-1 alpha) and MIP-1 beta can activate adhesion and induce migration of T cells in vitro, we investigated their expression in human liver allografts to determine whether they might be involved in regulating the recruitment of T cells to allografts in vivo. Both chemokines were expressed strongly by infiltrating leukocytes during rejection and could be detected immunohistochemically on biliary epithelium, an important target for T cell mediated graft damage. Both chemokines, but particularly MIP-1 beta, were detected on the vascular and sinusoidal endothelium of rejecting liver allografts, where they were coexpressed with the T cell beta 1-integrin receptor vascular cell adhesion molecule-1. In situ hybridization with complementary ribonucleic acid probes showed no MIP-1 alpha or MIP-1 beta mRNA in normal liver but dramatic expression of both chemokines in infiltrating leukocytes and graft endothelium during rejection. Expression was reduced after successful corticosteroid treatment of rejection but persisted in patients progressing to chronic rejection. Increased MIP-1 alpha and MIP-1 beta mRNA expression was already found in biopsies taken at the end of the transplant operation, suggesting that early induction of chemokines, possibly in response to graft reperfusion, might promote the subsequent development of graft rejection. These data demonstrate for the first time that MIP-1 alpha and MIP-1 beta are (1) expressed in human liver allografts, (2) produced by endothelial cells in vivo, and (3) induced early after transplantation. They suggest that MIP-1 alpha and MIP-1 beta produced by graft infiltrating leukocytes and graft endothelium might play a crucial role in regulating T cell recruitment to liver allografts in vivo.

Molecular interactions in the activation of effector and precursor cytotoxic T lymphocytes

Cell-cell interactions are influenced by parameters that cannot readily be studied using either intact cells or soluble molecules. Replacing one of the pair of interacting cells with an artificial cell surface construct allows novel insights to be gained into some of these parameters. Application of this approach to the study of CTL has helped to clarify the contrasting roles of some of the various receptors that are involved in recognition, adhesion and activation. In addition, it has revealed features of these receptor ligand interactions that help to explain how CTL are able to carry out effective immune surveillance and elimination of virus-infected or tumor cells. Although not discussed in this review, artificial cell surface constructs have also been effectively employed to study the interaction of TH cells with class II bearing surfaces. Class I protein and peptide antigen can be sufficient to mediate adhesion and activate CTL effector function through the TCR and CD8. In addition, interactions of other co-receptors with their ligands can act along with TCR and CD8 in a cascade of activated adhesion and co-stimulatory signal generation to allow adhesion and response when antigen and/or class I surface densities are too low to be sufficient by themselves to initiate response. The relative contributions of the various receptor/ligand interactions to a given CTL/target encounter will depend upon the affinity of the TCR for antigen and on the densities and types of ligands, including antigen, displayed on the target cell surface. It appears that the CTL has the ability to accomplish its task in a variety of ways, providing it with considerable flexibility in recognizing and eliminating antigen-bearing target cells. Thus, downregulation of any one particular ligand on a virus-infected or tumor cell does not allow escape from CTL surveillance provided that at least a low level of class I antigen remains present. The CTL is able to employ several co-receptors specific for ligands common to many cell types without being diverted from effective immune surveillance, since these receptors only become activated to mediate high-avidity adhesion when antigen is detected by the TCR. Cloned effector CTL are most amenable to studies of the kind reviewed here, since large numbers of homogenous cells can be obtained, antigen-specific adhesion can be readily measured and response is rapid and easily quantitated.(ABSTRACT TRUNCATED AT 400 WORDS)

Tyrosine kinases are involved with the expression of inducible nitric oxide synthase in human articular chondrocytes

The present study characterizes mechanisms involved with the induction of nitric oxide (NO) production, nitric oxide synthase (NOS) enzymatic activity and mRNA expression in human articular chondrocytes. Activation of chondrocytes with lipopolysaccharide (LPS) or IL-1 resulted in time- and dose-dependent increases in iNOS mRNA followed by increased NOS enzymatic activity and NO release. The protein tyrosine kinase (PTK) inhibitors herbimycin A or genistein reduced IL-1 or LPS-induced NO release and NOS enzymatic activity. This was associated with inhibition of iNOS mRNA expression as determined by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization. In contrast, inhibitors of protein kinase C (PKC) or protein kinase A (PKA) did not affect these responses. These results were confirmed in experiments with second messenger agonists where neither activation of PKC, nor increases in cyclic adenosine monophosphate (cAMP) or increased intracellular calcium levels were associated with the induction of iNOS mRNA or NO release. These results suggest that PKC, PKA and calcium-dependent signals are not required or sufficient for the stimulation of NO production. However, NO production is dependent on tyrosine kinases due to their role in the expression of iNOS mRNA.

