In the aftermath of the failed STEP trial, it was suggested that T cells activated in response to a rAd5 vaccine might represent an increased pool of potential targets for HIV infection, and the persistence of such activated cells may increase the susceptibility of vaccinated individuals to HIV infection. To explore this hypothesis, several laboratories evaluated the activation status of T cells in subjects who had received rAd5-based vaccines in the presence or absence of preexisting NAbs to rAd5 (5
). Many of these studies found no evidence of T-cell activation after rAd5 immunization in Ad5-seronegative or -seropositive subjects. However, in most of these studies, human peripheral blood mononuclear cells were not available for analysis during the first weeks after rAd5 immunization.
In the present study we evaluated T-cell activation status on a regular basis during the first 2 months after rAd5 prime or rAd5 boost immunization in a rhesus monkey model. Our results demonstrate that T-cell activation occurs during the first 14 days after rAd5 immunization. These findings are consistent with those of McElrath (9
), who demonstrated a similar transient activation of T cells in the days after a rAd5 immunization of human volunteers. Importantly, both vaccine-elicited antigen-specific CD4+
T cells and the total CD4+
T-cell population were transiently activated after the rAd5 immunization in the present nonhuman primate study.
Since the issue of vaccine-induced cell activation was raised in the wake of the findings of the STEP trial, attention has focused on rAd5-induced CD4+ T-cell activation as a consequence of vaccination. It is, however, important to know whether immunogen-induced T-cell activation occurs after administration of other vaccine modalities. In the present study we show that plasmid DNA vaccines do not activate CD4+ T cells and that rAd35 vaccines activate these cells, but less markedly than the rAd5 vaccines. These findings underscore the substantial differences between vectors in how they trigger T-cell responses and potential differences in their induction of global CD4+ T-cell activation.
Since the increased acquisition of HIV-1 in the STEP trial occurred in rAd5 vaccinated individuals who were Ad5 seropositive before vaccination (2
), it was important to assess immune activation after rAd5 administration in monkeys that were already Ad5 seropositive. Preexisting Ad5 immunity would best be generated by infecting monkeys with a wild-type, replication-competent adenovirus, since this would most closely model the acquisition of Ad5 immunity in humans. However, this could not be done because wild-type Ad5 does not replicate in rhesus monkeys (13
). Although a host-range mutant of Ad5 has been created and might be used to model the induction of preexisting anti-Ad5 immunity in monkeys, we chose to use monkeys that received a prior inoculum of a replication defective rAd5 vaccine construct. Although this mode of inducing anti-Ad5 immunity was not optimal, the titers of anti-Ad5 antibodies in the serum of the monkeys used in these studies (range, 1,094 to 4,212) clearly approximates the titers of anti-Ad5 antibodies observed in African human populations.
rAd5 vaccination also induced a transient T-cell activation in monkeys with preexisting immunity to Ad5. Although no further expansion of the hexon-specific CD4+ and CD8+ T cells was detected after a second rAd5 immunization, both the magnitude and the polyfunctionality of the Gag-specific T-cell responses were boosted. These results therefore indicate that preexisting immunity to Ad5 blunted rAd5-induced hexon-specific T cells responses but had no effect on the Gag-specific T-cell responses.
It is interesting that the SIV Gag-specific CD8+ T cells and the hexon-specific CD8+ T cells responded differentially after repeated rAd5 immunization. The SIV Gag gene fragment in the rAd5 immunogen was cloned downstream of the cytomegalovirus promoter and was therefore potently expressed by the vaccine construct. In contrast to that, the rAd5 hexon gene was likely expressed at a much lower level in the replication-defective rAd5 vector with E1 deleted. Therefore, the first rAd5 boost induced a higher frequency of SIV Gag-specific CD8+ T-cell responses than of Ad5 hexon-specific CD8+ T-cell responses. After the second rAd5 immunization, preexisting Ad5-specific NAbs may have partially neutralized the rAd5 vaccine vector, resulting in a lower effective dose of the vaccine. This lower effective dose of the second rAd5 immunization may have further boosted the Gag-specific CD8+ T-cell responses but may have had a limited effect on the hexon-specific CD8+ T-cell responses.
HIV transmission occurs most often across mucosal barriers and CD4+
T cells are among the first cells infected during transmission (4
). Activation of mucosal populations of T lymphocytes as a consequence of vaccination could certainly contribute to increasing the incidence of HIV transmission. To explore this possibility, we evaluated the kinetics of the activation status of total and antigen-specific mucosal T cells after rAd5 immunization in monkeys with preexisting immunity to Ad5. We found that parenteral rAd5 immunization induced lower-frequency SIV epitope-specific CD8+
T-cell responses and a delayed but more sustained total CD4+
T-cell activation in the colonic mucosa than in the peripheral blood. Such a sustained CD4+
T-cell activation may increase susceptibility to HIV transmission across a mucosal barrier. These results highlight the potential importance of evaluating mucosal T-cell populations in HIV-1 vaccine studies.