Viruses of Cats and Humans
The patient's symptoms and prognosis tell an all-too-familiar tale. The patient became infected several years before and suffered an acute illness before his immune system brought the virus under control. Then he was fine and lived without symptoms for several years, but during this time the virus was quietly weakening his immune system.
Now he is not eating, losing weight, reeling from high fever, terrible diarrhea, swollen lymph nodes, and suffering from chronic opportunistic infections. The patient is in the final throes of the terrible disease that has ravaged millions worldwideacquired immune deficiency syndrome (AIDS).
And though the story may sound familiar, the patient may not be. The patient in this imaginary case is a cat, suffering from the feline form of AIDS. Like the human disease, which is caused by the "lentivirus" human immunodeficiency virus (HIV), feline AIDS is caused by a lentivirus known as feline immunodeficiency virus (FIV).
That two such different species as humans and cats could suffer such similar diseases is perhaps not so surprising considering the similarity between the two viruses that cause these diseases. And FIV and HIV are very similar.
If you were to take an electron micrograph of human cells infected with HIV and cat cells infected with FIV and hold these two images right next to each other, you would see in both images small spiky virions with a visible pill-shaped capsid inside. They are the same size.
"It is impossible to tell them apart," says Professor John Elder, who has studied FIV since the mid-1980s at The Scripps Research Institute (TSRI), where he is currently supported by the hard work of Staff Scientist Aymeric deParseval and Research Associates Ying-Chuan Lin, Udayan Chatterji, and Sohela de Rozieres.
A Virus is Discovered and Isolated
It was in 1986, just a few years after the initial alarms had been raised about HIV, that FIV was discovered in California. In 1986, Niels Pedersen, who is currently director of the Center for Companion Animal Health at the University of California, Davis, and Janet Yamamoto, who is now a professor in the University of Florida's College of Veterinary Medicine, co-discovered FIV.
As the story goes, there was a kindly woman who took in straysmany strayshousing them in her large kennels. She noticed an odd thing. Several cats under her care became sick and eventually died seemingly as a result of sleeping in the same pen as one particular feral cat. So she contacted Pedersen, who took samples and eventually isolated a virion, which under the electron microscope looked like an RNA virus belonging to the lentivirus family.
Shortly thereafter, Elder began to collaborate with the Pedersen laboratory to work on the virus taken from that isolate and from another isolate from a cat belonging to former TSRI investigators Fred Hefron and Maggie So.
"They had a cat that came down with FIV, and we isolated the virus from it," says Elder.
Elder had been working at TSRI since the mid-1970s, and he was considered an expert in retroviruses like the newly discovered FIV. In fact, Elder and his laboratory had been working on retroviruses for 10 years by the time FIV was discovered in 1986.
"It was a natural progression for us to take our molecular tricks over there and try to find out what the structure of that virus was," says Elder.
The tricks Elder and his laboratory employed were basically those of molecular cloningthey were experts at isolating and amplifying the DNA of the virus so that it and its proteins could be studied. Basically, what this entailed was to extract the DNA from infected blood lymphocytes, chop it up with enzymes, insert the DNA pieces into phage (a virus that infects bacteria), and then infect bacteria with the phage.
Bacteria can easily be grown, providing a convenient way of amplifying the DNA. Elder and his laboratory then had only to look for the right pieces of DNA by labeling them with radioactive probes and separating them.
"We found one [piece] that contained what looked to be the whole virus," says Elder.
Then they took that piece of DNA and used it to transfect cells. They then observed those cells and found that they were productively replicating the virus, which they then isolated so that they could sequence it. Elder and his colleagues also made expression systems with many of the viral proteins so that they could be produced and purified for biochemical studies and other research.
Two Very Similar Viruses
Building on this initial work, Elder and his colleagues at TSRI and elsewhere have been able to characterize the genomic organization of FIV and, significantly, to compare it to that of its cousin HIV. FIV and HIV, it turned out, have a lot in common, which makes FIV a good model for studying an HIV-type infection.
The similarities between the two viruses go deeper than morphology. FIV, as a disease, represents as serious an epidemic for the wild and domestic cat populations in the world as HIV does for the human populations of the world.
Both are members of the lenti (slow) virus family and they contain many of the same characteristic genes and proteins. FIV, much like its human cousin HIV, has an RNA genome of around 10,000 bases that is packaged in a protein and lipid capsid and coat. HIV and FIV both code for a number of structural genes, which encapsulate the RNA and are produced by a gene called gag. In an infected cell, the virus produces a large Gag polyprotein that is later chopped up into its constituent pieces by an important viral enzyme called the protease.
The protease and a few other necessary enzymes are encoded by a viral pol gene. And HIV and FIV also encode env genes, which makes a glycoprotein that sticks up out of the coat of the virion and helps the virus infect cells.
At the amino acid level, Elder estimates, about 40 to 45 percent of the residues in FIV proteins are identical to those in HIV. "We found," says Elder, "[that the FIV and HIV sequences] were quite related."
In fact, he adds, some of the proteins are so similar that they have almost identical structures. Elder and his colleagues hope that their discoveries and successes in their research on FIV will shed light on the problem of HIV.
