The Lymphatic Headmaster of the Mast Cell-Related Splanchnic Inflammation in Portal Hypertension.

Portal hypertension is a common complication of liver disease, either acute or chronic. Consequently, in chronic liver disease, such as the hypertensive mesenteric venous pathology, the coexisting inflammatory response is classically characterized by the splanchnic blood circulation. However, a vascular lymphatic pathology is produced simultaneously with the splanchnic arterio-venous impairments. The pathological increase of the mesenteric venous pressure, by mechanotransduction of the venous endothelium hyperpressure, causes an inflammatory response involving the subendothelial mast cells and the lymphatic endothelium of the intestinal villi lacteal. In portal hypertension, the intestinal lymphatic inflammatory response through the development of mesenteric-systemic lymphatic collateral vessels favors the systemic diffusion of substances with a molecular pattern associated with damage and pathogens of intestinal origin. When the chronic hepatic insufficiency worsens the portal hypertensive inflammatory response, the splanchnic lymphatic system transports the hyperplasied intestinal mast cells to the mesenteric lymphatic complex. Then, an acquired immune response regulating a new hepato-intestinal metabolic scenario is activated. Therefore, reduction of the hepatic metabolism would reduce its key centralized functions, such as the metabolic, detoxifying and antioxidant functions which would try to be substituted by their peroxisome activity, among other functions of the mast cells.

Schistosome Egg Migration: Mechanisms, Pathogenesis and Host Immune Responses.

Many parasitic worms possess complex and intriguing life cycles, and schistosomes are no exception. To exit the human body and progress to their successive snail host, Schistosoma mansoni eggs must migrate from the mesenteric vessels, across the intestinal wall and into the feces. This process is complex and not always successful. A vast proportion of eggs fail to leave their definite host, instead becoming lodged within intestinal or hepatic tissue, where they can evoke potentially life-threatening pathology. Thus, to maximize the likelihood of successful egg passage whilst minimizing host pathology, intriguing egg exit strategies have evolved. Notably, schistosomes actively exert counter-inflammatory influences on the host immune system, discreetly compromise endothelial and epithelial barriers, and modulate granuloma formation around transiting eggs, which is instrumental to their migration. In this review, we discuss new developments in our understanding of schistosome egg migration, with an emphasis on S. mansoni and the intestine, and outline the host-parasite interactions that are thought to make this process possible. In addition, we explore the potential immune implications of egg penetration and discuss the long-term consequences for the host of unsuccessful egg transit, such as fibrosis, co-infection and cancer development.

Colitogenic effector T cells: roles of gut-homing integrin, gut antigen specificity and γδ T cells.

Disturbance of T-cell homeostasis could lead to intestinal inflammation. Naive CD4 T cells undergoing spontaneous proliferation, a robust proliferative response that occurs under severe lymphopenic conditions, differentiate into effector cells producing Th1- and/or Th17-type cytokines and induce a chronic inflammation in the intestine that resembles human inflammatory bowel disease. In this study, we investigated the key properties of CD4 T cells necessary to induce experimental colitis. α4ß7 upregulation was primarily induced by mesenteric lymph node (mLN) resident CD11b(+) dendritic cell subsets via transforming growth factor beta (TGFß)/retinoic acid-dependent mechanism. Interestingly, α4ß7 expression was essential but not sufficient to induce inflammation. In addition to gut-homing specificity, expression of gut Ag specificity was also crucial. T-cell acquisition of the specificity was dramatically enhanced by the presence of γδ T cells, a population previously shown to exacerbate T-cell-mediated colitis. Importantly, interleukin (IL)-23-mediated γδ T cell stimulation was necessary to enhance colitogenicity but not gut antigen reactivity of proliferating CD4 T cells. These findings demonstrate that T-cell colitogenicity is achieved through multiple processes, offering a therapeutic rationale by intervening these pathways.

[Microvascular effects of burn plasma transfer and therapeutic options in a rat model]. / Mikrovaskuläre Effekte des Verbrennungsplasma-transfers und therapeutische Optionen im Tiermodell.

