Treatment with anti-CR3 antibodies ED7 and ED8 suppresses experimental allergic encephalomyelitis in Lewis rats

The role of the mouse macrophage scavenger receptor in myelin phagocytosis

Myelin phagocytosis during Wallerian degeneration and immune-mediated demyelination depends on the action of mononuclear cells of the monocyte/macrophage system. The present study investigated the role of the macrophage scavenger receptor, a trimeric membrane glycoprotein, in myelin uptake by macrophages. Two in vitro models of myelin phagocytosis were used: an organ culture model of mouse peripheral nerves exposed to cocultured macrophages and a quantitative flow cytometric assay. Different concentrations of the monoclonal rat anti-mouse scavenger receptor antibody (2F8) were applied to these systems to selectively block the macrophage scavenger receptor. Concentration-dependent effects on macrophage migration and myelin uptake were seen when the macrophage scavenger receptor was blocked by the antibody 2F8. Low concentrations reduced myelin phagocytosis by the invading macrophages; higher concentrations completely abolished macrophage invasion of the nerves. Using a quantitative flow cytometric assay it was also shown that the 2F8 antibody inhibits phagocytosis of myelin in a dose-dependent manner. These data indicate that the macrophage scavenger receptor is involved in myelin phagocytosis by macrophages.

Leukemia inhibitory factor is expressed in astrocytes following cortical brain injury

The neuropoietic cytokine leukemia inhibitory factor (LIF) can act as a trophic factor, enhancing neuronal survival, and as a differentiation factor altering neuronal and glial gene expression. LIF also plays a role in the response to injury of the peripheral nervous system, as indicated by an increase in the amount of its mRNA within nonneuronal injury response in LIF knock-out mice. To determine if LIF is regulated after injury to the central nervous system, we surgically lesioned the cortex in adult rat brain. Using a quantitative RNAse protection assay, we find that LIF mRNA increases 30-fold following injury. The amount of this transcript goes up within 6 h after injury, reaches a peak at 24 h and returns to baseline by 7 days postlesion. In situ hybridization analysis reveals LIF transcript-containing cells scattered throughout the ipsilateral cortex close, but not immediately adjacent to the lesion site. Double-labeling with a variety of antibodies reveals that LIF mRNA is induced in GFAP-positive astrocytes as well as in a small number of microglial cells. The striking induction of LIF transcripts in glia suggests that this cytokine may play a key injury-response role in the CNS as it does in the PNS, where LIF has been demonstrated to regulate neuropeptide expression both in vivo and in vitro.

Macrophage antigen-1 positive cells are essential in the defense against Theiler's virus strain GD VII infection in the spinal cord

Acute encephalomyelitis caused by Theiler's virus strain GD VII resembles human poliomyelitis, and T cells are essential in eliminating the virus from the brain, though not from the spinal cord. We speculated that macrophage-lineage cells might play a crucial role in eliminating the virus from the spinal cord. To analyse the role of macrophage-lineage cells in the infection, antibodies specific for beta2 integrin, as well as an anti-leukocyte function antigen 1 (LFA-1) monoclonal antibody (MAb) and an anti-complement receptor type 3 (CR3) MAb were used to deplete the corresponding cell populations in Theiler's virus-infected mice. Infiltration of CD8+ T cells into the brain and spinal cord was inhibited by the administration of the anti-LFA-1 MAb, and viral replication was augmented only in the brain. The number of CD4+ T cells and macrophage antigen-1 (Mac-1[+]) cells in the brain and spinal cord were not decreased by anti-LFA-1 MAb treatment. Anti-CR3 MAb treatment led to decrease of Mac-1(+) cells in the brain and spinal cord. The viral replication in the spinal cord of anti-CR3 MAb treated mice was augmented, but not that in the brain. These results indicate that the defense mechanism against Theiler's virus strain GD VII is dependent on Mac-1(+) cells in the spinal cord.

Macrophage infiltration at the neuromuscular junction does not contribute to AChR loss and age-related resistance to EAMG

Aged rats resistant to acetylcholine receptor loss in passive transfer experimental autoimmune myasthenia gravis (EAMG) do not reveal infiltrating macrophages at the neuromuscular junction (NMJ) as observed in susceptible rats. It was investigated whether this age-related resistance is due to impaired macrophage function in these aged rats. Reconstitution of aged rats with bone marrow from young donors did not lead to macrophage infiltration, nor did it abolish resistance to EAMG. Subsequently, it was investigated whether macrophages are a primary cause of acetylcholine receptor (AChR) loss in EAMG or are attracted to the NMJ secondary to tissue damage. In lethally irradiated young susceptible rats infiltrating macrophages were absent from the NMJ. However, similar AChR losses were observed in irradiated and non-irradiated rats. These results suggest that macrophages do not contribute to acetylcholine receptor loss in the effector phase of passive transfer EAMG and that age related resistance to passive transfer EAMG is not primarily determined by the absence of infiltrating macrophages.

