The primary findings of this study indicate (1) M-CSF concentrations in the CSF have a relationship to cognitive impairment that is predictive of cognitive improvement (via MSK with trends for NPZ-8) in this small cohort; (2) at baseline, higher M-CSF levels were associated with decreased neuronal metabolism (lower NAA levels); a relationship that may be disrupted with the use of antiretroviral therapies; (3) neither baseline M-CSF levels nor the changes in M-CSF between time points were predictive of increases in NAA observed over the duration of the study; and (4) in nearly all subjects, increased NAA levels in many regions were observed within three months of initiating ART regimens.
Previous studies have observed the ability of M-CSF levels in the CSF to distinguish between dementia cohorts, as observed in this analysis (Gallo et al, 1990
; McArthur et al, 2004
). Moreover, this study indicated that M-CSF in the CSF demonstrated a direct relationship to composite neuropsychological scores (NPZ-8 at baseline), indicating those with higher levels had more cognitive impairment. Subjects who had improved NPZ-8 scores after 10 months of combination ART use tended to exhibit larger decreases in CSF M-CSF concentrations. Beyond the descriptive relationship, M-CSF concentrations in the CSF measured before therapy were capable of predicting improved MSK scores (with trends for NPZ-8) after 10 months of combination ART use. These results suggest CSF M-CSF levels (in the context of detectable viral levels) may be a useful predictor of active central nervous system CNS disease and the cognitive dysfunction caused by it. M-CSF levels may allow for a better assessment of those requiring ART medications with increased CNS penetration or specific efficacy against infected macrophages and microglia.
In contrast to the relationship found with cognition, baseline levels of M-CSF in both the CSF and blood were inversely correlated (using Spearman rank correlation) with NAA concentrations in many regions of the brain. It was interesting to discover a biologic marker associated with neuroaxonal metabolism in so many regions of the brain. MR studies to date have attempted to find such correlations with viral RNA, monocyte chemoattractant protein-1, CD4+ cell counts, and other molecules with varying degrees of success (Chang et al, 2002
). Notably, the strongest relationships at baseline and over time were observed within the subcortical regions within this study. It is possible that the stronger association in the basal ganglia and thalamus may be due to higher numbers of activated microglia in these areas (Kure et al, 1990
). However, the spectra as shown in may also suggest that a lack of MR field homogeneity could have weakened the association in cortical regions, especially those near the air-tissue interfaces, such as the frontal cortex (Kreis, 2004
). Regardless of this correlation observed at baseline, the relationship of CSF M-CSF to neuronal injury as measured by NAA was lost over time, indicating ART use may disrupt this association although subject drop-out may have contributed. Further analyses indicated that the changes in M-CSF and NAA between time points were not associated to one another, nor was M-CSF capable of predicting increases in NAA levels after 10 months of combination ART use.
Many MRS and MRSI studies have been performed on patients with HAD examining the neuronal integrity marker NAA, typically reported as a ratio to creatine levels in the same region rather than as an absolute concentration (Chang et al, 2004b
; Sacktor et al, 2005
). Speculation as to the changing nature of creatine levels in the brain due to HIV makes studies that can quantify absolute concentrations of NAA particularly relevant. Results in the present report indicate that NAA concentrations improved in PGM, PWM, and FWM regions of nearly all subjects and did so within the first 3 months of therapy, even though a cognitive response (NPZ-8) was not observed until months later. NAA may be a highly sensitive marker of neuronal dysfunction in the setting of unrestricted viral replication: one that indicates neuronal repair/recovery (e.g., reduced oxidative stress on mitochondria or remyelination in white matter regions) if therapy is initiated sufficiently early.
A limitation of this study includes a lack of history detailing drug abuse over the duration of the study, which could represent a confounding variable to these data and their interpretations. Also, the present study was not performed with respect to a specific antiretroviral therapy regimen and is representative of the many possible combinations of antiretroviral drugs currently used in treating HIV infection. The lack of consistency in regimens made further analysis or conclusions impossible due to the small sample sizes that had similar regimens. Future longitudinal studies should address the issue of inconsistent regimens since specific drugs such as zidovudine (AZT) may be more adept at reducing both the levels of M-CSF and viral production produced by macrophages (Kutza et al, 2000
). The ability of AZT to control those events may provide some explanation as to the rapid decline in severe dementia observed in the mid to late 1990s. Interestingly, M-CSF antagonists have been proposed as a potential mutation-resistant therapeutic target that could prevent the maintenance and establishment of a macrophage reservoir (Bosch et al, 2006
; Haine et al, 2006
; Kutza et al, 2000
In conclusion, the relationship of M-CSF levels to NAA levels and cognitive measures is indirect. The reduction of viral antigen and RNA by ART results in the reduced activation/infection of monocytes that can traffic into the brain (Aquaro et al, 2006
; Kaul et al, 2005
). Decreasing the amount of virus brought into the brain results in reduced neurotoxicity caused by viral proteins. The M-CSF receptor is also found on microglial cells, which when activated creates another possible avenue for the production of neurotoxic viral proteins and cytokines (Fixe et al, 1998
). Moreover, infected macrophages and microglia have increased cytokine production, such as tumor necrosis factor (TNF)-α. By reducing TNF-α production in the CNS, the extent of neuronal injury induced by radical oxygen species and glutamate excitatory pathways are greatly diminished. The argument that therapy disrupts the association between NAA and M-CSF by blocking those pathways is a valid interpretation, but it is possible that the loss of correlation is due to subject withdrawal from the CSF portion of the study. These results suggest that further study into the relationships of M-CSF to neuronal injury and cognitive impairment in larger cohorts of HIV+ individuals is warranted.