IN-111 LABELED LYMPHOCYTES - ISOTOPE DISTRIBUTION AND CELL-DIVISION

Since lymphocytes continue to proliferate and divide in vivo, it is im portant to determine the fate of a radionulide following lymphocyte la belling. Using the mixed lymphocyte reaction (MLR), we induced indium- 111 labelled lymphocytes from a specific in-bred rat strain (AS) to di vide and then observed the subsequent In-111 distribution between cell s and supernatant. L10 and L12.4 cells, which are allospecific CD4+ T lymphocytes from the AS rat, were stimulated in the MLR by antigen-pre senting cells from the August rat, a different strain. We labelled L10 or L12.4 lymphocytes on day 0, the first day of the stimulation cycle , and continued to culture the lymphocytes in vitro. The proliferation of the cells was estimated according to their increase in number. The distribution of In-111 between cell and supernatant fractions and bet ween viable and dead (but intact) cells was measured in the cell suspe nsion each day after labelling. The metabolic activity of In-111-label led lymphocytes was compared with control cells by measuring their upt ake of fluorine-18 fluorodeoxyglucose ([F-18]FDG). In-111-labelled lym phocytes showed a poor proliferative response compared with control ce lls 24-48 h after labelling but increased in number after this time. F rom 24 to 72 h, about 70% of In-111 was in the supernatant but only ab out 5%-10% was associated with intact dead cells. These dead cells ten ded to retain their In-111, losing less than 30% per day, suggesting t hat In-111 in the supernatant was the result of active elimination fro m viable cells, Moreover, 24 h after culture, considerably more In-111 was associated with viable than with dead lymphocytes, although over the next few days this distribution reversed. In-111-labelled lymphocy tes took up more [F-18]FDG than control cells at 24 h but not at 0 or 72-96 h; the maximum [F-18]FDG uptake coincided with the greatest redu ction in cell number. Furthermore, [F-18]FDG uptake correlated with th e initial In-111 burden in lymphocytes labelled with In-111 24 h previ ously. The results are consistent with active elimination of In-111 by In-111-labelled lymphocytes. The energy requirements for this are div erted away from cell division, thereby increasing the probability of c ell death, As lymphocytes become In-111 deplete, they recover their ca pacity to proliferate and their risk of death decreases, These finding s have important implications for In-111-labelled lymphocyte scintigra phy, suggesting that cells remaining viable immediately after labellin g will either subsequently die or alternatively eliminate the label.