Factors controlling T-cell migration across rat cerebral endothelium in vitro

The migration of lymphocytes through primary cultures of rat brain microvascular endothelial cell monolayers was examined in vitro by time-lapse videomicroscopy. Antigen-specific T cell line migration was dependent on the duration of culture (post-antigen stimulation) with exogenous interleukin-2 (IL-2). Peak migration (approximately 50% of T-cells during the 4 h migration assay) occurred after 4 days of culture with IL-2 but did not coincide with maximal expression of LFA-1, VLA-4 or the IL-2 receptor. On unstimulated endothelia antibody blockade of LFA-1 or ICAM-1 inhibited T-cell line migration to 8.0% and 6.8% of control values, respectively, whereas blocking VLA-4 and VCAM-1 had no effect. On IL-beta activated endothelium blocking LFA-1 and ICAM-1 was less effective (24.9% and 27.3% of control values, respectively) and blockade of VLA-4 and VCAM-1 brought about a reduction to 63.0% and 68.3% of controls respectively. Inhibition of IL-2-dependent proliferation with an IL-2 receptor blocking antibody also significantly inhibited T-cell migration to 22.2% of controls. Peripheral lymph node (PLN) lymphocytes could also be induced to migrate through untreated cerebral endothelial cell monolayers by cross-linking CD3 which was also time and IL-2-dependent with maximal migration (22.7%) occurring after three days in the presence of exogenous IL-2. Blocking LFA-or ICAM-1 resulted in a significant reduction in migration across IL-1 beta-activated endothelial cells to 17.4% and 20.9% of control values respectively although blocking the VLA-4/VCAM-1 interaction had no significant effect. Activation of PLN lymphocytes with concanavalin A for up to 5 days did not induce migration but when left in contact with the endothelial monolayer for 24 h migration reached 31.0%. These studies indicate that T-cells require a combination of signals to trigger the migratory phenotype which is necessary to enable them to penetrate the blood-brain barrier.

Resolution of CNS lesions following treatment of experimental allergic encephalomyelitis in macaques with monoclonal antibody to the CD18 leukocyte integrin

Experimental allergic encephalomyelitis (EAE) in macaques is an acute inflammatory and demyelinating disease of the central nervous system (CNS) which has been studied extensively as a model of the human demyelinating disease multiple sclerosis (MS). The in vivo administration of monoclonal antibodies against CD18, the common beta-chain of a leukocyte integrin, at the onset of clinical disease, significantly prolonged the survival of nine of 11 macaques (82%) and in some cases completely reversed the clinical appearance of disease. Treatment with anti-CD18 mAbs dramatically reduced the extent of inflammation in brain lesions as determined by magnetic resonance imaging (MRI). These improvements confirm that anti-CD18 mAbs are powerful anti-inflammatory agents in vivo and suggest that such mAbs may provide effective treatment of both demyelinating and inflammatory CNS diseases in man.

Therapy with antibody against leukocyte integrin VLA-4 (CD49d) is effective and safe in virus-facilitated experimental allergic encephalomyelitis

Experimental allergic encephalomyelitis (EAE) is facilitated in resistant BALB/c mice by intraperitoneal infection with an avirulent Semliki Forest virus (SFV-A7). Viral infection increases the incidence of EAE from 15-30% to 60-90% and speeds up appearance of paralysis from 24 to 14 days. In this paper, we describe treatment of virus-facilitated EAE with monoclonal antibodies (mAbs) against leukocyte and/or endothelial cell adhesion molecules. Therapy with mAb against ICAM-1 (intercellular adhesion molecule-1) had a modest effect, but caused hemorrhagic brain and spinal cord lesions. Therapy with mAb against Mac-1 (alpha M beta 2-integrin) was well tolerated but had no effect. Therapy with mAb against VLA-4 (alpha 4 beta 1-integrin) was safe, diminished both clinical and histopathological signs of EAE, decreased induction of VCAM-1 (vascular cell adhesion molecule-1) on brain vessels and diminished infiltration of VLA-4+ cells into the brain. The amount of viral antigen in the brain was not altered. We conclude that facilitation of leukocyte entry into the brain is a major mechanism for viral facilitation of EAE in the BALB/c mouse, and that facilitation can be inhibited by anti-adhesion therapy. This may have implications for treatment of relapses triggered by viral infections in multiple sclerosis.

