Increased activity of FIM in serum of mice during a Mycobacterium bovis (BCG) infection.

In mice given an intravenous injection of Mycobacterium bovis (BCG), the bacilli proliferated in the spleen, liver and lungs but the peritoneal cavity remained sterile. The numbers of blood monocytes and alveolar macrophages were increased during the first 2 weeks of the infection, whereas the number of peritoneal macrophages remained constant. To study whether factor-increasing monocytopoiesis (FIM) plays a role in the regulation of the monocytosis during the BCG infection, the activity of this factor in the serum of mice at various intervals during the infection was determined. Previous studies have shown that FIM stimulates monocyte production by its effect on the mitotic activity of monoblasts and promonocytes in the bone marrow. The FIM activity of the serum reached a maximum on Day 4 and remained elevated during the first 21 days of the BCG infection. Since FIM is synthesized and secreted by macrophages that have phagocytosed opsonized particles, it is highly probable that FIM occurring in serum originates from macrophages that have ingested BCG. The results of the present study led to the conclusion that FIM plays a role in the monocytosis developing during infection with BCG.