Human Immunodeficiency Virus Integration Efficiency and Site Selection in Quiescent CD4 ⁺ T Cells

Abstract

Until very recently, quiescent CD4؉ T cells were thought to be resistant to human immunodeficiency virus (HIV) infection. Subsequent studies, attempting to fully elucidate the mechanisms of resistance, showed that quiescent cells could become infected by HIV at low efficiency and form a latently infected population. In this study, we set out to identify the sites of viral integration and to assess the efficiency of the overall integration process in quiescent cells. Based on our results, HIV integration in quiescent CD4؉ T cells occurs in sites similar to those of their prestimulated counterparts. While site selections are similar, the integration process in quiescent cells is plagued by the formation of high levels of incorrectly processed viral ends and abortive two-long-terminal-repeat circles. Quiescent CD4ϩ T cells have been shown to be resistant to human immunodeficiency virus (HIV) infection, and the resistance is characterized by incomplete reverse transcription (82,83). However, the permissiveness of other nondividing cell types, such as resting T cells and macrophages, raised further questions regarding the nature of the block (10,26,56,65,67,(69)(70)(71). Later studies further elucidated which subsets of resting cells were refractory to infection. Truly quiescent cells in the G 0/1a phase were resistant to infection, while cells in the G 1b phase, characterized by high levels of RNA synthesis but not DNA synthesis, were susceptible to infection (43). These combined studies suggested that certain nondividing cells could support a productive infection. Subsequent studies were aimed at further examining the different steps of the viral life cycle in quiescent cells, as well as identifying potential cellular factors or the lack thereof that may block infection. One potential block to infection was hypothesized to be the lack of raw materials such as nucleotides. Treatment of quiescent cells with nucleosides resulted in increased reverse transcription but did not lead to a productive infection following stimulation of the cells (42, 61), suggesting that other factors may contribute to the resistance to productive infection. Recent studies examined the presence or absence of cellular factors responsible for the block in quiescent CD4 ϩ T cells. Both Murr1 and APOBEC3G have been shown to influence the viral life cycle in quiescent CD4 ϩ T cells (18, 29). However neither factor fully explains the lack of rescue of productive infection if quiescent cells are stimulated subsequent to infection.A more detailed examination of the viral life cycle in quiescent CD4 ϩ T cells can shed more light on the nature of the block presented by quiescent CD4 ϩ T cells. A series of recent studies looking at the different stages of the HIV life cycle in quiescent cells have further supported data from our earlier work (82, 83). More specifically, it has been shown that reverse transcription is inefficient in quiescent cells, generating fulllength viral transcripts that are very labile (half-life of 1 day) but are integration co...

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