Developmental expression and localization of MHC class I molecules in the human central nervous system

Recent animal studies have found neuronal expression of major histocompatibility complex (MHC) class I in the central nervous system (CNS). However, the developmental expression profiles of MHC class I in human CNS remain unclear. Here, we systemically evaluate the expression and subcellular localization of MHC class I molecules during human CNS development using immunohistochemistry and immunofluorescence. Between the age of 20-33 gestational weeks (GW), MHC class I expression was relatively absent in the cerebral cortex with the exception of a few neurons; however, expression increased rapidly in the cochlear nuclei and in the cerebellar cortical Purkinje cells while increasing slowly in the substantia nigra. Expression was also detected in some nuclei and nerve fibers of the brain stem including the ambiguus nucleus, the locus coeruleus and the solitary tract as early as 20 GW and persisted through 33 GW. These early-stage neural cells with MHC class I protein expression later developed neuronal morphology. 30-33 GW is an important period of MHC class I expression in neurons, and during this period, MHC class I molecules were found to be enriched not only in neuronal cell bodies and neurites but also in nerve fibers and in the surrounding stroma. No expression was detected in the adult brain with exception of the cerebrovascular endothelium. MHC class I molecules displayed greater postsynaptic colocalization in cerebellar Purkinje cells, in the lateral geniculate nucleus and in the cochlear nuclei. These results demonstrate diverse spatiotemporal expression patterns for MHC class I molecules in the prenatal human CNS and strongly support the notion that MHC class I molecules play important roles in both CNS development and plasticity.