Rapid quantitative method for measuring phagocytosis of Leishmania promastigotes using a double radiolabelling method

A double radiolabelling method is described for the measurement of phagocytosis of Leishmania major promastigotes in cultures of murine resident peritoneal macrophages. L. major promastigotes were radiolabelled during exponential growth in RPMI supplemented with [125I]5-iodo-2-deoxyuridine. They were used to infect sodium [51Cr]chromate-labelled macrophages. Phagocytosis was evaluated by measuring the radioactivity of the 125IUdR-labelled parasites detectable inside 51Cr-labelled macrophages by a Beckmann gamma 5500 counting system. This was able to count simultaneously, in two different windows the radioactivity of (a) the parasites and (b) the cells. The technique compares favorably with the conventional light microscopic technique and appears to be more sensitive, totally objective, and easy to use for the rapid analysis of multiple samples. Furthermore, the double radiometric method permits a more precise distinction between adherent and engulfed organisms than does the microscopic assay.