During natural
infections, Leishmania is in contact with a variety of mononuclear phagocytic cells in different tissues, including resident macrophages and monocytes mobilized to the site of
infection from the bone marrow and blood circulation. Because the functional capabilities of fully differentiated macrophages and blood monocytes differ, the outcome of
infection by Leishmania may depend upon the stage of differentiation of the host cells. To address this question, we evaluated Leishmania panamensis
infection of (1) the human promonocytic/histiocytic cell line U-937 before and after induction of differentiation by
phorbol myristate acetate; (2) fresh human peripheral blood monocytes; and (3) macrophages derived from monocytes by differentiation in vitro. Based on the percentage of cells infected and the number of parasites per cell, macrophages derived from monocytes or by induction of differentiation of U-937 cells were significantly more permissive to
infection by stationary-phase L. (Viannia) panamensis promastigotes than monocytes. Increasing time and maturation in culture prior to exposure to infective promastigotes was associated with the increased permissiveness of differentiated macrophages to
infection (P<0.05). The percentage of cells infected and number of amastigotes per cell increased with time postinfection for both monocytes and macrophages but remained significantly greater for macrophages. The increased expression of CD68, CD16, and
lysozyme, and decreased expression of
peroxidase by macrophages cultured for 5 days in vitro compared with fresh monocytes, whether adherent or in
suspension, supported the distinct maturation status of these cells.