Absolute numbers of lymphocytes subsets migrating through the compartments of the normal and transplanted rat spleen.

Lymphocyte migration is one of the basic principles of the immune system. Up to now lymphocyte migration experiments have been performed either in a quantitative way, determining whole organ recoveries of radiolabeled lymphocytes without histologic localization, or based on autoradiography which does not provide absolute numbers of immigrant lymphocytes. In this study the traffic of lymphocyte subsets through the splenic compartments: red pulp (RP), marginal zone (MZ), periarteriolar lymphatic sheath (PALS) and follicle was evaluated in absolute numbers. In normal spleens and splenic transplants fluorescein isothiocyanate (FITC)-labeled immigrant lymphocytes were localized and characterized immunohistochemically in cryostat sections by light microscopy. In addition morphometry of the splenic compartments was performed and the recovery of 51Cr-labeled lymphocytes in the spleen was determined. The combination of these methods allowed total numbers of immigrant subset cells to be calculated in individual splenic compartments. At 15 min about 17% of the injected B lymphocytes were found in the MZ. This is the largest fraction of an injected lymphocyte subset found in a single splenic compartment. At 24 h immigrant B cells were not only found in the follicle, but they had reached comparable numbers in the three compartments: follicle, RP and MZ. Most immigrant T lymphocytes were found in the PALS, which from 6 h after injection onwards contained more T cell immigrants than any single organ of the body. CD4+ and CD8+ lymphocytes showed a similar distribution throughout the splenic compartments at early time points. At 24 h CD4+ lymphocytes homed preferentially to the PALS, whereas CD8+ cells seemed to prefer the RP and MZ. Both CD4+ and CD8+ cells also migrated into the follicles. In regenerated splenic tissue after autotransplantation lymphocyte immigration was reduced in all compartments and to the MZ in particular. An impaired lymphocyte migration to the MZ in splenic transplants may be one reason for the lack of protection provided against bacterial infections. Thus examining lymphocyte migration in absolute numbers provides additional information which cannot be gained by determining labeling indices or percentages of lymphocyte subsets alone.