yielded hybrids in which the macrophage-specific traits were
often extinguished. More recently we have isolated hybrids of
this type which continue to secrete lysozyme and to express Fc
receptors and cell-specific antigens. Such hybrids, however, show a variant phenotype in that Fc receptors result in bind-ing of antibody-coated red cells, but fail to internalize these particles. The defect in ingestion is selective since the hybrids take up latex and immune complexes. These vari-ants should provide useful insights into normal cellular func-tion.
The lack of subset heterogeneity in mononuclear phagocytes makes it difficult to distinguish the parental origin in
mac-rophage X macmac-rophagelike hybrids. Species differences can be useful in this regard. The absence of a receptor for glyco-conjugates on the mouse macrophage line J774 has recently made it possible to rescue this marker by hybridization with rat alveolar macrophages.
Although several examples are known in which a genetically determined deficiency in the host, such as complement bio-synthesis or susceptibility to virus infection, is expressed by macrophages in culture, the cellular mechanisms remain to be characterized. The macrophage cell lines now available al-so lend themselves to mutagenesis and selection of variants with an interesting phenotype. It is, however, important to recognize that such variants are often complex and may express a variety of defects in addition to the one under investiga-tion.
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