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Bone mineral metabolism in T lymphocyte–deficient and –replete strains of rat

The immune and skeletal systems are known to interact. We have repeatedly shown that in contrast to in vitro data, the administration of T lymphocyte immunosuppressants, such as cyclosporin A, leads to an increase in bone resorption and a high turnover osteopenia. The purpose of this study was to characterize the bone metabolism of the T lymphocyte deficient Rowett athymic homozygous (rnu/rnu) nude rat. We wished to determine whether these rats share the bone abnormalities of cyclosporin A–treated rats. Eleven 10‐week‐old Sprague–Dawley rats and 12 similarly aged nude rats were studied over a 4‐week period. Metaphyseal cancellous bone histomorphometry was similar in the two groups of rats and only differed with regard to percentage eroded perimeter (lower in nude rats, p = 0.008) and longitudinal growth rate (49% lower in nude rats, p < 0.001). The nude rats had less body mass (p < 0.001) but nevertheless gained the same percentage of their body weight over the study period. The athymic rats had lower levels of serum 1,25‐dihydroxyvitamin D (p < 0.014) and serum osteocalcin (p < 0.009), and at the age of 14 weeks the nude rats had lower concentrations of serum creatinine (p = 0.001) and blood ionized calcium (p = 0.0002), yet serum PTH was similar throughout. RNA isolated from the contralateral tibias revealed that the nude group had lower steady‐state levels of osteocalcin mRNA despite similar rates of bone formation. In its entirety, the data suggest that T cell deficiency per se is not necessarily associated with high turnover osteopenia.