Background

Implementation science focuses on evaluating strategies for delivering evidence-based interventions to improve HIV prevention and treatment. The effectiveness of these implementation strategies is often context-dependent and reconciling the desire to produce generalizable knowledge in the face of these contextual interventions is a central challenge for implementation science researchers.

Methods

We provide an overview of the causal transportability theory and conceptualize context under this framework. We review how causal graphs can be used to illustrate the assumptions necessary to apply the results of a study to a new context, and we illustrate this approach using an example of a community adherence group intervention that aims to improve retention in HIV care. Finally, we discuss several key insights highlighted by the transportability theory that are relevant to implementation science researchers.

Results

By adopting causal transportability to consider how context may affect the success of an implementation strategy, researchers can formally diagnose when the results of a study are likely to generalize to a given setting. Moreover, selection diagrams can highlight what additional measurements would be needed in a target population to estimate the effect of an implementation strategy in that target population without having to repeat the initial study.

Conclusions

Transportability translates intuition about context-dependent interventions and external validity into actionable and testable insight.

Various ongoing trials seek to evaluate long-acting pre-exposure prophylaxis (PrEP) agents by showing that they are non-inferior to daily oral tenofovir disoproxil fumarate and emtricitabine. Trials comparing oral PrEP to new methods examine effectiveness in a setting where only one or the other is provided; however, a new product will probably be delivered in a context where oral PrEP is also available. The effectiveness of a new PrEP product is best measured by its potential effect in a context that also includes oral tenofovir disoproxil fumarate and emtricitabine as an option. We offer an alternative standard for long-acting products-a measure of the effectiveness of the new product in addition to oral tenofovir disoproxil fumarate and emtricitabine as compared with oral PrEP alone. We term this measure mosaic effectiveness. We illustrate scenarios where a novel product can fail to show non-inferiority but show substantial mosaic effectiveness, thus implying the public health value of the novel product even if it is less effective than oral PrEP. Regulatory standards should consider mosaic effectiveness, not just comparative effectiveness. We assert that measurements that combine rigor with public health relevance can accelerate progress against the HIV epidemic.

Subgroup analyses of randomized controlled trials guide resource allocation and implementation of new interventions by identifying groups of individuals who are likely to benefit most from the intervention. Unfortunately, trial populations are rarely representative of the target populations of public health or clinical interest. Unless the relevant differences between trial and target populations are accounted for, subgroup results from trials might not reflect which groups in the target population will benefit most from the intervention. Transportability provides a rigorous framework for applying results derived in potentially highly selected study populations to external target populations. The method requires that researchers measure and adjust for all variables that 1) modify the effect of interest and 2) differ between the target and trial populations. To date, applications of transportability have focused on the external validity of overall study results and understanding within-trial heterogeneity; however, this approach has not yet been used for subgroup analyses of trials. Through an example from the Iniciativa Profilaxis Pre-Exposición (iPrEx) study (multiple countries, 2007-2010) of preexposure prophylaxis for human immunodeficiency virus, we illustrate how transporting subgroup analyses can produce target-specific subgroup effect estimates and numbers needed to treat. This approach could lead to more tailored and accurate guidance for resource allocation and cost-effectiveness analyses.

Qualitative studies suggest that social relationships play an important role in HIV pre-exposure prophylaxis (PrEP) use, but there have been few quantitative assessments of the role of social relationships in PrEP uptake or adherence. We examined the association between disclosure of study participation or LGBT identity and PrEP use in the 1603 HIV-negative participants enrolled in the iPrEx OLE study. We also evaluated the association between LGBT social group involvement and PrEP use. Study participation disclosure to parents and LGBT identity disclosure to anyone in a participant's social network were associated with greater PrEP uptake. Study participation disclosure to partners was associated with higher probability of having protective PrEP drug concentrations compared [risk difference 0.15 95% CI (0.01, 0.30)]. For each additional type of LGBT organization a participant was involved in, the probability of PrEP uptake and having protective drug concentrations increased by 0.04 [95% CI (0.03, 0.06)] and 0.04 (95% CI (0.02, 0.07)] respectively. Overall, social context was associated with PrEP use in iPrEx OLE, and should be taken into consideration when designing future PrEP implementation programs.

Association of HIV-1-specific T-cell responses to infection risk in seronegative individuals is controversial. We quantified and phenotypically characterized gp120-specific T-cell responses in HIV-1 exposed, but uninfected subjects enrolled in the global Pre-exposure Prophylaxis Initiative (iPrEx) chemoprophylaxis trial. IFNγ ELISpot responses were detected in 24% of subjects irrespective of infection outcome. HIV-1 gp120 envelope-specific T-cell responses were more uniformly IFN-γ+TNF-α+Mip-1β+ in persistently seronegative subjects relative to subjects who later seroconverted (median frequency of 76.5% and 66.5%, respectively). IFNγ responses targeted the V2 loop for subjects who remained seronegative. HIV-1 gp120 envelope V2 loop-specific CD8 T-cell responses may help to protect against HIV-1 acquisition.

