Persistent immune activation is a characteristic and prognostic factor of HIV-1 infection and absent in nonpathogenic models of SIV infection occurring in natural hosts of SIV, such as sooty mangabeys, African green monkeys, and mandrills (38
). These observations suggest a crucial role for immune activation in HIV-1 pathogenesis. The underlying mechanisms, however, are not fully understood. Here we show that immune activation in HIV-1-infected individuals declines rapidly after initiation of HAART and that immune activation is directly associated with the level of HIV-1 viremia, as demonstrated previously (5
). These findings suggest a direct effect of HIV-1 on the activation of the immune system. In support of this model, we demonstrate that HIV-1 encodes multiple uridine-rich ssRNA TLR7/8 ligands that induce significant MyD88-dependent immune activation of pDCs and monocytes in vitro. This immune activation was suppressed by chloroquine, an inhibitor of lysosomal acidification known to abolish signaling of the intracellularly located TLRs (36
) and, in pDCs, in a dose-dependent manner, by a TLR7 antagonist. Furthermore, the activation of pDCs and monocytes resulted in a strong secondary activation of T cells. Taken together, these data demonstrate that components of HIV-1 itself can induce strong immune activation via TLR7 and TLR8, which can be blocked by interfering with TLR signaling.
TLRs play a crucial role in the recognition of foreign material, including viruses and bacteria (42
). More recently, it has been demonstrated that a uridine-rich sequence within the HIV-1 long terminal repeat, termed RNA40, can serve as a TLR7/8 ligand (17
) and that endosomally derived HIV-1 RNA can modulate pDC function via TLR7 in humans (3
). Here we demonstrate that multiple uridine-rich oligonucleotides are encoded by the ssRNA of HIV-1 gag
, and the gp160 gene. These ssRNA sequences, as well as synthetic ligands for TLR7 and TLR8, strongly activated pDCs and monocytes, resulting in a secondary activation of T cells. Although other receptors, such as RIG-1, have been shown to detect ssRNA (19
), the activation induced by the described HIV-1-derived ssRNA was dependent on the adaptor protein MyD88, which is essential in the signaling of TLRs, and was blocked by chloroquine. Activation was, furthermore, specifically suppressed in human pDCs in a dose-dependent manner by a previously described TLR7/TLR9-specific antagonist (IRS954). In accordance with the specificity of the TLR antagonist IRS954 for TLR7 and TLR9 (2
), the activation of TLR8+
monocytes by the HIV-1-derived oligonucleotides was not inhibited by IRS954 (17
). Taken together, these data demonstrate that the HIV-1-derived ssRNA oligonucleotides signaled through TLR7/8 and induced strong immune activation in vitro.
A recent study has demonstrated elevated levels of circulating LPS, an important component of the cell walls of gram-negative bacteria, in the plasma of chronically HIV-1-infected individuals (8
). LPS is a strong immunostimulatory ligand for TLR4 (34
). The level of LPS in chronic HIV-1 infection, which was significantly associated with increased immune activation, decreased following 48 weeks of antiretroviral therapy. These data suggest that intestinal microbial translocation, resulting from the breakdown of the mucosal barrier during HIV-1 infection (9
), can serve as a cause of immune activation in chronic HIV-1 infection (8
). In addition to these bacterium-derived TLR4 ligands, the rapid initial decline of immune activation associated with the suppression of HIV-1 replication following the initiation of HAART, described here and previously in other cohorts (15
), and the rapid increase in T-cell activation—within days—associated with the rebound of HIV-1 viremia following interruption of HAART (28
), indicate a direct effect of HIV-1 itself on immune activation. Interestingly, no elevation of plasma LPS levels is observed during acute HIV-1 infection (8
), while very strong immune activation is characteristic for this earliest phase of HIV-1 infection, resulting in the development of “flu-like” symptoms in the majority of infected individuals (22
), further suggesting a direct impact of the virus itself on the initial activation of the immune system.
These rapid changes in immune activation that follow HIV-1-viremia observed during HIV-1 infection and the identification of several HIV-1-encoded TLR ligands with significant immunostimulatory activity described here support a model that proposes a direct role of the virus in the activation of the immune system during HIV-1 infection. These data agree with the results of studies in other viral models demonstrating that TLRs play a crucial role in sensing PAMPs encoded by these viruses early in infection and in initiating the activation of the innate and subsequent adaptive immune response (1
). Furthermore, the persistent circulation of high levels of HIV-1 in chronic infection may directly contribute to the maintenance of increased immune activation, in conjunction with additional PAMPs resulting from pathogens that enter the host as a result of the weakening of the host's immune barriers, including opportunistic pathogens and the described intestinal microbial translocation through a compromised mucosal barrier (7
). Further studies will be needed to dissect the contributions of these different components to the immune activation observed in acute and chronic HIV-1 infection.
In conclusion, our data demonstrate that HIV-1 encodes multiple TLR ligands that can induce strong MyD88-dependent pDC and monocyte activation, as well as accessory cell-dependent T-cell activation. These data support a model in which HIV-1 directly contributes to the persistent immune activation observed during viremic HIV-1 infection and provide a rationale for the development of interventions aiming to reduce the immune activation induced by pathogen-encoded PAMPs.