Phagocytic Macrophages Research Articles

Displacement of platelets from blood to spleen following intravenous injection of liposomes encapsulating dichloromethylene bisphosphonate

Liposomes encapsulating dichloromethylene bisphosphonate (Cl2MBP-liposomes) have been shown to cause selective depletion of phagocytic macrophages. We have shown that intravenous injection of Cl2MBP-liposomes into mice induces an almost complete depletion of F4/80-positive cells (mature macrophages) in the liver and in the splenic red pulp, but not in the lung. Platelets in the mouse contain a large amount of 5-hydroxytryptamine (5HT; serotonin) and so, by measuring 5HT, it is possible to assess the translocation of platelets to tissues. The injection of Cl2MBP-liposomes was found to induce a prolonged and marked increase in 5HT that occurred selectively in the spleen. On the other hand, 5HT in the blood decreased by as much as 50%. These changes in 5HT corresponded well with each other in terms of both time course and dose–response relationship. To judge from measurements made at the peak of the response, the 5HT increase in the spleen corresponded to about 80% of the 5HT lost from the blood. Electron microscopic analysis revealed a great accumulation of platelets in the splenic cords. We have shown that aggregation and degranulation of platelets in the lung is involved in rapid anaphylactoid shock induced within 10 min of intravenous injection into mice of a lipopolysaccharide [Shibazaki, M., Nakamura, M., Endo, Y., 1996. Biphasic, organ-specific, and strain-specific accumulation of platelets induced in mice by a lipopolysaccharide from Escherichia coli and its possible involvement in shock. Infect. Immun. 64, 5290–5294; Endo, Y., Shibazaki, M., Nakamura, M., Takada, H., 1997. Contrasting effects of lipopolysaccharides (endotoxins) from oral black-pigmented bacteria and Enterobacteriaceae on platelets, a major source of serotonin, and on histamine-forming enzyme in mice. J. Infect. Dis. 175, 1404–1412]. In the present study, it was found that such shock was almost completely prevented in those mice in which platelets were displaced from the blood by Cl2MBP-liposomes. These results suggest that in the spleen the depletion of phagocytic macrophages may impair the function or structure of this organ. This may lead to the entry of platelets into the spleen in such large numbers as to reduce their level in the blood and result in their prolonged accumulation in the spleen. The Cl2MBP-liposome may be an excellent tool for the in vivo investigation of the role of platelets, as well as that of macrophages.

Effect of lipopolysaccharides of various composition from gram-negative bacteria on macrophage activity, oxidant metabolism, and microsomal activity of the liver

The study explores the effect of lipopolysaccharides of various composition from Gram-negative bacteria on phagocytic activity and oxygen-dependent metabolism of macrophages, intensity of lipid peroxidation, and activity of microsomal enzymes in mouse liver, and production of interleukin-1 and tumor necrosis factor by cultured peripheral blood mononuclears. It is found that the magnitude of inhibiting effect of lipopolysaccharides on microsomal oxidation is a mirror reflection of the degree of their prooxidant effect and activation of phagocytic and secretory macrophage functions, which confirms tight coupling of these processes under conditions of acute phase response.

4β, 15-Diacetoxyscirpenol induces Cytotoxicity and Alterations in Phagocytic and Fc-Receptor Expression functions in Chicken Macrophages In Vitro

4β, 15 Diacetoxyscirpenol (DAS) mycotoxin produced by Fusarium species was tested for detrimental effects on macrophage viability, phagocytosis, and Fc-receptor expression. Sephadex-elicited chicken abdominal cells were harvested to establish adherent macrophage monolayers on glass coverslips. Coverslips were then assigned randomly to treatment groups (0, 12.5 and 25 μg/mL DAS). Macrophage monolayers were exposed to treatments for 1 h, washed, and tested for various functional endpoints. Treatment with DAS resulted in decreased viability of macrophages (90.8% vs 81.5% vs 70.4% viable in the 0, 12.5 and 25 μg treatments, respectively) and decreased the percentage of macrophages phagocytizing sheep erythrocytes (81.6% vs 53.1% vs 46.0%. DAS also caused a decrease in the mean number of opsonized cells engulfed per phagocytic macrophage (5.7 vs 3.7 vs 2.9). A similar trend was observed using unopsonized sheep erythrocytes (15.4% vs 7.6% vs 5.5% phagocytic macrophages and 0.29 vs 0.11 vs 0.08 erythrocytes engulfed per macrophage). The incidence of Fc-receptor positive macrophages determined by sheep erythrocyte resetting was also decreased in DAS-treated macrophages as compared to the control (49.2% vs 32.7% vs 24.2%). The findings of this study demonstrate that DAS exposure causes a suppression in macrophage phagocytic function and therefore may alter the first line of immunological defense in chickens.

