Expression of nonclassical MHC class I (RT1-U) in certain neuronal populations of the central nervous system

Major histocompatibility complex (MHC) class I genes consist of classical ( Ia) and nonclassical (Ib) types. Recently, a set of structurally similar MH C class Ib genes in the rat, denoted RT1-U, was described. We here demonstr ate expression of RT1-U mRNA using highly stringent oligonucleotide in situ hybridization in several different neuronal populations, including differe nt motor nuclei and the substantia nigra in the rat MHC (c) and (n) haploty pes under normal conditions. The expression pattern for beta 2-microglobuli n mRNA was almost identical. In contrast, neuronal expression of classical MHC class I (RT1-A) was low. Interestingly, after mechanical nerve injury, glial cells predominantely upregulated expression of RT1-A, whereas neurona l expression of RT1-U remained unchanged. Neuronal expression of nonclassic al MHC class I may thus be important for immune surveillance in the nervous system.