Immunological defect and its correction in the osteopetrotic mutant rat.

Congenital osteopetrosis in the mutant rat "op" is accompanied by early atrophy of the thymus gland. The response of thymocytes to concanavalin A and phytohemagglutinin P was greatly diminished at age 28 days, even before pronounced thymic atrophy could be detected. The response of spleen cells to mitogens that stimulate thymus-derived (T) and bone-marrow-derived (B) cells was diminished as early as 35 days of age. A single injection of a suspension of normal bone marrow, which cures osteopetrosis, increased thymus weight and restored the normal responses of both thymus and spleen to various mitogens. These results add further support to the hypothesis that the thymus is involved in the pathogenesis of osteopetrosis.