Regenerating sciatic nerve does not utilize circulating cholesterol.

The rapid accumulation of myelin in the peripheral nervous system during the early postnatal period requires large amounts of cholesterol, a major myelin lipid. All of the cholesterol accumulating in the developing rat sciatic nerve is synthesized locally within the nerve, rather than being derived from the supply in lipoproteins in the systemic circulation (Jurevics and Morell, J. Lipid Res. 5:112-120; 1994). Since this lack of utilization of circulating cholesterol may relate to exclusion by the blood-nerve barrier, we examined the sources of cholesterol needed for regeneration following nerve injury, when the blood-nerve barrier is breached. One sciatic nerve was crushed or transected, and at various times later, the rate of cholesterol accumulation was compared with the rate of local in vivo synthesis of cholesterol within the nerve, utilizing intraperitoneally injected 3H2O as precursor. The accumulation of additional cholesterol in nerve during regeneration and remyelination could all be accounted for by that locally synthesized within the nerve. There was also an increase in cholesterol esters in injured nerve segments; in crushed nerves, these levels decreased during regeneration and remyelination, consistent with reutilization of cholesterol originally salvaged by phagocytic macrophages and Schwann cells. Thus, regeneration and remyelination following injury in sciatic nerve utilizes both salvaged cholesterol and cholesterol synthesized locally within the nerve, but not cholesterol from the circulation.

Apoptosis of microglia and oligodendrocytes after spinal cord contusion in rats.

Following spinal cord contusion in the rat, apoptosis has been observed in the white matter for long distances remote from the center of the lesion and is primarily associated with degenerating fiber tracts. We have previously reported that many of the apoptotic cells are oligodendrocytes. Here we show that the oligodendrocyte death is maximal at 8 days postinjury and suggest that loss of oligodendrocytes may result in demyelination of axons that have survived the initial trauma. There are two mechanisms that may account for the observed oligodendrocyte apoptosis. The apoptotic cell death may result from the loss of trophic support after axonal degeneration or it may be the consequence of microglial activation. The hypothesis that oligodendrocyte apoptosis is secondary to microglial activation is supported by our observations of microglia with an activated morphology in the same regions as apoptosis and apparent contact between some of the apoptotic oligodendrocytes and microglial processes. In addition to oligodendrocyte apoptosis, a subpopulation of microglia appears to be susceptible to apoptotic cell death as well, as evidenced by the presence of apoptotic bodies in OX42 immunopositive profiles. Thus, the population of apoptotic cells following spinal cord contusion is comprised of oligodendrocytes and putative phagocytic microglia or macrophages. Given the delayed time course of oligodendrocyte death, the apoptotic death of oligodendrocytes may be amenable to pharmacological intervention with subsequent improvement in functional recovery.

Tellurium causes dose-dependent coordinate down-regulation of myelin gene expression

