Mapping the Binding Site on CD8β for MHC Class I Reveals Mutants with Enhanced Binding

Structure, function, and immunomodulation of the CD8 co-receptor

Expressed on the surface of CD8+ T cells, the CD8 co-receptor is a key component of the T cells that contributes to antigen recognition, immune cell maturation, and immune cell signaling. While CD8 is widely recognized as a co-stimulatory molecule for conventional CD8+ αβ T cells, recent reports highlight its multifaceted role in both adaptive and innate immune responses. In this review, we discuss the utility of CD8 in relation to its immunomodulatory properties. We outline the unique structure and function of different CD8 domains (ectodomain, hinge, transmembrane, cytoplasmic tail) in the context of the distinct properties of CD8αα homodimers and CD8αβ heterodimers. We discuss CD8 features commonly used to construct chimeric antigen receptors for immunotherapy. We describe the molecular interactions of CD8 with classical MHC-I, non-classical MHCs, and Lck partners involved in T cell signaling. Engineered and naturally occurring CD8 mutations that alter immune responses are discussed. The applications of anti-CD8 monoclonal antibodies (mABs) that target CD8 are summarized. Finally, we examine the unique structure and function of several CD8/mAB complexes. Collectively, these findings reveal the promising immunomodulatory properties of CD8 and CD8 binding partners, not only to uncover basic immune system function, but to advance efforts towards translational research for targeted immunotherapy.

High-affinity CD8 variants enhance the sensitivity of pMHCI antigen recognition via low-affinity TCRs

CD8+ T cell-mediated recognition of peptide-major histocompatibility complex class I (pMHCI) molecules involves cooperative binding of the T cell receptor (TCR), which confers antigen specificity, and the CD8 coreceptor, which stabilizes the TCR/pMHCI complex. Earlier work has shown that the sensitivity of antigen recognition can be regulated in vitro by altering the strength of the pMHCI/CD8 interaction. Here, we characterized two CD8 variants with moderately enhanced affinities for pMHCI, aiming to boost antigen sensitivity without inducing non-specific activation. Expression of these CD8 variants in model systems preferentially enhanced pMHCI antigen recognition in the context of low-affinity TCRs. A similar effect was observed using primary CD4+ T cells transduced with cancer-targeting TCRs. The introduction of high-affinity CD8 variants also enhanced the functional sensitivity of primary CD8+ T cells expressing cancer-targeting TCRs, but comparable results were obtained using exogenous wild-type CD8. Specificity was retained in every case, with no evidence of reactivity in the absence of cognate antigen. Collectively, these findings highlight a generically applicable mechanism to enhance the sensitivity of low-affinity pMHCI antigen recognition, which could augment the therapeutic efficacy of clinically relevant TCRs.

CD8 Binding of MHC-Peptide Complexes in cis or trans Regulates CD8+ T-cell Responses

The coreceptor CD8αβ can greatly promote activation of T cells by strengthening T-cell receptor (TCR) binding to cognate peptide-MHC complexes (pMHC) on antigen presenting cells and by bringing p56^Lck to TCR/CD3. Here, we demonstrate that CD8 can also bind to pMHC on the T cell (in cis) and that this inhibits their activation. Using molecular modeling, fluorescence resonance energy transfer experiments on living cells, biochemical and mutational analysis, we show that CD8 binding to pMHC in cis involves a different docking mode and is regulated by posttranslational modifications including a membrane-distal interchain disulfide bond and negatively charged O-linked glycans near positively charged sequences on the CD8β stalk. These modifications distort the stalk, thus favoring CD8 binding to pMHC in cis. Differential binding of CD8 to pMHC in cis or trans is a means to regulate CD8⁺ T-cell responses and provides new translational opportunities.

