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In their recent discussion on the anti-inflammatory activity of ivermectin in sepsis, DiNicolantonio et al.  left open the question on how ivermectin may protect against COVID-19 initial symptoms and progression.
Insight was published by Li et al.  in their derivation of an ivermectin host protein and SARS-CoV-2 host protein interaction network (PPI). For this they metabolically labelled proteins in an ovarian cancer cell line and determined which proteins were upregulated and downregulated related to a 24 hour exposure to ivermectin versus no exposure. There were 4,447 identified proteins differentially regulated by ivermectin. When compared with the 284 host proteins known to be affected by SARS-CoV-2, this left 52 proteins in common, 50 of which were downmodulated. Only two proteins, HMOX1 and IL1F10 were upregulated by ivermectin.
This protein-protein interaction (PPI) network revealed EGFR at the center of the pathway with connections to mTOR/APOE, NFKB1/APP, AKT, MAPK1, and CASP3 through TGFB1 interacting with the protein ALB (albumin). BSG, recently shown to be absolutely essential for foam cell formation in macrophages  was also captured in the PPI network. Moreover, foam cell formation has been shown in macrophages to be mediated under the direction of EGFR  as well as for the foamy sebocytes of sebaceous glands which line the mucosal surfaces and may be an important site of viral entry . Foam cell formation is important in ho...
This protein-protein interaction (PPI) network revealed EGFR at the center of the pathway with connections to mTOR/APOE, NFKB1/APP, AKT, MAPK1, and CASP3 through TGFB1 interacting with the protein ALB (albumin). BSG, recently shown to be absolutely essential for foam cell formation in macrophages  was also captured in the PPI network. Moreover, foam cell formation has been shown in macrophages to be mediated under the direction of EGFR  as well as for the foamy sebocytes of sebaceous glands which line the mucosal surfaces and may be an important site of viral entry . Foam cell formation is important in host defense as it relates to induction of a foamy virus of humans which may protect against emerging RNA viruses .
Interestingly, foamy sebocytes have the distinct highly vacuolated morphology of the lipid body negative foamy macrophages (LB-FMs) which produce human endogenous retrovirus K102 (HERV-K102), the elusive, protector foamy virus of humans . In this regard, an examination of the GEO profiles [https://www.ncbi.nlm.nih.gov/geo/tools/profileGraph.cgi?ID=GDS3215:38935] for sebocytes reveals that sebocytes indeed express HERV-K102 (ERVK-7) along with BSG and bear all the relevant differentially expressed genes of the LB-FMs  including CD14, CD16 and CD68. Technically this definitively establishes the sebocytes as macrophages. As sebocytes undergo programmed cell death on day 7, just like the LB-FMs , this may be the first realization that sebocytes are LB-FMs which constitutively produce HERV-K102 particles. Thus, it seems HERV-K102 is positioned to provide the first line of defense against SARS-CoV-2 an emerging pandemic RNA virus. The relevance of this in terms of immunosenescence is discussed below.
The damage associated molecular pattern high mobility group box 1 (HMGB1) released by dying cells and which acts as a ligand stimulating TLR4 was also captured in the ivermectin SARS-Cov-2 host PPI network. DiNicolantonio et al, had in an earlier paper discussed how ivermectin may counteract sepsis by the inhibition of the LPS-like TLR4 responses in murine models .
However, Li et al  may have mistakenly labelled alpha-fetoprotein (AFP) as albumin (ALB) in the PPI network. First, 8 of the 8 sequenced peptides for ALB did not distinguish it from AFP which is related with 45% amino acid homology . Second, AFP is a better fit than ALB since 1) TGFB1 is known to bind and activate AFP [10,11], 2) AFP binds and inactivates CASP3 inhibiting apoptosis [12,13], 3) the main AFP signalling pathway involves PI3K/Akt/mTOR signalling  but also TGFB1 and MAPK pathways , 4) ovarian cancer cells express AFP but not ALB  and 5) SARS-CoV-2 infection upregulates AFP mRNA and protein in HuH-7 cells associated with hyperactivation of the Erbb2/EGFR pathway via PI3K/Akt/mTOR [17, and Dr. Ujjwal Neogi, personal communication]. Assuming that AFP replaces ALB in the PPI network of Li et al , it is remarkable then that ivermectin blocked AFP protein expression by 100% . This identifies ivermectin as a potent AFP antagonist. Overall, this PPI network is in agreement with previous reports that ivermectin binds EGFR and reverses drug resistance in cancer cells via EGFR/ERK/Akt/NFKB1  and that it may downregulate Akt/mTOR hyperactivity . Thus, this helps to explain the reports of anti-tumor activity of ivermectin in addition to its potential biological effects against SARS-CoV-2.
