SummaryThis study shows that in mice selectively depleted of neutrophils by treatment with a monoclonal antibody, RB6-8C5, listeriosis is severely exacerbated in the liver, but not in the spleen or peritoneal cavity during the crucial first day of infection. At sites of infection in the livers of neutrophildepleted mice, Listeria monocytogenes grew to large numbers inside hepatocytes. By contrast, in the livers of normal mice neutrophils rapidly accumulated at infectious loci and this was associated with the lysis of infected hepatocytes that served to abort infection in these permissive cells. In the spleen the situation was different, in that depletion of neutrophils did not result in appreciable exacerbation of infection. In this organ intact infected cells, many of which appeared to be fibroblastlike stromal cells, were found at loci of infection in the presence or absence of large numbers of neutrophils. This suggests that neutrophils are less effective at destroying L. monocytogenes-infected target cells in the spleen than in the liver. Consequently, at least during the first day, the organism remained free to multiply intracellularly in the spleen in cells that are permissive for its growth. Presumably, the same situation exists in the peritoneal cavity, because depleting neutrophils did not severely exacerbate infection initiated at this site.

Acinetobacter baumannii has emerged as a major cause of both community-associated and nosocomial pneumonia, but little is known about the cellular and molecular mechanisms of host defense against respiratory infection with this bacterial pathogen. In this study, we examined the role of neutrophils in host resistance to pulmonary A. baumannii infection in a mouse model of intranasal (i.n.) infection. We found that neutrophils were rapidly recruited to the lungs following i.n. inoculation of the pathogen and declined to baseline level upon clearance of the infection. Depletion of neutrophils using monoclonal antibody RB6-8C5 prior to infection resulted in an acute lethal infection that was associated with enhanced bacterial burdens in the lung (P < 0.05) and extrapulmonary dissemination to the spleen. The increased susceptibility to A. baumannii in neutropenic mice was associated with a delay in the mRNA expression and production of early proinflammatory cytokines such as tumor necrosis factor alpha, interleukin-6, keratinocyte chemoattractant protein, monocyte chemoattractant protein 1, and macrophage inflammatory protein 2 (MIP-2) in the lungs and development of severe bronchopneumonia and lymphoid tissue destruction in the spleen. Moreover, i.n. administration of the neutrophil-inducing chemokine MIP-2 to normal mice induced a pulmonary influx of neutrophils and significantly enhanced the clearance of A. baumannii from the lungs (P < 0.01). These results imply that neutrophils play a critical role in host resistance to respiratory A. baumannii infection.

The structure of the lipid A and core region of the lipopolysaccharide (LPS) from Francisella tularensis (ATCC 29684) was analysed using NMR, mass spectrometry and chemical methods. The LPS contains a β‐GlcN‐(1–6)‐GlcN lipid A backbone, but has a number of unusual structural features; it apparently has no substituent at O‐1 of the reducing end GlcN residue in the lipid part in the major part of the population, no substituents at O‐3 and O‐4 of β‐GlcN, and no substituent at O‐4 of the Kdo residue. The largest oligosaccharide, isolated after strong alkaline deacylation of NaBH4 reduced LPS had the following structure: where Δ‐GalNA‐(1–3)‐β‐QuiNAc represents a modified fragment of the O‐chain repeating unit. Two shorter oligosaccharides lacking the O‐chain fragment were also identified. A minor amount of the disaccharide β‐GlcN‐(1–6)‐α‐GlcN‐1‐P was isolated from the same reaction mixture, indicating the presence of free lipid A, unsubstituted by Kdo and with phosphate at the reducing end. The lipid A, isolated from the products of mild acid hydrolysis, had the structure 2‐N‐(3‐O‐acyl4‐acyl2)‐β‐GlcN‐(1–6)‐2‐N‐acyl1−3‐O‐acyl3‐GlcN where acyl1, acyl2 and acyl3 are 3‐hydroxyhexadecanoic or 3‐hydroxyoctadecanoic acids, acyl4 is tetradecanoic or (minor) hexadecanoic acids. No phosphate substituents were found in this compound. OH‐1 of the reducing end glucosamine, and OH‐3 and OH‐4 of the nonreducing end glucosamine residues were not substituted. LPS of F. tularensis exhibits unusual biological properties, including low endoxicity, which may be related to its unusual lipid A structure.

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The rate of growth of Listeria monocytogenes in the livers of mice infected intravenously with a lethal or sublethal inoculum of this facultative intracellular bacterium is greatly increased if neutrophils and other host cells are prevented from accumulating at foci of infection during the first 24 h by treatment with a monoclonal antibody (5C6) specific for the type 3 complement receptor of myelomonocytic cells. A histological examination of the livers of control mice showed that the accumulation of neutrophils at infectious foci resulted in the focal destruction of infected hepatocytes. In contrast, failure of neutrophils to accumulate at these sites in 5C6-treated mice allowed Listeria to multiply extensively in hepatocytes without destroying them. The results indicate that neutrophils play an important role in early defense against listeriosis in the liver by destroying infected hepatocytes, thereby reducing the opportunity for Listeria to multiply in permissive cells. In this way, neutrophils serve to break the chain of cell-to-cell spread of infection.

The results show that Listeria monocytogenes, Francisella tularensis, and Salmonella typhimurium are facultative intracellular bacteria with a capacity to invade and grow in nonphagocytic cells in vivo. In the liver, all of these pathogens were seen to invade and to multiply extensively in hepatocytes. In all three cases, inflammatory phagocytes were rapidly marshalled to foci of infection where they appeared to cause the destruction of infected hepatocytes, thereby releasing bacteria into the extracellular space, in which presumably they could be ingested and destroyed by the phagocytes. If phagocytic cells were prevented from accumulating at foci of liver infection by treatment of the mice with a monoclonal antibody (NIMP-R10) directed against the type 3 complement receptor of myelomonocytic cells, then lysis of hepatocytes failed to occur and bacteria proliferated unrestrictedly within them. Under these circumstances, otherwise sublethal infections became rapidly lethal. These findings strongly suggest that lysis of infected hepatocytes by phagocytic cells is an important general early-defense strategy against liver infection with at least three different intracellular bacteria.

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