Malignant osteoporosis and defective immunoregulation.

The linkage between immune cells and the osteoclast has become partially understood in the laboratory, but the full spectrum of clinical disorders of this relationship remain to be elucidated. We report a 29-month-old girl with recurrent infections and multiple fractures. Immune evaluation showed normal quantitative serum immunoglobulins but absent antibodies to the respiratory viruses and tetanus toxoid and decreased in vitro polyclonal-induced immunoglobulin production. Further analysis in vitro with separated lymphocyte populations showed normal B cell function but markedly increased suppressor T cell activity. The bone evaluation showed diffuse osteopenia on x-ray. Serum calcium, phosphorus, PTH, 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D were normal for age. Urinary calcium excretion (24 h) was, however, two times normal. An iliac crest biopsy confirmed the presence of extreme osteopenia with normal mineralization and numerous small atypical osteoclasts resorbing the bone. No circulating plasma resorptive activity was demonstrated. Calcitonin therapy markedly diminished the patient's hypercalciuria. We speculate that this patient's increased bone resorption, decreased bone formation, and suppressor activity may be linked by a common pathway involving the abnormal function of immune cells. Since no similar constellation of findings has been previously reported, this case may represent a new congenital disorder: severe osteopenia associated with increased osteoclast activity in association with a defect in T cell immunoregulation.