REGULATORY INFLUENCE OF THYMOPENTIN ON SPLENIC T CELL SETS OF THYMECTOMIZED AND AGED MICE1,2

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Abstract
Thymectomy of mice aged 6-8 weeks causes a disproportion of splenic T cell sets, the Ly123 set being relatively decreased and the Ly23 set relatively increased (Ly123 .dwnarw.: Ly23.uparw.). A similar disproportion of splenic T cell sets was found to occur spontaneously with advancing age (12-18 months). By PA-SRBC assay, the total number of splenic Lyt+ cells is not appreciably reduced by thymectomy or by aging, but the Thy-1+ cell count falls by about 40% according to both PA-SRBC and cytotoxicity assays. Thus, there is an increase in the number of Lyt+ cells expressing sub-threshold amounts of Thy-1. The following observations show that thymopentin (TP-5), a synthetic pentapeptide analog of thymopoietin, counteracts these changes in thymectomized and aged mice. As reported previously, the capacity of C3H/HeJ female mice to reject C3H/HeJ male skin was raised by thymectomy, or with age, the treatment with TP-5 substantially normalized the rejection response. Here we correlate these findings with changes in the profile of splenic T cell sets. The splenic T cell set profile of thymectomized B6-Tlaa male and female mice was essentially restored by TP-5. The Ly123.dwnarw.:Ly23.uparw. change caused by thymectomy was not associated with obviously altered proportions of Qa-1+ and Qa-1- subsets. Treatment of aged mice with TP-5 also prevented the onset of changes in splenic T cell sets that occur spontaneously with age. Thus, thymectomy and aging give rise to disproportions of splenic T cell sets, and in C3H female mice to a heightened capacity for male skin rejection, both effects being largely abrogated by the TP-5 derivative of thymopoietin.