Costs of immune response in cold-stressed laboratory mice selected for high and low basal metabolism rates

To study whether mounting an immune response is energetically costly, mice from two lines divergently selected for high (H-BMR) and low (L-BMR) basal metabolic rate (BMR) were immunized with sheep red blood cells. Their energy budgets were then additionally burdened by sudden transfer from an ambient temperature of 23 degreesC to 5 degreesC. We found that the immune response of H-BMR mice was lower than that of L-BMR mice. However, the interaction between line affiliation and ambient temperature was not significant and cold exposure did not result in immunosuppression in either line. At 23 degreesC the animals of both lines seemed to cover the costs of immune response by increasing food consumption and digestive efficiency. This was not observed at 5 degreesC, so these costs must have been covered at the expense of other components of the energy budget. Cold exposure itself elicited a considerable increase in food intake and the mass of internal organs, which were also heavier in H-BMR than in L-BMR mice. However, irrespective of the temperature or line affiliation, immunized mice had smaller intestines, while cold-exposed immunized mice had smaller hearts. Furthermore, the observed larger mass of the liver and kidneys in immunized mice of both lines kept at 23 degreesC was not observed at 5 degreesC. Hence, immunization compromised upregulation of the function of metabolically active internal organs, essential for meeting the energetic demands of cold. We conclude that the difficulties with a straightforward demonstration of the energetic costs of immune responses in these animals stem from the extreme flexibility of their energy budgets.