Complement activation by cell-associated immune complexes in contact sensitivity

Lymph node cells collected 4 days after painting the skin with picryl chloride activate the first components of the classical pathway of complement cascade, as shown by consumption of C4 of rabbit complement with total sparing of C5 and factor B activity. In contrast, lymph node cells collected 1 or 6 days after sensitization fail to do so. The ability of “4-day” cells to activate complement is inhibited by treating the cells with specific low-molecular-weight hapten, which is known to dissociate the immune complex present on the cell surface. When mouse serum was used as source of complement, a different behavior in complement activation between CBA/J and B.10.D2-New/SnJ serum was observed: “4-day” cells failed to consume CBA/J serum whereas a normal complement activation was detected when B.10.D2-New/SnJ serum was used. Using these two sera which differ in the level of C4, an inverse relationship between the ability of “4-day” cells to activate complement and their capacity to induce contact sensitivity when injected into the footpad of normal recipients was reported. Experiments performed using sera from C5 genetically deficient mice demonstrate that only the early complement components are involved, suggesting that membrane immune complexes are solubilized as a result of complement activation; on the other hand, membrane bound activated complement components could alter the immunizing potential of “4-day” cells.