INTRODUCTION: Thermal injuries with more than 20% of burned body surface area (BSA) lead to systemic shock with generalised oedema in addition to local tissue destruction. This condition, known as burn injury, is caused by immunmodulative mediators whose individual significance is not known in detail. We present an experimental model where plasma of burned animals (burn plasma) is transmitted to healthy animals, to trigger burn iniury without performing direct burn trauma. MATERIAL AND METHODS: The systemic oedema is measured by extravasation of fluorescent albumin in mesenterial venules of Wistar rats. In addition, leukocyte-endothelial interactions ("leukocyte rolling and sticking") is examined. RESULTS: The systemic capillary leak is induced by both direct thermal trauma as well as by infusion of burn plasma. This is evident even after plasma dilution (1% in Ringer's lactate) of the burn plasma. In addition, topical therapy for burned animals (donors) with cerium nitrate led to a significant reduction of plasma extravasation in receiver animals. In addition, systemic antioxidant therapy with high-dose vitamin C of receiver animals, led to a significant reduction of the capillary leak. Leukocyte-endothelial interactions are not significantly affected in either case. CONCLUSION: In summary, for the first time a reliable model of burn injury has been established, which eliminates mediator-independent effects. In addition, our studies show that antioxidant therapy with high doses of vitamin C and topical treatment with cerium nitrate both reduce the systemic capillary leak in receiver animals. Their positive influence could therefore soon be integrated in clinical treatment algorithms.

Monitoring of blood vessels and tissues by a population of monocytes with patrolling behavior.

The cellular immune response to tissue damage and infection requires the recruitment of blood leukocytes. This process is mediated through a classical multistep mechanism, which involves transient rolling on the endothelium and recognition of inflammation followed by extravasation. We have shown, by direct examination of blood monocyte functions in vivo, that a subset of monocytes patrols healthy tissues through long-range crawling on the resting endothelium. This patrolling behavior depended on the integrin LFA-1 and the chemokine receptor CX(3)CR1 and was required for rapid tissue invasion at the site of an infection by this "resident" monocyte population, which initiated an early immune response and differentiated into macrophages.

Endogenous hydrogen sulfide regulates leukocyte trafficking in cecal ligation and puncture-induced sepsis.

Hydrogen sulfide (H(2)S) is recognized increasingly as a proinflammatory mediator in various inflammatory conditions. Here, we have investigated the role of H(2)S in regulating expression of some endothelial adhesion molecules and recruitment of leukocytes to inflamed sites in sepsis. Male Swiss mice were subjected to cecal ligation and puncture (CLP)-induced sepsis and treated with saline (i.p.), DL-propargylglycine (PAG; 50 mg/kg, i.p.), an inhibitor of H(2)S formation or NaHS (10 mg/kg, i.p.), an H(2)S donor. PAG was administered 1 h before or after the induction of sepsis, and NaHS was given at the same time of CLP. Using intravital microcopy, we found that in sepsis, prophylactic and therapeutic administration of PAG reduced leukocyte rolling and adherence significantly in mesenteric venules coupled with decreased mRNA and protein levels of adhesion molecules (ICAM-1, P-selectin, and E-selectin) in lung and liver. In contrast, injection of NaHS up-regulated leukocyte rolling and attachment significantly, as well as tissue levels of adhesion molecules in sepsis. Conversely, normal mice were given NaHS (10 mg/kg, i.p.) to induce lung inflammation, with or without NF-kappaB inhibitor BAY 11-7082 pretreatment. NaHS treatment enhanced the level of adhesion molecules and neutrophil infiltration in lung. These alterations were reversed by pretreatment with BAY 11-7082. Moreover, expression of CXCR2 in neutrophils obtained from H(2)S-treated mice was up-regulated significantly, leading to an obvious elevation in MIP-2-directed migration of neutrophils. Therefore, H(2)S acts as an important endogenous regulator of leukocyte activation and trafficking during an inflammatory response.

Clostridium perfringens epsilon-toxin increases permeability of single perfused microvessels of rat mesentery.

