Paradoxical Effect of Anti-thymocyte Serum on the Thymus

Abstract

THE suppressive effects of anti-thymocyte serum (ATS) on cell-mediated immune responses^1–4 have been attributed to the selective depletion of recirculating T lymphocytes⁵. In this view, lymphoid tissues such as thymus which do not contain significant numbers of itinerant, blood-borne lymphocytes would be spared from the debilitating effects of ATS. The observation that the rate of recovery from the effects of ATS is substantially accelerated in the presence of an intact thymus^6,7 is consistent with this idea. That the thymus is spared is also supported by studies of the distribution of labelled ATS immunoglobulin (IgG), which indicate that antibody penetrates the thymus very poorly⁸. These considerations do not, however, directly bear on questions concerning possible alteration in immunological activity of thymus cells after the peripheral administration of ATS. The following experiments demonstrate that graft-vs-host (GVH) activity produced by thymus cells is substantially increased for several weeks after the peripheral administration of a single dose of ATS. As this effect was not produced by normal rabbit serum and was removed by absorption of ATS with thymocytes, it seems to represent a specific immunological consequence of administration of ATS.