by Gertrud U. Rey
There are several highly effective vaccines that block infection by human papillomaviruses (HPVs) and thereby prevent the cervical, anogenital, and head and neck cancers caused by these viruses. However, none of these vaccines are effective for the treatment of established HPV-induced tumors.
The success of the COVID-19 messenger RNA (mRNA) vaccines has inspired others to implement this vaccine modality for various additional applications. Traditional mRNA vaccines consist of an mRNA molecule that encodes a pathogen-derived protein (i.e., an “antigen”), which is known to trigger an immune response. Upon injection into a vaccine recipient, the mRNA enters cells and is translated by the host protein synthesis machinery into the encoded antigenic protein, which should then promote the desired immune response. Some vaccine mRNAs are “non-replicating” because they only encode the antigen of interest, and the amount of protein that is translated from the mRNA is limited to the amount of input vaccine mRNA. Other vaccine mRNAs are “self-replicating” because they also encode an RNA-dependent RNA polymerase in addition to the antigen of interest. Once the polymerase-encoding portion of the mRNA is translated into protein, the resulting polymerase enzyme can replicate any untranslated mRNA, thus amplifying the amount of vaccine mRNA. Because this approach produces more copies of the mRNA than the original input amount, self-replicating mRNA vaccines can be delivered in smaller doses than non-replicating vaccines. To make the vaccine less detectable by the immune system until the encoded antigen is translated, the mRNA can optionally be “modified,” for example by replacing every uracil base with a methylated version, as was done for the COVID-19 mRNA vaccines.
A recent article in Science Translational Medicine describes several mRNA vaccines that were designed for the therapeutic treatment of HPV-induced tumors. The vaccines consisted of 1) an unmodified non-replicating mRNA, 2) a modified non-replicating mRNA, or 3) an unmodified self-replicating mRNA. The mRNA molecule in each of the three vaccines encoded the antigenic HPV E7 protein, which is found in all HPV-associated cancers and plays a critical role in transforming normal cells into cancer cells. The mRNA also had the sequence of the type 1 herpes simplex virus glycoprotein D, which was added for its immune-boosting adjuvant properties. Translation of these two adjacently located sequences, gD and E7, would produce a “gDE7” fusion protein.
The authors tested these vaccines in mice with HPV-induced tumors to see if they could reduce or inhibit the growth of the tumors. Mice were first injected with HPV E7-expressing tumor cells (called TC-1 cells) to promote the development of tumors. Three days later they were immunized with a single dose of either of the three gDE7-encoding mRNA vaccines or a control vaccine that did not contain the gDE7 mRNA. Recipients of the control vaccine developed tumors that grew progressively until the mice had to be euthanized. In contrast, all of the mice immunized with a single dose of the modified non-replicating mRNA or unmodified self-replicating mRNA, and 80% of mice immunized with the unmodified non-replicating mRNA, survived and were completely tumor-free at the end of the 70-day observation period. To test the longevity of this effect, TC-1-injected and immunized mice were re-injected with TC-1 cells 90 days after the initial inoculation. All of the mice immunized with the modified non-replicating mRNA or unmodified self-replicating mRNA, and 50-80% of mice immunized with the unmodified non-replicating mRNA (depending on the vaccine dose) were protected from developing tumors after the second TC-1 injection. These results suggested that a single dose of any of the three vaccines, and in particular the modified non-replicating mRNA and unmodified self-replicating mRNA, were effective at reducing or completely inhibiting the growth of HPV-induced tumors in mice. Moreover, these protective effects were durable and even prevented relapse in most cases.
Helper T cells help activate cytotoxic T cells, which then directly kill infected cells and tumor cells. Using fluorescent antibodies specific for cytotoxic T cells, the authors also determined that mice that were protected from tumor development had higher levels of cytotoxic T cells than control mice, suggesting that these immune cells mediated the protection. To confirm that this observation wasn’t just a correlation and that the cytotoxic T cells actually had a positive functional effect, the authors repeated the above experiments in mice lacking either cytotoxic T cells or helper T cells. Cytotoxic T cell-deficient mice injected with TC-1 cells and immunized with any of the gDE7 vaccines showed progressive tumor growth and no survival within the 90-day observation period, similar to recipients of the control vaccine. Helper T cell-deficient mice immunized with any of the gDE7 vaccines showed partial suppression of tumor growth and 50-70% survival, suggesting the presence of some functional cytotoxic T cells that were presumably activated without the aid of helper T cells. Collectively, these results confirmed that cytotoxic T cells mediate the regression and inhibition of HPV-induced tumors in mice.
Although these results are exciting, I am eager to see how they will translate to humans. It will be particularly interesting to see whether the vaccines are effective at much later time points after tumor development, considering that people typically don’t receive cancer treatment within days or even weeks after tumor formation. Fortunately, mouse studies employing TC-1 tumor cells can be followed up directly in human clinical trials without the need for experiments in non-human primates. Treating HPV-induced tumors with therapeutic vaccines that can be delivered in one or more doses would be a very attractive alternative to current chemotherapeutic treatment options.
[For a more detailed discussion of this paper, I recommend TWiV 991].