Development of T cell lines sensitive to antigen stimulation

Immortalized T cells such as T cell hybridomas, transfectomas, and transductants are useful tools to study tri-molecular complexes consisting of peptide, MHC, and T cell receptor (TCR) molecules. These cells have been utilized for antigen discovery studies for decades due to simplicity and rapidness of growing cells. However, responsiveness to antigen stimulation is typically less sensitive compared to primary T cells, resulting in occasional false negative outcomes especially for TCRs having low affinity to a peptide-MHC complex (pMHC). To overcome this obstacle, we genetically engineered T cell hybridomas to express additional CD3 molecules as well as CD4 with two amino acid substitutions that increase affinity to MHC class II molecules. The manipulated T cell hybridomas that were further transduced with retroviral vectors encoding TCRs of interest responded to cognate antigens more robustly than non-manipulated cells without evoking non-antigen specific reactivity. Of importance, the manipulation with CD3 and mutated human CD4 expression was effective in increasing responsiveness of T cell hybridomas to a wide variety of TCR, peptide, and MHC combinations across class II genetic loci (i.e. HLA-DR, HLA-DQ, HLA-DP, and murine H2-IA) and species (i.e. both humans and mice), and thus will be useful to identify antigen specificity of T cells.

CD8+ T-cell specificity is compromised at a defined MHCI/CD8 affinity threshold

The CD8 co-receptor engages peptide-major histocompatibility complex class I (pMHCI) molecules at a largely invariant site distinct from the T-cell receptor (TCR)-binding platform and enhances the sensitivity of antigen-driven activation to promote effective CD8+ T-cell immunity. A small increase in the strength of the pMHCI/CD8 interaction (~1.5-fold) can disproportionately amplify this effect, boosting antigen sensitivity by up to two orders of magnitude. However, recognition specificity is lost altogether with more substantial increases in pMHCI/CD8 affinity (~10-fold). In this study, we used a panel of MHCI mutants with altered CD8-binding properties to show that TCR-mediated antigen specificity is delimited by a pMHCI/CD8 affinity threshold. Our findings suggest that CD8 can be engineered within certain biophysical parameters to enhance the therapeutic efficacy of adoptive T-cell transfer irrespective of antigen specificity.

Ligand-engaged TCR is triggered by Lck not associated with CD8 coreceptor

The earliest molecular events in T cell recognition have not yet been fully described, and the initial T cell receptor (TCR) triggering mechanism remains a subject of controversy. Here, using TIRF/FRET microscopy, we observe a two-stage interaction between TCR, CD8, and MHCp. There is an early (within seconds) interaction between CD3ζ and the coreceptor CD8 that is independent of the binding of CD8 to MHC, but that requires CD8 association with Lck. Later (several minutes) CD3ζ-CD8 interactions require CD8-MHC binding. Lck can be found free or bound to the coreceptor. This work indicates that the initial TCR triggering event is induced by free Lck.

Extensive diversification of MHC in Chinese giant salamanders Andrias davidianus (Anda-MHC) reveals novel splice variants

A series of MHC alleles (including 26 class IA, 27 class IIA, and 17 class IIB) were identified from Chinese giant salamander Andrias davidianus (Anda-MHC). These genes are similar to classical MHC molecules in terms of characteristic domains, functional residues, deduced tertiary structures and genetic diversity. The majority of variation between alleles is found in the putative peptide-binding region (PBR), which is driven by positive Darwinian selection. The coexistence of two isoforms in MHC IA, IIA, and IIB alleles are shown: one full-length transcript and one novel splice variant. Despite lake of the external domains, these variants exhibit similar subcellular localization with the full-length transcripts. Moreover, the expression of MHC isoforms are up-regulated upon in vivo and in vitro stimulation with Andrias davidianus ranavirus (ADRV), suggesting their potential roles in the immune response. The results provide insights into understanding MHC variation and function in this ancient and endangered urodele amphibian.

CD8αα and -αβ isotypes are equally recruited to the immunological synapse through their ability to bind to MHC class I

Bimolecular fluorescence complementation was used to engineer CD8 molecules so that CD8αα and CD8αβ dimers can be independently visualized on the surface of a T cell during antigen recognition. Using this approach, we show that CD8αα is recruited to the immunological synapse almost as well as CD8αβ, but because the kinase Lck associates preferentially with CD8αβ in lipid rafts, CD8αα is the weaker co-receptor. During recognition of the strong CD8αα ligand H2-TL, CD8αα is preferentially recruited. Thus, recruitment of the two CD8 species correlates with their relative binding to the available ligands, rather than with the co-receptor functions of the CD8 species.

