Immunological enhancement with a low dose of cyclophosphamide in aged mice.

Aged mice treated with a low dose of cyclophosphamide (CY) showed significantly enhanced immune capacity in cellular proliferation and antibody response. In these mice, total cell numbers were increased both in the thymus and spleen, compared to those in non-treated mice. Treatment with a low dose of CY induced apoptosis of thymocytes in the atrophic thymus of the aged mice, being followed by an increase of proliferation of thymocytes and leading to an increase of thymocytes and splenic T cells. Treatment with a high dose of CY also induced apoptosis in the thymus, but suppressed the proliferative capacity, therefore not leading to an enhancement of immune capacity. In young mice, however, CY suppressed immune capacity regardless of the dose. Thymocytes and splenic T cells of young mice were more susceptible to CY than those of old mice and were decreased in number after the treatment with even a low dose of CY.