Exposure of developing rats to a diet containing elemental tellurium systemically inhibits cholesterol synthesis at the level of squalene epoxidase. At high tellurium exposure levels (>0.1% in the diet), there is an associated segmental demyelination of the PNS. Low levels of dietary tellurium (0.0001%) led to in vivo inhibition of squalene epoxidase activity in sciatic nerve, and inhibition increased with increasing exposure levels. With increasing dose and increasing exposure times, there was an increasing degree of demyelination and increasing down-regulation of mRNA levels for myelin P 0 protein, ceramide galactosyltransferase (rate-limiting enzyme in cerebroside synthesis), and HMG-CoA reductase (rate-limiting enzyme in cholesterol synthesis). Because these were all down-regulated in parallel, we conclude there is coordinate regulation of the entire program for myelin synthesis in Schwann cells. An anomaly was that at early time points and low tellurium levels, mRNA levels for HMG-CoA reductase were slightly elevated, presumably in response to tellurium-induced sterol deficits. We suggest the eventual down-regulation relates to a separate mechanism by which Schwann cells regulate cholesterol synthesis, related to the need for coordinate synthesis of myelin components. Levels of mRNA for the low-affinity nerve growth factor receptor (indicator of alterations in axon-Schwann cell interactions) and for lysozyme (marker for phagocytic macrophages) were both up-regulated in a dose- and time-dependent manner which correlated with the presence of segmental demyelination. Levels of mRNA coding for myelin-related proteins were down-regulated at low tellurium exposure levels, without demyelination or up-regulation of nerve growth factor receptor. This suggests the down-regulation is related to the tellurium-induced cholesterol deficit, and not to the loss of axonal contact associated with early stages of demyelination or to the entry of activated macrophages.

Factors influencing phagocytic response of macrophages from the sea bass Dicentrarchus labrax L.): An ultrastructural and quantitative study

Background Although phagocytosis is considered the main nonspecific defense mechanism in fish, the significance of the influence of bacterial properties and assay conditions on this mechanism has not been established. We report the influence of leukocyte source, bacterial species, presence or absence of a bacterial wall, bacterial status (live or dead), and bacterial opsonization on in vitro phagocytosis by sea bass (Dicentrarchus labrax L.) macrophages. Methods Leukocytes were challenged with bacteria under different conditions and ultrastructurally studied. The number of phagocytic macrophages and the number of bacteria ingested per macrophage were quantified by microscopical counting, and the data obtained were statistically analyzed following general linear models. Results Macrophages from sea bass peritoneal exudate showed a greater capacity to engulf bacteria than did those isolated from blood, which, in turn, had greater engulfment properties than did those isolated from head-kidney. Greater phagocytic activity was observed in the assays carried out with Escherichia coli than in those carried out with Salmonella typhimurium, especially when the TA1950 S. typhimurium strain (with wall) was used. Macrophages showed greater phagocytic activity against live bacteria. Bacterial opsonization had a variable effect on phagocytosis. Sea bass macrophages did not always ingest the same number of bacteria. Conclusions Sea bass macrophages have strong bacterial phagocytic activity. Significant differences were found that had been influenced by bacterial properties as well as by assay conditions. Positive correlation between the number of phagocytic macrophages and the number of bacteria ingested per phagocyte was observed. Anat. Rec. 248:533-541, 1997. © 1997 Wiley-Liss, Inc.

Factors influencing phagocytic response of macrophages from the sea bass (Dicentrarchus labrax L.): an ultrastructural and quantitative study.

Although phagocytosis is considered the main nonspecific defense mechanism in fish, the significance of the influence of bacterial properties and assay conditions on this mechanism has not been established. We report the influence of leukocyte source, bacterial species, presence or absence of a bacterial wall, bacterial status (live or dead), and bacterial opsonization on in vitro phagocytosis by sea bass (Dicentrarchus labrax L.) macrophages. Leukocytes were challenged with bacteria under different conditions and ultrastructurally studied. The number of phagocytic macrophages and the number of bacteria ingested per macrophage were quantified by microscopical counting, and the data obtained were statistically analyzed following general linear models. Macrophages from sea bass peritoneal exudate showed a greater capacity to engulf bacteria than did those isolated from blood, which, in turn, had greater engulfment properties than did those isolated from head-kidney. Greater phagocytic activity was observed in the assays carried out with Escherichia coli than in those carried out with Salmonella typhimurium, especially when the TA1950 S. typhimurium strain (with wall) was used. Macrophages showed greater phagocytic activity against live bacteria. Bacterial opsonization had a variable effect on phagocytosis. Sea bass macrophages did not always ingest the same number of bacteria. Sea bass macrophages have strong bacterial phagocytic activity. Significant differences were found that had been influenced by bacterial properties as well as by assay conditions. Positive correlation between the number of phagocytic macrophages and the number of bacteria ingested per phagocyte was observed.

