RESUMO
BACKGROUND: CD56+ T cells previously have been identified as potentially cytotoxic lymphocytes, and relative numbers are increased in some infectious diseases. PATIENTS AND METHODS: Relative proportions of CD56+ T cells were measured by flow cytometry in groups of renal transplant patients differing in cytomegalovirus (CMV) status of donor (D) and recipient (R). These measurements were related to episodes of CMV viremia. RESULTS: Patient groups in which recipients (R+) or donors (D+/R-) were CMV+ had significantly higher proportions of CD56+ T cells (5.11 ± 0.69% and 5.42 ± 1.01%, respectively) than the D-/R- group (1.9 ± 0.35%; P = 0.0018 and 0.017, respectively). In the high-risk D+/R- group, it was found that patients who had post-transplant CMV viremia had higher levels than those who remained CMV negative (9.09 ± 2.34% vs. 3.16 ± 1.22%; P = 0.01). CD56+ T cells from R+ and D+/R- groups had higher proportions of both CD4+ and CD8+ cells than the D-/R- group. When activation markers were examined, some CD56+ T cells from both CMV+ groups had a TEM phenotype, with significantly more expressing CD45RO and NKG2C, and less expressing CD28, CD62L, CD127, and CD161 compared to the D-/R- group. Some CD56+ T cells showed specificity for CMV antigens and similar proportions of CD8+ cells were positive for class I HLA-CMV tetramers containing immunodominant CMV peptides compared to the majority CD56- T cells. CONCLUSION: The results show significant increases in proportions of CD56+ T cells in relation to CMV infection in renal transplant patients and suggest that these cells have a cytotoxic function against CMV-infected cells.
Assuntos
Antígeno CD56/sangue , Infecções por Citomegalovirus/imunologia , Transplante de Rim , Complicações Pós-Operatórias/imunologia , Linfócitos T Citotóxicos/metabolismo , Adulto , Idoso , Biomarcadores/sangue , Estudos Transversais , Infecções por Citomegalovirus/etiologia , Feminino , Citometria de Fluxo , Humanos , Masculino , Pessoa de Meia-Idade , Doadores de TecidosRESUMO
PURPOSE: We looked for herpes simplex virus types 1 and 2 (HSV-1 and HSV-2, respectively), varicella zoster virus (VZV), Epstein-Barr virus (EBV) and cytomegalovirus (CMV) DNA in Malawian adults with clinically suspected meningitis. METHODS: We collected cerebrospinal fluid (CSF) from consecutive adults admitted with clinically suspected meningitis to Queen Elizabeth Central Hospital (QECH), Blantyre, Malawi, for a period of 3 months. Those with proven bacterial or fungal meningitis were excluded. Real-time polymerase chain reaction (PCR) was performed on the CSF for HSV-1 and HSV-2, VZV, EBV and CMV DNA. RESULTS: A total of 183 patients presented with clinically suspected meningitis. Of these, 59 (32 %) had proven meningitis (bacterial, tuberculous or cryptococcal), 39 (21 %) had normal CSF and 14 (8 %) had aseptic meningitis. For the latter group, a herpes virus was detected in 9 (64 %): 7 (50 %) had EBV and 2 (14 %) had CMV, all were human immunodeficiency virus (HIV)-positive. HSV-2 and VZV were not detected. Amongst those with a normal CSF, 8 (21 %) had a detectable herpes virus, of which 7 (88 %) were HIV-positive. CONCLUSIONS: The spectrum of causes of herpes viral meningitis in this African population is different to that in Western industrialised settings, with EBV being frequently detected in the CSF. The significance of this needs further investigation.
