Elite and posttreatment controllers, two facets of HIV control

Purpose of review

The quest for HIV-1 cure could take advantage of the study of rare individuals that control viral replication spontaneously (elite controllers) or after an initial course of antiretroviral therapy (posttreatment controllers, PTCs). In this review, we will compare back-to-back the immunological and virological features underlying viral suppression in elite controllers and PTCs, and explore their possible contributions to the HIV-1 cure research.

Recent findings

HIV-1 control in elite controllers shows hallmarks of an effective antiviral response, favored by genetic background and possibly associated to residual immune activation. The immune pressure in elite controllers might select against actively transcribing intact proviruses, allowing the persistence of a small and poorly inducible reservoir. Evidence on PTCs is less abundant but preliminary data suggest that antiviral immune responses may be less pronounced. Therefore, these patients may rely on distinct mechanisms, not completely elucidated to date, suppressing HIV-1 transcription and replication.

Summary

PTCs and elite controllers may control HIV replication using distinct pathways, the elucidation of which may contribute to design future interventional strategies aiming to achieve a functional cure.

INTRODUCTION

Soon after the discovery of HIV-1 as the cause of AIDS, a rare group of people living with HIV (PLWH) who did not seem to progress towards overt immunosuppression was identified. These rare patients were initially called ‘long-term nonprogressors’ and defined by their ability to maintain high CD4⁺ T-cell counts over many years in the absence of antiretroviral therapy (ART) [1]. Over the past decades, a much smaller fraction of ‘nonprogressor’ was found to be able to fully suppress plasmatic HIV-1 levels, and called ‘elite controllers’ [2].

Not long after ART became widely available, a third group of individuals called posttreatment controllers (PTCs) was identified [3–5]. In contrast to most PLWH who experience viral rebound post-ART cessation [6], PTCs are able to maintain HIV-1 suppression for months or years despite discontinuing antiretroviral therapy.

To date, elite controllers and PTCs are considered to represent two facets of HIV-1 control. These two groups of individuals may suppress HIV-1 replication using distinct, nonmutually exclusive mechanisms, the characterization of which remains to be fully elucidated. In the present review, we propose to summarize some of the key observations from clinical or basic science research on the possible mechanisms involved in the viral control achieved in elite controllers and PTCs. 

Box 1.

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CLINICAL AND IMMUNOVIROLOGICAL FEATURES OF ELITE CONTROLLERS

Elite controllers are a rare group of PLWH who, in the absence of ART, suppress plasmatic viral load under the limit of detection of standard assays (typically <50 copies/ml). The proportion of elite controllers is less than 1% of PLWH in multiple cohorts [7–10]; interestingly, female gender [10] and black ethnicity [10–12] were independently associated to an increased proportion of natural controllers, whereas age at HIV-1 diagnosis do not seem to be associated with HIV-1 control [13]. Longitudinal studies suggested that elite controllers may suppress HIV-1 after a variable amount of time from seroconversion (median 16.7 months, 22% of controllers from the first HIV-RNA determination) [14] but tend to show a lower zenith viral load and a higher CD4 T-cell count during primary HIV-1 infection (PHI) when compared with typical progressors [15]. Duration of viral control appears to be relatively long in elite controllers, with more than 70% of individuals still suppressed after a median of 6 years of follow-up [16]. Nevertheless, recent data suggested that on the longer period, a consistent proportion of elite controllers may show signs of disease progression [17], immune activation [18,19], and, possibly, adverse clinical outcomes [20,21]. However, whether ART is beneficial in these PLWH is still debated [22,23], and a case-by-case evaluation is suggested by current guidelines [24–26].

The exceptional clinical and virological characteristic of these PLWH led to an extensive research activity focused on determining the features associated with natural HIV-1 control. Initial observations underscored the presence of deletions in viral genes (nef[27,28] or more recently, vpr[29]) or reduced in-vitro replication capacity [30,31] of HIV-1 isolates recovered from some elite controllers, arguing towards a potential lack of viral fitness in these individuals [30]. However, additional studies showed that replication-competent HIV-1 can be isolated from elite controllers [32], and full genome sequencing of proviruses from these patients revealed genetically intact HIV-DNA [33^▪▪], suggesting that viral factors alone could not entirely explain the natural control of HIV-1 infection. In addition, low-level viremia [34], viral evolution [35,36] and persistent viral replication in lymphoid tissues [37,38] could be observed in either elite controllers or macaque models of elite control, suggesting that residual viral production, mostly occurring in lymphoid tissues, may still be detected in natural controllers despite an undetectable plasmatic viral load.

