The localization of antigen in lymph node follicles of congenitally athymic nude rats.

We have examined the postulated dependence on T cells of follicular retention of antigen by studying antigen retention in the draining lymph nodes of congenitally athymic, nude rats after local injections of horseradish peroxidase (HRP). The lymphoid tissues of these rats contained germinal centres and follicular dendritic cells (FDC) that were ultrastructurally identical to those seen in euthymic rats and expressed the differentiation antigen MRC OX2. Nude rat FDC captured and retained locally injected antigen on their surfaces, but as with euthymic rats, only in the presence of previously injected anti-HRP antibody. This demonstrates that the FDC mature both morphologically and functionally in the absence of a thymus or T cells. However, in contrast to euthymic rats, there was no detectable antigen retention in nude rats that had been actively immunized by repeated intraperitoneal injections with HRP for 3 months. The lower number of germinal centres observed in athymic animals compared with their euthymic littermates could thus be explained by deficient production of specific antibody of the isotype necessary for follicular localization of environmental antigens.