Early Phases of COVID-19 Are Characterized by a Reduction in Lymphocyte Populations and the Presence of Atypical Monocytes

Abstract

Background: Severe acute respiratory syndrome coronavirus 2 is a recently discovered pathogen responsible of coronavirus disease 2019 (COVID-19). The immunological changes associated with this infection are largely unknown.

Methods: We evaluated the peripheral blood mononuclear cells profile of 63 patients with COVID-19 at diagnosis. We also assessed the presence of association with inflammatory biomarkers and the 28-day mortality.

Results: Lymphocytopenia was present in 51 of 63 (80.9%) patients, with a median value of 720 lymphocytes/µl (IQR 520-1,135). This reduction was mirrored also on CD8+ (128 cells/µl, IQR 55-215), natural killer (67 cells/µl, IQR 35-158) and natural killer T (31 cells/µl, IQR 11-78) cells. Monocytes were preserved in total number but displayed among them a subpopulation with a higher forward and side scatter properties, composed mainly of cells with a reduced expression of both CD14 and HLA-DR. Patients who died in the 28 days from admission (N=10, 15.9%), when compared to those who did not, displayed lower mean values of CD3+ (337.4 cells/µl vs 585.9 cells/µl; p=0.028) and CD4+ cells (232.2 cells/µl vs 381.1 cells/µl; p=0.042) and an higher percentage of CD8+/CD38+/HLA-DR+ lymphocytes (13.5% vs 7.6%; p=0.026).

Discussion: The early phases of COVID-19 are characterized by lymphocytopenia, predominance of Th2-like lymphocytes and monocytes with altered immune profile, which include atypical mononuclear cells.

Keywords: COVID-19; SARS-CoV-2; immune profiling; inflammation; monocytes; peripheral blood mononuclear cells.

Figure 1

Violin plots showing lymphocyte and monocyte subpopulations (reported as median value) assessed in 63 patients with COVID-19 at diagnosis and 10 controls (CTR). (A) Natural killer cell subpopulations (B) CD4+ cells activation; (C) CD8+ cells activation (D, E) CD4+ cells polarization; (F) Monocytes subpopulations. [Th1-like (CXCR3+CCR6-): Type 1 T helper-like cells; Th2-like (CXCR3-CCR6-): Type 2 T helper-like cells; Th17-like (CXCR3-CCR6+): T helper 17-like cells; Tregs: regulatory T cells; NK: natural killer cells]. t-test was used to calculate statistical difference. (** p value <0.01; *** p value <0.005).

Figure 2

Morphologic and phenotypic differences between monocytes from a healthy donor (left) and a COVID-19 patient (right): (A) flow cytometry dot plot showing normal and high-SC monocytes; (B) monocytes from May-Grunwald Giemsa stained blood smears (100X magnification, Leica Microscope DMLS); (C) histograms showing median fluorescence intensity (MFI) of CD14, CD16, and HLA-DR on monocytes from healthy donors (HD, n=10), normal and atypical monocytes from COVID-19 patients (n=14). Non parametric Kruskal Wallis test was used to determine significant difference. Kolmogorov-Smirnov test was used to assess normal distribution of samples. (* p value <0.005).