Edited by: Eric Huseby, University of Massachusetts Medical School, USA
Reviewed by: Ross M. Kedl, University of Colorado Denver, USA; Salvatore Valitutti, INSERM, France
Specialty section: This article was submitted to T Cell Biology, a section of the journal Frontiers in Immunology
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T cell immunity relies on the generation and maintenance of a diverse repertoire of T cell antigen receptors (TCRs). The strength of signaling emanating from the TCR dictates the fate of T cells during development, as well as during the immune response. Whereas development of new T cells in the thymus increases the available TCR repertoire, clonal selection during the immune response narrows TCR diversity through the outgrowth of clonotypes with the fittest TCR. To ensure maintenance of TCR diversity in the antigen-selected repertoire, specific mechanisms can be envisaged that facilitate the participation of T cell clonotypes with less than best fit TCRs. Here, we summarize the evidence for the existence of such mechanisms that can prevent the loss of diversity. A number of T cell-autonomous or extrinsic factors can reverse clonotypic hierarchies set by TCR affinity for given antigen. Although not yet complete, understanding of these factors and their mechanism of action will be critical in interventional attempts to mold the antigen-selected TCR repertoire.
Adaptive immunity provides formidable defense against an antigenically unpredictable array of infectious microbes and transformed cells. Recognition of diverse antigens relies on the generation and maintenance of a correspondingly diverse repertoire of antigen receptors, generated somatically and distributed clonally on lymphocytes (
Both chains of the heterodimeric αβ T cell receptor (TCRαβ, referred to here as TCR) are generated by a recombinatorial process of variable (V) and junctional (J), and in the case of the TCR β chain, also diversity (D), gene segment rearrangement. This process can create a conservatively estimated diversity of 1015 different TCRs (
The intrinsic ability of each TCR to recognize antigenic pMHCII complexes is largely responsible for the disparate behavior of distinct CD4+ T cell clonotypes during the immune response, both in terms of expansion (
In addition to its involvement during priming, the available evidence suggests that TCR signal strength contributes to clonotypic selection during memory formation and recall responses. Several studies have demonstrated selection for CD4+ T cell clonotypes with stronger pMHCII tetramer binding or functional avidity during memory formation in response to immunization or infection (
Which parameters of immunization or infection can affect TCR clonotypic composition independently of TCR affinity, and indeed to what degree they can overcome TCR affinity-based hierarchies, remains poorly understood. It is clear, however, that such parameters have the potential to reduce the gap in TCR signal strength between high- and low-affinity clonotypes. This property may be critical in ensuring the necessary clonotypic diversity in the CD4+ T cell response. Here, we focus on CD4+ T cells (as the significant amount of knowledge on CD8+ T cells is reviewed elsewhere) and review the current knowledge of the factors that can modify the response of distinct T cell clonotypes that would otherwise be set by TCR affinity.
The cell-autonomous effect of intrinsic TCR affinity on CD4+ T cell clonal expansion and selection is often taken to imply that CD4+ T cells responding to antigen are oblivious to other CD4+ T cells, including those with shared reactivity. However, the behavior of a given CD4+ T cell may be strongly affected by other CD4+ T cells, either through competition or regulation.
