The neuropathogenesis of listeria encephalomyelitis in sheep and mice

Human Listeriosis

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen that can cause severe invasive infections upon ingestion with contaminated food. Clinically, listerial disease, or listeriosis, most often presents as bacteremia, meningitis or meningoencephalitis, and pregnancy-associated infections manifesting as miscarriage or neonatal sepsis. Invasive listeriosis is life-threatening and a main cause of foodborne illness leading to hospital admissions in Western countries. Sources of contamination can be identified through international surveillance systems for foodborne bacteria and strains' genetic data sharing. Large-scale whole genome studies have increased our knowledge on the diversity and evolution of L. monocytogenes, while recent pathophysiological investigations have improved our mechanistic understanding of listeriosis. In this article, we present an overview of human listeriosis with particular focus on relevant features of the causative bacterium, epidemiology, risk groups, pathogenesis, clinical manifestations, and treatment and prevention.

Listeria monocytogenes at the interface between ruminants and humans: A comparative pathology and pathogenesis review

The bacterium Listeria monocytogenes (Lm) is widely distributed in the environment as a saprophyte, but may turn into a lethal intracellular pathogen upon ingestion. Invasive infections occur in numerous species worldwide, but most commonly in humans and farmed ruminants, and manifest as distinct forms. Of those, neuroinfection is remarkably threatening due to its high mortality. Lm is widely studied not only as a pathogen but also as an essential model for intracellular infections and host-pathogen interactions. Many aspects of its ecology and pathogenesis, however, remain unclear and are rarely addressed in its natural hosts. This review highlights the heterogeneity and adaptability of Lm by summarizing its association with the environment, farm animals, and disease. It also provides current knowledge on key features of the pathology and (molecular) pathogenesis of various listeriosis forms in naturally susceptible species with a special focus on ruminants and on the neuroinvasive form of the disease. Moreover, knowledge gaps on pathomechanisms of listerial infections and relevant unexplored topics in Lm pathogenesis research are highlighted.

Two Prevalent Listeria ivanovii subsp. ivanovii Clonal Strains With Different Virulence Exist in Wild Rodents and Pikas of China

Listeria ivanovii subsp. ivanovii is an intracellular bacterium distributed widely in nature, causing the listeriosis in ruminants and humans. Previous researches had isolated 116 strains of L. ivanovii subsp. ivanovii from wild rodents and pikas of different regions in China, and the predominant sequence types were ST1 and ST2. In this study, we first investigated the biological characteristics and virulence of these two clonal strains including motility, metabolism and virulence in cells and mouse model. The results demonstrated the ST1 strains exhibited motility, wide metabolic activity and hypervirulence, whereas the ST2 strains showed non-motility, relative lower metabolic activity and virulence. Considering the transmissible ability from wild rodents and pikas to ecological environment, the L. ivanovii subsp. ivanovii with potential pathogenicity to humans and ruminants should be monitored.

Histopathology of Listeria Meningitis

Listeria monocytogenes meningitis is the third most common cause of bacterial meningitis in adults and has high mortality and morbidity rates. We describe the clinical course and score brain pathology of 5 patients who died of listeria meningitis. All patients were immunocompromised and ages ranged between 48 and 76 years. Three cases were confirmed by cerebrospinal fluid culture; one was confirmed by brain culture; and one diagnosis was based on a positive blood culture and neuropathological findings. Mild inflammation of meningeal arteries was found in 3 of 5 cases (60%). Moderate/severe ventriculitis was seen in 4 of 4 cases (100%), abscesses in 3 of 4 cases (75%), mild vascular inflammation in 4 of 5 cases (80%), mild/moderate hemorrhage in 2 of 4 cases (50%), mild/moderate thrombosis of meningeal artery in 3 of 5 cases (60%), and 1 case (25%) showed a moderate infarct. The inflammatory cells present in the meninges were characterized by a mix of monocytes, macrophages, and neutrophils and removal of apoptotic inflammatory cells by macrophages (efferocytosis). Gram stain showed intra- and extracellular presence of rod-shaped bacteria in 3 cases. Pathological examination was characterized by moderate to severe ventriculitis, abscesses and abundant efferocytosis which has been suggested to be exploited by L. monocytogenes for cell-to-cell spread.

