Abstract: Accumulating evidence indicates that immune proteins major histocompatibility complex I (MHCⅠ) molecules are expressed by healthy neurons. Based on the study of related knockout animals, the neuronal MHCⅠ molecules have been proved to play important roles in synaptic formation, remodeling and plasticity, which are significantly different from their well-acknowledged immune functions. Using cultured hippocampal neurons, we show MHCⅠ proteins are expressed before neurite outgrowth. Knockout of MHC class I in cultured neurons or blocking of MHCⅠmolecules by MHCⅠ antibody increases the branch number of neurons. Blocking of pirB, which works with MHCⅠin immune system, mimics the phenotype of increased dendritic branching. Our results imply that endogenous MHCⅠplays a role in limiting the neurite outgrowth during the development of hippocampal neurons.

Key words: MHCⅠ, hippocampal neurons, neurite outgrowth