Immunocompetence as a Component of Energy Budgets and
Life-history Strategies of Mammals I

Funded by:
HHMI and the Committee on Institutional Studies and Research, MSU


Traditionally, the energy budgets of animals have included the factors of basal metabolic rate, locomotion, reproduction, ingested energy, energy lost in feces, and energy lost in urine.  The cost of defense of the organism has seldom been considered, especially within the context of the energetic cost of maintaining immunocompetence.   If there is a cost of immunity, then that cost needs to be considered in animal budgets.  Also, it needs to be determined how animals meet the cost of maintaining immunity.  When an infection or disease is encountered, do animals simply ingest more food energy to meet the energetic cost of mounting an immune challenge?  Or, do animals reduce energy expenditure on other physiological functions such as growth and reproduction?  The answers to questions such as these are essential to understanding how mammalian species, including humans, cope with the many pathogens encountered through their lifetime.  This is especially true of populations that are stressed in some way (e.g., poor nutrition, low food availability, crowding).



Energy assimilated = kJ ingested - kJ excreted - kJ egested -

    kJ Basal Metabolism - kJ Locomotion - kJ Reproduction - kJ Immunity?

 

Question 1:  Should energy expenditure for immune functions be incorporated into energy budget equations?

My students and I are testing two primary null hypotheses:

  1. There is no significant energy cost associated with maintenance of immunity.
  2. There is no significant energy cost associated with mounting an immune response.


Is there a significant energetic cost associated
with maintaining an immune system?

 

Adult male white-footed mice Peromyscus leucopus) were given injections of Cyclophosphamide (CY), an immunosuppressant, or saline every other day.  The resting (RMR) and daily (DMR) metabolic rates were periodically measured.  White 
blood cell counts (WBC) were made at the beginning and end of the experiment.  The animals were dissected and the wet and dry masses of organs  measured.



 

 

Stephen Compton prepares the metabolic chamber for measurement of the oxygen consumption of a white-footed mouse. 
 
 

 


 
 
   

 Lee Webb collects blood by heart puncture for measurement of  blood parameters.


 
 
 
Brandon Kellie uses a hemacytometer to count  white blood cells.

 
 
 
 

Stephen injects an animal with Cyclophosphamide.
 



There were no significant differences in the resting metabolic rates of animals whose immune system was suppressed (CY) compared with those whose immunity was normal (control).  There were also no significant differences in the masses of the intestinal, vital, or reproductive organs.
 
There was no significant difference in daily rates of energy use by animals whose immune system had been suppressed compared with those whose immune system was not compressed.

CONCLUSION:  There was no significant cost associated with maintaining the immune system.  Further study is needed, however, using more complete immunosuppression.


 

Question 2:  Is there a significant energy cost of mounting an
immune response?

 
Adult male white-footed mice were injected with sheep red blood cells (SRBC) at the beginning of the experiment to challenge the humoral branch of the immune system.   Near the end of the experiment the animals were injected with phytohemagglutinen (PHA) to challenge the cell-mediated branch of the immune system.  Resting and daily metabolic rates were measured periodically throughout the experiment.  Animals were  dissected and the wet and dry masses of the vital, intestinal, and reproductive organs were measured.


 

Stephen and Lee trap for white-footed mice. 

Animals were dissected and the reproductive organs removed.



 

Stephen and Dr. Derting get the oxygen analyzer running again.



Injections of SRBC and PHA resulted in a significant increase in WBCs.  There was no significant increase in resting or daily metabolic rates, however.  Thus, animals did not accommodate the cost of mounting a mild immune response through increases in energy ingestion.
Energy allocation to the small intestine and the testes was significantly less in the immuno-challenged males.  Thus, trade-offs in energy allocation between the immune system and the reproductive and digestive systems may fuel the cost of mounting an immune response.


CONCLUSION:  Animals met the cost of meeting a mild immuno-challenge by reducing energy expenditure to the reproductive system.  There was no significant increase in the total amount of energy used, however, during the immuno-challenge.