In the wake of the recently reported modest success of the RV144 Thai vaccine trial, interest has turned to the potential contribution of vaccine-elicited CD4+ T cell and antibody responses to protection against HIV-1 acquisition. In the current study, we found that different vectors generated virus-specific CD4+ T cell responses of different magnitudes and with different functional profiles. Of the vaccine regimens that we evaluated, DNA prime/rMVA boost immunization elicited the highest frequency virus-specific CD4+ T cell responses, and these cells had polyfunctional repertoires. A substantial expansion of the virus-specific CD4+ T cell responses was also seen in plasmid DNA primed monkeys after rAd5 boosting.
Immediately after a SHIV-89.6P challenge, the virus-specific CD4+
T cell populations in these vaccinated monkeys decreased, likely as a consequence of the well described loss of CD4+
T cells during the first weeks following a pathogenic CXCR4-tropic SHIV infection (Nishimura et al., 2005
). Although these virus-specific CD4+
T cell responses decreased following SHIV-89.6P challenge, highly significant differences were observed between the magnitudes of these responses in the control monkeys and groups of monkeys receiving heterologous prime-boost vaccinations. These results demonstrate that immunization by DNA prime, heterologous vector boost induced high frequency HIV-1- and SIV-specific CD4+
T cell responses that are partially preserved following an intravenous infection with a pathogenic CXCR4-tropic SHIV.
We also evaluated the expression of memory- and mucosal homing-associated molecules on SIV-specific CD4+ T cells following vaccination and following SHIV-89.6P challenge. We found that after a priming immunization very few of the virus-specific CD4+ T cells expressed β7, the mucosal homing-associated molecule. Moreover, a homologous or heterologous boosting immunization did not expand the proportion of β7+ virus-specific CD4+ T cells. Therefore, virus-specific CD4+ T cells elicited by vaccination expressed low levels of this mucosal homing molecule. A detailed memory phenotypic analysis showed that majority of the virus-specific CD4+ T cells elicited by vaccination were CD28+ central memory T cells, and no significant differences were detected in the relative representation of these CD28+CD4+ T cells in the various groups of animals following vaccination and following challenge. Therefore, these results suggest that these T cell vaccine strategies generated CD28+ memory CD4+ T cell populations that home to the lymphoid tissue.
We have previously shown that both the magnitude and functional profile of the virus-specific CD8+
T cells generated by vaccination were associated with control of viral replication following SHIV-89.6P challenge (Sun et al., 2008
). In the current study, we found that neither the magnitudes nor the functional profiles of the virus-specific CD4+
T cells generated by vaccination were associated with a preservation of CD4+
T cells or control of viral replication following SHIV-89.6P challenge. It is now clear that CD8+
T cell responses are associated with the containment of HIV/SIV replication in acute infection and they are also important for the maintenance of viral control during chronic infection (Borrow et al., 1997
; Schmitz et al., 1999
T cells provide help for B cell responses and maintain effective cytotoxic T lymphocytes (CTL) (Kalams and Walker, 1998
). Functional CD4+
T cells are also required at the time of immune priming for the development of long-term memory CD8+
T cells (Janssen et al., 2003
; Shedlock and Shen, 2003
). Therefore, although we did not document a direct association between the magnitudes of the virus-specific CD4+
T cell population generated by vaccination and the control of viral replication post-challenge, we previously reported that a survival advantage was associated with the magnitude of the virus-specific CD4+
T cell responses generated by vaccination and also associated with the preservation of the virus-specific CD4+
T cell responses following SIVmac251 infection (Sun et al., 2006
). The difference between these findings in the setting of a SHIV-89.6P and an SIVmac251 infection may reflect differences in the pathogenic consequences of a CXCR4-tropic and a CCR5-tropic lentivirus.
We used archived PBMCs from 51 rhesus monkeys from previously reported studies that received different homologous or heterologous prime-boost immunizations. There are therefore caveats that should be acknowledged when interpreting these findings. The differences in the immunization and challenge schedules for each experimental group of monkeys in these studies might have influenced the functional T cell data. In addition, the administration of the IL-2/Ig plasmid during DNA vaccine priming in the DNA/DNA and DNA/rMVA groups may have led to augmented immune responses that were capable of controlling viremia and preventing disease progression following a SHIV-89.6P challenge. Nevertheless, the findings in the present study demonstrate that vaccine strategies that include recombinant MVA or recombinant Ad5 vectors can elicit robust CD4+ T cell responses.
In the current study we found that rhesus monkeys primed with rAd5 showed a dramatic expansion of virus-specific CD4+ T cell responses following an envelope protein boost. In addition, boosting with MF59-adjuvanted protein elicited a further expansion of the polyfunctional Env-specific CD4+ T cell responses, with 3/4 of the responses made up of either cytokine double positive or triple positive CD4+ T cells. Further, these polyfunctional Env-specific CD4+ T cell responses were well preserved 12 weeks following the protein boost immunization. These results indicate that a protein boost immunization induced durable, high frequency envelope-specific CD4+ T cell responses with polyfunctional repertoires.