Interaction of cells with immune complexes: adherence, release of constituents, and tissue injury.

Neutrophils are essential mediators of tissue damage in many forms of immune complex-induced injury. In vitro, they have been shown to release some of their content of injurious constituents upon reaction with immune complexes (Fig. 10). If the complexes are distributed along a nonphagocytosable surface, degranulation to the exterior of the cell is observed. When the complexes were phagocytized, however, degranulation into the phagocytic vacuole, and some loss of enzymes into the surrounding medium, occurred. This may have resulted from a momentary opening of the vacuole to allow ingestion of additional particles, as was demonstrated with the electron microscope. This phenomenon was particularly noticeable when the particles were relatively large. Far more immune complex is required to induce release when in a phagocytosable form than when on a nonphagocytosable membrane. Neutrophils may be attracted to sites of immune complex deposition in many parts of the body (arteries, heart, skin, brain, kidney, joints) by complement-mediated processes. In some situations, e.g. in the joint fluid, they would encounter free immune complexes, phagocytose them, and release enzymes. In many others, in which immune complexes may be distributed along surfaces, such as in the glomerulus, adherence of neutrophils may also lead to release of injurious constituents (proteases, collagenase, elastase, permeability factors) capable of digesting and injuring the tissues.

Release of vasoactive amines from rabbit platelets induced by sensitized mononuclear leukocytes and antigen.

The immunological release of vasoactive amines from rabbit platelets by a mechanism requiring blood leukocytes has been described. The reaction involved leukocytes from an immunized rabbit, antigen, and platelets and did not require the complement system. The leukocytes appeared to be mononuclear, had the ability to adhere to glass, and were found in greater numbers in the blood than in spleen, lymph nodes, thoracic duct lymph, bone marrow, and peritoneal cavity washings. Part or all of the effect on the platelets appeared to result from active release of a soluble factor from the leukocytes, which induced vasoactive amine release from the platelets. The platelets were not lysed during this reaction and the release of vasoactive amines required the active participation of metabolic pathways in the platelet.

Activation and desensitization of platelets by platelet-activating factor (PAF) derived from IgE-sensitized basophils. I. Characteristics of the secretory response.

The secretion of vasoactive amines from rabbit platelets induced by the platelet-activating factor (PAF) derived from IgE-sensitized rabbit basophils, was examined. The secretion required calcium has previously been shown to be noncytotoxic and was optimal in both rate and extent at 37 degrees C and pH 7.2. Different temperature-sensitive steps were rate limiting for secretion above or below 20 degrees C. The rate of secretion was dependent upon the concentration of PAF and also of platelets. Maximal rates were observed with relatively low concentrations of platelets (2.5 X 10(8)/ml), sharply contrasting with other platelet stimuli such as C3 or thrombin. The extent of secretion was dependent upon PAF concentration until a maximum of 50 or 60% of the serotonin was released and then declined with increasing amounts of PAF. This was interpreted to result from the platelets becoming desensitized to the PAF, a process that shuts off the secretion. Such a desensitization was demonstrated and was shown to be stimulus specific, i.e., other stimuli could still induce secretion from PAF-desensitized platelets. PAF extracted with ethanol from the albumin to which it is usually bound during preparation, exhibited similar characteristics, except that secretion of up to 90% of the serotonin was induced. The extracted PAF thus seemed less able to induce the desensitization. Its use did provide important evidence that populations of rabbit platelets are relatively homogenous in their ability to respond to PAF.

Immunological induction of increased vascular permeability. II. Two mechanisms of histamine release from rabbit platelets involving complement.

