Polyadenylic-polyuridylic acid complexes, a potent adjuvant to the immune response, were tested for action on thymic-influenced and bone marrow-derived lymphocytes in model systems deficient in one or the other of these cells.

Adult mice, thymectomized at birth or mice treated with heterologous antithymocyte serum produced 90–95% fewer splenic rosette-forming cells than normal mice in response to an injection of sheep erythrocytes. Intravenous injection of complexes of homoribopolynucleotides, polyadenylic and polyuridylic acids, poly A:U with SRBC restored immunologic competence to NTx- or ATS-treated mice such that they produced normal or near normal levels of splenic RFC. In addition, injection of poly A:U enabled NTx mice to reject allogeneic skin grafts at the same rate as control mice with an intact thymus. Further reduction in residual thymocytes by combining neonatal thymectomy with ATS treatment reduced the number of anti-SRBC RFC induced by poly A:U.

Lethally irradiated mice which received SRBC, excess bone marrow cells, and as few as 40,000 thymic lymphocytes were stimulated by poly A:U to produce RFC. No adjuvant effect was observed when irradiated mice received excess thymic lymphocytes and low doses of bone marrow cells with poly A:U.

The results suggested that the adjuvant action of poly A:U was exerted on the thymic-influenced, antigen-reactive cell and that restoration of immunocompetence to NTx- or ATS-treated mice was caused by amplification of a small number of residual antigen-reactive cells which were influenced by the thymus in utero before thymectomy, or which survived treatment with ATS.

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