Binding of Mycobacterium avium-Mycobacterium intracellulare to human leukocytes

We examined nonopsonic binding of Mycobacterium avium-Mycobacterium intracellulare (MAI) by human leukocytes. Macrophages (M phi) avidly bound fluorescently labeled MAI in the absence of serum proteins. Binding appeared to be mediated by a lineage-specific, proteinaceous receptor on M phi, since (i) binding of labeled bacteria could be competitively inhibited by unlabeled MAI, (ii) treatment of M phi with trypsin ablated the ability of M phi to bind MAI, and (iii) the capacity to bind MAI was observed on monocytes, M phi, and stimulated polymorphonuclear cells but not on lymphocytes or unstimulated polymorphonuclear cells. The receptor for MAI appeared mobile in the plane of the membrane, since spreading of M phi on a carpet of immobilized, unlabeled MAI down modulated binding of labeled MAI added in suspension. The receptor required neither calcium nor magnesium for activity and appeared different from other known receptors for intracellular pathogens.