Listeria monocytogenes spreads within the brain by actin-based intra-axonal migration

Listeria monocytogenes rhombencephalitis is a severe progressive disease despite a swift intrathecal immune response. Based on previous observations, we hypothesized that the disease progresses by intra-axonal spread within the central nervous system. To test this hypothesis, neuroanatomical mapping of lesions, immunofluorescence analysis, and electron microscopy were performed on brains of ruminants with naturally occurring rhombencephalitis. In addition, infection assays were performed in bovine brain cell cultures. Mapping of lesions revealed a consistent pattern with a preferential affection of certain nuclear areas and white matter tracts, indicating that Listeria monocytogenes spreads intra-axonally within the brain along interneuronal connections. These results were supported by immunofluorescence and ultrastructural data localizing Listeria monocytogenes inside axons and dendrites associated with networks of fibrillary structures consistent with actin tails. In vitro infection assays confirmed that bacteria were moving within axon-like processes by employing their actin tail machinery. Remarkably, in vivo, neutrophils invaded the axonal space and the axon itself, apparently by moving between split myelin lamellae of intact myelin sheaths. This intra-axonal invasion of neutrophils was associated with various stages of axonal degeneration and bacterial phagocytosis. Paradoxically, the ensuing adaxonal microabscesses appeared to provide new bacterial replication sites, thus supporting further bacterial spread. In conclusion, intra-axonal bacterial migration and possibly also the innate immune response play an important role in the intracerebral spread of the agent and hence the progression of listeric rhombencephalitis.

Animal models of listeriosis: a comparative review of the current state of the art and lessons learned

Listeriosis is a leading cause of hospitalization and death due to foodborne illness in the industrialized world. Animal models have played fundamental roles in elucidating the pathophysiology and immunology of listeriosis, and will almost certainly continue to be integral components of the research on listeriosis. Data derived from animal studies helped for example characterize the importance of cell-mediated immunity in controlling infection, allowed evaluation of chemotherapeutic treatments for listeriosis, and contributed to quantitative assessments of the public health risk associated with L. monocytogenes contaminated food commodities. Nonetheless, a number of pivotal questions remain unresolved, including dose-response relationships, which represent essential components of risk assessments. Newly emerging data about species-specific differences have recently raised concern about the validity of most traditional animal models of listeriosis. However, considerable uncertainty about the best choice of animal model remains. Here we review the available data on traditional and potential new animal models to summarize currently recognized strengths and limitations of each model. This knowledge is instrumental for devising future studies and for interpreting current data. We deliberately chose a historical, comparative and cross-disciplinary approach, striving to reveal clues that may help predict the ultimate value of each animal model in spite of incomplete data.

Rhombencephalitis Caused by Listeria monocytogenes in Humans and Ruminants: A Zoonosis on the Rise?

Listeriosis is an emerging zoonotic infection of humans and ruminants worldwide caused by Listeria monocytogenes (LM). In both host species, CNS disease accounts for the high mortality associated with listeriosis and includes rhombencephalitis, whose neuropathology is strikingly similar in humans and ruminants. This review discusses the current knowledge about listeric encephalitis, and involved host and bacterial factors. There is an urgent need to study the molecular mechanisms of neuropathogenesis, which are poorly understood. Such studies will provide a basis for the development of new therapeutic strategies that aim to prevent LM from invading the brain and spread within the CNS.

