RITs were labeled with the Alexa Fluor 647 Labeling Kit (Invitrogen) for 3.5 hrs and purified according to manufacturer’s instructions. Harvested cells were incubated for 30, 60 and 120 min at 37°C with 2 μg/ml of SS1P-Alexa647 or RG7787-Alexa647 and processed as previously described (17 (link)). Fluorescence intensity was analyzed on a FACSCalibur.