MS is a chronic autoimmune disease characterized by the immune system attacking the protective sheath (myelin) that covers and protects nerve fibers in the central nervous system (CNS). The precise cause of MS is unknown, but it is speculated to arise from an environmental event, such as viral infection, that can trigger an autoimmune response in genetically predisposed individuals ^[1][128]. For instance, people who experience Epstein–Barr virus infection are a reportedly 32 times more likely to develop MS than non-infected counterparts ^[2][129]. No other known risk factor (such as HLA-DR15 allele homozygosity) had as strong of a correlation with MS as EBV infection and other viruses did not show a significant correlation with MS, including cytomegalovirus (CMV), a herpesvirus that is transmitted through the saliva, similar to EBV. As previously discussed, MS patients typically express high levels of MSRV and other HERVs belonging to the HERV-W family. Specifically, there is a 1.5- to 3-fold increase in HERV-W Env expression, which acts as a TLR4:CD14 agonist and induces the expression of various pro-inflammatory cytokines to promote neuroinflammation ^[3][130].

Interestingly, it was found that EBV glycoprotein 350 (EBVgp350), the major envelope protein of EBV, can stimulate the expression of HERV-W in astrocytes, monocytes, and B cells, likely through the NF-κB pathway ^[4][131]. This could explain the link between EBV, HERV-W, and the development of MS; wherein, EBV infection stimulates HERV-W expression, which in turn, induces a pro-inflammatory response through TLR4 activation and subsequently drives autoimmunity. This “dual virus hypothesis” relies on the presence of EBV infection shortly before the development of MS. However, while MS patients are significantly likely to have had an EBV infection at one point in time, there is not a direct link between active EBV infection and MS development. It may be possible that once activated by EBV, HERV-W does not rely on EBV infection for its maintenance and may perpetuate its upregulation through a gradual increase in neuroinflammation and degeneration. This is supported by data that show a return of HERV-W expression to baseline following successful intervention (determined by a reduced inflammatory signature and neurodegeneration) with IFNβ therapy, suggesting that HERV-W expression relies on a consistent supply of inflammation ^[5][132].

While HERV-W Env antigen is detectable in the serum of 73% of MS patients (and 0% of healthy controls), it does not appear that HERV-W antigen levels are directly correlated to the severity of the symptoms ^[6][72]. However, from the same study, the HERV-W DNA copy number in PBMCs is increased in chronic progressive MS patients versus relapsing-remitting MS (RRMS) patients and healthy controls, suggesting that HERV-W proviruses undergo expression and re-integration events during active MS. A separate study using an RNA-sequencing approach to more acutely detect the presence and expression levels of specific HERV loci in MS patients revealed that patients with a higher Expanded Disability Status Scale (EDSS) score (indicating a more severe MS condition) have a higher number of active HERV-W loci compared to relapsing and low score EDSS patients ^[7][133]. Interestingly, they also identified 18 other families of HERVs that were more highly expressed in MS patients compared to healthy controls. This suggests that the activation of HERV-W may contribute to the secondary activation of numerous groups of HERVs through the mechanisms described above.

Based on these findings, a monoclonal antibody (GNbAC1) against HERV-W Env was developed for the treatment of MS and showed no signs of toxicity in early clinical testing. In 2021, it completed a randomized phase 2b study with 270 participants (clinical trial: NCT04480307). Although the primary endpoint was not met (reduction in the cumulative number of T1 gadolinium-enhancing (GdE) lesions) at 24 weeks, the monoclonal antibody was found to exert anti-neurodegenerative effects based on other criteria, such as measures of brain atrophy, myelin integrity, and the number of chronic T1 hypointense lesions ^[8][134]. Following these findings, a new phase 2 study will be conducted with an extended dose range (instead of doses ranging from 6–18 mg/kg, doses will range from 18–54 mg/kg) to see if higher concentrations can elicit a significant response in the endpoint criteria.

