Blood biomarkers of long COVID: A cross-sectional evaluation of key cytokines and the coordination of inflammation

Introduction: Long COVID is a complex pathophysiological condition linked to a heterogenous symptom profile. Dysregulation of 1) immunological and inflammatory responses, 2) endothelial/vascular function, and 3) metabolic and clotting processes have been identified; however, there remains a lack of clinically validated diagnostic biomarkers. The study aim was to investigate dysregulation of cytokines, chemokines and growth factors in confirmed Long COVID cases grouped by disease severity. Methods: One-hundred-and-twenty-seven participants provided venous blood samples for cytokine profiling utilising Luminex assays. Participants were grouped into those with no knowledge of SARS-CoV-2 infection (n = 21), acute infection within 3-months (n = 22), spontaneous resolvers (>3-months, n = 21) and those with confirmed Long COVID (n = 63). Long COVID severity was grouped according to the post-COVID-19 functional status (PCFS) and defined as those with negligible (4A; n = 9), mild (4B; n = 20), moderate (4C; n = 24), and severe limitations (4D; n = 10). Cytokines were assessed for normality using Shapiro-Wilk tests and differences measured between groups using Kruskal-Wallis tests. Pairwise correlation and principal component analysis were performed. Results: Interleukin-12 (p40/p70) was 22% greater in the Long COVID group (Median [IQR]: 58 [49] pg/mL, p = 0.046) and 30% greater in the acutely infected group (64 [36] pg/mL; p = 0.039) against non-infected controls (45 [25] pg/mL). Following PCFS classification, fibroblast growth factor-basic was 25% lower in Long COVID 4B versus spontaneous resolvers (24 [7] vs. 32 [24] pg/mL; p = 0.025). Pairwise correlation differences demonstrated that three Long COVID subgroups had strong, positive associations between cytokines when compared to non-infected controls; however, PCA was unable to determine any coordinated cytokine variation that was group specific. Discussion and conclusions: Elevated interleukin-12 (p40/p70) and reduced basic fibroblast growth factor suggest persistent immune activation and possible subgroup-specific Long COVID; however, these findings were marginally significant and exploratory. A coordinated but partial immunological/inflammatory profile may exist in some of those with Long COVID, suggesting a rationale for subgrouping upon a biological, rather than self-report basis but larger longitudinal studies are required to confirm this hypothesis.