Differential responses of human mononuclear phagocytes to mycobacterial lipoarabinomannans: Role of CD14 and the mannose receptor

CD14 is a signaling receptor for both gram-negative bacterial lipopolysaccharide (LPS) and mycobacterial lipoarabinomannan (LAM) that lacks terminal mannosyl units (AraLAM). In contrast, terminally mannosylated LAM (ManLAM) binds the macrophage mannose receptor (MMRc), although the ability of the MMRc to serve as a signaling receptor has not been previously reported. We compared the abilities of AraLAM and ManLAM to induce distinct responses in two monocytic cell populations, freshly isolated human peripheral blood monocytes (PBM) and monocyte-derived macrophages (MDM). The responses examined were chemotaxis and transient changes in free cytosolic calcium ([Ca2+](in)). We found that AraLAM but not ManLAM was chemotactic for both PBM and MDM, Migration of these cells in vitro to AraLAM mas specifically blocked by an anti-CD14 monoclonal antibody, suggesting that CD14 mediates the chemotactic response to AraLAM. Subsequently, we found that AraLAM induced a transient rise in [Ca2+](in) levels within a subpopulation of PBM but not MDM. This response was blocked by anti-CD14 antibodies, In contrast, ManLAM induced a transient rise in [Ca2+](in) levels within a subpopulation of MDM but not PBM. This response was blocked by either anti-CD14 or anti-MMRc antibodies. These data suggest that the MMRc can serve as a signaling receptor and that coligation of both CD14 and the MMRc is required to elicit a specific response, Thus, one response to LAM (chemotaxis) can be elicited solely by engaging CD14, whereas a different response (changes in [Ca2+](in) levels) depends on both the differentiation state of the cells and concomitant engagement of CD14 and the MMRc.