Major histocompatibility complex class I (MHCI) genes were discovered unexpectedly in healthy CNS neurons in a screen for genes regulated by neural activity. In mice lacking just 2 of the 50+ MHCI genes H2-Kb and H2-Db, ocular dominance (OD) plasticity is enhanced. Mice lacking PirB, an MHCI receptor, have a similar phenotype. H2-Kb and H2-Db are expressed not only in visual cortex, but also in lateral geniculate nucleus (LGN), where protein localization correlates strongly with synaptic markers and complement protein C1q. In KbDb-/- mice, developmental refinement of retinogeniculate projections is impaired, similar to C1q-/- mice. These phenotypes in KbDb-/- mice are strikingly similar to those in β2 m-/-TAP1-/- mice, which lack cell surface expression of all MHCIs, implying that H2-Kb and H2-Db can account for observed changes in synapse plasticity. H2-Kb and H2-Db ligands, signaling via neuronal MHCI receptors, may enable activity-dependent remodeling of brain circuits during developmental critical periods.
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