Thymic hormones and immunopeptides

The role of the thymus and its hormones in immunity is discussed. The known characteristics of various thymic factors, their mode of action and possible clinical applications are also reviewed. A brief discussion of other immunopeptides which are liberated by lymphocytes is also included.

Age, thymic involution, and circulating thymic hormone activity

Circulating thymic hormone activity and thymic histology were studied in patients undergoing open heart surgery. Plasma thymic hormone activity was measured using a bioassay based upon thymocyte antigen induction on null mouse lymphocytes. Activity was highest at 15-30 yr of age and declined thereafter, being negligible after the sixth age decade. The age-related decline of circulating thymic hormone activity correlated, in general, with progressive thymic involution. However, hormone activity was detected in plasma from some cases with advanced involution, suggesting that the normal young thymus may have considerable functional reserve.

Effect of a human serum thymic factor on hydrocortisone-treated thymocytes

A human serum thymic factors (SF) stimulated cyclic adenosine-3' ,5'-monophosphate (cAMP) synthesis in normal mouse thymocytes. Such stimulation was no longer observed when thymocytes were depleted of hydrocortisone (HC)-sensitive cells. It was concluded that SF selectively stimulate cAMP in HC-sensitive cells. Furthermore, incubation of thymocytes with SF enlarged the population of HC-resistant thymocytes. These results suggest that SF might act on HC-sensitive thymocytes increasing their cellular cAMP level and inducing their transformation in HC-resistant cells.

Serum thymic factor and the neuromuscular transmission

The effect of Serum Thymic Factor (FTS) on neuromuscular transmission has been studied in vivo by electromyographic assay. As no modification of muscle tetanus pattern was detected, the involvement of this factor in the pathology of thymus disorders leading to myasthenia is unlikely.

Structural study of circulating thymic factor: a peptide isolated from pig serum. II. Amino acid sequence

The amino acid sequence of a circulating thymic factor (FTS, facteur thymique sérique) isolated from pig serum has been established as less than Glu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn-OH. This sequence was obtained by Edman analysis both on the intact peptide and after tryptic digestion. The COOH-terminal residue was identified by carboxypeptidase A digestion. A synthetic peptide was prepared which showed the same biological activities as native pig circulating thymic factor, thus confirming the proposed primary structure.

Structural study of circulating thymic factor: a peptide isolated from pig serum. I. Isolation and purification

A circulating thymic factor (FTS) has been characterized by a bioassay based on its ability to render theta-negative rosette-forming cells theta-positive and azathioprine-sensitive. FTS was sequentially purified and finally isolated from 1000 liters of pig serum by ultrafiltration, gel filtration, and ion exchange chromatography. Its amino acid composition and apparent molecular weight estimated by Sephadex G-25 chromatography, indicate that FTS is a nonapeptide of composition lysine, aspartic acid (or asparagine), serine 2, glutamic acid (or glutamine) 2, glycine 2, and alanine.

Thymosin-induced human serum factor increasing cyclic AMP

Thymosin has virtually no in vitro effect on cyclic AMP levels in thymocytes and seems not to react with either the beta-adrenergic receptor or with the putative human serum thymic factor (SF) receptor. However, when thymosin is injected into patients lacking SF activity, it induces the appearance of a serum factor which increases cellular cyclic AMP levels in thymocytes in vitro. This factor appears to act on a membrane site distinct from the beta-adrenergic receptors.

Bioassay determinations of thymopoietin and thymic hormone levels in human plasma

Thymopoietin is a thymic hormone that induces differentiation of thymocytes from precursor cells which arise in hemopoietic tissues. This paper describes a sensitive in vitro assay for the induction of Thy 1.2 antigen on null lymphocytes from germ-free athymic (nu/nu) mice. The sensitivity and specificity of the bioassay were increased by adding high concentrations of ubiquitin (a nonspecific inducer) to the induction incubations. The bioassay was sufficiently sensitive to detect thymopoietin at less than 0.25 ng/ml. A dose-response relationship was shown between thymopoietin concentration and the percentage of cells induced to express Thy 1.2 antigen. When normal human plasma was assayed, induction was registered with activity corresponding to thymopoietin at greater than 1 ng/ml in plasma from infants or young adults. Activities in the thymopoietin range of 0.25 ng/ml were registered with plasma from healthy subjects over 50 years of age. Thymectomy was followed by loss of this inductive activity from the plasma. This bioassay permits clinical studies on T (thymus-derived) cell inducers released by the human thymus into the circulation.