Two mechanisms have been described whereby rabbit platelets in vitro may be induced to release their contained histamine by the reaction of antigen and antibody. Both processes require the participation of the complement system. In the first, the adherence of platelets to particulate antigens such as zymosan or erythrocytes which have fixed complement through the third component was followed by histamine release. Plasma lacking C6 activity was fully active in this system. In the second mechanism, the reaction of soluble antigen with antibody in the presence of plasma also caused release of histamine from platelets and platelet clumping was observed. The release process, which appeared to follow the adherence of platelets to the immune complex, required the action of C6 and perhaps the later-acting components. No evidence could be obtained that a soluble factor was produced which caused the release. Instead, an inhibitor of the release process was detected after incubation of antigen, antibody, and plasma. Antibody preparations capable of giving complement-dependent PCA reactions in rabbits were also shown to induce the release of histamine from platelets in vitro.

Macrophage stimulation by bacterial lipopolysaccharides. I. Cytolytic effect on tumor target cells.

Bacterial lipopolysaccharides (LPS) stimulate mouse peritoneal macrophages to kill tumor cells in vitro. Lysis is confined to tumor cells where it is nonspecific; both allogeneic and syngeneic cells being susceptible. Stimulation by LPS appears to be due to direct interaction between LPS and macrophages and does not involve participation by lymphocytes. After exposure to LPS, a latent period must elapse before macrophages can lyse tumor cells. The cytolytic mechanism requires contact between target cells and viable effector cells which maintain their lytic capacity for a sustained period and can kill on repeated occasions. The generation of a macrophage cytolytic effect by LPS is critically dependent upon the absolute number of macrophages which must be sufficient to produce confluent monolayers. These findings indicate that LPS stimulation of macrophages in vitro represents a valuable model system for the study of the mechanisms of macrophage stimulation and of the mediation of tumor cell death.

Activation of stimulus-specific serine esterases (proteases) in the initiation of platelet secretion. I. Demonstration with organophosphorus inhibitors.

The effect of organophosphorus inhibitors of serine esterases (proteases) on secretion from washed rabbit platelets was examined. Five noncytotoxic stimuli were employed: collagen, thrombin, heterologous anti-platelet antibody (in the absence of complement), rabbit C3 bound to zymosan, and platelet activating factor derived from antigen-stimulated, IgE-sensitized rabbit basophils. Diisoprophyl phosphofluoridate, three series of p-nitrophenyl ethyl phosphonates, and a series of cyclohexyl phenylalkylphosphonofluridates were all found to be inhibitory to the platelet secretion. These are irreversible inhibitors of serine proteases but in this system were only inhibitory if added to the platelets concurrently with the stimuli. Pretreatment of either the platelets or the stimuli with the inhibitors followed by washing, was without effect on the subsequent reaction. This suggested the involvement of stimulus-activatable serine proteases in the secretory process. The concept was supported by finding that nonphosphorylating phosphonates or hydrolyzed phosphonates or phosphonofluoridates were without inhibitory action. The effect of a series of phosphonates or phosphonoflouridates in inhibiting each stimulus exhibited a unique activity-structure profile. The demonstration of such unique profiles with four series of inhibitors for each of the five stimuli was interpreted as demonstrating that a specific activatable serine protease was involved in the platelet secretory response to each stimulus.

Immunological induction of increased vascular permeability. I. A rabbit passive cutaneous anaphylactic reaction requiring complement, platelets, and neutrophils.

Passive cutaneous anaphylaxis (PCA) reactions were produced in rabbits by antibodies to bovine serum albumin. Two types of antibodies were found, each inducing increased vascular permeability, but by means of different mediation systems. One of these antibodies required the presence of complement, platelets, and neutrophils for the induction of the PCA reaction, which was inhibited by antihistamine. This antibody was heat stable, sedimented in the 7S region, and was found in both fast and slow electrophoretic fractions of rabbit gamma-globulin. Homocytotropic antibody was also detected. The PCA reactions induced by this type of antibody did not require platelets or neutrophils and were not inhibited in rabbits depleted of C3 with cobra venom factor. The lesions were, however, prevented by administration of antihistamine.

Acute immune complex disease in rabbits. The role of complement and of a leukocyte-dependent release of vasoactive amines from platelets.

