Comparative Analysis of Lymphocyte Populations in Post-COVID-19 Condition and COVID-19 Convalescent Individuals

Abstract

Reduced lymphocyte counts in peripheral blood are one of the most common observations in acute phases of viral infections. Although many studies have already examined the impact of immune (dys)regulation during SARS-CoV-2 infection, there are still uncertainties about the long-term consequences for lymphocyte homeostasis. Furthermore, as persistent cellular aberrations have been described following other viral infections, patients with "Post-COVID-19 Condition" (PCC) may present similarly. In order to investigate cellular changes in the adaptive immune system, we performed a retrospective analysis of flow cytometric data from lymphocyte subpopulations in 106 patients with confirmed SARS-CoV-2 infection who received medical care at our institution. The patients were divided into three groups according to the follow-up date; laboratory analyses of COVID-19 patients were compared with 28 unexposed healthy controls. Regarding B lymphocyte subsets, levels of IgA + CD27+, IgG + CD27+, IgM + CD27- and switched B cells were significantly reduced at the last follow-up compared to unexposed healthy controls (UHC). Of the 106 COVID-19 patients, 56 were clinically classified as featuring PCC. Significant differences between PCC and COVID-19 convalescents compared to UHC were observed in T helper cells and class-switched B cells. However, we did not detect specific or long-lasting immune cellular changes in PCC compared to the non-post-COVID-19 condition.

Keywords: COVID-19; SARS-CoV-2; immune system; long-COVID; lymphocyte subpopulations; post-COVID-19 condition (PCC); post-acute COVID syndrome.

Figure 1

Representative gating for detection of T, B, and natural killer (NK) cells (A), as well as B cell subpopulations (B). For quantification of lymphocyte subsets, fluorochrome-conjugated monoclonal antibodies against CD45, CD3, CD4, CD8, CD16/CD56, T cell receptor (TCR) γδ, CD19, and CD38 were used. For gating to determine proportions of B cell subpopulations, monoclonal CD19, CD20, CD38, CD138, CD21, CD27, IgG, IgA, and IgM antibodies were applied.

Figure 2

Box plots of the 25th to 75th percentile of B cell subpopulation counts (CD21 + low B, IgG + CD27+ B, IgA + CD27+ B, and IgM + CD27+ B cells) as well as Natural Killer (NK) cells and natural killer T cells (NKT). The middle line represents the median, and the upper/lower whiskers represent the max/min value within 1.5× the 75th/25th interquartile range, respectively. N = 28 healthy individuals, group 1 (G1) = 85–150 days after symptom onset (n = 21), group 2 (G2) = 151–210 days after symptom onset (n = 46), group 3 (G3) = 211–320 days after onset (n = 39). Statistical testing was performed using the Shapiro–Wilk normality test and Mann–Whitney U test. * p ≤ 0.05, ** p ≤ 0.01.

Figure 3

Box plots of the 25th to 75th percentile of T and B cell counts, as well as T cell subpopulations (T helper, cytotoxic T, activated T helper, activated cytotoxic T, and T cell receptor [TCR] γδ T cells), switched B cells, and transitional B cells. The middle line represents the median, and the upper/lower whiskers represent the max/min value within 1.5× the 75th/25th interquartile range, respectively. N = 28 healthy individuals, group 1 (G1) = 85–150 days after symptom onset (n = 21), group 2 (G2) = 151–210 days after symptom onset (n = 46), group 3 (G3) = 211–320 days after onset (n = 39). Groups 1-3 are colored orange, control group is depicted in green. Statistical testing was performed using the Shapiro–Wilk normality test and Mann–Whitney U test. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001.