Previous studies have shown that monocyte production during an inflammatory response is controlled by the factor increasing monocytopoiesis (FIM), secreted by macrophages at the site of inflammation. The inflammatory reaction to latex particles and a saline-soluble extract of Listeria monocytogenes (SEL), expressed as the number of monocytes in the circulation and of macrophages at the site of inflammation, was about twice as strong in C57BL/10 mice compared with CBA mice. This raised the question as to the mechanism underlying these differences. One possibility might be that these mouse strains differ with respect to the production of FIM, but this cannot be the case because the maximum levels of FIM activity in the serum of both C57BL/10 and CBA mice given latex or SEL intraperitoneally were almost the same; however, the courses of FIM activity in the two strains after intraperitoneal latex were not exactly synchronous. Another possibility is that the sensitivity of monocyte precursor cells for FIM differs. Evidence for the latter was provided by the finding that the intravenous injection of sera with FIM activity obtained from C57BL/10 and from CBA mice into the C57BL/10 mice evoked monocytosis, whereas CBA mice did not respond to these sera. Earlier studies showed that an increase of monocytes after the injection of serum containing FIM reflects increased monocyte production. Taken together, the results of the present study demonstrate that one of the mechanisms underlying the genetic control of the inflammatory response is, rather than enhanced FIM synthesis, the ability of monocyte precursors in the bone marrow to respond to FIM by increased monocyte production.

The regulatory mechanisms that determine the course of an inflammation induced by an intraperitoneal injection of kaolin were investigated in Listeria-susceptible CBA and Listeria-resistant B10 mice. The magnitude of the granulocyte inflammatory response in the peritoneal cavity was high in B10 mice (area under the curve; AUC0-48 h: 210.9 x 10(6) granulocytes/mouse x h) and lower in CBA mice AUC0-48 h: 136.8 x 10(6) granulocytes/mouse x h), whereas the reverse was seen for the granulocyte response in the peripheral blood (AUC0-48 h: 30.5 and 80.7 x 10(6) granulocytes/mouse x h, respectively). With respect to the presence of humoral factors that affect the number of granulocytes in the circulation, sera of both mouse strains sampled 24 h after the kaolin injection had granulocytosis-inducing effect in CBA recipient mice and did not induce a response in the B10 recipient mice. This divergent sensitivity to serum factors inducing granulocytosis is consistent with the difference in the blood granulocyte response of B10 and CBA mice but does not explain the divergent inflammatory responses in the peritoneal cavity. Computer simulation showed that at least two factors must be taken into consideration to explain the differences in the inflammatory response, i.e., a factor regulating the release of granulocytes from the bone marrow and a factor governing the rate of granulocyte efflux from the site of inflammation.