3. CD16+ Monocytes and Spike Protein Persistence
Remarkably, the spike protein has been found to persist in human CD16+ monocytes circulating in the blood as much as 15 months after infection with SARS-CoV-2 [20]. Spike persistence was associated with long COVID symptoms, and it was suggested that persistent spike presence could explain lingering symptoms. This was not reflecting an active infection, as only fragmented SARS-CoV-2 RNA was found in these PASC patients. This finding is mysterious, as 15 months seems too long for either a protein or a messenger RNA molecule to survive.
It is possible that this feat is achieved through a process that includes reverse transcription of the mRNA into DNA [21]. A recentin vitro study demonstrated that human liver cancer cells are able to convert the mRNA from the COVID vaccines into DNA within six hours of exposure [22]. Cancer cells are known to often express high levels of long interspersed nuclear element-1 (LINE-1), a retrotransposon that is capable of reverse transcribing mRNA into DNA. Expression of LINE-1 is higher in tumors with p53 mutations [23].
Furthermore, and remarkably, tumors release extracellular vesicles containing retroelements that can be taken up by circulating monocytes, especially under inflammatory conditions [24]. This suggests a mechanism by which the CD16+ monocytes could acquire the capability to reverse transcribe mRNA. Alternatively, tumor cells could be releasing exosomes containing mRNA coding for the spike protein, which is then taken up directly by the circulating monocytes and translated into protein [25].
Furthermore, the CD16+ monocytes themselves are likely to be long-lived. The CD16+ subset of circulating monocytes is known as the “inflammatory” subset, because they typically release higher amounts of inflammatory cytokines such as tumor necrosis factor-α ( TNF-α), and low levels of the anti-inflammatory cytokine IL-10, in response to toll-like receptor (TLR) stimulation, compared to “classical” CD16-monocytes. Normally, they make up 10-20% of the circulating monocyte pool, but their numbers are expanded in association with inflammatory conditions.
MicroRNAs are short single-stranded non-coding RNA molecules that function in post-transcriptional regulation of gene expression. MiR-146a in particular is a well-known marker for a “senescent” phenotype. The basal level of this miRNA is significantly higher in CD16+ monocytes than in the classical monocytes. Senescence is a long-lived state of irreversible proliferative arrest. The senescent monocyte remains alive for an extended period, continually releasing inflammatory cytokines [26].
Another mechanism by which the spike protein could persist long-term would be through its misfolding into a protease-resistant form. The spike protein is a glycoprotein, and glycoproteins from viruses have been shown to facilitate the spreading of proteopathic seeds. In a seminal experiment, cells propagating tau aggregates were transfected with a vector coding for the spike protein. The S1 segment was identified in lysates of transfected cells, and also showed up in extracellular vesicles secreted by these cells. ACE2-equipped HEK cells served as recipients, and it was demonstrated that the presence of spike protein expression in the source cells significantly increased the number of recipient cells with induced aggregates [27].
In a study investigating the durability of spike protein production following vaccination, abundant spike protein was still present in germinal centers in draining lymph nodes 16 days after the second vaccine, and spike antigen was still present as late as 60 days after the second vaccine [28]. A 2022 study by Bansal et al. showed that the spike protein appeared in circulating exosomes 14 days after the first mRNA vaccine dose, and that spike-containing exosomes were still detectable four months later. They argued that these exosomes played an essential role in the induction of antibodies [29].