Journal of Comparative Physiology B https://doi.org/10.1007/s00360-018-1195-9 ORIGINAL PAPER Ecoimmunology in degus: interplay among diet, immune response, and oxidative stress Natalia Ramirez‑Otarola1,2 · Mauricio Sarria2 · Daniela S. Rivera1 · Pablo Sabat1,2 · Francisco Bozinovic1 Received: 24 July 2018 / Revised: 13 November 2018 / Accepted: 22 November 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract The relationships between immunity, oxidative stress, and diet have not often been studied together. Despite this, it has been shown that dietary proteins can have effects on the functioning of the immune system and the oxidative status of animals. Here we evaluated the effects of dietary proteins on the response to an antigen and oxidative status of Octodon degus (Rodentia). We acclimated adult individuals to high-protein and low-protein diets and evaluated several aspects of the acute phase response and variables associated with oxidative status. After the immune challenge, animals acclimated to the highprotein diet had more inflammatory proteins and body mass losses than the group acclimated to a low-protein diet. Overall, the immune challenge increased the production of inflammatory proteins, total antioxidant capacity, lipid peroxidation, and duration of rest periods. In contrast, we did not find an interaction between diet and the challenge with the antigen. Overall, our results do not reveal an enhanced response to an antigen nor effects on the oxidative status of degus individuals subjected to a high-protein diet. Keywords Lipopolysaccharide · Oxidative stress · Protein · Rodent · Ecoimmunology · Diet Introduction What an animal eats defines its biological success (Bozinovic and Martínez del Río 1995). Food habits and diet selection are associated with traits that are affected from molecular to ecological scales. In fact, dietary features are strongly associated with metabolic rates (McNab 1988; Cruz-Neto and Bozinovic 2004; Sabat et al. 2010), digestive efficiency (Martínez del Río and Stevens 1989; Bozinovic Communicated by H. V. Carey. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00360-018-1195-9) contains supplementary material, which is available to authorized users. * Natalia Ramirez-Otarola nat.rotarola@gmail.com 1 Departamento de Ecología, Center of Applied Ecology and Sustainability (CAPES), Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Avenida Libertador Bernardo O’Higgins #340, 6513677 Santiago, Chile 2 Departamento de Ciencias Ecológicas, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile and Muñoz-Pedreros 1995; Ramirez-Otarola et al. 2011), and behavioral traits such as exploration and life history (Barnes et al. 1976; Sih and Christensen 2001). Recent studies have suggested that dietary chemical composition also influences the maintenance and functioning of the immune system of animals (Lee et al. 2006; Klasing 2007; Catalan et al. 2011; Venesky et al. 2012; Martel et al. 2014). Specifically, it has been shown that immune function requires the participation of specific dietary nutrients, in addition to energy, for its activation (Klasing 1998, 2007). In this way, Klasing (1998) has proposed that the interaction between diet and the immune response is mainly modulated by the proportion of specific nutrients present in the diet. In particular, dietary proteins have been shown to strongly influence the maintenance and functioning of normal immunocompetence (Latshaw 1991; Woorward 1998; Li et al. 2007; Schmid-Hempel 2011). Accordingly, several studies conducted using rodent model species (e.g., Mus musculus) have documented that reductions in dietary protein intake are highly associated with attenuated immune responses (Jennings et al. 1992; Venesky et al. 2012; Martel et al. 2014). The generation of an immune response is an expensive process, which not only demands many resources (e.g., proteins) but also involves high energetic costs (Mendes et al. 13 Vol.:(0123456789) Journal of Comparative Physiology B 2006; Costantini and Moller 2009; Hasselquist and Nilsson 2012; Hegeman et al. 2012; Sköld-Chiriac et al. 2015). The activation of immune cells, such as phagocytic cells, result in an increase in levels of reactive oxygen species (ROS) that not only destroy pathogens, but also damage the host tissue (Hampton et al. 1998; Babior 1999; Segal 2005; Sorci and Faivre 2009). Recently, it has been postulated that the damage caused by the presence of ROS represent one of the highest costs to the immune response (Costantini and Møller 2009; Monaghan et al. al. 2009; van de Crommenacker et al. 2010) and could be a key factor