Interleukin-6 and interleukin-10 are expressed in organs of normal young and old mice

Interleukin-6 (IL-6) is a pleiotropic inflammatory cytokine, also endowed with inflammation-inhibiting properties. The status of interleukin-10 (IL-10) as an anti-inflammatory cytokine is more solidly established. The roles of IL-6 and IL-10 in the context of organ physiology, and their possible modulation by the aging process, are not satisfactorily understood. The purpose of this work was to characterize organ IL-6 and IL-10 expression in different cellular compartments in mice, under steady-state and stress conditions. The former was evaluated by immunohistochemistry (IHC) and enzyme-linked immunosorbent assay (ELISA) analyses of organ lysates (LYS) (addressing the intracellular compartment), while the latter was assessed by ELISA analyses of organ-conditioned media (CM), obtained after 48 hrs of organ culturing (addressing the potential of cytokine secretion/diffusion). Under steady-state conditions, the overall level of IL-6 and IL-10 expression was relatively low in both age groups (exceptionally, IHC staining demonstrated an enhanced expression of these cytokines in the heart, skeletal muscle and brain of young mice). Much more elevated levels of IL-6 and IL-10 expression were demonstrated in organ CM, possibly emphasizing the role of these cytokines in the context of organ stress. This was most characteristically shown in the highly specialized organs (heart, skeletal muscle and kidney) and liver of old mice, as compared with the other lymphoreticular organs (lungs, spleen, small intestine) tested. Thus, IL-10 was markedly upregulated in the highly specialized organs, while IL-6 was considerably reduced in the lymphoreticular organs. In addition, aging appears to be associated with altered patterns of intracellular expression and secretion/diffusion potentials of IL-6 and IL-10 in the heart and skeletal muscle, as demonstrated by reduced IHC staining on one hand, and an increased detection in organ CM, on the other. These findings may contribute to a better understanding of the unique functions of organ IL-6 and IL-10 in various age groups, and suggest an important role in organ response to stress in old age.