The social environment and IL-6 in rats and humans

Inflammatory cytokine levels predict a wide range of human diseases including depression, cardiovascular disease, type 2 diabetes, autoimmune disease, general morbidity, and mortality. Stress and social experiences throughout the lifecourse have been associated with inflammatory processes. We conducted studies in humans and laboratory rats to examine the effect of early life experience and adult social position in predicting IL-6 levels. Human participants reported family homeownership during their childhood and current subjective social status. Interleukin-6 (IL-6) was measured from oral mucosal transudate. Rats were housed in groups of three, matched for quality of maternal care received. Social status was assessed via competition for resources, and plasma IL-6 was assessed in adulthood. In both humans and rats, we identified an interaction effect; early social experience moderated the effect of adult social status on IL-6 levels. Rats that experienced low levels of maternal care and people with low childhood socioeconomic status represented both the highest and lowest levels of IL-6 in adulthood, depending on their social status as young adults. The predicted interaction held for non-Hispanic people, but did not occur among Hispanic individuals. Adversity early in life may not have a monotonically negative effect on adult health, but may alter biological sensitivity to later social experiences.