By depletion of C3 from rabbits undergoing acute experimental immune complex disease with an anticomplementary factor in cobra venom, it has been possible to demonstrate that deposition of the complexes in arteries and glomeruli does not require the complement components reacting after C2. Immunological reactions, in which platelets release their vasoactive amines, have been examined in rabbits undergoing immune complex disease. A correlation was obtained between the presence of a complement-independent reaction which required blood leukocytes, antigen and platelets, the deposition of immune complexes, and the induction of glomerulonephritis. C3 depletion did, however, have a marked alleviating effect on the severity of the arterial lesions. Neutrophil accumulation and the subsequent necrotizing arteritis were prevented. In contrast, the character and severity of the glomerulonephritis was not altered by depletion of later-acting complement components.

Leukocyte-dependent histamine release from rabbit platelets. The role of IgE, basophils, and a platelet-activating factor.

We have studied the leukocyte-dependent mechanism of histamine release (LDHR) from rabbit platelets, a complement-independent mechanism which has been implicated in the deposition of immune complexes in acute serum sickness of rabbits. It was found by chromatography and passive transfer of serum from immunized rabbits that the antibody responsible for the LDHR was of IgE type. By electron microscope study of the reaction, the leukocyte involved in agglutination of platelets and release of their histamine content was identified as the basophil. Upon addition of antigen, basophils sensitized with IgE degranulated, released their histamine content and a platelet-activating factor (PAF) that caused aggregation of platelets and release of their histamine. Conditions of preparing and preserving PAF activity and some properties of this factor have been elucidated. LDHR must, therefore, be considered as an immediate hypersensitivity-type mechanism which may link allergic reactions with immunologic disease associated with severe structural injury.

Activation of platelets by platelet-activating factor (PAF) derived from IgE-sensitized basophils. II. The role of serine proteases, cyclic nucleotides, and contractile elements in PAF-induced secretion.

Secretion of serotonin from platelets induced by platelet-activating factor (PAF) derived from antigen-stimulated, IgE-sensitized rabbit basophils was studied to further characterize the biochemical requirements. Inhibition of secretion with diisopropylphosphofluoridate (DFP) was observed if the DFP was present during the reaction, but not if platelets or PAF were pretreated with the inhibitor. This suggested a role for an activatable serine protease in the secretion. Supporting evidence came from the observation that other protease inhibitors and a variety of low molecular weight amino acid esters were also inhibitory. TAMe was most effective, and AGLMe and LeuMe were inactive, indicating a specificity for different esters. Secretion was reduced by agents that increased intracellular cyclic AMP (cAMP), but enhanced by alpha-adrenergic stimulation, which reduced the levels of cAMP. Concurrent with PAF-induced secretion, a reduction in cAMP levels was observed. No effect of cyclic GMP or cholinergic stimulation was found. Secretion was inhibited by colchicine and enhanced by cytochalasin B, suggesting a role for microfilaments and microtubules. The effects of these three systems on PAF-induced secretion indicate the basic uniformity of the secretory process in platelets (and other cells) whatever the stimulus. The uniqueness of the reaction apparently lies in the stimulus-receptor interaction and the nature of the serine protease which is activated.

Enhancement of platelet response to immune complexes and IgG aggregates by lipid A-rich bacterial lipopolysaccharides.