Neuropathogenesis of naturally occurring encephalitis caused by Listeria monocytogenes in ruminants

Listeriosis is a serious food-borne disease with increasing frequency in humans and ruminants. Despite the facts that in both hosts, listeriosis can occur as rhombencephalitis and ruminants are a reservoir of Listeria monocytogenes (LM) strains pathogenic for humans, little work has been done on the pathogenesis in ruminants. This study investigates the neuropathogenesis of listeric encephalitis in over 200 natural cases in cattle, sheep and goats by analyzing anatomical distribution, severity, bacterial load and temporal evolution of the lesions. Our results suggest that LM gains access to the brainstem of all three species via axonal migration not only along the trigeminal nerve, but also along other nerves. The ensuing encephalitis does not remain restricted to the brainstem. Rather, LM spreads further from the brainstem into rostral brain regions likely by intracerebral axonal migration. Significant differences in severity of the lesions and bacterial load were found between cattle and small ruminants, which may be caused by species-specific properties of antibacterial immune responses. As histopathological lesions of human rhombencephalitis caused by LM strongly resemble those of ruminants, the disease likely has a similar pathogenesis in both hosts.

Listeria monocytogenes: epidemiology, human disease, and mechanisms of brain invasion

Listeria monocytogenes is a facultative intracellular bacterium that has predilection for causing central nervous systemic infections in humans and domesticated animals. This pathogen can be found worldwide in the food supply and most L. monocytogenes infections are acquired through ingestion of contaminated food. The main clinical syndromes caused by L. monocytogenes include febrile gastroenteritis, perinatal infection, and systemic infections marked by central nervous system infections with or without bacteremia. Experimental infection of mice has been used for over 50 years as a model system to study the pathogenesis of this organism including the mechanisms by which it invades the brain. Data from this model indicate that a specific subset of monocytes, distinguished in part by high expression of the Ly-6C antigen, become parasitized in the bone marrow and have a key role in transporting intracellular bacteria across the blood-brain barriers and into the central nervous system. This Minireview will summarize recent epidemiologic and clinical information regarding L. monocytogenes as a human pathogen and will discuss current in vitro and in vivo data relevant to the role of parasitized monocytes and the pathogenetic mechanisms that underlie its formidable ability to invade the central nervous system.

Axonal transport of Listeria monocytogenes and nerve-cell-induced bacterial killing

Listeria monocytogenes (L. monocytogenes) can cause fatal brainstem encephalitis in both sheep and humans. Here we review evidence that the bacteria can be incorporated into axons following a primary cycle of replication in macrophages/dendritic cells after subcutaneous injection in projection areas of peripheral neurons. The molecular mechanisms for the rocketing of L. monocytogenes in the cytosol by asymmetric cometic tails and the utility of this phenomenon for bacterial migration intraaxonally both in retro- and in anterograde directions to reach the central nervous system are described. The role of the immune response in the control of L. monocytogenes spread through peripheral neurons is highlighted, and a mechanism by which bacteria may be killed inside infected neurons through a nitric oxide-dependent pathway is pointed out.

Invasion of the central nervous system by intracellular bacteria

Infection of the central nervous system (CNS) is a severe and frequently fatal event during the course of many diseases caused by microbes with predominantly intracellular life cycles. Examples of these include the facultative intracellular bacteria Listeria monocytogenes, Mycobacterium tuberculosis, and Brucella and Salmonella spp. and obligate intracellular microbes of the Rickettsiaceae family and Tropheryma whipplei. Unfortunately, the mechanisms used by intracellular bacterial pathogens to enter the CNS are less well known than those used by bacterial pathogens with an extracellular life cycle. The goal of this review is to elaborate on the means by which intracellular bacterial pathogens establish infection within the CNS. This review encompasses the clinical and pathological findings that pertain to the CNS infection in humans and includes experimental data from animal models that illuminate how these microbes enter the CNS. Recent experimental data showing that L. monocytogenes can invade the CNS by more than one mechanism make it a useful model for discussing the various routes for neuroinvasion used by intracellular bacterial pathogens.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

