The EuroVacc 02 phase I trial has evaluated the safety and immunogenicity of a prime-boost regimen comprising recombinant DNA and the poxvirus vector NYVAC, both expressing a common immunogen consisting of Env, Gag, Pol, and Nef polypeptide domain from human immunodeficiency virus (HIV)-1 clade C isolate, CN54. 40 volunteers were randomized to receive DNA C or nothing on day 0 and at week 4, followed by NYVAC C at weeks 20 and 24. The primary immunogenicity endpoints were measured at weeks 26 and 28 by the quantification of T cell responses using the interferon γ enzyme-linked immunospot assay. Our results indicate that the DNA C plus NYVAC C vaccine regimen was highly immunogenic, as indicated by the detection of T cell responses in 90% of vaccinees and was superior to responses induced by NYVAC C alone (33% of responders). The vaccine-induced T cell responses were (a) vigorous in the case of the env response (mean 480 spot-forming units/106 mononuclear cells at weeks 26/28), (b) polyfunctional for both CD4 and CD8 T cell responses, (c) broad (the average number of epitopes was 4.2 per responder), and (d) durable (T cell responses were present in 70% of vaccinees at week 72). The vaccine-induced T cell responses were strongest and most frequently directed against Env (91% of vaccines), but smaller responses against Gag-Pol-Nef were also observed in 48% of vaccinees. These results support the development of the poxvirus platform in the HIV vaccine field and the further clinical development of the DNA C plus NYVAC C vaccine regimen.
The control of HIV transmission remains one of the most pressing public health priorities for the 21st century. All past experience suggests that a vaccine will be the only intervention able to control the HIV epidemic. Although many successful antiretroviral drugs have been developed with enormous impact on HIV-associated morbidity and mortality, access to antiretroviral therapy remains limited to only 5% of the total population of HIV-infected subjects in developing countries (1). Even in the face of open access to therapy, HIV transmission continues.
Over the past 10 yr there has been a progressive increase in resources for vaccine research, allowing the generation of several candidate vaccines capable of stimulating anti-HIV immunity (2–4). The ideal HIV vaccine should be safe, thermostable, able to elicit both humoral (antibody) and cellular (both CD4 and CD8 T cells) effector functions, and to induce durable protective immunity (2–5). The goal of the “antibody” vaccines is to induce neutralizing antibodies to provide immunity that prevents or limits infection; so far, these have been recombinant envelope protein vaccines (2–4). One envelope protein vaccine has been tested in a phase III efficacy clinical trial, without any protective effect (2–4). This candidate, as well as other tested envelope protein vaccine candidates, induces antibodies with neutralizing activity against laboratory-adapted strains of HIV-1, but these antibodies are largely ineffective against primary HIV-1 isolates. Strategies to develop high titres of neutralizing antibodies with broad activity against primary HIV-1 isolates remain elusive.
In contrast, relatively significant advances have been made in the development of “T cell” vaccine candidates. T cell vaccines are unlikely to prevent infection but may control HIV replication after infection, leading to attenuation of HIV disease (6, 7). The scientific rationale for developing T cell vaccines is based on several observations, including: (a) the presence of vigorous CD4 and CD8 HIV-1–specific T cell responses in HIV-1–infected subjects that are long-term nonprogressors (8); (b) the in vivo observation in the simian immunodeficiency virus (SIV) model of AIDS in macaques that the depletion of CD8 T cells is associated with rapid loss of control of virus replication (9); and (c) the recent observation that CD4 and CD8 T cell responses endowed with multiple functional capacities, particularly IL-2 secretion and proliferation in addition to typical effector functions (cytotoxic activity and secretion of IFN-γ, TNF-α, and MIP-1β), are associated with better control of virus replication (10–12).
Most T cell vaccines use virus vectors to induce T cell immunity, particularly adenovirus and poxvirus vectors (3, 4, 13). Adenovirus vectors used alone or in combination with plasmid DNA-based vaccines have been shown to induce vigorous T cell responses (14). However, the high seroprevalence to adenoviruses in target populations remains a major issue for adenovirus vectors, even though recent data seem to indicate that vector immunity may be circumvented by higher vaccine doses and/or combination with DNA-based vaccines (3, 4, 6, 13). Vector immunity seems to be a lesser problem for poxvirus vector–based T cell vaccines, due to the diminishing prevalence of a vaccinia-experienced population.
The efficiency of poxvirus vectors in eliciting T cell responses has been variable (15). Limited immunogenicity (17% response) has been shown in a study using recombinant DNA in combination with a modified vaccinia virus Ankara (MVA) expressing Gag protein (consensus of HIV-1 clade A) and several immunodominant CD8 T cell epitopes (16). A more recent study using the same vaccines at a higher dose showed enhanced immunogenicity (17). However, the response rate based on ex vivo functional analysis (IFN-γ ELISPOT and proliferation) remained limited at a 40% response, and the T cell responses induced were exclusively due to CD4 T cells. Similarly, the ex vivo immunogenicity of a canarypox candidate vaccine, ALVAC, currently being tested in combination with an Env protein vaccine in a phase III clinical trial, was in the range of 30% (18). This work describes the immunogenicity of a prime-boost regimen composed of recombinant DNA and the poxvirus vector NYVAC, both expressing common immunogens consisting of Env, Gag, Pol, and Nef proteins of the HIV-1 clade C isolate CN54. We performed a comprehensive functional analysis of the vaccine-induced T cell responses, including quantification of T cell responses by IFN-γ ELISPOT and functional characterization of CD4 and CD8 T cell responses (IFN-γ, IL-2, and TNF-α secretion, proliferation, and degranulation activity) by polychromatic flow cytometry and epitope mapping. Our results indicate that the DNA/NYVAC vaccine combination induced ex vivo T cell responses in 90% of immunized volunteers and that these responses were vigorous, polyfunctional, broad, and durable.