Macrophage phagocytosis of myelin in vitro determined by flow cytometry: phagocytosis is mediated by CR3 and induces production of tumor necrosis factor-alpha and nitric oxide

Demyelination of axons in the central nervous system (CNS) during multiple sclerosis (MS) and its animal model experimental allergic encephalomyelitis (EAE) is a result of phagocytosis and digestion by macrophages (M phi) and the local release of inflammatory mediators like tumor necrosis factor-alpha (TNF-alpha) and nitric oxide (NO). We have investigated the process of myelin phagocytosis by M phi in vitro using flow cytometric analysis. The binding and uptake of CNS-derived myelin was dose dependent, was abolished in the presence of EDTA and was enhanced after opsonization with complement. The phagocytosis of opsonized myelin could be inhibited by antibodies directed against complement receptor type 3 (CR3). Furthermore, CR3 also contributes to phagocytosis of non-opsonized myelin, e.g. under serum-free conditions. The phagocytosis of CNS-derived myelin induced the production of substantial amounts of TNF-alpha and NO by the M phi. Our results indicate an important role for CR3 in myelin phagocytosis. The induction of TNF-alpha and NO which accompanies this phagocytosis may further contribute to the overall process of demyelination during MS or EAE.

Lymphocyte targeting of the central nervous system: a review of afferent and efferent CNS-immune pathways

The central nervous system (CNS) in considered to be an immunological privileged site. However, inflammatory reactions in response to virus infections, in multiple sclerosis (MS) and in experimental autoimmune encephalomyelitis (EAE) suggest that there are definite connections between the CNS and the immune system. In this review, we examine evidence for afferent and efferent pathways of communication between the CNS and the immune system, the pivotal role of regional lymph nodes in T-cell mediated autoimmune disease of the CNS, and the factors involved in lymphocyte targeting of the CNS. Afferent pathways of lymphatic drainage of the brain are well established in a variety of species, especially rodents. Fluid and antigens appear to drain along perivascular spaces populated by immunocompetent perivascular cells. Drainage pathways connect directly via the cribriform plate to nasal lymphatics and cervical lymph nodes. Soluble antigens draining from the brain induce antibody production in the cervical lymph nodes. Using a model of cryolesion-enhanced EAE, we review the role of lymphatic drainage and cervical lymph nodes in the enhancement of cerebral EAE. If a brain wound in the form of a cryolesion is produced 8 days post inoculation (dpi) of antigen in the induction of acute EAE, there is a 6-fold increase in severity of cerebral EAE by 15 dpi. Removal of the cervical lymph nodes significantly reduces such enhancement of EAE. These findings suggest that drainage of antigens from the brain to the cervical lymph nodes, in the presence of activated lymphocytes in the meninges or CNS, results in an enhanced second wave of lymphocytes targeting the brain. In examining the efferent immune pathway by which lymphocytes home to the CNS, several studies have characterized the phenotype of infiltrating T lymphocytes by the use of immunocytochemistry or FACS analysis. T-cells infiltrating the CNS are recently activated/memory lymphocytes typified by their high expression of CD44, LFA-1 and ICAM-1 and low expression of CD45RB in the mouse. Following the induction of EAE in susceptible mice, ICAM-1 and VCAM-1 are dramatically upregulated on CNS vessels; lymphocytes bind to such vessels via the interaction of their known ligands, LFA-1/Mac-1 and alpha 4-integrins, at least in vitro. It appears that alpha 4-integrin plays a key role in lymphocyte recruitment across the blood-brain barrier and may be a major factor in lymphocyte targeting of the CNS. Definition of factors involved in the afferent and efferent connections between the CNS and the immune system may clarify mechanisms involved in immune privilege of the CNS and may open significant therapeutic opportunities for multiple sclerosis.

