DIFFERENTIAL CYTOKINE REGULATION OF COMPLEMENT C3, C4, AND FACTOR-B SYNTHESIS IN HUMAN INTESTINAL EPITHELIAL-CELL LINE, CACO-2

In the intestinal tract, the local synthesis of C3 and components of b oth the classical (C4) and alternative (factor B) C activation pathway has previously been demonstrated in vivo. However, the cellular sourc e of this local C synthesis has not been identified. In this study, we demonstrated the syntheses of C3, C4, and factor B in the human colon ic adenocarcinoma cell line Caco-2, which is regarded as a good experi mental model of normal human intestinal epithelial cells. The results of metabolic labeling experiments indicated that the intra- and extrac ellular molecular sizes and subunit structures of Caco-2-derived C3, C 4, and factor B were compatible with previously reported values for th ese components in other cells. The functional activities of C3 and C4 in the supernatants were also demonstrated by hemolytic titration assa y. Furthermore, C syntheses in this line were independently up-regulat ed by several human cytokines: C3 synthesis was dose-dependently enhan ced by the addition of IL-1beta or TNF-alpha; C4 synthesis was enhance d by the addition of IL-6 or IFN-gamma in the same manner; and the add ition of IL-1beta or IL-6 also induced a dose-dependent increase in fa ctor B synthesis. These enhancing effects were confirmed to be specifi c for individual cytokines by experiments using anti-human cytokine an tibodies. It is likely that intestinal epithelial cells are local prod uction sites of C3, C4, and factor B, and that local C syntheses in th e intestine are independently regulated by several cytokines, derived from monocytes/macrophages and T cells resident in the mucosal microen vironment.