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.