The main objectives of the EuroVacc 02 (EV02) trial were to evaluate the safety and immunogenicity of the prime-boost regimen, DNA C plus NYVAC C, compared with NYVAC C alone. The DNA and the poxvirus vector NYVAC both expressed fused Gag-Pol-Nef and the gp120 subunit of Env of the HIV-1 clade C isolate, CN54 (Fig. 1 A).
The design was open for participants and clinical investigators, without a placebo control, and 40 volunteers were randomized to receive DNA C or nothing on day 0 and at week 4, followed by NYVAC C at weeks 20 and 24 (Fig. 1 B). The participants received 2 × 2-ml injections of DNA C (1.05 mg per ml and a total dose of 4.2 mg) intramuscularly in the right and left vastus lateralis and a 1-ml injection of NYVAC C (107.7 CCID50 per ml) intramuscularly in the deltoid. The primary immunogenicity endpoints were measured at weeks 26 and 28 by the quantification of T cell responses using the IFN-γ ELISPOT assay. The T cell responses were also measured on day 0 and at weeks 5, 20, 24, and 48. Comprehensive analyses of the demographics of the trial population and of the safety of the vaccine regimens have been described (unpublished data and reference 19). The results indicated that both DNA C and NYVAC C candidate vaccines are safe and well tolerated.
Proportion of responders after vaccination with DNA C plus NYVAC C compared with NYVAC C alone
Although 40 subjects entered the EV02 study, only 35 had completed vaccination. 20 subjects were randomized to the DNA C plus NYVAC C group and 15 to the NYVAC C–alone group. As mentioned above, T cell responses were measured at different time points during the 48-wk study using the IFN-γ ELISPOT assay. The primary immunogenicity endpoints were, however, evaluated on the basis of the proportion of subjects with positive vaccine-induced T cell responses at weeks 26 and 28. There was a clear and significant difference (P = 0.003) in the proportion of subjects with positive vaccine–induced T cell responses within the two study groups. The proportion of responders was 90% (18 out of 20) in the DNA C plus NYVAC C group compared with 40% (6 out of 15) in the NYVAC C–alone group (Fig. 2 A).
Based on an intention-to-treat analysis of all 40 participants (assuming that missing equals no response) 83% (19 out of 23) in the DNA C plus NYVAC C group responded compared with 35% (6 out of 17) in the NYVAC C–alone group (difference 47%; 95% confidence interval 20–75%; P = 0.0034). It is important to note that one of the six responders in the NYVAC C–alone group had a detectable response (in the range of 200 spot-forming units [SFU]/106 cells) not only at weeks 26 and 28, but also at weeks 5 and 20 before vaccination. Therefore, although this subject had to be considered positive at weeks 26 and 28, the T cell response observed was clearly nonspecific and was not considered for analyses. On the basis of the above considerations, the proportion of subjects with vaccine-induced specific T cell responses was 33% (5 out of 15) in the group vaccinated with NYVAC C alone. The assessment of vaccine-induced T cell responses at different time points has indicated that the proportion of responders after the DNA C vaccination was low, for instance 10% at week 5 after two vaccinations and 35% at week 20 before NYVAC C boost (Fig. 2 B). Furthermore, the proportion of responders in the DNA C plus NYVAC C group mostly peaked (17 out of 20) at week 24, 4 wk after the first NYVAC C boost, and the proportion of responders was still 80% at week 48, 6 mo after the completion of the vaccination (Fig. 1 B). Only two subjects within the NYVAC C–alone group maintained positive vaccine–induced T cell responses at week 48 (Fig. 2 B).
Magnitude of vaccine-induced T cell responses
Vaccine-induced T cell responses were assessed using the IFN-γ ELISPOT assay after the stimulation of blood mononuclear cells with a panel of 464 peptides (15 mers overlapping by 11 amino acids) grouped in eight pools (50–60 peptides per pool). The peptides encompassed the Env, Gag, Pol, and Nef proteins of HIV-1 and were designed based on the sequence of the immunogens expressed by the DNA and NYVAC that were derived from the CN54 clade C isolate. The magnitude and the distribution of vaccine-induced T cell responses against Env, Gag, Pol, and Nef HIV-1 proteins in all the responders within the two study groups are shown in Fig. 3 A.
Vaccine-induced T cell responses were predominantly directed against Env in the DNA C plus NYVAC C and NYVAC C–alone groups. At weeks 26/28 (primary endpoints), Env-specific responses were observed in 21 out of 23 responders (91%) in both groups, whereas Gag, Pol, and Nef vaccine–induced T cell responses were observed in 11 out of 23 of volunteers (48%). Transient Gag- and Pol-specific T cell responses were found in two additional volunteers (ES16 and EU09) at week 24, and Nef-specific T cell responses were only found in one volunteer (EU07) in the NYVAC C–alone group (Fig. 3 A). The responses against Gag, Pol, and Nef were generally transient and substantially lower in magnitude compared with the Env-specific responses.
At week 26, the median (mean ± SD) Env-specific IFN-γ–secreting T cells was 299 (480 ± 339) SFU/106 cells within the study group vaccinated with DNA C plus NYVAC C compared with 131 (139 ± 69) SFU/106 cells within the group vaccinated with NYVAC C alone (Fig. 3 B). The differences in the magnitude of T cell response between the two groups were significant (P = 0.013). At week 28, the numbers were 246 (324 ± 196) SFU/106 cells for the DNA C plus NYVAC C and 76 (78 ± 24) SFU/106 cells for the NYVAC C–alone group (P = 0.005) (Fig. 3 B). The majority (>80%) of Gag-, Pol-, and Nef-specific responses were only detected at weeks 26 and 28 in the DNA C plus NYVAC C group and were not measurable at week 48 (Fig. 3 C and not depicted). These responses were measurable only at week 26 in the NYVAC C–alone group (Fig. 3 C). The median magnitude of Gag-, Pol-, and Nef-specific T cell responses was ∼100 SFU/106 cells (Fig. 3 C).
