No abstract

Human C5a, a complement-derived anaphylatoxin, is a potent mediator of human leukocyte chemotaxis. Using a homogeneous preparation of C5a that was 125I-labeled, we have demonstrated the presence of a specific cellular receptor for this glycoprotein on intact human polymorphonuclear leukocytes. Cellular uptake of the radiolabeled ligand occurred rapidly and the rate of dissociation was extremely slow. Cellular binding was saturable with respect to 125I-labeled C5a, and half-saturation occurred at a concentration of 3-7 X 10-9 M.

Measurable complement activation resulting in the formation of both C3a and C5a anaphylatoxins was observed in 12 patients undergoing maintenance dialysis treatment with cuprophan hollow fiber dialyzers. Specific radioimmunoassay measurements demonstrated that these patients displayed significantly elevated levels of C3a antigen in the venous (outflow) line of the dialyzer after only 2 min of dialysis. During hemodialysis, venous plasma C3a levels continued to increase and became maximally elevated after 15 min. Thereafter, C3a concentrations gradually declined, suggesting that the rate of complement activation abates with continued cuprophan hemodialysis. Complement activation, as judged by venous plasma C3a levels, was also temporally correlated with hemodialysis leukopenia. The factor believed to be responsible for pulmonary vascular leukosequestration, C5a, could also be detected in venous plasma, but levels of this antigen were not strikingly elevated until the later stages of dialysis. By contrast, six patients dialyzed with polyacrylonitrile dialyzers failed to exhibit hemodialysis leukopenia and displayed only very modest increases in their plasma C3a levels during the initial phases of hemodialysis. These observations provide direct evidence that anaphylatoxin formation during hemodialysis is a transient phenomenon and indicate that the biocompatibility of dialysis membranes, as reflected by their complement activating potential, may be significantly different.

Virulent group A streptococci have been found to express a cell-surface factor that has the capability of inactivating complement-derived chemotactic factors. To determine the mechanism ofaction ofthis factor, we examined the interaction of purified inactivator with pure C5a chemotaxin. Ligand-receptor binding studies demonstrated that streptococcal chemotactic factor inactivator (SCFI)-treated C5a expressed a greatly reduced ability to bind to receptors of polymorphonuclear leukocytes as compared with native C5a.The inactivation of C5a occurred by a nonstoichiometric and temperature-dependent process. NaDodSO4/PAGE analysis indicated that SCFI mediated a small decrease in the molecular weight of C5adArg, and sequencing of the carboxyl terminus of inactivated C5a demonstrated that a six-residue peptide was lost. The release of discrete peptide fragments from denatured bovine serum albumin upon prolonged incubation with SCFI was indicative of endoprotease activity. Although denatured bovine serum albumin was inefficiently cleaved, native bovine serum albumin and other native proteins were highly resistant to SCFI proteolysis; this indicated that activity was specific in nature.

We observed complement activation in 15 adults undergoing total cardiopulmonary bypass. Plasma levels of C3a were significantly elevated (P < 0.0001) at the beginning of the procedure, and they continued to increase steadily thereafter. At the end of the procedure, C3a levels were more than five times higher than preoperative levels. Plasma levels of C5a (a factor that binds avidly to neutrophils) did not change significantly during cardiopulmonary bypass. Instead, there was significant neutrophilia (P = 0.03) during bypass, and significant transpulmonary neutropenia (P = 0.0002) occurred when cardiopulmonary circulation was reestablished at partial bypass. The neutropenia is consistent with pulmonary-vascular sequestration of C5a-activated granulocytes. We also found that incubation of blood with the nylon-mesh liner of bubble oxygenators, as well as vigorous oxygenation of whole blood, promotes conversion of complement. We conclude that the complement-derived inflammatory mediators C3a and C5a produced during extracorporeal circulation may contribute to the pathogenesis of "post-pump syndromes."