mediating the costs and long-term consequences of immunocompetence (Hasselquist and Nilsson 2012). Evaluations of the relationship between levels of pro-oxidants and the immune response, however, have led to contradictory results. For instance, some studies report that immune challenge produces an increase in oxidative species (Bertrand et al. 2006; Costantini and Dell’Omo 2006; Hõrak 2007; Torres and Velando 2007) and a decrease in antioxidant capacity (Bertrand et al. 2006; Costantini and Dell’Omo 2006). In contrast, other studies have failed to demonstrate a significant relationship between oxidative damage and immune function (Alonso-Alvarez et al. 2004; Hõrak et al. 2006; Cohen et al. 2009; Pérez-Rodríguez et al. al. 2008; Costantini and Møller 2009). These discrepancies are likely due to the fact that some studies quantified only the levels of pro-oxidants (Torres and Velando 2007) or of antioxidants (Hõrak et al. 2006; Tummeleht et al. 2006). Furthermore, such differences may be explained by which part of immunity was evaluated (constitutive vs. induced) or by the co-occurrence of constraints that work at the same time as the immune response (Cram et al. 2015; Eikenaar et al. 2018).The evaluation of both oxidant and antioxidant levels is necessary to estimate the association between oxidative stress and the immune response (Costantini and Verhulst 2009). In this study, we evaluated the immune response, oxidant levels, and resistance to oxidative stress in response to dietary protein level, in adult individuals of the social rodent Octodon degus or degu. We hypothesized that since immune defense is the final protection against parasites, adequate provision of dietary macromolecules is necessary to maintain immunocompetence. As such, an increase in the percentage of dietary proteins consumed should increase total antioxidant capacity and oxidative damage should decrease. Also, the presence of proteins in an organism’s diet should enhance responses to specific antigens. As an alternative hypothesis, we proposed that animals can effort a high immune response at constant levels of oxidative stress because of an improved antioxidant capacity. 13 Materials and methods Animal capture and experimental setting The experiments were run on 32 males of the species Octodon degus (Rodentia, Octodontidae) captured between April and May of 2016 in Rinconada de Maipú in central Chile (33°23′S, 70°31′W). In the laboratory, all animals were maintained in individual cages containing a bedding of hardwood chips at an ambient temperature of 22 ± 2 °C and with a photoperiod of 12L: 12D; water and food (commercial rabbit pellets) were provided ad libitum. These acclimation conditions were maintained for a period of 10 days. Once the acclimation period was over, the males were randomly assigned to one of the two experimental diets. The diets (Table 1) consisted of: alfalfa pellet which was the lowprotein diet (LP; n = 16) and rabbit commercial pellet which was the high-protein diet (HP; n = 16). The diet treatments were provided for a period of 15 days (dietary acclimation). It has been reported that these two experimental diets are relevant in terms of energetics, producing differential metabolic responses in degus (Veloso and Bozinovic 2000a, b). After dietary acclimation, each group was split again into experimental and control subgroups. The experimental group was subjected to an injection of lipopolysaccharide (LPS purified from Salmonella enterica, Sigma; 500 µg/kg; Nemzek et al. 2003) while the other subgroup (control) was injected with a saline solution (0.9%). LPS is the immunogenic component of the cell wall of Gram-negative bacteria and induces an inflammatory response in degus (Nemzek et al. 2003, 2008; Ramirez-Otarola et al. 2018). The final volume of inoculations (LPS or saline solution) was 200 µl, and this was administered intraperitoneally between 18:00 and 19:00. Body mass data collection and body temperature Body mass (mb) was measured before (0 h) and 24 h after inoculations. Additionally, body temperature (Tb ± 0.2 °C) was recorded before inoculations (0) and at 12, 15, 19, and 24 h after the immune challenge. Measurements were Table 1 Chemical composition of experimental diets (proximal chemical analysis methods) Nutritional parameter High-protein diet Low-protein diet Energy (KJ/g) Dry matter (%) Proteins (%) Crude fiber (%) Water (%) Carbohydrates (%) 18.4 90.6 20.0 16.5 13.0 40.3 17.5 90.8 12.4 25.0 14.0 39.0 Journal of Comparative Physiology B made on the abdominal area using a non-contact digital laser thermometer (VeraTemp). These data allowed us to estimate the