TLR7 enables cross-presentation by multiple dendritic cell subsets through a type I IFN-dependent pathway

Abstract

Conjugation of TLR agonists to protein or peptide antigens has been demonstrated in many studies to be an effective vaccine formula in inducing cellular immunity. However, the molecular and cellular mediators involved in TLR-induced immune responses have not been carefully examined. In this study, we identify Type I IFN and IL-12 as critical mediators of crosspriming induced by a TLR7 agonistantigen conjugate. We demonstrate that TLR7-driven cross-priming requires both Type I IFN and IL-12. Signaling through the IFN-␣␤R was required for the timely recruitment and accumulation of activated dendritic cells in the draining lymph nodes. Although IL-12 was indispensable during cross-priming, it did not regulate DC function. Therefore, the codependency for these 2 cytokines during TLR7-induced cross-priming is the result of their divergent effects on different celltypes. Furthermore, although dermal and CD8␣ ؉ DCs were able to cross-prime CD8 ؉ T cells, Langerhans cells were unexpectedly found to potently cross-present antigen and support CD8 ؉ T-cell expansion, both in vitro and in vivo. Collectively, the data show that a TLR7 agonistantigen conjugate elicits CD8 ؉ T-cell responses by the coordinated recruitment and activation of both tissue-derived and lymphoid organ-resident DC subsets through a Type I IFN and IL-12 codependent mechanism. (Blood. 2011;118(11): 3028-3038) IntroductionActivation of the innate immune system is a prerequisite to initiating adaptive immune responses. A major pathway eliciting these responses is the recognition of foreign bodies through Toll-like receptors (TLR), which results in the activation of antigen presenting cells (APC) and the production of a variety of proinflammatory mediators. 1 Previously, we showed that conjugation of a synthetic agonist targeting TLR7 to protein antigens results in a highly immunogenic vaccine that potently generates protective CD8 ϩ T-cell responses. 2 TLR7 is an intracellular receptor that recognizes single-stranded RNA molecules and detects RNA viruses such as the influenza virus. Stimulation of TLR7 has been shown in both mice and humans to result in vigorous production of multiple pro-inflammatory cytokines, including Type I IFN and IL-12. 3 The induction of Type I IFN and IL-12 is of particular interest given abundant evidence in the literature establishing these 2 cytokines as critical mediators of CD8 ϩ T-cell activation. 4,5 Type I IFN comprises a group of various IFN proteins, notably IFN␣ and IFN␤. IFNs are induced primarily during viral infections and have been shown to promote natural killer (NK), Type I helper T-cell (Th1), and CTL responses, 4 which are critical to combat viral infections through the elimination of virus-infected cells. Similarly, IL-12 also promotes the development of Th1 and CTL-mediated immunity. [6][7][8] However, the production of IL-12 is primarily associated with bacterial and parasitic infections. 7,9 The role of IL-12 during viral infections remains unclear as some reports indicate that CD8 ϩ T-cell responses elicit...

Activated NKT Cells Can Condition Different Splenic Dendritic Cell Subsets To Respond More Effectively to TLR Engagement and Enhance Cross-Priming

“…For cross-priming CD8 + T cell responses, attention has been focused on introducing Ags to CD8a + DCs, or equivalent cells in humans, through targeting cell surface receptors, such as CD205 (54)(55)(56)(57). Other investigators are conjugating Ags directly to TLR ligands that target these same cells (58)(59)(60). However, we showed that through exposure to different stimuli, including from NKT cells, different DC subsets can become involved.…”

Targeting dendritic cells—why bother?

“…Conversely, several studies suggest that multiple DC subsets are required to induce optimal T-cell immunity. 73,74 These and other studies also describe a dependency on type I IFN in raising efficient immune responses, which can be produced by various cell types such as pDCs, myeloid DCs, monocytes, or stromal cells. [75][76][77][78] Together with the controversy over which DC subset may or may not present the best target, this argues against targeting vaccines to single APC subsets and justifies the question: "Targeting DCs-why bother?…”

TLR7 Triggering with Polyuridylic Acid Promotes Cross-Presentation in CD8α+ Conventional Dendritic Cells by Enhancing Antigen Preservation and MHC Class I Antigen Permanence on the Dendritic Cell Surface

“…Type I IFNs are induced primarily during viral infections and have been shown to promote NK cell, Th1 cell, and, in particular, CTL responses through stimulation of Ag cross-priming (71) and DC maturation (72)(73)(74)(75). It has been reported that type I IFNs are crucial factors regulating the accumulation of DCs in lymph nodes and their maturation into activated APCs during a TLR7-driven response (76). They are also required for intratumoral accumulation of the CD8a + cDC subset (48) and for their ability to develop antitumoral immunity (75) + T cell responses to cell-associated Ags.…”