Sickness behaviors due to inflammation, such as social withdrawal and disinterest in food, overlap greatly with depression behaviors but are attenuated when infection is cleared (Dantzer et al., 2008). Altered regulation of this adaptive behavioral response to immune challenge by chronic illness or psychosocial stress contributes to depression (Maes et al., 2009 and Dantzer et al., 2008). For example, patients with chronic inflammatory diseases such as multiple sclerosis,

rheumatoid arthritis and asthma can be up to 6 times more likely to develop depression than healthy individuals (Moussavi et al., 2007). Depressed patients also show markers of inflammation, including elevated levels of cytokines and their soluble receptors in serum and cerebrospinal fluid, the most consistently elevated being IL-6 (Maes KU57788 et al., 1997 and Dowlati et al., 2010). Inflammatory markers are also elevated in rodent stress models—chronic stress causes an elevation in serum and brain cytokines including IL-6 and Interleukin-1β (IL-1β) (Sukoff

Rizzo et al., 2012, Voorhees et al., 2013 and Koo and Duman, 2008). In both humans receiving immunotherapy and animal models of inflammation, administration of pro-inflammatory cytokines produces depression and anxiety-like behaviors (Bonaccorso et al., 2001, Bonaccorso et al., 2002, Anisman et al., 2002 and Sakic et al., 2001). While some studies have CDK assay shown that antidepressant medications reduce peripheral inflammation (Kubera et al., 2001a and Kubera et al., 2001b), others suggest the opposite (Hannestad et al., 2011 and Maes et al., 2012), resulting in a shift in drug development efforts that focus on the use of more direct anti-inflammatory agents to promote resilience. Recent studies form a growing

body of evidence that supports the existence of individual differences in inflammatory response to stress and subsequent physiological and behavioral vulnerability. Here, others we will discuss peripheral markers characteristic of vulnerability and resilience to stress as well as central mechanisms that contribute to inflammation-mediated behavioral outcomes. Several reports examine changes in immune cell localization and reactivity driven by stress exposure in rodents. Many of these studies utilize a social stress model similar to CSDS called social disruption stress (SDR). SDR involves chronic disruption of established social hierarchies in cages of male mice. Male cagemates establish a social hierarchy such that one mouse is the dominant, alpha male and the remaining males are codominant or subordinate (Avitsur et al., 2009). Once a day for a total of six days, a novel, dominant intruder mouse previously screened for aggressive behavior is placed into the housing cage for a period ranging from hours to overnight (Avitsur et al., 2001). The dominant intruder repeatedly attacks and defeats the resident mice, eliciting submissive behaviors.