Cytokine profiles of SARS-Cov-2 infected and uninfected individuals
Blood samples from 128 participants were stimulated overnight with a mega pool of 470 SARS-CoV-2 peptides and the level of IFN-γ was measured in the plasma (see Methods, Online Repository). Significant differences were observed in the magnitude of IFN-γ responses between individuals with previous PCR-confirmed infection and suspected infection (mean IFN-γ = 274pg/ml and 145pg/ml, respectively) and individuals with no history of exposure (mean IFN-γ = 26pg/ml) (Figure 1A). In the cohorts of PCR positive individuals and those with suspected infection/exposure, 97% of individuals (28/29) and 80% of individuals (24/30) respectively demonstrated a positive response. This was in comparison to 20% (14/69) of donors from the cohort of unexposed individuals. A positive response (>24pg/ml) was determined by Youden’s Index (see Methods).
Additional SARS-CoV-2-induced cytokines were measured by multiplex array (see Methods) to determine whether other biomarkers could more accurately distinguish natural infection from non-infection. A clear distinction in the magnitude of interleukin-2 (IL-2) responses was observed between individuals with previous PCR-confirmed infection and suspected infection (mean IL-2 = 328pg/ml and 275pg/ml, respectively) and individuals with no known history of exposure (mean IL-2 = 9pg/ml) (Figure 1B). From the cohort of previously infected donors, all 29 individuals demonstrated a positive IL-2 response, in marked contrast to only 1/69 from the unexposed cohort. In the cohort with suspected infection/exposure, 77% of individuals demonstrated a positive response (>19.9pg/ml).
Receiver-operating characteristic (ROC) curves were generated for IL-2 and IFN-γ. For IL-2, the area under the curve (AUC) value was 0.9950 (P <0.0001) (Figure 2A). For IFN-γ, the AUC value was 0.9465 with a P value of <0.0001 (Figure 2B). Additionally, Youden’s Index was used to measure the potential effectiveness of each biomarker. For IL-2, a sensitivity of 98.6% and a specificity of 100% was achieved. For IFN-γ, a sensitivity of 79.7% and specificity of 96.6% was attained.
Significant differences were also observed for interleukin-13 (IL-13) and interleukin-6 (IL-6) between the PCR positive and uninfected groups (Supporting Figure 1 A-B). Neither were as accurate as IL-2 or IFN-γ at differentiating between cohorts (AUC = 0.6669 for IL-13 and 0.6717 for IL-6, Supporting Figure 1 C-D). No other cytokine examined could discriminate between previously infected and unexposed individuals (TNFα, MIP-1β, IL-12p70, IL-4, IL-5, IL-10, G-CSF, IL-17A, IL-7, IL-8, IL-1β, MCP-1, GM-CSF - data not shown).
These results demonstrate unequivocally that IL-2 is the most accurate biomarker for distinguishing SARS-CoV-2 infected from uninfected individuals.