In parallel, increasing number of genetic and functional studies have been performed to uncover potential immunological mechanisms associated with elite controllers. Initial genetic observations identified certain HLA variants, collectively referred to as ‘protective HLAs’, as over-represented in elite controllers [39–43]. Mechanistic analyses proposed that these ‘protective’ HLAs were more efficient in presenting conserved [42] and highly networked [44^▪] HIV-1 epitopes to CD8 T cells, suggesting that HLA class-I-restricted CD8 T-cell responses represented one of the crucial immunological mechanisms by which elite controllers control HIV-1 replication.

However, other parameters may also play a role, and viral control in elite controllers might not totally rely on ‘protective HLA’ alleles. Indeed, the fact that PLWH harboring ‘protective’ HLAs do not necessarily control HIV-1 replication and that elite controllers may lose HIV-1 control following superinfection [45–47] suggests that the genetic milieu alone might not be sufficient to achieve viral suppression. Moreover, clinical evidences revealed that up to one-third of elite controllers do not express the so-called ‘protective’ HLAs [48–51] and may still display anti-HIV CD8 T-cell responses [48,49] suggesting that an efficient antiviral response might be achieved also in absence of ‘protective’ HLA alleles. Finally, the evidence that some elite controllers, harbouring or not ‘protective’ HLAs, do not show consistent CD8 T-cell-mediated anti-HIV responses while controlling viral replication suggested that class-I HLA-restricted CD8 T-cell-mediated immunity may not be the only immunological factor associated to the control of HIV-1 replication observed in elite controllers [50,52].

Therefore, a huge research effort has also been invested to further dissect immunological determinants associated to natural control of HIV-1. In particular, several studies observed superior attributes of HIV-specific CD8 T cells in elite controllers for what concern in frequency [53], breadth [48], polyfunctionality [49,54,55], stemness potential [56,57] and trafficking to lymphoid tissues [58] when compared with those of chronic progressors (reviewed in detail in the Rutishauer and Trautmann study). More recently, HIV-specific CD4 T-cell-mediated responses were also characterized in these patients, and were found to be quantitatively and qualitatively superior to those of natural progressors in terms of proliferative potential [59], polyfunctionality [60], T-follicular helper functions [61,62] and cytotoxicity [63,64]. Finally, multiple groups investigated the features of humoral and innate immunity in elite controllers, identifying peculiarities in terms of IgG-mediated seroneutralization activity [65], antibody functional profile [66], natural killer (NK) responses [67–69], and antigen-presenting cells functional profile [70,71]. Of note, the contribution of the innate components in the natural control of HIV is reviewed in details in the Calvet and Martin-Gayo study.

To further explore the mechanisms associated with the control of viral replication, several authors exploited the macaque model of natural viral control, and observed that depletion of CD8 T cells was sufficient to trigger SHIV rebound, demonstrating that CD8 T-cell-mediated antiviral immunity is most likely crucial to maintain viral suppression [72]. This view is also supported from the observation that elite controllers eventually losing the control on viral replication preferentially show an impairment in HIV-1-specific CD8 T cell compared with elite controllers persistently suppressing viral load [73^▪▪,74,75]. However, supplemental longitudinal studies are needed to comprehensively characterize the balance between viral replication dynamics and specific antiviral immune features.

To summarize, elite controllers represent a heterogeneous group of PLWH who display a long-lasting natural control of HIV-1 infection, associated in most of the individuals to efficient CD8 T-cell responses arising from a favorable genetic background.

