Implications of the SILCAAT and ESPRIT trials and the future for HIV immunotherapy.BACKGROUND: RATIONALE FOR THE SILCAAT AND ESPRIT TRIALS
The results of the SILCAAT and ESPRIT trials of interleukin-2 (IL-2), two of the largest and most costly trials of a potential HIV therapy, were presented in 2009. These studies involved over 5800 volunteers with HIV from 25 countries, and cost an estimated $100 million. In order to best understand the reasons why such an effort was undertaken, we shall briefly summarise the rationale that led to these studies, recap the trials themselves, and discuss the potential for HIV immunotherapy in the post-SILCAAT/ESPRIT era.
The first abnormality of cytokine function described in patients with HIV infection was a reduced production of IL-2 . Our understanding of the immunopathogenesis of HIV is still incomplete but it is evident that there is widespread immune dysregulation affecting not just T cells, but also B cells, NK cells and macrophages. The increased cell activation in HIV infection, in part controlled by CD4+CD25+ T regulatory cells, also helps drive infection of cells by HIV, and cell death . The common chain cytokines, in particular IL-2, IL-7, IL-12 and IL-15, help govern the process of T cell proliferation, function and cell death, and also HIV-specific immune responses. The production of these cytokines is markedly perturbed in HIV infection, with reductions in IL-2, IL-12 and IL-15, and increases in IL-7.
It was soon established that the functional deficit of IL-2 occurred even in the majority ofasymptomatic patients with HIV, and that the degree of reduced IL-2 production predicted the subsequent decline in CD4+ cell counts [3,4], and also predicted subsequent HIV-associated morbidity and mortality . Conversely, it has recently been shown that HIV controllers express higher levels of certain cytokines, including IL-2, than those who progress normally . These findings prompted investigation into the effect of IL-2 administration to patients on antiretroviral therapy (ART), and found that there was a marked rise in CD4 counts compared to the use of ART alone [7-9]. Other studies more closely defined the minimal likely effective dose, which was important in order to abrogate the considerable side effects of IL-2 therapy [10,11].
In the pre-HAART era, the predominant clinical phenotype was AIDS-associated disease and death in those with a CD4 count below 200 cell [mm.sup.2]. When HAART was introduced, it was thought that if treatment could raise the absolute CD4 count above this figure, or prevent it falling below 200, and help normalise the percentages, then almost all excess morbidity and mortality in HIV-infected individuals might be avoided. It was only after SILCAAT and ESPRIT were begun that strong evidence emerged for commencing HAART at higher CD4 counts.
THE SILCAAT AND ESPRIT TRIALS
Hence the stage was set for large (Phase 3) studies to determine the clinical utility of IL-2. The initial sponsor of SILCAAT, Chiron, failed to gain accelerated approval from the Food and Drug Administration (USA) for their subcutaneous recombinant human IL-2 product, Proleukin, on the basis of CD4 cell count changes alone. The NIAID took over the running of the trials. Participants were assigned at random to receive either HAART alone or HAART plus subcutaneous Proleukin over several 5-day cycles. The ESPRIT study enrolled people with early-stage infection (CD4+ T cell counts at or above 300 cells/[mm.sup.3]), and the SILCAAT study enrolled volunteers with later-stage HIV infection (CD4+ T cell counts between 50 and 299 cells/[mm.sup.3]). The primary endpoint of both trials was to compare the effects of IL-2 with no IL-2 on disease progression and death in HIV-infected individuals on combination antiretroviral therapy. These two trials confirmed that IL-2 infusions would increase the CD4 cell count number, and over the trial period of 7-8 years, the CD4 cell count rise was higher in the group receiving IL-2 compared to the controls (median rise of 53 and 159 cells/[mm.sup.3] in the SILCAAT and ESPRIT groups, respectively). In both the SILCAAT and ESPRIT studies, however, the incidence of HIV-associated opportunistic diseases and death was not significantly different for participants in either treatment arm .
