While the use of STI to stimulate and boost the immune system by re-exposure to viral antigens has not been proved effective and may be complicated by enduring T cell loss and enhanced resistance [4, 21], there is still interest in judicious suspension of therapy in order to reduce cost and, more particularly, side effects of therapy. Additionally, patients may stop treatment for personal reasons or because of toxicities. Our observation of increased CSF NFL in three of eight subjects raises a previously unexamined potential concern when interrupting antiretroviral treatment – the possibility of nervous system injury – although further studies are needed to confirm the frequency of this finding and, more particularly, to understand its pathobiological and clinical implications.
The neurofilament is a major structural element of neurons, found most conspicuously in larger neurons and their myelinated axons. It is composed of a triplet protein of which the light subunit (NFL) is an essential component of the neurofilament core. Its main function is to maintain the axonal calibre and it thereby has a crucial role in the structural and functional integrity of axons and in their capacity to rapidly conduct nerve impulses . Increased CSF NFL concentrations are thought to reflect principally injury of myelinated axons, and a clear association has been found between the presence of white matter changes and increased CSF NFL levels in patients with Alzheimer's disease and subcortical vascular dementia . CSF NFL is also increased in several other neurodegenerative disorders linked to demyelination and/or axonal degeneration, including active multiple sclerosis and amyotrophic lateral sclerosis [5, 10].
CSF NFL increased substantially in three of our eight subjects studied after stopping antiretroviral therapy, thereby indicating brain injury in this setting. The mechanisms responsible for this rise in NFL remain to be determined. Indeed, from a virological perspective, it is uncertain whether the increase in NFL is simply a complication of established viremia or is related more particularly to the abrupt surge in viral replication that follows treatment interruption. In other words, was this neurological damage due simply to higher levels of HIV replication or due to the "shock" associated with a rapid resurgence after interruption? Perhaps favouring the latter is the finding of elevations in NFL in these subjects without an AIDS diagnosis and CD4 counts above 200 cells/μL, contrasting somewhat with our earlier study that showing CSF NFL elevations chiefly in subjects with AIDS or with CNS opportunistic infections . Normal CSF NFL concentration in one patient measured before treatment initiation gives further support to a rapid viral rebound as the triggering event.
As seen by others , plasma viral load increased rapidly and was already detected at the first follow-up (median 18 days) after treatment interruption. Subsequently, CSF viral load increased after a short interval in the subjects with sufficient observations to define these temporal relationships. In a larger experience, that included some of these same subjects, we have noted this delay and also found that an increase in CSF lymphocytosis developed in more than half of subjects [16, 17].
The current study also shows that STI leads to increases in CSF and plasma neopterin levels and that these elevations peak later than plasma and CSF HIV RNA levels. Neopterin is an unspecific marker of immune activation that is largely derived from activated macrophages and microglia . CSF neopterin is increased in patients with ADC and has also been found to be a predictive marker of ADC . However, CSF neopterin is also frequently increased in HIV-infected patients without neurological complications, with higher levels found in severely immunocompromised patients than in those with CD4-cell counts above 200 cells/μL . In a previous study, we found an association between increased NFL and CSF neopterin concentrations . This led to our suggesting that immune activation was important in the neuronal injury and release of NFL in this setting. While elevations in CSF neopterin were found in all the subjects with increased NFL and there was the suggestion of temporal association, we also observed subjects with elevated CSF neopterin and normal, unchanging CSF NFL. Hence, further studies are needed to examine this association.
Whatever the underlying mechanism, the presumed axonal damage was subclinical, and no concomitant neurological deterioration was detected during the follow-up period either on the basis of symptoms or on clinical or more formal, though brief, quantitative testing. Based on this initial study, it remains unknown whether this axonal injury has clinical importance. The subject number was small and the follow-up period, for the most part, of limited duration. Hence, longer-term impact on neurological function could not be discerned, though one subject was followed for two years off treatment and did not exhibit neurological change over that time. With one exception (this same subject, 6011, with ADC 0.5 ), our subjects were neurologically normal and without prior evidence of ADC. Theoretically, for patients with ADC, resurgence of viremia and CNS infection might be more hazardous, and stopping treatment should be undertaken with particular caution. Whether treatment approaches with repeated cycles of treatment interruptions also can initiate repeated damage to the brain is an open question.
The present study also does not establish whether axonal injury is an acute short-term event after treatment interruption or if it is more chronic. In herpes simplex type 1 (HSV-1) encephalitis, CSF NFL levels increase to a maximum approximately 8–14 days after the onset of neurological symptoms, and only slowly decrease thereafter, with abnormal levels still detected as long as 3–10 months later . Similar observations, with slow normalization of CSF NFL concentrations, have also been made after focal brain ischemia and after acute relapses of multiple sclerosis . Wallerian degeneration, with an anterograde degeneration of axons and disruption of the axonal cytoskeleton, has been suggested as a principal cause of this delayed and long-lasting CSF NFL increase in these settings. The metabolic degradation and half-life of NFL in CSF is also not established.
The temporal delay between viral replication (as evidenced by plasma and more particularly CSF HIV RNA levels) and the onset of axonal disturbance (measured by NFL) may also imply that Wallerian degeneration is involved in STI. If the neuronal soma is the primary site of action, the axonal changes should be a later event. A direct effect on axons in myelinated tracts or spinal roots could also be hypothesised. Alternatively, there may be a lag between HIV replication and onset of neural injury, perhaps involving immunopathological pathways that lag behind the surge in viral replication. The magnitude of the CSF viral load increase, however, did not directly predict the development of elevated CSF NFL; as noted earlier, there were subjects with high CSF viral load without change in NFL levels.