CLINICAL AND IMMUNOVIROLOGICAL FEATURES OF POSTTREATMENT CONTROLLERS

PTCs represent a small subset of HIV-infected individuals able to maintain control of HIV-1 replication after stopping ART. Initial case-reports identifying rare patients maintaining viral suppression despite ART withdrawal [3,4] were followed by several cohort studies proposing that PTCs may represent a novel model of ART-free viral suppression. After the initial description of 14 PTCs from the VISCONTI study [5], the CASCADE collaboration [76], the SPARTAC trial [77,78] and, more recently, the CHAMP cohort [79] provided a deeper characterization of these individuals.

The majority of identified PTCs were male, even though this likely reflect the demographic characteristics of the original cohorts [5,76,78,79]; notably, whether PTCs are overrepresented amongst HIV-infected female individuals and individuals of non-Caucasian ethnicity remains to be further elucidated. The observed frequency of PTCs ranged between 2.4 and 15.6% of patients undergoing treatment interruption, and exceed the expected frequency of elite controllers (<1%) despite the heterogeneity in the inclusion criteria [80]. Compared with elite controllers, PTCs rarely harbored the aforementioned ‘protective’ HLAs, displayed higher viral loads and lower CD4⁺ T-cell counts during PHI [5,77], and showed a more labile persistence of viral control (median 89 weeks, 22% of patients still suppressed after 5 years) [5,79].

Collectively, these studies highlighted that PTCs were probably distinct from elite controllers, and would likely not have been able to naturally control viral replication without ART. The impact of ART in promoting HIV-1 control after treatment discontinuation emerges also from the observation that early-treated PLWH have higher chances of achieving the PTC status when compared with patients treated later during the disease [79]. Indeed, ART initiation during acute infection might favor the development of viral control reducing the size of latent reservoir [81] and preserving immune functions [82]. However, multiple reports described HIV-1 rebound in patients treated days postinfection [83–86], demonstrating that even extremely early ART alone is not sufficient to prevent the establishment of a functional reservoir, and suggesting that ‘too-early’ therapy might paradoxically dampen the development of an effective and appropriate immune response. Moreover, PTCs treated during chronic infection were also identified [87], highlighting that the unknown functional modifications promoting viral control after ART initiation may arise also in patients with more advanced disease. Therefore, additional factors, not exclusively related to timing of ART initiation, are needed to prevent viral rebound. Given the importance of immune response in the natural control of HIV-1 infection, multiple studies investigated whether specific immunological features might be associated to viral suppression in PTCs.

Initial evidence from the VISCONTI cohort suggested that HIV-specific CD8 T cells from PTCs displayed a lower activation status, reduced frequency and reduced suppression of autologous CD4 T-cell infection when compared with those from elite controllers [5]. Nevertheless, additional data suggested that PTCs may be a heterogeneous population in terms of antiviral cellular immunity, as patients harboring effective HIV-specific CD8 T-cell mediated responses could be observed [88,89^▪,90]. More recently, this heterogeneity was observed also for what concern humoral responses, with some PTCs showing effective antibody-mediated antiviral functions [89^▪,91^▪▪], whereas others not displaying any functional HIV-specific serological response [91^▪▪] and, in extreme cases, showing seroreversion [92]. The drivers behind the variable antiviral immune responses detected in PTCs remain unclear to date, but preliminary evidences suggested that heterogeneous in-vivo exposure to viral antigens might be involved in shaping the immunological profile [91^▪▪].

Of note, these observations arose from individuals analyzed during the phase of virological control, and may thus not reflect the processes mechanistically associated to the suppression of HIV-1 rebound. In this view, multiple studies aimed to identify biomarkers able to predict HIV-1 suppression before ATI. Given the paucity of PTCs detected to date, most of these studies focused on time-to-viral-rebound as a surrogate endpoint of acquired HIV-1 control, and explored biomarkers associated with this outcome. Several virological [93,94,95^▪], immunological [96,97] and metabolic biomarkers [98] were proposed; however, to date, none of these markers was clearly validated, and ATI trials including patients recruited on the presence of one [99,100] or more [101] of these markers have led to inconclusive results. In this view, recently described animal models of PTCs may provide valuable insights in dissecting the processes associated to HIV-1 suppression and in validating new biomarkers to select patients to be included in ATI trials [102].