Epsilon-toxin, the primary virulence factor of Clostridium perfringens type D, causes mortality in livestock, particularly sheep and goats, in which it induces an often-fatal enterotoxemia. It is believed to compromise the intestinal barrier and then enter the gut vasculature, from which it is carried systemically, causing widespread vascular endothelial damage and edema. Here we used single perfused venular microvessels in rat mesentery, which enabled direct observation of permeability properties of the in situ vascular wall during exposure to toxin. We determined the hydraulic conductivity (L(p)) of microvessels as a measure of the response to epsilon-toxin. We found that microvessels were highly sensitive to toxin. At 10 microg ml(-1) the L(p) increased irreversibly to more than 15 times the control value by 10 min. At 0.3 microg ml(-1) no increase in L(p) was observed for up to 90 min. The toxin-induced increase in L(p) was consistent with changes in ultrastructure of microvessels exposed to the toxin. Those microvessels exhibited gaps either between or through endothelial cells where perfusate had direct access to the basement membrane. Many endothelial cells appeared necrotic, highly attenuated, and with dense cytoplasm. We showed that epsilon-toxin, in a time- and dose-dependent manner, rapidly and irreversibly compromised the barrier function of venular microvessel endothelium. The results conformed to the hypothesis that epsilon-toxin interacts with vascular endothelial cells and increases the vessel wall permeability by direct damage of the endothelium.

Neutrophil extravasation in rat mesenteric venules induced by the chemotactic peptide N-formyl-methionyl-luecylphenylalanine (fMLP), with special attention to a barrier function of the vascular basal lamina for neutrophil migration.

The present study was performed to investigate morphologically the process of neutrophil extravasation induced by the synthetic bacterial peptide N-formylmethionyl-leucyl-phenylalanine (fMLP) in venules of the rat mesentery by the combined use of intravital microscopy and transmission electron microscopy (TEM). Special attention was given to the interaction of the neutrophils with the endothelial cells and endothelial basal lamina. By intravital microscopy, the rolling and adhesion of leukocytes were observed within 3 min in preparations activated by fMLP. Neutrophils remained in the vascular wall for more than 30 min even after penetration of the endothelium. In this period, neutrophils migrating between endothelial cells and their basal lamina were often observed by TEM. After 40 min application of fMLP, some of the migrating neutrophils parted from the vessel wall into the surrounding connective tissues. There were neutrophils which passed through the small pore of the basal lamina with a cytoplasmic constriction. These findings indicate that the endothelial basal lamina acts as a physical barrier against neutrophil extravasation, thus resulting in the transient retainment of neutrophils beneath the endothelial cells of the venular wall.

Reduction in intestinal leukocyte adherence in rat experimental endotoxemia by treatment with the 21-aminosteroid U-74389G.

OBJECTIVES: To investigate leukocyte adherence in intestinal venules in experimental endotoxemia after treatment with the 21-aminosteroid U-74389G. DESIGN AND SETTING: Prospective, randomized, controlled animal study in an experimental laboratory. SUBJECTS: Twenty-one male Wistar rats weighing 190 +/- 40 g. INTERVENTIONS: The rats were divided equally into three groups: (a) control group, (b) endotoxemia (5 mg/kg lipopolysacharide from Escherichia coli O55:B5), and (c) endotoxemia and U-74389G administration 30 min before (3 mg/kg) and 60 min after endotoxin challenge (1.5 mg/ kg). MEASUREMENTS AND MAIN RESULTS: The distal small intestine of the animals was examined using intravital fluorescence videomicroscopy 2 h after endotoxin challenge. Leukocytes were stained in vivo by means of rhodamine 6G. In the endotoxemic animals we observed a fourfold increase in the count of firmly adherent leukocytes in submucosal post-capillary and collecting venules. Treatment with the 21-aminosteroid U-74389G significantly attenuated the count of sticking leukocytes in the collecting venules (control, 61 +/- 10 cells/mm2; lipopolysaccharide, 237 +/- 42 cells/mm2; U-74389G 125 +/- 9 cells/mm2; p < 0.05). In these venules leukocyte rolling behavior was comparable to that in the control group without endotoxin challenge. CONCLUSIONS: Administration of U-74389G, which has radical scavenging properties, attenuates leukocyte adherence in selected populations of intestinal venules which is found increased during endotoxemia. Thus, 21-aminosteroids may have an impact in the treatment of endotoxin-induced intestinal injury.

Association between kinin B(1) receptor expression and leukocyte trafficking across mouse mesenteric postcapillary venules.

