Non-opsonic attachment of Bordetella bronchiseptica mediated by CD11/CD18 and cell surface carbohydrates

Porcine atrophic rhinitis associated with Bordetella bronchiseptica is characterized by a severe inflammatory response in the mucosa of the nasal turbinates. Initial infiltrates of polymorphonuclear leukocytes (PMN) are followed by accumulations of mononuclear cells. In this report, we have investigated the interaction between porcine PMN and B. bronchiseptica. PMN incubated in PBS with a fluorescently labeled hemagglutinating porcine isolate, but not a non-hemagglutinating variant, had high levels of cell-associated fluorescence as determined by flow cytometry. Light microscopy indicated that most cell-associated bacteria were ingested. Transmission electron microscopy confirmed the presence of intracellular bacteria, which were contained within membrane-bound phagosomes. A monoclonal antibody specific for the leukocyte integrin polypeptide CD18 partially inhibited attachment of B. bronchiseptica to normal PMN but not to PMN genetically deficient in CD11/CD18 integrins. Higher levels of inhibition occurred when bacteria and normal PMN were co-incubated in the presence of galactose, N-acetyl-D-galactosamine, N-acetyl-D-glucosamine, mannose and methyl alpha-D-mannopyranoside. D-glucose, L-fucose, alpha-lactose and sialic acid had no inhibitory effect. We conclude that B. bronchiseptica is readily ingested by porcine PMN in the absence of complement and antibody and that internalization is mediated by multiple adhesion mechanisms, including CD18- and carbohydrate-dependent ones.

Identification of a granulocytotropic Ehrlichia species as the etiologic agent of human disease

Six patients from northern Minnesota and Wisconsin with a febrile illness accompanied by granulocytic cytoplasmic morulae suggestive of ehrlichial infection were identified. Two patients died, and splenic granulocytes of one patient contained cytoplasmic vacuoles with organisms ultrastructurally characteristic of ehrlichiae. From one patient, a 1.5-kb DNA product was amplified by PCR with universal eubacterial primers of 16S rDNA. Analysis of the nucleotide sequence of the amplified product revealed 99.9 and 99.8% similarities with E. phagocytophila and E. equi, respectively, neither of which has previously been known to infect humans. From the variable regions of the determined sequence, a forward primer specific for three organisms (human granulocytic ehrlichia, E. phagocytophila, and E. equi) and a reverse primer for these ehrlichiae and E. platys were designed. By nested PCR with amplification by the universal primers and then reamplification with the specific primers described above, the expected 919-bp product was generated from the blood of the index patient and three additional patients. Blood from these four patients and two more patients with granulocytic morulae contained DNA which was amplified by nested PCR involving a combination of a universal primer and the human granulocytic ehrlichia-E. phagocytophila-E. equi-E. platys group-specific primer. This apparently vector-borne human granulocytic ehrlichia has only 92.5% 16S rDNA homology with E. chaffeensis. Nested PCR with group-specific primers did not amplify E. chaffeensis DNA, and E. chaffeensis-specific primers did not amplify DNAs of the human granulocytic ehrlichia. Thus, six patients were shown to be infected by an Ehrlichia species never previously reported to infect humans.

Interleukin-8 enhances nonoxidative intracellular killing of Mycobacterium fortuitum by human granulocytes

The results of this study show that recombinant interleukin-8 (IL-8) enhances the intracellular killing of Mycobacterium fortuitum by human granulocytes. This chemokine did not stimulate the phagocytosis of M. fortuitum by granulocytes at various bacterium-to-cell ratios. The killing process was not affected by the NADPH oxidase inhibitor diphenyleneiodonium bisulfate, which indicates that recombinant IL-8 stimulates oxygen-independent mycobactericidal mechanisms of granulocytes. IL-8 did not stimulate H2O2 production in granulocytes but primed the cells for enhanced H2O2 production upon stimulation with preopsonized M. fortuitum. In sum, the chemokine IL-8 not only is involved in the recruitment of granulocytes to the site of infection but also facilitates the elimination of microorganisms by increasing the efficiency of the bactericidal activity of granulocytes.

Electron microscopic identification of parvovirus virions in erythroid and granulocytic-line cells in a patient with human parvovirus B19 induced pancytopenia

In this short report we describe a patient with human parvovirus B19 (HPV B19)-induced transient pancytopenia. Parvovirus virions were seen by electron microscopy in both erythroid and granulocytic precursors. Erythroid cells are not the only targets in these cases. We draw attention to this disorder so that physicians involved with hematological disorders and transplantation be more aware of this infection.

