Age at onset of major depressive disorder predicts reductions in NK cell number and activity

BACKGROUND: Major depressive disorder (MDD) has been associated with altered immunologic parameters including reductions in natural killer cell activity (NKCA). It remains largely unknown, however, whether alterations in immune function characterize homogeneous sub-groups of MDD. The present study addressed the question of whether age at onset of index episode and/or duration of the present episode of MDD predicted alterations in NKCA and NK cell number. METHODS: Participants met DSM-IV criteria for MDD. Age at onset of MDD, duration of the present episode, demographics, and comorbidity were obtained by SCID for all subjects (n = 36). Severity and symptom pattern of MDD was assessed by the Hamilton Depression Rating Scale. NKCA was measured using a standard chromium-release cytotoxicity assay and NK number assessed by flow cytometry. RESULTS: Age at onset of MDD significantly predicted variance in NK cell number and NKCA. Consistent with previous studies, sleep disturbance and psychomotor retardation possessed significant explanatory power for variance in NK cell number and NKCA, respectively. LIMITATIONS: Measures of age at onset of MDD and duration of the present episode were obtained by self-report and thus recall bias may attenuate the reliability of the present findings. The present study design also precludes conclusions regarding the temporal association between alterations in NK cells and MDD. CONCLUSIONS: We propose that immunologic alterations, characterized by a suppression of NKCA and NK cell number concomitant with proinflammatory processes, may constitute an immunologic phenotype unique to early-age-onset depression and may be salient factors in the pathogenesis of depression.