*3.4. Limitations of Ex Vivo Manipulation and the In Vivo Administration of DCs*

Previous immunotherapeutic strategies have used the direct administration of ex vivo pulsed autologous DCs to activate T cell populations. One of the representative APCbased administrations was first approved by the Food and Drug Administration (FDA) as a cancer vaccine (Sipuleucel-T; Dendreon, CA, USA) in 2010 for late-stage castration refractory prostate cancer. This method includes APC isolation from patient blood, the coincubation of APCs with prostatic acid phosphatase antigen and a granulocyte-macrophage colony-stimulating factor (GM-CSF), and reperfusion into the patient [17]. To develop an engineering manipulation of DCs using whole TCLs, the ex vivo differentiation of monocyte-derived autologous DCs was achieved by the incorporation of GM-CSF, IL-4, and additional stimuli components (e.g., LPS or TNF-α) to increase the potency of DC activation. Pulsing DCs by incubating them with TCLs also facilitates the production of mDCs [83,84]. Therefore, the vaccination platform involving ex vivo DC pulsing has also been applied to several cancer types, and the potential immunological response against specific cancers, with suitable safety for clinical trials, has been demonstrated. The ex vivo manipulation of DCs using melanoma-derived antigenic TCLs effectively induced signals for melanoma-associated antigen-1 (MAGE-1)-specific CTL responses, and two out of sixteen patients showed long-lasting immune responses over 6 months by successfully modulating antitumor immunity [85]. When using a HOCl-treated TCL mixture (derived from three ovarian tumor lines), DCs also exhibited Th1-dependent antitumor effects and tumor growth delays in stage II/IV ovarian cancer patients [47].

However, the therapeutic efficacy of the ex vivo manipulation and in vivo administration of DCs depends upon the administration route [86], sufficient numbers of delivered DCs [87], and the DC subset [88,89]. It has been reported that more than 90% of ex vivo engineered DCs died or were lost to non-targeted sites, and therefore, only a small fraction of the delivered DCs could home in on a lymph node, resulting in an insufficient T cell response [90,91]. Additionally, the optimization of ex vivo culture conditions, the expansion process, and the loading efficiency of tumor antigens for proper antigen presentation, are all required [92,93]. Due to these technical limitations in obtaining sufficient in vivo immune responses, direct injections of TCLs targeting in vivo resident DC populations without ex vivo DC control have been extensively studied to facilitate antigen-specific immune responses against cancer. Therefore, recent progress in biomaterial-mediated in vivo TCL administration and successful T cell pathway activation by antigen-presenting DCs are emphasized in the following sections.