Amplification of superoxide anion generation in phagocytic cells by HIV-1 infection

Amplification of superoxide (O2-) generation by HIV-1 infection was examined in two human myeloid-monocytic cell lines. The level of O2- generation in HL-60 after infection became significantly higher than that of the steady-state. A similar phenomenon was also shown in U937, but only after acquisition of O2- generation ability by differentiation to macrophages. By means of the NADPH oxidase-coupled responses in infected cells, we reconstituted the O2(-)-generating machinery in cell-free system. The results suggested that cytosolic factor(s) exerted by infection might be responsible for the amplification of O2- generation. Thus, HIV-1 infection could elevate the level of oxidative stress in macrophages which might play an important role in disease progression.

In vitro proliferation of a canine granulocytic Ehrlichia

Canine granulocytic Ehrlichia sp., an agent which parasitizes the neutrophilic leukocytes in dogs, was transiently propagated in vitro. Dogs were experimentally inoculated with blood containing canine granulocytic Ehrlichia. Bacteremias in experimentally infected dogs varied from 1.2 to 9.3% granulocytes infected. Granulocytes from experimentally infected dogs were harvested and cultured in the presence of RPMI 1640 medium supplemented with fetal bovine serum, conditioned medium, and HEPES buffer. The percentages of granulocytes containing ehrlichial morulae increased significantly with time for 2 to 4 days, with at least one culture from each dog achieving 20% of granulocytes infected. Granulocytes taken from infected dogs early in bacteremia yielded cultures with the greatest percentage of infected cells. By 5 days post-infection the percentage of infected granulocytes decreased as did leukocyte viability. Attempts to maintain the in vitro cultures for prolonged periods by addition of uninfected granulocytes failed to increase the number of infected host cells, suggesting that no new infections were initiated and that observed increases in the percentage of infected cells in in vitro cultures were due to growth of the organism in granulocytes that were infected in vivo.

Lung surfactant suppresses oxygen-dependent bactericidal functions of human blood monocytes by inhibiting the assembly of the NADPH oxidase

Surfactant is known to lower the surface tension in alveoli and affects the antibacterial functions of alveolar and peritoneal macrophages. We investigated the effects of surfactant on the bactericidal functions and oxidative metabolism of human blood monocytes and granulocytes. Monocytes incubated with surfactant ingest this material and subsequently exhibit an impaired ability to kill ingested bacteria. Granulocytes incubated with surfactant do not ingest this material, and their bactericidal functions are not affected. However, granulocytes that have ingested surfactant-coated Staphylococcus aureus display an impaired ability to kill these bacteria. Moreover, in monocytes and granulocytes that contain surfactant--the latter by ingestion of surfactant-coated S. aureus--the intracellular production of H2O2 is impaired due to inhibition of the assembly of the NADPH oxidase. Together these results demonstrate that surfactant inside monocytes and granulocytes inhibits the capacity of these cells to kill bacteria intracellularly by impairing oxygen-dependent killing mechanisms.

Intracellular bactericidal activity of fosfomycin against staphylococci: a comparison with other antibiotics

The intracellular bactericidal activity of fosfomycin was compared to that of other anti-staphylococcal agents. Two staphylococcal strains (one isolate of Staphylococcus aureus and one isolate of Staphylococcus epidermidis) were incubated in vitro with human granulocytes isolated from volunteers. After phagocytosis and removal of residual extracellular bacteria, the cells were resuspended in the presence of clinically relevant concentrations of fosfomycin, teicoplanin, vancomycin and rifampin. The number of bacteria surviving intracellularly was determined by colony counts after osmotic lysis of the granulocytes at different time intervals. Incubation with fosfomycin resulted in a mean reduction of intracellular bacteria by approximately two logs of ten within 18 h for both staphylococcal strains. Although less effective than rifampin, fosfomycin showed a significantly higher intracellular activity than the glycopeptide antibiotics in this model.

