Depression and reduced natural killer cytotoxicity: a longitudinal study of depressed patients and control subjects.

Cross-sectional studies have demonstrated that natural killer (NK) cell activity is reduced in depression. To extend these observations and examine further the association between severity of depressive symptoms and values of NK activity, this study used a longitudinal case-control design and assessed NK cytotoxicity at intake and at follow-up 6 months after discharge from the hospital in depressed patients and control subjects. From acute hospitalization to follow-up, depression scores significantly (P < 0.01) decreased following treatment in the depressed patients but did not change in the control subjects. NK activity significantly (P < 0.05) increased from intake to follow-up in the depressives while lytic activity did not change in the controls. At intake NK activity was significantly (P < 0.01) reduced in the depressed patients as compared to values in the controls, while at follow-up cytotoxicity was similar between the two groups. These longitudinal data suggest that a reduction of NK cytotoxicity is temporally associated with the state of acute depression.