In this issue of the JCI
) and in a previous study (11
), Zhang et al. have identified p21Waf1/Cip1/Sdi1
(p21) as a gene product that can influence the sensitivity of HSCs to HIV-1 infection. HSCs are one of a few cell types, others being undifferentiated monocytes and unstimulated CD4+
T cells, that resist HIV-1 infection despite the presence of HIV-1 receptors (12
). In the case of HSCs, CD4 expression appears low, and previous work indicates a major block to HIV-1 infection of HSCs is at virus entry, since infection can be achieved using pseudotyped HIV-1 virions that carry a different viral envelope (14
). However, later work also suggests that a second block to HIV-1 replication is imposed by p21 because its depletion could enhance sensitivity to pseudotyped HIV-1 vectors by about 2- to 4-fold (11
). The new work in this issue extends this finding and shows that low-level–spreading HIV-1 replication can be obtained in HSCs if these cells are transfected with siRNAs that deplete p21. Additionally, Zhang et al. present a number of experiments designed to illuminate the molecular mechanism underlying the enhancement of HIV-1 replication in HSCs upon p21 depletion (10
So, is the effect of p21 on HIV-1 infection a direct effect on the incoming virus or a secondary effect of modulating cell physiology? The normal cellular functions of the p21 protein suggest that either could be the case. Notably, p21 is a cyclin-dependent kinase (CDK) inhibitor (15
) and particularly targets CDKs that are active in the G1 phase of the cell cycle. In doing so, p21 inhibits cell-cycle progression in some cellular types, including HSCs (16
). Although cell-cycle progression is not absolutely required for HIV-1 infection, quiescent cells (in G0 phase) are known to be very poor targets for HIV-1 infection, at least in part because reverse transcription is not completed (17
). Additionally, p21 binds to factors involved in DNA repair pathways (18
). The requirement for DNA repair activities in HIV-1 infection, particularly at integration, has been debated for some years, and recent findings suggest that some DNA repair activities may actually inhibit HIV-1 infection, perhaps by targeting viral DNA prior to integration into the target cell genome (19
). Overall, therefore, there are at least 2 plausible models by which p21 could inhibit HIV-1 infection.
Zhang et al. (10
) argue for a direct effect of p21 on incoming HIV-1. Data in the study hint that p21 binds to an incoming subviral complex containing HIV-1 matrix and integrase, known as the HIV-1 preintegration complex (PIC). This is consistent with the notion that the effect could be a direct one on viral components. However, caution is warranted here because this type of experiment is notoriously difficult and artifact prone. Nonetheless, it appears that the effect of p21 depletion is specific to HIV-1 infection, as low-level replication in HSCs of the simian immunodeficiency virus SIVmac251 is unaffected by p21 depletion. The lack of effect on a related virus is more difficult, albeit not impossible, to reconcile with a model in which the effect of p21 depletion on HIV-1 replication has an indirect effect on cell physiology. It would be useful to know whether SIVmac251 PICs associate with p21 and what viral determinants govern the apparently discordant effects of p21 on HIV versus SIVmac251 replication — this would provide strong clues as to an underlying mechanism. If the effects of p21 are direct, it might be possible to show that resistance can be induced in other cellular contexts by ectopic expression of p21. For now, the central role of p21 in cell physiology, and likely a complex cascade of events that accompany its depletion, urge circumspection in formulating models to explain why it appears to inhibit HIV-1 replication. The authors do show that p21-depleted cells do not begin to proliferate until some time after siRNA transfection, but this does not compellingly refute the possibility that p21’s effects on HIV-1 infection are indirect because the physiological changes that lead to DNA synthesis and cell division obviously precede the events themselves.