| Abstract: | Previous studies have shown that plasma membranes of murine lymphocytes and lymphoid tumor cells can reversibly inhibit the growth of both normal and transformed lymphocytes. The inhibitor can be extracted with organic solvents and has properties consistent with it being a lipid or lipid-like component of the membrane. This report identifies a series of cloned macrophage hybridoma cell lines, obtained by fusion of splenic adherent cells and the P388D1 line, which have very high levels of lipid-like growth-inhibitory molecules. Furthermore, a survey of seven cloned lines indicated that the macrophages fell into two distinct groups with regard to their level of growth-inhibitory activity. Group 1 lines had little or no inhibitory activity when cells were examined for their effect on a B lymphocyte proliferative response. Organic extracts from these macrophages had inhibitory activity (on a per cell basis) comparable to that seen with extracts of the P388D1 parental cell line and lymphoid tumor cells. In contrast, relatively low numbers of Group 2 macrophages could profoundly inhibit B macrophage proliferation. The growth-inhibitory activity was quantitatively recovered in organic extracts of the macrophages. Although the precise nature of the lipid moiety remains undefined, the data argue against the involvement of oxidized cholesterol. These findings indicate that lipid-like inhibitors of cell growth are present and functional in these macrophage cell lines. In addition, the results demonstrate that the inhibitory activity found in plasma membranes and liposomes is present and active in the membranes of intact cells, which is in contrast to the possibility that the inhibitor is an artifact generated during subcellular fractionation. Thus, the inhibitor is likely to have a physiologic role in growth control and in macrophage-mediated immunoregulation, probably acting via a mechanism involving cell-cell contact. |