In Fig. 3 D, one observes the individual maximum responses at week 26 or 28 (each “step” corresponds to the maximum response measured in each responder) and the percentage of participants per arm with a maximum response up to a certain value. In the NYVAC C–alone group, the maximum response was just above 200 SFU/106 cells, and 40% had a maximum response of 100 or fewer. In the DNA C plus NYVAC C group, the maximum response was almost 1,100 SFU/106 cells, and 33% had a maximum response of >600 SFU/106 cells.
Distribution of the vaccine-induced T cell responses in CD4 and CD8 T cell populations
The distribution of vaccine-induced T cell responses in CD4 and CD8 T cell populations was assessed in 3 out of 5 responders of the NYVAC C–alone group and in 16 out of 18 responders of the DNA C plus NYVAC C group. Only volunteers with IFN-γ ELISPOT responses in the range of 100 SFU/106 blood mononuclear cells or above were characterized using polychromatic flow cytometry. The vaccine-induced T cell responses were mediated by CD4 T cells in all the investigated 19 responders (3 in the NYVAC alone and 16 in the DNA C plus NYVAC C groups). However, vaccine-induced CD8 T cell responses were observed in addition to CD4 T cell responses in 1 out of 3 responders in the NYVAC C–alone group and in the 8 out of 16 (50%) responders in the DNA C plus NYVAC C group (not depicted). Representative flow cytometry profiles of Env-specific IFN-γ–secreting CD4 and CD8 T cells in six out of the seven responders with both CD4 and CD8 T cell responses vaccinated with DNA C plus NYVAC C are shown in Fig. 4 A.
The magnitude of vaccine-induced CD4 and CD8 T cell responses was either similar or higher in CD8 T cells (Fig. 4 A). The characterization of vaccine-induced CD4 and CD8 T cell responses was performed mostly for Env-specific responses as well as for the Gag-specific responses when the magnitude was >100 SFU/106 cells. Of note, the polychromatic flow cytometry analysis allowed us to provide an independent confirmation of the responses assessed using the IFN-γ ELISPOT assay. The frequencies of IFN-γ–secreting T cells measured by both assays were compared in 17 responders, with a very high correlation between the frequencies measured by the ELISPOT assay and by flow cytometry (Fig. 4 B).
Functional profile of vaccine-induced CD4 and CD8 T cell responses
The panel of T cell functions analyzed included IL-2, TNF-α, and IFN-γ secretion and proliferation for both CD4 and CD8 T cells, and degranulation activity for CD8 T cells. Env- and Gag-specific CD4 and CD8 T cell functions were analyzed using polychromatic flow cytometry (Fig. 5).
T cell functions were analyzed after stimulation with Env- or Gag-derived peptide pools. Representative functional profiles are shown in responders EU11, ES02, and ES08 vaccinated with DNA C plus NYVAC C (Fig. 5 A). These three responders had both vaccine-induced CD4 and CD8 T cell responses, and responders ES02 and ES08 also had Gag-specific in addition to Env-specific responses.
The simultaneous analysis of three functions allowed the assessment of the quality of the vaccine-induced CD4 and CD8 T cell responses. On the basis of the analysis of IL-2 and IFN-γ and TNF-α secretion, seven distinct Env- and Gag-specific CD4 T cell populations were identified (Fig. 5 B). Vaccine-induced CD4 T cell responses had a polyfunctional profile, with ∼60% of CD4 T cells exhibiting two or three functions (Fig. 5 B). Furthermore, vaccine-induced CD4 T cells efficiently proliferated after stimulation with the Env- and Gag-derived peptide pools (Fig. 5 A).
Similar to CD4 T cells, vaccine-induced CD8 T cells were highly polyfunctional. The simultaneous measure of four functions (IL-2, IFN-γ, and TNF-α secretion, and degranulation activity) allowed the identification of 15 functionally distinct CD8 T cell populations. About 70% of vaccine-induced Env- and Gag-specific CD8 T cells exhibited more than one function (Fig. 5 B). Finally, vaccine-induced CD8 T cells were endowed with proliferation capacity after Env- and Gag-specific stimulation (Fig. 5 A).
Collectively, these results indicated that vaccination with DNA C plus NYVAC C induced polyfunctional Env- and Gag-specific CD4 and CD8 T cell responses.
Phenotypic profile of vaccine-induced CD4 and CD8 T cell responses
Phenotypic analysis of vaccine-induced T cell responses was performed in volunteer ES26 vaccinated with DNA C plus NYVAC C. Both Env-specific CD4 and CD8 T cells were induced after vaccination. Blood mononuclear cells of volunteer ES26 were collected at different time points (weeks 24, 28, and 48), stimulated with Env peptide pools for 16 h, and stained with CD4, CD8, CD45RA, CCR7, IL-2, and IFN-γ antibodies. Previous studies have shown that CD45RA and CCR7 define functionally distinct populations of memory antigen-specific CD4 and CD8 T cells (20–23). The totality (single IL-2 plus dual IL-2/IFN-γ plus single IFN-γ) of Env-specific CD4 T cells was CD45RA−CCR7−, and the phenotypic profile and percentage of Env-specific CD4 T cells remained unchanged over time (Fig. 6).
The Env-specific CD8 T cells (dual IL-2/IFN-γ plus single IFN-γ) were almost equally distributed within CD45RA−CCR7− and CD45RA+CCR7− cell populations at week 24 (Fig. 5). However, there was a progressive loss of the CD45RA−CCR7− Env-specific CD8 T cell population over time, and ∼90% of the vaccine-induced CD8 T cells were CD45RA+CCR7− at week 48 (Fig. 6).
