discovery of TRIM 5 alpha

Human immunodeficiency virus type 1 (HIV-1) causes AIDS in humans, and to a lesser extent, in chimpanzees (1, 2). However, not long after the discovery of HIV-1, scientists realized that certain primate species were resistant to HIV-1 infection. In particular, monkeys from Africa and Asia, referred to as Old World monkeys, could not be infected with HIV-1 and did not develop AIDS (3). This discovery brought both excitement and frustration. The block to HIV-1 replication in Old World monkey cells hindered efforts to develop an animal model for testing drugs and vaccines. On the other hand, Old World monkeys had evolved for millions of years in Africa--the epicenter of the current HIV-1 epidemic. Perhaps exposure to past HIV-1-like epidemics led to the emergence of an antiviral defense that protected them against HIV-1.



Determining the cause of HIV-1 resistance in Old World monkey cells stymied HIV researchers for nearly two decades. An early view was that the block resulted from expression of an incompatible receptor on the surface of Old World monkey cells. However, identification of the HIV-1 co-receptor in the mid-1990s disproved this hypothesis. Subsequent studies demonstrated that HIV-1 could enter Old World monkey cells, but a block that targeted the viral capsid prevented the establishment of a permanent infection (4).

Using a genetic screen, we identified TRIM5alpha as the primary block to HIV-1 replication in Old World monkey cells (5). The expression of rhesus monkey TRIM5alpha in human cells potently inhibited HIV-1 infection and prevented the accumulation of reverse transcripts. Importantly, reducing the expression of TRIM5alpha in rhesus monkey cells with small interfering RNA relieved the block to HIV-1.

We initially hypothesized that TRIM5alpha functioned as a cofactor necessary for capsid uncoating. However, subsequent findings argued against this hypothesis. First, knocking down human TRIM5alpha showed no effects on HIV-1 replication in human cells. Second, rodent cells, which do not express TRIM5alpha, supported HIV-1 infection if engineered to express an appropriate receptor. Finally, human TRIM5alpha does not associate with the HIV-1 capsid in biochemical assays. Thus, TRIM5alpha appeared to have evolved primarily as an inhibitory factor aimed at thwarting viral replication, rather than a host factor co-opted by HIV-1 to promote infection.

Further evidence that TRIM5alpha functions primarily as a modulator of innate immunity against retroviruses emerged from comparing the sequences of TRIM5 alpha orthologs from different primate species. We found dramatic length variation, and an unusually high ratio of nonsynonymous to synonymous changes in the C-terminal domain of TRIM5alpha orthologs (6) suggesting that TRIM5alpha has been subjected to strong positive selective pressure during primate evolution. Furthermore, episodic changes in the TRIM5alpha C-terminal domain coincide with periods of retroviral epidemics (6). Indeed, a recent report suggests that selective changes occurred in the TRIM5alpha lineage during acquisition of resistance to an ancient retrovirus. These changes may have had the unfortunate consequence of attenuating TRIM5alpha potency against HIV-1 (7).

Does TRIM5alpha have the ability to block infection by other retroviruses? We found that TRIM5alpha from various Old World monkey species conferred potent resistance to HIV-1, but not SIV (5). New World monkey TRIM5alpha proteins, in contrast, blocked SIV but not HIV-1 infection. Human TRIM5alpha inhibited N-MLV and EIAV replication (8, 9). Thus, the variation among TRIM5 orthologs accounts for the observed patterns of post-entry blocks to retroviral replication among primate species.

To determine why Old World monkey TRIM5alpha but not human TRIM5alpha, potently blocks HIV-1, we systematically altered the human sequence to more closely resemble the monkey sequence. Remarkably, we found that a single amino acid determines the antiviral potency of human TRIM5alpha (10). If a positively charged arginine residue in the C-terminal domain of human TRIM5alpha is either deleted or replaced with an uncharged amino acid, human cells gain the ability to inhibit HIV-1 infection (11). Perhaps some humans have already acquired this change and are naturally resistant to HIV-1 infection.

How does TRIM5 inhibit infection? Following viral entry into the host cell, the capsid core, which encases the viral RNA, must disassemble to allowreverse transcription of the viral RNA into DNA (see the figure). Host factors that mediate capsid uncoating are presumed to exist, but have not been identified.

Because early studies demonstrated that sequences within the capsid determined susceptibility to the block, we asked if TRIM5 associated with the capsid. The association of TRIM5 with the capsid was dependent on the C-terminal domain and the association was necessary for restriction (12). TRIM5 proteins from various Old World monkey species bound the HIV-1 capsid; however, TRIM5 variants that did not restrict HIV, such as New World monkey TRIM5, did not associate with the capsid cores. Human TRIM5 exhibited a very weak association with the HIV-1capsid cores, explaining the lower potency of human TRIM5 in blocking HIV-1 infection (12).

Why does association of TRIM5 with the viral capsid inhibit infection? Previous studies of HIV-1 capsid mutants suggest that capsid disassembly may be a temporally regulated process with either too rapid or too slow disassembly compromising viral infectivity (13). By following the fate of viral cores in the cytosol just after viral entry, we found that TRIM5 caused capsid cores to undergo rapid, and premature, disassembly (12, 14). Accelerated uncoating of the capsid correlated with the ability of TRIM5 variants from different species to restrict HIV-1, SIV, and N-MLV infections. Future studies are needed to determine how TRIM5 promotes rapid disassembly of capsid and why accelerated disassembly is detrimental to infection. Perhaps accelerated disassembly of the retroviral capsid prematurely exposes the viral RNA or viral enzymes to degradation.

The discovery of TRIM5 not only answered a long-standing question in the HIV field, it also revealed a new pathway that protects cells from retroviral infection. The human genome encodes more than 50 members of the TRIM family. Recently, TRIM25 was shown to be essential for RIG-I-mediated antiviral activity (15) and TRIM family members such as TRIM1, TRIM19 (PML), and TRIM22, may block other viruses (16).

At a time when policy-makers and the public express frustration over our inability to produce an HIV vaccine, it is hoped that the discovery of TRIM5 will precipitate new ideas for how to protect human hosts from retroviral infection. Perhaps in the case of retroviruses, innate intracellular immunity mediated by factors like TRIM5 and APOBEC play a particularly crucial role. Efforts aimed at enhancing these innate immune defenses may ultimately prove to be more effective at protecting humans from HIV than vaccine strategies aimed primarily at stimulating humoral or cellular