Rheumatoid arthritis (RA) is a chronic autoimmune disease that is associated with autoantibodies against various self-epitopes present in the joints. Interestingly, it is speculated that upregulation of HERV expression, and specifically translation, may act as a trigger for the development of autoantibodies. Evidence suggests that RA patients have cross-reactive antibodies towards HERV-derived antigens with regions of high structural homology to “host” antigens (proteins of non-viral origin) present in the joints ^[9][10][135,136]. For instance, HERV-K10 Gag1 protein shares an antigenic region with Collagen II, which is highly expressed in joints, and Freimanis et al. demonstrated that human fibroblast-like synoviocytes derived from a RA patient displayed several fold higher expression of HERV-K10 gag1 compared to samples from osteoarthritis (OA) and healthy donors ^[9][135]. The increase in HML-2 expression was confirmed by Reynier et al., who demonstrated that both types of HML-2 (HML-2 type I and type II) are significantly upregulated in the synovial fluid of RA patients compared to OA patients and healthy controls, with a higher HML-2 type I viral load associated with increased disease activity in RA patients ^[10][136]. The increased expression of HML-2 in RA patients found by Freimanis et al. was also reflected by an increased titer of autoantibodies against a recombinant peptide derived from the antigenic region of HERV-K10 Gag1 that shares homology with Collagen II, suggesting a direct link between elevated HML-2 expression, anti-HML-2 autoantibodies, and anti-Collagen II autoantibodies ^[9][135]. A similar study by Nelson et al. also demonstrated a significantly higher titer of anti-HERV-K10 antibodies in RA patients compared to controls ^[11][137]. However, cross-reactivity of the anti-HML-2 antibodies against Collagen II was not performed by Freimanis et al. beyond the initial bioinformatic analysis to identify homologous regions. Additionally, it is unknown if the elevated HML-2 expression and autoantibodies occurs prior to or following the development of RA, as pro-inflammatory cytokine treatment of fibroblast-like synoviocytes induces the expression of HML-2 and therefore implies that HML-2 activity could simply be the result of RA. Therefore, future studies on the precise timing of HML-2 upregulation in RA patients are needed to address this question.

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease that is characterized by a dysregulation in both the innate and adaptive immune systems, leading to the development of autoantibodies, typically those against nuclear components ^[12][138]. These antinuclear antibodies (ANAs) bind to DNA, RNA, and nucleic acid:protein complexes and exacerbate pro-inflammatory signaling, tissue infiltration by immune cells, and ultimately, tissue destruction ^[13][14][139,140]. Due to previous associations between HERV expression and SLE, Tokuyama et al. implemented RNA-sequencing on PBMCs derived from SLE patients and healthy controls to examine HERV expression on a locus-specific level using the software package they developed, ERVmap ^[15][16][141,142]. They identified over 100 unique ERV loci that are overexpressed in SLE patients and found that the total ERV read count significantly correlated with measures of disease severity, such as anti-nuclear, anti-double stranded DNA, anti-ribonucleoprotein (anti-RNP), and anti-Smith (Sm) anti-bodies. A separate study using the software package Telescope to characterize locus-specific HERV expression was released around the same time and confirmed these results, with over 300 loci significantly upregulated in SLE patients compared to controls in contrast to 10 downregulated loci ^[12][138]. Following these findings, the Tokuyama group analyzed the HML-2 subfamily due to its high degree of coding competence compared to other families and found that 4 of the 12 HML-2 loci (HERV-K102, -K106, -K110, and -K115) with envelope-coding sequences are significantly upregulated in SLE PBMCs ^[15][141]. Interestingly, these loci appear to be human-specific, with no known homology to other primate genomes. Of the four loci, HERV-K102 displayed significant correlation with anti-RNP titers. Based on this, Tokuyama et al. produced a recombinant envelope SU peptide based on HERV-K102 and assayed for autoantibodies against it in SLE patients and found that anti-HERV-K102 antibody titers correlated with the ISG signature of circulating PBMCs ^[15][141].

