This study explored whether treatment intensification with an additional antiretroviral drug having a different mechanism of action from the drugs already being taken—in this case the integrase inhibitor raltegravir—would reduce CSF biomarkers of immune activation and CSF HIV-1 RNA. Our underlying mechanistic hypotheses centered on the capacity of raltegravir to further inhibit low levels of viral replication in the CNS despite clinically measured suppressive therapy and thereby reduce the residual intrathecal immune activation. Our main outcomes were CSF neopterin and CSF HIV-1 RNA measured by SCA, but we also measured CSF T-cell activation, WBC count, CSF-blood albumin ratio, and the QNPZ-4 index of neurological performance. None of these measures showed improvement with raltegravir augmentation in this well-treated group of subjects with low baseline CSF viral burden and immunoactivation. This was similar to the absence of systemic effects on the HIV-1 viral load in plasma and blood immune activation markers in this study and in larger studies [25
], which also did not detect an effect of intensification, with the exception of one that found an increase in 2–long terminal repeat circles, suggesting possible inhibition of low-level replication in a subset of patients [44
]. The only measured changes in T-cell activation showed higher rather than lower levels in the intensified group, and in each case the level of difference was small and might be explained by the multiple comparisons.
There are at least 3 possible explanations for the lack of observed effect of intensification in this small study.
First, raltegravir might not appreciably inhibit CNS infection. Although there is little direct study documenting the CNS efficacy of raltegravir in isolation, pharmacokinetic studies, including one involving some of the subjects included in this study [27
], have shown that the drug commonly achieves CSF levels in the low therapeutic range, albeit well below those in plasma [28
]. In addition, depending on interpretation of therapeutically effective concentrations, drug exposure was likely sufficient to inhibit CNS replication, although the period of treatment may possibly have been too short to detect an effect.
Second, the underlying hypothesis might not be sound: there may be little or no continued HIV-1 replication within the CNS, and local viral replication may not cause the persistent immunoactivation in the CSF. This explanation actually has 2 parts related to (1) residual ongoing viral replication in the CNS and (2) the mechanism of continued intrathecal immunoactivation. With respect to continued infection in treated patients, pathological studies do not generally detect active infection at autopsy in treated patients [47
]. However, this is the first study to use a very sensitive SCA method to examine residual CSF HIV-1 RNA in very well-treated subjects, and it shows that indeed even at baseline there was very little CSF virus despite detectible residual virus in many of the plasma samples, similar to that in other studies [25
]. Thus, overall, HIV-1 RNA was detected in only 8 of 56 CSF samples (14%) compared with 32 of 50 (64%) plasma samples. Using the rough estimation method described earlier of subtracting .1 copies from the limits of detection for each sample below these limits, the median CSF copy number was .2 copies/mL while that of the plasma was near .9 copies/mL, a CSF-plasma relationship not much different from the usual 1:10 ratio found in untreated patients, although higher than in patients failing therapy in which the ratio is closer to 1:100 [6
]. Overall, the very low levels of CSF HIV-1 RNA and the lack of further reduction by intensification suggest that there was indeed little active CNS viral replication in these subjects, with the general caveat that even our SCA applied to CSF may be insensitive to low-level infection within the brain parenchyma.
In untreated infection, virus detected in CSF may originate either from the bloodstream (so-called transitory or noncompartmentalized infection) or from local sources in the CNS (autonomous or compartmentalized infection) [9
]. To further elucidate the origins of the very low-level infection in treated subjects, it will be crucial to similarly explore the origins of CSF HIV found in these patients. While technically formidable, this will be essential to understanding the meaning of detecting CSF HIV in treated patients, particularly patients with higher amounts of virus than noted here. Is it spillover from the residual plasma virus or from long-lived memory CD4+
T cells, or is it released from local resident cells, macrophage-microglia, or even astrocytes? This issue may take on greater importance in relation to therapeutic viral eradication.
Stimulation of CSF neopterin by local infection is supported by the association of this pteridine with CNS infection and disease [11
] and with previous observations that an increase in these low levels in CSF correlates with viral escape [10
]. Thus, it is likely that local infection can “drive” CSF neopterin concentrations. However, the low residual levels seen in the broader experience of treated patients [11
] may also reflect other processes, including a continued immunological abnormality in the absence of local infection [49
Third, the underlying hypothesis might not have actually been addressed because of the particular makeup of the subject group with minimal CNS infection and immunoactivation that left little room to discern a therapeutic effect. In addition to the very low levels of virus in CSF, CSF neopterin concentrations were also quite low, similar to our previous control experience (mean 5.3 nmol/L, SD 2.2), although higher than a larger series of HIV-uninfected individuals without neurological disease (4.2 nmol/L, SD 0.8) [11
]. Whether the unanticipated low levels observed in our subjects related to their duration of treatment, their drug combinations (some of which included more than 3 drugs), treatment adherence, self-selection for a CNS intensification study, or other factors, they may have comprised an atypical or extreme group. This issue needs to be examined by further application of SCA to a larger cohort.
Nonetheless, our findings are consonant with a previous study of treatment intensification using either enfurvitide, maraviroc, or lopinavir/ritonavir over a shorter period of time (8 weeks) [50
]. Although the proportion of CSF samples in which HIV-1 could be detected and the levels of HIV-1 detected were higher in that study using a different method for HIV-1 RNA detection, still no change in the CSF neopterin was detected.
In summary, despite the favorable characteristics of raltegravir for CNS treatment, we found no evidence that intensification reduced either intrathecal immunoactivation or CSF HIV-1 RNA in our subjects. This is not only consistent with the previous CSF report but also with systemic effects of treatment intensification [25
]. A remarkable facet of our findings was the low amount of CSF viral RNA and neopterin detected in the subject group. This suggests that treatment can indeed be effective in reducing the CNS viral burden and intrathecal immune activation. It remains to be seen whether this is common or whether our subjects were indeed an unusual group.