Association between lymphocyte and monocyte subsets and cognition in children with HIV

Background This study assesses the relationships between lymphocyte and monocyte subsets and intelligence quotient (IQ) scores in antiretroviral therapy (ART)-naive, HIV-infected Thai children without advanced HIV disease. Findings Sixty-seven ART-naive Thai children with CD4 between 15-24% underwent cognitive testing by Weschler intelligence scale and had 13 cell subsets performed by flow cytometry including naive, memory and activated subsets of CD4+ and CD8+ T cells, activated and perivascular monocytes and B cells. Regression modelling with log10 cell count and cell percentage transformation was performed. Median age (IQR) was 9 (7–10) years, 33% were male, CDC stages N:A:B were 1:67:31%, median CD4% and count (IQR) were 21 (18–24)%, 597 (424–801) cells/mm3 and HIV RNA (IQR) was 4.6 (4.1-4.9) log10 copies/ml. Most (82%) lived at home, 45% had a biological parent as their primary caregiver, and 26 (49%) had low family income. The mean (SD) scores were 75 (13) for full scale IQ (FIQ), 73 (12) for verbal IQ (VIQ) and 80 (14) for performance IQ (PIQ). Adjusted multivariate regression analysis showed significant negative associations between B cell counts and FIQ, VIQ and PIQ (p < 0.01 for all); similar associations were found for B cell percentages (p < 0.05 for all). Conclusions High B cell counts and percentages were strongly associated with poorer FIQ, VIQ and PIQ scores. Prospective, long-term assessment of cell subsets and determination of relevant B cell subpopulations could help further elucidate associations between lymphocyte subsets and neurocognitive development.


Introduction
HIV causes neurocognitive impairment possibly by transmigration through the blood brain barrier of activated T cells and infected monocytes triggering a cascade of inflammatory processes [1]. HIV encephalopathy, a severe form of neurocognitive impairment in children, has become less common since antiretroviral therapy (ART) became widely available. However, chronic stable cognitive and behavioral deficits remain, predicting academic impairments in school [2]. Limited studies in US children have suggested that certain lymphocyte subsets may predict poorer cognition [3,4]. Here we assess the relationships between lymphocyte and monocyte subsets and intelligence quotient (IQ) scores in ART-naive, HIVinfected Thai children without advanced HIV disease.
Children's characteristics were described by median, interquartile range (IQR) or percentages, as appropriate. Each cell subset percentage and count was used as a continuous covariate. Regression modelling with both log 10 transformed cell count and cell percentage as outcome variables was performed against IQ scores, and adjustment was made for age, gender, HIV RNA, education of caregiver and socioeconomic status. Analyses were performed with Stata 12.1 (Statacorp, College Station, Tx, USA). The level of significance was based on P value of less than 0.05.
The study was approved by the following institutional review boards in Thailand: the Ministry of Public Health, Chulalongkorn University, Khon Kaen University, Bamrasnaradura Infectious Disease Institute, Chiangrai Prachanukroh Hospital and Queen Savang Vadhana Memorial Hospital.
Multivariate regression analysis was performed to evaluate associations between cell subsets and IQ scores ( Table 2). After adjusting for age, gender, HIV RNA, education of caregiver and income, significant negative associations between B cell counts with FIQ, VIQ and PIQ were observed (p < 0.01 for all). Similar negative associations were found for B cell percentages (p < 0.01 for FIQ and PIQ; p < 0.05 for VIQ). There was a marginal inverse association between memory cytotoxic T cell count and VIQ (p = 0.048). The other cell subsets showed no significant associations with IQ scores. Step father/mother or foster mother 3 (4.5) Education of primary caregiver PMTCT prevention of mother to child transmission.

