Graduate Student Koch Institute at MIT, United States
Disclosure(s):
Leena Hamad, MPhil: No financial relationships to disclose
Introduction/Rationale: Recent clinical trials have shown that messenger RNA vaccination against neoantigens can activate long-lasting CD8+ T cells against solid tumors. However, frequent doses appear to be needed to maintain this response. It is uncertain whether vaccination alone reliably recruits effector cells into tumors, particularly when antigen expression is low. We hypothesize that mRNA vaccination primes anti-tumor CD8+ cells, but this alone is not sufficient to recruit and sustain these cells within tumors.
Methods: We used a murine lung adenocarcinoma model; intratracheal lentiviral Cre recombinase was used to initiate KrasG12D/+; Trp53-/- tumors expressing two neoantigens. Mice were dosed intramuscularly with neoantigen-encoding mRNA. We analyzed CD8+ cell trafficking via flow cytometry, in vivo cytotoxicity assays, ex vivo transwell assays, and immunohistochemistry. Intravenous fluorescent labeling was used to track CD8+ T cell recruitment into tumors.
Results: In tumor-bearing mice, we observed a strong presence of neoantigen-specific CD8+ cells in the spleen one week post-vaccination. The majority of these cells were CX3CR1+ short-lived effectors. This was not seen among other antigen-experienced CD8+ cells in the spleen. However, these vaccine-primed effector cells mainly localized to the lung vasculature with limited tissue infiltration. In naïve mice, 99% of transferred neoantigen-pulsed splenocytes were cleared from the spleen within 20 hours.
Conclusion: Our data suggest that mRNA vaccination activates a strong effector population capable of clearing targets presenting neoantigens. However, these cells are ineffectively recruited from the vasculature, a potential barrier to strong anti-tumor immunity. We believe this is either due to a natural progression into short-lived effector cells after priming or inadequate signaling for tissue entry. Our current work aims to explore whether introducing a pro-migratory chemokine axis in the lung may improve recruitment of vaccine-primed cells into tumors.
