In both HIV and SIV infections, it has been shown that the most rapid and extensive loss of CD4 T cells occurs in the gut. As a consequence, a theory has emerged positing that gut CD4 T cell depletion plays a central causative role in driving HIV pathogenesis; the proposed mechanism is that gut wall integrity becomes compromised, leading to the leakage of normally friendly bacteria from the digestive tract and into systemic circulation, which in turn contributes to persistent immune activation and, ultimately, progression to AIDS. However, some scientists have remained skeptical of this theory, suggesting instead that gut CD4 T cell depletion is an effect of HIV infection but not necessarily the primary cause of disease progression. The skeptics have gained some support from studies showing that severe gut CD4 T cell depletion occurs during acute SIV infection in monkey species that experience no apparent ill effects from the virus (sooty mangabeys and African green monkeys). A paper published recently in the Journal of Virology now shows that the opposite phenomenon is also possible: a modified SIV that does not cause loss of gut CD4 T cells nevertheless causes persistent immune activation and progression to simian AIDS in rhesus macaques.
Richard Jefferys is the coordinator of the Michael Palm HIV Basic Science, Vaccines & Prevention Project Weblog at the Treatment Action Group (TAG). The original blog post may be viewed here.