How this connects to immunosenescence and COVID-19 pathogenesis is as follows. First immunosenescence was defined in the new immunosenescence paradigm of 2015 as the failed lytic release of human endogenous retrovirus K102 (HERV-K102) protector foamy virus particles from LB-FMs . Failure to release the protector foamy virus by lysis on day 7 was proposed to be due to active AFP. In part the activity of AFP relates to the DHEA/cortisol ratio which notably declines with age and/or with onset and progression of chronic illness. In this regard, DHEA but not DHEA-S was shown to bind and inactivate AFP blocking its ability to inhibit programmed cell death in primary cells . As well, AFP contains glucocorticoid response elements in its promoter  and is upregulated by corticosteroids in humans . Accordingly, AFP activity relates in part to the DHEA/cortisol ratio and may help explain the age/stress connection with immunosenescence and COVID-19 susceptibility. In COVID-19 hospitalized patients, 70% showed elevated cortisol levels between days 1-2 and 3-5  consistent with a role of SARS-Cov-2 infection mediating a heightened risk of immunosenescence.
Although the PPI network elaborated by Li et al  was not informative for p53, possibly due to its inactivation in ovarian cancers which are notably aggressive, nevertheless, in vivo and in vitro evidence suggests SARS-CoV-2 downmodulates TP53 [24,25]. One of the most important malignancy conferring genes downregulated by TP53 is AFP [26,27]. Pathogenic viruses such as Hepatitis B Virus may specifically target p53 resulting in the upregulation of AFP such as by HBx protein . On the other hand, silencing AFP leads to the upregulation of TP53  and the downmodulation of TGFB1 . SARS-CoV-2 also strongly upregulates TGFB1 in monocytes and macrophages in vitro  and in vivo associated with COVID-19 severity . An emerging picture of failed lytic release of the protector HERV-K102 particles from foamy macrophages by SARS-CoV-2 appears to involve the upregulation of TGFB1 which may bind and activate AFP (with  or without  upregulation at the protein/mRNA level) and the concomitant downregulation of TP53.
In addition to failed lytic release of HERV-K102 particles from LB-FMs, there is also evidence for the inhibition of foam cell formation in vitro. For example, as shown in ACE2 transfected A549 cells, not only is TP53 significantly downmodulated by SARS-CoV-2 infection but also BSG . Moreover, the mevalonate pathway which generates cholesterol for the production of vacuoles and HERV-K102 particles is uniquely negatively affected by SARS-CoV-2 infection amongst respiratory viruses . As well, hypoxia inducing factor -1A which induces foam cell formation in response to hypoxia was downregulated by SARS-CoV-2 infection in HuH-7 cells . In vivo Chow et al  have shown that BSG is downmodulated when bronchoalveolar lavage fluids (BALF) from healthy controls are compared with patients with severe COVID-19.
The induction of HERV-K102 proviral genomic RNA is initiated by interferon-gamma (IFNG) which induces the transcription factor Interferon Regulatory Factor 1 (IRF1) which binds to the HERV-K102 LTR . DDX6 is needed for the export of the RNA genomes to the cytoplasm for incorporation into particles . An alternative activation of IRF1 is through the LPS or LPS-like response via TLR4 . However here, this response not only downmodulates IFNG signalling but also interferes with lipogenesis needed for foam cell formation and HERV-K102 particle production . Consistent with this notion, while IRF1 is upregulated in BALF in patients with severe COVID-19 [24,31], DDX6 is downmodulated . Indeed, in a 33 differentially expressed gene (DEG) classification system of COVID-19 sepsis into 3 groups according to clinical parameters and mortality, the coagulopathic category that had the highest levels of D-dimer and ferritin, had the highest mortality at 42.3 %. Here the downregulation of DDX6 was part of the classification DEGS for this category . Potentially this implies little if any HERV-K102 particle production or foam cell formation. The inflammopathic group featured the highest levels of CRP and IL-6 and presented with 17. 9 % mortality . Here elevated LCN2 was part of the defining DEGS for this group, and LCN2 was shown to be upregulated on day 7 associated with the induction of apoptosis in sebocytes . Potentially this may indicate particle production but failed lytic release of the HERV-K102 particles. The adaptive category with 0 % mortality, younger age and the best clinical laboratory findings featured the upregulation of TP53BP1 which is involved in apoptosis . All together the sepsis endotype groups appear to categorize COVID-19 risks based on HERV-K102 activity: adaptive (0% mortality): no issues with the production and release of HERV-K102; inflammopathic (17.9% mortality): issues with the release of HERV-K102 particles but production might be okay; and coagulopathic (42.3 % mortality) and possible problems with HERV-K102 particle production and thus little foam cell formation. Clearly it will be important to determine HERV-K102 particle production (release versus macrophage content) for the different categories of sepsis endotypes.
