Indium-111 labelled lymphocytes: Isotope distribution and cell division

Since lymphocytes continue to proliferate and divide in vivo, it is important to determine the fate of a radionulide following lymphocyte labelling. Using the mixed lymphocyte reaction (MLR), we induced indium-111 labelled lymphocytes from a specific in-bred rat strain (AS) to divide and then observed the subsequent In-111 distribution between cells and supernatant. L10 and L12.4 cells, which are allospecific CD4+ T lymphocytes from the AS rat, were stimulated in the MLR by antigen-presenting 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 between viable and dead (but intact) cells was measured in the cell suspension each day after labelling. The metabolic activity of In-111-labelled lymphocytes was compared with control cells by measuring their uptake of fluorine-18 fluorodeoxyglucose ([F-18]FDG). In-111-labelled lymphocytes showed a poor proliferative response compared with control cells 24-48 h after labelling but increased in number after this time. From 24 to 72 h, about 70% of In-111 was in the supernatant but only about 5%-10% was associated with intact dead cells. These dead cells tended to retain their In-111, losing less than 30% per day, suggesting that In-111 in the supernatant was the result of active elimination from 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 lymphocytes 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 reduction in cell number. Furthermore, [F-18]FDG uptake correlated with the initial In-111 burden in lymphocytes labelled with In-111 24 h previously. The results are consistent with active elimination of In-111 by In-111-labelled lymphocytes. The energy requirements for this are diverted away from cell division, thereby increasing the probability of cell death, As lymphocytes become In-111 deplete, they recover their capacity to proliferate and their risk of death decreases, These findings have important implications for In-111-labelled lymphocyte scintigraphy, suggesting that cells remaining viable immediately after labelling will either subsequently die or alternatively eliminate the label.