A gerbil model for rhombencephalitis due to Listeria monocytogenes

Rhombencephalitis due to Listeria monocytogenes is a frequent complication of human listeriosis, inducing a high mortality and severe neurological sequelae despite antibiotic therapy. However, there is no animal model which consistently reproduces clinical rhombencephalitis. Here, we present a model of Listeria rhombencephalitis in gerbils. Animals were inoculated in the middle ears with a low infective dose of L. monocytogenes, thus creating prolonged otitis media with persistent bacteremia. Gerbils developed a severe rhombencephalitis with circling syndrome, paresia, ataxia, rolling movements. The invasion of the central nervous system was visualized on living animals by resonance magnetic imaging and characterized by bacterial growth in the brain, reaching about 10(7) bacteria in the rhombencephalum by day 12 of infection. The histological lesions were mainly located in the brainstem, and consisted in coalescent, necrotic abscesses with perivascular sheaths, mimicking those observed in human rhombencephalitis. Bacteria were detected by electronmicroscopy inside infectious foci, either free in necrotic material or inside inflammatory cells, mainly polymorphonuclear cells. This gerbil model of Listeria rhombencephalitis will be useful to study the molecular mechanisms allowing bacteria to cross the blood-brain barrier, and to evaluate the intracerebral efficacy of antibiotics.

A review of Listeria monocytogenes and listeriosis

Following the initial isolation and description in 1926 Listeria monocytogenes has been shown to be of world-wide prevalence and is associated with serious disease in a wide variety of animals, including man. Our knowledge of this bacterial pathogen and the various forms of listeriosis that it causes has until recently been extremely limited, but recent advances in taxonomy, isolation methods, bacterial typing, molecular biology and cell biology have extended our knowledge. It is an exquisitely adaptable environmental bacterium capable of existing both as an animal pathogen and plant saprophyte with a powerful array of regulated virulence factors. Most cases of listeriosis arise from the ingestion of contaminated food and in the UK the disease is particularly common in ruminants fed on silage. Although a number of forms of listeriosis are easily recognized, such as encephalitis, abortion and septicaemia, the epidemiological aspects and pathogenesis of infection in ruminants remain poorly understood. The invasion of peripheral nerve cells and rapid entry into the brain is postulated as a unique characteristic of its virulence, but relevant and practical disease models are still required to investigate this phenomenon. This review offers an up to date introduction to the organism with a description of virulence determinants, typing systems and a detailed account of listeriosis in animals. Experimental and field papers are reviewed and further sections deal with the diagnosis, treatment and control of listeriosis in animals. A final part gives an overview of listeriosis in man.

Phenotypic characterization of the cells of the inflammatory response in ovine encephalitic listeriosis

The brainstem (pons cerebri and medulla oblongata) of 22 sheep aged between 6 months and 3 years which had developed clinical signs of central nervous system dysfunction were examined. Histopathological changes characterized by microabscesses, focal gliosis and perivascular cuffing compatible with natural infection with Listeria monocytogenes were present. The brains were examined by lectin histochemistry and immunohistochemistry with markers for T lymphocytes (CD4+ and CD8+ subsets), B lymphocytes, mononuclear phagocytes (including macrophages, ramified microglia, activated microglia and amoeboid microglia), astroglia and L. monocytogenes. These methods allowed semiquantitative analyses of the frequency of the different cell types in the brain lesions. The distribution of listerial antigen in the lesions was variable but always sparse. Mononuclear phagocytes and neutrophils appeared to be the most numerous inflammatory cells in the affected areas of the brainstem. T lymphocytes (CD8+ and CD4+ subsets) and B lymphocytes also played a part in the inflammatory process, in addition to activated astrocytes.

Studies of the detection of Listeria monocytogenes by culture and PCR in cerebrospinal fluid samples from ruminants with listeric encephalitis

A total of 14 cerebrospinal fluid (CSF) samples from ruminants clinically suspected of suffering from listeric encephalitis were examined by polymerase chain reaction (PCR) for the detection of Listeria monocytogenes (L. m.). Of these samples, 11 were examined bacteriologically. Although the clinical diagnosis was confirmed in eight of 11 ruminants by histological and/or bacteriological examination of the brains, L. m. was only detected in one of the CSF samples using PCR, and in none by culture. The PCR-positive CSF sample was obtained from a sheep which had been treated with antibiotics prior to CSF sampling. From these findings, it was concluded that L. m. only occasionally gains access to the meningoventricular system in the course of listeric encephalitis of ruminants and that a reliable aetiological in vivo diagnosis of listeric encephalitis generally cannot be based on the detection of L. m. in the CSF of affected ruminants.