Modulation of leukotriene generation by invasive bacteria

The effect of invasive bacteria on the release of proinflammatory mediators (oxygen radicals, leukotriene release) from human polymorphonuclear neutrophils was studied. Bacterial stimuli were used including genetically cloned invasive Yersinia enterocolitica strains 108-P (bearing the phagocytosis-resistance plasmid) and 108-C (plasmidless variant), Listeria monocytogenes [SLCC 5779 (inv-) and NCTC 7973 (inv+)] as well as an Escherichia coli K 12 strain (pRI 203) in which the inv gene of Y. pseudotuberculosis was cloned. When human polymorphonuclear granulocytes were studied as target cells the inv+ as well as the inv- strains were phagocytosed to a comparable amount with the exception of the L. monocytogenes strain (inv+). Among the invasive strains E. coli HB 101 (pRI 203) was the most active to trigger polymorphonuclear leucocytes (PMN) for oxygen radical production. Preincubation of the cells with bacteria and subsequent stimulation with the Ca ionophore A23187 or opsonized zymosan suppressed the chemiluminescence response to a different degree. The various bacterial strains did not induce leukotriene release from endogenous arachidonic acid. Subsequent stimulation of the infected cells with Ca ionophore or opsonized zymosan led to an altered pattern of the combined amounts of leukotriene B4 (LTB4), 20-OH- and 20-COOH-LTB4 as well as the ratio of LTB4 versus 20-OH and 20-COOH-LTB4. Infection of the cells also reduced strain dependently the number of LTB4-receptor sites. Our data suggest that bacterial uptake modulates the inflammatory response of granulocytes (e.g. chemiluminescence response, leukotriene generation).

Severe in vitro inhibition of erythropoiesis and transient stimulation of granulopoiesis after bone-marrow infection with eight different HIV-2 isolates

OBJECTIVE

To correlate severe anaemia and frequent neutropenia in HIV-2-infected patients with an inhibitory effect on bone-marrow progenitors common to several HIV-2 isolates.

DESIGN

The effects of eight HIV-2 isolates on early (BFU-E) and late (CFU-E) erythroid progenitors and on granulomonocytic (CFU-GM) progenitors, produced in long-term bone-marrow cultures (LTBMC), were studied.

METHODS

Absolute numbers of BFU-E, CFU-E and CFU-GM per culture flask were calculated weekly for each HIV-2-infected LTBMC using semi-solid clonogenic assays, and compared with those obtained in mock-infected LTBMC. Levels of significance for comparisons were determined by an analysis of variance (ANOVA).

RESULTS

Pooled data from 24 series of LTBMC (three series for each HIV-2 isolate) revealed 80 and 100% inhibition of BFU-E and CFU-E on days 6 and 12 of LTBMC, respectively, while transient stimulation of CFU-GM was observed between days 14 and 20 of LTBMC, followed by total inhibition on day 30.

CONCLUSIONS

These results confirm a direct inhibitory effect of HIV-2 on in vitro haematopoiesis. The similar pattern of erythroid progenitor inhibition obtained from seven out of eight isolates suggests that the inhibitory effect on erythropoiesis is a feature common to a large number of HIV-2 isolates, which correlates with clinical findings.

Regulation of expression of the reovirus receptor on differentiated HL60 cells

The reovirus receptor on mammalian cells has not been fully characterized and controversy exists over the nature of this receptor. We report here that the expression of this receptor is dependent on the differentiation status of a human promyelocytic leukaemia cell line (HL60). Phorbol treatment of HL60 cells for 24 h, at a concentration range of 160 nM down to 1 nM, led to differentiation of these cells towards monocytes and a loss of approximately 80% of their ability to bind reovirus in a fluorescence assay. These cells also lost their susceptibility to T1 and T3 reovirus infection. DMSO treatment for 24 h at a concentration of 1.25% (v/v) led to differentiation towards granulocytes. This was accompanied by an increase of approximately 15% in binding of reovirus to these cells. After being infected by T1 or T3 reovirus, the granulocytes produced higher titres of progeny virus than did untreated HL60 cells. Similar differences were noted when virus binding to HL60 cells was assayed using radiolabelled reovirus. These effects were not detected when murine L fibroblasts were treated with DMSO or phorbol. ATCC-derived murine R1.1 cells did not bind reovirus. Competition data indicated that there may be two reovirus receptors on HL60 cells, and that T1 can bind to only one receptor whereas T3 can bind to both receptors. Our data also suggested that the beta-adrenergic receptor was unlikely to act as the reovirus receptor on HL60 cells.

