On Partial Identification of the Pure Direct Effect

In causal mediation analysis, nonparametric identification of the pure (natural) direct effect typically relies on, in addition to no unobserved pre-exposure confounding, fundamental assumptions of (i) so-called "cross-world-counterfactuals" independence and (ii) no exposure- induced confounding. When the mediator is binary, bounds for partial identification have been given when neither assumption is made, or alternatively when assuming only (ii). We extend existing bounds to the case of a polytomous mediator, and provide bounds for the case assuming only (i). We apply these bounds to data from the Harvard PEPFAR program in Nigeria, where we evaluate the extent to which the effects of antiretroviral therapy on virological failure are mediated by a patient's adherence, and show that inference on this effect is somewhat sensitive to model assumptions.Comment: 24 pages, 4 figure

Genomic Sites of Human Immunodeficiency Virus Type 2 (HIV-2) Integration: Similarities to HIV-1 In Vitro and Possible Differences In Vivo

Retroviruses have distinct preferences in integration site selection in the host cell genome during in vitro infection, with human immunodeficiency virus type 1 (HIV-1) integration strongly favoring transcriptional units. Additionally, studies with HIV-1 have shown that the genomic site of proviral integration may impact viral replication, with integration in heterochromatin associated with a block in viral transcription. HIV-2 is less pathogenic than HIV-1 and is believed to have a lower replication rate in vivo. Although differences in integration site selection between HIV-2 and HIV-1 could potentially explain the attenuated pathogenicity of HIV-2, no studies have characterized integration site selection by HIV-2. In this study, we mapped 202 HIV-2 integration sites during in vitro infection of peripheral blood mononuclear cells with a primary HIV-2 isolate. In addition, we assayed for in vivo proviral integration within heterochromatin in 21 HIV-1-infected subjects and 23 HIV-2-infected subjects, using an alphoid repeat PCR assay. During in vitro infection, HIV-2 displayed integration site preferences similar to those previously reported for HIV-1. Notably, 82% of HIV-2 integrations mapped to Refseq genes, and integration strongly favored regions of the genome with high gene density and high GC content. Though rare, the proportion of HIV-2 subjects with evidence of proviral integration within heterochromatin in vivo was higher than that of HIV-1-infected subjects. It is therefore possible that integration site selection may play a role in the differences in HIV-1 and HIV-2 in vivo pathogenesis

Long-Term Intrapatient Evolution During HIV-2 Infection

HIV-2 disease progression and transmission is attenuated compared to HIV-1, yet prospective studies examining HIV-2 intrapatient evolution have been limited. We examined viral sequence evolution in the C2V3C3 region of the viral env gene in 8 HIV-2 infected individuals from Dakar, Senegal, over the course of approximately 10 years. To compare results to HIV-1 infection, we reanalyzed data from our previous study that examined intrapatient evolution in HIV-1 infected individuals from the same population. HIV-2 sequences from early and late timepoints were phylogenetically intermixed for all subjects, and no distinct trends were observed in terms of increases or decreases in fragment size or number of N-linked glycosylation sites. In homologous env C2V3 sequence, rates of viral divergence and diversification were slower in individuals infected with HIV-2 than individuals with HIV-1. This data indicates that viral evolution occurs slowly in HIV-2 infection, which is consistent with the slow disease progression observed in HIV-2 infection, and supports the notion that viral evolution may be a relevant correlate for disease progression

Time-Dependent Predictors of Loss to Follow-Up in a Large HIV Treatment Cohort in Nigeria

Background: Most evaluations of loss to follow-up (LTFU) in human immunodeficiency virus (HIV) treatment programs focus on baseline predictors, prior to antiretroviral therapy (ART) initiation. As risk of LTFU is a continuous issue, the aim of this evaluation was to augment existing information with further examination of time-dependent predictors of loss. Methods: This was a retrospective evaluation of data collected between 2004 and 2012 by the Harvard School of Public Health and the AIDS Prevention Initiative in Nigeria as part of PEPFAR-funded program in Nigeria. We used multivariate modeling methods to examine associations between CD4+ cell counts, viral load, and early adherence patterns with LTFU, defined as no refills collected for at least 2 months since the last scheduled appointment. Results: Of 51 953 patients initiated on ART between 2004 and 2011, 14 626 (28%) were LTFU by 2012. Factors associated with increased risk for LTFU were young age, having nonincome-generating occupations or no education, being unmarried, World Health Organization (WHO) stage, having a detectable viral load, and lower CD4+ cell counts. In a subset analysis, adherence patterns during the first 3 months of ART were associated with risk of LTFU by month 12. Conclusions: In settings with limited resources, early adherence patterns, as well as CD4+ cell counts and unsuppressed viral load, at any time point in treatment are predictive of loss and serve as effective markers for developing targeted interventions to reduce rates of attrition