After its identification in December 2019 in Wuhan, China, COVID-19 has rapidly spread and evolved into a pandemic over a few months. The pathophysiology of this infection is not completely understood; however, the causative agent SARS-CoV-2 shares 79% genomic similarity with SARS-CoV. SARS-CoV infects host cells expressing the angiotensin-converting enzyme 2 (ACE-2) receptor, including airway epithelium, alveolar lining, endothelial cells, and alveolar macrophages. SARS-CoV-2 is postulated to infect cells via the ACE-2 and transmembrane protease serine 2 (TMPRSS2) receptors, leading to host cell death while triggering monocyte/macrophage activation, T-cell activation, cytokine release, and B-cell mediated antibody production. Furthermore, studies have demonstrated that SARS-CoV is able to infect and replicate in peripheral blood mononuclear cells.[12,13] Transcriptomic analysis of peripheral blood in COVID-19 patients, however, has not demonstrated presence of SARS-CoV-2 RNA in hematopoietic cells. Activation of genes in the proapoptotic and p53 signaling pathway was identified in the peripheral blood mononuclear cells of patients with SARS-CoV-2.
Laboratory abnormalities reported in COVID-19 patients include lymphopenia (>40% patients), leukocytosis, leukopenia, neutrophilia, monocytosis, and eosinopenia.[4–6] These laboratory abnormalities were also reported in SARS-CoV patients. Lymphopenia is the predominant hematologic finding associated with SARS-CoV-2 in the literature and is reported to predict disease severity. Several hypotheses exist regarding mechanism of lymphopenia in COVID-19 patients, including direct viral toxicity due to ACE-2 receptor expression on lymphocytes, cytokine-induced lymphocyte apoptosis, and metabolic products causing lymphocyte inhibition. In our cohort of cases, absolute lymphopenia by CBC was only seen in 2 cases, but flow cytometric analysis revealed absolute T-cell count reduction in 6 of 7 cases. All patients with decreased T cells were admitted to the intensive care unit and required mechanical ventilation. Lymphocyte subset alterations are reported to be associated with disease activity in COVID-19 patients, and flow cytometry is a sensitive modality to detect that. No specific CBC abnormalities were identified in COVID-19 cases compared to the control group in this study.
There is limited literature on peripheral smear findings in COVID-19 patients. Zini et al reported presence of APHA, prominent abnormal neutrophilic granulation, monolobate neutrophils, granulocytic left shift, large abnormal platelet morphology, apoptotic cells, and few reactive lymphocytes. These abnormalities were not quantified in their study. Interestingly, the neutrophilic morphologic abnormalities almost entirely disappeared after 1 week of antiviral/anti-inflammatory treatment in a subset of cases. Our study is in concordance with that of Zini et al in that there is conspicuous presence of APHA including monolobate neutrophils in the peripheral blood of COVID-19 cases compared to control cases. Pelger-Huët anomaly is a benign hereditary condition where mutations in the lamin B receptor (LBR) result in hyposegmented neutrophils with dense chromatin. LBR plays an important role in maintaining the structure of nuclear membranes. Acquired causes of this anomaly include myelodysplastic syndrome, infections like tuberculosis, HIV/AIDS, influenza A, mononucleosis, parvovirus, and drugs such as immunosuppressive agents and some antibiotics. There was no evidence of medications associated with APHA identified in our patient cohort. In the 2 cases with HIV/AIDS, the viral loads were undetectable. The mechanism behind APHA is unclear. Some hypotheses include acquired mutations in LBR gene and accelerated apoptosis. In light of evidence regarding enriched apoptotic activity by SARS-CoV-2, that hypothesis is a possibility. APHA is not reported in association with SARS-CoV.
Barring myeloid neoplasms, left shift of granulocytes is conventionally interpreted as a sign of bacterial infection. Neutrophil kinetics in bacterial infections is thought to trigger mobilization of marrow reserves, resulting in left shift of granulocytes. Of the 8 patients with left shift in our study, only 2 patients had concurrent bacterial infections. SARS-CoV-2 induced cytokine release causing neutrophil migration akin to bacterial infections is a possibility. Direct myelotoxicity caused by the virus or marrow overproduction in a background of increased peripheral cell turnover are other hypotheses. Leukoerythroblastosis, defined as circulating immature granulocytes and nucleated RBCs with or without anemia, is typically associated with marrow infiltrative processes but may also represent marrow response to stressors like hypoxia, peripheral destruction/sequestration, or sepsis. Peripheral blood leukoerythroblastosis in a COVID-19 patient was reported by Mitra et al This was noted in 1 case in our cohort as well. Given the lack of any other evidence suggestive of an underlying myeloid neoplasm or malignancy causing a myelophthisic process, this may represent marrow stress and response to the viral infection.
Atypical/reactive lymphocytes in the peripheral smear is the hallmark of some infections, such as infectious mononucleosis, Bordetella pertussis, and hantavirus.[24–26] Infectious mononucleosis shows a spectrum of reactive/pleomorphic lymphocytes, while hantavirus shows predominantly Downey type III cells or immunoblasts. These are not specific findings and may be seen in varying numbers in other infections, autoimmune diseases, and malignancies. Peripheral smear review of 32 COVID-19 cases from Singapore reported presence of circulating reactive lymphocytes with predominantly lymphoplasmacytoid morphology in 72% of cases. Our study also shows a spectrum of atypical lymphocytes in almost all COVID-19 cases, with plasmacytoid lymphocytes present in greater frequency in COVID-19 cases compared to the control group. However, variant lymphocytes comprise a minority of lymphocytes (<10%) in most patients. This finding may be similar to SARS-CoV, where reactive lymphocytes were not identified in significant numbers by Chng et al and seen in only about 15% cases by Lee et al. Quantitation of reactive lymphocytes was not performed in other SARS-CoV or COVID-19 studies.
In summary, we report CBC and peripheral smear findings on admission in 12 symptomatic patients who tested positive for COVID-19 and compared them to a control group. To the best of our knowledge, this is the first study to quantify individual morphological findings on peripheral smear in COVID-19 cases. We acknowledge the limitations of our study, especially the number of peripheral smears available for review. Larger series of cases with peripheral smear review during and after treatment will be of interest, to study their potentially transient nature and correlation with disease activity.
Am J Clin Pathol. 2020;154(3):319-329. © 2020 American Society for Clinical Pathology