Complement proteins and macrophages. 1. Quantitative estimation of factor B produced by mouse peritoneal macrophages

Factor B, the complement alternative pathway serine proteinase, is a major constitutive protein synthesized and secreted by resident and elicited mouse macrophages

Factor B, the complement alternative pathway serine proteinase, a class III gene product of the major histocompatibility complex, is a major constitutive secretion product of mouse mononuclear phagocytes. This glycoprotein was synthesized and secreted by macrophages as a doublet of Mr 90,000 and 93,000 polypeptides that were immunoprecipitable with antibodies raised to human serum factor B, and that were indistinguishable from plasma factor B by immunoreactivity, peptide mapping, and molecular weight. Macrophage factor B was cleaved and activated to factor Bb- and Ba-like fragments by factor D and cobra venom factor. Some conversion of macrophage factor B to Bb-sized fragments occurred spontaneously in the conditioned culture medium after several hours. Factor B represented approximately 0.5% of newly synthesized protein and 4-6% of the secreted protein of resident peritoneal macrophages and macrophages elicited with thioglycollate broth, pyran copolymer, NaIO4, bacillus Calmette-Guerin, or Corynebacterium parvum. We detected synthesis of factor B immediately upon explanting these macrophages in culture; synthesis continued for several days in culture. The rate of secretion of factor B, as a proportion of total protein secretion in culture, remained constant with time. By radioimmunoassay, factor B antigens accumulated in the 24-h macrophage-conditioned culture medium at 2-10 nM, and was present in cell lysates at 4-8 nmol per 10(6) cells. We detected synthesis of factor B in bone marrow-derived macrophages as early as 5 d of culture. The P388D1 macrophage line synthesized factor B, but mouse L cells did not. In contrast, apolipoprotein E, another secreted protein of macrophages, was secreted by resident and thioglycollate-elicited macrophages but not by freshly harvested pyran copolymer-activated macrophages. Its synthesis was initiated at day 9 in culture of bone marrow-derived macrophages. These data support the classification of factor B as a constitutive biosynthetic and secreted protein of immature and mature macrophages in various states of activation. Production of factor B was modulated by treatment of macrophages in vivo or in culture with bacterial lipopolysaccharide endotoxin, which increased the synthesis, secretion, and accumulation of factor B up to 11-fold.

Rapid Isolation of Endogenous Inhibitor of Factor B

An inhibitor of mouse factor B was purified from lysate of L cells by a human factor B-affinity column. The purified inhibitor was found to be homogeneous with a molecular weight of about 25,000 and identical to the 25K protein isolated from L cell-lysate by the coprecipitation method with anti mouse factor B. Hemolytic titration of the activity and PAGE-analysis of the inhibitor-factor B complex showed that 1 mol of inhibitor reacted with 1 mol of mouse factor B and inactivated its hemolytic activity.

Endogenous inhibitors of factor B

proteins which were able to bind noncovalently with mouse factor B were found in cells that are nonsecretors of factor B such as mouse-established monocytic cells and L cells but not in peritoneal resident macrophages. These proteins were isolated from lysates of L cells and separated into four distinct proteins by preparative SDS-polyacrylamide gel electrophoresis, with molecular weights of 25K , 28K , 33K, and 35K . The individual proteins formed a complex with purified mouse factor B at a molecular ratio of 1: 1 and inhibited its hemolytic activity. Proteins 25K and 28K inhibited the hemolytic activity of an activated form of factor B combined with cobra venom factor as well as that of the native form. These inhibitors did not affect the hemolytic activity of the second component of complement in mouse serum. The inhibitory activity of the 25K protein was partially inhibited by antiserum raised against it in rabbits.

Synthesis of complement by macrophages and modulation of their functions through complement activation

During the last decade considerable progress has been made to characterize intimate functional links between macrophages, a major cellular component of immunoinflammatory responses, and the complement system representing the major humoral mediator of inflammation. Macrophages of various species and tissue sites have been shown to synthesize and release most of the complement components providing these cells with their own "pericellular" complement system. Circumstantial evidence for the assembly of both classical and alternative pathway convertases has been adduced. An intricate network of feedback loops involving endogenous and extrinsic factors operates to adjust complement production to acute requirements, for example augmenting production in the face of accelerated turnover at sites of inflammation, and returning it to baseline levels once the inflammatory stimulus has subsided, in order to maintain a fine-tuned balance. The molecular mechanisms underlying regulation of complement synthesis by macrophages are beginning to be elucidated by use of gene technology. On the other hand, complement activation products exert a number of effects on macrophages via specific surface receptors causing internalization of offending agents, microbes, and immune complexes, promotion of intracellular killing, controlling migration behavior, inducing release of potent biologic substances such as lysosomal enzymes, arachidonic acid metabolites, and interleukin 1. In these interactions, two important humoral mediator systems of inflammation, the complement system and the arachidonic acid cascade, are functionally linked at the level of the macrophage. Stimulation of the release of immunomodulating compounds from macrophages invoke a role for complement in immune regulation. This multifaceted interplay is of particular importance considering the mobility of macrophages that allows them to gain almost unrestricted access to sites of ongoing immunoinflammatory responses. The time seems to have come to abandon the petrified thinking in socalled systems as, for instance, humoral versus cellular, specific versus unspecific, and to proceed to interlocking functions guided by physiology proper.

