Lymphopenia and Cytokine Storm in COVID-19: Comparative Analysis of T Cell Subsets and Serum Cytokine Profiles Across Disease Severity Groups

Abstract

COVID-19 disease severity ranges from asymptomatic infection to critical illness, with immunological dysregulation emerging as a central determinant of outcomes. Italy and Spain were among the countries most severely affected in the first wave of the pandemic (February-May 2020), providing extensive clinical cohorts for immunological characterisation. This study investigated T lymphocyte subset dynamics and serum cytokine profiles across COVID-19 severity groups in a multicentre Southern European cohort. A total of 160 RT-PCR-confirmed COVID-19 patients were enrolled and stratified into mild (n=40), severe (n=40), and critical (n=40) groups, with 40 age- and sex-matched healthy individuals as controls. Flow cytometric immunophenotyping was performed for CD3+, CD4+, CD8+, CD19+, and NK cell populations. Serum concentrations of IL-6, IL-10, IL-1β, TNF-α, IFN-γ, and IL-17A were quantified by multiplex immunoassay. CD4+ and CD8+ T cell counts declined progressively with disease severity (critical group: CD4+ 180±45 cells/μL, CD8+ 120±38 cells/μL). IL-6 showed the most dramatic elevation in critical patients (24.1-fold vs. healthy controls). A strong negative correlation was observed between IL-6 levels and CD4+ T cell counts (r = -0.78, p < 0.001). These findings provide cellular and molecular evidence for concurrent lymphopenia and cytokine storm characterising severe COVID-19, with implications for anti-IL-6 immunomodulatory therapeutic strategies.