Assuntos
Infecções por Herpesviridae/virologia , Herpesviridae/isolamento & purificação , Meningite Viral/virologia , Adulto , Citomegalovirus/isolamento & purificação , DNA Viral/líquido cefalorraquidiano , Feminino , Herpesviridae/genética , Infecções por Herpesviridae/diagnóstico , Infecções por Herpesviridae/epidemiologia , Herpesvirus Humano 1/isolamento & purificação , Herpesvirus Humano 2/isolamento & purificação , Herpesvirus Humano 3/isolamento & purificação , Herpesvirus Humano 4/isolamento & purificação , Humanos , Malaui/epidemiologia , Masculino , Meningite Viral/diagnóstico , Meningite Viral/epidemiologiaRESUMO
OBJECTIVE: To evaluate a sensitive and specific, real-time PCR assay with internal control for Chlamydia trachomatis and Neisseria gonorrhoeae DNA detection in urine specimens. METHODS: The diagnostic performance of a laboratory-developed quadruplex assay (LDQA) targeting the cryptic plasmid and MOMP genes of C trachomatis, the porA pseudogene of N gonorrhoeae and a synthetic internal control was assessed using 1028 urine specimens. The LDQA was compared with the Roche COBAS Taqman CT test and the COBAS Amplicor NG assay with supplemental confirmation tests. The subsequent performance of the LDQA in detecting N gonorrhoeae was monitored in comparison with bacterial culture from swabs. RESULTS: 88 (8.6%) urines were determined as C trachomatis positive in the diagnostic evaluation. LDQA sensitivity and specificity were calculated to be 100% and 99.9%, respectively, for C trachomatis. The LDQA showed high specificity with isolates of other Neisseria species and gave complete concordance with resolved data for N gonorrhoeae detection. However, the incidence of N gonorrhoeae infection was low, with 17 (1.7%) positive patients. A post-implementation audit of 14 316 patients gave the LDQA N gonorrhoeae urine PCR protocol (porA, OPA, 16s rDNA) a sensitivity of 96.9% and specificity of 99.8% in comparison with bacterial culture from swabs. CONCLUSIONS: The LDQA was found to be an effective method for the detection of C trachomatis and N gonorrhoeae DNA in urine samples, and the PCR protocol has replaced bacterial culture for the screening of N gonorrhoeae in asymptomatic men and women in the laboratory.
Assuntos
Infecções por Chlamydia/diagnóstico , Chlamydia trachomatis/isolamento & purificação , DNA Bacteriano/urina , Gonorreia/diagnóstico , Neisseria gonorrhoeae/isolamento & purificação , Reação em Cadeia da Polimerase/métodos , Adolescente , Adulto , Idoso , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Reação em Cadeia da Polimerase/normas , Sensibilidade e Especificidade , Adulto JovemRESUMO
PURPOSE: Cytomegalovirus (CMV) retinitis classically occurs in advanced human immunodeficiencyvirus (HIV) infection but is rare in other forms of immunosuppression. The authors report a case of CMV retinitis in an HIV-negative man with idiopathic CD4 lymphocytopenia (ICL). This is the first such case to be confirmed by polymerase chain reaction (PCR) of aqueous humor. METHODS: Case report. RESULTS: A 69-year-old retired Chinese seaman presented with gradual visual deterioration. He was a diet controlled diabetic on regular steroids for presumed asthma. Examination showed no diabetic eye disease but confirmed acute retinal necrosis (ARN). Anterior chamber tapping of the aqueous humor was PCR positive for CMV. HIV antibody and RNA tests were negative but his full blood count revealed lymphocytopenia, with a low CD4+ subset. He responded to a 3-week course of intravenous ganciclovir therapy followed by suppressiveoral valganciclovir. CONCLUSIONS: CMV is associated with sight-threatening retinitis in HIV infection at CD4+ counts below 50 cells/microL and in transplant recipients or heavily immunosuppressed patients. Systemic steroids are a risk factor for clinical disease in these groups. It is extremely rare to report CMV eye disease in previously healthy individuals. This case illustrates that the condition does occur in association with ICL. Corticosteroids may be implicated in disease reactivation. Molecular METHODS are necessary to confirm the diagnosis.
Assuntos
Retinite por Citomegalovirus/virologia , Soronegatividade para HIV , Terapia de Imunossupressão , Síndrome de Necrose Retiniana Aguda/virologia , Idoso , Antivirais/uso terapêutico , Humor Aquoso/virologia , Contagem de Linfócito CD4 , Citomegalovirus/genética , Citomegalovirus/isolamento & purificação , Retinite por Citomegalovirus/diagnóstico , Retinite por Citomegalovirus/tratamento farmacológico , DNA Viral/análise , Ganciclovir/análogos & derivados , Ganciclovir/uso terapêutico , Humanos , Masculino , Reação em Cadeia da Polimerase , Síndrome de Necrose Retiniana Aguda/diagnóstico , Síndrome de Necrose Retiniana Aguda/tratamento farmacológico , ValganciclovirRESUMO
In the 1980s the outcome of patients with herpes simplex encephalitis was shown to be dramatically improved with aciclovir treatment. Delays in starting treatment, particularly beyond 48 h after hospital admission, are associated with a worse prognosis. Several comprehensive reviews of the investigation and management of encephalitis have been published. However, their impact on day-to day clinical practice appears to be limited. The emergency management of meningitis in children and adults was revolutionised by the introduction of a simple algorithm as part of management guidelines. In February 2008 a group of clinicians met in Liverpool to begin the development process for clinical care guidelines based around a similar simple algorithm, supported by an evidence base, whose implementation is hoped would improve the management of patients with suspected encephalitis.