Taken together, PTCs represent a group of individuals that do not share the same attributes than elite controllers in terms of genetic background, viral dynamics during PHI and duration of control on viral replication. Initial studies suggested that this population might have peculiar immunological features, with a reduced preponderance of cellular and humoral antiviral responses. However, a marked heterogeneity emerged among these patients, suggesting that multiple mechanisms may be involved in achieving and maintaining viral suppression.

HIV-1 VIRAL RESERVOIR IN ELITE CONTROLLERS AND POSTTREATMENT CONTROLLERS

In addition to the characterization of clinical and immunological features of elite controllers and PTCs, many studies focused on elucidating multiple aspects of the HIV-1 reservoir in these groups of patients.

HIV-1 reservoir in elite controllers is significantly smaller in terms of total [103,104] and genetically intact proviruses [33^▪▪,105] when compared with chronic progressors and to ART-treated patients, despite being subjected to the same processes of clonal expansion detected in the most of PLWH [33^▪▪,38,90]. In addition, the combined study of proviral genetic intactness and integration sites highlighted that intact, but not defective, HIV-1 genomes in elite controllers were predominantly located in DNA regions significantly distant from actively transcribing chromatin, suggesting that selection processes, most likely mediated by the immune system, eliminated proviruses able to produce HIV-1 proteins and favorably positioned for transcription [33^▪▪]. Of note, both intact and defective proviruses from elite controllers showed a lower frequency of mutations associated with ongoing immune pressure when compared with ART-treated patients [106^▪▪], suggesting that the elimination of actively transcribing proviruses in elite controllers may be followed by a phase in which HIV-1 replication reaches a ‘dead-end’ of poorly inducible, transcriptionally silent proviruses, unable to further escape host immunity. Coherently, previous observations showed that elite controllers mostly have a poorly inducible reservoir [107]. Nevertheless, as mentioned above, viral evolution can still be detected in a subset of elite controllers despite a suppressed plasmatic viral load, suggesting that additional research efforts are needed to elucidate whether heterogeneity might be detected in reservoir composition and dynamics among natural controllers.

For what concern PTCs, the study of reservoir characteristics is less extensive; however, multiple reports [5,108] suggested that total HIV-DNA in peripheral blood is significantly lower in these patients when compared with ART-treated individuals. Of note, total intact HIV-DNA was also significantly lower in PTCs but patients with intact reservoir as high as 40% of the total were still able to control HIV-1 rebound [108]. Moreover, integration site analysis confirmed that HIV-1 reservoir from PTCs, including those harboring replication-competent proviruses, can expand in large clones despite persistent control of viral replication [90,108]. Interestingly, the observed reservoir size in PTCs did not reach the extremely low levels observed in currently reported examples of HIV-1 cure [109–111], and is similar to the one observed in very-early-treated patients not able to control viral replication [86]. Therefore, additional factors, independent of total or intact reservoir size assessed in blood, may be associated to virological control in PTCs. The VISCONTI study analyzed the cellular distribution of HIV-1-infected cells in blood, and observed an enrichment among transitional-memory CD4 T cells in PTCs, suggesting that these patients may differ in terms of cells constituting HIV-1 reservoir [5]. In addition, indirect evidence, coming from the association between low cell-associated HIV-RNA levels and longer time-to-viral-rebound [93,95^▪] suggested that viral reservoir in PTCs may show limited transcriptional activity (reviewed more extensively in the Pasternak et al. study). Of note, whether this restricted HIV-1 expression is the consequence of selection processes eliminating proviruses favorably positioned for transcription, in analogy with what observed in elite controllers, or of a reservoir ‘silencing’ caused by other factors is currently unknown. Recently, the integration site landscape of HIV-1 reservoir from two PTCs was compared to that of two progressors, and no particular distribution suggestive of an active selection was identified [89^▪]. On the other side, additional longitudinal observations analyzing simultaneously integration sites and intactness of proviruses suggested that selection against favorably positioned intact proviruses might occur also in PTCs [112]. However, large-sized studies comprehensively characterizing the HIV-1 reservoir in PTCs, especially focusing on cellular and tissue distribution, IS landscape and intactness of proviruses are missing, and would provide useful insights in deciphering the viral dynamics occurring in these patients.