According to Liao et al.  and Ren et al.  there are two major subsets of macrophages in BALF from severe COVID-19 patients (sBALF) which appear to be alternative forms of the LB-FMs. These cells comprise over 50% of the major cell types in sBALF . As well, it should be appreciated that in sBALF but interestingly, not BALF from patients with moderate COVID-19, the myeloid cells have more SARS-CoV-2 RNA than the epithelial cells, despite the absence of expression of ACE2 and TMPRSS2 . From Liao et al, both macrophage subsets appear to exhibit epithelial mesenchymal transition (EMT, the malignant phenotype), and this is associated with glycolysis, apoptosis resistance, and dysfunctional p53 suggesting immunosenescence in both major groups. Group 3 may express more TGF-1 and exhibits more notable TGFBR dysfunction, which are also features of the malignant phenotype. While the Group 2 (Macro c2) features inflammation associated with mir155 expression which promotes M1 proinflammatory polarization , the Group 3 (Macro c1) which may be 1.7 times more dominant  features coagulation which is associated with higher PPARG and NR1H3 expression. NR1H3 is the liver x receptor alpha which controls cholesterol homeostasis and is considered anti-inflammatory . PPARG encodes the protein peroxisome proliferator activated receptor gamma (glitazone receptor) which regulates fatty acid storage, glucose metabolism, and adipocyte/foam cell differentiation and is also considered anti-inflammatory . When compared with the sepsis endotype categories , it appears Group 3 may be contributing to the coagulopathic highest mortality category (42.3%), while Group 2 may be contributing to the inflammopathic category with significant mortality (17.9%). The finding that TGFB1 may be more upregulated in the highest mortality category is consistent with the notion that TGFB1 binds and activates AFP [10, 11], and that AFP may drive immunosenescence as originally proposed  and COVID-19 mortality. Thus, SARS-CoV-2 infection itself may exacerbate pre-existing immunosenescence in parallel with its upregulation of cortisol .
Additional clinical evidence is consistent with SARS-CoV-2 targeting HERV-K102 particle production. Lieberman et al.,  showed that both MX1 (elevates on day 7 with apoptosis of sebocytes) and IRF1 (induces HERV-K102 proviral genomic RNA ) were significantly upregulated in nasopharyngeal swabs when SARS2 positive samples were compared with those that were SARS2 negative. On the other hand, when a comparison was made of high SARS2 viral load to low, neither MX1 nor IRF1 were elevated, suggesting SARS2 at higher inoculums may block apoptosis, HERV-K102 particle upregulation and/or that pre-existing immunosenescence may allow for higher viral loads.
As mentioned in the earlier paper by DiNicolantonio et al , Zhang et al had demonstrated that ivermectin prevented the lethality of intraperitoneal injection of LPS in mice where the IC50 was 4 mg/kg . According to DiNicolantonio et al  this extrapolates to 18 mg for a 70 kg adult. A recent clinical trial (preprint) used 15 mg/70 kg and although underpowered for mortality, showed significance for ivermectin inhibition of CRP, D-dimers and ferritin in patients with severe COVID-19 . As mentioned above, the TLR4 signalling which results in elevated IRF1 appears to abolish IFNG signalling and lipogenesis  needed for HERV-K102 genome expression . Accordingly, this sepsis inducing TLR4 signalling may qualify under the umbrella of immunosenescence as it also leads to the failed lytic release of HERV-K102 particles through blocking foam cell formation. Thus, accumulating evidence suggests ivermectin may be useful for the treatment of all stages of COVID-19 as all stages may be related to immunosenescence.
In summary, from the PPI network, ivermectin may counteract the effects of SARS-CoV-2 in the dysregulation of the EGFR/ERBB2 pathway involved in foam cell formation of LB-FMs through its antagonism to AFP and/or the EGFR/ERBB2 mediated hyperactivity of the PI3K/Akt/mTOR pathway Ivermectin appears to reverse or prevent immunosenescence and EMT which re-establishes the HERV-K102 innate protector system. Since BSG has been demonstrated to mediate SARS-CoV-2 entry into cells , it appears that SARS-CoV-2 particularly with progression to severe COVID-19, may directly target the LB-FMs and sebocytes to block HERV-K102 particle production and/or release. Interestingly, HERV-K102 may also be the target of another pandemic RNA virus, HIV-1 [45,46]. Potentially this further substantiates the importance of HERV-K102 and ivermectin for pandemic preparedness.
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