Scientists in the U.K. have discovered how poxviruses such as the monkeypox virus (MPXV) and variola viruses evade host cell defenses by exploiting a protein, cyclophilin A (CypA), that puts the brakes on a natural cellular antiviral protein called human tripartite motif protein 5α (TRIM5α), which then allows the pathogens to replicate and spread. Results from the study, headed by a team at the University of Oxford, the University of Cambridge, and the Pirbright Institute, point to a new therapeutic approach that may be more durable than current treatments, and which could feasibly use existing immunosuppressants such as cyclosporin A (CsA) and other antiviral drugs that target cyclophilin A.

Initial experiments reported by the researchers suggested that these drugs can restrict replication and spread of poxviruses. They suggest that this approach to treatment, whereby the drug does not directly target the virus, could make it more difficult for poxviruses to evolve drug resistance. And because the protein-hijacking mechanism is the same across many poxviruses, cyclophilin A-targeting drugs could be effective in treating a range of diseases.

“The drugs we identified may be more durable than the current treatment for monkeypox—and we expect will also be effective against a range of other poxviruses including the one that causes smallpox,” said research lead Geoffrey L. Smith, FRS FMedSci FRSB, who conducted the work in the department of pathology at the University of Cambridge, the Dunn School of Pathology, University of Oxford, and the Pirbright Institute. Smith and colleagues reported on their findings in Nature, in a paper titled, “TRIM5α restricts poxviruses and is antagonized by CypA and the viral protein C6,” in which they concluded that their results “warrant testing of CsA derivatives against orthopoxviruses, including monkeypox and variola.”

Smallpox has been eradicated as a disease since 1979, but the virus that causes it, variola, is still being held in two high-security labs—one in the United States and one in Russia. The threat of variola virus being used in bioterrorism has led to a drug, tecovirimat, being licensed to treat smallpox. Vaccinia virus (VACV), the live vaccine used to eradicate smallpox, is also currently being used to immunize at-risk populations against monkeypox virus, the cause of monkeypox (mpox), the team explained. “VACV, cowpox virus (CPXV), MPXV, camelpox virus (CMLV), and variola virus (VARV), the cause of smallpox, are all orthopoxviruses and are immunologically cross-protective.” Tecovirimat has been used to treat severe cases of mpox over the last year, but this has resulted in the emergence of multiple drug-resistant strains of the monkeypox virus.

Once a poxvirus infects a host cell it has to defend itself from attack by cellular proteins that would restrict virus replication and spread. The newly reported research started with the simple observation that vaccinia virus infection causes a reduction in the level of TRIM5α in human cells. To find out why, the team engineered human cells to lack TRIM5α and found that in these cells the virus replicated and spread better, indicating that TRIM5α has anti-viral activity.

The team next identified the vaccinia virus protein that TRIM5α targets. They also discovered that the virus has two defenses against attack by TRIM5a. First, it exploits another cellular protein, cyclophilin A, to block the antiviral activity of TRIM5α, and second, it makes a protein, C6, that induces destruction of TRIM5α. “… the antiviral activity of TRIM5α is countered by the proviral activity of CypA, which in turn is antagonized by CsA and derivatives that prevent the binding of CypA to its viral target, the poxvirus capsid protein L3,” the team stated. L3 is highly conserved in orthopoxviruses, and the team showed that L3 from VACV, MPXV, and VARV binds to human CypA and human TRIM5α, and the former interaction is prevented by CsA and derivatives. Moreover, they wrote, “Like L3, C6 is highly conserved in orthopoxviruses, and C6 orthologues from VACV, CPXV, CMLV, MPXV, and VARV all bind to human TRIM5α and induce its degradation, despite most of these proteins deriving from viruses that are not endemic in humans.”

Existing drugs target cyclophilin A, and when the team tested a series of these drugs against a range of poxviruses, including monkeypox, they found that the drugs exhibited antiviral effects, effectively by making the virus more sensitive to TRIM5α. “The role of CypA in antagonizing the antiviral activity of TRIM5α provides a route to antiviral drug development for orthopoxviruses such as MPXV and VARV,” the team noted. “CsA and the non-immunosuppressive derivatives alisporivir and NIM811, interrupt the interaction of CypA and L3, and thereby reverse the proviral activity of CypA and enhance TRIM5α-mediated restriction. These compounds are therefore antiviral in the presence of CypA and TRIM5α and can restrict the replication and spread of orthopoxviruses.”

“There are various drugs that target cyclophilin A, and because many of them have gone through clinical trials we wouldn’t be starting from scratch but repurposing existing drugs, which is much quicker,” said Smith. “Our results were completely unexpected. We started the research because we’re interested in understanding the basic science of how poxviruses evade host defenses and we had absolutely no idea this might lead to drugs to treat monkeypox virus and other poxviruses.

The authors further noted that in contrast with tecovirimat, which does not block replication and needs a functional immune system to remove virus-infected cells alongside drug treatment, CsA, alisporivir, and NIM811 hinder virus replication by targeting a cellular protein, “making the emergence of drug resistance difficult.” Both of the non-immunosuppressive derivatives have also progressed in development to Phase II clinical trials, and so there is also an indication of their safety. “Therefore, clinical testing of these drugs against MPXV is warranted,” the team stated.

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