The reports keep coming in. This update is all about muscle autoantibodies. A group out of the UK compared auto-antibodies from 84 Covid-19 patients with 32 individuals who had been in the intensive care unit for other reasons. The Covid-19 group was subdivided into 24 who had mild symptoms that did not require hospitalization, 25 with severe Covid-19 whose blood was drawn while in the intensive care, and 35 who had severe Covid-19 whose blood was drawn 3-6 months into recovery.  Groups 1-4 were scored via the WHO ordinal Covid-19 severity scale.
|1||no||Covid||2||acute||3||severe||6 mo||4||mild||6 mo|
The bottom line is that these antibodies are staining areas that also contain the cytoskeleton protein β-actin.
There is a light area of disagreement with X-axis title in Figure 2 that is a bar graph that lists the percentage of patients in the four groups as a function of the independent variable of 0, 1, 2, and 3 auto antibodies. The heart and parietal cell auto antibody staining looks very much like actin staining with rhodamine phalloidin. Actin and α-actinin are proteins common to parietal cells and the three types of muscle. An autoantigen against α-actinin released from airway or GI tract epithelial cells may cross react with α-actinin isoforms found in smooth, skeletal and cardiac muscle. The Wikipedia actin page list the six isoforms of mammalian actin. This particular Wikipedia page has grown quite extensively in its coverage of the many binding partners of actin in multiple tissue types. The “tissue specific” auto antibodies that very much align where we expect to see actin, could be recognizing actin or anything that binds to actin.
A group out of the United Kingdom took a different approach:
- They purchased recominant human IgG1 monoclonal antibodies made against Covid-19 spike protein S1 and S2 domains and SARS-CoV-2 nucleoprotein. 
- They purchased the following purified proteins: cardiolipin, actin, myelin basic protein (MBP), tropomyosin, ganglioside GM1, insulin, liver microsomes, transglutaminases (tTGs), enolase, beta-amyloid protein, tau protein, somatotropin, human serum albumin (HSA), and dipeptidylpeptidase.
- The ability of human and rabbit Covid-19 antibodies to react with their assortment of purified proteins was tested. They used optical density (OD) using an ELISA assay.
- Portions of the sequences of Covid-19 proteins were aligned with cytoplasmic actin
|Antigens||Spike protein OD||% reactivity||Nucleo|
|% reactivity||Envelope protein OD||% reactivity||Membrane protein OD||% reactivity|
|SARS-CoV-2||3.4||100 ++++||3.76||100 ++++||3.68||100 ++++||3.78||100 ++++|
|Actin||0.74||17.6 +||1.1||27.1 ++||0.78||18.0 +||0.95||22.2 +|
|NFP||1.98||56 ++++||0.42||8.5 +||0.27||3.7 –||2||51.1 +++|
|Alpha-myosin||0.72||17.0 +||0.89||21.3 +||0.36||6.2 +||0.94||21.2 +|
|Tropomyosin||0.21||1.2 –||0.25||3.8 –||0.25||3.1 –||0.58||12.1 +|
|Int epi cells||0.49||9.9 +||0.46||9.6 +||2.13||56.0 ++++||0.95||22.2 +|
|SARS-CoV-2 antigen||SARS-CoV-2 sequence||Mapped start to end||Actin sequence||ID (%)|
|Chain A, Spike protein||GKIQDSLSST||16–25||GSILASLS-T*||60|
|Chain A, Spike protein||STEKSNII||85–92||STMKIKII*||63|
|Chain A, Spike protein||IGAGICAS||697–704||IGGSILAS*||63|
|Chain A, Spike protein||PS–GRLVPR||1,210–1,217||PSIVGR–P||60|
|Chain A, Nucleoprotein||SSSTKKS||15–21||SSSLEKS||71|
|Chain A, Nucleoprotein||TEGALNTPK||90–98||TEAPLN-PK||67|
|Chain A, Spike protein||SVLYNSASFSTF||33–44||SIL—ASLSTF**||58|
|Chain B, Spike protein||SVLYNSASFSTF||48–59||SIL—ASLSTF**||58|
|Chain C, Spike protein||SVLYNSASFSTF||48–59||SIL—ASLSTF**||58|
|Chain E, Spike protein||SVLYNSASFSTF||37–48||SIL—ASLSTF**||58|
|Chain E, Spike receptor binding domain||SVLYNSASFSTF||48–59||SIL—ASLSTF**||58|
The human cytoplasmic β-actin as protein blasted against the human database. β-actin shares 94-99% sequence homology to the other five isoforms of mammalian actin.
Since we have been examining cardiac tissue in Figure 1 and 2, Figure 3 shows the alignment between and cardiac actin. The top line is the sequence of β-actin, the third cardiac actin, and the middle the sequence homology. A “+” indicates substitution with a similar amino acid . Regions that share homology with Covid protein are highlighted. Note that these overlapping regions also share homology with cardiac actin. Table 5 from reference  contains some homology between non spike proteins and β-actin. These include some non structural proteins and helicases.
Coivd-19 patient sera certainly seem to have antibodies that bind to actin in a variety of tissue sections.  This report stopped short of showing that these polyclonal antibodies actually recognized actin.  Another report demonstrated binding of human recombinant monoclonal antibodies can recognize purified actin.  This report offered some nice speculations as to molecular mimicry with the spike protein and other Covid proteins. The implication of the latter report are interesting in that they suggest the potential for developing a Long Covid auto immune disorder from the vaccine. Finally, autoantibodies against an extremely abunant protein like actin imply a major immune response to even a slight injury that releases actin into the circulation. The high degree of homology between actin isoforms of actin only more problematic.
- Richter AG, Shields AM, Karim A, Birch D, Faustini SE, Steadman L, Ward K, Plant T, Reynolds G, Veenith T, Cunningham AF, Drayson MT, Wraith DC. Establishing the prevalence of common tissue-specific autoantibodies following severe acute respiratory syndrome coronavirus 2 infection. Clin Exp Immunol. 2021 Jun 3. free article
- Vojdani, A., Vojdani, E., & Kharrazian, D. (2021). Reaction of Human Monoclonal Antibodies to SARS-CoV-2 Proteins With Tissue Antigens: Implications for Autoimmune Diseases. Frontiers in immunology, 11, 617089. PMC free article