Of note, the changes in phenotype and in the percentage of Env-specific CD8 T cells were observed only for vaccine-induced CD8 T cells because the phenotype and the percentage of EBV/CMV-specific CD8 T cell responses assessed in blood samples collected at the same time points in volunteer ES26 remained unchanged (Fig. 6). Similar results were obtained in three additional volunteers.
Epitope mapping of vaccine-induced CD4 and CD8 T cell responses
Identification of epitopes recognized by vaccine-induced CD4 and CD8 T cell populations was performed in nine volunteers, eight belonging to the DNA C plus NYVAC C arm and one to the NYVAC C–alone arm. Characterization was limited to the Env-specific responses. After the initial screening using Env-derived peptide pools, identification of the peptides recognized was performed by testing the reactivity of blood mononuclear cells against the relevant peptides in a matrix setting using the IFN-γ ELISPOT assay. After this analysis, 19 different Env-derived peptides were identified in the nine volunteers studied, and further characterization of the vaccine-induced CD4 and CD8 T cell populations recognizing these peptides was performed using polychromatic flow cytometry (Table I and Fig. 7).
A variable number of peptides, ranging from 2 to 8, were recognized in each volunteer, with a mean of 4.2 peptides (Fig. 7 A). 10 out of 19 peptides identified in the nine volunteers corresponded to epitopes that have already been described either in subjects with chronic HIV-1 infection or in vaccine studies performed in mice and humans (Table I). However, nine potential novel epitopes were identified that have not been previously described or reported, according to the Los Alamos database (Table I) (24).
Representative flow cytometry profiles of vaccine-induced Env-specific CD4 and CD8 T cells recognizing individual peptides are shown in Fig. 7 (B and C). In particular, we had the opportunity to perform fine epitope mapping of peptide LTKKNYSENSSEYYR recognized by CD8 T cells in seven volunteers (six belonging to the DNA C plus NYVAC C group and one to the NYVAC C–alone group). By using a set of overlapping peptides, we demonstrated that the epitope recognized by vaccine-induced CD8 T cells corresponded to the following sequence: YSENSSEYY (Fig. 7 C). Two representative examples of the YSENSSEYY epitope mapping in volunteers ES26 and ES2 are shown in Fig. 7 C. The YSENSSEYY epitope was restricted by HLA-A*0101, as demonstrated by the detection of a vaccine-induced well-defined population of YSENSSEYY-specific CD8 T cells using the relevant HLA-A*0101–YSENSSEYY pentameric complex (Fig. 7 D).
Durability of vaccine-induced T cell responses
The duration of the study in the original protocol was 48 wk. However, to gain insights on the long-term durability of the vaccine-induced T cell response, the protocol was subsequently amended to assess the T cell responses at week 72, i.e., 1 yr after the last immunization. The protocol was amended only in Lausanne and, after institutional review board approval, blood was collected at week 72 from volunteers that were originally enrolled at the Lausanne site, which had a positive IFN-γ ELISPOT response at week 48. We analyzed 13 volunteers (11 belonging to the DNA C plus NYVAC C group and 2 to the NYVAC C–alone group) at week 72. None of the two volunteers belonging to the NYVAC C alone group had a positive IFN-γ T cell response at week 72 (Fig. 8).
9 out of the 11 volunteers belonging to the DNA C plus NYVAC C group had a positive IFN-γ T cell response at week 72. Of interest, the magnitude of the IFN-γ T cell response observed at week 72 was not significantly different (P = 0.09) from that measured in the eight volunteers with responses at all three time points (Fig. 8, A and B).
The changes in the composition of vaccine-induced, functionally distinct CD4 and CD8 T cell responses were compared at weeks 28, 48, and 72 by performing the simultaneous measures of multiple functions using polychromatic flow cytometry.
No substantial changes were observed in the frequency of the seven functionally distinct CD4 T cell populations over time (Fig. 8 C). Only a drop in the range of 30% was observed in the polyfunctional (IL-2/IFN-γ/TNF-α and IL-2/TNF-α) CD4 T cell populations between weeks 28 and 48, whereas the frequency of these populations remained stable between weeks 48 and 72 (Fig. 8 C).
With regard to the vaccine-induced CD8 T cell responses, a decrease in the frequency of functionally distinct CD8 T cell populations was observed over time (Fig. 8 C). In particular, three CD8 T cell populations (dual IFN-γ/TNF-α, dual CD107a/ IFN-γ, and single IFN-γ) decreased >50% between weeks 28 and 72 (Fig. 8 C).
Vaccine-induced antibody response against gp-140
Vaccine-induced IgG antibodies against gp140 CN54 were assessed at different time points during the immunization regimen. The induction of IgG anti-gp140 CN54 was assessed in an ELISA assay. Only a small number of volunteers (25%) had a measurable antibody response at week 26 in the NYVAC C–alone group, whereas a large percentage (75%) of volunteers had measurable IgG anti-gp140 antibodies in the DNA C plus NYVAC C group (P = 0.007; not depicted). In addition to the differences in the percentage of responders between the two study groups, the magnitude of the antibody response was also significantly greater in the DNA C plus NYVAC C group compared with the NYVAC C–alone group (P = 0.006; not depicted). Vaccine-induced antibodies failed to show any neutralizing activity in the three neutralizing assays tested (see Materials and methods for the description of the assays; not depicted).
