Seasonal rhythmicity in lymphocyte blastogenic responses of mice persists in a constant environment

Young C57BL/6 mice were housed in an environment in which the ambient temperature (22.5 degrees C +/- 1 degree) and photo-period were constant for the duration of the observations. At weekly intervals, animals were sacrificed, and splenic single cell suspensions were tested for percentage of viable cells. The functional capacity of T and B lymphocytes was assessed in vitro by the mitogen-induced incorporation of tritiated thymidine by dividing cells. The lymphocytes were incubated in RPMI 1640 containing 10% fetal calf serum for 68 hr at 37 degrees C. T and B lymphocytes were stimulated with phytohemagglutinin-P and concanavalin A and with lipopolysaccharide, respectively. Seasonal rhythmicity in the incorporation of tritiated thymidine was exhibited by both lymphocyte subpopulations, with peak responses two to five times higher in March to April 1978 and February to March 1979 than during the previous two Decembers. Factors such as birth date of the mice and components of the culture medium had no influence on the results. Because the annual cycles persisted in the absence of known environmental signals or Zeitgebers, they bear similarities with circannual rhythms. It is suggested that a relationship may exist in mice and in other species between seasonally depressed immune functions and increased incidence of infectious agents.