A Deadly Embrace: Hemagglutination Mediated by SARS-CoV-2 Spike Protein at Its 22 N-Glycosylation Sites, Red Blood Cell Surface Sialoglycoproteins, and Antibody

Abstract

Rouleaux (stacked clumps) of red blood cells (RBCs) observed in the blood of COVID-19 patients in three studies call attention to the properties of several enveloped virus strains dating back to seminal findings of the 1940s. For COVID-19, key such properties are: (1) SARS-CoV-2 binds to RBCs in vitro and also in the blood of COVID-19 patients; (2) although ACE2 is its target for viral fusion and replication, SARS-CoV-2 initially attaches to sialic acid (SA) terminal moieties on host cell membranes via glycans on its spike protein; (3) certain enveloped viruses express hemagglutinin esterase (HE), an enzyme that releases these glycan-mediated bindings to host cells, which is expressed among betacoronaviruses in the common cold strains but not the virulent strains, SARS-CoV, SARS-CoV-2 and MERS. The arrangement and chemical composition of the glycans at the 22 N-glycosylation sites of SARS-CoV-2 spike protein and those at the sialoglycoprotein coating of RBCs allow exploration of specifics as to how virally induced RBC clumping may form. The in vitro and clinical testing of these possibilities can be sharpened by the incorporation of an existing anti-COVID-19 therapeutic that has been found in silico to competitively bind to multiple glycans on SARS-CoV-2 spike protein.

In Silico Analysis of the Multi-Targeted Mode of Action of Ivermectin and Related Compounds

“…When considering the binding affinities of ivermectin to NTD sites, it is significant that glycan bindings from SARS-CoV-2 and other coronaviruses to host cells are generally weak when univalent but orders of magnitude stronger when multivalent [4,12,29]. Thus, ivermectin, the molecular dimensions of which span approximately 2 × 1 nm [136] (with the length of the spike protein being ~20 nm [137,138]), could block clinically relevant multivalent bindings from the spike protein to host cells by steric interference, even if its actual bindings to some glycan sites on the spike protein were somewhat weaker than predicted.…”

“…In particular, the SA-cleaving enzyme hemagglutinin esterase (HE) is expressed by the human betacoronaviruses that cause the common cold, OC43 and HKU1, but not by SARS-CoV, SARS-CoV-2 and MERS, the three deadly strains in that viral family [39][40][41]. It has been proposed that vascular occlusion, central to the morbidities of COVID-19 [42][43][44][45][46][47], is initially triggered by the clumping and snagging of SARS-CoV-2 with blood and endothelial cells, and that HE expressed by the common cold betacoronaviruses may limit these morbidities [4].…”

“…Obstruction of the binding between SARS-CoV-2 spike protein and the ACE2 receptor on target human cells has been one focus of therapeutic intervention [1,2]. Although viral fusion to host cells and replication occur via ACE2, SARS-CoV-2 and several other viral strains, including other betacoronaviruses, initially attach to host cells via more abundantlydistributed glycoconjugate host binding sites [3,4]. Notable among these are sialic acid (SA) and the transmembrane glycoprotein receptor CD147, which are widely distributed in blood, endothelial and several other cells in the human body [4] and provide additional therapeutic targets.…”

“…Although viral fusion to host cells and replication occur via ACE2, SARS-CoV-2 and several other viral strains, including other betacoronaviruses, initially attach to host cells via more abundantlydistributed glycoconjugate host binding sites [3,4]. Notable among these are sialic acid (SA) and the transmembrane glycoprotein receptor CD147, which are widely distributed in blood, endothelial and several other cells in the human body [4] and provide additional therapeutic targets. Nicotinic acetylcholine receptors, which are densely distributed in neuronal tissue and in cytokine-secreting inflammatory cells (for example macrophages and mast cells), may provide an additional attachment point for SARS-CoV-2 due to a "toxin-like" epitope on the viral spike protein and may thus provide druggable targets as well [5][6][7].…”

Alveolar macrophages: Achilles’ heel of SARS-CoV-2 infection

“… 31 For instance, the NTD of SARS-CoV-2 S1 subunit has a sialic acid-binding pocket that can interact with various sialoproteins or glycoproteins. 32 , 33 Consistently, although ACE2 is used to adsorb SARS-CoV-2, tumor cell-derived microparticles (T-MPs) retain approximately 10% capacity to adsorb viral particles under ACE2-deficient conditions. 16 …”