Early local generation of C5a initiates the elicitation of contact sensitivity by leading to early T cell recruitment

We have shown previously that an early complement C5-dependent cascade is r equired to recruit T cells to elicit 24-h contact sensitivity (CS) response s. In this paper, we have characterized molecular events of this early requ ired cascade by biochemically analyzing extracts of mouse ears undergoing e licitation of CS, Chemotactic activity was found after local Ag challenge, in CS ear extracts early (by 1 h), in CS ear extracts late (through 24 h), in previously immunized mice, but not in ears of vehicle-immunized or non-i mmune challenged mice. The early chemotactic activity at 2 h was likely cau sed by C5a, because it was neutralized in vitro. by anti-C5a Ab, was inacti ve on C5aR-deficient (C5aR(-/-)) macrophages, and was absent In C5-deficien t mice. The activity was present in T cell-deficient mice, but elaboration was Ag-specific. This T cell-independent, Ag specific elaboration of C5a ea rly in CS ear responses likely led to T cell recruitment, because subsequen t local IFN-gamma mRNA and protein expression, as markers of T cell arrival and activation, began by 4 h after Ag challenge. In contrast to early C5a chemotactic activity, late chemotactic activity 24 h after Ag challenge was unaffected by anti-C5, was active on C5aR(-/-) macrophages, was T cell-dep endent, and by ELISA appeared largely due to chemokines (macrophage-inflamm atory protein-1 alpha and 1 beta, IFN-gamma-inducible protein-10, and monoc yte chemoattractant protein-1). Importantly, early generation of C5a was re quired for T cell recruitment because C5aR(-/-) mice had absent 24-h CS, Ta ken together, these findings indicate an important linkage of C5a as a comp onent of early activated innate immunity that is required for later elicita tion of acquired T cell immunity, probably by facilitating the initial recr uitment of T cells into the Ag-challenged local site in CS responses.