In a recent study posted to the medRxiv* preprint server, researchers compared serum cytokine expression among healthy individuals and those with PASC [post-acute sequelae of coronavirus disease 2019 (COVID-19)].
Among several COVID-19 patients, symptoms persist for approximately three months after the acute phase of infection, referred to as PASC, long COVID (LCOVID), or long-haul COVID. PASC symptoms include post-exertional malaise, fatigue, cardiac and respiratory symptoms, digestive symptoms, and neurological symptoms. The symptoms vary among individuals, and the underlying mechanisms of which have not yet been well-characterized.
About the study
In the present study, researchers assessed cytokine expression among PASC patients and compared it to those among healthy individuals.
Serum samples were obtained from 12 PASC patients, and 15 healthy individuals between May 2020 and December 2021, and their peripheral blood mononuclear cells (PBMCs) were isolated for performing multiplexed cytokine assays. The assays included interleukin (IL)-1β, 2, 4, 5, 6, 8, 10, 12, 13, 17, 33, interferon-gamma (IFNγ), tumor necrosis alpha (TNFα) and soluble IL-2 receptor (sIL-2R).
Individuals who were either uninfected or were SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2)-infected but recovered without any PASC comprised the healthy individual group. LCOVID patients were individuals whose symptoms persisted beyond the acute infection phase, whose symptoms resolved but recurred subsequently, or who developed novel symptoms three months after the initial SARS-CoV-2 infection.
Further, the team investigated whether the cytokine expression was associated with gender, for which cytokine expression among men and women of the healthy individual group and that between the PASC group females and the healthy group females were compared.
Results and discussion
PASC patients showed a 100% reduction in IL-8 and IFNγ expression with significant reductions in the expression of IL-2, 4, 6, 13, and 17. Therefore, the team proposed immune exhaustion as the driving factor of PASC, with a complete IL-8 and IFNγ reduction preventing the pulmonary tissues and other tissues from recovering post-acute COVID-19, which decreased the capability to prevent subsequent COVID-19, both contributing to the PASC symptomatology.
PASC patients showed a 70% reduction in IL-6 expression, >40% decreases in IL-2, 13, and 17 levels, and a 26% reduction in IL-4 expression. However, there were no significant differences in the serological expression of TNFα, sIL-2R, IL-1β, IL-5, 10, and 12 between the two groups. In the healthy individual group, 12 (out of the 13) cytokines tested did not significantly differ by gender, with only a 42% lower expression of IL-2 among healthy men than healthy women.
Comparing cytokine expression between the PASC group (all women) and the healthy group women (n=8), again, IFNγ and IL-8 expression reduced by 100%. In addition, IL-6, il-2, IL-13 and IL-4 expressions were lower by 72%, 55%, 59%, 44% among PASC women.
Of note, the observed reduction in IL-5 expression (26%) among PASC women showed statistical significance when only females of both cohorts were considered, which dropped to 14% with no significance when both the groups were compared, including both genders. On the contrary, the alterations in TNFα, sIL-2R, IL-1β, 10, and 12 expressions remained non-significant irrespective of the genders considered.
IL-8 is produced by many cell types, including fibroblasts, epithelial cells, endothelial cells, lymphocytes, mast cells, and macrophages, and IL-8 expression is induced partially by IL-1β expression. IL-8 is involved in natural killer (NK) cell and neutrophil recruitment to tissues inflamed during acute COVID-19 for SARS-CoV-2-infected cell elimination and promotion of wound healing. Therefore, 100% IL-8 reduction in PASC individuals could be responsible for a few PASC symptoms.
IFNγ is expressed by NK cells of the innate immunological system and the cluster of differentiation (CD)4+ T lymphocytes and CD8+ T lymphocytes (CTL) of the adaptive immunological system post- SARS-CoV-2 antigen-targeted immunity development. IL-12 and IFNγ drive T helper 1 (Th1) lymphocyte differentiation and subsequent TNFα and IL-2 expression. The lack of IFNγ expression in the sera of PASC patients indicated severe immune exhaustion or dysfunction.
The reduced expression of ILs-2,4 among PASC patients indicated that lesser T lymphocytes differentiated into Th2 cells and lower expression of cytokines secreted by Th2 cells, including IL-5, 6, 9, and 13. The IL-6 cytokine is mainly involved in Th17 cell differentiation, and therefore the lower IL-6 expression in the present study indicated incomplete Th17 cell differentiation, with resultant lower IL-17 expression.
Overall, the study findings showed significantly lower IL-8 and IFNγ cytokine expression in PASC patients, indicating that immune exhaustion perpetuates PASC. The study findings could aid in developing techniques for intervening and potentially restoring immune function concerning PASC.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.