Fibronectin induces phosphorylation of a 120-kDa protein and synergizes with the T cell receptor to activate cytotoxic T cell clones

Fibronectin (FN) has been shown to act as a costimulator in both CD4+ and CD8+ T cell activation through the T cell receptor (TcR). Consistent with previous studies, we found that FN is able to both enhance the maximal amount of TcR-triggered degranulation and lower the threshold for activation. The density of immobilized anti-CD3 or anti-TcR required to induce degranulation and tyrosine phosphorylation of cellular proteins by several cytotoxic T lymphocyte clones is quantitatively about tenfold lower in the presence of FN. We further demonstrate that FN alone stimulates transient tyrosine phosphorylation of a 120-kDa protein (pp120) in CD8+ T cells and when FN is coimmobilized with substimulatory amounts of anti-CD3 or anti-TcR there is a synergistic response, resulting in prolonged and enhanced phosphorylation of pp120. To determine if FN acts as a costimulator in CD8+ cells solely through mediating adhesion events or if it also transduces signals in T cells we conducted remote stimulation experiments. Degranulation was induced when FN and sub-stimulatory anti-CD3 were presented on separate surfaces, indicating that FN induces independent transmembrane signals capable of augmenting TcR-induced signals resulting in a functional response. Both FN plus TcR-induced tyrosine phosphorylation of pp120 and degranulation are inhibited by RGD-containing peptides, implying that an RGD-dependent FN receptor is mediating phosphorylation of pp120 and enhancing TcR-mediated degranulation.

Alloantigenic recognition of artificial glycosyl phosphatidylinositol-anchored HLA-A2.1

Alloantigen presentation by GPI-reanchored variants of the human class I MHC molecule HLA-A2.1 was studied in human cellular systems. To this end, we generated chimeric coding sequences for two GPI-modified HLA-A2.1 heavy chain derivatives. In these chimeras, the coding sequence for the HLA-A2.1 heavy chain was fused in-frame to alternative overlapping sequences from the 3'-end of human DAF containing the GPI-modification signal sequence. The encoded polypeptides HLA-A2.1:DAF-S and HLA-A2.1:DAF-L differed by 53 amino acids of additional DAF sequence in the latter. Both were detected on stably transfected C1R cell surfaces by HLA-A2.1-specific mAb, and their GPI-modification was confirmed by PI-PLC enzymatic cleavage. Immunoprecipitation analysis of surface-biotinylated C1R transfectants revealed heterodimeric association for both HLA-A2.1:DAF-L and HLA-A2.1:DAF-S heavy chains with beta 2m. Alloantigenic stimulation by, and cytotoxic recognition of, both HLA-A2.1:DAF-S/C1R and HLA-A2.1/CIR cells was observed; however, HLA-A2.1:DAF-L/C1R cells could not serve as allostimulators or allotargets. These findings establish that polymorphic human class I MHC molecules can function, when artificially GPI-reanchored, as alloantigenic targets. Moreover, the data suggest that the sequence bridging the HLA-A2 extracellular domain and the membrane can influence alloantigenic presentation.

Lysis of allogeneic and syngeneic target cells by primary cytotoxic T lymphocytes induced in vitro by Drosophila melanogaster cells

Incubation of unprimed splenocytes directly with synthetic peptides induces CD8+ primary CTL in vitro. The ability of peptide-pretreated LPS blasts or H-2Db-expressing Drosophila melanogaster cells to induce peptide-reactive primary CTL was analysed using synthetic peptides corresponding to distinct and overlapping amino acids in SV40 T antigen Db-restricted T-cell epitopes I, II-III, and V. Peptide-pretreated, but not untreated, LPS blasts induced strong peptide-reactive CTL in splenocytes of naive C57 BL/6 mice. The reactivity of these CTL was indistinguishable from that of CTL induced by direct stimulation of spleen cells with peptides. In contrast, effector cells induced by untreated or peptide-pretreated drosophila cells expressed equivalent cytolysis both against untreated and peptide-pretreated syngeneic target cells. They also strongly lysed the allogeneic (H-2d) P815 target cells. The cytolytic activity was expressed by CD8+ T lymphocytes. However, the lack of specificity and restriction was probably due to stimulation of LAK cells as well. The CTL-inducing potential of the drosophila cells was stable at 46 degrees C. Peptide-activated splenocytes cultured for 1 month in medium supplemented with spent medium from drosophila growth culture were 75% B cells and 25% CD4+ T cells. Taken together, these results indicated that LPS blasts are efficient APC for induction of primary CTL and that drosophila cell-derived molecule(s) are capable of inducing broad-reactive effector cells and may also assume the function of leukocyte growth factor(s).

Evidence for differential intracellular signaling via CD4 and CD8 molecules

Although both the CD4 and CD8 molecules enhance antigen responsiveness mediated by the T cell receptor (TCR), it is not known whether CD4 and CD8 initiate similar or different intracellular signals when they act as coreceptors. To characterize the early signals transmitted by CD4 and CD8, both CD4 and CD8 alpha were expressed in the same murine T cell hybridoma. In the double positive transfectants, CD4 and CD8 associated with equal amounts of p56lck (Lck), and both molecules enhanced interleukin 2 (IL-2) production equivalently when cross-linked with suboptimal levels of anti-TCR antibody. However, in an in vitro kinase assay, cross-linking CD4 initiated fourfold greater kinase activity compared with CD8 cross-linking. In the same assay, when CD4 or CD8 was cross-linked to the TCR, novel phosphorylated proteins were found associated with the TCR/CD4 complex but not with the TCR/CD8 complex. Consistent with this data, antiphosphotyrosine immunoblotting revealed greater tyrosine phosphorylation of intracellular substrates after TCR/CD4 cross-linking compared with TCR/CD8 cross-linking. Additionally, a specific protein kinase C inhibitor (RO318220) inhibited CD8-mediated enhancement of IL-2 production far more effectively than CD4-mediated enhancement. Thus, it appears that CD8 alpha may depend more on a protein kinase C-mediated signaling pathway, whereas CD4 may rely on greater tyrosine kinase activation. Such differential signaling via CD4 and CD8 has implications for thymic ontogeny and T cell activation.