Macrophage-melanocyte heterokaryons. II. The activation of macrophage DNA synthesis. Studies with inhibitors of RNA synthesis

Mouse peritoneal macrophages, which do not synthesize DNA in vitro, were fused with melanocytes, a mouse cell strain which proliferates rapidly in vitro. DNA synthesis was induced in macrophage nuclei 2-3 hr after fusion and occurred irrespective of the number of macrophage nuclei present per melanocyte nucleus in each heterokaryon. 50-80% of macrophage nuclei initiated DNA synthesis in the 3-7 hr period after fusion. The activation of most 11-12-day chick red cell nuclei in melanocyte cytoplasm took longer than 10 hr. The lag before DNA synthesis may reflect the heterochromatin content of each nucleus. Studies with actinomycin showed that heterokaryon RNA synthesis was essential for subsequent macrophage DNA synthesis. This RNA was synthesized 1-4 hr before the DNA and was unlikely to be ribosomal RNA, since it was insensitive to <0.1 microg/ml actinomycin. Melanocytes and macrophages were treated before fusion with actinomycin and bromotubercidin to bring about a more selective inhibition of RNA synthesis. Macrophages pretreated for 1 hr with 5 microg/ml of actinomycin showed less than 20% of control RNA synthesis in the first 4 hr after fusion, but a normal activation of macrophage DNA synthesis. Pretreatment of melanocytes for 3-7 hr with 5 microg/ml bromotubercidin, a reversible inhibitor of RNA synthesis, prevented macrophage DNA synthesis without affecting macrophage RNA synthesis in the heterokaryons (81% of control). These studies showed that only melanocyte RNA synthesis was essential for the production of macrophage DNA. The exposure of one cell partner to actinomycin before fusion caused cross-toxicity of the untreated nucleus after fusion. Bromotubercidin, an adenosine analogue which is incorporated into RNA, did not give rise to such cross-toxicity after fusion. Once the macrophage nucleus becomes activated in the heterokaryon it becomes less sensitive to the action of actinomycin.

Macrophage-melanocyte heterokaryons. I. Preparation and properties

High yields of mouse macrophage-melanocyte heterokaryons and macrophage-macrophage homokaryons were obtained through the virus-induced fusion of cells spread on a glass surface. After fusion there was a striking reorganization of cellular architecture by means of a colcemid-sensitive process. Heterokaryons were isolated through the use of differential trypsinization and many underwent division to form melanocyte-like hybrids. The selective uptake of dextran sulfate by macrophages served as a useful cytoplasmic marker in identifying hybrids. Many characteristic macrophage properties were altered in the heterokaryons. Within an hour of fusion macrophage nuclei became swollen, nucleoli were more prominent, and increased nuclear RNA synthesis occurred. 3 hr after fusion, a wave of DNA synthesis took place in the previously dormant macrophage nuclei. The fate of typical macrophage markers was examined in both heterokaryons and homokaryons. Macrophage homokaryons continued to exhibit active phagocytosis of sensitized erythrocytes, whereas this capacity was lost irreversibly in heterokaryons. The loss of phagocytic activity of heterokaryons occurred at an exponential rate and was accelerated by high concentrations of calf serum. Another macrophage surface marker, a divalent cation-dependent adenosine triphosphatase (ATPase), could be demonstrated histochemically on heterokaryons. Shortly after fusion, it was present in discrete regions, but it became more diffuse and disappeared within a day. Acid phosphatase-positive secondary lysosomes and retractile lipid droplets disappeared from heterokaryons but continued to accumulate in macrophage homokaryons. These observations indicate that typical macrophage properties cease to be expressed in heterokaryons, and melanocyte functions presumably predominate in heterokaryons and hybrids.

Comparison of the tryptic peptides and amino acid composition of the beta polypeptide chains of the three common haptoglobin phenotypes

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Recombination of heavy and light chains from human immunoglobulins

The recombination of reduced, alkylated heavy and light chains from three IgG, two IgM and one IgA pathological protein and from normal IgG has been quantitatively studied.

All γ, α and μ chains showed some recombination with all κ and λ chains tested. One of the IgG and two of the IgM proteins showed more than 50 per cent recombination and in these there was evidence of a selective reconstitution of autologous chain pairs; the structural features which determine re-association appear to be independent of the electrophoretic mobility, or antigenic and allotypic specificity of the interacting chains. The chains of normal IgG and of three myeloma proteins showed a low percentage recombination and these autologous chain pairs did not combine preferentially. It appears that immunoglobulin chains show a variable ability to regain their native configuration after isolation.

Two pathological κ chains which were identical in electrophoretic mobility and Inv specificity but not equivalent in re-association behaviour showed well marked differences in tryptic peptide patterns. This indicates that the number of chemically distinct human light chains exceeds the forty variants previously postulated on the basis of antigenic, allotypic and electrophoretic differences.