Tumor vaccines represent a promising therapeutic approach, but thus far have achieved only limited success in the clinic. The major challenge is to find a means of overcoming inhibitory immune regulatory mechanisms and eliciting effective T-cell responses to antigens preferentially expressed by tumor cells. Here we show that the stimulatory capacity of dendritic cells (DCs) and the magnitude of adaptive immunity are critically regulated by the suppressor of cytokine signaling (SOCS) 1 in DCs. Silencing SOCS1 in antigen-presenting DCs strongly enhances antigen-specific anti-tumor immunity. Our findings indicate that SOCS1 represents an inhibitory mechanism for qualitatively and quantitatively controlling antigen presentation by DCs and the magnitude of adaptive immunity. This study has implications for understanding the regulation of antigen presentation and for developing more effective tumor vaccines by silencing the critical brake in antigen presentation.

Regulatory T cells (T reg ) suppress autoreactive immune responses and limit the efficacy of tumor vaccines; however, it remains a challenge to selectively eliminate or inhibit T reg . In this study, A20, a negative regulator of the TLR and TNFR signaling pathways, was found to play a critical role in controlling the maturation, cytokine production, and immunostimulatory potency of dendritic cells (DC). A20-silenced DCs with the spontaneous and enhanced expression of costimulatory molecules and proinflammatory cytokines have contrary effects on T cell subsets: inhibiting T reg and hyperactivating cytotoxic T lymphocytes and T-helpers that produced IL-6 and TNFα, infiltrated tumors, and were refractory to T reg -mediated suppression. Hence, this study not only identifies A20 as a critical antigen presentation attenuator in control of antitumor immune responses during both the priming and effector phases, but also provides a novel strategy to supersede T reg -mediated suppression in an antigen-specific manner, reducing the need to directly target T reg .

DC-based tumor vaccine research has largely focused on enhancing DC maturation/costimulation and antigen presentation in order to break tolerance against self tumor-associated antigens. DC immunization can activate autoreactive T cells but rarely causes autoimmune pathologies, indicating that self tolerance at the host level is still maintained in the vaccinated hosts. This study in mice reveals a novel regulatory mechanism for the control of self tolerance at the host level by DCs through the restriction of positive cytokine feedback loops by cytokine signaling inhibitor SOCS1. The study further finds the requirement of persistent antigen presentation by DCs for inducing pathological autoimmune responses against normal tissues and tumor, which can be achieved by silencing SOCS1 to unleash the unbridled signaling of IL-12 and the downstream cytokine cascade. However, the use of higher-affinity self peptides, enhancement of DC maturation, and persistent stimulation with cytokines or TLR agonists fail to break tolerance and induce pathological antitumor immunity. Thus, this study indicates the necessity of inhibiting SOCS1, an antigen presentation attenuator, to break self tolerance and induce effective antitumor responses.

BackgroundCurrent efforts to develop HIV vaccines that seek to stimulate immune responses have been disappointing, underscoring the inability of natural immune responses to control HIV-1 infection. Here we tested an alternative strategy to induce anti-HIV immune responses by inhibiting a host's natural immune inhibitor.Methods and FindingsWe used small interfering RNA (siRNA) to inhibit suppressor of cytokine signaling (SOCS) 1, a key negative regulator of the JAK/STAT pathway, and investigated the effect of this silencing on the ability of dendritic cells (DCs) to induce anti-HIV-1 immunity. We found that SOCS1-silenced DCs broadly induced enhanced HIV-1 envelope (Env)-specific CD8+ cytotoxic T lymphocytes and CD4+ T helper cells, as well as antibody responses, in mice. Importantly, SOCS1-silenced DCs were more resistant to HIV Env-mediated suppression and were capable of inducing memory HIV Env-specific antibody and T cell responses. SOCS1-restricted signaling, as well as production of proinflammatory cytokines such as interleukin-12 by DCs, play a critical role in regulating the anti-HIV immune response. Furthermore, the potency of HIV DNA vaccination is significantly enhanced by coimmunization with SOCS1 siRNA expressor DNA.ConclusionsThis study demonstrates that SOCS1 functions as an antigen presentation attenuator to control both HIV-1-specific humoral and cellular responses. This study represents the first, to our knowledge, attempt to elicit HIV-specific T cell and antibody responses by inhibiting a host's antigen presentation attenuator, which may open a new and alternative avenue to develop effective therapeutic and prophylactic HIV vaccines.

Dendritic cell (DC)-based tumor vaccines have only achieved limited clinical efficacy, underscoring the limitation of stimulatory strategies to elicit effective cytotoxic T lymphocyte (CTL) responses against self-tumor-associated antigens. Here, we investigate the role of human suppressor of cytokine signaling 1 (SOCS1), a feedback inhibitor of the Janusactivated kinase/signal transducer and activator of transcription signaling pathway, in regulating antigen presentation by human DCs (hDC). We find that human SOCS1 (hSOCS1)-silenced DCs have an enhanced stimulatory ability to prime self-antigen-specific CTLs in vitro and in a severe combined immunodeficient-hu mouse model. Human CTLs activated by SOCS1-silenced DCs, but not wild-type DCs, have an active lytic activity to natural antigen-expressing tumor cells. We further find that the capacity of hDCs to prime CTLs is likely controlled by SOCS1-restricted production and signaling of proinflammatory cytokines, such as interleukin-12. These results indicate a critical role of hSOCS1 in negatively regulating the immunostimulatory capacity of DCs and imply a translational potential of this alternative SOCS1 silencing strategy to develop effective DC vaccines. [Cancer Res 2009;69(20):8076-84]