T lymphocytes are necessary for the peripheral phase of B lymphocyte maturation

Until recently, B lymphocyte maturation was considered to be independent of the thymus and T lymphocytes. However, using nude animals, which lack the functional thymus, we have shown that T lymphocytes are required for the peripheral phase of B lymphocyte maturation. We showed that the proportion of immature B lymphocyte subsets (CD90highIgMhigh and CD90highIgMlow) was significantly increased, whereas that of mature B lymphocyte subsets (CD90?IgMlow and CD90?IgMhigh) was decreased in the peripheral blood and lymph nodes of nude rats. In addition, the expression of functionally important surface molecules MHC class II, ICAM-1, CD44 and L-selectin was significantly down-regulated both on immature and mature B lymphocyte subsets. The implantation of thymic tissue under the kidney capsule of nude rats alleviated the block in B lymphocyte maturation and normalized of the defective expression of surface molecules. Comparable effects were seen after the adoptive transfer of T lymphocytes. This shows that in nude rats B lymphocytes do not mature properly due to the lack of T cell help and that T lymphocytes are required for the peripheral phase of B lymphocyte maturation, as well as for the appropriate expression of surface molecules. This should be considered when treating patients with T lymphocyte deficiencies.