Using intravital microscopy, we examined the role played by B(1) receptors in leukocyte trafficking across mouse mesenteric postcapillary venules in vivo. B(1) receptor blockade attenuated interleukin (IL)-1beta-induced (5 ng intraperitoneally, 2 h) leukocyte-endothelial cell interactions and leukocyte emigration ( approximately 50% reduction). The B(1) receptor agonist des-Arg(9)bradykinin (DABK), although inactive in saline- or IL-8-treated mice, caused marked neutrophil rolling, adhesion, and emigration 24 h after challenge with IL-1beta (when the cellular response to IL-1beta had subsided). Reverse transcriptase polymerase chain reaction and Western blot revealed a temporal association between the DABK-induced response and upregulation of mesenteric B(1) receptor mRNA and de novo protein expression after IL-1beta treatment. DABK-induced leukocyte trafficking was antagonized by the B(1) receptor antagonist des-arg(10)HOE 140 but not by the B(2) receptor antagonist HOE 140. Similarly, DABK effects were maintained in B(2) receptor knockout mice. The DABK-induced responses involved the release of neuropeptides from C fibers, as capsaicin treatment inhibited the responses. Treatment with the neurokinin (NK)(1) and NK(3) receptor antagonists attenuated the responses, whereas NK(2), calcitonin gene-related peptide, or platelet-activating factor receptor antagonists had no effect. Substance P caused leukocyte recruitment that, similar to DABK, was inhibited by NK(1) and NK(3) receptor blockade. Mast cell depletion using compound 48/80 reduced DABK-induced leukocyte trafficking, and DABK treatment was shown histologically to induce mast cell degranulation. DABK-induced trafficking was inhibited by histamine H(1) receptor blockade. Our findings provide clear evidence that B(1) receptors play an important role in the mediation of leukocyte-endothelial cell interactions in postcapillary venules, leading to leukocyte recruitment during an inflammatory response. This involves activation of C fibers and mast cells, release of substance P and histamine, and stimulation of NK(1), NK(3), and H(1) receptors.

Simvastatin inhibits leukocyte-endothelial cell interactions and protects against inflammatory processes in normocholesterolemic rats.

Simvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, has been shown to lower serum cholesterol levels and normalize endothelial cell function. Moreover, HMG-CoA reductase inhibitors exert beneficial effects in coronary artery and cerebrovascular diseases. We examined the effects of simvastatin on leukocyte-endothelial cell interaction in vivo by intravital microscopy. Simvastatin (12.5 or 25 microg per rat) was given 18 hours before study. Superfusion with the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 50 micromol/L) significantly increased leukocyte rolling from 12+/-2 to 60+/-8 leukocytes per minute, increased adherence to the mesenteric endothelium from 1.8+/-0.5 to 17+/-1.2 leukocytes per 100 microm of venular length, and raised leukocyte transmigration from 2.5+/-1.0 to 10+/-2 leukocytes per perivessel area (P<0.01). Similar results were obtained with thrombin (0.5 U/mL) superfusion of the mesentery. In contrast, pretreatment with simvastatin (25 microg per rat IP) significantly attenuated L-NAME-stimulated leukocyte rolling, to 12+/-2 (P<0.01); adherence, to 5+/-0.5 leukocytes per 100 microm (P<0.01); and leukocyte transmigration, to 3.5+/-1.5 leukocytes per perivessel area (P<0.01). Similar results were obtained in thrombin-superfused mesenteries. Moreover, immunohistochemical analysis demonstrated significantly increased P-selectin expression on the mesenteric venular endothelium after superfusion with either L-NAME (P<0.01) or thrombin (P<0.01), which was significantly attenuated by simvastatin. These results clearly demonstrate that simvastatin is a potent and effective endothelium-protective agent that reduces leukocyte-endothelial cell interactions independently of its well-known lipid-lowering effects. This effect was found to be at least partially mediated via downregulation of P-selectin expression on the microvascular endothelium. Thus, HMG-CoA reductase inhibitors like simvastatin have important anti-inflammatory effects besides their well-known lipid-lowering action.

Human eotaxin induces eosinophil extravasation through rat mesenteric venules: role of alpha4 integrins and vascular cell adhesion molecule-1.

