At the initial visit, 19 HIV-positive patients were neurocognitively normal (NP-nml) and 15 met criteria for HAND, including six (40%) MND and nine (60%) ANI (Table 1). Those with and without HAND were well matched demographically and on HIV disease and treatment variables (Table 1). Patients with MND had significantly higher plasma sCD163 than ANI (P = 0.04) or NP-nml (P = 0.02) (Fig. 1a). Plasma or CSF sCD14, CSF neopterin, CSF sCD163 or CSF MCP-1 levels were not significantly different between neuropsychological-nml, ANI and MND (data not shown).
When overall neurocognitive performance was summarized using Global Deficit Scores (GDS), 21 HIV-positive patients were identified, as GDS normal and 13 (38%) were GDS impaired at the initial visit (Table 2). GDS-impaired patients had higher plasma sCD163 than those who were not impaired [median (IQR) 1401 ng/ml (1057–2258) versus 955 ng/ml (586–1313); Wilcoxon P = 0.028] (Fig. 1b).
HIV-positive participants were examined at two visits at least 6 months apart [median 16.1 (IQR 6.8–31.9)]. Of the 13 patients who were GDS-impaired at the first visit, 10 remained impaired at the second visit, and all but one of the 21 GDS-unimpaired patients remained normal. All patients remained HIV-undetectable on their ART regimens. Whereas plasma sCD163 levels dropped in patients who were stably GDS-unimpaired across visits (P < 0.032) (Fig. 1c), levels remained elevated in those who remained GDS-impaired (P = 0.50) (Fig. 1d).
When examining individual neuropsychological domains as read-outs we found elevated sCD163 in patients with impaired executive functions and learning (Fig. 1e and f). Patients who were impaired in the learning domain had significantly higher sCD163 in plasma than learning normal HIV-infected patients (Fig. 1e, P = 0.005). Furthermore, patients with executive dysfunction had significantly higher sCD163 in plasma than executive normal HIV-infected patients (Fig. 1f, P = 0.029). These results are consistent with observations by others that executive and learning functions are impaired in HIV-infected patients with effective ART .
Following the observations above, we sought to determine if these markers changed or stabilized with long-term viral suppression [at least 6 months median 16.1 (IQR 6.8–31.9)]. Plasma sCD163 at visit A (1343.0 ± 161.4 ng/ml) significantly decreased when compared with plasma sCD163 at visit B (1122.9 ± 123.7 ng/ml) (Table 3, P = 0.01, paired t-test). CSF neopterin at visit A (181.8 ± 15.7 nmol/ml) was significantly lower when compared with CSF neopterin at visit B (123.2 ± 17.8 nmol/ml) (Table 3, P = 0.05, paired t-test). Plasma and CSF sCD14 and sCD163 and MCP-1 in CSF did not significantly change between visits (Table 3).
In addition to sCD163 in plasma we found that neopterin in CSF is elevated in GDS impaired HIV-infected patients compared with GDS normal HIV-infected patients (P = 0.03) (Fig. 2a). Patients who were impaired in the executive domain had significantly higher neopterin in CSF than executive normal HIV-infected patients (P = 0.037) (Fig. 2b). We did not detect a significant difference between plasma or CSF sCD14, CSF MCP-1, or CSF sCD163 between neurophysiologically impaired and neurophysiologically normal HIV-infected patients (data not shown). In HIV-infected patients, plasma sCD163 and CSF neopterin levels were significantly correlated (Spearman rho = 0.37, P = 0.03) (Fig. 2c).
Here we have demonstrated that neuropsychological impairment in virologically suppressed HIV-infected individuals was associated with a sustained elevation of the monocyte/macrophage activation marker sCD163 as compared to NP-nml patients, in whom sCD163 levels declined. We report elevated levels of sCD163 in patients with impaired GDS and MND demonstrating persistent monocyte/macrophage activation in HIV-related neuropsychological impairment despite virologic suppression.
Even milder HIV-associated neurocognitive disorders such as MND can have considerable impact on daily functioning. The burden of this functional decline is amplified by the extended survival that many of these individuals can expect on effective antiretroviral therapy. Deficits in everyday, ‘real world’ functioning affect only a subset of individuals and are generally only evident in more complex tasks such as financial management, meal preparation, and medication adherence. HIV-associated neurocognitive impairment has been established as a robust risk factor for unemployment and decreased job performance , nonadherence to antiretroviral medications , and impairment in certain driving abilities .
