Restorative effects of thymosin polypeptides on purified protein derivative--dependent migration inhibition factor production by the peripheral blood lymphocytes of adult thymectomized guinea pigs.

Thymosin fraction 5 restored the capacity of peripheral blood lymphocytes (PBL) from thymectomized adult guinea pigs sensitized with purified protein derivative (PPD) to produce migration inhibition factor (MIF) as measured in an agarose droplet microassay with peritoneal exudate cells. PBL from sham-thymectomized and normal PPD-sensitized guinea pigs did not lose their capacity to make MIF in response to PPD and were unaffected by thymosin polypeptides. Splenic and lymph node lymphocytes of thymectomized guinea pigs retained their antigen responsivity , suggesting that thymectomy did not affect fully matured, localized memory cells, but caused an accumulation in the circulation of partially matured, antigen-sensitive lymphocytes that could release MIF only in the presence of thymic factors. Various thymosin polypeptides were tested in this system and only thymosin alpha 1, an acidic polypeptide of 28 amino acid residues, showed consistent activity. This polypeptide, which has a molecular weight of 3,108, did not inhibit macrophage migration by itself, but 10-1,000 ng/ml was able to completely restore the MIF-producing capability of PBL from thymectomized guinea pigs. A standardized method for pooling and cryopreserving guinea pig PBL was developed, allowing multiple assays to be performed with a single pool of cells. The specificity of this bioassay system has made it a useful tool in evaluating the thymosin alpha 1 activity present in various thymosin preparations, and in validating biological activity of thymosin alpha 1 preparations made by synthetic or recombinant DNA techniques.