Assuntos
Gerenciamento Clínico , Encefalite Viral/terapia , Adulto , Antivirais/uso terapêutico , Encefalite por Herpes Simples/tratamento farmacológico , Encefalite Viral/diagnóstico , Encefalite Viral/epidemiologia , Encefalite Viral/patologia , HumanosRESUMO
BACKGROUND: Over the last 15 years, bacterial meningitis has received considerable attention, including national guidelines, whilst viral central nervous system (CNS) infections have been relatively neglected. A recent pilot study suggested that management of patients with suspected viral encephalitis was often suboptimal. AIM: To examine the relative incidence, clinical features and management of suspected acute CNS infections in adults across the NHS North West Region. DESIGN: A multicentre cross-sectional retrospective cohort study at 10 hospitals across the region over 3 months (from September to December 2007). Following a screen of all patients who had cerebrospinal fluid (CSF) analysis or received intravenous aciclovir and/or third-generation cephalosporin, those with clinical features suspicious of a CNS infection were included. Management was compared with the national meningitis and regional encephalitis guidelines. RESULTS: Three hundred and eighty-five patients were screened; 217 patients had a suspected CNS infection and 44 (20%) had a CNS infection: 18 aseptic meningitis (one herpes simplex virus [HSV]-2), 13 purulent meningitis (four Streptococcus pneumoniae) and 13 encephalitis (three HSV-1). The median (range) time from admission to suspicion of CNS infection and to LP was longer for patients with encephalitis than meningitis [4 (0.3-312) vs. 0.3 (0.1-12) h, P<0.001, and 23 (4-360) vs. 12 (2-48) h, P=0.042, respectively]; and the median time to treatment was longer for aciclovir than cephalosporin [7 (0.5-312) vs. 3 (0.3-312) h, P=0.002]. DISCUSSION: Encephalitis was as common as purulent meningitis, and HSV as common as Streptococcus pneumoniae. However, the management of patients with encephalitis was worse than meningitis. National encephalitis guidelines are needed.
Assuntos
Infecções Bacterianas do Sistema Nervoso Central/tratamento farmacológico , Viroses do Sistema Nervoso Central/tratamento farmacológico , Doença Aguda , Adolescente , Adulto , Idoso , Idoso de 80 Anos ou mais , Infecções Bacterianas do Sistema Nervoso Central/diagnóstico , Infecções Bacterianas do Sistema Nervoso Central/epidemiologia , Viroses do Sistema Nervoso Central/diagnóstico , Viroses do Sistema Nervoso Central/epidemiologia , Inglaterra/epidemiologia , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Estudos Retrospectivos , Resultado do Tratamento , Adulto JovemRESUMO
BACKGROUND: Few studies have investigated the impact of chronic hepatitis B and C infection on antiretroviral therapy (ART) outcomes in sub-Saharan Africa. Hepatotoxicity may be a particular concern in co-infected patients taking nevirapine-stavudine-lamivudine. METHODS: We conducted a prospective cohort study of 300 Malawian adults starting ART and describe one-year ART outcomes according to viral hepatitis status. RESULTS: At baseline, patients had advanced HIV disease (29.3% were in WHO stage 4; mean CD4 = 157 cells/microL; mean log(10)HIV-1 RNA = 5.24 copies/ml). Co-infection with hepatitis B, C and B + C were present in 6.7%, 5.7% and 1.7% respectively. At 50 weeks, all-cause mortality was 43 (14.3%). Sixteen (5.3%) had transferred to another unit. Eight (2.7%) were lost to follow up. Sixteen (5.3%) had stopped ART. 217 (72.3%) were alive on ART, of whom 82.5% had an HIV-1 RNA <400 copies/ml at week 50. During the first 50 weeks of ART, severe hepatotoxicity (liver enzyme values >5 times upper level of normal) occurred in 9%, but did not result in any ART discontinuations. Clinical hepatitis or jaundice was not observed. There were no significant differences in occurrence of hepatotoxicity, other side effects, mortality, severe morbidity, immune reconstitution or virological failure between hepatitis B and/or C co-infected patients and those who were not. Viral hepatitis co-infection was not associated with severe hepatotoxicity, mortality, severe morbidity or virological failure in multivariate analyses. CONCLUSION: Our data suggest that screening for viral hepatitis B and C and liver enzyme monitoring may not require high priority in ART programmes in sub-Saharan Africa.