The advent of TCR-transgenic T cells permitted the artificial increase of the precursor frequency of T cell clones reactive with a given antigen. At unphysiologically high precursor frequencies, memory development of antigen-specific monoclonal CD4+ T cells was found to be severely compromised due to intraclonal competition (
Starting at more physiological precursor frequencies, monoclonal CD4+ T cells responding with relatively low functional avidity to an H2-Ab-restricted epitope from the lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP), failed to enter the memory pool when the GP epitope was expressed recombinantly in
In another TCR-transgenic system, low-avidity CD4+ T cell clonotypes in a semipolyclonal TCRβ-transgenic population reactive with an epitope from the surface glycoprotein (SU), encoded by the envelope (
Competition between clonotypes with shared antigen reactivity may drive even more extreme differences in outcome if we consider the composition of the antigen-naïve TCR repertoire. Although current estimates suggest that reactivity for a given antigenic pMHCII complex is shared by, on average, 100 CD4+ T cells in the preimmune repertoire, this number varies substantially for different antigenic pMHCII complexes in the same individual from a theoretical one to experimentally observed several hundred (
Although CD4+ T cell clonotypes with shared ability to recognize a particular antigenic pMHCII complex will compete for access to it, there are a number of additional factors that have been proposed to determine the outcome of clonotypic competition. These include multiple costimulatory factors, such as members of the B7 or TNF families of cytokines and costimulatory molecules and their receptors (
Competition at the level of costimulation operates among T cell clonotypes with shared antigen reactivity, but may not be restricted to them. Expression of the receptors for many costimulatory or inhibitory molecules is typically induced on effector CD4+ T cells (
The preimmune CD4+ T cell pool also contains a significant proportion of regulatory T (Treg) cells, characterized by the expression of the transcription factor Foxp3, and naturally endowed with suppressive activity (
A generally suppressive environment created by Treg cell action would raise the activation threshold for all clonotypes indiscriminately. However, the effect of Treg cells would block the activation of lower avidity clonotypes preferentially, as their threshold may fall below that for effective participation in the response. Indeed, Treg cell activation by IL-2 secreted by the first few effector T cell clonotypes may, in principle, prevent further recruitment of other clonotypes (
Another layer of complexity regarding Treg cell-mediated modulation of clonotypic diversity is shared antigen reactivity between Treg cells and effector CD4+ T cells. Although high-avidity effector CD4+ T cell clonotypes can be efficiently suppressed by Treg cell that do not share antigen reactivity (
The overall strength of TCR signal a T cell receives is determined by the TCR affinity for a given pMHCII complex, but it is also affected by the amount or nature of the pMHCII complex itself. Increasing amounts of antigenic pMHCII complexes will prime an increasing number of clonotypes as the activation threshold of lower avidity clonotypes is progressively reached (
Early work by Malherbe et al. first demonstrated the powerful effect of the co-administered adjuvant on the overall avidity and clonotypic composition of the CD4+ T cell response to immunization with a fixed amount of purified pigeon cytochrome c (PCC) protein (
Although the choice of adjuvant or vaccine vector can have a profound effect on the clonotypic composition of the elicited T cell response, the underlying mechanisms remain unclear and thus the outcome is not always predictable. Nevertheless, some shared properties of vaccines that induce high-avidity CD4+ T cells can be postulated (
If we consider the speed and numerical size of the response as an indicator of the efficiency of antigenic pMHCII presentation, the results of these studies would suggest that fast and strong antigen presentation kinetics result in preferentially high-avidity CD4+ T cell responses. This observation would be consistent with a model where the outcome of competition between clonotypes is determined by at least two parameters: first, timing of recruitment, with the higher-avidity clonotypes starting their response earlier; and second; the duration of the response as a whole (Figure
Any effect of antigen delivery, production, and presentation will inevitably also depend on the type of APC. The potential of distinct APC types to instruct fundamentally different fates in T cells is perhaps best exemplified by thymic selection, where developing T cells interact with diverse subsets of thymic APCs in discrete thymic microenvironments, resulting in positive and negative selection (
Professional APCs, including dendritic cells (DCs), macrophages, and B cells, differ wildly in terms of absolute numbers, anatomical location as well as the way they capture and process exogenous antigen (
In addition to capturing antigen administered as a vaccine or produced in other cell types, professional APCs may also synthesize antigenic proteins for loading onto MHC II molecules, particularly in the setting of infection. Numerous infectious microbes have been documented to infect professional APCs, including the notable examples of LCMV infection of mouse DCs, Epstein–Barr virus (EBV) infection of human B cells, and
As well as professional APCs, a variety of hematopoietic and non-hematopoietic cell types are increasingly implicated in MHC II-mediated antigen presentation (
These studies collectively support a role for the type of APC in shaping the clonotypic composition of the CD4+ T cell response. However, given the large number of variables potentially dictating the quantity and quality of antigen presentation by different APC types, it is perhaps unsurprising that a clear correlation with the clonotypic composition of the induced T cell response has not yet been made.
Besides the affinity of the TCR for pMHCII complexes, the total amount of TCR signaling a CD4+ T cell will receive will vary according to the amount of TCR molecules expressed on the T cell surface, as well as the relative capacity of the TCR signaling complex to initiate the signaling cascade. Both of these parameters can change during the course of the immune response and may therefore affect the clonotypic hierarchy.