Enhancement of humoral and cellular immunity by vitamin E after embryonic exposure

In the present study, the amnion of turkey and chicken embryos were injected 3 d prior to hatch with different levels of vitamin E (VE). In Experiments 1 and 2, turkey embryos received 10, 20, and 30 IU of VE. In Experiment 3, broiler embryos received 10 IU VE. In all three experiments, sham-injected control embryos (0 IU VE) received 300 microL of saline. In Experiments 1 and 2 (turkey embryos), 20 and 30 IU of VE reduced (P < or = 0.05) percentage hatchability below that of controls. At hatch, poults exhibited a dose related increase (P < 0.05) in plasma VE levels. Mean BW gain up to 35 d and relative bursa of Fabricius and spleen weights were not different among treatment groups. When challenged at 7 d posthatch, total (P < 0.05) and IgM (P < 0.08) anti-SRBC antibodies were higher in 10 IU VE poults than in controls. Immunoglobulin G levels did not differ among the treatment groups. Poults in the 10 IU VE group had higher (P < 0.002) numbers of Sephadex-elicited inflammatory exudate cells, as well as a greater percentage of phagocytic macrophages (P < 0.0001). Additionally, the numbers of SRBC per phagocytic macrophage were greater (P < 0.001), than in control poults at 4 wk of age. In Experiment 3, chick embryos exposed to 10 IU VE, exhibited no differences in hatchability, BW gain, or bursal and splenic weights from the sham-exposed group. However, total and IgM antibody responses against SRBC were greater (P < 0.01) in the 10 IU VE group at 7 d postinjection. A secondary SRBC challenge given at 14 d after primary injection resulted in higher total (P < 0.07) and IgG (P < 0.04) antibody responses in the 10 IU VE chicks than in the controls. Similarly, broiler chicks (10 IU VE) had more Sephadex-elicited abdominal exudate cells (P < 0.07), and greater macrophage phagocytic potential (P < 0.0001). In ovo VE exposure (10 IU) also increased nitrite production (P < 0.04) by chick macrophages. The results from this study demonstrated an enhanced antibody and macrophage response and suggest that in ovo exposure with VE may improve posthatch poult and broiler quality.

Quinolinate Immunoreactivity in Experimental Rat Brain Tumors Is Present in Macrophages but Not in Astrocytes

Experimental tumors of the central nervous system were investigated with antibodies to quinolinate to assess the cellular distribution of this endogenous neurotoxin. In advanced F98 and RG-2 glioblastomas and E367 neuroblastomas in the striatum of rats, variable numbers of quinolinate immunoreactive cells were observed in and around the tumors, with the majority being present within tumors, rather than brain parenchyma. The stained cells were morphologically variable, including round, complex, rod-shaped, and sparsely dendritic cells. Neuroblastoma and glioma cells were unstained, as were neurons, astrocytes, oligodendrocytes, ependymal cells, endothelial cells, and cells of the choroid plexus and leptomeninges. Glial fibrillary acidic protein immunoreactivity was strongly elevated in astrocytes surrounding the tumors. Dual labeling immunohistochemistry with antibodies to quinolinate and glial fibrillary acidic protein demonstrated that astrocytes and the cells containing quinolinate immunoreactivity were morphologically disparate and preferentially distributed external and internal to the tumors, respectively, and no dual labeled cells were observed. Lectin histochemistry withGriffonia simplicifoliaB4isolectin andLycopersicon esculentumlectin demonstrated numerous phagocytic macrophages and reactive microglia in and around the tumors whose distribution was similar to that of quinolinate immunoreactive cells, albeit much more numerous. Dual labeling studies with antibodies to quinolinate and the lectins demonstrated partial codistribution of these markers, with most double-labeled cells having the morphology of phagocytes. The present findings suggest the possibility that quinolinate may serve a functional role in a select population of inflammatory cell infiltrates during the immune response to brain neoplasms.

Histological, histochemical, and fine structural observations on the lymph node of the common seal (Phoca vitulina) and the grey seal (Halichoerus grypus).

