Bone marrow-derived mononuclear phagocytes autoregulate mannose receptor expression

This study extends our previous observation that surface mannose receptor expression by pure populations of CSF-1-dependent bone marrow-derived macrophages increases with time (Clohisy, D.R., Bar-Shavit, Z., Chappel, J.C., and Teitelbaum, S.L. (1987) J. Biol. Chem. 262, 15922-15929). We presently find, however, that the progressive enhancement of ¹²⁵I-mannose-bovine serum albumin (¹²⁵I-Man-BSA) binding per cell reflects cell number rather than duration of culture. In fact, macrophages plated at high density 8-fold more ¹²⁵I-Man-BSA than do their low density counterparts, with no difference in receptor-ligand affinity. Furthermore, cells cultured at high-density are ultimately subjected to lower levels of exogenously provided macrophage growth factor, and fewer are in interphase. By obtaining synchronous populations of quiescent bone marrow macrophages, however, we demonstrate that neither cell cycling nor attendant levels of colony stimulating factor-1 influence mannose receptor expression. Our next series of experiments established that density-related mannose receptor expression reflects removal, by marrow macrophages, of a 'down-regulating' factor contained in culture medium. To this end, we treated mononuclear phagocytes with either macrophage- or control-conditioned medium and found that, via a fetal calf serum-residing protein(s), only control medium is capable of noncompetitively reducing ¹²⁵I-Man-BSA binding in a dose-dependent manner. Moreover, reconstituted 20-40% (NH₄)₂SO₄-precipitable fractions derived from either sham-conditioned medium or fetal calf serum are capable of down-regulating mannose receptor expression. Alternatively, the same fraction obtained from macrophage-conditioned medium contains no such activity. Finally, initial characterization of the down-regulating factor reveals it to be acid-activable and trypsin-sensitive, yet resistant to heating to at least 80°C, ribonuclease A, or freezing and thawing. We conclude that bone marrow macrophages up-regulate expression of their own plasma membrane mannose receptor by inactivating a noncompetitive, serum-residing inhibitory protein(s).