Modification of immunologic and hematologic variables by method of CO2 euthanasia

Background and Purpose: The major goal was to determine whether variations in the method of CO2 euthanasia would induce significant immunologic differ ences. Methods: Young adult C57BL/6 mice (n = 40) were euthanized, using four regi mens: 70% CO2/30% O-2; 70% CO2/30% O-2 --> 100% CO2; 100% CO2-naive chamber ; and 100% CO2 pre-charged chamber. Time to recumbency and euthanasia and b ody, liver, lung, spleen, and thymus masses were determined. Blood and sple en were further evaluated for leukocyte, lymphocyte, and thrombocyte counts , erythrocyte characteristics, distribution of lymphocyte subpopulations, s pontaneous and mitogen-induced blastogenesis, complement activity, and cyto kine production. Results: Time to euthanasia was five- to eightfold longer in mice exposed t o 70% CO2/30% O-2 than that for any other group. There were slight increase s in mean erythrocyte volume (MCV) and mean erythrocyte hemoglobin (MCH) fo r all groups, compared with those for the 100% CO2 pre-charged group. Circu lating cytotoxic T (CD8(+)) lymphocyte percentages and numbers, and spontan eous blastogenesis of leukocytes in blood and spleen, also were affected by euthanasia method. Conclusions: The method of CO2 euthanasia can result in significant differe nces in immunologic/hematologic variables, Thus, consistency in euthanasia procedures may be important in accurate interpretation of research data.