change in Tb following inoculation. Blood sampling For all individuals, blood samples (200 µl) were obtained from the lateral saphenous vein (UBC Animal Care Guidelines, Parasuraman et al. 2010) before the immune challenge and 24 h after inoculations. The total handling time from the initial restraint of an animal to the completion of blood collection did not exceed 2 min. Blood samples were centrifuged at 21,380×g for 6 min at 4 °C, and the plasma was stored at − 80 °C until subsequent assays (RamirezOtarola et al. 2018). Haptoglobin was assessed from blood samples collected at 28 dph, 12 h ± 30 min (between 19:38 and 21:15) after inoculation with antigen or PBS. Blood samples were collected within 10 min of entering the aviary and samples were centrifuged within 30 min of collection, Haptoglobin was assessed from blood samples collected at 28 dph, 12 h ± 30 min (between 19:38 and 21:15) after inoculation with antigen or PBS. Blood samples were collected within 10 min of entering the aviary and samples were centrifuged within 30 min of collection. Haptoglobin assay Haptoglobin is an acute phase protein present in a wide range of taxa (Delers et al. 1988; Matson 2006; Matson et al. 2006), and increased levels indicate an inflammatory response (O’Reilly and Eckersall 2014). Haptoglobin binds to hemoglobin that is released during pathological conditions, inhibiting microbe iron uptake (Jayle et al. 1956; Wicher and Fries 2006). We determined the levels of this protein after inoculation using a commercial assay kit (TP-801, Tridelta Development LTD, Ireland) following the kit instructions. Briefly, 7.5 µl of each sample was transferred to a blank 96-well plate. Then, 100 µl of the hemoglobin stock solution was added to each microwell, and the microplate was agitated gently to mix the plasma and hemoglobin. After that, 140 µl of a stock solution of chromogen was added to each microwell and were incubated for 5 min at room temperature. The absorbance of each well was measured at 630 nm in 96-well plate spectrophotometer (Multiskan GO, Thermo Scientific, USA). The standard curve included haptoglobin concentrations of: 2.5, 1.25, 0.625, 0.313, 0.156, and 0.078 mg/ml. Finally, haptoglobin activity in the samples was calculated based on the standard curve. Plasma lipid peroxidation and total antioxidant capacity Lipid peroxidation in animals causes cellular damage and oxidative stress. Lipid peroxidation is the oxidative degradation of lipids resulting in the generation of Malondialdehyde (MDA) as a secondary compound (Dasgupta and Klein 2016). Plasma lipid peroxidation was estimated by spectrophotometric determination of Thiobarbituric Acid Reactive Substances (TBARS) with a commercial assay kit (OxiSelect STA- 330, Cell BiosLAb, Inc.), before (0 h) and after inoculations (24 h). TBARS allows one to quantify the MDA levels in plasma samples. The thiobarbituric acid reacts with MDA forming a fluorescent compound that can be detected in a spectrophotometer (Dasgupta and Klein 2016). In brief, 100 µl of plasma was transferred, separately, to microcentrifuge tubes. After that, 100 µl of SDS Lysis Solution was added to each microcentrifuge tube, and then they were mixed gently. The tubes were incubated for 5 min at room temperature. Then 250 µl of TBA reagent was added to each tube and mixed thoroughly, and each tube was incubated for 45 min at 95 °C. After the incubation period, each tube was allowed to cool down at room temperature. Finally, 200 µl of each microcentrifuge tube was transferred to a 96-well microplate and the absorbance of each microplate was read at 532 nm. The standard curve contained MDA concentrations ranging from 0 to 125 µM. Total Antioxidant Capacity is a biological defense system against ROS and other reactive species that prevent their formation and neutralize them once formed (Dasgupta and Klein 2016). The plasmatic antioxidant system consists of macromolecules that avoid ROS damage; these macromolecules include superoxide dismutase and endogenous (e.g., glutathione and acid uric) and exogenous antioxidants (like vitamins). Additionally, this system includes a repair system to recover the function of damaged cells (Dasgupta and Klein 2016). An organism’s entire antioxidant system can be evaluated by estimating total antioxidant capacity (TAC). Here, TAC was estimated using a commercial assay kit (Oxiselect STA-360, CellBioslabs, Inc). Briefly, 200 µl of each sample was transferred to a 96-well microplate. Then, 180 µl of reaction buffer was added to each well and mixed thoroughly. The initial absorbance of each sample was read at 490 nm. Next, 50 µl