Role of CD8 in aberrant function of cytotoxic T lymphocytes

Challenges and future perspectives of T cell immunotherapy in cancer

Since the formulation of the tumour immunosurveillance theory, considerable focus has been on enhancing the effectiveness of host antitumour immunity, particularly with respect to T cells. A cancer evades or alters the host immune response by various ways to ensure its development and survival. These include modifications of the immune cell metabolism and T cell signalling. An inhibitory cytokine milieu in the tumour microenvironment also leads to immune suppression and tumour progression within a host. This review traces the development in the field and attempts to summarize the hurdles that the approach of adoptive T cell immunotherapy against cancer faces, and discusses the conditions that must be improved to allow effective eradication of cancer.

Analysis, Isolation, and Activation of Antigen-Specific CD4(+) and CD8(+) T Cells by Soluble MHC-Peptide Complexes

T cells constitute the core of adaptive cellular immunity and protect higher organisms against pathogen infections and cancer. Monitoring of disease progression as well as prophylactic or therapeutic vaccines and immunotherapies call for conclusive detection, analysis, and sorting of antigen-specific T cells. This is possible by means of soluble recombinant ligands for T cells, i.e., MHC class I-peptide (pMHC I) complexes for CD8(+) T cells and MHC class II-peptide (pMHC II) complexes for CD4(+) T cells and flow cytometry. Here we review major developments in the development of pMHC staining reagents and their diverse applications and discuss perspectives of their use for basic and clinical investigations.

Modulation of tumor immunity by soluble and membrane-bound molecules at the immunological synapse

To circumvent pathology caused by infectious microbes and tumor growth, the host immune system must constantly clear harmful microorganisms and potentially malignant transformed cells. This task is accomplished in part by T-cells, which can directly kill infected or tumorigenic cells. A crucial event determining the recognition and elimination of detrimental cells is antigen recognition by the T cell receptor (TCR) expressed on the surface of T cells. Upon binding of the TCR to cognate peptide-MHC complexes presented on the surface of antigen presenting cells (APCs), a specialized supramolecular structure known as the immunological synapse (IS) assembles at the T cell-APC interface. Such a structure involves massive redistribution of membrane proteins, including TCR/pMHC complexes, modulatory receptor pairs, and adhesion molecules. Furthermore, assembly of the immunological synapse leads to intracellular events that modulate and define the magnitude and characteristics of the T cell response. Here, we discuss recent literature on the regulation and assembly of IS and the mechanisms evolved by tumors to modulate its function to escape T cell cytotoxicity, as well as novel strategies targeting the IS for therapy.

T cell sensitivity and the outcome of viral infection

The importance of CD8(+) T cells in the control of viral infections is well established. However, what differentiates CD8(+) T cell responses in individuals who control infection and those who do not is not well understood. 'Functional sensitivity' describes an important quality of the T cell response and is determined in part by the affinity of the T cell receptor for antigen. A more sensitive T cell response is generally believed to be more efficient and associated with better control of viral infection, yet may also drive viral mutation and immune escape. Various in vitro techniques have been used to measure T cell sensitivity; however, rapid ex vivo analysis of this has been made possible by the application of the 'magic' tetramer technology. Such tools have potentially important applications in the design and evaluation of vaccines.

Antagonism of HIV-specific CD4+ T cells by C-terminal truncation of a minimum epitope

Antagonism of T cell responses by variants of the cognate peptide is a potential mechanism of viral escape from immune responses and may play a role in the ability of HIV to evade immune control. We show here a rarely described mechanism of antagonism by a peptide shorter than the minimum length epitope for an HIV p24-specific CD4+ T cell clone. The shorter antagonist peptide-MHC complex bound the T cell receptor (TCR), albeit with lower affinity than the full-length agonist peptide. Prior work showing the crystal structure of the peptide-MHC complex revealed a unique glycine hinge near the C-terminus of the agonist peptide, allowing the generation of full-length antagonist peptide lacking the hinge. These results confirm the dependence of productive TCR engagement on residues spilling out from the C-terminus of the MHC binding groove and show that partial engagement of the TCR with a truncated, low-affinity ligand can result in T cell antagonism.