Previous prime-boost clinical studies of candidate T cell vaccines for HIV have suggested that adenovirus vectors were more immunogenic than poxvirus vectors in terms of cellular immune responsiveness. In the EV02 trial, we demonstrate that DNA plus NYVAC is a highly immunogenic prime-boost regimen, with durable responses up to 1 yr after vaccination. The primary analysis of the vaccine-induced T cell responses was performed using a validated IFN-γ ELISPOT assay. Polychromatic flow cytometry was used to characterize the phenotypic and the functional profiles of the vaccine-induced T cell populations. Furthermore, both the IFN-γ ELISPOT assay and polychromatic flow cytometry were instrumental in analyzing the breadth of the vaccine-induced T cell responses and for performing epitope mapping. All the analyses have been performed on frozen blood mononuclear cell samples, and the evaluation of the immunogenicity has been assessed in the IFN-γ ELISPOT assay after a conventional overnight stimulation of the blood mononuclear cells, with the panel of peptide pools encompassing Env, Gag, Pol, and Nef of HIV-1 clade C CN54.
The majority (18 out of 20, 90%) of the volunteers immunized with the DNA C plus NYVAC C regimen had a positive response using the IFN-γ ELISPOT assay at weeks 26 or 28, the primary endpoints of the study. The percentage of responders in the NYVAC C–alone group was 33%. These results were superior to previous studies that have evaluated the immunogenicity of DNA plus poxvirus (MVA) vaccine regimens in which the percentage of responders was 17% (low dose DNA) and ∼40% (high dose DNA) when conventional overnight stimulation was performed to analyze IFN-γ–secreting cells (as used in EV02) (16, 17). Responses were observed in the eight volunteers in the high dose DNA plus MVA study only using a cultured ELISPOT assay (5-d stimulation) able to detect weak vaccine-induced T cell responses (16, 17). The immunogenicity observed in EV02 was also higher than that observed in studies using Ad5-based vaccine strategies and in the same range of studies using DNA plus Ad5 regimens (unpublished data).
Vaccine-induced IFN-γ–secreting cells were detected only in 7 out of 20 volunteers after DNA C vaccination and before NYVAC C boosting. However, despite the fact that vaccine-induced T cell responses were not measurable in the majority of volunteers after the two DNA vaccinations, the DNA priming was clearly responsible for the substantial increase in the immunogenicity and magnitude (see below) of the vaccine-induced T cell responses in the DNA C plus NYVAC C group compared with the NYVAC C–alone group. These results indicate that the assessment of immunogenicity after immunization with DNA alone is not a reliable measure of the priming ability of DNA candidate vaccines.
Collectively, these results indicate that the DNA C plus NYVAC C regimen is superior to the NYVAC C–alone regimen and has a degree of immunogenicity comparable to that of other promising candidate vaccines that have entered large phase II/IIB clinical studies (3, 4, 13).
The magnitude of the vaccine-induced T cell responses as measured by the frequency of IFN-γ–secreting cells using the ELISPOT assay was substantially higher (three- to fourfold) in the DNA C plus NYVAC C group compared with the NYVAC C–alone group. The magnitude of the vaccine-induced T cell responses observed after DNA C plus NYVAC C immunization was superior to that observed in previous DNA plus MVA studies (17) and comparable to that of Ad5 candidate vaccines and to the DNA plus Ad5 vaccine combination (unpublished data).
The vaccine-induced T cell responses were predominantly (in 100% of responders) mediated by CD4 T cells. However, CD8 T cell responses were also found in 47% of responders. Furthermore, both vaccine-induced CD4 and CD8 T cell responses were predominantly directed against Env (the median magnitude of Env T cell responses in the DNA C plus NYVAC C group vs. the NYVAC C–alone group was 299 and 131 SFU/106 cells, respectively). T cell responses against Gag, Pol, and Nef were detected in 48% of volunteers and had lower magnitude (the median magnitude of Gag-, Pol-, and Nef-specific T cell responses was ∼100 SFU/106 cells).
The finding of predominant CD4 T cell responses after immunization with DNA C plus NYVAC C is consistent with previous studies (3, 4, 17) investigating the T cell responses induced by poxvirus candidate vaccines. The predominant CD4 T cell response after immunization with vaccines including poxviruses seems to differentiate these latter from Ad5-based candidate vaccines and the DNA plus Ad5 vaccine regimen that appear to induce more balanced CD4 and CD8 T cell responses (unpublished data). Three factors may have contributed to the dominant CD4 T cell and Env-specific responses: (a) the DNA priming that seems to favor the development of the CD4 T cell response; (b) the monovalent nature of the NYVAC C construct in which Env, Gag, Pol, and Nef are expressed within the same vector; and (c) the construction of the NYVAC vector with regard to Env, which is expressed in a secreted form. The use of multivalent instead of monovalent DNA and Ad5-based vaccines may reduce the Env immunodominance in favor of more balanced Env, Gag, and Pol responses. The secreted form of Env is likely to be responsible for favoring the exogenous pathway of antigen presentation and thus stimulation of CD4 T cells.
In natural infection studies, it has been reported that Gag-specific CD8 T cell responses are associated with better control of HIV disease in individuals with chronic HIV-1 infection, whereas Env-specific CD8 T cell responses are associated with poor control (25). However, it is not clear from this study whether (a) Gag-specific CD8 T cell responses are the cause or rather the effect of lower levels of virus replication, and (b) results generated in the nonhuman primate model have shown that the presence of Env- and Gag-specific vaccine-induced T cell responses in animals immunized with DNA plus Ad5 expressing Env and Gag conferred better protection from disease after infection compared with animals immunized with DNA plus Ad5 Gag-expressing vaccines (26).
The DNA C plus NYVAC C vaccine regimen induced CD4 T cell responses in 100% of the immunized individuals and CD8 T cell responses in 50% of vaccines. In this regard, it is important to mention that the presence of the vaccine-induced CD4 T cell responses strongly correlated with the containment of viremia in macaques exposed to the highly pathogenic SIVmac251 after immunization with the DNA-SIV-gag-env plus NYVAC-SIV-gag-pol-env (27). Therefore, vaccine-induced CD4 T cell responses may be effective in the attenuation of HIV disease.