The recent seal death epizootic prompted interest in their immune system, for which no current morphological data were available. Lymph nodes from adult harbor seals (Phoca vitulina) and grey seals (Halichoerus grypus) were investigated by light microscopy, electron microscopy, and lectin histochemistry. No significant differences in the lymph node morphology were found between the two species, and the overall organization of the nodes comprises of capsule, trabeculae, cortex, paracortex, and medulla. Capsule and trabeculae are composed of tightly packed collagen and elastic fibrils and are rich in fibroblasts, myofibroblasts, and smooth muscle cells. Unmyelinated nerve fibers are common. The cortex contains numerous secondary follicles with well-developed germinal centers and paracortical areas with high endothelial venules. Antigen-presenting cells and phagocytic macrophages were abundantly present. The medullary cords contain numerous plasma cells. Fibroblastic reticulum cells are common throughout the parenchyma and transverse the sinus. Marginal, radial, and medullary sinuses are lined by littoral cells. The visceral lining of the sinuses is marked by macrophages and by numerous mast cells. The morphology of seal lymph nodes does not differ significantly from that of terrestrial mammals. Earlier functional conclusions concerning seal lymph nodes are substantiated by this morphological study.

Cellular inflammatory response after spinal cord injury in Sprague-Dawley and Lewis rats.

The distribution of microglia, macrophages, T-lymphocytes, and astrocytes was characterized throughout a spinal contusion lesion in Sprague-Dawley and Lewis rats by using immunohistochemistry. The morphology, spatial localization, and activation state of these inflammatory cells were described both qualitatively and quantitatively at 12 hours, 3, 7, 14, and 28 days after injury. By use of OX42 and ED1 antibodies, peak microglial activation was observed within the lesion epicenter of both rat strains between three and seven days post-injury preceding the bulk of monocyte influx and macrophage activation (seven days). Rostral and caudal to the injury site, microglial activation plateaued between two and four weeks post-injury in the dorsal and lateral funiculi as indicated by morphological transformation and the de-novo expression of major histocompatibility class II (MHC II) molecules. Similar to the timing of microglial reactions, T-lymphocytes maximally infiltrated the lesion epicenter between three and seven days post-injury. Reactive astrocytes, while present in the acute lesion, were more prominent at later survival times (7-28 days). These cells were interspersed with activated microglia but appeared to surround and enclose tissue sites occupied by reactive microglia and phagocytic macrophages. Thus, trauma-induced central nervous system (CNS) inflammation, regardless of strain, occurs rapidly at the site of injury and involves the activation of resident and recruited immune cells. In regions rostral or caudal to the epicenter, prolonged activation of inflammatory cells occurs preferentially in white matter and primarily consists of activated microglia and astrocytes. Differences were observed in the magnitude and duration of macrophage activation between Sprague-Dawley (SD) and Lewis (LEW) rats throughout the lesion. Increased expression of complement type 3 receptors (OX42) and macrophage-activation antigens (ED1) persisted for longer times in LEW rats while expression of MHC class II molecules was attenuated in LEW compared to SD rats at all times examined. Variations in the onset and duration of T-lymphocyte infiltration also were observed between strains with twice as many T-cells present in the lesion epicenter of Lewis rats by 3 days post-injury. These strain-specific findings potentially represent differences in corticosteroid regulation of immunity and may help predict a range of functional neurologic consequences affected by neuroimmune interactions.

Modulation of the phenotypic and functional properties of phagocytic macrophages by wear particles from orthopaedic implants.

An attempt was made to assess the local chronic inflammatory response in patients with failed orthopaedic implant that is clinically associated with osteolysis, bone and bone marrow necrosis. The main objective was to analyse the heterogeneity of the macrophage functional subsets in the bone-implant interface membrane and to evaluate their possible role in the development of an erosive inflammatory lesion within the bone. Immunohistology was performed on 21 specimens of the bone-implant interface obtained from 17 patients during revision arthroplasty, and synovial membranes from rheumatoid (RA, n=4), and osteoarthritis (OA, n=4) patients. Three well-characterized monoclonal antibodies (MAb) recognizing antigenic determinants on specific functional subsets of macrophages (Mphi) were used. RFD1 (interdigitating reticulum cells/antigen presenting cells, (APC), RFD7 (mature phagocytic macrophages), and RFD9 epithelioid cells and foreign body giant cells (FBGC). RFD1 was expressed on a variable number of perivascular and synovial lining Mphi in both RA and OA synovia, at a frequency of 25%-40%. In cases with total joint replacements, the interface showed a marked increase in the expression of RFD1 (20%-90%). A considerably greater percentage of RFD1 positive Mphi and FBGC was noted in the interfaces from cases with a high level of detectable metal particulate wear debris (mean 80%, range 60%-90%) than in cases with polyethylene wear debris (mean 30%, range 0%-50%), p 0.0001. RFD7 labelled most tissue Mphi in each group. Immunoreactivity for RFD9 was restricted to FBGC in all cases analysed. The finding of elevated expression of RFD1 on metal-containing Mphi and FBGC in the bone-implant interface suggests an increase in antigen-presenting phenotype and indicates that metal particles have more impact in the induction of immune-mediated responses. Such responses are characterized by sustained cellular hyperreactivity and phenotypic changes in Mphi subsets.

