The effect of season on inflammatory response in captive baboons

Abstract

Introduction: Highly seasonal animals demonstrate predictable changes in immune function that coincide with changes in photoperiod. Little is known about the effect of season on immune response in baboons. The objective of this study was to determine the effect of season on inflammatory response in baboons.

Materials and methods: Peripheral blood mononuclear cell cytokine response following immune stimulation and serum markers of inflammation were assessed during each season in two groups of young male baboons: one housed under natural light and one in a controlled environment of 12 hours light:12 hours dark.

Results: A seasonal immune rhythm was evident in both groups, with a greater TNF-α and IL-6 response to stimulation and serum CRP concentration in June and September compared with December.

Conclusions: Season is an important experimental confounder, and therefore, time of year should be controlled when designing studies and analyzing data from immune studies in baboons.

Figure 1

Serum concentration of c-reactive protein (Panel A) and interleukin-6 (Panel B) in young male baboons housed outdoors in natural light (gray bars) or indoors in a controlled environment of 12 hours light: 12 hours of dark (black bars). There was no effect of housing conditions on CRP concentration so the groups were combined to compare seasonal effects. Serum CRP was decreased in December as compared to in June (P<0.05, Figure 1). Serum IL-6 concentration varied by season (P<0.001) as well as by housing (P<0.001), with no interaction between the two factors. In the outdoor baboons IL-6 concentration was significantly greater than that of the animals housed indoors at all timepoints (P<0.001). In addition, IL-6 concentration in the outdoor animals was highest during the coldest time of year (December) and lowest in June, the warmest time collected. In the outdoor animals the IL-6 concentration was greatest in December while in the indoor animals IL-6 concentration was greatest in September (Figure 1).

Figure 2

Serum cortisol, plasma ACTH and plasma alpha-melanocyte stimulating hormone (α-MSH) concentration. There was an interaction between the effect of housing and season on serum cortisol, therefore each housing group was analyzed separately. There was no effect of season on serum cortisol concentration in animals housed either in natural or controlled environment. Plasma ACTH (all months) and serum cortisol concentration (June, September, December) were greater in outdoor animals. There was no effect of housing or season on plasma α-MSH concentration.

Figure 3

Increase in TNF-α (Panels A and C) or IL-6 (Panels B and C) release from peripheral blood mononuclear cell (PBMC) after stimulation with lipopolysaccharide (LPS, Panels A and B) or unmethylated CpG oligodeoxynucleotide (Panels C and D). A robust seasonal rhythm was present in baboons housed outside and those under a constant 12 hours light: 12 hours dark. LPS stimulation caused the greatest increase in cytokine release in June and September, and least response in December. In contrast, CpG had the greatest effect in June, with December showing the least response. Baboons housed in the controlled environment had a greater TNF- α release secondary to LPS treatment in March and a greater IL-6 release in September in response to CpG compared to than those housed in a natural environment.