The stimulation of DNA synthesis in mouse (C57BL) macrophages explanted in vitro was demonstrated after treatment with conditioned medium or infection with SV40. In the latter case, induction of SV40 T antigen was detected before TdR-3H incorporation. Even though all macrophages were infected (T antigen-positive), they exhibited considerable pleomorphism, accompanied by functional differences. Permanent lines of SV40-transformed macrophages were eventually established, and one clone was isolated which replicates indefinitely and has many properties of primary macrophages: high acid phosphatase and phagocytic activity, lysozyme production, and specific antigenic determinants. These cells differ from normal macrophages in that they contain the SV40 genome, can be trypsinized, and do not require conditioned medium for continued replication.

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