Cytokine response following vaccination
To investigate whether IL-2 was also an accurate biomarker for identifying vaccine-induced T cell responses, 32 individuals were recruited to donate blood samples prior to SARS-CoV-2 vaccination and following the first and/or second vaccine doses (see Methods for vaccine details).
A marked and consistent increase in the magnitude of IL-2 responses was observed between pre- and post-vaccination in previously unexposed individuals. The mean IL-2 response increased by ~18-fold from 12pg/ml prior to vaccination to 203pg/ml and 216pg/ml after the 1st and 2ndvaccine doses, respectively (Figure 3A). Following vaccination, 94% of individuals demonstrated a positive IL-2 response (>37.1pg/ml).
In the cohort of previously infected individuals, all donors demonstrated a positive IL-2 response following the 1st and 2nd doses of vaccine (Figure 3B). Vaccination did not boost the magnitude of responses from the levels of IL-2 seen pre-vaccination, and the mean level of IL-2 produced by these donors, although slightly higher than the cohort with no previous exposure, was not significantly increased (277pg/ml vs 209pg/ml).
An ROC curve was generated from the data obtained from previously unexposed individuals (Figure 3C). The AUC value was 0.9896 (P <0.0001). The test sensitivity was 94.3% and the specificity was 95.5%.
Significant differences were similarly observed in the magnitude of IFN-γ responses between pre- and post-vaccination in previously unexposed individuals (Figure 4A). Mean IFN-γ levels increased by ~10-fold following vaccination. In the pre-vaccinated group, 5% of individuals demonstrated a positive IFN-γ response (>42.3pg/ml), in noticeable contrast to 88% and 81% following 1 or 2 vaccine doses. A sensitivity of 84.9% and specificity of 95.2% was achieved (Figure 4B).
The magnitude of the IL-13 response was small (<10pg/ml), but the differences between pre- and post-vaccination were significant (Figure 5A). After the 1st vaccine dose, 85% of donors showed a positive IL-13 response (>2.3pg/ml), dropping to 67% in donors having received 2 doses. The magnitude of the IL-10 response was again small, but a significant difference was observed between pre-vaccinated samples, and samples taken after the 2nd vaccine dose only (Figure 5B). All samples gave a positive IL-10 result following 2 vaccinations, however, so did 31% of the unvaccinated samples (>7.8pg/ml). Cytokines TNFα, IL-12p70, IL-4, IL-5, and GM-CSF were also measured but none were effective at differentiating unvaccinated from vaccinated individuals (data not shown).
These results confirm that IL-2 is the most accurate biomarker for distinguishing unvaccinated and vaccine-induced T cell responses.
Finally, we evaluated whether there was any correlation between the magnitude of the vaccine-induced IL-2 response and the number of days that had passed since receiving the second vaccine. The IL-2 response remained elevated (>20pg/ml) up to 80 days post-vaccination and did not significantly decrease over time (p = 0.224; Figure 6). This suggests that within ~80 days, the vaccine-induced T cell response is not waning.