T cell receptor binding kinetics required for T cell activation depend on the density of cognate ligand on the antigen-presenting cell

CD8(+) T cells recognize peptides of eight to nine amino acid residues long in the context of MHC class I molecules on the surface of antigen-presenting cells (APCs). This recognition event is highly sensitive, as evidenced by the fact that T cells can be activated by cognate peptide/MHC complex (pMHC) at extremely low densities (1-50 molecules). High sensitivity is particularly valuable for detection of antigens at low density, such as those derived from tumor cells and intracellular pathogens, which can down-modulate cognate pMHCs from the surface of APCs to evade recognition by the adaptive immune system. T cell activation is only triggered in response to interactions between the T cell receptor (TCR) and the pMHC ligand that reach a specific half-life threshold. However, interactions with excessively long half-lives result in impaired T cell activation. Thus, efficient T cell activation by pMHC on the surface of APCs requires an optimal dwell time of TCR-pMHC interaction. Here, we show that, although this is a requirement at low cognate pMHC density on the APC surface, at high epitope density there is no impairment of T cell activation by extended TCR-pMHC dwell times. This observation was predicted by mathematical simulations for T cell activation by pMHC at different densities and supported by experiments performed on APCs selected for varied expression of cognate pMHC. According to these results, effective T cell activation depends on a complex interplay between inherent TCR-pMHC binding kinetics and the epitope density on the APC.

The beta1 and beta3 integrins promote T cell receptor-mediated cytotoxic T lymphocyte activation

Recognition by CD8+ cytotoxic T lymphocytes (CTLs) of antigenic peptides bound to major histocompatibility class (MHC) I molecules on target cells leads to sustained calcium mobilization and CTL degranulation resulting in perforin-dependent killing. We report that beta1 and beta3 integrin-mediated adhesion to extracellular matrix proteins on target cells and/or surfaces dramatically promotes CTL degranulation. CTLs, when adhered to fibronectin but not CTL in suspension, efficiently degranulate upon exposure to soluble MHC.peptide complexes, even monomeric ones. This adhesion induces recruitment and activation of the focal adhesion kinase Pyk2, the cytoskeleton linker paxillin, and the Src kinases Lck and Fyn in the contact site. The T cell receptor, by association with Pyk2, becomes part of this adhesion-induced activation cluster, which greatly increases its signaling.

Human CD8 co-receptor is strictly involved in MHC-peptide tetramer-TCR binding and T cell activation

Although there has been extensive analysis on the capacity of MHC-peptide tetramers to bind antigen-specific TCR, there have been comparatively few studies regarding the role of the CD4 and CD8 co-receptors in binding and activation by these multimeric molecules. Here, we start from the observation that different antibodies against human CD8 exert opposite effects on MHC-peptide tetramer binding to the TCR: tetramer staining was enhanced by OKT8 antibody, while it was blocked with SK1 antibody. We used these different anti-CD8 antibodies to modulate CD8 function during tetramer staining of Melan-A/MART1-specific CTL clones. We show that CD8 action could be variably modulated during all the phases of interaction, indicating that CD8 participates in both the initial association of the TCR with MHC-peptide tetramers and the stability of this interaction. While the blocking effect of anti-CD8 antibodies was mostly exerted during the initial binding of the TCR with MHC-peptide tetramers, the enhancing effect was exerted by augmenting the duration of this interaction. Blocking anti-CD8 antibodies were also capable of preventing tetramer-mediated T cell activation. The possibility of variably affecting MHC-peptide tetramer binding and T cell activation using anti-CD8 antibodies confirms the critical role exerted by the CD8 co-receptor in this interaction and supports the notion that TCR engagement by MHC-peptide ligands typically involves CD8.

