Although the prenatal hippocampus displays deficits in cellular proliferation/migration and volume, which are later associated with memory deficits, little is known about the effects of trisomy 21 on postnatal hippocampal cellular development in Down syndrome (DS). We examined postnatal hippocampal neuronal profiles from autopsies of DS and neurotypical (NTD) neonates born at 38-weeks’-gestation up to children 3 years of age using antibodies against non-phosphorylated (SMI-32) and phosphorylated (SMI-34) neurofilament, calbindin D-28k (Calb), calretinin (Calr), parvalbumin (Parv), doublecortin (DCX) and Ki-67, as well as amyloid precursor protein (APP), amyloid beta (Aβ) and phosphorylated tau (p-tau). Although the distribution of SMI-32-immunoreactive (-ir) hippocampal neurons was similar at all ages in both groups, pyramidal cell apical and basal dendrites were intensely stained in NTD cases. A greater reduction in the number of DCX-ir cells was observed in the hippocampal granule cell layer in DS. Although the distribution of Calb-ir neurons was similar between the youngest and oldest NTD and DS cases, Parv-ir was not detected. Conversely, Calr-ir cells and fibers were observed at all ages in DS, while NTD cases displayed mainly Calr-ir fibers. Hippocampal APP/Aβ-ir diffuse-like plaques were seen in DS and NTD. By contrast, no Aβ1–42 or p-tau profiles were observed. These findings suggest that deficits in hippocampal neurogenesis and pyramidal cell maturation and increased Calr immunoreactivity during early postnatal life contribute to cognitive impairment in DS.

Although Down syndrome (DS), the most common developmental genetic cause of intellectual disability, displays proliferation and migration deficits in the prenatal frontal cortex (FC), a knowledge gap exists on the effects of trisomy 21 upon postnatal cortical development. Here, we examined cortical neurogenesis and differentiation in the FC supragranular (SG, II/III) and infragranular (IG, V/VI) layers applying antibodies to doublecortin (DCX), non-phosphorylated heavy-molecular neurofilament protein (NHF, SMI-32), calbindin D-28K (Calb), calretinin (Calr), and parvalbumin (Parv), as well as β-amyloid (APP/Aβ and Aβ1–42) and phospho-tau (CP13 and PHF-1) in autopsy tissue from age-matched DS and neurotypical (NTD) subjects ranging from 28-weeks (wk)-gestation to 3 years of age. Thionin, which stains Nissl substance, revealed disorganized cortical cellular lamination including a delayed appearance of pyramidal cells until 44 wk of age in DS compared to 28 wk in NTD. SG and IG DCX-immunoreactive (-ir) cells were only visualized in the youngest cases until 83 wk in NTD and 57 wk DS. Strong SMI-32 immunoreactivity was observed in layers III and V pyramidal cells in the oldest NTD and DS cases with few appearing as early as 28 wk of age in layer V in NTD. Small Calb-ir interneurons were seen in younger NTD and DS cases compared to Calb-ir pyramidal cells in older subjects. Overall, a greater number of Calb-ir cells were detected in NTD, however, the number of Calr-ir cells were comparable between groups. Diffuse APP/Aβ immunoreactivity was found at all ages in both groups. Few young cases from both groups presented non-neuronal granular CP13 immunoreactivity in layer I. Stronger correlations between brain weight, age, thionin, DCX, and SMI-32 counts were found in NTD. These findings suggest that trisomy 21 affects postnatal FC lamination, neuronal migration/neurogenesis and differentiation of projection neurons and interneurons that likely contribute to cognitive impairment in DS.

Mounting evidence suggests the brain’s own immune cells, microglia, and perivascular macrophages play important roles in immune surveillance/inflammation, but also support proliferation/growth of gliomas, possibly through interactions with tumor stem cells. Immune cells are increasingly regarded as potential targets for cancer therapy, however, the immune microenvironment has not been well-studied in pediatric embryonal CNS tumors. In a series of 13 medulloblastomas, 7 PNETs and 6 ATRTs, we quantified the number of cells immunopositive for macrophage/microglia Ionizing calcium binding adaptor molecule 1 [Iba1], CD68 (expressed on both “M1” antitumor/proinflammatory macrophages and “M2” protumor/anti-inflammatory macrophages), CD163 (“M2” macrophages), CD3 (T-cell marker), CD4 (helper T cell), CD8 (cytotoxic T cell) and CD20 (B-cell marker) and correlated these findings with the stem-cell marker nestin and the Ki67/MIB1 proliferation index. Significant results were as follows: Across groups, CD68 showed a strong Spearman rank-order correlation with both CD163 (r=0.610, p=0.0007) and nestin (r=0.577, p=0.0017), while CD3 correlated strongly with CD8 (r=0.601, p=0.004), CD4 (r=0.526, p=0.005) and CD20 (r=0.499, p=0.008). Between groups, the number of CD163-positive cells was significantly higher in PNETs compared to medulloblastomas (p=0.035). Within the medulloblastoma group, CD68 showed a significant relationship with nestin (r=0.595, p=0.0387) and Ki67 (r=0.606, p=0.0333), and there was a robust correlation between CD4 and CD8 values (r=0.846, p=0.0000002). Within PNETs, there was a strong negative correlation between CD163 and nestin (r=-0.883, p=0.0000002), while CD68 correlated with CD4 (r=0.803, p=0.006). Within ATRTs, nestin and Ki67 were strongly correlated (r=1.000, p=0.0028) as were CD68 and CD163 (r=0.943, p=0.017). These data suggest a role for blocking embryonal CNS tumor-associated macrophages by inhibiting the CSF1R pathway (perhaps in conjunction with CD8-positive T cells which could antagonize the immunosuppressive function of tumor-associated macrophages) or reprogramming them from an M2 to an M1 phenotype (particularly in PNETs, which had the highest number of CD163-positive cells).