of copper ion reagent was added to initiate the reaction, and then the reactions were incubated for 5 min in an orbital shaker. After the incubation period, we added 50 µl of a stop solution to each well to cease the reaction. Final absorbance was read at 490 nm. Sickness behavior and food intake data collection Sickness behavior is a stereotypical response to inflammation (Hart 1988; Aubert 1999) and includes a reduction in 13 Journal of Comparative Physiology B activity and food intake (Hart 1988; Aubert 1999). The behavior of males was recorded with a video camera (Handycam HDR CX220) mounted on a tripod in front of each cage. The recordings were made 15 h post-immune challenge and lasted 30 min. The following behaviors were recorded: (1) crouching: characterized by a hunched posture with lowered head and hidden feet for a period of 60 s (number of periods in this position), (2) locomotion: movement from one end of the cage to the other (number of movements), and (3) eye closure: eyes closed for a period of 30 s (number of periods). These behaviors have been reported as part of the inflammatory response of degus (Ramirez-Otarola et al. 2018). We chose to evaluate the sickness behavior at 15 h (10:00 a.m.) after the challenge because this frame time is within the main period of activity of this species (Fulk 1976; Yáñez and Jaksic 1978; Iriarte et al. 1989; Lagos et al. 1995; Kenagy et al. 2002). Food intake was measured for a period of 24 h after inoculation. Specifically, a known amount of food was provided to each animal before inoculations. Then, the remaining food and feces were collected. We separated the remaining food from the feces and weighed these separately. The amount of food consumed was calculated from the differences between the initial amount of food and the remaining, after correction for water content. Statistical analysis We first examined whether basal estimates (i.e., prior to the LPS-challenge) of oxidative variables were correlated with each other and with body mass, using Pearson correlations. We tested the effect of diet and LPS-injection using an analysis of covariance using the baseline values (before injection) as a co-variate for each variable (Hegemann et al. 2012). Because these analyses revealed a non-significant effect of the co-variate, this term was dropped from the final models. The effect of diet and LPS-injection on body mass loss (i.e., difference between body mass before and after LPS-injection) was evaluated using a two-way ANOVA. Food intake and levels of haptoglobin were evaluated using a two-way analysis of variances where diet and LPS-injection were the independent variables. The basal levels of total antioxidant capacity (before inoculation) were evaluated using a oneway ANOVA where diet was the independent variable. The effect of diet and LPS-injection on TAC (after inoculation) was evaluated using a two-way ANOVA. Levels of TBARS were evaluated before and after inoculation. Prior to inoculation, the effect of diet on levels of TBARS was evaluated using a one-way analysis of variance. The levels of TBARS after inoculation were evaluated using a two-way ANOVA; diet and LPS-injection were the independent variables. Body temperature at the five-time points was compared using a two-way repeated measures ANOVA with the effect of diet 13 and LPS-injection as the main factors and hour after inoculations as a random factor. When ANOVAs yielded significant results, we used a posteriori Tukey tests to evaluate the specific differences between experimental groups. Sickness behavior was analyzed using parametric and nonparametric analyses. Locomotion was evaluated using a two-way ANOVA with diet and LPS-injection as main factors. The number of times the animals were in the crouching state and had their eyes closed was transformed using aligned rank transformations (ART; Wobbrock et al. 2011). Using this approach, we conducted a factorial analysis of nonparametric data to evaluate the interactions between factors. First, we transformed the aligned data of each effect and then ranked the data. Ranked aligned data were then analyzed using a factorial analysis of variance. To evaluate for specific differences among treatments, a Tukey post hoc test was used. We always included the interaction between main effects in all analysis. The assumptions of normality of the residuals were assessed in the final linear models using R (http://www.R-project.org/), see Zuur et al. (2010). To meet the assumption of normality, body mass, food intake, Hp, TAC, and TBARS were log-transformed. Additionally, Tb data were subjected to an inverse function