AbstractThe outcome of T cell activation is determined by mechanisms that balance Ca²⁺ influx and clearance. Here we report that murine CD4 T cells lacking Neuroplastin (Nptn^−/−), an immunoglobulin superfamily protein, display elevated cytosolic Ca²⁺ and impaired post-stimulation Ca²⁺ clearance, along with increased nuclear levels of NFAT transcription factor and enhanced T cell receptor-induced cytokine production. On the molecular level, we identified plasma membrane Ca²⁺ ATPases (PMCAs) as the main interaction partners of Neuroplastin. PMCA levels were reduced by over 70% in Nptn^−/− T cells, suggesting an explanation for altered Ca²⁺ handling. Supporting this, Ca²⁺ extrusion was impaired while Ca²⁺ levels in internal stores were increased. T cells heterozygous for PMCA1 mimicked the phenotype of Nptn^−/− T cells. Consistent with sustained Ca²⁺ levels, differentiation of Nptn^−/− T helper cells was biased towards the Th1 versus Th2 subset. Our study thus establishes Neuroplastin-PMCA modules as important regulators of T cell activation.