Stimulation of complement production in mice by N alpha-(N-acetylmuramyl-L-alanyl-D-isoglutamine)-N epsilon-stearoyl-L-lysine

N alpha-(N-acetylmuramyl-L-alanyl-D-isoglutamine)-N epsilon-stearoyl-L-lysine, a synthetic muramyl dipeptide analog, stimulated the production of the third component of complement (C3) in mice. The serum concentration of C3 was elevated significantly by subcutaneous treatment with a single dose (10 to 100 micrograms per mouse) of the adjuvant 24 h before assay of the serum. Thereafter, the concentration decreased gradually with time and returned to the normal level on day 4 to 5. Immunoelectrophoretic analysis of the serum revealed that the decrease in serum C3 could not be accounted for by the cleavage to C3a and C3b. By intermittent treatment with the adjuvant on every fifth day, a significant increase in serum C3 was repeated. However, no continuous retention of the serum level of C3 was established even during continuous treatment with the adjuvant once a day for 10 consecutive days. Instead, in this case, the level of C3 increased repeatedly at almost 5-day intervals.

Human monocyte spreading induced by activated factor B of the complement alternative pathway: differential effects of Fab' and F(ab')2 antibody fragments directed to C5, C6, and C7

Human peripheral blood mononuclear phagocytes are induced by activated Factor B (Bb) of the complement alternative pathway to undergo morphological shape changes in vitro which have been described as "spreading." The spreading reaction induced by Bb has previously been shown to depend upon the enzymatic activity of Bb and to be inhibited by Fab' antibody fragments directed to C5 (but not anti-C3 Fab'). The possibility that Bb may exert its effect on monocytes by initiating assembly of terminal complement complexes comprised of C5b, 6, 7, C5b-8, or C5b-9 was addressed in the present study. The effects were tested of Fab' and F(ab')2 antibody fragments directed to C5, C6, C7, and C8 and to neoantigens expressed in the assembling terminal complement complexes on the monocyte spreading reaction induced by Bb. Differential effects of monovalent Fab' and divalent F(ab')2 antibody fragments were observed. Anti-C5, C6, and C7 Fab' were found to inhibit the spreading reaction induced by Bb in an immunologically specific manner. Divalent F(ab')2 fragments directed to these same proteins (but not to C3, C4, C8, or C9) induced monocyte spreading in the complete absence of Bb or other recognized inducing agents. Monocyte spreading induced by hybridoma immunoglobulin (Ig) directed to C5 and C7 was found to be correlated with the binding of 10(6) molecules Ig per cell. These findings support the notion that C5, C6, and C7 (or an analogous system of cellular proteins) are associated with the surface of human peripheral blood monocytes and that these proteins may play a role in certain reactions by which mononuclear phagocytes are induced to altered states of cellular physiology.

In vivo changes in complement induced with peptidoglycan-polysaccharide polymers from streptococcal cell walls

In rats injected with an arthropathogenic dose of streptococcal cell wall fragments, serum hemolytic activity decreased over the first 24 h and was then elevated from days 2 through 6 after injection. Hemolytic activity was again elevated at days 16 and 40. Levels of activity of alternative complement pathway, C3, and factor D were also altered.

Complement proteins and macrophages. III. The production of factor B by particle-ingesting macrophages

A kinetic study of factor B secretion by particle-ingesting macrophages showed that the ingestion of zymosan but not latex lead to an early suppression of factor B secretion followed by a significant enhancement. This dual response was proportional to the number of ingested zymosans. After a suppressive period of 24-48 hr, the ingested zymosan stimulated the synthesis of factor B. The response of acid phosphatase synthesis to ingested zymosan was the same as that of factor B, while the synthesis of lactic dehydrogenase proceeded without an early suppression. These results suggest that factor B should be considered an enzyme released into the lysosomes of macrophages.

Complement proteins and macrophages. II. The secretion of factor B by lipopolysaccharide-stimulated macrophages

The secretion of factor B by mouse peritoneal macrophages was found to be enhanced following in vivo or in vitro stimulation with lipopolysaccharide (LPS). The intravenous administration of LPS to mice of various strains caused an increased release of factor B but not the release of acid phosphatase by the peritoneal macrophages obtained from the stimulated mice. In vitro stimulation of cultured macrophages with LPS resulted in an enhanced secretion of both factor B and acid phosphatase. The dose-dependent augmentation of factor B secretion by LPS was found in the macrophages from LPS-responsive C3H/HeN mice, whereas the macrophages from LPS-unresponsive C3H/HeJ mice did not respond to either phenol-extracted LPS or butanol-extracted LPS. The ability of LPS to cause the enhancement of factor B secretion by macrophages was abolished by alkali or acid treatment of LPS, indicating that its lipid A part was responsible for the observed effect.