Progression of relapsing-remitting demyelinating disease does not require increased TCR affinity or epitope spread

What's the Catch? The Significance of Catch Bonds in T Cell Activation

One of the main goals in T cell biology has been to investigate how TCR recognition of peptide:MHC (pMHC) determines T cell phenotype and fate. Ag recognition is required to facilitate survival, expansion, and effector function of T cells. Historically, TCR affinity for pMHC has been used as a predictor for T cell fate and responsiveness, but there have now been several examples of nonfunctional high-affinity clones and low-affinity highly functional clones. Recently, more attention has been paid to the TCR being a mechanoreceptor where the key biophysical determinant is TCR bond lifetime under force. As outlined in this review, the fundamental parameters between the TCR and pMHC that control Ag recognition and T cell triggering are affinity, bond lifetime, and the amount of force at which the peak lifetime occurs.

Relationship of 2D Affinity to T Cell Functional Outcomes

T cells are critical for a functioning adaptive immune response and a strong correlation exists between T cell responses and T cell receptor (TCR): peptide-loaded MHC (pMHC) binding. Studies that utilize pMHC tetramer, multimers, and assays of three-dimensional (3D) affinity have provided advancements in our understanding of T cell responses across different diseases. However, these technologies focus on higher affinity and avidity T cells while missing the lower affinity responders. Lower affinity TCRs in expanded polyclonal populations almost always constitute a significant proportion of the response with cells mediating different effector functions associated with variation in the proportion of high and low affinity T cells. Since lower affinity T cells expand and are functional, a fully inclusive view of T cell responses is required to accurately interpret the role of affinity for adaptive T cell immunity. For example, low affinity T cells are capable of inducing autoimmune disease and T cells with an intermediate affinity have been shown to exhibit an optimal anti-tumor response. Here, we focus on how affinity of the TCR may relate to T cell phenotype and provide examples where 2D affinity influences functional outcomes.

CTL-Derived Exosomes Enhance the Activation of CTLs Stimulated by Low-Affinity Peptides

Cytotoxic T cells (CTLs) bind to peptides presented by MHC I (pMHC) through T cell receptors of various affinities. Low-affinity CTLs are important for the control of intracellular pathogens and cancers; however, the mechanisms by which these lower affinity CTLs are activated and maintained are not well understood. We recently discovered that fully activated CTLs stimulated by strong-affinity peptides in the presence of IL-12 are able to secrete exosomes that, in turn, stimulate bystander CTLs without requiring the presence of antigen. We hypothesized that exosomes secreted by high-affinity CTLs could strengthen the activation of low-affinity CTLs. Naive OT-I CD8+ cells were stimulated with altered N4 peptides of different affinities in the presence or absence of Exo. The presence of Exo preferentially increased cell proliferation and enhanced the production of IFNγ in CTLs stimulated by low-affinity peptides. The expression of granzyme B (GZB) was augmented in all affinities, with higher GZB production in low-affinity stimulated CTLs than in high-affinity stimulated ones. Exosomes promoted the rapid activation of low-affinity CTLs, which remained responsive to exosomes for a prolonged duration. Unexpectedly, exosomes could be induced quickly (24 h) following CTL activation and at a higher quantity per cell than later (72 h). While exosome protein profiles vary significantly between early exosomes and their later-derived counterparts, both appear to have similar downstream functions. These results reveal a potential mechanism for fully activated CTLs in activating lower-affinity CTLs that may have important implications in boosting the function of low-affinity CTLs in immunotherapy for cancers and chronic viral infections.

CD4 T Cell Affinity Diversity Is Equally Maintained during Acute and Chronic Infection

TCR affinity for peptide MHC dictates the functional efficiency of T cells and their propensity to differentiate into effectors and form memory. However, in the context of chronic infections, it is unclear what the overall profile of TCR affinity for Ag is and if it differs from acute infections. Using the comprehensive affinity analysis provided by the two-dimensional micropipette adhesion frequency assay and the common indirect affinity evaluation methods of MHC class II tetramer and functional avidity, we tracked IA^b GP_61-80-specific cells in the mouse model of acute (Armstrong) and chronic (clone 13) lymphocytic choriomeningitis virus infection. In each response, we show CD4 T cell population affinity peaks at the effector phase and declines with memory. Of interest, the range and average relative two-dimensional affinity was equivalent between acute and chronic infection, indicating chronic Ag exposure did not skew TCR affinity. In contrast, functional and tetramer avidity measurements revealed divergent results and lacked a consistent correlation with TCR affinity. Our findings highlight that the immune system maintains a diverse range in TCR affinity even under the pressures of chronic Ag stimulation.