Env- and Gag-specific CD4 and CD8 T cell responses induced by DNA C plus NYVAC C vaccination were polyfunctional. Recently (8, 10, 11), the term polyfunctional has been used to define T cell responses that, in addition to typical effector functions such as secretion of IFN-γ, TNF-α, and MIP-1β, as well as cytotoxic activity, comprise T cell populations also able to secrete IL-2 and retain antigen-specific proliferation capacity, whereas the term “only-effector” defines T cell responses/populations with typical effector functions but lacking IL-2 and proliferation capacity. Of interest, several studies (11, 28–31) have demonstrated that polyfunctional and not only-effector T cell responses were associated with protective antiviral immunity (32). The DNA C plus NYVAC C immunization therefore induced the best functional profile of virus-specific CD4 and CD8 T cells capable of controlling virus replication in several chronic virus infections, such as cytomegalovirus, Epstein-Barr virus, herpes simplex virus, and HIV-1 in patients with nonprogressive disease (8, 10, 33). These data are consistent with our previous study demonstrating that vaccine-induced CD8 T cells after vaccination with DNA C plus NYVAC C were highly polyfunctional and that almost 75% of these cells had four or five functions based on IFN-γ, TNF-α, MIP-1β, IL-2, and CD107a (34).
The polyfunctional vaccine-induced CD4 T cell populations were CD45RA−CCR7−, a phenotype of cells with effector functions but also with the ability to secrete IL-2 and endowed with proliferation capacity (20–23, 29, 30, 33). The phenotype of the vaccine-induced CD4 T cells remained unchanged over time. The vaccine-induced CD8 T cells were either CD45RA−CCR7− or CD45RA+CCR7−. This phenotypic profile defines memory CD8 T cells at intermediate and advanced stages of differentiation and is consistent with our recent study using CD27 and CD45RO (34). The presence and rapid appearance after immunization of CD45RA+CCR7− CD8 T cells is of interest. This phenotype has been proposed to define effector CD8 T cells at advanced stages of differentiation (21). Furthermore, virus-specific CD45RA+CCR7− CD8 T cells have been found in controlled chronic virus infections such as CMV and EBV (21, 29, 30), and a correlation between the percentage of this cell population and virus control has also been shown in HIV-1 infection (8). The majority of the vaccine-induced CD45RA−CCR7− CD8 T cell population disappeared over time, whereas ∼90% of vaccine-induced CD8 T cells were CD45RA+CCR7− at week 48, 6 mo after the last immunization. Therefore, the DNA C plus NYVAC C vaccination likely induces the generation of a long-lived population of memory CD8 T cells.
Of interest, the DNA C plus NYVAC C vaccination induced a broad T cell response, with a mean 4.2 epitopes recognized per volunteer. About 50% of epitopes identified in the EV02 study have not previously been described according to the Los Alamos database (24), and approximately two out of three of the total epitopes identified were located in constant regions.
A critical component of the effectiveness of vaccines is their ability to induce long-lasting immunity. The results obtained in EV02 study after vaccination with DNA C plus NYVAC C are extremely promising. More than 70% of volunteers have still measurable vaccine-induced T cell responses 1 yr after the last vaccination and, more importantly, the magnitude of the T cell responses is not substantially changed compared with 1 mo after the completion of the vaccination regimen.
Poxvirus vectors have traditionally represented an interesting platform in the HIV vaccine arena because of (a) their large and successful use in the veterinary field, (b) extensive safety data in humans, (c) their use in vaccine platforms for other infectious diseases and cancer, and (d) their facility to be manipulated and to be inserted with large gene fragments (4). However, disappointing results on their immunogenicity in humans had seriously cast doubt on the validity of the poxvirus platform and the rationale for their further development in the HIV vaccine arena. The present data generated within the EV02 study have clearly demonstrated that a poxvirus candidate vaccine, NYVAC, in combination with DNA is highly immunogenic, induces vigorous and broad T cell responses, comprising of both CD4 and CD8 T cell responses, which are polyfunctional, and more importantly, this vaccine regimen induces long-lasting T cell immunity.
These promising results support the further development of the poxvirus platform and the move of the DNA C plus NYVAC C vaccine regimen into larger clinical trials.
Materials And Methods
DNA immunogens GagPolNef (GPN) and Env.
RNA- and codon-optimized GPN and Env gene vector inserts were designed by GENEART AG using the GeneOptimizer software package. Clade B/C' GPN and Env (gp120) sequences were designed based on sequence information derived from a 97CN54 provirus clone (sequence submitted to GeneBank) (35). The HIV C clade isolate CRF_70 B/C' 97CN54 was collected from Sinkiang Province in China, biologically characterized, and sequenced in a collaboration of China CDC (the Chinese Academy for Preventive Medicine) and the European Commission–funded research cluster CHIVAC 1–3 in an attempt to generate region-specific candidate vaccines. This and subsequently detected, closely related variants represent at least 60% of HIV infections in China and are anti-genetically also close to other C clade isolates from India and South and East Africa. The CN54 Env construct comprises 1,500 nucleotides encoding an artificial signal peptide (MDRAKLLLLL LLLLLPQAQ), followed by gp120 CN54 (nucleotides 5,673–7,109). The 5′ part of the 4,254 nucleotide CN54 GPN polygene construct encodes the group-specific antigen (nucleotides 167–1,651) with a G2A modification rendering this polyprotein myristylation deficient. The Gag coding sequence is followed by a 952-bp (nucleotides 1,444–2,406) fragment encoding the 5′ part of pol, including a D577N mutation leading to an inactivation of the viral protease. A 618-bp fragment encoding a scrambled Nef variant (5′ end of nucleotides 8,170–8,469 linked to 3′ end of nucleotides 8,470–8,787) was fused to the 3′ end of pol-coding sequence replacing the active site of the reverse transcription. The 3′ pol reading frame (nucleotides 2,527–3,591) lacking the integrase gene was extended by the 3′ end of the scrambled Nef gene. The sequence stretch (nucleotides 2,407–2,514) encoding the active site of the reverse transcription (amino acids 1,382–1,417 in GPN) was translocated to the 3′ end of the polygene construct, resulting in an open reading frame encoding the ∼160-kD nonglycosylated artificial GPN polyprotein. Both genes were placed under direct control of CMV IE promoter/enhancer to generate pORT1a-GPN and pORT1a-gp120, lacking any antibiotic resistance gene and instead using a repressor titration system for plasmid selection (Cobra Biomanufacturing Plc.) (36). The genetic stability of both plasmids was evaluated in a DH1lacdapD host strain up to 39 cell generations and controlled by double-strand DNA sequencing (GENEART AG).
