The prevalence of HPV infection in our study subjects is high: 70% (262/373) at baseline and 70% (78/111) infected during follow-up, with a cumulative prevalence of 91% (340/373). This is as expected for a high-risk cohort of adolescents, judged by their self-reported sexual and other risk behaviors, and for HIV patients in the U.S. [30
]. While clearance of HPV infection is frequent among HIV-positive adolescents on HAART, continued acquisition and persistence of HPV infections are common with and without HAART therapy, as previously reported [21
]. HPV prevalence and incidence for most types tend to be slightly lower among HIV-negative than HIV-positive females in general (although this is not statistically significant in most cases--see Table footnote); however, our data suggest that there is no immediate effect of HAART on HIV-infected adolescents, especially with regard to high-risk and vaccine-type HPV infection prevalence, persistence, or clearance. Additionally, there is no clear pattern of HPV infection or clearance with respect to immune reconstitution (based on CD4+
T-cell counts) from HAART.
The present data corroborate previous findings that there is no immediate positive effect of HAART on high-risk HPV incidence and clearance, despite immune reconstitution through the increase of CD4+
T-cells after treatment. On the contrary, we observed higher prevalence and incidence of possible carcinogenic and low-risk HPV types in the after HAART initiation period. Thus, prevention of HPV acquisition is important, especially in vulnerable populations such as sexually active adolescents. Recently, the FDA approved Merck's HPV vaccine, Gardasil [39
], for use in girls and women aged 9-26, and the European Medicines Agency approved GlaxoSmithKline's HPV vaccine, Cervarix [40
]. However, since there is no known therapy for HPV infections, measures to control acquisition or persistence of HPV infections (mainly other possible carcinogenic HPV types), especially in HIV/HPV co-infected females, would reduce the cervical cancer burden. Of note, high-risk types other than HPV 16 and 18 (targeted in both vaccines) occurred more frequently in the before and after HAART initiation periods. Thus, although the vaccines might be effective for particular 16 and 18 strains, innovative approaches are needed to examine cross-protection against other high-risk types that might be of epidemic concerns in HIV-positive populations both on and off HAART. Interestingly, the prevalence of HPV 16 was quite high, but the incidence was not that high compared to the other types and remained similar among HIV-negative participants and HIV-positive participants before and after HAART. Several studies have reported that the prevalence of HPV 16 was lower in HIV-positive women than in the general population and that other high-risk types are also present in these populations [41
]. Most of these studies, however, are cross-sectional and lack prospective data, even for a shorter period like ours. While speculative, our data could indicate that HPV type 16 is more prevalent among the participants and their partners overall and thus may be transmitted during the earlier sexual encounters, whereas other HPV types may be in circulation in this population with higher incidence, but because they do not persist longer, their cumulative prevalence is lower.
The small size of our overall study sample limits our ability to perform analyses that would fully answer some relevant questions, such as the impact of HAART on type-specific clearance and persistence. Thus we present only counts of clearance rather than the rates. Additionally, the number of visits in which HPV data were obtained varied considerably from person to person, both during and before/after HAART initiation period, and may have introduced a systematic bias. However, the median number of visits for the two time periods was similar. The follow-up time periods might be shorter, especially to detect clearance with two consecutive negative visits, but they are relevant as they span the initiation of HAART and examine the immediate effect of HAART. The median follow-up time after HAART initiation in the study was 427.5 days. Most (70-90%) HPV infection in healthy women clears between 12-24 months, and the 13-month follow-up time may not be long enough to observe a significant difference and thus may lead to our result. Although we observed the reconstituted immunity (increase in CD4+ count), it is possible that it may take a longer time to clear any infection, including HPV. The long-term period cannot be assessed from this study; however, it would be important in the future to study the long-term effect of HAART, especially in adolescents as they live to adulthood.
Further, it is possible that the use of cervical lavage samples, rather than cervical exfoliated cells samples, may have resulted in a slightly lower sensitivity for HPV detection, although we do not expect this to unduly affect our study results. Since HPV testing was performed semi-annually, it is possible that a proportion of infections of less than six-month duration were missed. Although this may affect the prevalence and incidence endpoints, more emphasis was placed on evaluation of HPV infections that persisted, since transient infections are less frequently associated with significant high-grade precancerous outcomes. The genotyping of HPV type was performed using a standard, validated method; however, some of the types were combined with a common probe for the analysis. This made it challenging to separate some of the HPV types, such as types 6 and 11, which are included in the quadrivalent HPV vaccine, from types 42 or 44. Additionally, persistence might be overestimated if the infections were different at consecutive visits but were detected by the generic probe, but clearance would be more conservative. However, high-risk and low-risk types still could be clustered for the analyses. In essence, participants of the study were high-risk adolescents, many of whom were likely recently infected with HPV. Thus, this will likely reduce the bias of long-term persistence in this population, specifically among those who were HPV-positive at baseline.
Besides HIV infection, the study participants were co-infected with other STIs [42
], which may cumulatively or independently affect the immune system separately from HAART therapy. Although other factors, including age, sex, race, socio-economic status, smoking status, health-care, and host genetics, are associated with acquisition and clearance of HPV infection, these variables were not adjusted due to the small sample size. For instance, we did not observe any statistically significant associations (data not shown) with the incidence, prevalence, or clearance of type-specific HPV by baseline smoking status (40% of participants reported ever smoking). While these factors may be important, the present study focused only on the effect of HAART. All female adolescents who adhered to their medication were included and were censored either on the visit-date that they reported not taking their medication or at the end of the study. While validation of self-reports is not always reliable, this seems like an appropriate proxy for measuring their adherence, given the study design in this adolescent population. There are reports that Protease Inhibitor (PI) might inhibit the degradation of p53, which forms part of the mechanism of HPV persistence; however, we did not see any significant association of PI-based HAART (at baseline). Also, several participants changed their HAART regimen during follow-up, which makes further analysis of this association difficult, and the incidence of clearance post-HAART was limited.