There remains an ongoing debate as to whether viral replication persists in the context of suppressive HAART [12
]. This question is critical in the continuing efforts toward viral eradication, and multiple studies, including raltegravir intensification studies, have attempted to address the issue [15
In this study, raltegravir intensification for 24 weeks did not decrease plasma RNA more than placebo. This finding is consistent with those in 2 other recent studies of raltegravir intensification [16
]. However, several important differences deserve comment. Our study focused on immunologic nonresponders, and the duration of intensification was longer (24 weeks) than in the other studies. We found that both groups had a significant decrease in plasma RNA, probably because subjects in each group were in a relatively early phase of viral suppression (2.6 years) [41
]. More importantly, and in contrast to other studies, we focused on the host responses to HIV in both the blood and the gut, assuming that these responses might be a more sensitive measure of low-level viral replication [20
]. We hypothesized that higher levels of viral replication would result in higher levels of T cell activation and perhaps higher levels of HIV-specific T cell responses, with these effects being most evident in the GALT, which is thought to be the major reservoir of HIV during HAART [42
]. Our data consistently failed to reveal any effect of raltegravir intensification on any of these measurements.
However, we acknowledge that our study did not fully rule out the presence of ongoing viral replication in the context of effective HAART. Subjects in our study were not required to have detectable plasma RNA at baseline; indeed, only 19 of 28 subjects had plasma RNA that was detectable at baseline with an ultrasensitive assay (lower limit of detection, <.3 copy/mL). Thus, it is possible that had we selected only subjects with plasma RNA detectable at baseline with an ultrasensitive assay, we may have observed a decrease in plasma viremia with raltegravir intensification. However, results from such a study were recently published and did not show an effect with intensification [16
]. Moreover, additional studies, including measurement of 2–long terminal repeat circles [15
] and measurement of viral persistence in other tissues, such as GALT, are necessary to completely rule out the presence of ongoing viral replication [43
]. In a recent randomized clinical trial of raltegravir intensification, Buzón and colleagues found that raltegravir may have an effect on both virus and T cell dynamics in individuals who are taking a protease inhibitor as part of their HAART regimen [15
]. Of note, we observed similar findings in our study. When we limited our analysis of residual viremia to subjects taking a protease inhibitor as part of their HAART regimen, the proportion of subjects with undetectable plasma RNA at week 12 was higher in the raltegravir group than in the placebo group (100% vs 44%, respectively; P
= .04). These data suggest that residual viral replication may occur in an anatomic compartment that is less accessible to protease inhibitors. Further studies will be necessary to confirm these findings.
Another limitation of this study, and of this particular area of research, is the lack of uniformity in the definition of “immunologic nonresponders.” Although we examined subjects with CD4+ T cell counts of <350 cells/mm3 despite viral load suppression for ≥1 year, it is possible and indeed likely that the biology associated with having a low CD4+ T cell count during relatively early HAART may prove to be different from that in long-term, immunologic nonresponders. This is another area that requires further study.
Immunologic nonresponders may have a larger latent reservoir than responders, and most of this reservoir—a disproportionate amount—resides in the gut [42
]. Moreover, immunologic nonresponders may have insufficient or ineffective HIV-specific responses in this critical location where most of the residual virus exists. The degree of immune reconstitution after HAART has been shown to be related to the degree of local fibrosis in lymph nodes and the gut [10
]. Immunologic nonresponders may be caught in a continuous, self-sustaining cycle of increased immune activation, greater damage and fibrosis to the gut, and increased microbial translocation [44
]. To elucidate further the determinants of immunologic nonresponse, cross-sectional studies comparing the size of the latent reservoir and HIV-specific mucosal responses in immunologic responders and nonresponders will be necessary.
Although multiple studies have defined the immunologic correlates of virus control in untreated persons, to our knowledge there has not been an extensive study examining the immunologic correlates of viral persistence in the context of long-term antiretroviral therapy. In our study, we considered the level of viremia and cell-associated virus in the peripheral blood as well as other possible determinants, including the level of immune activation and the magnitude of HIV specific T cell responses. In a secondary analysis, we observed negative correlations between mucosal HIV-specific T cell responses and measurements of the cellular reservoir; however, these findings are preliminary and require confirmation by other, larger studies. Although the mechanisms accounting for these associations cannot be fully addressed in this cross-sectional analysis, strong mucosal T cell responses might contribute to rapid clearance of virus during HAART, and presumably this is testable. Moreover, this hypothesis could coexist with a scenario in which HIV-infected cells continue to produce virus but are susceptible to immune clearance. Approaches aimed at expanding HIV-specific CD4+ and CD8+ T cell responses in the gut mucosa may accelerate clearance of the viral reservoir. The next logical step would be to pursue therapeutic vaccine studies using HIV vaccines that elicit strong mucosal T cell responses in HAART-treated patients.
In summary, we found that suboptimal CD4+
T cell gains during long-term HAART are not likely to be due to low-level ongoing viral replication, at least as detected in peripheral blood. Other therapeutic options for this challenging patient population will be needed. In the context of complete or near complete viral suppression, mucosal HIV-specific T cell responses may be a determinant of the size of the latent reservoir. This latter observation provides support for future studies aimed at using therapeutic vaccines to affect the size of the reservoir [46