NFM Cross-Reactivity to MOG Does Not Expand a Critical Threshold Level of High-Affinity T Cells Necessary for Onset of Demyelinating Disease

Of interest to the etiology of demyelinating autoimmune disease is the potential to aberrantly activate CD4⁺ T cells due to cross-recognition of multiple self-epitopes such as has been suggested for myelin oligodendrocyte glycoprotein epitope 35-55 (MOG_35-55) and neurofilament medium protein epitope 15-35 (NFM_15-35). NFM_15-35 is immunogenic in C57BL/6 mice but fails to induce demyelinating disease by polyclonal T cells despite having the same TCR contact residues as MOG_35-55, a known encephalitogenic Ag. Despite reported cross-reactivity with MOG-specific T cells, the polyclonal response to NFM_15-35 did not expand threshold numbers of MOG_38-49 tetramer-positive T cells. Furthermore, NFM lacked functional synergy with MOG to promote experimental autoimmune encephalomyelitis because NFM-deficient synonymous with knockout mice developed an identical disease course to wild-type mice after challenge with MOG_35-55 Single-cell analysis of encephalitogenic T cells using the peptide:MHC monomer-based two-dimensional micropipette adhesion frequency assay confirmed that NFM was not a critical Ag driving demyelinating disease because NFM_18-30-specific T cells in the CNS were predominantly reactive to MOG_38-49 The absence of NFM contribution to disease allowed mapping of the amino acids required for encephalitogenicity and expansion of high-affinity, MOG-specific T cells that defined the polyclonal response. Alterations of N-terminal residues outside of the NFM_15-35 core nonamer promoted expansion of high-affinity, MOG_38-49 tetramer-positive T cells and promoted consistent experimental autoimmune encephalomyelitis induction, unlike mice challenged with NFM_15-35 Although NFM_15-35 is immunogenic and cross-reactive with MOG at the polyclonal level, it fails to expand a threshold level of encephalitogenic, high-affinity MOG-specific T cells.

Low-affinity CD4+ T cells are major responders in the primary immune response

A robust primary immune response has been correlated with the precursor number of antigen-specific T cells, as identified using peptide MHCII tetramers. However, these tetramers identify only the highest-affinity T cells. Here we show the entire CD4+ T-cell repertoire, inclusive of low-affinity T cells missed by tetramers, using a T-cell receptor (TCR) signalling reporter and micropipette assay to quantify naive precursors and expanded populations. In vivo limiting dilution assays reveal hundreds more precursor T cells than previously thought, with higher-affinity tetramer-positive T cells, comprising only 5-30% of the total antigen-specific naive repertoire. Lower-affinity T cells maintain their predominance as the primary immune response progresses, with no enhancement of survival of T cells with high-affinity TCRs. These findings demonstrate that affinity for antigen does not control CD4+ T-cell entry into the primary immune response, as a diverse range in affinity is maintained from precursor through peak of T-cell expansion.

Conserved Region C Functions To Regulate PD-1 Expression and Subsequent CD8 T Cell Memory

Expression of programmed death 1 (PD-1) on CD8 T cells promotes T cell exhaustion during chronic Ag exposure. During acute infections, PD-1 is transiently expressed and has the potential to modulate CD8 T cell memory formation. Conserved region C (CR-C), a promoter proximal cis-regulatory element that is critical to PD-1 expression in vitro, responds to NFATc1, FoxO1, and/or NF-κB signaling pathways. Here, a CR-C knockout mouse was established to determine its role on PD-1 expression and the corresponding effects on T cell function in vivo. Deletion of CR-C decreased PD-1 expression on CD4 T cells and Ag-specific CD8 T cells during acute and chronic lymphocytic choriomeningitis virus challenges, but did not affect the ability to clear an infection. Following acute lymphocytic choriomeningitis virus infection, memory CD8 T cells in the CR-C knockout mouse were formed in greater numbers, were more functional, and were more effective at responding to a melanoma tumor than wild-type memory cells. These data implicate a critical role for CR-C in governing PD-1 expression, and a subsequent role in guiding CD8 T cell differentiation. The data suggest the possibility that titrating PD-1 expression during CD8 T cell activation could have important ramifications in vaccine development and clinical care.