Assuntos
Fármacos Anti-HIV/administração & dosagem , Terapia Antirretroviral de Alta Atividade/métodos , Infecções por HIV/complicações , Infecções por HIV/tratamento farmacológico , Hepatite B Crônica/complicações , Hepatite C Crônica/complicações , Adulto , Fármacos Anti-HIV/efeitos adversos , Terapia Antirretroviral de Alta Atividade/efeitos adversos , Estudos de Coortes , Feminino , HIV-1 , Humanos , Malaui , Masculino , Gravidez , Estudos Prospectivos , Resultado do TratamentoRESUMO
Hepatitis B (HBV) and HIV co-infection is common in resource-poor settings. A recent study from Malawi revealed poor correlation between hepatitis B surface antigen (HBsAg) point-of-care tests and reference tests in patients co-infected with HIV. We studied a cohort of 300 Malawian adults entering a treatment programme for HIV. Sera were tested for HBsAg first using the Determine rapid test and re-tested using a commercial enzyme immunoassay (EIA). All tests were done under optimal conditions in Liverpool, UK. Sera from all 25 patients positive for HBsAg using the rapid test and from 50 who were negative, were re-tested using the EIA, with complete concordance of results. The kappa correlation was 1, specificity 100% (93-100%) and sensitivity 100% (86-100%) compared to the reference test. Patients had advanced immune suppression (mean CD4=175 cells x 10(6)/l). In a non-field setting, the results of point-of-care Determine rapid hepatitis B tests appear reliable in patients with HIV-1 co-infection.
Assuntos
Infecções por HIV/epidemiologia , Antígenos de Superfície da Hepatite B/sangue , Vírus da Hepatite B/isolamento & purificação , Hepatite B/diagnóstico , Adulto , Estudos de Coortes , Feminino , Hepatite B/imunologia , Humanos , Malaui/epidemiologia , Masculino , Kit de Reagentes para Diagnóstico , Sensibilidade e Especificidade , Testes Sorológicos/métodos , Virologia/métodosRESUMO
BACKGROUND: Coinfection with hepatitis B (HBV) or hepatitis C (HCV) adversely affects the prognosis of HIV infection and vice versa, and results in complex interactions with antiretroviral therapy. These infections are common in sub-Saharan Africa but there are few data on prevalence of coinfection. All three components of the most common ART regimen used in Africa, stavudine, lamivudine and nevirapine, can cause hepatic problems and lamivudine resistant HBV is known to emerge after HBV monotherapy in coinfected patients. Point of care (POC) tests for HBV and HCV are widely used but have not been validated in field tests in sub-Saharan Africa. METHODS: Prospective observational study of sequential adult inpatients in medical wards of a large urban teaching hospital in Malawi in 2004. Comparison of demographic risk factors with HIV antibody status determined using local double POC test protocols, and with HBsAg and HCV antibody prevalence as estimated in a reference laboratory in Liverpool, UK. Results of locally performed POC tests for HBV using Determine HBsAg (Abbott) and for HCV antibody using HCV-SPOT (Genelabs) were compared with results of reference methods in the UK. RESULTS: Of 226 adults (39% male), median (range) age 35 (14-80) years, 81% had a history of traditional scarification, 12% a history of blood transfusion and 11% a history of jaundice. HIV antibodies were present in 76.1%, HBsAg in 17.5% and HCV in 4.5%, with HIV/HBV coinfection in 20.4% and HIV/HCV coinfection in 5% of those with HIV. There was no correlation between prevalence of any of the three viruses and demographic risk factors or presence of either of the other two viruses. Point of care tests gave misleading results with prevalence estimates of 38% for HBV and 4.5% for HCV. For both of these POC tests the performance indices were unacceptable for individual patient management or epidemiological survey purposes. CONCLUSIONS: The high prevalence of hepatitis/HIV coinfections may impact on treatment with antiretroviral therapy, especially if there are unintended interruptions of therapy, and studies are needed to document the possible clinical impact on ART programmes. The poor performance of POC tests for HBV and HCV may be due to local operational problems or to unexpected technical issues not revealed by early validation tests. These tests are widely used in resource poor settings and should be revalidated in prospective field studies in areas of the tropics with high HIV prevalence rates.
