Shared Genomic Architecture Between Schizophrenia and Multiple Sclerosis Identifies an Un-Drugged HCAR1 Neuroimmune Checkpoint

Multiple Sclerosis (MS) pathogenesis is contingent upon the hyper-proliferative infiltration of peripheral macrophages across the blood-brain barrier. While front-line therapeutics, such as Dimethyl Fumarate, achieve clinical efficacy by agonizing the HCAR2 immune cooling switch, the tandemly duplicated HCAR1 lactate sensor has remained entirely unexplored. Here, by cross-referencing MS and Schizophrenia (SCZ) genomic architectures, we identify a massive shared structural fracture strictly localized to the HCAR tandem regulatory domain. We demonstrate that this locus acts as a highly specific neuroimmune ignition switch: it drives disease susceptibility but is unequivocally unassociated with MS severity or classical systemic autoimmune phenotypes (Crohn's Disease, Lupus, Rheumatoid Arthritis, and Psoriasis). Crucially, utilizing high-resolution eQTL mapping in purified human immune lineages, we reveal that the shared MS/SCZ risk allele drives a profound, state-independent transcriptomic collapse of HCAR1 exclusively in peripheral macrophages. This enhancer failure renders activated macrophages physically "lactate blind"--unable to sense their own glycolytic exhaust to engage the cAMP-suppressing negative feedback loop required to halt immune proliferation. By bridging psychiatric genetics and neuroimmunology, this study reframes the HCAR tandem array as a master neuroimmune bifurcation point and introduces the un-drugged HCAR1 lactate brake as a critical therapeutic checkpoint for arresting demyelinating disease.