The identification of CD8+ T cell correlates of immune control in HIV infection has remained an elusive yet profoundly important goal that will immeasurably advance our quest for an effective vaccine. However, the heterogeneity that exists between individuals and between CD8+ T cell populations represents a major barrier to the precise identification of these attributes. To understand the mechanistic basis of the immune protection mediated by CD8+ T cells, we conducted a comprehensive study of CD8+ T cell populations specific for B27-KK10 that seem to confer favorable outcomes in infected individuals, as indicated by the literature and our data showing an inverse correlation between the magnitude of p24-specific CD8+ T cell responses and cVL. We found that B27-KK10–specific CD8+ T cells exhibited a more polyfunctional profile, demonstrated increased rates of clonal turnover associated with characteristics of senescence, and displayed superior functional avidity when compared with other HIV-specific CD8+ T cells. These features are interlinked and characterize protective CD8+ T cell–mediated immunity.
Polyfunctional capacity, or the ability to produce high levels of several soluble factors simultaneously, is emerging as a key factor of T cell effectiveness. Soluble factors secreted by T cells like cytokines, cytotoxins, or chemokines are central to the immune response, exerting direct effector capacities, or enabling further T cell recruitment or expansion. Two recent studies indicate that vaccine efficacy depends on the induction of polyfunctional T cells in both mice and humans (22
). Our data suggest that CD8+
T cells that achieve optimal control of HIV-1 replication also display polyfunctional capacities, in keeping with the original work from Betts et al. showing the maintenance of polyfunctional CD8+
T cells in long-term nonprogressors (21
). Collectively, these data provide further support for the consideration of T cell polyfunctionality as a correlate of protective immunity.
Functional avidity, as defined in this paper, is a measure of the efficiency of antigen recognition by CD8+
T cells. In previous reports, high avidity CD8+
T cells have been shown to initiate the lysis of target cells more rapidly at any given antigen density and to be more effective at mediating viral clearance than low avidity cells (34
). Thus, the efficiency with which CD8+
T cells are stimulated and exert their antiviral effects to eradicate HIV-infected cells is dependent on functional avidity. In addition, recent work in mouse models suggests a correlation between T cell functionality and avidity, so that the quality of the CD8+
T cell response in terms of cytokine profile is enhanced for highly avid T cells (37
). Our results are in line with these findings. The superior polyfunctionality of B27-KK10–specific CD8+
T cell populations may therefore directly relate to their higher functional avidity, and these two features together may enable the more efficient eradication of HIV-infected cells. The high avidity of B27-KK10–specific CD8+
T cell populations may also explain the general immunodominance of this response over other populations and its importance in bearing the brunt of the immunological impact of HIV replication. This is in keeping with recent work suggesting a correlation between functional avidity and the magnitude of CD8+
T cell responses in HIV-1 infection (38
), as well as with studies showing that immunodominant clonotypes have higher avidities compared with subdominant clonotypes in chronic CMV and EBV infections (39
). Thus, among the diversity of HIV-specific CD8+
T cells, it seems that those clonotypes that exhibit high levels of functional avidity will become immunodominant, presumably through competitive effects, and display superior functionality, thereby orchestrating more effective control of HIV replication.
However, high functional avidity also implies a stronger exposure of the cognate CD8+
T cells to activation signals derived from a given concentration of antigen. This effect will result in proportionately greater levels of CD8+
T cell stimulation and proliferation, thereby causing the increased rate of clonal turnover within B27-KK10–specific CD8+
T cell populations. However, CD8+
T cells, like any other immune cells, have a limited replicative lifespan in vivo (41
). This implies that they can be driven toward replicative senescence over time with sustained activation and proliferation, as suggested both by our observations with B27-KK10–specific CD8+
T cell populations at the clonal level and in other persistent viral infections (39
). Mathematical modeling has highlighted the potential role for cellular senescence in shaping the CD8+
T cell response to a persistent infection (42
). Similarly, the incapacity of immunodominant HIV-specific CD8+
T cell populations to proliferate has been associated with progressive HIV disease (43
). Our findings are also in line with work in mouse models showing that persisting lymphocytic choriomeningitis infection can lead to the disappearance of clonotypes with a near complete switch in the epitope-specific repertoire between acute and chronic infection (44
), and that high dose antigen can be accompanied by the emergence of low avidity clonotypes (45
). Collectively, this suggests that functional avidity can drive clonal turnover and senescence, and entails the need for continuous renewal of high avidity T cell clonotypes to maintain optimal control of HIV replication.
Although the case of patient 02.011 did not involve clonal senescence but loss of antigenic stimulation caused by viral escape, it offers an interesting illustration of the disastrous consequences that the irreversible disappearance of dominant CD8+
T cell clonotypes can have on the control of HIV replication. Constant output from the pool of naive T cell precursors is therefore a prerequisite to maintain an effective control of viral replication, to either replace important but senescent clonotypes or adapt the response to viral variations when possible. Recent data from the mouse models of persistent viral infection indicate that new naive cells are continuously being recruited during the chronic phase and that lack of thymic output results in the decline of virus-specific CD8+
T cells (46
). However, as HIV-infected individuals reach advanced stages of the infection, their thymic output and the number of naive cells decline considerably (47
), which will affect their ability to renew senescent clones with high turnover, resulting in suboptimal control of HIV replication despite the presence of subdominant HIV-specific CD8+
T cell populations with lower rates of clonal turnover. Our work emphasizes the importance of studying clonal dynamics in the interpretation of cellular immune data. Although much work has concentrated on finding immunologic correlates of protection through the assessment of phenotypic markers on HIV-specific CD8+
T cells at a given time point, our study indicates that this is unlikely to have any predictive value regarding the outcome for viral control, because new clonotypes with different characteristics may be generated thereafter. The capacity to renew important clonotypes will be fundamental for the outcome of HIV-1 infection.
The identification of functional avidity as a central feature of the protective nature and life span of CD8+ T cells raises a question as to which factors determine the recruitment of high avidity clonotypes into any given antigen-specific CD8+ T cell population. Many variables, such as TCR avidity and density, CD8 coreceptor-mediated effects, membrane flexibility, and molecular topography can contribute to the overall functional avidity of a CD8+ T cell population. The study of B27-KK10–specific CD8+ T cell clonotypes described in this paper hints at another potential determinant. Specifically, we observed a high degree of clonotypic diversity at the level of primary sequence across the CDR loops when compared, for example, with A2-SL9– or B7-GL9–specific CD8+ T cell populations. One could speculate that a high degree of TCR diversity within antigen-experienced T cells provides the foundation to establish high avidity populations, as clonotypes with highly avid TCRs may arise more easily from such a large source of possibilities. Further work is necessary to clarify the potential relationship between functional avidity and TCR diversity, and to reveal new insights into the genetic, biophysical, and structural bases of these effects.
In conclusion, the comprehensive analysis of a unique CD8+ T cell population, known to mediate superior control of HIV-1 replication, enabled the identification of distinct attributes that characterize the protective immunity mediated by these cells. These findings represent a substantial step forward in our understanding of several allied immune phenomena and illuminate the biological basis of CD8+ T cell efficacy in HIV infection with associated implications for the evaluation of potential vaccine immunogens.