IL-10 production by adult human derived microglial cells

Microglia, a population of central nervous system (CNS) macrophages, have been demonstrated to support immune accessory and effector functions in the CNS. Numerous studies support the role of microglia in CNS development and pathology, where activation of microglia is consistently noted. The current study investigated microglial immune functions under basal and activation conditions and assessed the ability of interleukin-10 (IL-10), added exogenously or produced by microglia, to down-regulate microglial functions. This report demonstrates that microglia from the adult human brain produce IL-10 following interferon-gamma/lipopolysaccharide activation. Functionally, recombinant human IL-10 down-regulated basal HLA-DR expression by microglia and inhibited, in a dose-dependent response, the ability of microglia to stimulate CD4+ T-cells in antigen presentation assays. These data, together with recent observations of the inhibition of experimental allergic encephalomyelitis (EAE) following IL-10 administration and reduced CNS infection by Listeria monocytogenes after anti-IL-10 treatment, suggest that IL-10 production by microglia may have important immune-regulatory functions in CNS disease and disease models.

Transfer of type II collagen-induced arthritis from DBA/1 to severe combined immunodeficiency mice can be prevented by blockade of Mac-1

Collagen-induced arthritis in susceptible mice is widely accepted as an experimental model for human rheumatoid arthritis (RA). We have investigated the role of the Mac-1 integrin beta 2 in the development and maintenance of arthritis by means of in vivo administration of 5C6 monoclonal antibody (mAb) to block this receptor. Injection of a single dose of 5C6 mAb (0.5 mg, intraperitoneally) prior to the expected onset of collagen-induced arthritis in DBA/1 mice diminished the severity of subsequent disease in these animals, as assessed both clinically and histologically (P < 0.01, chi 2). In the DBA/1 to severe combined immunodeficiency (SCID) transfer model of arthritis, the incidence of clinical arthritis was significantly reduced in SCID mice receiving maintained 5C6 treatment commencing the day prior to administration of donor splenocytes. Histological evaluation of joints from animals without clinically evident arthritis confirmed the absence of an inflammatory infiltrate in 22/27 joints examined. In a minority of these joints, however, synovial hyperplasia was apparent. In contrast, delaying antibody administration until 10 days after donor spleen cell transfer failed to protect three of five SCID recipients. These results confirm a functional role for Mac-1 in the generation of collagen-induced arthritis in mice. Since mAb 5C6 is non-cytotoxic, its action must be by blockade of the interactions between Mac-1 and its natural ligand(s). Our findings support the hypothesis that cells expressing Mac-1 play an important role in the induction and maintenance of joint damage in collagen-induced arthritis.

Aggravation of experimental allergic encephalomyelitis (EAE) by administration of nitric oxide (NO) synthase inhibitors

Macrophages constitute a large proportion of the inflammatory cells that infiltrate the central nervous system (CNS) of animals with EAE. Through the production of inflammatory mediators these infiltrating macrophages can contribute to the regulation of the immune reaction within the CNS, that eventually results in neurological deficits associated with EAE. NO, a free radical produced by macrophages and other cell types, has been put forward as such an immune mediator. In the present study we show that macrophages isolated from the CNS of Lewis rats with clinical signs of EAE produce elevated amounts of NO. We treated rats, in which EAE was induced, with N(omega) -nitro-L-arginine-methylester or N(g)-monomethyl-L-arginine, inhibitors of NO synthase, either systemically via intraperitoneal injection, or intracerebrally via a cannula placed in the lateral ventricle. Both treatments resulted in a marked aggravation of clinical signs of EAE. These data point to an important role of NO, produced by infiltrating macrophages, as an immune-suppressor in the disease process during EAE.

The role of macrophage subpopulations in autoimmune disease of the central nervous system

In this review the role of various subpopulations of macrophages in the pathogenesis of experimental autoimmune encephalomyetitis is discussed. Immunohistochemistry with macrophage markers shows that in this disease different populations of macrophages (i.e. perivascular cells, microglia and infiltrating blood-borne macrophages) are present in the central nervous system. These subpopulations partially overlap in some functional activity while other activities seem to be restricted to a distinct subpopulation, indicating that these subpopulations have different roles in the pathogenesis of encephalomyelitis. The studies discussed in this review reveal that immunocytochemical and morphological studies, combined with new techniques such as in situ nick translation and experimental approaches like the use of bone marrow chimeras and macrophage depletion techniques, give valuable information about the types and functions of cells involved in central nervous system inflammation. The review is divided in three parts. In the first part the experimental autoimmune encephalomyelitis model is introduced. The second part gives an overview of the origin, morphology and functions of the various subpopulations. In the third part the role of these subpopulations is discussed in relation to the various stages (i.e. preclinical, clinical and recovery) of the experimental disease.