Thymic activity in severe combined immunodeficiency diseases

Thymic function was evaluated by quantitation of circulating thymic factor in patients with several forms of severe infantile immunodeficiency diseases. Direct quantitation of thymic factor in serum of patients with severe combined immunodeficiency revealed heterogeneity of this syndrome by this parameter, as was also shown by study of susceptibility of the marrow cells to differentiation in vitro. Thymic factor was not detectable in one patient with severe combined immunodeficiency, but was present in normal or near-normal concentrations in three others. Circulating levels of this hormonal activity were also not detectable in a patient with DiGeorge athymic syndrome. Following marrow or fetal liver transplantation, which corrected the severe combined immunodeficiency thymic factor levels either increased slightly or did not change appreciably. Fetal thymic transplantation, which together with fetal liver transplantation corrected the immunodeficiency in one patient with severe combined immunodeficiency, was associated with increase of thymic factor to normal levels. Fetal thymus transplantation alone, which was employed to correct the immunodeficiency of DiGeorge athymic syndrome, caused an increase in thymic factor activity to normal or near normal levels in this patient.

[Biochemical characterization of a circulating thymic factor]

A serum thymic factor has been isolated from pig serum. Its amino-acid sequence has been determined by Edman technique, modified by Gray: Gln-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn. A nonapeptide, synthesized on the basis of this sequence is active in the rosette assay used for the isolation of the biological product, both in vitro (10(-15) M) and in vivo (0.1 ng per Mouse).

Effects of antithymic reticuloepithelial cells serum on the levels of circulating thymic factor and on the sensitivity to azathioprine of spleen spontaneous rosette-forming cells

Swiss mice treated with an antithymic reticuloepithelial cells serum (ATRES) showed a drastic and prolonged depression of the serum thymic factor. A similar but less pronounced effect was also observed following the administration of the antithymocyte (ATS) and the antilymphocyte (ALS) sera. Conversely, the azathioprine sensitivity of spleen spontaneous rosette-forming cells was highly modified by the ATRES but not by the ATS or the ALS. The probable mechanisms of such effects are discussed.

Immunological aspects of psoriasis. II. Dissociated impairment of thymus-dependent lymphocytes

We have investigated lymphocyte subpopulations (six different markers) and T cell functions (mitogen responses and serum thymic factor determination) in twenty patients with psoriasis compared with thirty-five healthy subjects. Normal results were found for B cell markers, but significantly lower numbers of T cells were assessed by E-rosettes and anti HTLA serum. Mitogen responses were significantly higher in psoriasis than in normal subjects of the same age. These findings could be in favour of a T cell subpopulation defect involved in an auto-immune pathogenesis of psoriasis.

Quantification of two basic proteins related to a thymic hormone (LSH) and their occurrence in serum of normal and cancerous adults

A quantitative assay for a thymic hormone (LSHr) was developed and utilized for biologic materials. The protein was isolated from tissue extracts or biologic fluids by precipitation with 20% (NH4)2SO4 and reprecipitation with 75% methanol with subsequent disc-electrophoresis on polyacrylamide gel. The proteins on the gel were stained with amido black. One of the two protein bands was readily identified with LSHr by its electrophoretic mobility and agar immunodiffusion tests. Comparison of the intensity of the protein bands obtained from biologic tissues with those of graded amounts of the pure LSHr allowed quantitative estimation. Another basic protein of the serum is found along with LSHr by the above fractionation procedure; it was tentatively named GEM 126 on the basis of its electrophoretic mobility. The total quantity of each of these serum basic proteins and their ratios to each other showed no correlation to any of 20 clinical and laboratory parameters in either healthy or cancerous adults. The lack of correlation between the quantity of serum protein presumed to be LSHr and cancer susceptibility suggests this thymic hormone is not deficient in patients with cancer. Treatment of female cancer patients with radiation and chemotherapy caused a decrease in serum GEM 126 when compared to healthy adults, and untreated or surgically treated cancer patients.