We have earlier reported that diverse collagen vascular diseases, including arthritis, arteritis, thrombosis, myocarditis, myositis, sialo-/dacryoadenitis and dermatitis develop with the advent of autoantibodies in transgenic rats carrying the LTR-env-pX gene of human T lymphocyte virus type I (LTR-env-pX rats). To clarify the pathogenesis of these collagen vascular diseases, immunological features of LTR-env-pX rats were examined. In LTR-env-pX rats affected with these diseases, expression of CD80/86 on both tissue-infiltrating and peripheral T cells increased, compared with findings in non-transgenic rats with experimental inflammatory diseases. CD80/86 was also upregulated on peripheral T cells in LTR-env-pX rats prior to the development of diseases. Lymphocytes from LTR-env-pX rats showed an increase in autologous proliferation and were hyperreactive against several mitogens, including concanavalin A, immobilized anti-CD3 antibodies, and superantigens in vitro. Antigen-specific immune response was also enhanced in LTR-env-pX rats. The collective evidence indicates that lymphocytes of LTR-env-pX rats constitutively express surface molecules related to T cell activation and are immunologically hyperresponsive. Bone marrow cell transfer from LTR-env-pX rats to lethally irradiated non-transgenic rats revealed that these immunologically pre-activated and hyperresponsive lymphocytes play a critical role in the pathogenesis of several collagen vascular diseases, especially of dermatitis in LTR-env-pX rats.