Evidence for in vivo degradation of C3a anaphylatoxin by mast cell chymase. I. Nonspecific activation of rat peritoneal mast cells by C3ades Arg.

The anaphylatoxin C3a (125I-HC3a) is extensively degraded when exposed to isolated rat mast cells (RMC). Degradation occurs without prior stimulation of these cells. The protease responsible for C3a degradation has been identified as chymase. Mixed peritoneal cells, containing equal numbers of mast cells compared with incubates containing more highly purified RMC, promoted less degradation of the C3a molecule than did the purified RMC. These variable levels of spontaneous activation (ie, chymase release) of RMC in vitro were concluded to be a function of the cellular handling and isolation procedures. No degradation occurred when low levels of 125I-HC3ades Arg (10(-8) mol/l [molar]) were introduced in the peritoneum of a rat, unless the mast cells were stimulated by prior introduction of specific activators. Evidence that the enzyme being released in the peritoneal cavity of rats was chymase was provided both by its appearance after adding specific mast cell activators (ie, compound 48/80 and anti-IgE [gamma E immunoglobulin]) and by inhibition with chymostatin. Because 125I-HC3ades Arg at low levels (10(-8) mol/l) was not degraded in the rat peritoneum, expression of chymase by the rat mast cells could be monitored in situ during mast cell stimulation. 125I-HC3ades Arg was rapidly converted (in 2 to 5 minutes) to smaller fragments in the peritoneal cavity of the rat after either 10 micrograms of compound 48/80 or anti-IgE was injected. Introduction of higher levels of HC3a or HC3ades Arg (2.5 to 5.0 x 10(-6) mol/l) to the peritoneal cavity of the rat stimulated both chymase release and HC3a degradation without other mast cell activators being present. HC3ades Arg was equally as effective in vivo as HC3a in stimulating the rat peritoneal mast cell to release chymase. It was concluded that the mechanism of RMC activation by C3a (C3ades Arg) was nonspecific and similar to the process by which polyamines and polycations stimulate mast cells, as C3a is a highly cationic molecule. The fact that C3a may in turn be destroyed in minutes by the recruited protease (chymase) defines a potentially important physiologic control mechanism. This cellular control process, demonstrated here for rat mast cells, may function in other animals, including man, for regulating tissue levels of factors such as the anaphylatoxins that are potentially capable of mast cell activation through nonspecific (polycationic) mechanisms.