Preliminary data from current studies underway in UK (University of Cambridge) and also In Hong Kong (University of Hong Kong) that have yet to be published are indicating that the new emerging subvariants of BA.2 such as the BA.2.2 and the BA.2.3 are totally different from the initial Omicron BA.1 and BA.1.1 variants especially in terms of pathogenesis.
It has been found that the newer emerging BA.2 subvariants are far more aggressive in terms of attacking the lower respiratory tract and the lungs and tends to replicate extremely faster in lung tissues.
Hence it can be seen in all the new cases of infection, acute respiratory distress syndrome (ARDS) and severe lung damage is being witnessed and also the need for oxygen supplementation.
A study published a few days ago found that the SARS-CoV-2 virus attacks T cells including CD4 cells just like HIV and causes immunodeficiency problems.
Since its emergence in December 2019, SARS-CoV-2, the etiology of coronavirus disease 2019 (COVID-19), quickly spread to the majority of countries in the world and posed great threats to public health. The virus shares 79.5% genome identity with SARS-CoV-1 and also uses angiotensin-converting enzyme 2 (ACE2) as a cell entry receptor.1,2,3,4,5 Typical clinical symptoms of COVID-19 patients include fever, fatigue, dry cough, and pneumonia, whereas around 20% of the severe cases may die of multi-organ failure.6,7,8,9
Apart from the respiratory system, multiple organs including the immune system of COVID-19 patients were also targeted by SARS-CoV-2 infection. Notably, lymphopenia was observed in 83.2% of the patients on admission, and fatal infections were associated with more severe lymphopenia over time.6,7,8 Lymphocytes (particularly T cells) play a central role in the human immune system, a decrease of which would result in immune suppression and serious complications.10
It has been proposed that viral-induced lymphopenia might be due to direct infection, cytokine-mediated cell death, tissue sequestration of lymphocytes, or suppression of the bone marrow or thymus for T-cell generation.11 In the case of MERS-CoV, apoptosis induced by direct viral infection of T cells has been observed in vitro, which possibly explained lymphopenia in MERS patients.11 SARS-CoV-1 viral particles were also observed in multiple leukocytes from an autopsy study, suggesting that direct infection might account for the decrease in lymphocytes.12
Similarly, SARS-CoV-2 particles or proteins were also found in the spleen and lymph nodes from a study of 91 deceased COVID-19 cases, suggesting an infection of lymphocytes.13 Furthermore, in COVID-19 immune landscape depicted by single-cell RNA-seq studies, SARS-CoV-2 viral RNA has been found in multiple immune cells, including myeloid cells with phagocytic activity (neutrophil and macrophage) and lymphocytes without phagocytic activity (T, B, and NK cells).14,15
Notably, SARS-CoV-2 RNA-positive immune cells did not co-express the entry factors ACE2 and TMPRSS2, or other hypothesized entry co-factors.14,15 It is speculated that cell-associated SARS-CoV-2 viral positivity may represent a mixture of replicating virus, immune cell engulfment, and virions or virally infected cells attached to the cell surface.14,15
It has been shown that SARS-CoV-2-infected human monocytes, monocyte-derived macrophages, and dendritic cells in vitro, which potentially plays a major role in COVID-19 pathogenesis.16,17 However, whether SARS-CoV-2 infects lymphocytes, which do not express ACE2, to result in lymphopenia is still unknown.
This knowledge gap also brings difficulty for our understanding of how lymphocytes lost the ability to control viral infection. Here, we provided evidence that activated T lymphocytes could be infected by SARS-CoV-2 in an ACE2-independent manner. The infection leads to pronounced T-cell apoptosis in vitro or in patients with COVID-19. Our findings shed light on the understanding of SARS-CoV-2 infection-induced lymphopenia.