The effect of the common lipid moiety of bacterial LPS on secretion from washed human platelets has been studied. The lipid A-rich LPS of S. minnesota R595 and a lipid A preparation both potentiated platelet serotonin secretion in response to IgG aggregates or immune complexes up to 50-fold but had little effect in the absence of IgG. Lipid A has been shown to bind immune aggregates, raising the possibility that its mechanism of action involved effective enlargement or insolubilization of the aggregates. IgG aggregates of dimer to tetramer size were shown to be platelet simuli, equivalent on a weight basis to larger soluble aggregates. The effect of both sizes of aggregates on platelets were equally enhanced by the LPS, indicating that increased size of aggregates alone could not account for the effect of LPS. Similarly, because lipid A-rich LPS enhanced platelet response to already insoluble immune complexes, its mechanism of action cannot simply be insolubilization of immune aggregates. These LPS did not enhance platelet stimulation by antiplatelet antibody, monosodium urate crystals, or thrombin and only slightly enhanced stimulation by insoluble human skin collagen. This indicates some stimulus specificity in the ability of LPS to increase platelet secretion. The enhancement of cell response to immune complexes by the common lipid region of LPS may represent a mechanism for the diverse effects of LPS in vivo and in vitro.

Phagocytosis of senescent neutrophils by human monocyte-derived macrophages and rabbit inflammatory macrophages.

An in vitro system to investigate the ability of macrophages to recognize and ingest senescent polymorphonuclear neutrophils has been used that uses chromium-labeled neutrophils and staining for myeloperoxidase (MPO). Human monocyte-derived macrophages obtained from in vitro cultures were able to recognize "aged" but not freshly isolated 51Cr-labeled human neutrophils and ingest them. Freshly isolated monocytes did not exhibit this property. Because the aged neutrophils were greater than 95% viable, death did not appear to be a prerequisite for recognition and ingestion. Serum was not required for the aging of the neutrophils, and when serum was used, different concentrations did not appear to effect the aging process; that is, neutrophils aged in different concentrations of serum were ingested equally. Phagocytosis of senescent neutrophils by macrophages occurred in a time-dependent manner and was also dependent on the number of neutrophils added. Monocyte-derived macrophages first exhibited the ability to phagocytose senescent neutrophils on the 3rd d of culture, with the percentage of active macrophages increasing through day 7. In experiments with rabbit mononuclear phagocytes, immune complex-induced inflammatory macrophages from the lung but not resident bronchoalveolar macrophages or peripheral blood monocytes were found to be capable of recognition and ingestion of senescent rabbit neutrophils. These data suggest that the monocyte maturation process, akin to that seen during inflammation, is necessary in vitro before macrophages recognize and remove senescent neutrophils.

Chemotactic activity generated from the fifth component of complement by plasma kallikrein of the rabbit.

Rabbit plasma kallikrein incubated with rabbit C5 resulted in the generation of chemotactic and secretagogue activity for rabbit neutrophils. This effect on C5 appeared to be due to kallikrein itself and not to a contaminating enzyme, because it could be inhibited by anti-kallikrein IgG or by soybean trypsin inhibitor to the same extent the kinin generation by the same kallikrein preparation was inhibited by these agents. The chemotactic response was consistent with the generation of a C5a-like peptide from C5 because the effect could be partially inhibited by carboxypeptidase N and was related to the generation of a small (approximately 14,000 mol wt) fragment of C5. No direct chemotactic response was detectable for kallikrein, activated Hageman factor, high-molecular weight kininogen, or intact C5. Incubation of Kallikrein, high-molecular weight kininogen, and Hageman factor together, so that activation of all three proteins occurred, did not results in the generation of detectable chemotactic activity.

Priming of neutrophils for enhanced release of oxygen metabolites by bacterial lipopolysaccharide. Evidence for increased activity of the superoxide-producing enzyme.

