High Frequencies of Senescent and Exhausted Natural Killer Cells and SARS-CoV-2-Specific Memory CD8+ T Cells Distinguish Critically Ill from Asymptomatic Unvaccinated COVID-19 Patients

COVID-19 disease severity in unvaccinated individuals spans a striking continuum from complete absence of symptoms to life-threatening critical illness, yet the immunological determinants driving this divergence remain poorly understood. While Natural Killer (NK) cells and CD4+/CD8+ T cells mount distinct responses against SARS-CoV-2, the phenotypic and functional signatures distinguishing protective immunity from pathogenic immune dysfunction have yet to be defined. In the present study, we present a comparative analysis of NK cell and SARS-CoV-2-specific T cell immunity in unvaccinated symptomatic (SYMP) versus asymptomatic (ASYMP) COVID-19 patients. Using multicolor flow cytometry, we interrogated key immune checkpoints, the senescence marker CD57, differentiation status (CD45RA/CCR7), exhaustion marker PD-1, and activation markers HLA-DR and CD38, across NK and T cell subsets from SYMP patients, ASYMP patients, and Healthy Donors (HD). SYMP COVID-19 patients harbor significantly elevated co-expression of exhaustion and senescence markers on both NK and T cells relative to ASYMP patients and HD controls. Within the T cell compartment, SYMP patients exhibit broad dysregulation across naïve, central memory, effector memory, and terminally differentiated effector memory subsets, with CD57 enriched most prominently on CD8+ TEM and CD8+ TEMRA populations. While cytokine profiles did not significantly differ between ASYMP and SYMP patients, both groups showed markedly elevated pro-inflammatory TNF-α, IFN-γ, IL-6, IL-8, and IL-17 compared to HD controls, underscoring systemic immune activation as a hallmark of SARS-CoV-2 infection regardless of clinical severity. Taken together, these findings suggest that NK and T cell exhaustion and senescence are correlates of severe COVID-19, highlighting potential targets for future mechanistic and therapeutic investigation.