Thymic hormones bioassays

Serum thymic hormone levels, based on either the bioassay of Bach and Dardenne or the method of Twomey and colleagues, have now been determined for a variety of primary and secondary immunodeficiency disorders (summarized in Table 1). 

A great variety of patients with primary immunodeficiency disorders have now been studied with both bioassay procedures (Bach et al., 1972, 1975 Incefy et al, 1977 Lewis et al, 1978 Iwata et al., 1981). Serum thymic hormone levels are lower than age-matched normal levels for all patients studied with DiGeorge s syndrome, and a high proportion of patients with severe combined immunodeficiency. In patients vdth the complete Di-George s syndrome, thymic hormone levels are always undetectable, whereas in the partial form, FTS-like bioactivity is measurable but lower than normal. In some cases in which thymus grafi ing was utilized as a therapeutic modality, serum FTS-like bioactivity became detectable as early as 1 day after transplantation (Incefy et al., 1977) and eventually returned to normal (Lewis et al, 1978). These results have suggested that the increase in serum thymic hormone levels resulted from production by the transplanted thymus. In patients with SCID, regardless of the bioassay employed, thymic hormone levels were found to be either undetectable or much lower than that of age-matched normal donors. In rare cases serum FTS-like bioactivity... 

Another major problem with the currently available serum bioassays is that they do not detect significant serum bioactivity in subjects beyond the fifth decade of life. This makes it impossible to establish whether serum thymic hormone levels in patients with diseases more commonly associated with old age, such as rheumatoid arthritis, or the more common cancers are lower than those of age-matched healthy donors. 

Serum thymosin Uj levels detected by RIA are approximately 10 times higher than those of circulating thymulin levels as determined by RIA (see Section 7.2.3). In addition, there are several differences between serum Tuj levels and thymulin levels in association with aging. First, the 3-fold decline of serum Tuj levels with age was not as dramatic as the 30-fold decline of serum thymic hormone activity determined in the various bioassays or in the thymulin RIA. In addition, serum Toj levels tend to drop abruptly in childhood and remain constant after age 20 and well past the sixth decade of life. 

The initial Taj RIA that employs rabbit hetero mtiserum has been used to evaluate serum levels in children with primary immunodeficiency disorders (D. Wara et al., 1982). With this assay, 11 of 13 children with combined immunodeficiency, 5 to 7 children with ataxia-telangiectasia and all 3 patients with Wiscott-Aldrich syndrome exhibited serum Taj levels below the mean of the normal age-matched donors. Such findings were similar to those reported using the Bach-Dardenne bioassay (Iwata et al., 1981). A major inconsistency, however, was in children with DiGeorge s syndrome. Whereas all such children manifested low serum thymic hormone bioactivity, only 3 of 6 exhibited levels below the mean of normals using the Taj RIA and serum levels were actually quite high in 2. 

Since the various thymic hormones have only recently entered clinical trials, there is little information available on circulating levels following parenteral administration in man. In their initial description of the rosette-azathioprine bioassay, Dardenne and Bach (1973) reported that after the intravenous injection of a crude thymosin fraction to adult thymectomized mice, transient serum thymic hormone bioactivity was demonstrable and peaked at 2 hours postinjection and disappeared after 48 hours. More recent studies using the rosette-azathioprine bioassay have indicated that FTS itself disappears rapidly from blood after intravenous administration, with a half-life of 15 minutes (Bach et al., 1978). The half-life of FTS could be prolonged by preincubation with serum from thymectomized mice or by binding to carboxymethyl cellulose. 

Several indirect arguments have suggested that cyclic AMP (cAMP) may be a second messenger for thymic hormone action. These include the observations that cAMP in the Bach-Dardenne bioassay acts synergistically upon rosette-forming cells with thymulin (Bach and Bach, 1973). In addition, the inductive effects of thymic hormones can readily be mimicked by a variety of products that increase cellular cAMP and conversely can be inhibited by products that decrease cAMP (Scheid et al., 1973, 1975 Astaldi et al., 1978). 

Another major problem that has plagued the field of thymic hormone research is that specific bioassays for thymic hormones are not available. Indeed, the thymic hormones have been aptly termed hormones in search of a bioassay (Bach and Carnaud, 1976). It is quite simple to add a thymic factor to a routine immune assay and look for a positive or negative influence. However, unless the target cell population for such studies is well defined, the results are likely to be difficult to interpret in terms of normal physiological mechanisms. This latter point no doubt explains many of the inconsistencies reported in the thymic hormone field. 

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