November 2009 Articles

Back to the drawing board

It is not certain why the STEP trial failed, and why it even increased the risk of HIV infection in those individuals with pre-existing Ad-5-specific immunity. One possibility is that the expansion of Ad-5-specific CD4 memory T cells could have created a permissive environment for HIV infection [11]. Nevertheless, two recent studies could not confirm a correlation between the numbers of expanded Ad-5-specific T cells and an increased risk of HIV infection in vaccinees with pre-existing Ad-5 immunity [12,13]. This suggests that other flaws undermined the success of the Ad-5/HIV vaccination strategy and that new concepts need to be considered in order to reach the finish line of successful vaccination.

Several studies have demonstrated that CD8⁺ cytotoxic T cell (CTL) responses play a crucial role in the initial containment and early control of HIV or SIV replication [10,14-16]. In primary infections, a potent anti-HIV or SIV specific CTL response is detected at a time when a humoral response is weak or nonexistent, suggesting that CD8⁺ effector cells are key controllers of early viremia [15,16]. Nevertheless, SIV-macaque models demonstrated that measuring cellular immune responses to an SIV vaccine, using in vitro assays on blood samples, does not faithfully predict the efficacy of the vaccine. Low levels of protection against SIV were observed in the face of rather vigorous vaccine-induced immune responses [17]. Also, live, attenuated SIV infection provided potent protection against wild-type SIV, whereas only weak anti-SIV immune responses were measured in vitro [18,19], suggesting that vaccine-induced protection against HIV/SIV might be critically dependent on localized, likely mucosal-specific immune responses in vivo.

Consistent with the importance of the local response, and further underscoring the central, early contribution of CD8 T cells, are the results from nonhuman primates demonstrating that depletion of CD8 T cells in vivo resulted in enhanced AIDS viral replication during the acute phase, increased loss of CCR5⁺CD45RA^-CD4 T cells in the intestine, reduced humoral responses and, ultimately, in an accelerated lethal outcome of the disease in the infected monkeys [20-26].

The efficacy of virus-specific CD8 CTLs to control viral replication and/or prevent AIDS led to the concept that an effective vaccine should lead to the induction of a strong and early viral-specific CD8 immune response that, although unable to provide sterilizing protection, ought to be able to control the viral load and delay or prevent the onset of disease, as well as reduce the potential for secondary transmission.

A major step forward: providing proof of concept

Unlike naїve T lymphocytes and central memory T cells (T_CMs), effector memory T cells (T_EMs) have the ability to reside long-term in non-lymphoid tissues, such as the gut mucosa, and to provide immediate effector function, without the pre-requisite of an initial proliferation step [27]. This suggests that the generation of vaccine-induced mucosal HIV-specific T_EMs might provide front-line protection to prevent the initial early phase of HIV infection.

An important recent study by Hansen et al. [28] provides direct support for this new paradigm. They demonstrated that Rhesus macaques inoculated with rhesus cytomegalovirus (RhCMV) vectors containing an SIV Gag, Rev-Tat-Nef fusion protein, and Env were protected from progressive wild-type SIV infection after repeated limiting-dose intrarectal challenges. Moreover, the protected animals that were repeatedly infected via the mucosal route showed no evidence of systemic infection months later, suggesting that the infection was contained locally, well before extensive viral replication or systemic spreading of the virus. Interestingly, and in contrast to the CMV vector-based protection that generated a high frequency of SIV-specific T_EMs, an Ad-5-boost vaccine regimen, using the same limiting-dose intrarectal challenge protocol, instead induced a strong T_CM-biased immune response that did not provide protection against SIV challenge.

Turning the spotlight on T_EMs as the mucosal frontline defenders

Earlier SIV/macaque model studies also showed that partial successful SIV protection was reached when immunization was performed via the intranasal or oral route [29,30], further stressing the issue that the induction of mucosal immunity might be key for a successful HIV/AIDS vaccination strategy. Most HIV infections are acquired across a mucosal barrier, and the mucosal immune system forms the main HIV reservoir, the most important target for HIV replication, and it endures the major early T cell losses [5,6,8]. The depletion of mucosal T cell responses is probably the result of direct infection of T cells and immune exhaustion due not only to chronic activation by HIV, but also to activation by intestinal bacteria. The rapid and severe depletion of mucosal CCR5⁺CD4 memory T cells [5,6,8] results in a considerable translocation of bacteria across the mucosal barrier [31]. Especially critical is the loss of Th17 CD4 memory cells, a major contributor to impaired barrier function [32]. Together these processes contribute to the induction of chronic systemic immune activation and T cell exhaustion.

In order to generate protective immunity, future HIV vaccines will probably have to include the induction of viral-specific T_EM populations at relevant mucosal entry surfaces as one of the absolute goals. A successful protective immune response will need to be able to sense a low dose of viral epitopes endogenously produced by naturally infected cells, and to eliminate those infected cells before extensive viral replication and the loss of mucosal CD4 memory T cells occurs. This may require specific immunization strategies other than systemic immunization, and the development of more accurate assays to distinguish mucosal HIV-specific T cell responses from those of peripheral cells. But most of all, we need to understand and identify the mechanisms and factors that can direct vaccine-induced differentiation and maintenance of highly effective and long-lived mucosal T_EMs that can contribute to form the first line of defense and destroy the pathogen before it gets a chance to destroy its host.