Inflammation is a significant component of Alzheimer's disease pathology. While neuroprotective microglia are important for containment/clearance of Amyloid plaques and maintaining neuronal survival, Alzheimer inflammatory microglia may play a detrimental role by eliciting tau pathogenesis and accelerating neurotoxicity. Regulatory T cells have been shown to suppress microglia-mediated inflammation. However, the role of regulatory T cells in ameliorating the proinflammatory immune response in Alzheimer's disease requires further investigation. Forty-six patients with Alzheimer disease, 42 with mild cognitive impairment and 41 healthy controls were studied. The phenotypes of peripheral regulatory T cells were assessed with multicolour flow cytometry. Regulatory T cells were co-cultured with responder T cells and proliferation was determined by 3H-thymidine incorporation. In separate experiments, regulatory T cells were added to induced pluripotent stem cell-derived pro-inflammatory macrophages and changes in interleukin-6/tumour necrosis-alpha transcripts and protein levels were measured. Freshly isolated regulatory T cells were expanded ex vivo in the presence of CD3/CD28 expander beads, interleukin-2 and rapamycin to promote their suppressive function. We found that the suppressive function of regulatory T cells on responder T-cell proliferation was compromised at the Alzheimer disease stage, compared with mild cognitive impairment and healthy controls. CD25 mean fluorescence intensity in regulatory T-cell population was also reduced in Alzheimer dementia patients. Regulatory T cells did not suppress pro-inflammatory macrophages at baseline. Following ex vivo expansion, regulatory T-cell suppression of responder T-cell proliferation and pro-inflammatory macrophage activation increased in both patients and controls. Expanded regulatory T cells exerted their immunoregulatory function on pro-inflammatory macrophages through a contact-mediated mechanism. In conclusion, regulatory T-cell immunophenotype and function are compromised in Alzheimer's disease. Following ex vivo expansion, the immunomodulatory function of regulatory T cells is enhanced even at advanced stages of Alzheimer's disease. Restoration of regulatory T-cell function could be explored as a means to modulate the inflammatory status of Alzheimer's disease.