The localization of antigen in lymph nodes and its relation to specific antibody-producing cells. II. Comparison of iodine-125 and tritium labels.

A branched multichain polypeptide of the type (Tyr,Glu)-pAla--pLys was synthesized from L-lysine, L-tyrosine, L-glutamic acid and tritium labelled DL-alanine; the final product, H-A--L, had a specific radioactivity about 3 mc/mg. Its immunological behaviour in mice was compared with that of another preparation of (T,G)-A--L trace labelled with I at a specific radioactivity of 2 mc/mg. The H-A--L proved to be only very weakly immunogenic compared with I-A--L. This was not attributable to its radioactivity, but probably to its relatively low tyrosine content. However, detailed autoradiographical studies of the localization of the two materials in the draining lymph nodes after injection into the footpads of previously primed and of unprimed mice revealed no qualitative differences between their behaviour, which resembled that previously described for I-A--L in similar experiments. When a preparation of (T,G)-A--L, labelled on the same molecules with both H and I, was studied in respect of gross retention of each label in lymph nodes, selective retention of H relative to I was observed. This was explained by greater susceptibility to peptidase activity of L-tyrosine residues at the ends of the side chains compared with that of the underlying polymeric DL-alanine. It is concluded that in studies of the fate of iodine-labelled peptides or proteins detection of the radioactive label is likely to indicate the presence of intact molecules or of large portions of them, but that failure to detect iodine cannot be taken to denote their absence. The autoradiographs suggested the existence of fine channels containing antigen penetrating through the cortical and intermediate zones of lymph nodes.