2.2.2. Oxidation

The oxidation of source tumor cells prior to the preparation of TCLs could facilitate necrosis and augment the immunogenicity of the antigenic components in TCLs by increasing oxidative stress (Figure 2E). Through this modulation, DCs could boost the uptake of antigenic danger signals and antigen processing mechanisms [44]. Therefore, hypochlorous acid (HOCl)-mediated oxidation is used for the generation of effective TCL contents, since protein chlorination enhances proteolytic vulnerability and improves the immunogenicity of the antigenic components [45]. HOCl-mediated oxidation also produces aldehyde-modified antigens with higher immunogenicity than that of unmodified antigens [46]. Chiang et al. [47] compared the in vitro efficacy of DCs pulsed with various TCLs obtained by HOCl-mediated oxidation, UVB irradiation, and six freeze–thaw cycles. Here, both HOCl-mediated oxidation and UVB irradiation efficiently induced the necrosis of tumor cells expressing ovalbumin (OVA) antigens, and the MHC-1-dependent presentation of the peptide SIINFEKL was achieved in DCs treated with the TCLs. In vivo tumor suppression in ID8 ovarian tumor models also demonstrated the enhanced immunogenic capability of the antigen contents in TCLs obtained from HOCl-mediated oxidation.

As a more stable molecule than HOCl, squaric acid (SqA) has been clinically approved for the treatment of skin papillomas [32]. SqA was also shown to induce the complete necrosis of source tumor cells and induce subsequent chemical changes in tumor antigens by combining with them via mechanisms including redox alteration, additional crosslinking, and aggregation through the reactive functional group. The resulting DAMPs from SqA-treated TCLs stimulated DCs, and these activated DCs elicited significant cytokine (IL-12 and IFN-γ) secretion and antigen presentation ability, indicating a more potent Th1 response.