The membrane attack complex of complement mediates peripheral nervous system demyelination in vitro

The present study used cocultures of rat dorsal root ganglia (DRG) and peritoneal macrophages to define the role of activated complement components during demyelination. The complement cascade was activated in vitro by treatment of the cultures with natural rat serum and lipopolysaccharides. Complement activation was examined by detection of the membrane attack complex of complement (MAC) with an antibody directed against rat C5-9. Detection of MAC in vitro by immunoelectron microscopy was associated with morphological changes of the myelin sheath. The sheath's regular structure was disrupted. Myelin lamellae were split and showed signs of decompaction. These changes were followed by a selective macrophage attack on myelin sheaths resulting in demyelination. Schwann cell viability was not affected by complement activation. Axons and sensory ganglion cells also survived this attack. The specificity of the complement effect was tested in experiments using treatment regimens with natural rat serum or lipopolysaccharides alone. In these experiments, no morphological changes of the myelin sheath were observed as well as no macrophage attack on myelin.

Both anti-CD11a (LFA-1) and anti-CD11b (MAC-1) therapy delay the onset and diminish the severity of experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is a demyelinating disease of the central nervous system (CNS) induced in rodents by activated CD4+ T cells specific for various myelin proteins such as myelin basic protein and proteolipid protein. The disease is characterized by breach of the blood-brain barrier, perivascular infiltration of leukocytes into the CNS, local inflammation and demyelination in the form of plaques. In this study, we evaluated the effect of administration of antibodies to two members of the beta 2 integrin sub-family of adhesion molecules, CD11a and CD11b, on the onset and progression of EAE. CD11a and CD11b are involved in cell-cell interactions leading to T cell and macrophage extravasation to inflammatory sites and T cell activation. Our results show that anti-CD11a antibodies could completely block the induction of EAE and anti-CD11b antibodies significantly delayed the onset and diminished the severity of clinical signs of EAE even when injections were initiated at the first appearance of clinical signs.

Anti-CD11b/CD18 antibodies reduce inflammation in acute colitis in rats

The influx of monocytes and neutrophils into the inflamed tissue could be an important aspect in the pathogenesis of inflammatory bowel disease (IBD). A membrane protein involved in the monocyte/neutrophil adherence to endothelium is CD11b/CD18 or alpha M beta 2 (complement receptor type 3 = CR3). In the present study the role of CD11b/CD18 in experimental IBD was studied by treatment with ED7 and OX42, two MoAbs against CD11b/CD18. Colitis was induced in rats by a single, rectal administration of 30 mg 2,4,6-trinitrobenzene sulfonic acid (TNBS) dissolved in ethanol 30%. Two hours before and 3 days after induction of colitis, the animals were given an i.v. dose of 0.5 mg of either ED7 or OX42 in 1 ml PBS. Controls received PBS or an irrelevant MoAb. Four days after the last treatment with the antibodies, the rats were killed, and macroscopic damage scores of the colon were determined. Macrophages and granulocytes were studied by immunohistochemistry and quantified by Interaktives Bild Analysen System (IBAS), and myeloperoxidase (MPO) activity in colonic tissue was measured. After treatment with ED7 and OX42 the mean damage score of the colon was reduced from 4.2 in IBD animals to 1.0 and 1.3, respectively. Smaller areas of ulcerations and a decrease in the number of ulcerations were observed compared with PBS-treated rats. Furthermore, the amount of infiltrating monocytes and leucocytes in the submucosa was enormously reduced, as well as MPO activity in the colonic tissue. These results show that treatment with MoAbs against CD11b/CD18 reduces clinical signs of experimental IBD in rats by a partial blockade of infiltrating macrophages and granulocytes.

Complement depletion affects demyelination and inflammation in experimental allergic neuritis

The effect of systemic complement depletion by cobra venom factor (CVF) on experimental allergic neuritis (EAN) was studied in rats immunized with variable amounts of bovine peripheral nerve myelin. Low-dose myelin EAN rats treated with CVF i.p. (n = 10) had lower clinical scores (0.3 +/- 0.7 vs. 1.1 +/- 1.1), less demyelination (0.4 +/- 0.8 vs. 1.9 +/- 1.1) and inflammation (0.6 +/- 1.2 vs. 2 +/- 1) than EAN animals treated with i.p. saline (n = 10). Endoneurial infiltrates had fewer ED1-positive (phagocytic) macrophages (0.4 +/- 0.5 vs. 1.6 +/- 1.1) and CD11bc-positive (expressing iC3b receptor or CR3) cells (1 +/- 0.8 vs. 2.5 +/- 0.8) (mean +/- S.D.) detected by immunocytochemistry. This effect was partially abrogated by immunizing animals with a higher dose of myelin. Our studies suggest that complement may play a role in the recruitment of macrophages into the endoneurium and in opsonizing myelin for phagocytosis.