We leveraged data from the Preexposure Prophylaxis Initiative (iPrEx), a global trial of preexposure chemoprophylaxis against human immunodeficiency virus type 1 (HIV-1) infection, to compare T-cell activation between those who remained negative for HIV-1 and those who became infected during the trial. The frequency of CD38(+)HLA-DR(+) CD8(+) T cells was greater in those who seroconverted, relative to the frequency in those who remained uninfected (1.30% vs 0.82%, respectively; P = .005). This translated to an odds ratio of 4.26 (95% confidence interval, 1.54-11.78) for the association between CD8(+) T-cell activation and infection with HIV-1. T-cell activation may be a biomarker for elevated HIV-1 infection risk.

We assessed the role of depressive symptoms on adherence to daily oral FTC/TDF for HIV PrEP in cisgender men who have sex with men (MSM) and transgender women who have sex with men (TGW) using data from the iPrEx OLE study. A marginal structural logistic regression model was used to estimate the effect of time-varying CES-D scores on having protective levels of drug concentration, adjusting for confounding by sexual practices over time, prior adherence, and baseline demographic characteristics. We found a non-monotonic relationship between CES-D score and odds of protective FTC/TDF levels in MSM. We found evidence that the effect of depression on adherence varied between MSM and TGW, and that depressive symptoms did not contribute greatly to decreased adherence on a population scale. We recommend that depressive symptoms not preclude the prescription of PrEP, and that MSM and TGW be studied separately.

Objective

To evaluate the effects of HIV preexposure prophylaxis (PrEP) with tenofovir disoproxial fumurate (TDF)/emtricitabine (FTC) on kidney function and kidney tubular health.

Design

The Iniciativa Profilaxis Pre-Exposicion open-label extension (iPrEx-OLE) study enrolled former PrEP trial participants to receive open-label TDF/FTC. This study included 123 iPrEx-OLE participants who demonstrated PrEP adherence.

Methods

We compared estimated glomerular filtration rate calculated using serum creatinine (eGFRcr), serum cystatin C (eGFRcys), and in combination (eGFRcr-cys), and a panel of 14 urine biomarkers reflecting kidney tubular health before and 6 months after PrEP initiation.

Results

At baseline, mean eGFRcr, eGFRcys, and eGFRcr-cys were 108.3, 107.0, and 111.1 ml/min per 1.73 m, respectively. Six months after PrEP initiation, eGFRcr declined by -4% (95% CI: -5.7 to -2.4%), eGFRcys declined by -3.3% (95% CI: -8.3 to 1.9%), and eGFRcr-cys declined by -4.1% (95% CI: -7.5 to -0.7%). From the urine biomarker panel, α1-microglobulin and β2-microglobulin increased by 22.7% (95% CI: 11.8--34.7%) and 14.1% (95% CI: -6.1 to 38.6%), whereas chitinase-3-like 1 protein and monocyte chemoattractant protein-1 decreased by -37.7% (95% CI: -53.0 to -17.3%) and -15.6% (95% CI: -31.6 to 4.2%), respectively. Ten of the 14 urine biomarkers, including albumin, had estimated changes of less than 12% with wide confidence intervals.

Conclusion

Six months of PrEP with TDF/FTC was associated with decreases in eGFRcr and eGFRcys. We also observed for the first time changes in flour of 14 urine biomarkers reflecting kidney tubular health. These findings demonstrate that PrEP has direct effects on eGFR and the proximal tubule.

HIV-1-specific T-cell responses in exposed seronegative subjects suggest that a viral breach of the exposure site is more common than current transmission rates would suggest and that host immunity can extinguish subsequent infection foci. The Preexposure Prophylaxis Initiative (iPrEx) chemoprophylaxis trial provided an opportunity to rigorously investigate these responses in a case-control immunology study; 84 preinfection peripheral blood mononuclear cell samples from individuals enrolled in the iPrEx trial who later seroconverted were matched with 480 samples from enrolled subjects who remained seronegative from both the placebo and active treatment arms. T-cell responses to HIV-1 Gag, Protease, Integrase, Reverse Transcriptase, Vif, and Nef antigens were quantified for all subjects in an IFN-γ enzyme-linked immunospot (ELISpot) assay. IFN-γ responses varied in magnitude and frequency across subjects. A positive response was more prevalent in those who remained persistently HIV-1-negative for Gag (P = 0.007), Integrase (P < 0.001), Vif (P < 0.001), and Nef (P < 0.001). When correlated with outcomes in the iPrEx trial, Vif- and Integrase-specific T-cell responses were associated with reduced HIV-1 infection risk [hazard ratio (HR) = 0.36, 95% confidence interval (95% CI) = 0.19-0.66 and HR = 0.52, 95% CI = 0.28-0.96, respectively]. Antigen-specific responses were independent of emtricitabine/tenofovir disoproxil fumarate use. IFN-γ secretion in the ELISpot was confirmed using multiparametric flow cytometry and largely attributed to effector memory CD4+ or CD8+ T cells. Our results show that HIV-1-specific T-cell immunity can be detected in exposed but uninfected individuals and that these T-cell responses can differentiate individuals according to infection outcomes.