COMPARING HIV-1 CONTROL IN ELITE CONTROLLERS AND POSTTREATMENT CONTROLLERS

Although the mechanisms responsible for viral control in elite controllers are still not fully understood, a number of evidences have demonstrated that these individuals are characterized by a strong immune response against HIV-1, with a clear involvement of an effective CD8 T-cell-mediated immunity. The enhanced immune response detected in elite controllers may lead to HIV-1 control by clearing HIV-infected cells and selecting for those that harbor HIV-DNA in less transcriptionally favorable positions, increasing the barrier to HIV-1 reactivation. Thus, the ‘locked’, poorly inducible HIV-1 reservoir observed in elite controllers might be the consequence of the peculiar immune response detected in these individuals.

On the other hand, PTCs is a more recently described group, which has not been extensively characterized from an immunological and virological standpoint. Initial evidence pointed out that, compared with elite controllers, these patients may rather display modest HIV-specific CD8 T-cell-mediated immune responses during viral control, and may thus take advantage of additional mechanisms to suppress HIV-1 replication, possibly involving currently unknown immunological mediators able to eliminate HIV-1-infected cells. The elucidation of these factors may be pivotal to design further cure strategies. However, the consistent evidence showing that PTCs would probably not have been able to control HIV-1 without an initial course of antiretroviral therapy suggests that the modifications occurring after ART introduction might be involved per se in promoting viral suppression. ART initiation is associated to an extensive immunological remodeling [113–115], whose effect might also favor HIV-1 latency instauration [116,117] by promoting effector-to-memory transition of infected cells [118]. PTCs may, thus represent the extreme end of this process: the paucity of antiviral immune responses detected ex vivo in these patients might be the consequence of a poor expression of HIV-1 in vivo, because of an excessive ‘silencing’ of HIV-1 transcription in infected cells occurring after ART initiation. Interestingly, limited levels of initiated transcripts were found prior to ATI in PTCs when compared with noncontrollers, even though these observations are not normalized on the levels of intact HIV-DNA [119]. Additional longitudinal studies, possibly involving new techniques able to simultaneously identify integration sites, intactness of proviral DNA and transcriptional activity [120^▪▪] will help to elucidate whether or not immune selection mechanisms occur in these patients, and how the interplay between HIV-1 expression and antiviral response leads to acquired HIV-1 control (Fig. 1).

FIGURE 1.

Characteristics of elite controllers, posttreatment controllers and antiretroviral therapy-treated patients. Top panel: elite controllers show a strong, effective immune response against HIV-1, primarily based on CD8 T cells. PTCs, on the contrary, may display a less pronounced antiviral immunity, even though great heterogeneity has been detected in the immunological features of these individuals. Middle panel: both elite controllers and PTCs harbor a significantly lower frequency of HIV-1-infected cells in blood when compared with ART-treated patients. Lower panel: HIV-1 reservoir in elite controllers is dominated by poorly inducible proviruses showing low transcriptional activity. Features of HIV-1 reservoir in PTCs are largely unknown but based on preliminary data and on paucity of antiviral immune responses detected ex vivo in these patients, we can speculate that a reduced transcriptional activity of intact reservoirs might be associated to natural control of HIV-1 replication in this group. ART, antiretroviral therapy; PTCs, posttreatment controllers.

CONCLUSION

The development of effective cure strategies could exploit the immunovirological mechanisms naturally occurring in elite controllers and/or PTCs, which may represent two complementary models of ART-free HIV-1 remission. The thorough investigation and comparison of these exceptional individuals may open previously undisclosed possibilities to identify new pathways associated with HIV-1 suppression, and may elucidate the mechanisms needed to develop novel intervention strategies aiming at achieving a cure for HIV-1.

Acknowledgements

None.

Financial support and sponsorship

This work was supported by Swiss National Science Foundation Grant 320030_200912 and by Freedom Forever Association to M.P. and by an educational grant from FONDATION MACHAON to R.B.

Conflicts of interest

There are no conflicts of interest.

REFERENCES AND RECOMMENDED READING

Papers of particular interest, published within the annual period of review, have been highlighted as:

• ▪ of special interest

• ▪▪ of outstanding interest