Assuntos
Infecções por HIV/epidemiologia , Hepatite B/epidemiologia , Hepatite C/epidemiologia , Hospitais de Ensino/estatística & dados numéricos , Adolescente , Adulto , Idoso , Idoso de 80 Anos ou mais , Feminino , Anticorpos Anti-HIV/sangue , Infecções por HIV/complicações , Infecções por HIV/diagnóstico , Hepatite B/complicações , Hepatite B/diagnóstico , Antígenos de Superfície da Hepatite B/sangue , Hepatite C/complicações , Hepatite C/diagnóstico , Anticorpos Anti-Hepatite C/sangue , Humanos , Malaui/epidemiologia , Masculino , Pessoa de Meia-Idade , Sistemas Automatizados de Assistência Junto ao Leito , Prevalência , Garantia da Qualidade dos Cuidados de Saúde , Fatores de RiscoRESUMO
This report presents the characterization of three mouse monoclonal antibodies (mAb) reactive with the guinea-pig leucocyte common antigen (LCA); CD45 in the human nomenclature. One, IH-1, reacted with LCA on all leucocytes. The other two were more restricted: IH-2 recognized only the 220,000, 210,000 and 195,000 MW isoforms, and IH-4 the 220,000, 210,000 MW isoforms. Both IH-2 and IH-4 reacted with all B cells and all Kurloff cells [the putative guinea-pig natural killer (NK) cell]. IH-2, but not IH-4, reacted with monocytes and macrophages. Neither reacted with neutrophils. Most thymocytes expressed low levels of the IH-2 and IH-4 epitopes, with those expressing high levels located predominantly within the medulla. Most (90%) CD4+ T cells from newborn guinea-pigs expressed high levels of the IH-2 and IH-4 epitopes; this percentage decreased with age to 70% in 2-year-old animals. We have demonstrated that CD4+ T cells which express low levels of the IH-2 epitope also express low levels of the IH-4 epitope. CD8+ T cells can be divided into two subsets by IH-4 but not IH-2. The reactivities of IH-2 and IH-4 are remarkably similar to those of human anti-CD45RB and anti-CD45RA antibodies respectively. Analogies with man and other species suggest important functional differences for subpopulations of guinea-pig T lymphocytes defined by anti-CD45R antibodies.
Assuntos
Antígenos Comuns de Leucócito/análise , Subpopulações de Linfócitos T/imunologia , Envelhecimento/imunologia , Animais , Anticorpos Monoclonais/imunologia , Linfócitos B/imunologia , Linfócitos T CD4-Positivos/imunologia , Eletroforese em Gel de Poliacrilamida , Cobaias , Técnicas Imunoenzimáticas , Camundongos , Camundongos Endogâmicos BALB C , Linfócitos T/imunologia , Timo/imunologiaRESUMO
Ten cases of hepatitis A were associated with a public house whose barman had chronic non-infectious diarrhoea and had served drinks while incubating hepatitis A himself. Eight cases had drunk in the public house in the two weeks before the barman became ill, had no other risk factors for hepatitis A infection, and were regarded as primary cases. Two other cases had visited the pub during the same period, but were classified as secondary cases because they became ill later and were contacts of primary cases. Sexual transmission was excluded. Fomite transmission by contamination of glasses was a more likely route of spread of the infection among the eight primary cases than food or contaminated surfaces in the toilet.
