Dendritic cells process synthetic long peptides better than whole protein, improving antigen presentation and T‐cell activation

<jats:p>The efficiency of antigen (<jats:styled-content style="fixed-case">A</jats:styled-content>g) processing by dendritic cells (<jats:styled-content style="fixed-case">DC</jats:styled-content>s) is vital for the strength of the ensuing <jats:styled-content style="fixed-case">T</jats:styled-content>‐cell responses. Previously, we and others have shown that in comparison to protein vaccines, vaccination with synthetic long peptides (<jats:styled-content style="fixed-case">SLP</jats:styled-content>s) has shown more promising (pre‐)clinical results. Here, we studied the unknown mechanisms underlying the observed vaccine efficacy of <jats:styled-content style="fixed-case">SLP</jats:styled-content>s. We report an in vitro processing analysis of <jats:styled-content style="fixed-case">SLP</jats:styled-content>s for <jats:styled-content style="fixed-case">MHC</jats:styled-content> class I and class II presentation by murine <jats:styled-content style="fixed-case">DC</jats:styled-content>s and human monocyte‐derived <jats:styled-content style="fixed-case">DC</jats:styled-content>s. Compared to protein, <jats:styled-content style="fixed-case">SLP</jats:styled-content>s were rapidly and much more efficiently processed by <jats:styled-content style="fixed-case">DC</jats:styled-content>s, resulting in an increased presentation to <jats:styled-content style="fixed-case">CD</jats:styled-content>4<jats:sup>+</jats:sup> and <jats:styled-content style="fixed-case">CD</jats:styled-content>8<jats:sup>+</jats:sup> <jats:styled-content style="fixed-case">T</jats:styled-content> cells. The mechanism of access to <jats:styled-content style="fixed-case">MHC</jats:styled-content> class I loading appeared to differ between the two forms of <jats:styled-content style="fixed-case">A</jats:styled-content>g. Whereas whole soluble protein <jats:styled-content style="fixed-case">A</jats:styled-content>g ended up largely in endolysosomes, <jats:styled-content style="fixed-case">SLP</jats:styled-content>s were detected very rapidly outside the endolysosomes after internalization by <jats:styled-content style="fixed-case">DC</jats:styled-content>s, followed by proteasome‐ and transporter associated with Ag processing‐dependent <jats:styled-content style="fixed-case">MHC</jats:styled-content> class I presentation. Compared to the slower processing route taken by whole protein <jats:styled-content style="fixed-case">A</jats:styled-content>gs, our results indicate that the efficient internalization of <jats:styled-content style="fixed-case">SLP</jats:styled-content>s, accomplished by <jats:styled-content style="fixed-case">DC</jats:styled-content>s but not by <jats:styled-content style="fixed-case">B</jats:styled-content> or <jats:styled-content style="fixed-case">T</jats:styled-content> cells and characterized by a different and faster intracellular routing, leads to enhanced <jats:styled-content style="fixed-case">CD</jats:styled-content>8<jats:sup>+</jats:sup> <jats:styled-content style="fixed-case">T</jats:styled-content>‐cell activation.</jats:p>