A study of monkeys infected with HIV's simian equivalent and then given antiretrovirals (ARVs) suggests that the drugs may act indirectly by enabling a state of protective anti-HIV immunity.
The study also suggests that the brain impairment observed in HIV infection may be due at least partially to the toxicity of the immune messenger chemical interferon-alfa. This may mean that people with HIV-related cognitive dysfunction are, in essence, experiencing the same toxicity as people with hepatitis B and C given artificial interferon as a drug.
The findings are published in the journal AIDS.
The research, which treated the monkeys with tenofovir and nelfinavir, two antiretrovirals (ARVs) with low penetration into the central nervous system (CNS), found that the drugs nonetheless almost completely suppressed the viral load in the monkeys’ brains. This finding, seen in some though not all other studies, may be caused by there being fewer HIV-infected lymphocytes in the rest of the body to migrate into the brain.
However the researchers also suggest that ARVs, while not disabling virus directly, are able to establish an immune climate in the brain akin to that induced by a live vaccine. This then prevents HIV replication and further damage.
The researchers found a variety of inflammatory immune changes brought on in the brain after HIV infection. These modified into a more protective immune profile when ARVs were administered. In, particular there were more ‘effector’ CD4 and CD8 cells that actually prevented ongoing viral infection, and fewer memory cells held in reserve as ‘bystanders’.
The changes observed, according to the researchers, “resemble the phenotype observed in live-attenuated vaccine protocols.”
The research
Researchers from the Scripps Research Institute in La Jolla, California infected eight rhesus macaques with simian immunodeficiency virus (SIV) and then eight weeks later put four on a regimen of nelfinavir and tenofovir, and four on placebos.
Viral load, the electrical activity of brain cells, and immune cell counts in the blood were established while the animals were alive. Eight weeks after the monkeys started taking antiretrovirals, they were given a fatal dose of anaesthetic and immune cells autopsied from their brain. The HIV viral RNA in cells was measured and the immune cells were differentiated into types by means of flow cytometry. The levels of a number of different cytokines in different parts of the brain were also measured.
The results
The blood viral load fell to undetectable levels (below 400) in three of the four monkeys given ARVs by seven weeks and was near-undetectable in the fourth. Five measurements of viral load in different parts of the brain were also taken in each animal: in only one of 20 measurements in treated animals was viral load detectable, compared with eight out of 20 in untreated animals.
Brain cells in treated animals exhibited higher levels of electrical activity that they did before infection, whereas in untreated animals there was no change.
All HIV-infected animals had higher levels of the common infection-scavenging immune cells called macrophages in their brain than they did before infection, regardless of treatment. Treatment, however, produced significant differences in the type of CD4 and also CD8 cells seen.
Treated animals had higher levels of so called ‘effector memory’ CD4 and CD8 cells in their brain (and liver) and lower levels of ‘central memory’ cells, while untreated animals had more central memory cells and fewer effector memory cells. The opposite pattern was seen in the spleen, where treated animals had more central memory and fewer effector memory cells than untreated ones.
When animals were infected with HIV the levels of several cytokines such as interferon-alfa and CCL5, also called RANTES, rose. When treated, levels of interferon-alfa decreased but levels of CCL5 remained high or increased further.
The initial level and falls in level on treatment of interferon-alfa were particularly great in the brain region called the hippocampus, which is essential for the processing and retention of memories.
Implications
These results document a direct improvement in brain cell activity in monkeys given antiretrovirals and suggest several mechanisms for the brain damage and recovery seen.
Effector-memory cells are activated immune cells that secrete anti-virus cytokines such as interferon-gamma and also recruit other cells to kill off virus-infected cells. Central memory cells are longer-lived ‘bystander’ cells primed to recognise future infections. What immune activity they do have involves inflammatory activity and the direct killing of virus-infected cells. One theory of how HIV causes AIDS is that the body’s inability to control viral replication fixes the immune system into a central-memory, inflammatory state which exhausts T-cell reserves and causes organ damage.
After treatment, the researchers hypothesise, fewer of the short-lived effector-memory cells in treated animals are destroyed in the brain and are therefore able to exert better immune surveillance; conversely more of the long-lived central memory cells emigrate to the spleen to be archived properly, rather than remain in the brain to damage nerve cells. The result would be better control of virus in the brain with less cell damage, even in situations where there is little ARV drug in the CSF.
Interferon-alfa, when used therapeutically, “often results in neurological deficits and behavioural alterations”, the researchers note, and interferon-alfa levels sink back to normal after treatment. Conversely, CCL5, whose levels remain high, can block the spread of HIV because it attaches to the CCR5 co-receptor, to which HIV also bonds in order to gain entrance into cells. Artificial analogues of CCL5, such as maraviroc, are now used as HIV drugs.
In short, the researchers hypothesise, antiretroviral treatment, even if it does not penetrate the blood-brain barrier, induces a protective antiviral climate in the brain which may protect it against further neural damage.
However, they warn, “a later introduction of antiretrovirals may not have such a beneficial effect, given the chronicity of virus-host interaction and its resultant effects on the brain.”
They argue that early treatment could be particularly important in terms of protecting the nervous system because it has “the potential for the development of a long-lasting protective environment.”
Marcondes MCG et al. Early antiretroviral treatment prevents the development of central nervous system abnormalities in simian immunodeficiency virus-infected rhesus monkeys. AIDS 23(10):1187-95. 2009.