Evaluation of functional thymic hormones in Arabian horses with severe combined immunodeficiency.

Arabian horses with severe combined immunodeficiency disease (SCID) were evaluated for thymic hormone activities using thymic extracts and sera. Extracts prepared from thymus of SCID horses were able to increase the number of spleen cells responding to sheep red blood cells in irradiated, bone marrow-reconstituted mice. In addition, ultrafiltrates prepared from sera of these immunodeficient horses, which contained material with molecular weight of less than 50,000 Daltons could (a) induce a population of human bone marrow precursor cells to differentiate into cells bearing SRBC receptors and form spontaneous E-rosettes, a characteristic of T lymphocytes, and (b) stimulate cyclic adenosine monophosphate (AMP) synthesis in mouse thymocytes. Based on in vivo and in vitro effects, it was concluded that the defect of these Arabian horses with severe combined immunodeficiency disease did not involve a complete thymic hormone inadequacy.