Eotaxin is a potent eosinophil-specific CC-chemokine, which has been shown to play a role in the selective induction of eosinophil accumulation in a number of allergic models of inflammation. Many aspects of the mechanism by which eotaxin induces eosinophil accumulation in vivo remain unresolved. In the present study, we investigated the direct effect of synthetic human eotaxin on leucocyte/endothelial cell interactions within rat mesenteric venules, as quantified by intravital microscopy. Topical eotaxin (30 pmol) induced rapid firm adhesion and extravasation of leucocytes within the rat mesentery, the extravasated leucocytes all being eosinophils, as determined by histological analysis. Whilst eotaxin was unable to stimulate the interaction of rat eosinophils with vascular cell adhesion molecule-1 (VCAM-1) under static conditions in vitro, eotaxin-induced responses in vivo were significantly suppressed by anti-alpha4 integrin and anti-VCAM-1 monoclonal antibodies (mAbs). The anti-alpha4 integrin mAb, HP2/1 (3.5 mg/kg), inhibited the eotaxin-induced firm adhesion and extravasation, 60 min postapplication of the chemokine, by 89% and 84%, respectively. In the same set of experiments, the anti-VCAM-1 mAb, 5F10 (3.5 mg/kg), inhibited leucocyte adhesion and extravasation by 61% and 63%, respectively. These results demonstrate that eotaxin-induced migration of eosinophils through rat mesenteric venules in vivo is dependent on an alpha4 integrin/VCAM-1 adhesion pathway, the significance of which may only be evident under flow conditions and/or following the ligation of other adhesion molecules expressed on eosinophils.

Animal model of vascular inflammation.

Inflammatory mechanisms play a central role in the pathology of a variety of conditions ranging from atherosclerosis, arthritis, cancer and Alzheimer's disease. Under normal conditions the inflammatory response initiates protective actions, but triggers tissue damage under pathological conditions. Acute or chronic inflammation is mediated by nascent expression of a host of proteins such as the cytokines interleukins (IL), tumor necrosis factor (TNF), and interferons. Currently available in vitro or in vivo methods do not offer the specificity to probe the complex inflammatory cascade. We developed an animal model in which a single injection of the proinflammatory cytokines TNF-alpha and IL-1 beta in live rodents initiates a rapid inflammatory reaction which can be monitored by video microscopy and electron microscopy. This model exhibits the characteristic feature of inflammatory reaction such as adhesion and transmigration of leukocytes, and activation and degranulation of platelets and mast cells. This model is applicable to inflammatory processes in the peripheral and cerebral vasculature including the blood-brain barrier disruption in Alzheimer's disease. The animal model of inflammation reported here may prove to be a valuable tool in investigating the pathophysiology of a number of inflammatory diseases and identifying potential targets as well as agents for therapy.

Image enhancement of the in vivo leukocyte-endothelium contact zone using optical sectioning microscopy.

A major determinant of the strength of leukocyte [white blood cell (WBC)] to endothelium [endothelial cell (EC)] adhesion is the contact area formed between the two cells, which is often obscured by out-of-focus information inherent to intravital microscopy. To improve visualization of the WBC-EC contact zone, techniques of optical sectioning microscopy were developed to enhance brightfield images of WBC-EC adhesion in postcapillary venules of the mesentery of the rat. A 50x/1.0 NA objective was held in a piezoelectric mount that was computer-driven, and video images were obtained by digitizing images from a CCD camera while focusing through the vertical direction in 1 micron steps over a depth of 16 microns. Using measurements of the microscope's optical transfer function, deconvolution of the central image was performed in the Fourier domain using the technique of singular value decomposition with Tikhonov-Miller regulation to remove out-of-focus information. Measurement of the length of the WBC-EC contact zone (LC) in the original images yielded values on the order of 4.32 +/- 1.08 microns (mean +/- SD). The enhanced images showed a significantly 35% smaller LC equal to 2.78 +/- 0.70 micron. Topical application of the chemoattractant f-met-leu-phe resulted in a 26% increase in LC to 3.49 +/- 0.72 micron, thus suggesting that upregulation of adhesion molecules on the WBC membrane results in the recruitment of additional membrane area from surface ruffles into the zone of adhesion. Other advantages of the deconvolution were to visualize structural characteristics of the microvascular wall and parenchymal tissue in greater detail. Thus, brightfield optical sectioning microscopy may provide a valuable tool for in vivo studies of the microvasculature, and serves as a useful alternative to fluorescence microscopy without the undesirable effects of exogenous fluorophores and exposure to ultraviolet radiation.