Detection of cytomegalovirus (CMV) in granulocytes by polymerase chain reaction compared with the CMV antigen test

To compare the sensitivity and suitability of detection of active cytomegalovirus (CMV) infection by using monoclonal antibodies against CMV antigen (antigen test to detect antigenemia) and the polymerase chain reaction (PCR; to detect viral DNA) in granulocytes, 19 heart and 2 lung transplant recipients were closely monitored by these tests for at least 3 months after transplantation. All patients were CMV seropositive or had a seropositive donor. In total, 201 samples were tested; 46 were positive by both tests, 9 samples showed only antigenemia, 54 samples were positive by PCR only, and 102 samples were negative by both tests. PCR was positive earlier after transplantation in eight patients, whereas antigenemia was positive earlier after transplantation in one patient. In another four patients, both tests were positive at the same time. PCR was, on average, positive for a longer period of time. Discordant results showing a positive antigen test and a negative PCR were partly due to sampling error; some were positive by PCR on retesting. Samples which were negative by the antigen test and positive by PCR were taken at the beginning or at the end of an active CMV infection. In two patients, no active CMV infection was detected by the antigen test, cultures of urine and saliva, or serology, although PCR was positive for a long period of time in the two patients.

Effects of equine infectious anemia virus on hematopoietic progenitors in vitro

Direct effects of equine infectious anemia virus (EIAV) on hematopoiesis in vitro were studied. Bone marrow mononuclear cells from clinically normal horses were incubated with 100 TCID50 of EIAV/10(7) cells. These cells were cultured to assay for colonies derived from erythroid progenitors, granulocyte/monocyte progenitors, and fibroblastic progenitors. The EIAV had a selective suppressive effect on the erythroid progenitors. Colony-forming units-erythroid were suppressed to 80% of that for medium controls (P = 0.011). Burst-forming units-erythroid were suppressed to 70% of that for medium controls (P = 0.003). Significant effect was not apparent on colony-forming units-granulocyte/macrophage or on colony-forming units-fibroblastic.

Human immunodeficiency virus infection in central nervous system and myeloid-monocytic lineage

We established persistent infection with a strain of human immunodeficiency type 1, HTLV-IIIB, in a ++promyelo-monocytic cell line, ML-1 (CD4 antigen nearly negative and CD4 mRNA negative), and a promonocytic cell line, THP-1 (CD4 antigen positive). Different reactions of giant cell formation were found after cocultivation of infected and uninfected cells of ML-1, HL-60, THP-1 and U-937 cell lines with uninfected and infected MOLT4 (a T-lymphoma cell line). Factors affecting the cellular tropism of human immunodeficiency virus were discussed.

Abortive human cytomegalovirus infection in patients after allogeneic bone marrow transplantation

Infection with human cytomegalovirus (HCMV) after allogeneic bone marrow transplantation (BMT) was studied in 12 HCMV seronegative recipients of marrow from seropositive donors by weekly monitoring of cultures, expression of HCMV antigenemia (pp65) in granulocytes, polymerase chain reaction (PCR) on HCMV-DNA in granulocytes and IgM and IgG anti-HCMV antibodies. Eight patients remained negative in all tests as did 33 HCMV seronegative recipients of marrow from seronegative donors. In four patients, a transient expression of HCMV antigen pp65 in granulocytes from peripheral blood, together with a positive PCR on HCMV-DNA from the same samples were found without positive cultures, seroconversion or expression of other HCMV antigens in granulocytes. The data indicate the presence of an abortive HCMV infection in these four patients.

Antibodies and complement enhance binding and uptake of HIV-1 by human monocytes

OBJECTIVE

To characterize antibody- and complement-mediated binding and uptake of HIV-1 by human monocytes.

DESIGN

The first step in the infection of the monocyte by HIV-1 is binding of the virus to the susceptible cell. Procedures were designed to assess the influence of anti-HIV-1 antibodies and complement on this binding, and to study the process of internalization following binding.

METHODS

Human monocytes were incubated with fluorescein-labelled purified HTLV-IIIB virions and human sera with high-titre anti-HIV-1 antibodies and/or complement. Binding and uptake of virus by the monocytes was measured as fluorescence per cell by flow cytometry.

RESULTS

Binding of purified HIV-1 to monocytes was increased by complement and, to a lesser extent, by anti-HIV-1 antibodies. Uptake of HIV-1 bound to the monocyte appeared to be mediated by antibodies and was increased further by the presence of complement. Complement alone, however, resulted in the uptake of only a small part of the bound virus.

