Experimental (high-dose) group: high-dose NK cells (>5×109) and MSCs (>5×109), i.v. infusion once every 2 days for a total of 5 timesConventional dose group: conventional dose NK cells (>3×109) and MSCs (>3×109), i.v. infusion once every 2 days for a total of 3 timesPreventive dose group: preventive dose NK cells (>3×109) and MSCs (>3×109), i.v. infusion once every week for a total of 1 time