Construction of recombinant NYVAC vector.
The NYVAC vector expressing Gag/Pol/Nef and Env of clade C HIV-1 97CN54 was used as described previously (35). Plasmids containing codon-optimized clade C HIV-1 gagpolnef and gp120 genes (pMA60gp120C/gagpolnef-C-14) were used. Functional expression of the donor genes has been demonstrated by Western blot analysis of the proteins produced by the premaster seed lot. Expression levels of the GagPolNef polygene and the Env gene were similar. The description of the recombinant NYVAC vector expressing Env, Gag, Pol, and Nef has been described elsewhere (37). The characteristics and immunogenic potential of NYVAC C has been documented in mice (37).
DNA and NYVAC HIV-1 clade clinical lots.
Good manufacturing production clinical lots of DNA C and NYVAC C were manufactured by Cobra Biomanufacturing Plc. and sanofi pasteur.
At weeks 0 and 4, one group was “primed” with DNA, and one group received nothing. At weeks 20 and 24, all volunteers were immunized with NYVAC. The DNA pORT-gp120 and pORT-gpn plasmids were mixed before administration in an equimolar fashion (final total DNA concentration, 1.05 mg/ml). Volunteers were injected i.m. with 2 ml DNA in each upper leg (4.2 mg/4 ml total per person). The NYVAC vector expressing GPN and Env was administered i.m. in the left upper arm (107.7 pfu/ml total). The study was approved by the institutional review boards of the Centre Hospitalier Universitaire Vaudois, University of Lausanne, Switzerland, and of St Mary's Hospital, Imperial College, London.
Synthetic peptides and peptide–MHC class I complex.
All peptides used in this study were HPLC purified (>80% purity). Overlapping peptides (15 mers with 11 amino acids overlap; n = 474) spanning the entire Gag/Pol/Nef polygene, and the Env clade C of HIV-1 97CN54 (Synpep Corporation) was grouped in eight pools as follows: Gag1 60 peptides (Cg1-Cg240), Gag2 61 peptides (Cg244-Cg486), Gag/Pol 60 peptides (Cgp485-Cp721), Pol1 61 peptides (Cp725-Cpn817 and Cnp1017-Cp1161), Pol2 61 peptides (Cp1165-Cp1403), Nef 49 peptides (Cn838-Cnp1030), Env1 49 peptides (CN9-CN249), and Env2 63 peptides (CN253-CN485). In addition, Env peptides were either rearranged in a matrix setting for the fine epitope mapping analyses or used as single peptides. NYSENSSEY, YSENSSEYY, and SENSSEYYR were obtained from the peptide facility at the University of Lausanne. Furthermore, a set of peptides (n = 28) most frequently recognized in CMV, EBV, and flu infection (CEF pool) (38, 39) was used as an additional positive control. The HLA-A*0101-YSENSSEYY peptide–MHC pentameric complex was purchased from PROIMMUNE.
ELISPOT assays were performed at weeks 0, 5, 20, 24, 26, 28, and 48. In addition, 13 volunteers recruited in Lausanne were also evaluated at week 72. ELISPOT assays were performed as per the manufacturer's instructions (BD Biosciences). In brief, cryo-preserved blood mononuclear cells were rested for 8 h at 37°C, and then 200,000 cells were stimulated with peptide pools (1 μg of each single peptide) in 100 μl of complete media (RPMI plus 10% FBS) in quadruplicate conditions. Media only was used as negative control. 200 ng/ml of staphylococcal enterotoxin B (SEB) was used as a positive control on 50,000 cells, and stimulation with the CEF pool (on 200,000 cells) was used as an additional (antigen-specific) positive control. Results are expressed as the mean number of SFU/106 cells from quadruplicate assays. Only cell samples with >80% viability after thawing were analyzed, and only assays with <50 SFU/106 cells for the negative control and >500 SFU/106 cells after SEB stimulation were considered valid.
Flow cytometry analysis.
