HIV, TB and ART: the CD4 enigma

1 This is a development of a presentation given by Brian Williams at the ‘Modelling and Quantitative Research Priorities’ meeting of the TB Modelling and Analysis Consortium (TB MAC), funded by the Bill and Melinda Gates Foundation, 28th and 29th September 2012, Johannesburg, South Africa Abstract The concentration of CD4+ T-lymphocytes (CD4 count) in a person’s plasma is widely used to decide when to start HIVpositive people on anti-retroviral therapy (ART) and to predict the impact of ART on the future course of HIV and tuberculosis (TB). However, CD4+ cell-counts vary widely within and among populations and depend on many factors besides HIV-infection. The way in which CD4 counts decline over the course of HIV infection is neither well understood nor widely agreed. We review what is known about CD4 counts in relation to HIV and TB and discuss areas in which more research is needed to build a consensus on how to interpret and use CD4 counts in clinical practice and to develop a better understanding of the dynamics and control of HIV and HIV-related TB.