At steady-state, numbers of surface TCR molecules are maintained at constant levels by balancing production, recycling, and degradation of individual components of the TCR-CD3 signaling complex (
The molecular details of TCR downregulation have been elucidated using short
To what extend TCR downregulation reduces the ability of a T cell to receive further antigenic stimulation, particularly in chronic
Other than changes in levels and organization of the surface TCR-CD3 complex, accumulating evidence suggests that the TCR signaling capacity in response to antigenic stimulation is modified by the strength of TCR signaling received prior to antigenic encounter. Such signaling originates from recognition of self pMHCII complexes, but may also include environmental antigens from ubiquitous sources or unrelated infections, and has the ability to tune the TCR signal strength (
A major function of self pMHCII interaction is to desensitize or eliminate, through negative selection, strongly autoreactive CD4+ T cells (
The precise relationship between affinity for self or antigenic pMHCII complexes and clonal expansion in response to antigen is not fully delineated and may depend on additional parameters. In a simple model, clonotypes with the strongest reactivity with self pMHCII complexes will also display the strongest reactivity with antigenic pMHCII complexes, likely as a result of stronger TCR binding to the latter complexes (
The proposed dominant effect of self-reactivity on CD4+ T cell responsiveness to antigenic stimulation (
In the same way, relative importance of either self or antigenic pMHCII complexes will also depend on the affinity of individual clonotypes for either type of TCR stimulation (Figure
An overwhelming amount of data supports a role for strong TCR signaling in CD4+ T cell expansion and differentiation during priming. Continuous TCR signals also contribute to successful CD4+ T cell memory formation (
Central to the ability of the host to mount a strong T cell response is also the ability to regulate potential over-reactivity and many cell-autonomous layers of negative regulators have been described. The most extreme form of negative regulation is death of T cells as a direct result of strong or persistent TCR signaling (
TCR signal strength can heavily skew Th differentiation to or away from particular Th subsets (
The relative ability of different Th subsets to form a stable memory population has not been systematically compared and may vary according to the infection or immunization. For example, maintenance of Tfh cells is thought to require persistent antigen and germinal center B cells (
Thus, CD4+ T cell clonotypes will, to a certain degree, assort into distinct Th functional subsets, each of which may exhibit differential capacity to persist into memory, in turn shifting the clonotypic composition of the CD4+ T cell response over time. For instance, a low-affinity clonotype may be preferentially enriched in a chronic response due to its skewed differentiation into a Th subset with increased numerical stability.
The relative stability of Th subsets, and by extension of the clonotypes that preferentially differentiate into those subsets, can additionally be affected by extrinsic infection-related factors. CD4+ T cells clonotypes with the highest affinity for antigenic pMHCII complexes will outcompete other lower-affinity clonotypes, but may be lost due to preferential infection in the case of T cell-tropic viruses (
The energetically costly generation of a diverse TCR and BCR repertoire underpins the evolutionary success of adaptive immunity as it is critical for its function (
Although a single naïve T cell can give progeny that differentiates into multiple functional subsets (
As the number of available TCRs is finite and relatively small in comparison to the number of potential antigens, TCR cross-reactivity is essential to broaden immune coverage by the naïve repertoire (
Another potential advantage of diversity in the CD4+ T cell response to a particular antigen relates to the clonal nature of TCR distribution. Even if sufficient for protective immunity, a single clonal T cell family may inherit the characteristics of the single thymic selection event imprinted onto the founder cell of that family. A polyclonal response will comprise clones from separate thymic selection events, life-histories and likely different patterns of TCR responsiveness, self-reactivity, and survival potential.
The emerging picture is of numerous factors that maximize diversity in the CD4+ T cell response, by alleviating any disadvantage of lower-affinity clonotypes. Some of these factors are extrinsic to T cells and can be manipulated in our attempts to induce protective CD4+ T cell immunity or prevent autoimmunity. However, any intervention toward TCR repertoire “engineering” will necessitate deeper understanding of the type of TCR diversity that best fits a particular response, and this should be the focus of future investigation.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.