Efficient T cell activation requires an optimal dwell-time of interaction between the TCR and the pMHC complex

Cytotoxic T cell (CTL) activation by antigen requires the specific detection of peptide-major histocompatibility class I (pMHC) molecules on the target-cell surface by the T cell receptor (TCR). We examined the effect of mutations in the antigen-binding site of a Kb-restricted TCR on T cell activation, antigen binding and dissociation from antigen.These parameters were also examined for variants derived from a Kd-restricted peptide that was recognized by a CTL clone. Using these two independent systems, we show that T cell activation can be impaired by mutations that either decrease or increase the binding half-life of the TCR-pMHC interaction. Our data indicate that efficient T cell activation occurs within an optimal dwell-time range of TCR-pMHC interaction. This restricted dwell-time range is consistent with the exclusion of either extremely low or high affinity T cells from the expanded population during immune responses.

Altered peptide ligand-mediated TCR antagonism can be modulated by a change in a single amino acid residue within the CDR3 beta of an MHC class I-restricted TCR

The Ag receptor of cytotoxic CD8+ T lymphocytes recognizes peptides of 8-10 aa bound to MHC class I molecules. This Ag recognition event leads to the activation of the CD8+ lymphocyte and subsequent lysis of the target cell. Altered peptide ligands are analogues derived from the original antigenic peptide that commonly carry amino acid substitutions at TCR contact residues. TCR engagement by these altered peptide ligands usually impairs normal T cell function. Some of these altered peptide ligands (antagonists) are able to specifically antagonize and inhibit T cell activation induced by the wild-type antigenic peptide. Despite significant advances made in understanding TCR antagonism, the molecular interactions between the TCR and the MHC/peptide complex responsible for the inhibitory activity of antagonist peptides remain elusive. To approach this question, we have identified altered peptide ligands derived from the vesicular stomatitis virus peptide (RGYVYQGL) that specifically antagonize an H-2Kb/vesicular stomatitis virus-specific TCR. Furthermore, by site-directed mutagenesis, we altered single amino acid residues of the complementarity-determining region 3 of the beta-chain of this TCR and tested the effect of these point mutations on Ag recognition and TCR antagonism. Here we show that a single amino acid change on the TCR CDR3 beta loop can modulate the TCR-antagonistic properties of an altered peptide ligand. Our results highlight the role of the TCR complementarity-determining region 3 loops for controlling the nature of the T cell response to TCR/altered peptide ligand interactions, including those leading to TCR antagonism.

The role of T-cell receptor dimerization in T-cell activation

T-cell specificity is encoded in single T-cell receptors (TCRs) but monovalent interactions with peptide bound to the major histocompatibility complex (MHC) may not sufficiently account for the complexities associated with T-cell activation. This review proposes that TCRs undergo dimerization before activation and that this property might be essential for both T-cell antagonism and T-cell specificity, and may be pivotal for T-cell survival versus T-cell activation.

Differential Requirements for NF-κB and AP-1 trans-Activation in Response to Minimal TCR Engagement by a Partial Agonist in Naive CD8 T Cells

We investigated the basis for partial reactivity of naive CD8 T cells expressing an alloreactive transgenic TCR in response to a mutant alloantigen. When unstimulated APCs were used, IFN-γ as well as IL-2 and cell proliferation were observed in response to wild-type Ag, whereas mutant Ag induced only IFN-γ. DNA binding and reporter gene assays showed that the response to mutant Ag involved NF-κB, but not AP-1 activation, whereas wild-type Ag activated both transcription factors. Increasing the contribution of costimulatory signals by using LPS-activated APCs partially corrected the activation by mutant Ag, because proliferation and weak IL-2 production could be measured. This also led to AP-1 activation, albeit with delayed kinetics, in response to mutant Ag. To explain how engagement of the same TCR by distinct ligands results in different T cell responses, it may be proposed, in line with models stressing the importance of the kinetics of Ag/TCR interaction, that two types of signals be distinguished: a “fast” short-lived signal is sufficient to activate NF-κB; whereas a “slow” signal obtained after prolonged TCR engagement is required for AP-1 activation. Failure to activate AP-1 in limiting conditions (unstimulated mutant APC) was partially corrected by increasing costimulation.

