Relation of Surface Phagocytosis to the Fibrinous Character of Acute Bacterial Exudates

Studies on the cellular immunology of acute bacteremia. I. Intravascular leucocytic reaction and surface phagocytosis

Evidence has been presented that the introduction of large numbers of bacteria into the blood stream causes a widespread intravascular reaction, characterized by the sticking of leucocytes to the endothelium of capillaries, arterioles, and venules. The adherent granulocytes promptly become motile and thus potentially phagocytic. This intravascular leucocytic response affords a rapid and efficient mobilization of a vast number of active phagocytes within the blood stream. In some of the smaller vessels of both the systemic and pulmonary circulation the reaction is accompanied by the deposition of what appears to be intravascular fibrin. Direct observation by the rabbit ear chamber technique has revealed that leucocytes thus mobilized in small peripheral vessels are capable of phagocyting fully encapsulated Friedländer’s bacilli in the absence of antibody. Ingestion of the encapsulated blood-borne bacteria results from surface phagocytosis and occurs primarily in those vessels in which the flow of blood is either slowed or has temporarily stopped altogether. Leucocytes can be seen to phagocyte the organisms by first trapping them against the walls of the vessels or against adjacent leucocytes. Bacteria caught in the interstices of the intravascular "fibrin" may likewise be immobilized and readily phagocyted. Thus granulocytes, without the aid of opsonins, are able to ingest and destroy encapsulated blood-borne bacteria by the same mechanisms that operate in extravascular tissues. It is concluded from these studies that intravascular surface phagocytosis by polymorphonuclear leucocytes supplements the well known phagocytic activities of the reticulo-endothelial cells and therefore serves as an important defense of the host in acute infections caused by encapsulated bacteria and complicated by bacteriemia.

A quantitative study of the kinetics of blood clearance of P32-labelled Escherichia coli and Staphylococci by the reticuloendothelial system

1. The clearance of P³²-labelled heat-killed E. coli and staphylococci from the blood follows an exponential function of the time, and the bacteria are phagocytized principally by the RES of the liver and spleen. 2. The rates of clearance of equivalent number of E. coli from the blood is rapid in rabbits and slow in mice and appears to be related to the level of antibodies in the serum of these animals. 3. Unlike E. coli, staphylococci are cleared rapidly and efficiently by the RES from the blood of mice which have a sufficient level of serum antibody against these bacteria. 4. The numbers of bacteria, phagocytized by the liver or the spleen respectively, depend upon the rate of clearance and the extent of opsonization of the bacteria. Rapidly cleared, well opsonized E. coli are removed almost exclusively by the liver, while less efficiently phagocytized bacteria are also cleared by the spleen in large numbers. 5. The rate of clearance of E. coli and the efficiency with which they are phagocyted by the RES in mice have been shown to be directly related to the level of antibody in the serum. 6. Treatment of mice with S. typhi or Serratia marcescens endotoxins increases the rate of clearance of E. coli from the blood and the level of antibody against E. coli in the serum. The enhanced clearance of E. coli can be transferred to normal mice by the serum of endotoxin-treated mice.

Phagocytosis of Porphyromonas gingivalis and Prevotella intermedia by human polymorphonuclear leukocytes in clots formed from human plasma or purified fibrinogen

We have studied the polymorphonuclear leukocyte (PMN) phagocytosis of Porphyromonas gingivalis (Bacteroides gingivalis) and Prevotella intermedia (Bacteroides intermedius) in three-dimensional fibrin meshworks formed by either clotting human plasma or by clotting purified fibrinogen with human serum incorporated. PMN phagocytosis of rhodamine isothiocyanate-labelled P. gingivalis in the fibrinogen clot was 43 +/- 6% and in the plasma clot was 62 +/- 10%; for P. intermedia the values were 15 +/- 10% and 63 +/- 10%, respectively. This showed that phagocytosis in the plasma clot system was significantly higher than in the fibrinogen clot system. However, increasing the concentration of serum in the fibrinogen clot abolished this difference, suggesting that opsonin levels determine PMN function in this assay. Labelling the bacteria with a different fluorochrome, fluoroscein isothiocyanate, reduced the level of PMN phagocytosis in both three-dimensional systems but phagocytosis was unaffected when PMN were adherent to glass. It would appear that PMN phagocytosis in three-dimensional systems can vary depending on the composition of the clot and the fluorochrome used to label the bacteria.

Effect of iron compounds on antibacterial function of human polymorphs and plasma

Human plasma was bactericidal for small numbers of Klebsiella pneumoniae, but larger numbers grew slowly over a period of 24 h. Human polymorphs in a clot of autologous plasma had a bacteriostatic effect on relatively large numbers of bacteria for up to 24 h and were much more effective than plasma alone. The bactericidal effect of plasma could be abolished by saturating the plasma albumin and hemopexin with hematin, the haptoglobin with hemoglobin, and the transferrin with Fe3+. Stimulation of bacterial growth in the presence of polymorphs depended on the degree of saturation of the plasma proteins which bind the Fe3+ or heme compounds. Hematin bound to albumin appeared to be readily available to K. pneumoniae. Free hemoglobin stimulated bacterial growth but the organism could not utilize the haptoglobin-hemoglobin complex. In the presence of polymorphs the addition of Fe3+ to give 60% saturation of the transferrin with iron led to rapid growth after a long delay (19 h). Progressive increases in saturation above 60% gave correspondingly quicker growth, suggesting that the higher the saturation of transferrin, the easier it is for the bacteria to acquire iron. The antibacterial effect of the plasma appears to be critically dependent on the availability of iron for the bacteria, and this in turn affects the ability of the polymorphs to control bacterial growth.

