Human Monocytes CD36 and CD16 Are Signaling Molecules. Evidence from Studies Using Antibody-induced Chemiluminescence As a Tool to Probe Signal Transduction

A panel of monoclonal antibodies (mAb) directed against human monocyte surface antigens was tested for their capacity to mediate signal transduction by measuring luminol-enhanced chemiluminescence (CL). The response patterns fell into three categories. The mAb Mo4, OKM3, OKM6 and antibodies specific for Fc receptor (FcR) type I and II did not mediate signal transduction directly or were weak triggers, but upon second-order cross-linking by goat anti-mouse immunoglobulin (Ig) F(ab')2 or rabbit anti-mouse Ig, a strong CL response was induced. The mAb recognizing different epitopes on CD11b (complement receptor type III alpha chain) were unique in their ability to induce a CL response either by direct stimulation or by second-order cross-linking. The mAb 3G8 recognizing FcR type III (FcRIII; CD16) on a monocyte subpopulation and CD36-specific monoclonals directly elicited a CL response. A response of similar magnitude was obtained with 3G8 F(ab')2 or with intact 3G8. Furthermore, elutriation-centrifugation-purified monocytes were stimulated by 3G8 to a similar extent as unseparated mononuclear cells, whereas lymphocytes did not respond. This suggests that a FcRIII-expressing monocyte subpopulation may mediate effector functions, including the generation of reactive oxygen species, via FcRIII triggering. Our finding that anti-CD36 F(ab')2 directly induces an oxidative burst is the first evidence that CD36 itself is a trigger molecule. CD36, which is thought to interact with erythrocytes infected with Plasmodium falciparum and with thrombospondin, may constitute a signal-transducing element and thus may have functions extending beyond mediation of adherence. The present CL system constitutes a simple assay for detecting mAb directed against monocyte surface signalling elements. Probing mAb for signal transduction requires suspended cells and antibodies in the fluid phase in order to avoid inadvertent FcR triggering, which may occur when cells are stimulated by surface-adherent whole antibodies.