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].