Early and late consequences of damage to the stem cell compartment following SARS-CoV-2 infection

This application has been reviewed in a previous cycle and we would like to thank reviews for very positive comment. However, to our surprise one reviewer stated that this proposal is not based on our original hypothesis. To address this misunderstanding the role of Nlrp3 inflammasome as a trigger of cytokine storms COVID19 was proposed in the literature for a first time by our team (Ratajczak MZ, Kucia M. SARS-CoV-2 infection and overactivation of Nlrp3 inflammasome as a trigger of cytokine "storm" and risk factor for damage of hematopoietic stem cells. Leukemia. 2020 Jul;34(7):1726-1729. doi: 10.1038/s41375-020-0887-9) and subsequently several groups confirmed this hypothesis and highly cited our initial report. COVID19 or SARS-CoV-2 virus is single-stranded RNA virus, whose infection can be asymptomatic or lead to damage of several vital organs and a fatal complication involving “cytokine storm”, which results in uncontrolled hyperactivation of the immune response by innate immunity cells. The major concern is that we still cannot foresee late complications of this infection including direct or indirect effects on stem cell compartment. SARS-CoV-2 may enter human cells after binding to the angiotensin-converting enzyme 2 (ACE2) receptor and utilizes its surface spike protein (SP) for attachment and entry into the target cells. It has been demonstrated that ACE2 receptor is highly expressed on hematopoietic stem cells (HSCs) and endothelial progenitors (EPCs) isolated from adult hematopoietic organs as well on developmental early precursors of these cells. Its expression increases with more primitive phenotype of cells and it may explain that our group noticed its high expression in addition to HSC and EPC on human very small CD133+CD34+Lin-CD45– cells, which can be specified as reported by us and others into functional HSCs and EPCs. COVID19 after binding to ACE2 may hyperactivate Nlrp3 inflammasome as we recently demonstrated in cells at different level of specification into hematopoietic and endothelial lineage. This may lead to pyroptotic death of the cells exposed to virus SP. Moreover, this could lead also as we postulated of an initiation of “cytokine storm” by innate immunity cells. Evidence accumulates that COVID19 infection despite a fact that it manifests primarily as a respiratory syndrome has significant impact on other organs including the hematopoietic system and endothelium leading to several complications. To support this a large percentage of infected patients, suffer from lymphopenia and thrombocytopenia as well as from damage of endothelium that promotes hypercoagulability. Nevertheless, there are still not very well-known mechanisms how virus affect human stem cells and damage them by productive or abortive infection. It is well known that the innate immune response and activation of the Nlrp3 inflammasome are important defense mechanisms during the first days of infection, until acquired immunity responds with the production of antibodies. However, as mentioned above hyperactivation of this intracellular protein complex in innate cells may induce a cytokine storm or may lead to their death of other cells in mechanism of pyroptosis. Virus may also damage cells by lysis or theoretically what we hypothesize may stay after entry into long living stem cells in a latent form and become activated when immune system becomes impaired. Our group postulated a possibility that damage of stem cells for hemato/endothelial lineage may occur mainly by hyperactivation of Nlrp3 inflammasome after binding of viral SP to ACE2 expressed on these cells. Similar role may play interaction of SP with Toll like receptor-4 (TLR4). Our group and group of Dr. Hal Broxmeyer has demonstrated that exposure of umbilical cord blood-derived HSCs to SP protein decreases viability and in vitro clonogenicity of these cells. We also observed similar effect on proliferation of human EPC. Based on this a central hypothesis of our proposal is that COVID19 infection may damage by SP-ACE2 or SP-TLR4 interaction stem cells from hematopoietic/endothelial lineage which contributes to early and late consequences of this infection.

RNAseq database 

Patient COVID - PRJNA1167900 - https://www.ncbi.nlm.nih.gov/bioproject/PRJNA1167900

scRNA-seq of VSELs: PRJNA1126429 (https://www.ncbi.nlm.nih.gov/sra/PRJNA1126429)

scRNA-seq of HSC: PRJNA1128409 (https://www.ncbi.nlm.nih.gov/sra/PRJNA1128409).