Merkel cell carcinoma (MCC) is a relatively new addition to the expanding category of oncovirus-induced
cancers. Although still comparably rare, the number of cases has risen dramatically in recent years. Further complicating this trend is that MCC is an extremely aggressive
neoplasm with poor patient prognosis and limited treatment options for advanced disease. The causative agent of MCC has been identified as the merkel cell polyomavirus (MCPyV). The MCPyV-encoded large T (LT)
antigen is an
oncoprotein that is theorized to be essential for virus-mediated
tumorigenesis and is therefore, an excellent MCC
antigen for the generation of antitumor immune responses. As a foreign
antigen, the LT
oncoprotein avoids the obstacle of immune tolerance, which normally impedes the development of antitumor immunity. Ergo, it is an excellent target for anti-MCC
immunotherapy. Since
tumor-specific CD8+ T cells lead to better prognosis for MCC and numerous other
cancers, we have generated
a DNA vaccine that is capable of eliciting LT-specific CD8+ T cells. The
DNA vaccine (pcDNA3-CRT/LT) encodes the LT
antigen linked to a damage-associated molecular pattern,
calreticulin (CRT), as it has been demonstrated that the linkage of CRT to
antigens promotes the induction of
antigen-specific CD8+ T cells.
RESULTS: The present study shows that
DNA vaccine-induced generation of LT-specific CD8+ T cells is augmented by linking CRT to the LT
antigen. This is relevant since the
therapeutic effects of the pcDNA3-CRT/LT
DNA vaccine is mediated by LT-specific CD8+ T cells. Mice vaccinated with the
DNA vaccine produced demonstrably more LT-specific CD8+ T cells. The
DNA vaccine was also able to confer LT-specific CD8+ T cell-mediated protective and
therapeutic effects to prolong the survival of mice with LT-expressing
tumors. In the interest of determining the LT
epitope which most MCC-specific CD8+ T cells recognize, we identified the amino acid sequence of the immunodominant LT
epitope as aa19-27 (IAPNCYGNI) and found that it is H-2kb-restricted.
CONCLUSION: The results of this study can facilitate the development of other modes of MCC treatment such as
peptide-based
vaccines and adoptive transfer of LT-specific CD8+ T cells. Likewise, the MCC
DNA vaccine has great potential for clinical translation as the immunologic specificity is high and the treatment strategy can be exported to address other virus-induced
tumors.