T cell responses are critical to the containment of many viral infections. However, some infections, such as HIV-1, hepatitis C virus (HCV), and hepatitis B virus (HBV), can evade these responses and develop long-lasting persistence. A cardinal feature of these chronic viral infections is the development of immune dysfunction, or “exhaustion,” in the presence of ongoing antigen exposure (4
). This exhaustion is gradual and results in the hierarchical impairment of T cell effector functions, with an initial loss of proliferative capacity and interleukin-2 (IL-2) production and the late loss of the ability to secrete proinflammatory cytokines, such as gamma interferon (IFN-γ) (46
A number of inhibitory pathways have been shown to mediate immune exhaustion in murine models of viral infection (reviewed in references 38
). Several of these immunoregulatory networks, including programmed death 1 (PD-1), cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), T cell Ig and mucin-3 (TIM-3), and IL-10, have been shown to be upregulated in the setting of chronic HIV-1 infection in vivo
and to mediate a reversible virus-specific immune dysfunction in vitro
(reviewed in references 14
, and 38
). Although these different pathways have been suggested to be important in HIV-1 pathogenesis, only IL-10 has been shown to have an impact on disease progression and long-term outcome in HIV-1 infection (7
IL-10 was initially identified as an important mediator of T cell exhaustion and a crucial determinant of viral persistence in the lymphocytic choriomeningitis virus (LCMV) mouse model (6
). In this system, chronic infection of animals with LCMV can be cleared if the mice are treated with an IL-10 receptor α (IL-10Rα)-blocking antibody or by animals deficient in the IL-10 gene. IL-10 blockade or deficiency can also augment clearance of several other pathogens, including Listeria monocytogenes
, Mycobacterium avium
, and Trypanosoma cruzi
). The role of IL-10 in immune regulation, however, is complex, and it appears that this pleiotropic cytokine controls the delicate balance between the robust immune responses necessary for pathogen clearance and excessive inflammation that leads to collateral immune damage.
Studies of humans with naturally occurring polymorphisms in the promoter region of IL-10 show that changes associated with higher levels of IL-10 production are linked with asymptomatic chronic HBV infection (29
), whereas changes leading to lower IL-10 levels are found in those with more-rapid progression to hepatic fibrosis (27
). Similarly, IL-10 appears to critically regulate multiple aspects of HIV-1 pathogenesis. In chronic, untreated HIV-1 infection, IL-10 is increased in the plasma, with levels that closely correlate with HIV-1 viral load (5
). Additionally, our work previously showed that blockade of IL-10 signaling with an IL-10Rα antibody can restore both CD4 and CD8 T cell proliferation and cytokine secretion, suggesting that IL-10 may play a critical role in HIV-1-mediated immune exhaustion. IL-10 also directly modulates HIV-1 replication in peripheral blood mononuclear cell (PBMC) subsets with somewhat contrasting results depending on the model used (1
). However, genetic studies suggest that the anti-inflammatory effects of IL-10 may be protective in the setting of HIV-1-driven chronic immune activation. Individuals with genetic polymorphisms in the IL-10 gene that are associated with higher levels of IL-10 production show attenuated progression to AIDS, slower CD4 decline, and longer survival (7
). In addition, a recent publication demonstrated that higher levels of IL-10 expression were associated with significantly higher plasma viral loads in acute infection but not in chronic infection (30
). Taken together, these studies suggest that IL-10 can play significant and contrasting roles in HIV-1-associated immune dysfunction and disease progression.
Despite the key role of IL-10 in the regulation of inflammation, many of the fundamental details regarding the control of IL-10 production in HIV-1 infection remain poorly understood. Among these is the cellular source of IL-10, which is unclear. IL-10 has been reported to be produced by a variety of cell types, including B cells, T cells, NK cells, and monocytes, although reports of the source in humans have been conflicting, likely in part due to different assessment methods (5
). Regulatory T cell (Treg) subsets have been reported to mediate their immune inhibitory function via IL-10, but via an uncertain mechanism (35
In this study, we examined the role of Tregs in IL-10-mediated inhibition of virus-specific CD4 T cell responses in chronic HIV-1 infection. We show that monocytes are the primary producer of IL-10 in ex vivo PBMCs from individuals with untreated HIV-1 infection and that this production is tightly regulated by Tregs, which themselves produce little IL-10 ex vivo. We thus demonstrate a novel mechanism by which regulatory T cells inhibit HIV-1-specific immune responses by modulating the cytokine secretion pattern of a major cell subset involved in both innate and adaptive immunity, shifting it toward an inhibitory profile with high levels of IL-10 and low levels of TNF-α (IL-10hi/TNF-αlo).