Application of the popliteal lymph node assay to evaluate an immunosuppression protocol.

There exists a variety of experimental techniques which are used to evaluate host immunity. None of these in vitro assay systems are optimal, since they are performed ex vivo. Here is described an in vivo model, using four rat strain combinations and a modification of the popliteal lymph node assay, which quantifies the immune response to blood transfusion, in a sensitive and reproducible fashion. The subcutaneous injection of a 5 x 10(5) or 10 x 10(5) (depending on the rat strain) X-irradiated allogeneic lymph node cells into the hind footpad of a rat results in an increase in the weight of the draining popliteal lymph node harvested 7 days later. Injection of 100 x 10(5) X-irradiated cells results in the maximum nodal enlargement (fourfold). A dose of 50 x 10(5) cells results in lymph node enlargement just shy of maximum and so was selected for use in all experiments. When 50 x 10(5) nonirradiated allogeneic lymph node cells are injected, there is up to an eightfold increase in nodal size. To eliminate the effects of non-specific inflammation, as well as variation in node weights, a stimulation index (SI) was established. Transfusion of a rodent with blood from the same strain as the lymph node donor, 1 week prior to subcutaneous injection, resulted in a significantly lower SI, compared to the SI observed in untreated animals challenged with the same inoculum or third-party controls (P < 0.05). The significance of all comparisons was assessed by the Students' t test. This simple method of in vivo quantification of the immune response has obvious application to the evaluation of any experimental immunosuppression regimen.