Experimental study of drainage and granulation in response to the intra-articular injection of particles in rat knee joints

Reactive granulation and drainage of intraarticularly injected plastic particles in rat knee joints was examined by light and electron microscopy. The knee joints and associated iliac lymph nodes were excised at various intervals after the injection of latex beads (ϕ1 μm) or fluoresbrite particles (ϕ0.2μm or ϕl0μm) from 5min until 3 months after the injection. Particles in the lymphatic or blood vessels were successfully demonstrated by an enzyme-histochemical method ost of the particles were scattered on the surface of the synovial membrane, and some particles were phagocytosed by synovial lining (5'-nucleotidase staining). Five min after the injection, most of the particles were scattered on the surface of the synovial membrane, and some particles were phagocytosed by synovial lining cells. After 5 h, neutrophils had phagocytosed particles which adhered to fibrin in the joint cavity. Twelve h after the injection, after the neutrophils had died, those same particles were phagocytosed by macrophages in the joint cavity. One day after the injection, Fluoresbrite particles (ϕ0.2μm) phagocytosed by macrophages were found in the iliac lymph nodes, while latex particles (ϕ1 μm) were detected in the iliac lymph nodes 3 days after the injection. Some Fluoresbrite particles (ϕ10μm) were seen in the 5'-nucleotidase-positive lymphatic vessels in the synovial membrane. Three months after the injection, many macrophages filled with particles had formed granulation tissue in the synovial membrane, and macrophages containing phagoeytosed particles were also seen increasingly in the iliac lymph nodes. Our findings suggested that neutrophils and macrophages phagocytosed injected particles in the joint cavity, and that the macrophages brought the particles into the deep layer of the synovial membrane. Phagocytic macrophages also carried the particles to the iliac lymph nodes through lymphatic vessels in the synovial membrane. There were no morphological differences in the processes of granulation and drainage between the two different sized plastic particles (ϕ1 μm and ϕ0.2μm), except for the behavior of the macrophages phagocytosing the particles.

Role for the class A macrophage scavenger receptor in the phagocytosis of apoptotic thymocytes in vitro.

Numerous immature thymocytes undergo apoptosis and are rapidly engulfed by phagocytic thymic macrophages. The macrophage surface receptors involved in apoptotic thymocyte recognition are unknown. We have examined the role of the class A macrophage scavenger receptor (SR-A) in the engulfment of apoptotic thymocytes. Uptake of steroid-treated apoptotic thymocytes by thymic and inflammatory-elicited SR-A positive macrophages is partially inhibited by an anti-SR-A mAb and more completely by a range of scavenger receptor ligands. Thymic macrophages from mice with targeted disruption of the SR-A gene show a 50% reduction in phagocytosis of apoptotic thymocytes in vitro. These data suggest that SR-A may play a role in the clearance of dying cells in the thymus.

Gadolinium chloride inhibits Kupffer cell nitric oxide synthase (iNOS) induction.

Kupffer cells (KC) are the phagocytic macrophages of the liver. The rare earth metal, gadolinium (GdCl3), is a lanthanide, which, after phagocytosis by the KC, has been found to alter various aspects of KC physiology. In this study, we describe for the first time that the in vivo administration of GdCl3 to rats decreases the release of NO by isolated rat KC in response to lipopolysaccharide. Western blot analysis shows decreased expression of both inducible nitric oxide synthase as well as total cellular calmodulin after GdCl3 treatment. Possible mechanisms for this phenomenon are suggested.