Chemokine expression in murine experimental allergic encephalomyelitis

Chemokines are a family of low molecular mass proteins with chemotactic and cell activating activities. Reverse transcription-polymerase chain reaction and Northern hybridization were used to examine their expression during murine experimental allergic encephalomyelitis (EAE), an autoimmune disease used as a model of multiple sclerosis. The mRNAs encoding RANTES, MIP-1 alpha, MIP-1 beta, TCA3 (I-309), IP-10, JE (MCP-1), KC (MGSA/gro), and MARC (MCP-3) were induced in the spinal cord 1-2 days before clinical signs were apparent. SDF, a cDNA predicted to encode a chemokine-like product, was expressed in normal as well as diseased spinal cords. No expression of C10 or MIP-2 was detected. Activated encephalitogenic T cells expressed message for RANTES, MIP-1 alpha, MIP-1 beta, and TCA3. These results define a subset of chemokines that may play an important role in the inflammatory process during murine EAE.

Macrophages in T cell line-mediated, demyelinating, and chronic relapsing experimental autoimmune encephalomyelitis in Lewis rats

About 50% of the mononuclear cells in the perivascular lesions in the central nervous system (CNS) of rats suffering from experimental allergic encephalomyelitis (EAE) are blood-borne macrophages. In this study we investigated the role of these macrophages in different variants of EAE, using a liposome-mediated macrophage depletion technique. Intravenously injected liposomes containing dichloromethylene diphosphonate (Cl2MDP) are ingested by macrophages and cause temporary and selective elimination of these cells. Macrophage depletion during EAE induced by a T cell line specific for myelin basic protein (MBP; T cell-EAE) suppresses development of neurological signs of EAE. T cell-EAE with pronounced demyelination as induced by an additionally injected MoAb directed against myelin oligodendrocyte glycoprotein (MOG) was also significantly ameliorated after macrophage depletion. During chronic relapsing EAE (CR-EAE) the occurrence of relapses was prevented or suppressed, provided that the liposomes were injected before the initiation of a putative relapse. A chronic progressive course of CR-EAE was not modified by Cl2MDP containing liposome treatment. Histologic examination of the CNS of liposome-treated animals confirmed decreased infiltration of macrophages into the parenchyma in the rats with T cell and AD-EAE, whereas T cells were still present.

Rat interleukin-2-activated natural killer (A-NK) cell-mediated lysis is determined by the presence of CD18 on A-NK cells and the absence of major histocompatibility complex class I on target cells

The precise mechanism by which target cells are recognized and subsequently lysed by interleukin-2-activated natural killer (A-NK) cells is poorly understood. In this study the role of major histocompatibility complex (MHC) class I and adhesion molecules in the recognition and lysis of tumor cells was investigated in a syngeneic Wag rat model. Preincubation of tumor cells with F(ab')2 fragments of anti-MHC class I monoclonal antibody (mAb) OX18 strongly enhanced the A-NK cell-mediated lysis. Also normal syngeneic cells such as T cells and A-NK cells became highly sensitive for lysis by A-NK cells after preincubation with mAb OX18. Two other mAb against MHC class I had no effect on lysis of target cells. These data indicate that masking of MHC class I on syngeneic tumor and normal cells by mAb OX18 is sufficient for A-NK cells to recognize target cells as non-self, resulting in lysis. In addition, we found that the presence of mAb against the beta 2 (CD18)-integrins blocked the lysis of all tumor cell lines by A-NK cells in 51Cr-release assays, also when target cells were preincubated with mAb OX18. Because of the absence of CD18 on most tumor cells we concluded that a CD18-associated integrin on A-NK cells is essential for lysis of target cells. These results show that in this syngeneic rat model CD18 on A-NK cells together with MHC class I on tumor cells determine A-NK cell-mediated lysis. Furthermore, we hypothesize that the anti-MHC class I OX18 recognizes an epitope on rat MHC class I which is, or is very close to, the restriction element determining A-NK cell-mediated lysis.