Structural basis of the CD8 alpha beta/MHC class I interaction: focused recognition orients CD8 beta to a T cell proximal position

In the immune system, B cells, dendritic cells, NK cells, and T lymphocytes all respond to signals received via ligand binding to receptors and coreceptors. Although the specificity of T cell recognition is determined by the interaction of T cell receptors with MHC/peptide complexes, the development of T cells in the thymus and their sensitivity to Ag are also dependent on coreceptor molecules CD8 (for MHC class I (MHCI)) and CD4 (for MHCII). The CD8alphabeta heterodimer is a potent coreceptor for T cell activation, but efforts to understand its function fully have been hampered by ignorance of the structural details of its interactions with MHCI. In this study we describe the structure of CD8alphabeta in complex with the murine MHCI molecule H-2D(d) at 2.6 A resolution. The focus of the CD8alphabeta interaction is the acidic loop (residues 222-228) of the alpha3 domain of H-2D(d). The beta subunit occupies a T cell membrane proximal position, defining the relative positions of the CD8alpha and CD8beta subunits. Unlike the CD8alphaalpha homodimer, CD8alphabeta does not contact the MHCI alpha(2)- or beta(2)-microglobulin domains. Movements of the CD8alpha CDR2 and CD8beta CDR1 and CDR2 loops as well as the flexibility of the H-2D(d) CD loop facilitate the monovalent interaction. The structure resolves inconclusive data on the topology of the CD8alphabeta/MHCI interaction, indicates that CD8beta is crucial in orienting the CD8alphabeta heterodimer, provides a framework for understanding the mechanistic role of CD8alphabeta in lymphoid cell signaling, and offers a tangible context for design of structurally altered coreceptors for tumor and viral immunotherapy.

An essential role for the stalk region of CD8 beta in the coreceptor function of CD8

The CD8alphabeta heterodimer is integral to the selection of the class I-restricted lineage in the thymus; however, the contribution of the CD8beta chain to coreceptor function is poorly understood. To understand whether the CD8beta membrane proximal stalk region played a role in coreceptor function, we substituted it with the corresponding sequence from the CD8alpha polypeptide and expressed the hybrid molecule in transgenic mice in place of endogenous CD8beta. Although the stalk-swapped CD8beta was expressed on the cell surface as a disulfide-bonded heterodimer at equivalent levels of expression to an endogenous CD8beta molecule, it failed to restore selection of CD8(+) class I MHC-restricted T cells and it altered the response of peripheral T cells. Thus, the stalk region of the CD8beta polypeptide has an essential role in ensuring functionality of the CD8alphabeta heterodimer and its replacement compromises the interaction of CD8 with peptide-MHC complexes.

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.

The crystal structure of CD8 in complex with YTS156.7.7 Fab and interaction with other CD8 antibodies define the binding mode of CD8 alphabeta to MHC class I

The CD8alphabeta heterodimer interacts with class I pMHC on antigen-presenting cells as a co-receptor for TCR-mediated activation of cytotoxic T cells. To characterize this immunologically important interaction, we used monoclonal antibodies (mAbs) specific to either CD8alpha or CD8beta to probe the mechanism of CD8alphabeta binding to pMHCI. The YTS156.7 mAb inhibits this interaction and blocks T cell activation. To elucidate the molecular basis for this inhibition, the crystal structure of the CD8alphabeta immunoglobulin-like ectodomains were determined in complex with mAb YTS156.7 Fab at 2.7 A resolution. The YTS156.7 epitope on CD8beta was identified and implies that residues in the CDR1 and CDR2-equivalent loops of CD8beta are occluded upon binding to class I pMHC. To further characterize the pMHCI/CD8alphabeta interaction, binding of class I tetramers to CD8alphabeta on the surface of T cells was assessed in the presence of anti-CD8 mAbs. In contrast to YTS156.7, mAb YTS105.18, which is specific for CD8alpha, does not inhibit binding of CD8alphabeta to class I tetramers, indicating the YTS105.18 epitope is not occluded in the pMHCI/CD8alphabeta complex. Together, these data indicate a model for the pMHCI/CD8alphabeta interaction similar to that observed for CD8alphaalpha in the CD8alphaalpha/pMHCI complex, but in which CD8alpha occupies the lower orientation (membrane proximal to the antigen presenting cell), and CD8beta occupies the upper position (membrane distal). The implication of this molecular assembly for the function of CD8alphabeta in T cell activation is discussed.