transformation, and locomotion data were square root transformed (Zuur et al. 2010). All statistical analyses were performed using Statistica for Windows 7 and presented as arithmetic mean ± standard error. Results Analysis of pre‑LPS‑injection oxidative variables and body mass Initial body mass was similar between dietary acclimations (F1,30 = 0.79; p = 0.38; Table 2). Similarly, TAC levels before Table 2 Body mass (mb), total antioxidant capacity (TAC), and thiobarbituric acid reactive substances (TBARS) measured before (0 h) lipopolysaccharide-injection (LPS-injection) in animals acclimated to the experimental diets LP Saline LPS HP Saline LPS Body mass 0 h TAC 0 h TBARS 0 h 246.9 ± 9.45a 242.34 ± 8.89a 6.01 ± 1.69a 12.6 ± 2.31a 0.26 ± 0.07a 0.28 ± 0.07a 246.16 ± 13.35a 256.99 ± 9.95a 6.55 ± 1.15a 6.28 ± 0.79a 0.16 ± 0.07b 0.11 ± 0.04b Values are expressed as mean ± standard error. LP and HP are lowand high-protein diets, respectively. Different letters denote significant differences among treatments (i.e., for each column) at p < 0.05 after a post hoc Tukey test. N = 8 animals per group Journal of Comparative Physiology B inoculations did not differ significantly between diet types (F1,30 = 0.008; p = 0.93; Table 2). The TBARS levels, before inoculations, were significantly lower in animals acclimated to the HP diet compared to animals in the other diet acclimations (F1,25 = 8.15; p = 0.009; Table 2). The correlation analysis showed that total antioxidant capacity and TBARS prior to the LPS-challenge were not significantly correlated (r = 0.25, p = 0.23). The same analysis revealed that TBARS were not correlated with body mass (r = − 0.05, p = 0.8). Finally, total antioxidant capacity and body mass were not significantly correlated (r = − 0.14, p = 0.51). Body mass and body temperature After inoculations, animals acclimated to the HP diet had the greatest body mass losses (F 1,27 = 7.38; p = 0.011; Fig. 1). In the same way, the greatest body mass losses were observed in animals inoculated with LPS (F1,27 = 16.4; p = 0.0004; Fig. 1). However, the interaction between diet and LPSinjection did not influence body mass loss (F1,27 = 0.11; p = 0.73). Body temperature after inoculations was not affected by diet (F 5,110 = 1.0; p = 0.93; Fig. 2), LPS-injection (F5,110 = 0.5; p = 0.75; Fig. 2), or the interaction between variables (F5,110 = 1.9; p = 0.1). Haptoglobin, TBARS and TAC determinations Haptoglobin levels after LPS-injection differed significantly between diet types (F1,21 = 4.73; p = 0.04); animals acclimated to the LP diet had the lowest levels (Fig. 3). LPS-injection significantly affected haptoglobin levels (F1,21 = 31.81; p > 0.001); animals subjected to LPS inoculations had the highest levels (Fig. 3). We observed a borderline statistical effect of the interaction between diet and LPS-injection on haptoglobin levels (F1,21 = 4.1; p = 0.056). After inoculations, diet did not affect TAC levels (F1,27 = 1.39; p = 0.25; Fig. 4); however, LPS-injection significantly affected levels of total antioxidant capacity (F1,27 = 49.87; p > 0.001); immune-challenged animals had the highest TAC levels (Fig. 4). Finally, the interaction between diet and LPS-injection did not have a significant effect on TAC levels (F1,27 = 3.2; p = 0.08). On the other hand, diet did not affect TBARS levels after inoculations, (F1,25 = 0.22; p = 0.64; Fig. 4). However, LPS-injection significantly increased TBARS levels (F1,25 = 5.9; p = 0.023; Fig. 4). The interaction between diet and LPS-injection did not significantly affect TBARS levels (F1,25 = 0.24; p = 0.63). Because TAC and TBARS levels were high in immune-challenged animals, we ran correlations of the two variables for animals in the LPS treatment. The correlation between TAC Fig. 1 Change in body mass and food intake (measured 24 h after inoculation). a Change of body mass in males acclimated to the high-protein (HP) and low-protein (LP) diets and b food intake in animals challenged with lipopolysaccharide (LPS) or saline solution (N = 8 animals per group). Different letters denote significant differences between control and immune-challenged animals for each dietary treatment. The asterisk denotes a significant difference between experimental diets. The level of significance is set at p < 0.05 and TBARS was significantly positive (r = 0.68, p = 0.01; Fig. 4). Sickness behavior and food intake data collection LPS inoculation signif icantly reduced locomotion (F 1,29 = 6.54; p = 0.016; Fig. 5), but neither diet (F 1,29 = 0.1; p = 0.75) nor interaction between diet and LPS-injection (F 1,25 = 1.02; p = 0.32;) affected locomotion. On