phytohemagglutinin (PHA) (Sigma-Aldrich Japan, Japan). After
5–7 days incubation at 37 1C in a humidified atmosphere with 5%
CO2, the cultured cells were centrifuged and the supernatant was
passed through 0.22-mm filter and 3 ml aliquots of the supernatant
were stored at 20 1C until use.