CD4 immunophenotyping in HIV infection

Recent figures from the World Health Organization (WHO) indicate that almost 33.2 million people are living with HIV infection worldwide, and over two-thirds of infected individuals are present in developing countries with limited resources (FIG. 1). In the region that has been hardest hit by AIDS — sub-Saharan Africa — the prevalence rate of HIV infection is 39%, yet only 17% of the HIV-infected individuals in this region had access to life-saving antiretroviral (ARV) therapy in 2007. Globally, more than 50% of those infected with HIV are women. Furthermore, recent reports from the WHO state that 1,800 babies are born with HIV every day because their mothers cannot access the necessary drugs or healthcare treatment. It is predicted that between 2008 and 2010 there will be 45 million new infections in resource-constrained and middle-income countries if no additional preventative methods are introduced. Figure 1 Global distribution of HiV-1 infection, December 2007 It is now well recognized that, in the majority of cases, the development of AIDS parallels the decline in CD4+ T cells, which play a vital role in the regulation of the immune response. HIV specifically targets and binds to the CD4 antigen and chemokine receptor 5 (CCR5) or CXC chemokine receptor 4 (CXCR4) on CD4+ T cells to replicate, ultimately causing the destruction and deterioration of the immune system. The monitoring of CD4+ T cells during the course of HIV infection is therefore a crucial component in the monitoring of disease progression and the response to ARV therapy. Over the past 25 years there have been major technological advances in the ways that CD4+ T cells are enumerated, with flow cytometry now generally regarded as the predicate technique. During this period, flow cytometry has progressed from large, expensive and complex fluorescence-activated cell sorting (FACS) machines that require highly specialized operating personnel, to smaller, more affordable bench-top instruments that can be used by individuals with minimal training. This shift was made possible in part by including multicolour analysis coupled with simplified gating technologies. More recently, several low-cost, point of care, micro-diagnostic technologies have been developed for use in rural settings, however, full validation of these technologies is still awaited. In many resource-constrained countries, including countries in sub-Saharan Africa, the cost of flow cytometers has prohibited the routine use of CD4 monitoring. However, the pressure of the AIDS pandemic has stimulated significant advances in the development of lower-cost methods for flow cytometry. This article will focus on flow cytometry as it relates to CD4 enumeration to determine the initiation and monitoring of ARV therapy.