CONCLUSIONS

Complement significantly increases the binding of HIV-1 to human monocytes, and a combination of antibodies and complement efficiently mediates uptake of HIV-1 by monocytes.

Evaluation of granulocytic ehrlichiosis in dogs of Missouri, including serologic status to Ehrlichia canis, Ehrlichia equi and Borrelia burgdorferi

Canine granulocytic ehrlichiosis was diagnosed in 37 dogs by finding ehrlichial morulae in 0.1 to 26.2% of their blood neutrophils and eosinophils. All 37 dogs had clinical signs of arthritis or muscular stiffness. Titer to Ehrlichia canis was determined in sera from 31 of the 37 dogs; 25 dogs had titer ranging from 1:20 to 1:5,120. In the other 6 dogs, titer to E canis was less than 1:10. The most common hematologic abnormality in these dogs, other than rickettsiemia, was thrombocytopenia. Granulocytes infected with ehrlichial organisms were not found in another 10 dogs that had clinical signs of arthritis or muscular stiffness. Of these 10 dogs, 3 had titer to E canis ranging from 1:40 to 1:320. Titer in the other 7 dogs was less than 1:10. Ehrlichial morulae were not found in the granulocytes of 18 healthy dogs. Of these 18 dogs, 9 had titer to E canis ranging from 1:20 to 1:5,120. Titer in the other 9 dogs was less than 1:10 Titer to Borrelia burgdorferi was determined in dogs with granulocytic ehrlichiosis, arthritic dogs without detected rickettsiemia, and in healthy dogs. Low titer determined by 2 laboratories was considered to be nonspecific reaction in all 3 groups of dogs and, thus, did not indicate that the arthritic disorders were attributable to canine borreliosis.

Analyses of Ehrlichia canis and a canine granulocytic Ehrlichia infection

Ehrlichia canis and canine granulocytic Ehrlichia sp. (CGE) infect canine monocytes and granulocytes, respectively. E. canis has been cultured in vitro and used to develop an immunofluorescence assay. CGE has not been cultured, and a serologic assay is not available. The sera of dogs infected with CGE were reported to react with E. canis by immunofluorescence. In this study, the temporal response of immunoglobulin G (IgG) was determined by an enzyme-linked immunosorbent assay (ELISA) with purified E. canis antigen in four dogs experimentally infected with E. canis, in two dogs experimentally infected with CGE, and in one dog infected with E. canis and subsequently infected with CGE. E. canis-infected dogs developed an IgG ELISA result of 1.5 or greater for the optical density signal/noise ratio by 2 months postinfection. CGE challenge of a dog with a previous E. canis infection induced an anamnestic increase in the IgG ELISA result; however, CGE infection alone did not induce a significant IgG ELISA response. Western immunoblot analysis showed that dogs infected with E. canis developed antibodies initially that reacted with low-molecular-mass proteins (30, 24, and 21 kDa) and subsequently with higher-molecular-mass proteins (160, 100, 78, 64, 47, and 40 kDa). In contrast, CGE-infected dogs showed reactions with the same higher-molecular-mass proteins of E. canis but, unlike E. canis-infected dogs, not with the low-molecular-mass proteins of E. canis. Of 10 serum samples collected in the field of Indonesia from dogs with tropical canine pancytopenia, all had an optical density signal minus noise value of 2.54 or greater in the IgG ELISA and reacted with E. canis antigen in a pattern similar to that of serum samples from dogs experimentally infected with E. canis in Western immunoblotting. This study suggests that the IgG ELISA and Western immunoblotting with purified E. canis as the antigen are useful in distinguishing between E. canis and CGE infections in dogs.

Influence by staphylococcal lipase on granulocyte metabolism and killing of bacteria

Lipase purified from Staphylococcus aureus interferes with granulocyte phagocytic killing of S. aureus but not with killing of Streptococcus pneumoniae or Streptococcus agalactiae group B. Lipase treatment of granulocytes did not influence bacterial adherence to the cells.

Human immunodeficiency virus infection in cells of myeloid-monocytic lineage

We established persistent infection with a strain of human immunodeficiency virus type 1, HTLV-IIIB, in a promyelomonocytic cell line, ML-1 (CD4 antigen nearly negative and CD4 mRNA negative), and a promonocytic cell line, THP-1 (CD4 antigen positive). Different reaction of giant cell formation was found after co-cultivation of infected and uninfected cells of ML-1, HL-60, THP-1 and U-937 cell lines with uninfected and infected MOLT4 (a T-lymphoma cell line).

