Are We Done Monkeying Around With TRIM5α?

In 2004, the TRIM5α protein from rhesus macaques was first identified as the restriction factor preventing HIV-1 infection in these monkeys.¹ Since then, the antiretroviral activity of a large number of primate TRIM5α orthologs has been examined. Collectively, these studies indicate that human TRIM5α is at the back of the pack when it comes to inhibiting HIV-1 infection. This has established the dogma that retroviruses have evolved to evade the TRIM5α ortholog present in species to which they are endemic. Following this thinking, because HIV-1 has learned to evade human TRIM5α, leveraging TRIM5α as an antiviral therapy will require expression of TRIM5α variants or orthologs not present in the human genome. This dogma is being challenged in the publication by Richardson and co-workers in this issue of Molecular Therapy.² The authors report that stabilization of human TRIM5α enables it to restrict HIV-1 infection in primary human T cells challenged with HIV-1 in a NOD/SCID mouse model. The findings suggest that a better understanding of the mechanisms of TRIM5α degradation may provide opportunities to reduce its turnover in a more targeted way that could be exploited for therapeutic purposes.

TRIM5α is a cellular protein that inhibits retroviral infection by binding to determinants present on the incoming capsid core. This ability to inhibit infection varies remarkably between members of the primate lineage and is thought to be responsible for the species-specific tropism of many retroviruses, including HIV-1 and other primate lentiviruses. The AIDS pandemic is strong evidence that our endogenous restriction factors, including TRIM5α, are not effective at inhibiting HIV-1 infection. Unlike other restriction factors, which are often targeted for degradation by viral proteins, the failure of human TRIM5α to inhibit HIV-1 seems to stem from inefficient recognition of the viral capsid. Numerous studies from many groups have revealed that genetic variability in the C-terminal SPRY domain defines the species-specific restriction spectrum observed among primate TRIM5α orthologs.³ In the case of humans, a previous encounter with a retrovirus appears to have forced intense selective pressure on our SPRY domain. Unfortunately, these changes have left us poorly equipped to inhibit HIV-1 infection, especially when compared to our primate cousins. Indeed, changing a single amino acid within the human TRIM5α SPRY to the corresponding rhesus residue (R332P) confers potent anti-HIV activity upon human TRIM5α.⁴

However, Richardson and colleagues' study reported in this issue shows that expressing human TRIM5α as a fusion protein to mCherry stabilizes the protein, which is otherwise turned over quite rapidly.⁵ This stabilization unexpectedly allows human TRIM5α to protect primary human T cells from infection. Although this method of stabilization is unlikely to be part of any therapeutic strategy, it establishes a critical proof of principle: human TRIM5α is capable of inhibiting HIV-1 infection in vivo. Clearly, this outcome is achieved with the benefit of performance enhancement. As noted above, however, prevailing dogma would suggest that inherent inadequacies of the human SPRY domain would preclude human TRIM5α from inhibiting HIV-1 replication in a meaningful way. If we accept that the mCherry–human TRIM5α fusion is not altering the affinity of the human SPRY for HIV-1 capsid, it seems that this dogma has been challenged.

With respect to SPRY domains and specificity, the data presented in the study by Richardson et al. suggest that the field should perhaps begin to look past the primate “SPRY domain envy” that has established the dogma that the human SPRY domain is a nonstarter in approaches designed to inhibit HIV-1 infection. Perhaps the most controversial part of the study is the observation that stabilized human TRIM5α inhibits HIV-1 to an extent comparable to that observed when using rhesus TRIM5α. Many TRIM5α aficionados will find this conclusion difficult to accept, given the depth of the literature in this area. However, debating the validity of this conclusion misses the larger point: human TRIM5α does not need to outperform rhesus TRIM5α to be therapeutically useful. It does need to inhibit HIV-1 in vivo, and the authors have provided the first evidence that this might be possible. If other studies recapitulate and advance this finding, how human TRIM5α stacks up against primate orthologs will be a moot point. Until gene therapy allows for the expression of TRIM5α orthologs of our choosing, we are forced to go to war with the SPRY domain we have, not the SPRY domain we wish we had. These results provide the first evidence that the human SPRY domain might not be as inherently incapable of recognizing HIV-1 as we have come to believe.

If human TRIM5α can indeed be leveraged to inhibit HIV-1 replication, what are the types of “performance-enhancing drugs” that might allow TRIM5α to make the jump to the “big leagues”? The mechanistic understanding of the restriction process developed using primate TRIM5α orthologs might be very useful in considering this question. For example, the molecular determinants driving the self-association and assembly of rhesus TRIM5α are established.^6,7 If the ability of human TRIM5α to self-associate is increased, this might be predicted to enhance TRIM5α binding to capsid in much the same way that immunoglobulin M utilizes avidity to overcome the low-affinity interactions inherent in many immunoglobulin M antibodies. Similarly, the authors have demonstrated that reducing turnover of human TRIM5α generates a pool of protein sufficiently large to allow restriction. To this point, most studies of TRIM5α degradation have focused on degradation occurring during restriction, although the mechanism of TRIM5α degradation appears to be different in the absence of restriction-sensitive virus.^5,7 These are all questions that are now worth asking. Hopefully, the answers will demonstrate that human TRIM5α can indeed play in the big leagues. Unlike professional athletes, few people will care if success requires the use of performance-enhancing drugs.