WHAT ARE THE IMPLICATIONS OF THE LACK OF EFFICACY OF IL-2 IN SILCAAT AND ESPRIT?
Twenty-eight years after the recognition of AIDS, the SILCAAT and ESPRIT trials have made us question the value of CD4 cell counts as a universally applicable biomarker. These studies have shown that amelioration of depressed CD4 cell levels does not always confer added benefit. It is unclear why IL-2 did not improve outcomes in SILCAAT and ESPRIT. It may have been that the expanded T cell repertoire comprised poorly functioning cells, and many consider this to be the likely explanation. IL-2 produces a polyclonal expansion of the existing pool of CD4+ cells, mainly increasing the numbers of naive or central memory phenotypes. In comparison, HAART also leads to a much larger expansion of pre-existing effector memory cells. It is the latter, rather than the former, that are better placed to contribute to HIV-antigen-specific recognition and killing. Furthermore, there is less expansion of CD25 + CD4+ cells, the eponymous T regulatory cells, than occurs due to HAART treatment. These T regulatory cells play an important role in suppression of immune activation generally, and especially in the HIV-infected host. Another consideration is that the toxicity of IL-2 might have mitigated against the benefits of using it. The difference in the toxicity profile between the intervention and the control group was, as expected, quite marked. IL-2 recipients in both trials exhibited a greater number of serious clinical events already known to be associated with IL-2, including cardiovascular disorders, injection site reactions and such psychiatric disorders as depression and suicidal behaviour. Of particular note in the ESPRIT trial was that participants in the IL-2 group had significantly more severe or life-threatening events than participants in the control group. There were 466 such events in the IL-2 group and 383 in the control group (hazard ratio 1.23, P= 0.003). It is also worth noting that a combined analysis of smaller studies, perfomed using single or dual antiretroviral therapy, suggested a trend towards less morbidity and mortality . A more recent study using IL-2 combined with HAART in patients with advanced HIV also showed this same trend . In all of these studies, however, it is likely that the small size and lack of power led to the apparent trends in clinical benefit that were not found in ESPRIT and SILCAAT. It emphasises the value of performing large, well-powered studies, rather than a series of smaller trials.
It should be remembered that the ESPRIT and SILCAAT studies were planned long before the SMART trial demonstrated the hazards of stopping HAART. In the SMART trial, much of the excess morbidity was associated with markers of chronic inflammation. It is interesting to speculate whether IL-2 might have promoted this chronic inflammatory process, which is thought to drive the ubiquitous non-HIV manifestations of disease that we observe today. Irrespective of the causes of the disappointing results from ESPRIT and SILCAAT, we now need to address the future potential and direction of HIV immunotherapy.
WHERE NOW FOR HIV IMMUNOTHERAPY?
There is a wide range of HIV immunotherapeutic strategies under investigation, and it is beyond the scope of this article to mention them all. Neither would such an exercise prove to be a reliable guide to the future: none of the current candidates have anywhere near as much evidence behind them as did IL-2 before the SILCAAT and ESPRIT trial results became known. Hence we shall highlight some of the main strategies and candidates under investigation.
At the start of the new millennium, there was a wide variety of approaches being planned for therapeutic vaccination against HIV . A number of trials have been conducted, and despite being based on sound rationales, results thus far have proved disappointing. These have included the use of priming with DNA vaccines, followed by boosting with the vaccine carried within a viral vector , and the use of one the most promising candidates along with pulsed IL-2 therapy . Further hope was raised for the outcomes of the adenovirus HIV vaccine construct used in the STEP prophylactic HIV vaccine trial . This vaccine construct stimulated strong and persistent HIV-antigen specific T cell responses in the majority of vaccines. If this potential prophylactic vaccine did not protect against HIV, it was still hoped, therefore, that it would demonstrate a therapeutic action against those who became infected. This did not prove to be the case, and the STEP trial failed on both accounts. This study showed that ELISPOT responses might demonstrate T cell recognition of HIV-specific antigen, but are not useful biomarkers to predict the efficacy of a candidate HIV therapeutic vaccine.