Listeria innocua isolated from a case of ovine meningoencephalitis

This paper reports a naturally occurring case of meningoencephalitis associated with Listeria innocua in a Polled-Dorset ewe. The ewe was one of a housed group of twenty-five, fed ad lib. on wrapped baled silage. L. innocua was isolated after one week from cold enrichment culture of brain and pituitary tissue. Its identity was confirmed by conventional biochemical tests, API Listeria (BioMerieux UK Ltd), the absence of hly and prfA genes using PCR assay and sequencing two variable regions of 16S rDNA. Histological examination demonstrated lesions of vasculitis and perivascular cuffing in the midbrain which were consistent with listeriosis although limited in distribution and severity.

Infection of murine fetal brain cell cultures with Listeria monocytogenes

The uptake of Listeria monocytogenes by different brain cells was studied in primary dissociated brain cell cultures derived from murine fetuses. In respect to the supposed intraaxonal migration of Listeria monocytogenes in the pathogenesis of listeric focal brain stem encephalitis, it was examined whether the bacterium was internalized by neurons. Infection rates of distinct cell types were determined by double immunofluorescence with antibodies against cell type-specific markers and the bacterial pathogen. Because of the changing composition of the cultures and time-dependent expression of the oligodendrocyte marker galactocerebroside (GC), infections were carried out on day 4, 6, 8, and 15 in vitro. Listeria monocytogenes was detected predominantly within macrophages. Astrocytes, oligodendrocytes, and fibronectin-expressing cells were infected to a lesser extent. The lowest rates of infection were observed in neurons. A tropism of Listeria monocytogenes for neurons was not detected in vitro.

Diagnostic ultrastructure of Listeria monocytogenes in human central nervous tissue

The pathogenesis of Listeria monocytogenes infection of the human central nervous system is poorly understood and ultrastructural aspects are not documented. A case of fatal human L. monocytogenes meningoencephalitis is described, in which diagnosis was confirmed by blood culture and in which special staining (Brown's) showed slender gram-positive bacilli in inflamed areas of the brainstem at autopsy. Electron microscopy of blocks rescued from formalin revealed rods, up to 2.5 microns long x 0.4 microns in diameter, with gram-positive type cell walls and distinctive conic ends, the latter being apparent in axial section only. The organism was either free within the cytoplasm or within endosomes or phagosomes of macrophages, smooth muscle cells, and endothelial cells. In one instance, one was seen adhering to the luminal aspect of the vascular endothelium. Central nervous system parenchymal cell infection was suspected but not confirmed. Intra-cytoplasmic bacteria were surrounded by an electron-transparent halo and, beyond that, a cloud of host-derived microfilaments, as previously described by others in vitro and in animals. The morphology of L. monocytogenes is compared with that of other intracellular pathogens and is found to be distinctive, suggesting the value of ultrastructural study in diagnosis. Such work could also help to solve outstanding pathogenetic questions of central nervous system invasion and spread.

Choroiditis and meningitis in experimental murine infection with Listeria monocytogenes

In a study of central nervous system involvement in experimental listeriosis 27 Swiss CD1 mice were inoculated subcutaneously with Listeria monocytogenes. Systemic infection developed, as shown by the isolation of Listeria monocytogenes and histopathological lesions in the spleen and liver. In the central nervous system a mixed inflammatory infiltration in the ventricular system, especially in the choroid plexus, and leptomeningitis were the most relevant lesions. Inflammatory lesions were associated with the presence of Listeria monocytogenes, as demonstrated by a positive anti-Listeria monocytogenes immunoperoxidase reaction within phagocytic cells. It is suggested that choroiditis and meningitis developed as a consequence of hematogenous dissemination of Listeria monocytogenes within mononuclear phagocytes and penetration of these cells into the ventricular system through the choroid plexus.

