The effect of multiparity and lactation periods on the graft versus host reactivity of thymocytes and splenocytes from aging C57BL mice.

The ontogeny of graft versus host (GvH) reactivity of thymocytes and splenocytes was tested in C57BL female mice from the day of birth up to the age of 24 months. In comparison with virgin mice, the effect of a different number of pregnancies, with or without lactation, on the GvH reactivity was examined in middle-aged and aging female mice. The most striking results were obtained with suckling, aging 18-month-old multiparas. After three or more pregnancies with lactation--regardless of the number of pregnancies without suckling--the weight and cell count of the thymus and GvH reactivities on both thymocytes and splenocytes were significantly increased and were comparable with those of middle-aged, 12-month-old virgins. The present results indicate that physiological lactation periods are responsible for the long-lasting, immunoenhancing effect of multiparity, and that the maternal thymus is involved in the mechanism of this phenomenon. A possible role of prolactin rejuvenation of the thymus in suckling multiparas is suggested.