br Conclusions br Outstanding Questions br Search strategy a

Conclusions
Outstanding Questions
Search strategy and Selection Criteria The following are the supplementary data related to this article.
Authors\' Contributions
Competing Interests
Introduction Dendritic cells (DCs) are professional antigen presenting cells (APCs) that can efficiently prime T cells. Both endogenous and exogenous purchase Phos-tag are efficiently presented by DCs in the context of major histocompatibility complex class I and II (MHC I and II)/peptide complexes. Among various types of APCs, DCs are the most efficient at cross-presenting antigens to T cells (Delamarre and Mellman, 2011; Jung et al., 2002; Segura and Villadangos, 2009), although the types and magnitude of T cell responses largely rely on the functional specialty and plasticity of DC subsets. T cell-mediated immunity plays crucial roles in therapeutic immunity against cancers and viral infections. The potent ability of DCs to cross-prime CD8+ T cells positions them as novel cellular targets for the rational design of vaccines. In line with this premise, Bonifaz et al. (2002, 2004) demonstrated that the efficiency of antigen cross-presentation by DCs, assessed by measuring the magnitude of antigen-specific CD8+ T cell responses, could be improved over 100-fold by targeting antigens to DEC205 in mice. This seminal observation has led many scientists to further study the biology of DC surface receptors and the use of the “DC-targeting vaccines” against cancers and viral infections. For more than a decade, researchers have been attempting to optimize DC-targeting vaccines by delivering antigens to different DC surface receptors. These receptors include c-type lectins (e.g., DEC205, DC-SIGN, CD207, LOX-1, DC-ASGPR, Dectin-1, DCIR, DCIR2, CLEC6, CLEC9A, and CLEC12A) (Bonifaz et al., 2004; Caminschi et al., 2008; Carter et al., 2006; Delneste et al., 2002; Dudziak et al., 2007; Duluc et al., 2014; Flacher et al., 2014; Flamar et al., 2013; Idoyaga et al., 2008, 2011; Kastenmuller et al., 2014; Lahoud et al., 2009; Li et al., 2012; Meyer-Wentrup et al., 2008; Ni et al., 2010; Sancho et al., 2008; Tacken et al., 2005, 2007, 2011; Weck et al., 2008), as well as non-lectin receptors, including CD40 (Chatterjee et al., 2012; Cohn et al., 2013; Flamar et al., 2013; Rosalia et al., 2015; Williams et al., 2012), mannose receptor (Tsuji et al., 2011), and integrins (Castro et al., 2008). Antigens delivered to DCs via each of these receptors have been reported to elicit certain levels of antigen-specific CD8+ T cell responses in vitro in humans and in vivo in mice or non-human primates (NHPs). However, it still remains unclear which targeted receptors are the most efficient at priming and boosting antigen-specific CD8+ and CD4+ T cell responses. Finding a specific DC surface receptor that permits us to efficiently evoke potent CD8+ and CD4+ T cell responses will be fundamental for the rational design of effective DC-targeting vaccines against cancers and viral infections. Recent preclinical (in NHPs) and clinical data of DEC205-targeting vaccines also suggest that efficient priming and activation of antigen-specific CD8+ cytotoxic T lymphocytes (CTLs) are still major challenges for the success of DC-targeting vaccines for cancer immunotherapy (Kastenmuller et al., 2014). However, it is also important to note that CD4+ T cells are crucial for the longevity of memory CD8+ CTL-mediated immunity (Janssen et al., 2003), which will determine the efficacy of vaccines in many circumstances. In this study, we first compared nine different human DC surface receptors for their ability to promote antigen cross-presentation to CD8+ T cells. We found that CD40 was the most efficient at priming and boosting antigen-specific CD8+ CTLs that were functional. We then compared CD40 with the two best DC lectins, LOX-1 and Dectin-1, for their ability to present antigens to CD4+ T cells. Interestingly, both LOX-1 and Dectin-1 were superior to CD40 at evoking antigen-specific CD4+ T cell responses. To assess the mechanistic insights of the functional dichotomy of CD40 versus lectins (e.g., LOX-1 and Dectin-1) in antigen presentation to CD8+ and CD4+ T cells, we have examined subcellular and intracellular trafficking of the three different receptor-bound antibodies in DCs. We further investigated the kinetics of antigen cross-presentation by DCs targeted with antigen via different receptors. Lastly, we were able to show that antigen targeting to CD40 results in potent CD8+ T cell responses in vivo using human CD40 transgenic (hCD40Tg) mice. This in vivo model further allowed us to conclude that CD40 is superior to Langerin, another lectin receptor, at evoking antigen-specific CD8+ T cell responses, while targeting antigen to Langerin resulted in greater levels of antigen-specific CD4+ T cell responses than targeting to CD40.