Role of interleukin 8 on leucocyte-endothelial cell adhesion in intestinal inflammation.

BACKGROUND: An important action of interleukin 8 (IL8) is stimulation of granulocytes. The object of this study was to assess the contribution of IL8 to the leucocyte-endothelial cell interactions associated with intestinal inflammation in the rat. METHODS: Two indomethacin injections (48 and 24 hours prior to the experiments) induced a longlasting ileitis in rats. The number of adherent and emigrated leucocytes, leucocyte rolling velocity, and shear rate were monitored in normal and inflamed mesenteric postcapillary venules. Some animals received a monoclonal antibody (MAb) against IL8 or CD11b/CD18 at 24 and 12 hours prior to the experiment. RESULTS: Indomethacin elicited a seven-fold increase in leucocyte adherence and a 5.4-fold increase in leucocyte emigration, while leucocyte rolling velocity was reduced by nearly 80%. The indomethacin induced increases in leucocyte adherence and emigration were significantly reduced (by 57% and 67%, respectively) while leucocyte rolling velocity was increased (to 63% of control) by the IL8-specific MAb. The level of inhibition seen with the IL8 MAb was similar to that associated with administration of a MAb directed against the leucocyte adhesion molecule CD11b/CD18. CONCLUSIONS: IL8 contributes to the leucocyte-endothelial cell interactions elicited in mesenteric venules by indomethacin.

The mononuclear cells of human mesenteric blood, intestinal mucosa and mesenteric lymph nodes: compartmentalization of NK cells.

The proportions of T cell subsets and Leu 7+ cells and the spontaneous cell-mediated cytotoxicity (SCMC) of isolated mononuclear cells have been determined across the mesenteric vascular bed and along the intestinal mucosal-mesenteric lymph node (MLN) axis in patients undergoing abdominal surgery. Whereas the proportion of T4+ and T8+ cells were similar in simultaneously taken PVB and mesenteric venous blood (MVB), the proportion of Leu 7+ cells was higher in MVB in 16 of 17 studies (15.4 +/- 6.8%, 10.8 +/- 5.1%). Additional studies showed that the proportions of lymphocyte subsets in peripheral arterial blood are the same as those in PVB. Thus, an enrichment of Leu 7+ cells occurs across the mesenteric vascular bed. Isolated intestinal and MLN mononuclear cells contained similarly high proportions of T4+ and T8+ cells as in PVB but Leu 7+ cells made up a minority subpopulation in intestinal (1.3 +/- 0.8%) and MLN mononuclear cells (1.0 +/- 0.9%). The SCMC of intestinal and MLN mononuclear cells was low and paralleled the proportion of Leu 7+ cells. Despite the higher proportions of Leu 7+ cells in MVB, the SCMC was less than that of PVB in eight patients with inflamed intestine and not significantly different from PVB in seven patients with normal intestines. These paradoxical findings were at least in part due to inhibitory factors in mesenteric plasma. In conclusion, NK cells appear to be largely confined within the vascular system and the enrichment of Leu 7+ cells across the mesenteric vascular bed suggests that this compartmentalization may be due to differences in the traffic of lymphocyte subpopulations through the intestinal mucosa and MLN.

[Comparison between the way of intravenous administration of flagellin from Salmonella typhosa to the margial vein in the ear and into the mesenteric superior vein. Preliminary note]. / Confronto tra la via di introduzione endovenosa di "flagellina" della Salmonella typhosa nella vena marginale dell'orecchio e nella vena mesenterica superiore. Nota preliminare

Rabbits receiving three injections of PFA intravenously in ear, after that first treatment gave a weak antibody response, after the second one a rapid and copious increase of agglutination titre; the third injection induced a very small increase. In rabbits injected with PFA in superior mesenteric vein the first injection produced a little effect, the second induced a little effect too; the third injection performed in auricular vein, gave a substancial rapid and copious antibody response. This behaviour is similar to that obtained after the second injection in the first set of experiments. The PFA injected two times in mesenteric vein caused certainly a massive phagocytosis of antigen by Kupffer cells. Nevertheless this copious phagocytosis can exercise only an action of "priming", not a response of secondary type. For this purpose it is necessary an injection in the ear vein to provoke a direct intervention of lymphonodal reticular phagocytes. The phenomenon can be related to the proteic nature of the antigen.