the other hand, crouching intervals increased significantly in animals inoculated with LPS (F1,29 = 8.75; p = 0.006; Fig. 5). However, crouching intervals were not affected by diet (F1,29 = 1.35; p = 0.25) or by the interaction between both factors (F1,29 = 1.02; p = 0.32). Finally, acclimation to the LP diet resulted in an increase in eye closure 13 Journal of Comparative Physiology B Fig. 2 Body temperature recorded at 0, 15, 17, 19, 20, and 24 h after lipopolysaccharide challenge (LPS) or saline inoculation. Animals (N = 8 per group) were acclimated to one of two different diets: lowprotein diet (LP) and high-protein diet (HP) Fig. 3 Haptoglobin levels in O. degus inoculated with lipopolysaccharide (LPS) or saline solution and acclimated to high-protein (HP) or low-protein (LP) diets (N = 8 animals per group). Different letters denote significant differences between treatments. The level of significance is set at p < 0.05 intervals (F1,29 = 5.85; p = 0.02; Fig. 5). In the same way, animals inoculated with LPS had increased eye closure intervals (F1,29 = 15.95; p > 0.001; Fig. 5). The interaction between diet and LPS-injection did not significantly affect eye closure intervals (F1,29 = 2.46; p = 0.13). Food intake was significantly reduced in immune-challenged animals (F1,30 = 8.68; p = 0.006; Fig. 1). However, neither diet (F 1,30 = 1.81; p = 0.19) nor the interaction between variables (F 1,30 = 1.57; p = 0.22) affected food intake. 13 Fig. 4 Levels of a thiobarbituric acid-reactive substances (TBARS) and b total antioxidant capacity (TAC) in animals inoculated with the antigen (lipopolysaccharide or LPS) or saline solution and acclimated to two contrasting diets (N = 8 animals per group). c Correlation between TAC and TBARS levels. Different letters denote significant differences between treatments after a post hoc Tukey test. The level of significance is set at p < 0.05 Journal of Comparative Physiology B Discussion Fig. 5 Sickness behavior. a Locomotion, b eyes closed, and c crouching in O. degus males inoculated with LPS or saline acclimated to the HP and LP diets (N = 8 animals per group). Different letters denote significant differences between treatments after a post hoc Tukey test. LP and HP are low- and high-protein diets, respectively, between treatments (diet and immune challenge). The level of significance is set at p < 0.05 It has been hypothesized that diet is a powerful selective agent shaping rates of energy expenditure in birds and mammals (Bozinovic and Martínez del Río 1995; CruzNeto and Bozinovic 2004). Here we tested the effect of dietary chemical composition on specific variables of the acute phase response and on general oxidative status. Overall, we found that the response, the immune response, produced an increase in TBARS levels (i.e., lipid peroxidation) and a rise in total antioxidant capacity in adult degu males. Also, the immune response included sickness behavior, increased haptoglobin levels, decreased body mass, and a reduced food intake. Although we did not record food intake before injections, similar food intake between the saline groups feeding either HP or LP diets suggests that food intake among diet groups was similar before injection. Hence the absolute decrease in food intake of immune-challenged animals we measured in HP and LP diet groups is likely not confounded by different food intake among diet groups before injection. Overall, in mammals, LPS inoculation usually induces an inflammatory response. Such responses are characterized by body mass loss, a reduction in general activity, decreased food intake, and increased levels of inflammatory proteins, such as haptoglobin (Sorci and Faivre 2009). Several studies have also demonstrated that dietary protein intake is associated with inflammatory responses (Jennings et al. 1992; Lee et al. 2006; Venesky et al. 2012; Martel et al. 2014). Specifically, a decrease in dietary protein intake is related to weak immune responses and reduced humoral responses (Good et al. 1976; Davis 1995; Demas et al. 2003). Our results show that animals exposed to LPS have decreased body mass and increased rest periods, irrespective of dietary acclimation. Indeed, high-protein content diets did not seem to enhance the inflammatory response in this species. A possible explanation for the absence of effects of dietary acclimation may be related to the quality of the proteins and/or the presence of specific amino acids in the diets (Bounous et al. 1983; Van Heugten et al. 1994). In this sense, it has been reported that dietary amino acids and different types of proteins influence immune function