Factors Affecting the In Vitro Activation of Resident Peritoneal Macrophages by Mouse Interferon

Abstract

Resident normal peritoneal macrophages (NpMØ) from BALB/c mice incubated in vitro in the continuous presence of 10² or 10³ units per ml of murine fibroblast derived interferon (MuIFN) became cytotoxic for tumor cells, L1210R (34% and 43% tumor cell reduction, respectively). Since L1210R cells are not sensitive to the actions of IFN, the direct effect of IFN on these cells was ruled out as a cause of tumor cell susceptibility to the cytotoxic effects of IFN activated MØs. IFN activity on NpMØ was not dependent on lipopolysaccharide (LPS). However, when IFN was present continuously, exogenously added LPS (2 ng/ml) acted synergistically with it to enhance the activation level of NpMØs. MØs treated only with LPS were not activated. When MØs were treated for only 24 h with 10³ units IFN/ml and then assayed for cytotoxicity against the tumor cell P815-2 in the absence of IFN, there was only an 8% decrease in cell numbers. However, when 200 or 750 pg of LPS/ml were present during the 24 h treatment with IFN, the decrease in tumor cell numbers rose to 23% and 43%, respectively. This synergistic effect of LPS was only necessary during the 24 h treatment step, and could be blocked by the addition of 0.5 μg Polymyxin B/ml. In contrast, MØs treated for 24 h with 10⁴ units IFN/ml and then assayed for cytotoxicity in the absence of IFN reduced tumor cell number by 58% and at this concentration of IFN the modulating effect was not dependent upon LPS. It was concluded that there were conditions under which IFN can activate MØs directly and independently of LPS, whereas under other situations both IFN and LPS could act synergistically and both were necessary to allow expression of extensive cytotoxicity by MØS towards tumor cells. In addition, under circumstances in which IFN could act independently of LPS, the additional presence of LPS would enhance the degree of MØ activation in a synergistic fashion. Thus, it was suggested that LPS acts synergistically with IFN possibly at two levels, that of enhancing the degree of macrophage activation per se and that of prolonging the time period or duration of this state.