Higher plasma sCD163 levels were specifically associated with impairments in the neuropsychological domains of executive functions and learning, two of the most common and consistent areas of cognitive impairment in the ART era . Global impairment is higher among asymptomatic patients in the ART versus pre-ART era, and this increase in global impairment is primarily driven by an escalation of deficits in episodic memory and executive functions .
We did not find significant levels of sCD163 in the CSF of HIV-infected patients with durable virologic suppression compared with seronegative individuals and sCD163 in CSF did not correlate with neurological impairment. Whether sCD163 is shed in the CSF as efficiently as plasma is not known. CNS macrophages including perivascular macrophages that are in the perivascular space, which is in contact with CSF, and some parenchymal microglia have CD163 on their surface [39,40]. The majority of monocytes express CD163 and activated CD14+CD16+ monocyte/macrophage are implicated as a significant source of sCD163 in plasma [9,39,41]. Our data and others on CD163 levels on CD14+CD16+ monocytes [39,40], as well as data demonstrating an association of HIV-DNA in circulating monocytes with HAND [31–33,42], suggest that the pathological events that led to nervous system injury may originate outside of the nervous system.
Our data are consistent with persistent monocyte/macrophage activation in HAND. The cause of persistent activation is not known, but likely results in part from bacterial translocation, coinfection (although our patients were CMV negative), and other factors. Because we did not find that sCD14 was a marker of neuropsychological impairment, future studies on monocyte/macrophage mechanisms of HIV-associated neuropsychological impairment may point to differential activation of monocyte/macrophages.
That plasma sCD163 correlates with neurological impairment is opportune, as plasma is easier to obtain than CSF. Additionally, plasma sCD163 measurements may guide Neuropsychologists in determining if a comprehensive neurocognitive test battery is needed. Elevated plasma sCD163 is found in other diseases with neurologic symptoms, but it is associated but not correlative with disease. This is likely because diseases like multiple sclerosis that have elevated sCD163 are not driven only by macrophages . sCD163 does correlate in Gaucher disease, hemophagocytic syndrome and macrophage activation syndrome , all of which are primarily macrophage driven. These data underscore the potential importance of chronic innate immune activation in general, and monocyte/macrophage in particular with regard to CNS disease during HIV.
Our previous study of 30 chronic and 14 early HIV-infected patients found an association of sCD163 with CD14+CD16+ monocytes and HIV activity . We also found a correlation of sCD163 with noncalcified plaques in 102 HIV-positive men  and with arterial inflammation in 27 HIV-infected patients with cardiac disease . Additionally, sCD163 was significantly increased in 65 obese patients compared with 30 normal-weight controls . Recently, there have been reports of correlations between patients who are obese or have cardiovascular disease with dementia. It is likely all of these contribute to HIV-associated neuropsychological impairment, wherein macrophages are central players.
Further studies on HIV-associated neuropsychological impairment should focus on CD14+CD16+ monocytes in blood that may drive HAND neuropathogenesis. In addition, research focused on chronic CNS inflammation and neuroimaging studies with plasma sCD163 may be informative. Overall, these data underscore the important role of immune activation in neurological impairment of HIV-infected patients. We suggest that sCD163, a marker of macrophage activation, is a potential biomarker in the plasma for neurocognitive impairment in HIV-infected individuals.
The author's contributions to this manuscript are as following: concept and design (T.H.B., R.J.E., K.C.W.) acquisition of data (T.H.B., A.W.), analysis and interpretation of data (T.H.B., S.W., S.L., R.J.E. and K.C.W.), drafting of manuscript (T.H.B. and K.C.W.), critical revision of the manuscript for important intellectual content (A.W., S.W., S.L. and R.J.E.), statistical analysis (T.H.B., S.W. and R.J.E.) and obtaining funding (S.W., S.L., R.J.E. and K.C.W.).
This work was supported by R01-NS082116 (T.H.B.), R01-NS37654 (K.C.W.), R01-NS40237 (K.C.W.), R01-MH58076 (R.J.E., S.L.), P30-MH62512 (R.J.E., S.L.), R21-MH85610 (R.J.E., S.L.), R01-MH073419 (S.P.W.). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
There are no conflicts of interest.
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