Assuntos
Surtos de Doenças , Hepatite A/transmissão , Restaurantes , Inglaterra/epidemiologia , Contaminação de Equipamentos , Feminino , Hepatite A/epidemiologia , Hepatite A/prevenção & controle , Hepatovirus , Humanos , Higiene , Masculino , Microbiologia da ÁguaRESUMO
A 31-year-old patient in remission of acute lymphoblastic leukaemia (ALL), receiving oral maintenance chemotherapy (6-mercaptopurine, methotrexate (MTX), cyclophosphamide), developed a monoclonal, Epstein-Barr virus (EBV)-related lymphoproliferative disorder (LPD). Treatment consisted of excisional biopsy and the discontinuation of maintenance chemotherapy. To our knowledge, this is the first such report in an adult. The histological similarity to previous reports of 'lymphomatoid granulomatosis' following paediatric ALL suggests that they are the same disease. MTX may play a central role in the development of LPD in this setting. Although it is a rare complication of ALL, EBV-related LPD should be considered in patients who develop lymphadenopathy.
Assuntos
Infecções por Herpesviridae/complicações , Herpesvirus Humano 4 , Transtornos Linfoproliferativos/virologia , Leucemia-Linfoma Linfoblástico de Células Precursoras/tratamento farmacológico , Infecções Tumorais por Vírus/complicações , Adulto , Protocolos de Quimioterapia Combinada Antineoplásica/uso terapêutico , Rearranjo Gênico , Humanos , Masculino , Reação em Cadeia da Polimerase/métodosRESUMO
The molecular epidemiology of varicella-zoster virus in London, England, between 1971 and 1995 was examined by using two informative polymorphic markers, variable repeat region R5 and a BglI restriction site in gene 54. Viruses from 105 cases of chickenpox and 144 of zoster were typed. Two alleles of R5, A and B, were found at prevalences of 89 and 6%, respectively. No difference in allele frequency between the zoster and chickenpox cases was found, and no change in the frequencies of these alleles was observed to occur over time. By contrast, a BglI restriction site (BglI+) was found with increasing frequency over time among cases of varicella (P < 0.005) and, to a lesser extent, cases of zoster. The BglI+ polymorphism was strongly associated (P < 0.0005) with zoster in subjects who had immigrated to the United Kingdom from countries with low adult immunity to varicella (LAIV). Sixty-three percent of the subjects with zoster who had emigrated from countries with LAIV carried the BglI+ virus, in contrast to 10% of adults who had grown up in countries with high adult immunity to varicella. The significance of these data, in view of the changing epidemiology of chickenpox, is discussed.
Assuntos
Varicela/epidemiologia , Herpes Zoster/epidemiologia , Herpesvirus Humano 3/genética , Adulto , Fatores Etários , Desoxirribonucleases de Sítio Específico do Tipo II , Emigração e Imigração , Feminino , Variação Genética , Herpes Zoster/imunologia , Herpesvirus Humano 3/classificação , Herpesvirus Humano 3/isolamento & purificação , Humanos , Londres/epidemiologia , Masculino , Epidemiologia Molecular , Polimorfismo Genético , Prevalência , Sequências Repetitivas de Ácido Nucleico , Mapeamento por Restrição , Sorotipagem , Fatores de Tempo , Reino UnidoRESUMO
The regression of Epstein-Barr (EB) virus-transformed B-cell outgrowth which is seen in experimentally-infected cultures of blood mononuclear (UM) cells from healthy seropositive donors can be abolished in medium containing the T-cell-suppressive agent cyclosporin A (CSA) at concentrations of 0.05 microgram/ml and above. CSA mediates its effect within the first 4 days post-infection of the UM cells and this prevents subsequent in vitro generation of the EB virus-specific cytotoxic-T-cell response which normally brings about regression. Regression can be fully restored by supplementing the CSA-treated culture with interleukin 2 (IL-2)-containing culture supernatants or indeed with purified IL-2 itself, suggesting that CSA mediates its effect in this system through inhibiting the endogenous production of IL-2 which is required to amplify the virus-specific cytotoxic response. "Spontaneous transformation" to EB virus genome-positive lymphoblastoid cell lines in noninfected cultures of UM cells from healthy seropositive donors, though rare in normal medium, is enhanced to such a degree in the presence of CSA that, for many donors, the phenomenon becomes titratable against input cell dose across the 2.0 X 10(6)-2.5 X 10(5) cells/culture range. Cell mixing experiments suggest that the spontaneously transformed cell lines which arise with such efficiency under these conditions do so not by direct in vitro outgrowth of progenitor cells transformed by the virus in vivo, but by a two-step mechanism involving virus release and secondary infection in vitro.