Evaluation of Free Light Chains (FLCs) Synthesis in Response to Exposure to SARS-CoV-2

The aim of this study is to assess the synthesis of kappa (kappa) and lambda (lambda) free light chains (FLCs) in the serum of patients with COVID-19. All the 120 serum samples were collected from patients with COVID-19 and from healthy controls (vaccinated and non-vaccinated against SARS-CoV-2). FLCs, IgG total, IgG4, IgG anti-Nucleocapsid (N), anti-spike S1 receptor binding domain (S-RBD) antibodies and IL-6 were measured according to the manufacturers' instructions. The concentrations of anti-N IgG, IgG total, IgG4 and IL-6 were elevated in the COVID-19 group in comparison to the vaccinated and non-vaccinated controls. The levels of anti-S-RBD IgG and kappa FLC were increased in COVID-19 and healthy vaccinated patients when compared to non-vaccinated controls. lambda FLC concentration was higher in the COVID-19 group than in the non-vaccinated group. The kappa:lambda ratio was lower in both COVID-19 and non-vaccinated groups in comparison to vaccinated controls. kappa FLC correlated with all tested parameters (anti-S-RBD IgG, anti-N IgG, lambda FLC, kappa:lambda ratio, IgG total, IgG4 and IL-6) except CRP, whereas lambda FLC correlated with all examined parameters except IgG4. Elevated levels of FLCs in COVID-19 and healthy vaccinated against SARS-CoV-2 patients, as well as the correlation between free light chains with specific anti-SARS-CoV-2 antibodies and IL-6, reflect hyperactivation of the immune system after contact with coronavirus. Furthermore, it seems that serum levels of FLCs might be used as predictive markers of COVID-19. Our findings suggest that free light chains are involved in SARS-CoV-2 infection. However, understanding the exact mechanism requires further investigation.