There are several other experimental approaches to immune-based therapy for HIV, and we will highlight two of these. HIV gp120, along with some malignant cells, secrete a compound, acetylgalactoaminidase (nagalase). Nagalase prevents the deglycosylation of vitamin D3 binding protein (Gc protein). Gc protein is involved in the production of macrophage-activating factor (MAF). A pilot study examined the use of Gc protein and MAF in 15 treatment-naive asymptomatic HIV patients. The nagalase levels in these patients rose to within the range for healthy controls, and the volunteers maintained their CD4 cell counts above 500 cells/[mm.sup.3] during 7 years of follow-up.
Another experimental approach examines the role of alloimmunity in HIV. The alloimmune response is the strongest immune response that occurs in humans, and is mediated by presentation of foreign antigens by human leukocyte antigens. There is evidence that this immune response might play a significant role in natural protection against HIV [19-21], and potentially might be used as a therapeutic approach.
Early-phase clinical trials are being conducted on a number of other interesting approaches in the area of HIV immunotherapeutic vaccination. These include dendritic cell immunotherapy intended to enhance the CD8+ T cell response to HIV . There were promising results from a study in non-human primates using infusion of overlapping peptides pulsed onto peripheral blood mononuclear cells ; a Phase 1 trial in humans is currently in progress. Although yet to reach people, pre-clinical trials of adoptive immunotherapy are under way, in which infusions of genetically engineered cells are used, for example genetic modification of the CCR5 receptors on T cells .
There is immunotherapeutic potential in a compound recently licensed as an anti-HIV drug, maraviroc. When maraviroc was added to optimal HAART therapy, there was a significantly greater rise in CD4 cell count compared to a control arm using optimal HAART alone . These CD4 cell elevations have been noted in a meta-analysis comparing CCR5 inhibitor-containing regimens with standard HAART . The mechanism behind these changes remains unclear, but might relate to a decrease in immune activation with the use of CCR5-blocking agents . There is currently interest in intensifying HAART with maraviroc, but the results of ESPRIT and SILCAAT must lend caution on relying upon the CD4 cell count changes alone; the scale of such a study to examine the effect of maraviroc on clinical endpoints is daunting.
The greatest barrier to the development of HIV immunotherapy remains in place: this is the lack of a biomarker for an effective HIV immunotherapy. It was hoped that CD4 cell count rises would have predicted clinical outcomes following the administration of IL-2, but in this instance the historical value of CD4 as a biomarker deserted us. In a similar fashion, ELISPOT responses that signal HIV antigen-specific T cell responses also have failed to translate into efficacy. Now appears the time to rebalance our effort in the quest for immune-based therapeutic or prophylactic approaches against HIV. Key pieces of the puzzle required for the understanding of the immunopathogenesis of HIV need to be found. This will require extensive programmes of work, some of which are under way in the form of large collaborative programmes. This 'return to basics' includes the essential investigation into why some non-human primates, such as the sooty mangabey, become infected with SIV but do not develop disease, despite high levels of replicating SIV . It appears likely that the disease-free species have a particular ability to 'tolerate' SIV, as evidenced by the ratio of TH-17 cells to T regulatory cells, in contradistinction to their diseased cousins who exhibit much more immune activation. Hence, maybe a future approach to HIV immunotherapy should be considering how we might 'dampen down' the immune response to HIV, rather than seeking to 'activate' it.
It is understandable, and often desirable, that speculative clinical trials of human immunotherapy candidates will continue to take place. This activity should not, however, displace the effort to improve our basic understanding of the human immune response to the virus. Only through this means can we design more rational clinical trials in the future.
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BARRY S PETERS (1,2) AND MIRIAM SAMUEL (2)
(1) Kings College London, (2) Guys and St Thomas' Hospital Trust, London
Correspondence to: Barry S Peters, Harrison Wing
St Thomas' Hospital, Westminster Bridge Road
London SE1 7EH, UK