Differential expression of the human CD8beta splice variants and regulation of the M-2 isoform by ubiquitination

The CD8alphabeta heterodimer functions as a coreceptor with the TCR, influencing the outcome of CD8(+) T cell responses to pathogen-infected and tumor cells. In contrast to the murine CD8B gene, the human gene encodes alternatively spliced variants with different cytoplasmic tails (M-1, M-2, M-3, and M-4). At present, little is known about the expression patterns and functional significance of such variants. We used quantitative RT-PCR to demonstrate differential mRNA expression patterns of these splice variants in thymocytes and in resting, memory, and activated primary human CD8(+) T cells. In total CD8(+) T cells, mRNA levels of the M-1 variant were the most predominant and levels of M-3 were the least detected. The M-4 isoform was predominant in effector memory CD8(+) T cells. Upon stimulation of CD8(+) T cells, the M-2 variant mRNA levels were elevated 10-20-fold relative to resting cells in contrast to the other isoforms. Curiously, the M-2 isoform was not expressed on the cell surface in transfected cell lines. Using fluorescent chimeras of the extracellular domain of mouse CD8beta fused to the cytoplasmic tails of each isoform, the M-2 isoform was localized in a lysosomal compartment regulated by ubiquitination of a lysine residue (K215) in its cytoplasmic tail. In contrast, upon short-term stimulation, the M-2 protein localized to the cell surface with the TCR complex. The relatively recent evolution of CD8B gene splice variants in the chimpanzee/human lineage is most likely important for fine-tuning the CD8(+) T cell responses.

Doubting the TCR coreceptor function of CD8alphaalpha

"The beginning of wisdom is found in doubting; by doubting we come to question, and by seeking we may come upon the truth." -Pierre Abélard. CD8 is a glycoprotein expressed on hematopoietic cells. Two isoforms of CD8, CD8alphabeta and CD8alphaalpha, have been identified that are distinct in their expression and function. Whereas CD8alphabeta serves as a T cell receptor (TCR) coreceptor to enhance the functional avidity and is constitutively expressed on MHC class I-restricted T cells, CD8alphaalpha marks T cells that are distinct from the conventional thymus-selected and MHC-restricted CD4(+) or CD8alphabeta(+) T cells. Inconsistent with a coreceptor function, CD8alphaalpha decreases antigen sensitivity of the TCR, and it can be transiently or permanently expressed on T cells, regardless of the MHC restriction of the TCR or the presence of conventional coreceptors. Together, these observations indicate that CD8alphaalpha on T cells marks a differentiation stage and that it likely functions as a TCR corepressor to negatively regulate T cell activation.

Kinetics of MHC-CD8 interaction at the T cell membrane

CD8 plays an important role in facilitating TCR-MHC interaction, promoting Ag recognition, and initiating T cell activation. MHC-CD8 binding kinetics have been measured in three dimensions by surface plasmon resonance technique using purified molecules. However, CD8 is a membrane-anchored, signaling kinase-linked, and TCR-associated molecule whose function depends on the cell membrane environment. Purified molecules lack their linkage to the membrane, which precludes interactions with other structures of the cell as well as signaling. Furthermore, three-dimensional binding in the fluid phase is biologically and physically distinct from two-dimensional binding across apposing cell membranes. As a first step toward characterizing the molecular interactions between T cells and APCs, we used a micropipette adhesion frequency assay to measure the adhesion kinetics of single mouse T cells interacting with single human RBCs coated with MHC. Using anti-TCR mAb we isolated and characterized the specific two-dimensional MHC-CD8 binding from the trimolecular TCR-MHC-CD8 interaction. The TCR-independent MHC-CD8 interaction has a very low affinity that depends on the MHC alleles, but not on the peptide complexed to the MHC and whether CD8 is an alphaalpha homodimer or an alphabeta heterodimer. Surprisingly, MHC-CD8 binding affinity varies with T cells from different TCR transgenic mice and these affinity differences were abolished by treatment with cholesterol oxidase to disrupt membrane rafts. These data highlight the relevance and importance of two-dimensional analysis of T cells and APCs and indicate that membrane rafts play an important role in modulating the affinity of cell-cell interactions.