Combination antiretroviral treatment (CART) inhibits the reproduction of HIV particles and reverses disease progression , but several factors call for alternative treatment strategies: high cost, persistent low-grade viral replication, incomplete normalization of T-cell functions, drug-related side-effects, and emerging primary HIV infections with multidrug-resistant virus.
Therapeutic immunization aims to attenuate disease progression by modulating HIV-specific immune responses, but the correlates of effective immunity remain to be defined [1–4]. So far, most immune response-inducing candidates have neither been linked with clinical benefit nor with the sustained control of viral replication [2,4,5]. However, a recent study by Lévy et al.  suggested that immunization in combination with CART and IL-2 improved HIV-specific immunity and lowered the viral set-point after stopping CART. Furthermore, Lu et al.  showed that autologous monocyte-derived dendritic cells (DC), pulsed ex vivo with inactivated autologous virus, induced a prolonged reduction of HIV RNA in a substantial number of patients, highlighting the potential of immunotherapy strategies targeting DC.
We have recently targeted DC in vivo with four modified HIV p24-like peptides (Vacc-4x) using the DC-stimulating granulocyte macrophage-colony stimulating factor as a local adjuvant in an open-label, randomized, dose-finding prospective phase II clinical trial with 40 patients on CART. Cellular immune responses were enhanced in 90% of the patients in a dose-related manner . Immunizations were followed by a 4-week treatment interruption to rule out side-effects and allow exposure to autologous virus, and CART was resumed for 8 weeks until week 38. Patients then interrupted CART for 14 more weeks, completing the original trial at week 52. The magnitude of the preceding Vacc-4x responses, irrespective of the Vacc-4x dose, was related to lower HIV-RNA set-points and lower CD4 T-cell declines at week 52 without any other confounding factors . Although these data need to be confirmed, a therapeutic HIV vaccine should fulfil at least two demands that we wanted to address in this follow-up study, namely the longevity of vaccine-related cellular immune responses and clinical efficacy.
All of the 37 eligible Vacc-4x patients were evaluated clinically with measurements of HIV RNA and CD4 T lymphocyte counts 109 (108–114) weeks (median, interquartile range) after enrolment in the Vacc-4x trial, i.e. 70 weeks after CART was stopped. Vacc-4x and HIV-specific T-cell responses in vitro and in vivo [delayed-type hypersensitivity (DTH) tests] were performed in the 32 patients who were able to participate within the assay period. One patient did not receive a DTH test because of a previous allergic reaction . The study was conducted with informed patient consent and was approved by the Norwegian Medicines Control Authority and the Regional Ethics Committee.
After completion of the original study at week 52, a resumption of CART was clinically indicated in two of the patients (5%). All participants were subsequently followed post-study every third month by their regular consultants at our policlinic. CART was resumed without knowledge of the Vacc-4x responses, either according to medical advice from the patient's physician based on current treatment guidelines [10,11] (n = 10), or on the patient's personal initiative to ensure better protection for partners (n = 2). At follow-up, 23 of the patients (62%) were still without CART with CD4 cell counts at 410 cells/μl (300–460) and HIV RNA at 82 000 copies/ml (31 000–200 000), whereas 14 (38%) had restarted effective CART after 41 weeks (22–49), with CD4 cell counts of 245 cells/μl (208–343) increasing to 375 cells/μl (325–515) at follow-up. One Vacc-4x low-responder (DTH < 25th percentile) died from cryptococcal meningitis after a diagnostic delay, approximately 11 months after CART was stopped, and death was judged to have no relation to Vacc-4x.
One of the aims of HIV immunotherapy is to alleviate the drug burden; trial follow-ups will therefore include CART-free periods. However, several studies have reported a drop in HIV-specific T-cell clones from blood during longer interruptions of CART [12,13]. We have recently questioned whether blood is the right test compartment in this particular situation, and pointed out the complexity of evaluating the durability of stimulated HIV-specific responses . In this follow-up study, we found that the T cell-specific responses to Vacc-4x persisted both quantitatively and qualitatively: 26 of the retested patients (84%) still had positive DTH reactions (n = 31), and 25 out of 32 (78%) had detectable Vacc-4x-specific T-cell proliferative responses in the peripheral blood mononuclear cells, no different from after completed immunization (90 and 80%, respectively) . Quantitatively, the Vacc-4x-specific T-cell proliferation and DTH reactions correlated (R = 0.65, P < 0.0001), and were unchanged in magnitude compared with study weeks 38 (CART stop) and 52 (end of study) (Wilcoxon paired tests, Fig. 1). This was true, independent of the Vacc-4x immunization dosages and whether patients had resumed CART or not (data not shown). However, the previously described Vacc-4x high dose advantage in terms of Vacc-4x-specific immune responses [8,9] persisted throughout (P < 0.04, Mann–Whitney; Fig. 1).
As a result of the paucity of HIV-related complications, we  and others  have used pseudomarkers for clinical progression in HIV as efficacy parameters, such as CD4 lymphocyte counts and HIV RNA. However, an analysis of these pseudomarkers was difficult at follow-up because a fraction of patients in every analytical subgroup had resumed CART for various reasons and at different timepoints. Clinical outcomes in relation to Vacc-4x immune responses were therefore evaluated by the time-dependent resumption of CART based on standard clinical care. When patients were stratified according to their post-immunization DTH responses at study week 38 before the last CART-stop, analogous to our previous study , DTH high-responders having DTH greater than the median remained CART-free longer than 2 years after enrolment, in a Kaplan–Meier analysis (P = 0.01, Cox's F-test; Fig. 1).
In conclusion, Vacc-4x peptides induced qualitatively and quantitatively durable cellular immune responses, even in the presence of HIV viremia. The magnitude of early Vacc-4x responses was the only discriminating factor as to whether the resumption of CART was indicated in clinical practice, although a placebo-controlled trial is required to prove a causative relationship. The fact that most of the patients were still CART-free approximately 1.5 years after completing immunization confirmed the safety of Vacc-4x. These observations altogether encourage further exploration of both Vacc-4x and other HIV peptide-based immunization regimens targeting DC.
The authors would like to thank M. Sannes and M. Jørgensen for excellent assistance.
Sponsorship: This work was supported by a grant from the Norwegian Parliament and by Global Alliance for Vaccines and Immunization (GAVI).
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