Discussion
The present study evaluated the associations between 13 peripheral blood mononuclear cell subsets and IQ scores in ART-naive, HIV-infected Thai children with mild to moderate immune suppression. Higher B cell percentages and counts were strongly associated with poorer FIQ, VIQ and PIQ. B-cell perturbation in particular over proliferation of aberrant B cell populations and immunoglobulins, and depletion of resting memory B cells, is a marker for HIVassociated immune deficiency. Such aberrations are linked to high HIV viremia and low CD4 nadir [7]. Although no published report exists on the association of B cells with neurodevelopment in HIV-infected children, higher B cell counts have been reported in children with autism and postulated to contribute to autoimmune response and neurodevelopment disorder in that disease [8]. B lymphocytes can enter the brain and they tend to display an activated phenotype. In HIV-infected patients, increased numbers of B cells in autopsied brain parenchyma and perivascular spaces are observed [9]; possibly playing a role in brain inflammation and insult leading to poorer neurocognition. The marginal significant association between higher memory CD8+ T cell counts with poorer  Regression analysis was performed with an outcome variable of log 10 transformed T cell subset percentages and counts, versus cognitive outcomes adjusted for potential confounders (age, gender, HIV RNA, education of primary caregiver and household income). IQ Intelligence quotient.
VIQ could be a coincidental finding seen in untreated HIV disease. Persistent exposure to HIV viremia can lead to preferential depletion of naive CD8+ T cells and skewed maturation of memory CD8+ T cells [10]. We did not observe a negative association between activated cytotoxic T cells and neurocognition as reported in a US study by Mekmullica et al. In that study, the relative absence of activated cytotoxic T cells, CD8 + HLA-DR + and CD8 + 38+ cells, in the first few months of life was strongly associated with favorable psychomotor outcomes in the first 2 years of life [3]. Activated cytotoxic T cells could migrate into the brain and cause production of proinflammatory cytokines and neurotoxins resulting in neuronal dysfunction [1]. However, Kapetanovic S, et al. recently reported higher activated cytotoxic T cell percentage, using the same surface markers, to have a neuroprotective effect in a group of older ART-treated, US children with advanced HIV disease [4]. The discrepancies in associations between T cell subsets and neurocognition in these two latter studies and our own study might partially be accounted for by differences in study population in age and HIV disease severity. Children in the Mekmullica study were newborn at the time of lymphocyte collection, whereas those in the study by Kapetanovic ranged from 1 year to teenagers, and were mostly heavily pretreated with advanced HIV disease. In contrast, our study was conducted in children with less advanced disease than those described by Kapetanovic. Although the Mekmullica study states that B cells were enumerated, the results were not included in the paper which might suggest that no significant association was found, but nevertheless precludes direct comparison with the results in our study.
It must be acknowledged that the peripheral blood B cell compartment contains several distinct B cell types, e.g., immature/transitional (CD10 + , CD27 -, activated/ mature (CD21 lo CD10 -), and resting memory (CD21 hi CD27 + ) [11]. None of these subsets were included in the measurements in this report and further studies will be necessary to determine the neurotoxic B cell subpopulation. The presence of activated monocytes has also been associated with neurocognitive impairment but we did not see this in our study. CD163 is a scavenger receptor expressed on monocytes and macrophages that are thought to be precursors of perivascular macrophages that traffic to the brain [6,12]. They are seen in higher frequencies in HIV-infected subjects compared to healthy controls, and increased frequencies are associated with HIV viremia and low CD4 count [12]. Therefore the weak negative association between these perivascular monocytes and PIQ may be due to untreated HIV disease.
Our study is limited by the small number of children. As previously noted, they were relatively well without advanced HIV disease and were not yet treated with ART. Age, ethnicity and environment can affect both cell subset distribution and neurocognition [2,5]. These factors may limit the generalizability of our data to other populations. We did not have data from age-matched HIV negative Thais to evaluate the relationship between these cell subsets and IQ. Inclusion of such data would also have allowed us to control for non-HIV confounders such as socioeconomic status and caregiver's education.
Nevertheless, we observed a strong association between higher B cells and poorer FIQ, VIQ and PIQ. Long term study of this PREDICT cohort of children randomized to immediate versus deferred ART initiation may enable us to further elucidate these relationships, and identify cellular subsets which could perhaps predict poor neurodevelopment. This in turn, could help identify children that would benefit from early ART and/or close monitoring and intervention to improve neurodevelopment.