THE USE OF THE LIPOPHILIC FLUOROCHROME CM-DII FOR TRACKING THE MIGRATION OF LYMPHOCYTES

In this study we examined the new cell dye CM-DiI for tracking the mig ration of lymphocytes from blood to lymph. This lipophilic marker inte rcalates in the plasma membrane like the PKH dyes and older DiI deriva tives. The stability and intensity of staining achieved with these dye s is better than most other fluorochromes or radioisotopes, yet they a re poorly soluble in aqueous solutions, which can make staining diffic ult, and they are not fixable in tissue sections. CM-DiI is reported t o have increased water solubility and it can be fixed using traditiona l aldehyde fixatives, making it feasible to detect labeled cells in hi stological sections, To determine the suitability of CM-DiI as a lymph ocyte marker, a labeling protocol was developed, We tested the ability of stained cells to recirculate in vivo. Following the intravenous in jection of CM-DiI positive cells, their recovery in lymph over 40 h wa s comparable to that of cells labeled with other fluorochromes or radi oisotopes, The kinetics of recirculation were also very similar, as la beled cells were detectable in lymph within 4 h of injection, and the peak percentage of labeled cells in lymph was generally observed betwe en 20-30 h, We also confirmed that CM-DiI is retained in the lymphocyt e membrane following routine paraffin processing, Thus CM-DiI does not appear to alter the process of lymphocyte recirculation, and it shoul d be a useful marker for tracking these cells.