"One very big parallel [between the two]," says Eder, "is that FIV uses the chemokine receptor CXCR4, like many strains of HIV."
Much like HIV, FIV requires the interaction of its surface glycoprotein molecules with CXCR4. And, also like HIV, FIV requires another receptor to maximize binding to the chemokine receptor. In HIV, the required receptor is called CD4. The exact receptor needed by FIV is, at the moment, not known.
"We're trying to figure out what it is," says Elder.
There are some major differences, of course, between FIV and HIV. Perhaps the biggest difference is that HIV cannot infect cats and FIV cannot infect humansin fact, there is zero evidence that FIV has ever infected humans at any time in the last 6,000 years, during which humans and cats have been living together.
For instance, there are differences in the accessory genes that the viruses use. FIV and HIV, like all lentiviruses, have a number of other "accessory" genes as well, with names like tat, rev, nef and vif. The roles of some of these genes are clear-cutrev, for instance, helps the viral genome get into and out of the nucleus.
Others are more mysterious. "[Scientists are still trying to figure out what vif does," says elder, referring to the acronym for the "viral infectivity factor" gene. "But both FIV and HIV need it."
At this level, the differences between the two viruses are more apparent. HIV has an accessory gene called nef, while FIV does not. And FIV has a different accessory gene called a dUTPase that HIV does not have. "This gene helps the virus get around in non-dividing cells," says Elder.
The differences can be explained by the fact that the two different viruses have had to adapt to different hosts. Each virus must get along in its own particular host, and given its high replication and mutation rate has had the opportunity to pick up genes that presumably help it make more virus.
HIV and FIV live in completely different host organisms, which offer different challenges to the viruses in terms of how they can replicate and survive in each species' lifecycle. For instance, the primary mode of transmission of FIV is through animal bites, whereas that of HIV is through heterosexual intercourse.
But for a scientist like Elder, what is really interesting is how these molecules might be targets for therapy.
Elder, his laboratory, and his collaborators at TSRI and other institutions are trying to make more broad-based inhibitors of the two viruses.
"We're trying to make drugs that are efficacious for HIV and FIV," says Elder. "The idea is that if we can make broad-based inhibitors that hit FIV, maybe they will hit more HIV subtypes as well."
They look, for instance, at the FIV integrase as a target for therapeutics. This enzyme, as its name implies, is responsible for integrating the viral genome into the DNA of the host cell. They also look at the FIV protease, the protein that processes the virus's long polyprotein strips into the pieces necessary to package FIV into new infectious virus particles.
Like the viruses themselves, the HIV and FIV proteases are very similar. They have a 32 percent amino acid identity and essentially the same three-dimensional structure.
"If I showed you two pictures of FIV protease and HIV protease, you couldn't tell them apart," says Elder. Yet, strangely, the two proteases respond differently to the same inhibitors. Common drugs that inhibit HIV protease and are used for treating AIDS do not work on the FIV protease at all.
This observation led Elder, in collaboration with TSRI investigators Chi-Huey Wong, Bruce Torbett, Arthur Olson, and several others, to examine the structures of FIV protease and HIV protease to see what subtle differences between them could cause such a great distinction. It was research associate Taekyu Lee in Wong's group who noticed a region of the FIV protease that was smaller than the corresponding region in HIV protease. This prompted Wong to replace the inhibitor residue that fits in that site with a smaller amino acid. When they did this, the potency of this inhibitor increased 1,000-fold for FIV.
"This was the best [inhibitor] we had ever seen against FIV," says Elder, adding that it was also efficacious against the wild-type HIV and nine out of thirteen protease-resistant HIV isolates tested. In other words, the molecular changes that allowed many variants of HIV to escape drug therapies were the same as those that made FIV distinct from HIV.
Another protein that he is looking at is the virus-surface glycoproteins encoded by the env gene. He is particularly interested in elucidating the rules that determine which cells a virus coated with a particular glycoprotein can enter.
Some FIV virions are able to infect one type of cell and not another, while others infect the other cells and not the first. What determines these specificities are the structure of the particular glycoproteinssome of them bind to CXCR4 directly, whereas others need the interaction of a co-receptor to aid in the binding. Elder and his colleagues are trying to work out how and why this is so.
There's No Place Like Hope
Finally, Elder and his laboratory also look at the development of vaccines in HIV and FIV.
In this arena, there was a highly publicized report a few months ago on the failure of one vaccine against HIV to prove efficacious in a large Phase-III clinical trial. A few months before this, however, one big success was reported by a small manufacturer of animal medicines in a quiet suburb of Kansas City. This company, Fort Dodge Animal Health, a division of Wyeth, reported in September that the U.S. Department of Agriculture provided license for manufacture of the first prophylactic vaccine against FIV. In its press release, the company reports that the vaccine, called "Fel-O-Vax® FIV," has an 84 percent efficacy rate.
While this is certainly good news for cats and cat lovers, it is also good news for those concerned about HIV, because it means that despite the recent failure of one HIV vaccine, it is possible to design a vaccine against a lentivirus similar to HIV.