Cellular metabolism as a basis for immune privilege

Abstract

We hypothesize that the energy strategy of a cell is a key factor for determining how, or if, the immune system interacts with that cell. Cells have a limited number of metabolic states, in part, depending on the type of fuels the cell consumes. Cellular fuels include glucose (carbohydrates), lipids (fats), and proteins. We propose that the cell's ability to switch to, and efficiently use, fat for fuel confers immune privilege. Additionally, because uncoupling proteins are involved in the fat burning process and reportedly in protection from free radicals, we hypothesize that uncoupling proteins play an important role in immune privilege. Thus, changes in metabolism (caused by oxidative stresses, fuel availability, age, hormones, radiation, or drugs) will dictate and initiate changes in immune recognition and in the nature of the immune response. This has profound implications for controlling the symptoms of autoimmune diseases, for preventing graft rejection, and for targeting tumor cells for destruction.

Figure 1

Metabolic modification of cell surface Fas expression. Changes in expression of Fas caused by removing glucose or adding insulin to the culture medium. The glucose is removed by incubating HL60 (human promyelocytic leukemia) cells in glucose free RPMI with the addition of 5 mM 2-Deoxyglucose. When cells were incubated with insulin, normal RPMI medium conditions were used with the addition of 100 μg/mL of insulin. The above conditions represent a 24 hour treatment period. This data is representative of five separate experiments. Each of the experiments showed the same general trends, however the experiments were done at different times and because the intensity of the fluorochromes varies with time, this makes direct statistical comparisons suspect. The level of Fas was detected using PE-conjugated anti-humanFas (CD95) antibodies (Pharmingen, California) and measured using a Coulter Elite Epics Flow Cytometer (Coulter, Hialeah, Florida) and FlowJo analysis software (Tree Star, Inc, Oregon).

Figure 2

Inhibition of CPT induces increased cell surface Fas expression. Levels of cell surface Fas on B16F1 melanoma cells in cultures with different concentrations of Etomoxir for 24 hours. Etomoxir blocks the mitochondria from using carbons from fat as fuel. Fas levels, normally low in melanoma cells, rise in the cells treated with Etomoxir. At a concentration of 500 μg/mL all the cells died. The level of Fas was detected using PE-conjugated anti-mouse Fas (CD95) antibodies (Pharmingen, California) and measured using a Coulter Elite Epics Flow Cytometer (Coulter, Hialeah, Florida) and FlowJo analysis software (Tree Star, Inc, Oregon).

Figure 3

Confocal Microsopy of L1210 (rapidly dividing cells) and L1210 DDP (slowly dividing cells) showing distribution of UCP2. The slowly dividing cells have substantial UCP2 within the cell and also have little to no cell surface Fas. In contrast, the rapidly dividing cells have UCP2 on or near the cell surface and have significant levels of cell surface Fas.