Resistance to apoptosis: mechanism for the development of HIV reservoirs

New insights into the physiological cell death program in mammalian cells have further confounded the central issue of T lymphocyte destruction during HIV infection. Although infected and uninfected cells die following infection, recent evidence indicates that infected and uninfected cells may have unique pathways controlling death. Two widely accepted models for apoptosis have been described, namely the intrinsic and extrinsic apoptotic pathway. In the extrinsic pathway, ligation of TNF death ligands to their receptors causes oligomerization of the death receptors and recruitment of adaptor proteins typically involving caspase 8 activation. In the intrinsic pathway, apoptotic signals converge on mitochondria to cause activation and subsequent release of cytochrome c, which leads to formation of a multiprotein complex containing Apaf-1, cytochrome c, dATP and procaspase 9. Expression of HIV proteins alters these pathways resulting in enhanced levels of apoptosis. Although HIV infection results in T cell apoptosis, under some circumstances a small fraction of CD4+ T cells and macrophages do not die following infection, indicating that this may be a critical step in the development of viral reservoirs. In addition, monocyte differentiation into macrophages leads to an apoptosis resistant phenotype characterized by upregulation of antiapoptotic molecules and lower levels of proapoptotic molecules. The development of these stable antiretroviral resistant cells represents the major obstacle in achieving a complete sterile cure. However, this provides a unique opportunity to further understand the regulation of apoptosis and may facilitate novel immune based therapies aimed at modifying apoptosis in HIV disease.