Following this finding, Tokuyama et al. moved to another aspect of innate immunity, neutrophils. A key driver of inflammation in SLE is neutrophil activity and the formation of neutrophil extracellular traps (NETs), an inflammatory marker of SLE that some speculate may be involved in the development of ANAs, and subsequently, SLE ^[17][18][19][143,144,145]. To examine if HERV-K102 Env-SU can activate neutrophils, Tokuyama et al. incubated recombinant Env-SU antigen in plasma from SLE patients or healthy controls. The immune complexes generated in the plasma from SLE patients, but not healthy controls, were able to activate neutrophil phagocytosis and induce the secretion of intracellular DNA in the form of NETs ^[15][141]. Overall, the authors suggest that HERV-K102 antigen and anti-HERV-K102 IgG from SLE patients form immune complexes that are readily phagocytosed by neutrophils and induce NET formation, which may contribute to autoimmunity and enhanced interferon signaling in SLE.

Pulmonary arterial hypertension (PAH) is a progressive disorder that is characterized by endothelial dysfunction and vasculature remodeling of the pulmonary arteries, leading to obstruction of the blood flow and increased resistance ^[20][21][146,147]. The increase in blood vessel wall resistance causes an elevation in pulmonary artery pressure that can overload the right ventricular, resulting in heart failure and death. Although PAH is known to correlate with various genetic and environmental factors, including mutations in the BMPR2 gene, the presence of pre-existing connect tissue diseases (SLE, rheumatoid arthritis, systemic sclerosis, etc.), HIV/schistosomiasis infection, and others ^[22][148], the majority of PAH cases are idiopathic (occur without a known cause) ^[23][149]. Recent research suggests that inflammation and autoimmunity are intrinsically linked to the development with PAH, with Saito et al. providing evidence of HERV-K as a novel initiator and sustainer of PAH that may be a suitable target for therapeutic intervention ^[24][25][26][150,151,152].

To determine if lung tissue from PAH patients contained viruses that were previously implicated in PAH pathology (HIV, Human herpesvirus 8, and hepatitis C virus), Saito et al. performed a metagenomic viral screen using next-generation sequencing and did not detect the presence of any exogenous viruses ^[26][152]. However, the authors observed a significant increase in HERV-K envelope and dUTPase mRNA in lung extracts from patients with PAH compared to healthy controls. Upon further inspection, they identified that HERV-K envelope and dUTPase proteins were primarily expressed in CD68+ macrophages that were present in the lung tissue and found that circulating monocytes from PAH patients exhibited higher levels of dUTPase mRNA compared to controls. These findings led the authors to explore the functional implications of dUTPase elevation by treating monocytes, pulmonary arterial endothelial cells (PAECs), and B cells with recombinant dUTPase. Following dUTPase treatment, all three cell types exhibited markers of inflammation or activation, including secretion of TNFα, IL1β, and IL6 by the monocytes and PAECs, and with CD69 expression and STAT3 signaling in the B cells. Additionally, the authors found that HERV-K dUTPase treated PAECs displayed increased vulnerability to apoptosis in an IL-6 independent manner in response to serum withdrawal, a hallmark of PAH PAECs. Lastly, as the authors found increased HERV-K dUTPase levels in the lungs and circulating monocytes of PAH patients, they decided to examine the impact of intravenous HERV-K dUTPase administration in a rat model to assess if HERV-K dUTPase elevation could initiate the development of PAH. They found that HERV-K dUTPase administration resulted in decreased pulmonary artery acceleration time, increased right ventricular systolic blood pressure, and right ventricular hypertrophy compared with saline-injected rat markers of early PAH development. Interestingly, the same group recently showed that monocytes overexpressing HERV-K dUTPase release this protein, incorporated into extracellular vesicles (EVs), and cause pulmonary hypertension in association with endothelial mesenchymal transition (EndMT) related to the induction of SNAIL/SLUG, IL-6, and VCAM1 ^[27][153].

While it is difficult to definitively prove that HERV-K elevation is the (or an) initiating factor in PAH, the authors identified that HERV-K dUTPase is elevated in PAH patients, demonstrated the ability of HERV-K dUTPase to induce inflammation through the activation of several relevant cell types, and established that HERV-K dUTPase administration alone can result in pathological indicators of PAH development ^[26][152]. Though there is a relative lack of literature on the relationship between HERVs and cardiovascular-related diseases, it is clear that HERV-K elevation can instigate and sustain inflammation within the vasculature network. It is likely that future studies can shed new light on the consequences of HERV reactivation in the context of heart inflammation and disease.