Interaction of Sulfonamide Derivatives with the TCR of Sulfamethoxazole-Specific Human αβ+ T Cell Clones

Drugs like sulfamethoxazole (SMX) or lidocaine can be presented to specific human αβ+ T cell clones (TCC) by undergoing a noncovalent association with MHC-peptide complexes on HLA-matched APCs. For a better understanding of the molecular basis of the recognition of such drugs by specific TCC, we investigated 1) the fine specificity of the recognizing TCR, 2) the dose-response relationship for the induction of proliferation or cytokine production, and 3) the mechanism of TCR triggering. For that purpose, we tested the reactivity of 11 SMX-specific CD4+ TCC and 2 SMX-specific CD8+ TCC to a panel of 13 different sulfonamide derivatives bearing the same core structure. Five of 13 clones recognized only SMX, while all other clones were responding to as many as 6 different compounds. Some of the compounds needed up to two orders of magnitude higher concentrations than SMX to stimulate TCC, thereby displaying features of weak agonists. Different clones showed clear differences in the minimal drug concentration required for the induction of a proliferative response. Therefore, weaker or stronger agonistic properties were not a characteristic of a given sulfonamide derivative but rather an intrinsic property of the reacting TCR. Finally, the number of down-regulated TCRs was a logarithmic function of the ligand concentration, implicating that specific T cells were activated by serial TCR engagement. Our data demonstrate that, despite the special way of presentation, nonpeptide Ag like drugs appear to interact with the TCR of specific T cells in a similar way as peptide Ags.

CD8 expression allows T cell signaling by monomeric peptide-MHC complexes

Physiologically, TCR signaling is unlikely to result from the cross-linking of TCR-CD3 complexes, given the low density of specific peptide-MHC complexes on antigen-presenting cells. We therefore have tested directly an alternative model for antigen recognition. We show that monomers of soluble peptide-MHC trigger Ca2+ responses in CD8alphabeta+ T cells. This response is not observed in CD8- T cells and when either the CD8:MHC or CD8:Lck interactions are prevented. This demonstrates that an intact CD8 coreceptor is necessary for effective TCR signaling in response to monomeric peptide-MHC molecules. We propose that this heterodimerization of TCR and CD8 by peptide-MHC corresponds to the physiological event normally involved during antigen-specific signal transduction.

The Efficiency of Antigen Recognition by CD8+ CTL Clones Is Determined by the Frequency of Serial TCR Engagement

Using H-2Kd-restricted CTL clones, which are specific for a photoreactive derivative of the Plasmodium berghei circumsporozoite peptide PbCS252–260 (SYIPSAEKI) and permit assessment of TCR-ligand interactions by TCR photoaffinity labeling, we have previously identified several peptide derivative variants for which TCR-ligand binding and the efficiency of Ag recognition deviated by fivefold or more. Here we report that the functional CTL response (cytotoxicity and IFN-γ production) correlated with the rate of TCR-ligand complex dissociation, but not the avidity of TCR-ligand binding. While peptide antagonists exhibited very rapid TCR-ligand complex dissociation, slightly slower dissociation was observed for strong agonists. Conversely and surprisingly, weak agonists typically displayed slower dissociation than the wild-type agonists. Acceleration of TCR-ligand complex dissociation by blocking CD8 participation in TCR-ligand binding increased the efficiency of Ag recognition in cases where dissociation was slow. In addition, permanent TCR engagement by TCR-ligand photocross-linking completely abolished sustained intracellular calcium mobilization, which is required for T cell activation. These results indicate that the functional CTL response depends on the frequency of serial TCR engagement, which, in turn, is determined by the rate of TCR-ligand complex dissociation.