Regulatory and T Effector Cells Have Overlapping Low to High Ranges in TCR Affinities for Self during Demyelinating Disease

Having regulatory T cells (Tregs) with the same Ag specificity as the responding conventional T cells is thought to be important in maintaining peripheral tolerance. It has been demonstrated that during experimental autoimmune encephalomyelitis there are myelin oligodendrocyte glycoprotein (MOG)--specific Tregs that infiltrate into the CNS. However, the affinity of naturally occurring polyclonal Tregs for any self-antigen, let alone MOG, has not been analyzed in the periphery or at the site of autoimmune disease. Utilizing the highly sensitive micropipette adhesion frequency assay, which allows one to determine on a single-cell basis the affinity and frequency of polyclonal Ag-specific T cells directly ex vivo, we demonstrate that at peak disease MOG-specific Tregs were progressively enriched in the draining cervical lymph nodes and CNS as compared with spleen. These frequencies were greater than the frequencies measured by tetramer analysis, indicative of the large fraction of lower affinity T cells that comprise the MOG-specific conventional T cell (Tconv) and Treg response. Of interest, the self-reactive CD4(+) Tconvs and Tregs displayed overlapping affinities for MOG in the periphery, yet in the CNS, the site of neuroinflammation, Tconvs skew toward higher affinities. Most of the MOG-specific Tregs in the CNS possessed the methylation signature associated with thymic-derived Tregs. These findings indicate that thymic-derived Treg affinity range matches that of their Tconvs in the periphery and suggest a change in TCR affinity as a potential mechanism for autoimmune progression and escape from immune regulation.

Cutting Edge: Resident Memory CD8 T Cells Express High-Affinity TCRs

Tissue-resident memory T (TRM) cells serve as vanguards of antimicrobial host defense in nonlymphoid tissues, particularly at barrier epithelia and in organs with nonrenewable cell types (e.g., brain). In this study, we asked whether an augmented ability to sense Ag complemented their role as early alarms of pathogen invasion. Using mouse polyomavirus, we show that brain-resident mouse polyomavirus-specific CD8 T cells, unlike memory cells in the spleen, progressively increase binding to MHC class I tetramers and CD8 coreceptor expression. Using the two-dimensional micropipette adhesion-frequency assay, we show that TRM cells in brain, as well as in kidney, express TCRs with up to 20-fold higher affinity than do splenic memory T cells, whereas effector cells express TCRs of similar high affinity in all organs. Together, these data demonstrate that TRM cells retain high TCR affinity, which endows them with the high Ag sensitivity needed for front-line defense against infectious agents.

Lower Affinity T Cells are Critical Components and Active Participants of the Immune Response

Kinetic and biophysical parameters of T cell receptor (TCR) and peptide:MHC (pMHC) interaction define intrinsic factors required for T cell activation and differentiation. Although receptor ligand kinetics are somewhat cumbersome to assess experimentally, TCR:pMHC affinity has been shown to predict peripheral T cell functionality and potential for forming memory. Multimeric forms of pMHC monomers have often been used to provide an indirect readout of higher affinity T cells due to their availability and ease of use while allowing simultaneous definition of other functional and phenotypic characteristics. However, multimeric pMHC reagents have introduced a bias that underestimates the lower affinity components contained in the highly diverse TCR repertoires of all polyclonal T cell responses. Advances in the identification of lower affinity cells have led to the examination of these cells and their contribution to the immune response. In this review, we discuss the identification of high- vs. low-affinity T cells as well as their attributed signaling and functional differences. Lastly, mechanisms are discussed that maintain a diverse range of low- and high-affinity T cells.

Force-Regulated In Situ TCR-Peptide-Bound MHC Class II Kinetics Determine Functions of CD4+ T Cells

We have recently shown that two-dimensional (2D) and force-regulated kinetics of TCR-peptide-bound MHC class I (pMHC-I) interactions predict responses of CD8(+) T cells. To test whether these findings are applicable to CD4(+) T cells, we analyzed the in situ 3.L2 TCR-pMHC-II interactions for a well-characterized panel of altered peptide ligands on the T cell surface using the adhesion frequency assay with a micropipette and the thermal fluctuation and force-clamp assays with a biomembrane force probe. We found that the 2D effective TCR-pMHC-II affinity and off-rate correlate with, but better predict the T cell response than, the corresponding measurements with the surface plasmon resonance in three dimensions. The 2D affinity of the CD4 for MHC-II was very low, approaching the detection limit, making it one to two orders of magnitude lower than the affinity of CD8 for MHC-I. In addition, the signal-dependent cooperation between TCR and coreceptor for pMHC binding previously observed for CD8 was not observed for CD4. Interestingly, force elicited TCR-pMHC-II catch-slip bonds for agonists but slip-only bonds for antagonists, thereby amplifying the power of discrimination between altered peptide ligands. These results show that the force-regulated 2D binding kinetics of the 3.L2 TCR for pMHC-II determine functions of CD4(+) T cells.