| Abstract: | It has been suggested that the developmental maturation of Leishmania major promastigotes can affect their interaction with human complement receptors. To study this, we measured the adhesion of metacyclic and logarithmic-phase L. major promastigotes to complement receptors expressed on primary macrophages, to recombinant receptors expressed on transfected cells, or to purified complement receptors in a cell-free system. We demonstrate that complement-opsonized promastigotes can bind to both Mac-1 and complement receptor type 1 (CR1) and that the transition of promastigotes from the noninfectious logarithmic phase of growth to the infectious metacyclic stage does not affect this interaction. Furthermore, we show that Mac-1 and CR1 can cooperate to mediate the efficient adhesion of complement-opsonized metacyclic promastigotes to cells expressing both receptors. On human monocyte-derived macrophages, Mac-1 appears to make a quantitatively greater contribution to this adhesion than does CR1, since blocking macrophage Mac-1 diminishes metacyclic promastigote adhesion to a greater extent than does blocking CR1. In addition, bovine monocytes lacking Mac-1 exhibit a dramatic decrease in complement-dependent promastigote adhesion, relative to normal monocytes. The predominance of Mac-1 in these interactions is due, at least in part, to the factor I cofactor activity of CR1, which facilitates the conversion of C3b to iC3b. The stable adhesion of complement-opsonized metacyclic promastigotes to Mac-1 is a prerequisite for phagocytosis by human monocyte-derived macrophages. Blocking Mac-1 on macrophages abrogates the majority of the complement-dependent phagocytosis of promastigotes, whereas blocking CR1 has no detectable effect on phagocytosis. In addition, bovine monocytes lacking Mac-1 exhibit a dramatic reduction in promastigote phagocytosis relative to normal bovine monocytes. We conclude, therefore, that the two complement receptors, Mac-1 and CR1, can cooperate to mediate the initial complement-dependent adhesion of metacyclic promastigotes to human monocyte-derived macrophages and that Mac-1 is the predominant complement receptor responsible for the phagocytosis of complement-opsonized metacyclic promastigotes. |
