Reproducibility of seasonal effects in antibody formation and host resistance in the outbred mouse.

The purpose of this study was to assess the reproducibility of any seasonal effects in the outbred CD1 mouse of antibody production to sheep red blood cells (SRBCs) and host resistance to the bacterium Listeria monocytogenes. A marked seasonal effect on antibody production was seen when 5- to 6-week-old female CD1 mice were studied on a weekly basis for a period of 2 years. Maintained on a 12:12 h light:dark schedule, animals were held for 12 days prior to experiment to insure physical condition and acclimatization to the lighting regimen. Beginning at 4 h after lights on (HALO) for day 1 and 2 HALO thereafter, groups of mice were (a) not treated, (b) administered a vehicle (corn oil, 1% methylcellulose, or distilled water) by oral gavage for 5 days, or (c) not treated, but given an intraperitoneal injection of cyclophosphamide 24 h prior to assay. On the fifth day, mice were injected with SRBCs intravenously. Four days later, antibody formation against SRBCs was measured using spleen cells. Circannual and seasonal rhythms were displayed by each group of animals, with greatest antibody production, indicated by the number of plaque-forming cells (PFCs)/million viable cells, in the Spring (range of double amplitude = 36-108%). The timing of these rhythms was reproducible from one year to the next. In contrast, the magnitude of the response in year 1 was significantly different from year 2 for animals given vehicle or not treated. Cyclophosphamide-treated mice had consistently low numbers of plaque-forming cells. Host resistance was studied in separate mice treated with vehicles at the same time as the antibody assay. On the third day of dosing, mice were injected intravenously with Listeria monocytogenes (LM) and monitored for death for 10 days. When analyzed by Kruskal-Wallis life table analysis, there was no overall effect of vehicle on survival for 1987 but there was an effect for 1988 and when data from both years were combined. Distilled water-treated mice had lower survival rates than the other two vehicles. Mice treated with distilled water displayed a circannual rhythm for survival for 1988 and for both years combined, in contrast to the other two vehicles. When each vehicle was analyzed separately for seasonal effect, a significant effect of season occurred for corn oil- and distilled water-treated animals. The greatest survival rate and longest survival time generally occurred in the months between July and December.