Cell-mediated immunity to intestinal infection

Specified pathogen-free B6D2F1 mice were orally infected with various doses of Listeria monocytogenes. Oral inocula containing more than 2.5 X 10(8) live organisms consistently initiated infection in the Peyer's patches (PP) of the small intestine. At lower doses the infection was sporadic, with many mice showing no apparent infection in the PP. The PP appeared to be the only site of tissue invasion and L. monocytogenes survival in the intestinal tissues, as no organisms were recovered from mucosa dissected free of all visible PP. Within the PP, the bacteria multiplied and the infection then disseminated to the mesenteric lymph node (MLN), liver, and spleen. However, bacteria were almost completely eliminated from all tissues, both systemic and gut-associated by 6 days postinfection. Mice given a primary L. monocytogenes infection by the oral route were highly resistant to subsequent intravenous or oral challenge. Likewise, sublethal intravenous infection rendered mice highly resistant to subsequent oral infection. In addition, lymphocytes from the PP, MLN, and spleens of mice recovering from a primary oral infection were able to adoptively transfer immunity to normal recipients. Finally, after oral infection, mice did not display peripheral delayed hypersensitivity to L. monocytogenes antigens until the organisms had penetrated to the spleen.

Spontaneous listeric encephalitis and neuritis in sheep. Light microscopic studies

Sixteen of 17 sheep with spontaneous listeric encephalitis had neuritis characterized by diffuse and focal intrafascicular and perineural accumulations of lymphocytes, plasma cells, macrophages and neutrophils in one or more cranial nerves. Nine sheep had extensive trigeminal neuritis which was usually unilateral. Brain lesions were mainly in the stem and were foci of macrophages or neutrophils or both, malacia, neutrophilic neuronophagia, vascular cuffing, and meningitis. Lesions in the brain and trigeminal ganglia were most severe on the same side as the affected trigeminal nerve. Gram-positive bacilli were in proximal parts of cranial nerves in foci of inflammatory cells and occasionally in morphologically intact nerve fibers. Organisms in the brain were in phagocytes in areas of inflammation and in scattered neurons and axons. The results were consistent with centripetal migration of the infectious agent along one or more branches of the trigeminal nerve to the brain and dissemination in the brain stem occurring, at least partly, along fiber tracts. Intraaxonal movement of bacteria probably is a mechanism involved in the pathogenesis of this disease.

Effects of antithymocyte and antimacrophage sera on the survival of mice infected with Listeria monocytogenes

Antisera prepared in rabbits against purified mouse thymocytes (antithymocyte serum; ATS) and peritoneal macrophages (antimacrophage serum; AMS) were injected intraperitoneally into Balb/c mice infected with the bacterium Listeria monocytogenes. When administered near the initiation of infection, the ATS significantly decreased the survival time of the animals and increased the mortality rate. When ATS was administered 6 days after a sublethal dose of L. monocytogenes had been inoculated, an overt disease did not evolve. ATS that significantly potentiated primary listeriosis also had high cytotoxicity titers for thymocytes and lymphoid cells from the peritoneal cavity. Although cytotoxic activity against peritoneal macrophages could be demonstrated in lower dilutions of the ATS, this activity did not appear to correlate with the effects of the sera on listeriosis. The injection of AMS did not enhance the infectious process. In some trials more deaths occurred among mice receiving normal rabbit serum than those receiving AMS. All of the AMS had cytotoxic titers against peritoneal macrophages, and the sera were usually inactive against thymocytes and peritoneal lymphoid cells. Listeria was isolated from fatally infected mice with nearly equal success in all of the serum-treated groups, and the serum treatments did not appear to alter the pattern of gross lesions. The afferent limb of the immune response was markedly affected by the presence of antibodies to lymphocytes. However, antibodies reacting with macrophages did not demonstrably enhance the Listeria process, which depends upon cellular immunity as the principal means of acquired host defense.