VP-1 stimulated perforin-, granzyme-, or CD107a-expressing- CD8+ T lymphocytes in Multiple Sclerosis patients undergoing natalizumab. Left Column: Results obtained during a 24 months period; grey dots indicate subject in whom virus activation in blood was not detected (JCV NEG); black dots indicate individuals in whom JCV DNA was detected in blood (JCV POS). Error bars depict first and third quartiles. Statistical significance is shown. Right Column: Representative results obtained in JCV NEG (A, C, E) or in JCV POS (B, D, F) patients. In the upper right corner the percentage of positive cells is indicated.