Macrophage turnover in inflamed connective tissue

Abstract

It has previously been shown that macrophages in various types of inflamed tissue proliferate, whereas macrophages on subcutaneously inserted glass cover slips showed little such activity. This discrepancy suggested that macrophage proliferation in inflammation might be a complex and variable process. The present investigation indicates that inflammatory macrophages that have entered the mitotic cycle fall into one of three categories; cells newly arrived from the circulation, cells forming a high turnover pool heavily dependent on recruitment from the bone marrow, and cells forming a self-sufficient population largely independent of marrow recruitment. In reactions provoked by subcutaneous insertion of cover slips, uptake of a pulse of [$^{3}$H]T by perivascular but haematogenous macrophages begins at 4 h but is not evident in macrophages adhering to the cover slip until 2 to 4 days. Contrary to previous belief, DNA synthesis in multinucleate giant cells does occur but not until the cells are about 14 days old. DNA synthesis is frequently synchronous in the nuclei of a given cell. The percentage of connective tissue and cover-slip macrophages undergoing DNA synthesis becomes equal at 4 to 6 days. Selective X-irradiation of the bone marrow or the reaction site showed that the cells remain in the tissues or on the cover slip for only one or two days and that the lesion is totally dependent on daily recruitment from the marrow. Turnover is highest in the first few days of the lesion. The newly arrived macrophages that monopolize these early lesions have the curious property of resisting removal from the granuloma by dissection, whereas established arrivals are teased out with ease. The presence or absence of DNA synthesis in viable macrophages is determined partly by factors within the cell and partly by its environment. This was shown by transferring coverslip macrophages from early lesions to more mature reactions and vice versa and to tissue culture. Early macrophages unable to enter the mitotic cycle retained this disability under all circumstances. Macrophages from older reactions that normally synthesize DNA continued to do so in tissue culture but ceased this activity on transfer to early inflammatory lesions, indicating the inhibitory effect of this particular environment. Peritoneal macrophages that synthesize DNA very poorly in the normal peritoneum and in tissue culture entered the mitotic cycle in large numbers when transferred to inflammatory reactions old enough to have ceased to inhibit DNA synthesis. Similarly, blood monocytes that show even less proliferation than peritoneal cells in vitro, exhibited intense DNA synthesis 2 days after implantation into inflamed subcutaneous tissue. Since small numbers of monocytes gave rise, within a few days, to large numbers of macrophages, an association was established between incorporation of [$^{3}$H]T into nuclear DNA and net increase in cell numbers. The technique of selective X-irradiation and dissection were applied to various types of dermal and subcutaneous inflammation. In reactions to bovine fibrinogen all DNA-synthesizing macrophages were found to be of the newly arrived type. More unexpectedly in even long-established reactions to B. pertussis vaccine, almost all the macrophages were again of this type, presumably because of the toxicity of the killed organisms. Granulomata due to incomplete Freund adjuvant were similar to the reactions provoked by subcutaneous cover slips in that once the lesion was established most of the macrophages belonged to a high-turnover pool continuously replenished by fresh recruits from the marrow. Carrageenan granulomata revealed the existence of a third type of macrophage population, largely independent of further marrow recruitment. Presumably the low toxicity of carrageenan permits the expression of macrophage longevity which augmented by occasional mitotic activity maintains a self-sufficient population.