To date, clinical manifestations of post-vaccination side effects or injuries have been described but very few mechanisms have been offered to explain these findings. In the present study, we investigated whether an S1 protein mechanism of inflammation similar to what we published in PASC might underlie the persistent, PASC-like symptoms that remain for months following vaccination with currently available vaccines in the US.
Given that ongoing viral replication may not be required for prolonged symptoms9 and given the overlap in symptomatology, we applied machine learning to a panel of previously published immune biomarkers to determine if an immune signature for post-vaccination PASC-like symptoms might exist. Using two algorithms (severity score and long hauler index) previously derived from these biomarkers8, we found that post-vaccination PASC-like symptoms were associated with an inflammatory profile with statistically significant elevations in CCL5, sCD40L, IL-6, and IL-8. Further these patients were classified as PASC using a single classifier and PASC with inflammation using a dual classifier. Elevated IL-8 was a unique marker relative to PASC in post-vaccination individuals with PASC-like symptoms. We recently found a statistically significant correlation between decreased IL-8 and improvement in the NYHA cardiac symptom score in PASC following treatment with a CCR5 antagonist and statin11.
Because of the similarities between PASC and patients with post-vaccination PASC-like symptoms, we examined whether S1 protein persistence might also occur in patients with post-vaccination PASC-like symptoms. We demonstrated a statistically significant elevation of S1 protein containing non-classical monocytes (NCM) and in S1-containing intermediate monocytes (IM) in post-vaccination PASC-like patients compared to normal controls. We sorted these CD16 + monocytes as previously performed and used mass spectroscopy to interrogate whether S proteins (S1 and or S2) were present in these highly mobile cells. We confirmed the presence of S1 sequences as well as S2 sequences in these monocytic subsetss. Further, we found amino acid mutations in all six patients analyzed; the impact of which remains to be elucidated.
NCM bind to fractalkine expressed on vascular endothelial cells through the expression of the fractalkine receptor CX3CR1 on the surface of NCM11. Fractalkine is also upregulated by IL-1, IFN-γ, and TNF-α12, cytokines that we have reported to be elevated in PASC9. CX3CR1 also provides a survival signal to non-classical monocytes through CX3CR1-dependent expression of the anti-apoptotic protein BCL213, 14. Stress and exercise mobilize non-classical monocytes including up to 4-fold with exercise15, 16. The interaction between fractalkine and CX3CR1 has been reported to be involved in the pathogenesis of atherosclerosis, vasculitis, vasculopathies, and inflammatory brain disorders17 and could also be contributing to a vascular endotheliitis in post-vaccination individuals with PASC-like symptoms. Vascular inflammation has been shown to expose the collagen surface and platelet activation/adherence by way of glycoprotein 1b-IX-V-receptor (GPIb-IX-V) with collagen-bound von Willebrand factor (vWF)18,19. Activated platelets also release soluble CD40 ligand (sCD40L) which leads to recruitment of both neutrophils and monocytes to the sites of vascular inflammation, thus activating the coagulation cascade20. Activated platelets also release CCL5/RANTES which binds to endothelial cells, promoting monocyte adhesion to inflamed endothelial tissues21 and acting as a chemoattractant for inflammatory cells. Studies in atherosclerosis have shown that CCR5 antagonists reduce non-classical monocyte recruitment to sites of atherosclerosis21,22. In addition, accumulation of non-classical monocytes can be reduced by statin treatment through reduction in fractalkine expression23,24. Interfering with these pathways may hold potential therapeutic targets for PASC and post-vaccination individuals with PASC-like symptoms10.
Further, activated platelets and endothelial cells also secrete (VEGF) which induces angiogenesis and microvascular hyperpermeability. VEGF contributes to vasculitic neuropathy and also promotes a pro-inflammatory-prothrombotic environment25. Given the elevated levels of sCD40L and the frequent co-expression of VEGF, this pathway may also contribute to the thrombosis seen in some individuals post-vaccination.
Taken together, these findings suggest a possible mechanism for the debilitating symptoms found in some patients weeks and months following vaccination. The findings that the immune profile and persistent S1 protein in CD16 + monocytes suggest that S1 protein persistence is a major contributor not only of symptoms in post-vaccination individuals with PASC-like symptoms but also may be a major contributor of PASC itself given that S1 alone delivered by vaccination can cause similar pathologic features.