The entry of lymphocytes into stimulated lymph nodes. The site of selection of alloantigen-specific cells.

During a primary response to antigen, lymph nodes (LN) receive antigen-specific lymphocytes (ASL) from the recirculatory pool via specialized postcapillary venules (SPCV). Preliminary experiments demonstrated a three-fold increase in entry of 51Cr-labeled thoracic duct lymphocytes (TDL) into the rat popliteal LN 3-4 days after alloantigenic stimulation. This increased entry was accounted for by an associated 4-5-fold increase in LN blood flow. It was unclear whether recruitment of ASL occurred within the SPCV lumen leading to selective entry of ASL--or, more conventionally, that entry of lymphocytes was a random process and that recruitment of ASL occurred in the extravascular space by selective retention. Thus a double-labeling technique was used to study the migratory characteristics of two populations of thoracic duct lymphocytes (TDL) that had been passaged from blood to lymph. The first population, passaged through a semiallogeneic animal, was negatively selected, and thus 99% depleted of ASL; the second population, passaged through a syngeneic animal, contained a normal complement of naive ASL. The two populations were combined before i.v. injection into syngeneic recipients. At 24 hr the right popliteal LN, which had previously been stimulated with the specific antigen, showed a preferential localization of the TDL containing ASL compared with the left (unstimulated) popliteal LN. This preferential localization was not present at 1 hr after injection. In the light of background data on the kinetics of lymphocyte traffic it is concluded that ASL were selected not at the luminal surface of SPCV but in the LN after they had left the bloodstream.