The absence of a specific marker in the blood and tissues of certain monkeys may be part of the key to understanding how they can be infected for years with SIV, the primate version of HIV, and never develop AIDS, according to research published by Tulane University pathologist Ivona Pandrea in the scientific journal Blood.

Pandrea compared the blood, lymph nodes and intestines of five primate species that do not develop AIDS when infected with SIV to the same tissues of those species that do develop AIDS as a result of SIV or HIV infection. She analyzed the presence of CCR5, an important marker found on CD4 cells, which are part of the immune system targeted by HIV and SIV. As the viruses kill the CD4 cells, AIDS develops. CCR5 allows the virus access to these cells. Pandrea found that the AIDS-resistant monkeys had fewer CD4 cells with CCR5 on them than the comparison group. As a result, the viruses do less damage to the immune systems of infected nonhuman primates and they do not develop AIDS.

Pandrea theorizes that the absence of CCR5 is due to the adaptation of AIDS-resistant monkeys to SIV infection over millennia, while the others only recently have been exposed to SIV or HIV so they have not developed resistance.