Immunological imprint of COVID-19 on human peripheral blood leukocyte populations

SARS-CoV-2 has triggered a pandemic that is now claiming many lives. Several studies have investigated cellular immune responses in COVID-19-infected patients during disease but little is known regarding a possible protracted impact of COVID-19 on the adaptive and innate immune system in COVID-19 convalescent patients.

We used multiparametric flow cytometry to analyze whole peripheral blood samples and determined SARS-CoV-2-specific antibody levels against the S-protein, its RBD-subunit, and viral nucleocapsid in a cohort of COVID-19 convalescent patients who had mild disease ~10 weeks after infection (n = 109) and healthy control subjects (n = 98). Furthermore, we correlated immunological changes with clinical and demographic parameters.

Even ten weeks after disease COVID-19 convalescent patients had fewer neutrophils, while their cytotoxic CD8⁺ T cells were activated, reflected as higher HLA-DR and CD38 expression. Multiparametric regression analyses showed that in COVID-19-infected patients both CD3⁺ CD4⁺ and CD3⁺ CD8⁺ effector memory cells were higher, while CD25⁺ Foxp3⁺ T regulatory cells were lower. In addition, both transitional B cell and plasmablast levels were significantly elevated in COVID-19-infected patients. Fever (duration, level) correlated with numbers of central memory CD4⁺ T cells and anti-S and anti-RBD, but not anti-NC antibody levels. Moreover, a "young immunological age" as determined by numbers of CD3⁺ CD45RA⁺ CD62L⁺ CD31⁺ recent thymic emigrants was associated with a loss of sense of taste and/or smell.

Acute SARS-CoV-2 infection leaves protracted beneficial (ie, activation of T cells) and potentially harmful (ie, reduction of neutrophils) imprints in the cellular immune system in addition to induction of specific antibody responses.