Surface phagocytosis; further evidence of its destructive action upon fully encapsulated pneumococci in the absence of type-specific antibody

Experiments recently reported (18) have been interpreted to indicate that surface phagocytosis plays no significant part in natural antipneumococcal defense. A repetition of these experiments has revealed: (a) that the cellular content of the leucocytic suspensions used in the phagocytic tests was of a different order of magnitude from that of the exudates which usually exist in infected tissues, (b) that the suspensions were too dilute to allow surface phagocytosis of pneumococci to occur, and (c) that the ratio of bacteria to leucocytes. was such that, when a sufficiently concentrated exudate was employed, the pneumococci injured the leucocytes and thus prevented phagocytosis from taking place. When conditions of the tests were suitably controlled, and conventional quantitative methods were employed to measure the end results of the phagocytic reaction, the essential observations relating to surface phagocytosis were fully confirmed. The significance of this non-antibody mechanism of defense in pneumococcal infections was thus further substantiated.

An experimental analysis of the curative action of penicillin in acute bacterial infections. II. The role of phagocytic cells in the process of recovery

Type I pneumococci injected into the leg muscles of otherwise normal mice reached a maximum total population of approximately 10(6) organisms. In mice rendered severely leucopenic by previous irradiation the maximum bacterial counts recorded were of the order of 10(9). Since the lesions in the latter animals were relatively acellular, the thousandfold difference in the two experiments represented a rough measure of the antibacterial action of the leucocytic exudate. The suppressive effect of the leucocytic exudate was shown by histologie studies to involve phagocytosis. The ingestion of pneumococci was clearly demonstrable within the first 12 to 18 hours. Accordingly, it was attributed to surface phagocytosis. In support of this conclusion was the finding that type III pneumococci reached a significantly higher total population in the myositis lesions than did type I. The type III strain used had been previously shown to be resistant to surface phagocytosis during active growth, whereas the type I strain was known to be susceptible throughout its growth phase. Evidence was also presented that the dense leucocytic exudate probably caused in addition a significant degree of bacteriostasis. When penicillin therapy was begun 9 hours after inoculation, the pneumococci were cleared from the lesions with equal rapidity regardless of the presence or absence of leucocytic exudate. At this early stage the pneumococci were multiplying rapidly in the lesions of both the irradiated and unirradiated mice and therefore were promptly killed by the direct action of the penicillin. When the start of treatment was delayed, however, until 24 hours after inoculation, the bacteria in both sets of lesions had already reached their maximum counts and therefore were presumably resistant to the bactericidal effect of the antibiotic. Under such circumstances the destruction of the bacteria was found to be significantly less prompt in the acellular lesions than in those with a normal cellular exudate. It is concluded from these findings that, in established pneumococcal myositis in mice, the curative effect of penicillin is due, not to the bactericidal action of the antibiotic alone, but rather to the combined effect of the drug and the cellular defenses of the host. The same conclusion also appears to be applicable to analogous acute infections in man, particularly when they are sufficiently advanced to be definitively diagnosed.

An experimental analysis of the curative action of penicillin in acute bacterial infections. I. The relationship of bacterial growth rates to the antimicrobial effect of penicillin

Three strains of pneumococcus (types I and III), equally sensitive to penicillin, have been shown to be killed by the antibiotic in vitro when grown either in enriched beef infusion broth or in a thin serous exudate. Killing of the bacteria resulted promptly when the penicillin was added during the logarithmic phase of growth but failed to occur if addition of the antibiotic was delayed until the later "stationary" growth phase. In analogous experiments with thick purulent exudates from established subcutaneous abscesses, the pneumococci failed to grow rapidly, and added penicillin exerted only a relatively slow bactericidal effect. The relevance of these in vitro observations to the curative action of penicillin was demonstrated in a systematic histologic study of the antimicrobial effect of the drug in experimental (type I) pneumococcal pneumonia. Evidence was obtained that at least two distinct processes are involved. The first, the direct bactericidal effect of the penicillin itself, was shown to operate in the outer edema zone of the spreading pneumonic lesion where the micro-organisms multiply rapidly in the thin serous exudate. The second, which predominates in the older more central portions of the lesion, was demonstrated to depend upon destruction of the pneumococci by phagocytosis. Here the bacteria, having presumably reached a relatively stationary phase of growth in the alveolar exudate, are resistant to the bactericidal action of the penicillin but are readily destroyed by the phagocytes.

The mechanisms by which macrophages phagocyte encapsulated bacteria in the absence of antibody

EVIDENCE HAS BEEN PRESENTED: (1) that macrophages from experimentally produced inflammatory exudates are capable of phagocyting fully encapsulated Type I pneumococci and group A Friedländer's bacilli in the absence of antibody, (2) that the principal mechanisms involved are those of surface phagocytosis, and (3) that the majority of pneumococci ingested by macrophages in antibody-free preparations are ultimately destroyed. The relationship of these phenomena to the mechanism of recovery in pneumococcal and Friedländer's bacillus infections has been briefly discussed.