Cytokine Biomarkers for Major Depressive Disorder are Gender-Specific: Implications for Biology and Therapeutics. [unreadable] [unreadable] Major Depressive Disorder (MDD) affects a significant fraction of the U.S. population, and is currently diagnosed solely on the basis of clinical presentation. It would therefore be useful to have objective MDD-specific biomarkers in clinically accessible biological fluids that could provide guidance not only for objective diagnosis but also for evidence of therapeutic efficacy. There is evidence that some pro-inflammatory cytokines can be found elevated in either cerobrospinal fluid (CSF) or plasma of MDD patients. Based on these observations, it has been suggested that some symptoms of MDD might be a consequence of "cytokine sickness". However, the association of cytokines with MDD has yet to be broadly tested.[unreadable] Hypothesis: Clinically, MDD can be distinguished in male and female patients. We have therefore hypothesized that while certain cytokines might serve as specific biomarkers for MDD, their distribution might be distinguished on the basis of gender.[unreadable] Methods:[unreadable] To test this hypothesis we have compared individual samples of CSF from a set of 20 well-annotated MDD with 19 age and sex-matched normal control patients. The samples were analyzed quantitatively for expression of 20 different cytokines and chemokines by both Searchlight ,Pierce; and ELISA assays, R&D Systems.[unreadable] Results:[unreadable] Of the 20 proinflammatory analytes tested on CSF, 18 analytes gave detectable signals. Of these, only two analytes were significantly different when comparing control males and females. These control, gender-independent analytes were TGFbeta1 :Cmale/Cfemale = 0.56; P=0.009; and IL-6 :Cmale/Cfemale = 0.48; P=0.025. Thus the average levels of these two analytes were 1.8 and 2.1-fold higher, respectively, in control females compared to control males. However, we find that to discriminate between MDD patients and healthy controls, it is necessary to be gender-specific. Thus, the analytes that are important for male MDD are less important for female MDD, and vice versa. For example, of the analytes tested, only three are useful in distinguishing female MDD from female Controls. These are TGFbeta1, GROalpha, and ICAM-1. By contrast, there are six analytes that are useful for distinguishing male MDD from male Control patients. These are TNFalpha, IL-6., IL-6R, MCP1, IP10 and L-selectin. In both male and female gender-specific cohorts, we find that there are some patients that do not fit into the pattern. This overlap has led to the need for classical cut points, and thus instances of candidate False Positive and False Negative patients. Our solution has been to devise a classical ROC curve based on novel algorithms which use data from multiple analytes to create a highly specific and highly sensitive candidate metric for gender-specific MDD.[unreadable] Discussion:[unreadable] These data clearly support the hypothesis that gender-specific cytokine biomarkers exist for Major Depressive Disorder. However, since only 40 patients were used for this study, it is evident that much additional discovery work needs to be done. With respect to the suggestion that depressive symptoms may be due in part to a kind of cytokine sickness, the data described here do not exclude this possibility. However, based on the gender differences in disease-specific cytokine expression, the possibility must be considered that MDD may not be the same disease in male and female patients. Finally, we suggest that mechanistic studies on cellular origins of these biomarkers may not only lead to important insights regarding the biological basis for MDD, but may also direct attention to new approaches to mechanism-based therapeutics. [unreadable] :