Effects of interferon-alpha on rhesus monkeys: a nonhuman primate model of cytokine-induced depression

Abstract

Background: Interferon (IFN)-alpha is an innate immune cytokine that causes high rates of depression in humans and therefore has been used to study the impact of cytokines on the brain and behavior. To establish a nonhuman primate model of cytokine-induced depression, we examined the effects of IFN-alpha on rhesus monkeys.

Methods: Eight rhesus monkeys were administered recombinant human (rHu)-IFN-alpha (20 MIU/m(2)) or saline for 4 weeks in counterbalanced fashion, and videotaped behavior, as well as plasma and cerebrospinal fluid (CSF), were obtained at regular intervals to assess behavioral, neuroendocrine, immune, and neurotransmitter parameters. Additionally, expression and activity of IFN-alpha/beta receptors in monkey peripheral blood mononuclear cells (PBMCs) were assessed.

Results: Compared with saline treatment, IFN-alpha administration was associated with persistent increases in anxiety-like behaviors and decreases in environmental exploration. In addition, IFN-alpha induced significant increases in plasma concentrations of corticotropin (ACTH), cortisol, and interleukin-6 that tended to diminish after chronic administration, especially in dominant animals. Interestingly, in three animals, depressive-like, huddling behavior was observed. Monkeys that displayed huddling behavior exhibited significantly higher plasma concentrations of ACTH and lower CSF concentrations of the dopamine metabolite homovanillic acid. Rhesus monkey PBMCs were found to express mRNA and protein for the IFN-alpha/beta receptor. Moreover, treatment of PBMCs with rHu-IFN-alpha led to induction of STAT1, one of the primary IFN-alpha-induced signaling molecules.

Conclusions: IFN-alpha evoked behavioral, neuroendocrine, and immune responses in rhesus monkeys that are similar to humans. Moreover, alterations in hypothalamic-pituitary-adrenal axis responses and dopamine metabolism may contribute to IFN-alpha-induced depressive-like huddling behavior.

Figure 1

Anxiety-like behavior, exploratory behavior, and locomotor activity during IFN-alpha or saline administration. IFN-alpha (20 MIU/m²) or saline was administered s.c. to 8 rhesus monkeys for 4 weeks in counterbalanced fashion, and behavior was assessed weekly by videotape at 2 hours post-injection. Compared to saline, IFN-alpha increased anxiety-like behaviors (A), decreased exploratory behaviors (B), and had no significant effect on locomotor activity (C). Data are presented as the mean (+/- SEM) at each time point. * - indicates a significant difference between treatment groups at indicated time points as revealed by post-hoc analyses (p<0.05). † - indicates the initiation of saline or IFN-alpha administration (day 1).

Figure 2

Depressive-like huddling behavior during IFN-alpha or saline administration. IFN-alpha (20 MIU/m²) or saline was administered s.c. for 4 weeks to 8 rhesus monkeys in counterbalanced fashion, and behavior was assessed weekly by videotape at 2 hours post-injection. Administration of IFN-alpha was associated with persistent huddling behavior in 3 animals (A). A photograph of a monkey huddling during IFN-alpha administration appears in B. Data are presented as the mean (+/- SEM) for these 3 animals at each time point during either saline or IFN-alpha treatment. * - indicates a significant difference between treatment groups at indicated time points as revealed by post-hoc analyses (p<0.05). † - indicates the initiation of saline or IFN-alpha administration (day 1).

Figure 3

Plasma ACTH, cortisol and IL-6 concentrations during IFN-alpha or saline administration. IFN-alpha (20 MIU/m²) or saline was administered s.c. to 8 rhesus monkeys for 4 weeks in counterbalanced fashion, and plasma was collected in weeks 1,2 and 4, 3 hours post-injection, for the analyses of plasma hormones and cytokines. Compared to saline, IFN-alpha induced significant increases in plasma ACTH (A), cortisol (B), and IL-6 (C) concentrations, which were attenuated over time. Data are presented as the mean (+/- SEM) at each time point. * - indicates a significant difference between treatment groups at indicated time points as revealed by post-hoc analyses (p<0.05). † - indicates the initiation of saline or IFN-alpha administration (day 1).

Figure 4

Plasma ACTH, cortisol and IL-6 concentrations during IFN-alpha or saline administration in dominant and subordinate monkeys. IFN-alpha (20 MIU/m²) or saline was administered s.c. for 4 weeks to 8 rhesus monkeys in counterbalanced fashion, and plasma was collected in weeks 1,2 and 4, 3 hours post-injection, for the analyses of hormones and cytokines. Compared to dominant monkeys (n=4) who had significant elevations in ACTH (A) and cortisol (B) at week 1 only, subordinate animals (n=4) had more persistent elevations in plasma ACTH (A), cortisol (B), and IL-6 (C) with increases in ACTH and cortisol at weeks 1 and 2, and IL-6 at weeks 1-3. Data are presented as the mean (+/- SEM) at each time point. * - indicates a significant difference between treatment groups at indicated time points as revealed by post-hoc analyses (p<0.05). † - indicates the initiation of saline or IFN-alpha administration (day 1).

Figure 5

Plasma ACTH, cortisol and IL-6 responses to IFN-alpha in monkeys that displayed depressive-like huddling behavior. IFN-alpha (20 MIU/m²) or saline was administered s.c. for 4 weeks to 8 rhesus monkeys in counterbalanced fashion, and plasma was collected in weeks 1,2 and 4, 3 hours post-injection, for the analyses of hormones and cytokines. Behavior was assessed weekly by videotape 2 hours post-injection. Monkeys that huddled (“Huddlers”, n=3) demonstrated significantly higher average plasma ACTH concentrations during the study than those that did not (“Non-Huddlers”, n=5). Data are represented as mean (+/- SEM). * - indicates a significant difference between groups p<0.05.

Figure 6

Depressive-like huddling behavior was associated with CSF HVA concentrations. IFN-alpha (20 MIU/m²) or saline was administered s.c. for 4 weeks to 8 rhesus monkeys in counterbalanced fashion, and CSF was collected from the cisterna magna at weeks 1, 2 and 4, 3 hours post-injection, for the analyses of the monoamine metabolites HVA, DOPAC, and 5-HIAA. Behavior was assessed weekly by videotape 2 hours post-injection. IFN-alpha significantly reduced average CSF HVA concentrations during the study compared to saline values in monkeys who displayed huddling behavior (“Huddlers”, n=3) in contrast to those that did not (“Non-Huddlers”, n=5). Delta (IFN-alpha minus saline) values are presented as mean (+/- SEM) (A). The average amount of time spent huddling during the study correlated with average CSF HVA concentrations during IFN-alpha administration (B). * - indicates a significant difference between groups (p<0.05).

Figure 7

Expression and activation of IFNAR1 in rhesus monkey PBMCs. IFNAR1 was expressed comparably in IFN-alpha naïve rhesus monkey and human PBMCs as detected by RT-PCR and Western blot. Control samples included a mouse cell line (HT22) for Western blot and a no-RT sample for RT-PCR. (A). Incubation of PBMCs from IFN-alpha-naïve rhesus monkeys and humans with rHu-IFN-alpha-A (5000 IU/ml) for 15,30, and 60 minutes activated pSTAT-1 expression to a similar degree as detected by flow cytometry (B) and Western blot (C).