LAUSR

Alterations in reward processing are a central feature of depression and may be influenced by inflammation. Indeed, elevated inflammation is associated with dysregulation in reward-related neural circuitry. However, the downstream behavioral manifestations of such impairments are rarely examined in humans. In this study, 42 healthy young adults (age 18–22, 31 female), provided blood samples and completed behavioral measures of two key aspects of reward – reward-related learning (Probabilistic Reward Task) and reward motivation (Effort Expenditure for Rewards Task) - before and one day after receiving an influenza vaccine. The influenza vaccine led to mild but significant increases in circulating IL-6 ( Mpre  = 1.16, SD = .94; Mpost  = 1.46, SD = 1.21; d  = 0.39). Contrary to hypotheses, increases in IL-6 predicted increases in reward learning, b  = 0.167, p  = 0.005. While change in IL-6 did not predict reward motivation, higher baseline levels of IL-6 predicted increased reward motivation following the vaccine for reward of moderate (versus low and high) magnitude, b  = 0.034, p  = 0.034. Results were not substantively altered when controlling for current depressive symptoms and sex. These findings suggest that reward-related learning is facilitated by modest increases in circulating inflammation, and that higher levels of IL-6 are also positively associated with reward motivation. While this is contrary to prior findings that inflammation dampens reward reactivity, it is possible that the relatively small increases in inflammation seen here may drive engagement with the environment.