Infection of granulocyte/monocyte progenitor cells with HIV1

In order to study whether cytopathic HIV1 infection of haemopoietic progenitor cells is involved in the derangement of haemopoiesis in patients with HIV1 infection, we infected enriched progenitor cells with HIV1, by addition of viral inoculate supernatants from HIV1-infected peripheral blood mononuclear cells or by coculture with HIV1-infected monocytes/macrophages. Progenitor cells were seeded into colony assays and single colonies were chosen for HIV1 mRNA determination by in situ hybridization. Growth of progenitors was not affected by infection. However, up to 42% of colonies of pluripotent progenitor cells (colony-forming unit/granulocyte-erythrocyte-monocyte; CFU-GEM) and committed progenitor cells CFU/granulocyte-monocyte (CFM-GM) contained HIV1 mRNA-expressing cells. In addition, we studied HIV1 infection of progenitor cells from the bone marrow of 6 patients with AIDS or AIDS-related complex. Two patients were negative, two had a few colonies expressing HIV1 mRNA in a minority of cells, and in the remaining two, up to 11% of CFU-GM contained HIV1-expressing cells. Thus, infection of progenitor cells with HIV1 was achieved experimentally in vitro and occurs in vivo. However, growth of progenitors after in vitro infection continues and therefore HIV1 infection does not seem to contribute directly to the reduced incidence of haemopoietic progenitor cells in vivo.

Hepatitis B virus binds to peripheral blood mononuclear cells via the pre S1 protein

The hepatitis B virus has been documented in hepatic and extrahepatic compartments, including bone marrow and peripheral blood cells. The viral protein involved in the attachment to human hepatocytes has been identified within the N-terminus of the pre S1 envelope protein. Using recombinant particles containing the pre S1, pre S2 and S encoded sequences, we studied which virus envelope protein is involved in binding to peripheral blood cells. Mononuclear cells of 20 healthy subjects bound 125I-labelled particles, with a S/N ratio higher than 2.5 (range 2.69-7.77). Binding was abolished by trypsinization. B lymphocytes and monocytes were found to bind viral particles much more efficiently compared to T cells and granulocytes. A monoclonal antibody (MA 18/7), recognizing the (27-49) pre S1 sequence, completely inhibited viral particle attachment to PBMC, while anti-pre S2 (Q 19/10) and anti-S (20/2) monoclonal antibodies had no effect. We conclude that the pre S1 sequence is involved in HBV attachment to PBMC and to hepatocytes. The nature of the cellular attachment site is unknown, but it might be a receptor for physiologic ligands, as occurs with other viruses.

The antibacterial activity of benzylpenicillin against Staphylococcus aureus ingested by granulocytes

Granulocytes that had ingested pre-opsonized Staphylococcus aureus for 3 min at 37 degrees C and then had been washed twice, were incubated in the presence of benzylpenicillin. The antibiotic showed a concentration-dependent effect on cell-associated S. aureus between 0.005 and 0.1 mg/l benzylpenicillin. The antibacterial effect on cell-associated S. aureus was equal to that on opsonized S. aureus in suspension. Microscopic studies with lysostaphin showed that the number of extracellular bacteria was negligible and that the decrease in the number of cell-associated bacteria reflected the true rate of intracellular killing of S. aureus. Contrary to the generally accepted opinion that benzylpenicillin does not act intracellularly in granulocytes, we were able to show intracellular antibacterial activity of benzylpenicillin against S. aureus under the experimental conditions we used.

Cellular isoform of the scrapie agent protein participates in lymphocyte activation

The scrapie agent protein (Sp33-37 or PrPSc) is the disease-associated isoform of a normal cellular membrane protein (Cp33-37 or PrPC) of unknown function. We report that normal human lymphocytes and lymphoid cell lines, but not erythrocytes or granulocytes, express PrPC mRNA and protein. PrPC is detectable on the surface of lymphocytes; the surface immunoreactivity is sensitive to phosphatidylinositol-specific phospholipase C, indicating glycosyl-phosphatidylinositol membrane anchorage. Lymphocyte PrPC surface abundance is increased by cell activation, and polyclonal antibodies to PrPC suppress mitogen-induced activation. We conclude that PrPC is a lymphocyte surface molecule that may participate in cell activation.