in mice (Bounous et al. 1983; Van Heugten et al. 1994). For example, dietary proteins such as whey or casein enhance the immune function of laboratory animals (e.g., Mus musculus) while other types of protein do not improve the immune response (Bounous et al. 1983; Van Heugten et al. 1994; Martel et al. 2014). Klasing and Roura (1991) have reported that in chicks the requirements for amino acids like methionine and lysine increase after an immune challenge, and this is probably due to the metabolic response induced by the immune challenge. As such, we cannot 13 Journal of Comparative Physiology B discard the possibility that the immune response of O. degus could be related to the quality of dietary proteins rather than to the amount of it. Additionally, it is possible that protein malnutrition may affect other types of immune responses, such as cellular-mediated immune responses. Indeed. Thomason et al. (2013) reported that diets with different amounts of protein differentially affect humoral responses and cellular defense. Specifically, these authors found that low-protein diets decreased the magnitude of the humoral response while improving the cellular response (Thomason et al. 2013). In this sense, our results showing that diet affected haptoglobin levels but not sickness behavior or body temperature, seems to support this idea. Thus, studying cellular responses seems necessary to obtain a complete view of the effects of dietary protein on the immune response of O. degus. Several studies have documented that immune challenge is associated with an increase in reactive species and a decrease in antioxidant protection that is also associated with the risk of oxidative damage (Costantini and Moller 2009; Hasselquist and Nilsson 2012; Raberg et al. 1998). In this study, we combined diet and an immune challenge to estimate the oxidative status of animals. We found that those degus that mounted an immune response had increased levels of lipid peroxidation and total antioxidant capacity (Fig. 4). Our analysis revealed that the increase in lipid peroxidation was positively correlated with an increase in total antioxidant capacity (Fig. 3), suggesting that animals were oxidatively balanced. This result contrasts with previous findings (Bertrand et al. 2006; Costantini and Dell’Omo 2006; Costantini and Moller 2009) where an immune challenge caused a decrease in antioxidant capacity 24 h after inoculation or had no effect on plasma antioxidant capacity (Cohen et al. 2009). A possible explanation for our results could be due to the fact that the expression and activity of antioxidant enzymes increases when cells are subjected to oxidative stress (Horak et al. 2007; Han et al. 2008). These types of compensatory mechanisms involve the synthesis of antioxidant enzymes which depend on the level of oxidative damage (Halliwell and Gutteridge 2007; Han et al. 2008). Specifically, very high levels of oxidants result in reduced antioxidant enzymatic activity due to damage in the molecular machinery that induces the expression of antioxidant enzymes (Han et al. 2008). In this sense, gradual and moderate elevation of oxidant levels may result in less oxidative stress due to concomitant up-regulation of the antioxidant defense (Monaghan et al. 2009; Sabat et al. 2017). Accordingly, this relationship between oxidants and antioxidant capacity is dependent on the magnitude of the immune challenge and/or the speed of the immune response (Monaghan et al. 2009; van de Crommenacker et al. 2010). In this way, the immune challenge of O. degus could have resulted in a gradual immune response causing the activation of 13 compensatory mechanisms and concomitant up-regulation of antioxidant enzymes. Overall, the evidence so far suggests that to achieve a neutral protein balance, the protein intake by individuals that are experiencing inflammation should be greater than the minimum protein intake of healthy individuals (Guadagni and Biolo 2009; Martel et al. 2014). In this sense, the evaluation of more extreme diets, in terms of protein content, are needed to evaluate the interaction between dietary protein, immune function and oxidative status. It is also necessary to determine the effects of specific dietary aminoacids on the immune response of individuals experiencing inflammation. Acknowledgements We are grateful to Carolina Contreras for her help and assistance with data collection. This study was funded by FONDECYT 3160133 to NRO, FONDECYT 3140395 to DSR and FB 00022014 to FB. Animals were captured with permits from SAG, Chile (No. 2355/2016). 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