Cryo-preserved blood mononuclear cells (1–2 × 106) were stimulated overnight in 1 ml of complete media containing 1μl/ml Golgiplug (BD Biosciences) and 0.5 μg/ml αCD28 antibodies (BD Biosciences) as described previously (29). For stimulation of blood mononuclear cells, individual peptides or peptide pools were used at 1 μg/ml for each peptide. SEB stimulation (200 ng/ml) served as positive control. For functional analyses (i.e., intracellular cytokine staining [ICS] and assessment of the degranulation activity/CD107a mobilization), the following antibodies were used in various combinations: CD4-FITC, -PerCP-Cy5.5, or -PB; CD8-PerCP-Cy5.5, -PB, or APC-Alexa 700; CD3-ECD; CD14-PB; CD16-PB; CD19-PB; IFN-γ-FITC or -APC; IL-2-PE or -APC; TNF-α-FITC or -PECY-7; and CD107a-FITC or -PE. All antibodies were from BD Biosciences, except CD8-APC-Alexa 700 (VWR International AG) and CD3-ECD (Beckman Coulter). Furthermore, dead cells were excluded using the violet LIVE/DEAD stain kit (Invitrogen). In addition, for phenotypic analyses, the following antibodies were used: CCR7-FITC, -APC, or -APC-Alexa-647; CD45RA-ECD or -PerCP; and CD127-PE or -APC-Alexa-647. CD45RA-ECD and CD127-PE were from Beckman Coulter. At the end of the stimulation period, cells were washed, permeabilized (FACS Perm 2 solution; BD Biosciences) and stained as described previously (29). Data were acquired on a FACSCalibur or on an LSRII three-laser (488, 633, and 405 nm) and analyzed using CELLQuest and DiVa software (Becton Dickinson), respectively. Of note, analyses were also performed using SPICE 4.1.5 software from Mario Roederer, Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health. The number of lymphocyte-gated events ranged between 105 and 106 in the flow cytometry experiments shown. With regard to the criteria of positivity of ICS, the background in the unstimulated controls never exceeded 0.01 to 0.02%. An ICS to be considered positive had to have a background of <20% of the total percentage of cytokine+ cells in the stimulated samples.
Ex vivo proliferation assay.
After an overnight rest, cryo-preserved blood mononuclear cells were washed twice, resuspended at 106/ml in PBS, and incubated for 7 min at 37°C with 0.25 μM CFSE (Invitrogen) as described previously (29). The reaction was quenched with 1 vol of FCS, and cells were washed and cultured in the presence of 0.5 μg/ml αCD28 antibody (BD Biosciences). 1–2 × 106 cells were then stimulated with HIV-1 peptide or peptide pools (20 ng/ml of each peptide). SEB stimulation (40 ng/ml) served as a positive control. At day 5, cells were harvested and stained with CD4-PerCP-Cy5.5 and CD8-APC. Cells were fixed with CellFix (BD Biosciences) and acquired on an LSRII. The number of lymphocyte-gated events ranged between 105 and 106.
Analysis of antibody responses.
The induction of HIV-specific antibodies was assessed using an ELISA assay. In brief, 1 μg/ml of recombinant gp140 of CN54 (provided by S. Jeffs, Imperial College, London, UK) in 100 mM Na2HCO3, pH 9.6, was coated on plates (Maxisorp; Nunc) for one night at 4°C. After elimination of the solution and blocking step with PBS, 5% nonfat dry milk, serum dilutions made in PBS, 5% milk, and 3% Tween 20 (beginning at 1/20) were added for 1 h at 37°C. Antibodies bound to the coated gp140 were revealed by an anti–human IgG–horseradish peroxidase conjugate (1/25 000, A1070; Sigma-Aldrich). The inhibitory activity of antibodies was assessed with three different assays. First, a multiple-round neutralization assay on PBMCs was performed with the homologous primary isolate CN54 using experimental conditions described previously (40). Second, antibody effects in a single-cycle infection of primary isolate Bx08 in the engineered cell line (TZMbl) was measured as reported (41). Third, the inhibition by antibodies of HIV-1 Bal multiplication in macrophages was determined according to the method developed previously (42).
An ELISPOT result was defined as positive if the number of SFU was ≥55 SFU/106 cells and greater or equal to fourfold the negative control. The primary immunogenicity endpoint was a positive ELISPOT result at weeks 26 and 28. Each participant was classified as a responder if there was at least one positive response against any of the HIV peptides at weeks 26 or 28, and as a nonresponder if responses at these weeks were all negative. The magnitude of a ELISPOT response was described as the sum of SFU of all positive responses and—assuming that there is no overlap in response across the eight peptide pools—expressed per 106 cells, either by peptide pool or overall, without subtraction of background. An ICS was considered as positive if background was <20% of the total percentage of cytokine+ cells in the stimulated samples. Of note, the background in the unstimulated controls never exceeded 0.01 to 0.02%. An antibody response was classified as present if the OD measured with sera collected after immunization was at least three times greater than the OD obtained with the corresponding preimmune serum.
Comparisons of categorical variables (e.g., the primary endpoint) were made using Fisher's exact test. The magnitude of an ELISPOT response and other continuous variables was compared between groups using nonparametric statistical tests (magnitude over time, paired Wilcoxon or Friedman test; comparison between randomization groups, Mann-Whitney test). For the comparison of flow cytometry and ELISPOT assay in measuring the frequency of IFN-γ–secreting T cells, generalized estimating equations modeling was used to consider within-participant dependencies. The level of statistical significance was 5% for all analyses, without adjustment for multiple comparisons.
We thank Ms. Song Ding from the EuroVacc Foundation for the support in the management of the trial. Special thanks to Drs. Marc Girard, Jaap Goudsmit, and Michel Klein for their key contributions in the EuroVacc EU-funded program. We are also thankful to all the EuroVacc program investigators. We are grateful to the volunteers participating in the study.
The Ev02 clinical trial has been sponsored by the EuroVacc Foundation. The EuroVacc program has been supported by the European Commission fifth framework program under research grants QLK2-CT-1999-01321, QLK2-CT-2001-01316, and QLK2-CT-2002-01431.
The authors have no conflicting financial interests.
Abbreviations used: EV02, EuroVacc 02; GPN, GagPolNef; ICS, intracellular cytokine staining; MVA, modified vaccinia virus Ankara; SEB, staphylococcal enterotoxin B; SFU, spot-forming unit(s); SIV, simian immunodeficiency virus.
A. Harari and P.-A. Bart equally contributed this work.