Exogenous antigens gain access to the major histocompatibility complex class I processing pathway in B cells by receptor-mediated uptake.

Professional antigen-presenting cells, such as macrophages, dendritic cells, or B cells, take up soluble, exogenous antigens (Ags) and process them through the class II pathway. Several reports have shown that phagocytic macrophages also process particulate or soluble forms of exogenous Ag via the class I pathway. By contrast, B cells normally do not process soluble, exogenous Ag by way of the class I pathway unless Ags are directly introduced into the cytoplasm. Here we report that B cells present exogenous Ag via the class I pathway when Ags are taken up by receptor-mediated endocytosis. Thus, specialized methods of Ag uptake such as phagocytosis or receptor-mediated endocytosis deliver exogenous Ag into the class I pathway of Ag processing and presentation.

REVIEW ■ : Activation of Microglia: A Dangerous Interlude in Immune Function in the Brain

Microglial cells are representatives of the immune system in the CNS parenchyma. Their most characteristic property is their ability to modify their behavior in response to diverse signals from other cells in a variety of experimental conditions and human diseases, both acute and chronic. The transformation from a quiescent state into phagocytic brain macrophages is under strict control and accompanied by the production of several secretory products. These include cytokines, excitatory amino acids, and reactive oxygen metabolites by which the activated microglial cells correspond with other cells of the brain and immune system. Thus, they represent an essential host defense and repair system, and may be responsible for tissue destruction and neuronal death, depending on the balance of activating and inhibitory signals. NEUROSCIENTIST 2:293-299, 1996

A transient phase of cell death in the developing medial forebrain of the perinatal ferret.

A transient bilateral population of cells immediately rostral to the ferret corpus callosum was examined at weekly intervals between embryonic day 28 and postnatal day 7. This region is tentatively identified as the medial forebrain apoptosis zone (MFAZ) because of the specific nature of cell death, and the limited area or zone in which it was observed. No other region within the brain or retina exhibited a similar pattern or amount of cell death. Only scattered apoptotic cells were found throughout the remainder of the brain-including the cerebral cortical plate, subplate, and white matter-with the exception of the ventricular zone of the lateral ventricles which contained a significant population of apoptotic cells. This study addressed three questions about apoptosis in the MFAZ: (1) does apoptotic cell death in this region signal the appearance of phagocytic macrophages, (2) does cell degeneration and phagocytosis in this region lead to the formation of an extracellular space analogous to the cavum septi pellucidi of rodents, and (3) what is the duration of degeneration, or clearance rate, of cell death in this defined region. The MFAZ was first found to contain apoptotic nuclei and macrophages late in gestation, at E34. Numbers of apoptotic nuclei and macrophages peaked one week later at birth in this area, but disappeared early in postnatal life. During this period, formation of a space rostral to the corpus callosum due to the removal of apoptotic cells was not observed. Finally, presence of apoptotic cells in the MFAZ over a period of > or = 9 days suggests a clearance rate of many hours. The close spatiotemporal correlation of the distribution of apoptotic cells and their removal by macrophages suggests that these are interrelated, and perhaps interdependent events.

Optimum treatment of intracellular infection.

The intracellular location of some micro-organisms has been early recognised as a critical point to explain failure of antibiotic therapy to eradicate such pathogens from infected hosts. Most often parasites invade 'professional' phagocytic cells, including neutrophils, monocytes and macrophages, by resisting the intracellular bactericidal phagolysosomal pathway. Alternatively, they may invade 'non-professional' phagocytic cells (cells with fewer phagocytic and bactericidal abilities) such as endothelial cells, or even cells without lysosomes such as erythrocytes. The intracellular activity of an antibiotic depends on several factors including its ability to reach the eukaryotic cell membrane, its subcellular localisation as compared to that of the parasite, the possibility that the intracellular milieu may partially inactivate its activity, and the susceptibility of the intracellular form of the parasite. In vitro and animal models have been developed to investigate antibiotic activity against intracellular pathogens. However, it should be emphasised that only data obtained from patients give reliable information to define the optimum antibiotic regimen.