Immune activation and cognition: Cognitive changes associated with il-6 in patients with schizophrenia, versus other psychiatric and medical disorders

A wealth of data exists, implicating an altered immune response in the aetiology of Schizophrenia. In this chapter, we review evidence that immune activation, measured in terms of cytokine response, is associated with variation in cognitive performance in schizophrenia, other psychiatric and non-psychiatric disorders, and healthy participants, including older ealthy participants. Our purpose in carrying out this review was: (1) to examine whether altered cytokine response resulted in functionally relevant changes in schizophrenia, and (2) to examine whether any association observed was different in patients versus other groups. Because of the volume of research published in this area we focused our study on one inflammatory marker – interleukin 6, the cytokine most reliably associated with schizophrenia. To achieve this we conducted a search on Pubmed to June 2017. Studies which measured serum or plasma levels of IL-6 and which examined their relationship to cognitive test scores were included. In total 88 studies were identified as meeting our criteria for inclusion. The picture which emerges from this review is that, in general, there is consistent evidence for an association between IL-6 and cognitive function, depending on the participant group. Most studies controlled for a wide range of confounders in their analysis, and multiple cognitive domains were implicated, including executive function, processing speed and verbal and visual memory. Where significant associations were observed, the vast majority of studies found higher IL-6 levels to be associated with cognitive impairment.