We investigated the capacity of bacterial endotoxin (lipopolysaccharide, LPS) to modify the oxidative metabolic response to membrane stimulation of human neutrophils. Neutrophils were pretreated for 60 min with LPS, 10 ng/ml, then stimulated by exposure to fixed immune complexes, the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (FMLP), or phorbol myristate acetate. Release of superoxide anion (O-2) was up to 7-times greater in cells preincubated with LPS, depending upon the stimulus used. Consumption of oxygen and release of hydrogen peroxide (H2O2) were similarly increased, using FMLP as stimulus. The enhancement was accompanied by a reduction in lag time and an increase in the rate of the response, but the duration of the oxidative events was not changed. The molecular basis for the augmented oxidative response of LPS-pretreated cells was investigated. Preincubation with LPS at 0 degrees C prevented priming, but preincubation in the presence of cycloheximide or chelation of extracellular calcium ion did not. Neutrophils preincubated with LPS had slightly decreased numbers of binding sites and equivalent binding affinity for radiolabeled FMLP. Possible changes in the enzyme responsible for the oxidative burst were analyzed by studying NADPH-dependent generation of O-2 by particulate fractions from cells preincubated with LPS or buffer, then stimulated before cell disruption. The fraction prepared from LPS-pretreated neutrophils exhibited greater release of O-2 over a wide range of concentrations of NADPH. The calculated apparent Km for NADPH was equivalent in the two fractions, but the Vmax was increased 2.5-fold in the subcellular fraction from LPS-pretreated cells. These results suggest that LPS could increase neutrophil-mediated host defense or the tissue damage associated with endotoxemia by enhancing the generation of oxygen metabolites by neutrophils. These results also support the concept that the neutrophil is not an end-stage cell in regard to function or metabolic activity.

Macrophage stimulation by bacterial lipopolysaccharides. II. Evidence for differentiation signals delivered by lipid A and by a protein rich fraction of lipopolysaccharides.

Stimulation of macrophages to lyse tumor cells is a property common to lipopolysaccharide (LPS) extracted from a variety of smooth and rough bacterial strains by several different preparative procedures. The relationship between macrophage stimulation and the structural characteristics of LPS is defined. In protein-free LPS, lipid A bears the stimulatory signal which results in the differentiation of elicited macrophages into killer cells. The polysaccharide moiety is neither stimulatory itself nor does it block the activity of complete LPS on macrophages. Extraction of LPS by the butanol or Boivin procedures produces preparations in which LPS is complexed through its lipid A moiety to a protein rich component, LAP. Isolated LAP delivers a macrophage differentiation signal which is independent of lipid A. The presence of these two structurally distinct constituents in the cell walls of gram-negative bacteria broadens the biological environments in which they can stimulate macrophages in vivo.

Mediation systems in bacterial lipopolysaccharide-induced hypotension and disseminated intravascular coagulation. I. The role of complement.

We have studied the role of complement in lipopolysaccharide (LPS)-induced hypotension and disseminated intravascular coagulation (DIC) by comparing the effects of injection of three preparations of LPS from E. Coli 0111:B4, S. minnesota Re595, and S. marcescens. Injections of nonlethal doses of these LPS preparations into normal rabbits produced decreases in mean arterial blood pressure during a 5-h period. When rabbits treated with cobra venom factor (CoF) to deplete C3 were injected with the various LPS preparations, mean arterial pressures fell at a rate and extent essentially identical to that observed in normal rabbits. Rabbits genetically deficient in C6 also demonstrated LPS-induced hypotensive changes. Only minimal, or no changes in plasma C3 levels or serum CH50 values were detected in normal rabbits after LPS injection. Hypotensive changes were also induced in rabbits when complement was rapidly activated by intravenous injection of CoF. In contrast to the hypotension induced by LPS, the fall in arterial pressure associated with the consumption of complement was short lived and required the rapid consumption of considerable amounts of C3. The occurrence of DIC noted in normal rabbits injected with each preparation of LPS was not inhibited in either rabbits treated with cobra factor or in C6-deficient rabbits. The DIC was most pronounced after injection of Re595 and S. marcescens LPS. Injection of the various LPS preparations produced a rapid disappearance of circulating neutrophils and mononuclear cells, which occurred with the same kinetics and to the same extent in normal, CoF-treated, and C6-deficient rabbits. Injection of either Re595 LPS or S. marcescens LPS produced a biphasic disappearance of circulating 51Cr-platelets. In contrast, injection of 0111:B4 LPS affected only slightly the rate of disappearance of 51Cr-platelets. Depletion of C3 by cobra factor treatment had no effect on the disappearance of platelets in animals injected with 0111:B4. In marked contrast cobra factor treatment greatly reduced the initial rapid disappearance of platelets in rabbits injected with either Re595 or S. marcescens LPS, but had no effect in the secondary disappearance phase.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells.

Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.

A hierarchical role for classical pathway complement proteins in the clearance of apoptotic cells in vivo.

The strongest susceptibility genes for the development of systemic lupus erythematosus (SLE) in humans are null mutants of classical pathway complement proteins. There is a hierarchy of disease susceptibility and severity according to the position of the missing protein in the activation pathway, with the severest disease associated with C1q deficiency. Here we demonstrate, using novel in vivo models of apoptotic cell clearance during sterile peritonitis, a similar hierarchical role for classical pathway complement proteins in vivo in the clearance of apoptotic cells by macrophages. Our results constitute the first demonstration of an impairment in the phagocytosis of apoptotic cells by macrophages in vivo in a mammalian system. Apoptotic cells are thought to be a major source of the autoantigens of SLE, and impairment of their removal by complement may explain the link between hereditary complement deficiency and the development of SLE.

Clearance of apoptotic cells by phagocytes.

Phagocytic clearance of apoptotic cells may be considered to consist of four distinct steps: accumulation of phagocytes at the site where apoptotic cells are located; recognition of dying cells through a number of bridge molecules and receptors; engulfment by a unique uptake process; and processing of engulfed cells within phagocytes. Here, we will discuss these individual steps that collectively are essential for the effective removal of apoptotic cells. This will illustrate our relative lack of knowledge about the initial attraction signals, the specific mechanisms of engulfment and processing in comparison to the extensive literature on recognition mechanisms. There is now mounting evidence that clearance defects are responsible for chronic inflammatory disease and contribute to autoimmunity. Therefore, a better understanding of all aspects of the clearance process is required before it can truly be manipulated for therapeutic gain.

Bacterial lipopolysaccharide suppresses the production of catalytically active lysosomal acid hydrolases in human macrophages.

Sub-microgram quantities of bacterial lipopolysaccharide (LPS) have been found to substantially reduce the intracellular catalytic activities of three representative lysosomal enzymes (namely, acid phosphatase, hexosaminidase, and beta-glucuronidase) in human monocyte-derived macrophages. This response was not associated with a concurrent increase in enzyme catalytic activity in the culture supernatant, and hence, could not be explained by mobilization of preformed material. By conducting experiments in the presence and absence of indomethacin, a cyclooxygenase inhibitor, the reduction in lysosomal enzyme catalytic activities was shown not to be dependent on the ability of LPS to induce prostaglandin E2 production. The response was not found to be the result of a more generalized LPS-dependent reduction in the ability of the cells to synthesize protein, since the presence of LPS in macrophage cultures did not appreciably affect the amount of [35S]methionine incorporated into total cellular proteins. A kinetic analysis of the effect of LPS on the down-regulation of enzyme catalytic activities indicated that this was an early response of the cells to LPS exposure. An investigation of the effects of blockade of enzyme catabolism (using the lysosomotropic weak-base, methylamine) indicated that the reduction of catalytic enzyme activities in response to LPS was probably due to a decreased rate of production of active product, rather than an enhanced rate of enzyme catabolism. This suggestion was confirmed by experiments in which the synthesis of pro-hexosaminidase (measured by biosynthetic labeling with [35S]methionine and specific immunoprecipitation of labeled pro-hexosaminidase) was found to be reduced by 42% after a 24-h exposure to LPS (although the synthesis of complement component C3 was stimulated by a factor of 4.5). It is suggested that the ability of LPS to regulate the functional expression of protein products contributes to changes in the overall functional status of these cells in response to this bacterial product.