To the Editor, Amid the recent worldwide coronavirus disease 2019 (COVID-19) outbreak caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), there has been increasing interest in the host-pathogen interaction and the resulting immune dysregulation. The role that the innate immune system plays in responding to SARS-CoV-2 is yet to be fully understood. The effect of this virus on the different lymphocyte populations is based on single-case reports and sporadic series [1–5]. Here, we report the findings from our evaluation of the cellular immune response in six patients infected with SARS-CoV-2 from New York City. We used flow cytometry to analyze helper and cytotoxic T cell populations, assess the antibody-secreting cells (ASCs) and examine the expression of activation markers. Briefly, we reviewed flow cytometry studies from six patients who were confirmed positive for the infection by SARS-CoV-2 using nucleic acid testing via RT-PCR of throat swab specimens using RT-PCR. All patients tested positive for SARS-CoV2 within 14 days of the flow cytometry study from our institution, a major tertiary academic center in New York City, which has been a hot spot of the COVID-19 pandemic. Originally, five of the flow cytometry studies were performed as a workup for hematological malignancies. Five peripheral blood and one bone marrow aspirate samples are included. Flow cytometry was performed by five-color analysis on the Navios Flow Cytometer (Beckman Coulter, Miami, FL), and data analysis was performed on the Kaluza Flow Cytometry Analysis Software (Beckman Coulter, Miami, FL). Leukocytes were stained with antibodies against CD3, CD4, CD8, CD19, CD38, and HLA-DR. Fluorochromes included fluorescein isothiocyanate (FITC), PE-cyanine 5 (PC5), PE-cyanine 7 (PC7), PE-Texas Red (ECD), and phycoerythrin (PE).

Our initial measurements defined both CD4+ and CD8+ T cell populations. We calculated the CD4: CD8 ratio. Then, we assessed the expression of CD38 and HLA-DR on both populations as surrogate markers of activation. To screen for ASCs, we elected to use a simplified two-antibody gating strategy by evaluating the expression of CD38 on the CD19+ cells taking into account that this phenotype is expressed in most ASCs regardless of their type. Four out of the six patients included in this study showed varying degrees of lymphopenia (Table 1). Four patients showed characteristically low CD4: CD8 ratio; these four patients showed bright expression of CD38 with partial/dim HLA-DR on the CD8+ T cells indicative of cellular activation (Fig. 1). Analysis of the CD19+ cells for CD38 expression showed no increase in the number of ASCs in any of the patients. The remaining two patients with normal CD4: CD8 ratio showed no expression of CD38 or HLA-DR on the T cells and no evidence of increased ASCs. Although five of the cases had a history of hematologic malignancy, only one case (patient 1) was found to have hematologic neoplasm at the time of the study. For this case, the abnormal population was detected by flow cytometry, whereby this patient had a large B-lymphoblast population (first-time diagnostic study). None of the patients was on treatment for malignancy at the time of study.